MD Antoni Bayes-Genis
email: abayesgenis@gmail.com.

James L Januzzi

A Mark Richards

Henrike Arfsten

Rudolf A Boer

Michele Emdin

Arantxa González

Tiny Jaarsma

Pardeep S Jhund

Christian Mueller

Rudolf A De Boer

Arantxa Gonzalez

Julio Nunez

Patrick Rossignol

Ivan Milinkovic

Giuseppe M C Rosano

Andrew Coats

Petar Seferovic

Julio Núñez

Giuseppe Mc Rosano

The 'Peptide for Life' Initiative: a call for action to provide equal access to the use of natriuretic peptides in the diagnosis of acute heart failure across Europe

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The Breathing Not Properly (BNP) study, published in 2002, kick-started the use of natriuretic peptides (NPs) as companion biomarkers for the diagnosis of heart failure (HF) [START_REF] Maisel | Breathing Not Properly Multinational Study Investigators. Rapid measurement of B-type natriuretic peptide in the emergency diagnosis of heart failure[END_REF]. More than just a clinical trial with positive results, it revolutionized a whole field and introduced biomarkers and the molecular phenotyping of the heart to algorithms in use for HF diagnosis and management. Ever since, the use of biomarkers to predict the onset of future HF, identify its presence when fully developed, and riskstratify affected patients has gathered much attention and inspired a tsunami of studies.

Indeed, the introduction of objective, non-invasive, biologically meaningful biomarkers to clinical assessment has considerably changed the way HF is diagnosed and monitored [START_REF] Mueller | Heart Failure Association of the European Society of Cardiology. Heart Failure Association of the European Society of Cardiology practical guidance on the use of natriuretic peptide concentrations[END_REF].

The setting of acute heart failure (AHF) diagnosis is where the evidence for the use of NPs is most robust. Twenty years after the BNP study using B-type natriuretic peptide (BNP), with similar evidence reported for amino-terminal BNP (NTproBNP), NPs have a central position in international guidelines for the diagnosis of AHF, and have thus been incorporated into the biomarker portfolios of a growing number of hospitals across Europe and the rest of the world.

Given the wealth of evidence, and the consensus in the HF community, one would assume that the use of NPs is widespread and that testing capacity is 1).

In this viewpoint article, the biomarkers working group of the HFA discusses the scientific evidence that supports the use of NPs to diagnose AHF in the emergency department, followed by the recommendations of international guidelines, and conclude with a call for action to increase awareness to promote equal access to NP analysis in HF diagnosis across Europe.

What does the science say?

BNP production in normal healthy individuals is minimal, with a level of about ≤10 pg/mL. In conditions of myocardial stretch, the induction of the BNP gene results in the production and secretion of prohormone proBNP1-108. This is cleaved into the biologically active BNP1-32 (usually referred to as BNP) and the biologically inert but biochemically more stable NTproBNP1-76 (usually referred to as NT-proBNP). Both 5), and soon confirmed by Bayes-Genis (6) and Januzzi [START_REF] Januzzi | The Nterminal Pro-BNP investigation of dyspnea in the emergency department (PRIDE) study[END_REF]. Subsequently, the International Collaborative of NTproBNP (ICON) study [START_REF] Januzzi | NT-proBNP testing for diagnosis and short-term prognosis in acute destabilized heart failure: an international pooled analysis of 1256 patients: the International Collaborative of NT-proBNP Study[END_REF] examined optimal applications of NTproBNP in 1256 acutely dyspnoeic patients. Patients with AHF had considerably higher NTproBNP concentrations compared with those without AHF (mean 4639 vs. 108 pg/mL, P < 0.001), and symptom severity correlated with NTproBNP concentrations (P = 0.008).

Optimal negative and positive predictive values were obtained by use of an ageindependent rule-out cut-off at 300 pg/mL and three age-stratified rule-in cut-off points (NTproBNP ≥450 pg/mL for age <50 years, ≥900 pg/mL for age 50-75 years, and ≥1800 pg/mL for age >75 years.). These cut-offs remain equally useful in AHF across the full spectrum of left ventricular ejection fraction. More recently, the ICON RELOADED (ICON: Re-evaluation of Acute Diagnostic Cut-Offs in the Emergency Department) (9) and BASEL-V (10) studies validated the age-specific NTproBNP cutoffs in contemporary cohorts. They found that the NTproBNP-supported strategy led to 14.5% fewer initial hospitalizations, 16.0% fewer admissions to cardiology, 12.5% fewer admissions to intensive care units, 3.2% fewer ED readmissions, and 21.6% fewer hospital readmissions (11)(Figure 2). The recent publication of the universal definition of HF proposes the incorporation of an objective measurement (elevated NP levels and/or imaging or haemodynamic derangements) in addition to the symptoms in the HF definition for making the diagnosis of HF more accessible to non-specialists and more reliable and consistent between observers, hospitals, and health care systems [START_REF] Bozkurt | Universal definition and classification of heart failure: A report of the Heart Failure Society of America, Heart Failure Association of the European Society of Cardiology, Japanese Heart Failure Society and Writing Committee of the Universal Definition of Heart Failure: Endorsed by Canadian Heart Failure Society, Heart Failure Association of India, the Cardiac Society of Australia and New Zealand, and the Chinese Heart Failure Association[END_REF].

What do the guidelines and the

A Call for Action

ESC member countries differ in their organization, delivery, and funding of health care, all of which ultimately lead to the uneven use of NP in the emergency department. With ample and consistent scientific evidence, and the highest recommendation by clinical guidelines, it is time to build strategies to help identify specific barriers to guideline implementation and to define actions to ensure that the majority of patients with acute dyspnoea have access to NP measurement in the emergency department. A timely and accurate diagnosis is crucial to enable early initiation of key lifesaving therapies. NPs should allow clinicians to choose the best diagnostic and treatment strategy more rapidly and to triage patients more confidently.

We present two additional pieces of evidence to support such a call for action initiative. First, the majority of patients are initially diagnosed with HF in the emergency department. An audit of the National Health Service in England showed that 80% of people diagnosed with HF receive the diagnosis following an acute hospital admission, despite half of them having had HF symptoms for up to 5 years before admission [START_REF] Bottle | Routes to diagnosis of heart failure: observational study using linked data in England[END_REF]. Second, the use of NPs has favourable effects not only on health outcomes, but also on costs. Contemporary data show that the use of NTproBNP decreases average inpatient management costs by 10.3% and reduces the total length of stay in the emergency department and hospital (11)(Figure 2). It is the right time for HFA to launch the "Peptide for Life" initiative (Figure 3) coordinated by the ESC, the HFA, and the NHFS, focused on countries where NP use in the emergency department is currently negligible.

Disclosures

  omnipresent. The Heart Failure Association (HFA) of the European Society of Cardiology (ESC) recently published the first edition of the HFA Atlas. It provides a contemporary description of HF epidemiology, resources, and management, and of the activities of the National Heart Failure Societies (NHFS) in ESC member countries, which comprise a total population of over 800 million people (3

  fragments and the precursor, proBNP1-108, are detected in the circulation[START_REF] Gaggin | Biomarkers and diagnostics in heart failure[END_REF].The majority of studies regarding the use of B-type class peptides have focused on the measurement of either BNP or NTproBNP. The study led by Maisel et al. (1) measured BNP levels in 1586 patients presenting to the emergency department with acute dyspnoea. Investigators found that patients with clinically diagnosed AHF had higher BNP levels compared with those without AHF (mean 675 vs. 110 pg/mL, P < 0.001). BNP was the best single predictor of a final diagnosis of AHF compared with individual history, physical examination, chest x-ray, and laboratory findings. The area under the curve (AUC) for BNP in receiver operating characteristic curve testing was 0.91for the diagnosis of AHF. A cutoff BNP value of 100 pg/mL had a sensitivity of 90% and a specificity of 76%. BNP was more accurate (83%) than either the National Health and Nutrition Examination Survey (NHANES) criteria (67%) or the Framingham criteria (73%), two established sets of criteria for HF diagnosis. Importantly, the best method of AHF diagnosis combined BNP and clinical findings. NTproBNP is cleared via different mechanisms and has a longer half-life than BNP (70 minutes vs. 20 minutes), but it is equivalent for the diagnostic evaluation of patients with suspected AHF, and likely more accurate in patients treated with neprilysin inhibitors (i.e. sacubitril/valsartan). The use of NTproBNP in the diagnosis of AHF was first demonstrated by Richards et al. (

  Universal Definition of HF say? NP testing aids in the diagnosis of AHF, and the use of these biomarkers is now embedded as a class I, level of evidence A, recommendation in ESC and American College of Cardiology/American Heart Association (ACC/AHA) clinical practice guidelines. A Class IA indication is the highest recommendation in international guidelines. The 2016 guidelines on HF from the ESC (12) state the following: 'Upon presentation to the emergency department or coronary care unit, a plasma NP level should be measured in all patients with acute dyspnoea and suspected AHF to help in the differentiation of AHF from non-cardiac causes of acute dyspnoea. NPs have high sensitivity, and normal levels in patients with suspected AHF makes the diagnosis unlikely'.The 2017 focused update of the 2013 guidelines of the ACC/AHA (13) states the following: 'In emergency settings, NP biomarker levels usually have higher sensitivity than specificity and may be more useful for ruling out than ruling in HF. Although lower values of NP biomarkers exclude the presence of HF, and higher values have reasonably high positive predictive value to diagnose HF, clinicians should be aware that elevated plasma levels for both NPs have been associated with a wide variety of cardiac and noncardiac causes'.

Figure 1 :

 1 Figure 1:Hospitals with availability of natriuretic peptides (BNP or NTproBNP) in

Figure 2 :

 2 Figure 2:Incorporation of natriuretic peptides in the diagnosis of acute

Figure 3 :

 3 Figure 3:"Peptide for Life" Initiative Logo.

Figure 3

 3 Figure 3

:

  Dr Bayes-Genis reports grants and personal fees from AstraZeneca, Abbott, Vifor, Boehringer-Ingelheim, Novartis, Roche Diagnostics and Critical Diagnostics. The UMCG, which employs Dr. De Boer has received research grants and/or fees from AstraZeneca, Abbott, Boehringer Ingelheim, Cardior Pharmaceuticals Gmbh, Ionis Pharmaceuticals, Inc., Novo Nordisk, and Roche. Dr. de Boer received speaker fees from Abbott, AstraZeneca, Bayer, Novartis, and Roche. Dr Jhund's employer the University of Glasgow has been remunerated by Novartis, Novo Nordisk and AstraZeneca for his work on clinical trials and he has received speakers fees and/or advisory board fees from Novartis, AstraZeneca, Boehringer Ingelheim and grants from Boehringer Ingelheim. Patrick Rossignol reports consulting for Bayer, G3P, Idorsia, and KBP; honoraria from Ablative Solutions, AstraZeneca, Bayer, Boehringer-Ingelheim, Corvidia, CVRx, Fresenius, Grunenthal, Novartis, Novo Nordisk, Relypsa Inc., a Vifor Pharma Group Company, Sanofi, Sequana Medical, Servier, Stealth Peptides, and Vifor Fresenius Medical Care Renal Pharma; Cofounder:

	CardioRenal.
	…


Yuri Bedjanian
email: yuri.bedjanian@cnrs-orleans.fr

Rate Constants for the Reaction of SO with NO 2 over the Temperature Range 220-960 K

Keywords: SO, NO 2, SO 2, kinetics, rate coefficient, temperature dependence

The kinetics of the reaction of SO radicals with NO 2 have been studied in a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer at nearly 2 Torr total pressure of helium and in the temperature range 220 -960 K. The reaction rate constant was determined under pseudo-first order conditions in excess of NO 2 over SO using two different methods for generation of the SO radicals,: k 1 = (6.7 ± 0.2)10 -12 exp(295 ± 10)/T) cm 3 molecule -1 s -1 (the uncertainties represent precision at the 2 level, the estimated total uncertainty on the rate constant being 15% at all temperatures). The present measurements indicate that the current recommendations for k 1 underestimate the rate constant at low stratospheric temperatures and overestimate it at the high temperatures characteristic of combustion.

INTRODUCTION

Reactions of SO radical, an intermediate in the oxidation of sulfur-containing compounds, are of considerable interest for both atmospheric and high temperature combustion chemistry. The title reaction, SO + NO 2  SO 2 + NO (1) is included in sulfur combustion submechanisms e.g. [START_REF] Cerru | Systematically reduced chemical mechanisms for sulfur oxidation and pyrolysis[END_REF] and in kinetic data evaluations for use in atmospheric modelling. [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF][START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I -gas phase reactions of O x , HO x , NOx and SO x species[END_REF] Another potential implication of the reaction at high temperatures is its involvement in the hot near-source volcanic plume chemistry. [START_REF] Roberts | Reaction rates control high-temperature chemistry of volcanic gases in air[END_REF] Surprisingly, despite the potential practical interest in the reaction at high temperatures, there are no experimental data for the reaction rate constant at temperatures higher than 363 K. [START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] Previously the rate constant of reaction (1) has been measured in three studies: by Clyne and Macrobert [START_REF] Clyne | Kinetic studies of free radical reactions by mass spectrometry. I. The reactions SO + NO 2 and ClO + NO[END_REF] and by Black et al. [START_REF] Black | Rate coefficients at 298 K for SO reactions with O 2 , O 3 , and NO 2[END_REF] at T = 298 K and by Brunning and Stief 5 in the temperature range 210 -363 K.

The values of k 1 at room temperature (about 1.4×10 -11 cm 3 molecule -1 s -1 ) reported in the three studies are in good agreement, however more temperature dependence measurements over an extended temperature range are clearly needed.

In the present study, using the discharge flow technique combined with mass spectrometry the absolute rate constant of reaction (1) has been measured as a function of temperature in an extended temperature range, T = 220 -960 K.

EXPERIMENTAL

Kinetic measurements have been conducted at nearly 2 Torr total pressure of Helium using a conventional fast-flow reactor combined with a molecular beam sampling electron-impact ionization mass spectrometer operated at 16-35 eV energy. [START_REF] Bedjanian | Rate constant of the reaction of F atoms with methane over the temperature range 220-960 K[END_REF][START_REF] Bedjanian | Temperature-dependent kinetic study of the reaction of hydroxyl radicals with hydroxyacetone[END_REF][START_REF] Bedjanian | Temperature-dependent rate constants for the reactions of chlorine atom with methanol and Br 2[END_REF] Total flow rate of helium of (12 -28) cm 3 atm s -1 gave linear flow velocities in the reactor of (1830 -3200) cm s -1 at the temperatures used. Depending on the temperature range, two different flow reactors were used. The high temperature reactor, used at T = 300 -960 K, consisted of a Quartz tube (45 cm length and 2.5 cm i.d.) with an electrical heater and water-cooled attachments (Figure S1, Supporting Information (SI)). [START_REF] Morin | Experimental study of the reactions of OH radicals with propane, n-pentane, and n-heptane over a wide temperature range[END_REF] The low temperature reactor (Figure S2, SI), used at

RESULTS AND DISCUSSION

Rate Constant of Reaction (1)

Two methods were employed for the absolute measurements of k 1 . In the first one, SO radicals were generated in the microwave discharge of SO 2 and kinetics of their consumption was monitored under pseudo-first order conditions in excess of NO 2 . The main complication in this chemical system is the contribution of SO 2 (from the source of SO and produced in reaction (1)) to the mass spectrometric signal of SO at m/z = 48 due to the dissociative ionization of SO 2 in the ion source of the mass spectrometer. In order to minimize this interference, the electron energies were lowered to approximately 16 eV, which reduced the SO 2 contribution at m/z = 48 to a few percent of the SO signal, obviously with some loss of detection sensitivity.

The temporal profiles of SO ([SO] 0 = (1.5 -4.0)10 11 molecule cm -3 ) monitored in excess of NO 2 (see Table 1 for the range of the NO 2 concentrations used) followed exponential The small, statistically non-significant intercepts in Figure 2 indicate a negligible rate of the heterogeneous loss of SO. All the pseudo-first order rate constants, k 1 ', were corrected for axial and radial diffusion of SO. [START_REF] Kaufman | Kinetics of elementary radical reactions in the gas phase[END_REF] The corrections typically did not exceed 10%. The results of the measurements of k 1 are shown in Table 1. The combined uncertainty on k 1 was estimated to be about 15% by adding in quadrature statistical error (≤ 2%) and those on the measurements of the absolute concentration of NO 2 (≤10%), flows (3%), pressure (2%) and temperature (1%).

[SO] (relative units) [START_REF] Clyne | Rate constant measurements for rapid reactions of ground state sulphur atoms[END_REF] with their subsequent detection at m/z = 95/97 (BrO + ) and 111/113 (SBr + ), respectively. The signal of BrO was found to be an order of magnitude smaller than that of SO indicating a rather low concentration of the oxygen atoms. In the reactive system, the small concentrations of O atoms, even if present, are removed with NO 2 , O + NO 2  NO + O 2 (5) k 5 = 6.1  10 -12 exp(155/T) cm 3 molecule -1 s -1 (T = 220 -950 K), [START_REF] Bedjanian | Rate constants of the reactions of O( 3 P) atoms with Br 2 and NO 2 over the temperature range 220-950 K[END_REF] and, in any case, do not interfere with the kinetics of SO. In contrast, S atoms can lead to the production of SO in reaction with NO 2 : [START_REF] Clyne | Atomic resonance fluorescence for rate constants of rapid bimolecular reactions. Part 7.-Sulphur atom reactions: S + O 2 → SO + O and S + NO 2 → SO + NO from 296 to 410 K[END_REF] S + NO 2  SO + NO (6) k 6 = 4.9  10 -11 exp(84/T) cm 3 molecule -1 s -1 (T = 296 -410 K).

k 4 = (9.5 ± 1.7)  10 -11 cm 3 molecule -1 s -1 (T = 298 K),
However, no observable signal was found at m/z = 111/113 (SBr + ), which indicates a negligible concentration of S atoms entering the main reactor from the microwave discharge of SO 2 under experimental conditions and in the configuration of the flow system used.

To be more confident that the source of SO radicals does not affect the measurements of k 1 , we conducted a series of experiments with the generation of SO in the reaction of oxygen atoms with an excess C 2 H 4 S (reaction 2). In addition, to avoid the problem of the contribution of SO 2 (reaction product) to the SO signal, in these experiments, the kinetics of SO 2 formation was monitored. The kinetics of the reaction product follows the following expression: 3 are given in Table 1. (1.37 ± 0.10)10 -11 cm 3 molecule -1 s -1 at T = (210 -363) K reported in the only previous temperature dependent study. [START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] The difference between absolute values of k 1 from the two studies increases at low temperatures reaching a factor of 1.8 at T = 220 K, the lowest temperature of the present work. It is difficult to establish the reason for the discrepancy between these two studies, conducted under similar experimental conditions and using the same methodology. One of the possible reasons could be the contribution of SO 2 to SO signal at m/z = 48 due to its dissociative ionization. As noted in the study of Brunning and Stief, [START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] to prevent the contribution all the measurements were made at electron energies less than or equal to 16 eV. Unfortunately, it was not indicated whether this was sufficient for complete elimination of the SO 2 signal at m/z=48, and if not, whether it was taken into account in the calculations. It can also be noted that in the study of Brunning and Stief,

[SO 2 ] = [SO] 0 -[SO] 0 ×exp(-k 1 't). Rearrangement of this expression results in [SO 2 ]  -[SO 2 ] = [SO 2 ]  ×exp(-k 1 't),

Comparison with Previous Data

  decay, [SO] = [SO] 0 ×exp(-k 1 '×t), where k 1 ' = k 1 ×[NO 2 ]+k w is the pseudo-first-order rate constant and k w corresponds to the loss rate of SO radicals in the absence of NO 2 in the reactor. Examples of first order plots observed at T = 270 K are shown in Figure 1. Examples of the dependencies of the pseudo-first order rate constant, k 1 ' = k 1 ×[NO 2 ]+k w , on concentration of NO 2 at different temperatures are shown in Figure 2.

Figure 1 .

 1 Figure 1. Typical decay profiles of SO in the presence of different concentrations of NO 2 observed at T = 270 K.

Figure 2 .

 2 Figure 2. Pseudo-first-order rate constant k 1 ' = k 1 ×[NO 2 ]+k w as a function of NO 2 concentration at selected temperatures. Partially shown error bars represent typical uncertainties (≤5%) on the determination of k 1 ′.

  where [SO 2 ]  = [SO] 0 and k 1 '  k 1 ×[NO 2 ], considering negligible heterogeneous loss of SO radicals. In experiments, the kinetics of SO 2 growth was monitored and [SO 2 ]  was determined upon total consumption of SO. Typical first-order plots are shown in Figure S3 (SI). Examples of the plots of the pseudo-first-order rate constants versus NO 2 concentration observed at different temperatures are shown in Figure 3. The values of k 1 determined in this series of experiments from the slopes of the straight lines like those in Figure

Figure 3 .

 3 Figure 3. Pseudo-first-order rate constant, k 1 ', obtained from the kinetics of SO 2 formation at selected temperatures as a function of NO 2 concentration.

Figure 4

 4 Figure 4 summarizes the results of the current measurements of the rate constant of reaction (1) in comparison with those from previous studies. Concerning the results from this study, one can note an excellent agreement between the measurements of the rate constant carried out with two different sources of SO. Least-squares analysis of the present temperature dependent data for k 1 yields the following Arrhenius expression in the temperature range T = 220-960 K:

Figure 4 .

 4 Figure 4. Summary of the data on the rate constant of the SO+NO 2 reaction. Partially shown representative error bars for k 1 values from this study correspond to the estimated systematic uncertainty of 15 %.

5 JPL NASA evaluation 2

 52 This work (SO kinetics) This work (SO 2 kinetics)

  [START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] a relatively narrow range of concentrations of the excess reagent, NO 2 , was used at low temperatures; [NO 2 ] was varied only by a factor of 3.2 and 2.6 at T = 248 and 210 K, respectively.Current recommendation of the NASA Panel for data evaluation, k 1 = (1.4 ± 0.2)10 -11 cm 3 molecule -1 s -1 at T = (210 -360) K, 2 shown as dashed line in Figure4, is based on the only temperature dependent study of Brunning and Stief.[START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] A similar expression is recommended by the IUPAC subcommittee on kinetic data evaluation.[START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I -gas phase reactions of O x , HO x , NOx and SO x species[END_REF] Furthermore, the temperature invariant value of k 1 , obtained at relatively low temperatures (210-363 K)[START_REF] Brunning | Kinetic studies of the reaction of the SO radical with NO 2 and ClO from 210 to 363 K[END_REF] is used also in the sulfur oxidation submechanisms for the modelling of the combustion processes at high temperatures. e.g.[START_REF] Cerru | Systematically reduced chemical mechanisms for sulfur oxidation and pyrolysis[END_REF] The present new measurements indicate that the currently recommended values of k 1 are underestimated at low stratospheric temperatures and overestimated at high temperatures relevant to combustion.ACKNOWLEDGEMENTSupport from the VOLTAIRE project (ANR-10-LABX-100-01) funded by ANR through the PIA (Programme d'Investissement d'Avenir) is gratefully acknowledged.

Table 1

 1 Reaction SO + NO The quoted errors represent the statistical 2 fit precision, the estimated total uncertainty on k 1 is nearly 15 %.

	Reactor surface c	Source of SO

2 : summary of the measurements of the rate constant T (K) a [NO 2 ] (10 12 molecule cm -3 ) k 1 (± 2) b (10 -11 cm 3 molecule -1 s -1 ) a 7-11 kinetic runs at each temperature. b c HW: halocarbon wax; Q: quartz.

SUPPORTING INFORMATION

Additional supporting information may be found online in the Supporting Information section at the end of the article.


Yuri Bedjanian
email: yuri.bedjanian@cnrs-orleans.fr

Rate Constant and Products of the Reaction of O( 3 P) Atoms with Thiirane over the Temperature Range 220-950 K

Keywords: Oxygen atom, Thiirane, SO, Rate constant, Non-Arrhenius

Rate constant and products of the reaction of O( 3 P)

atoms with thiirane over the temperature range 220-950 K

Yuri Bedjanian

INTRODUCTION

Reactions of thiirane (ethylene sulfide, C 2 H 4 S) with various atoms (O, H, Cl, S) are known to be rapid and proceed through S-atom abstraction leading to almost stoichiometric formation of ethylene and sulfur containing radicals. [START_REF] Murrells | Elementary reactions of the SCl radical. Part1.-Rate constants and mechanisms of the reaction Cl + C 2 H 4 S → SCl + C 2 H 4 , SCl + SCl → products and SCl + Cl 2 → SCl 2 + Cl[END_REF][START_REF] Van Roodselaar | The reactions of S( 3 P J ) atoms with thiiranes[END_REF][START_REF] Lee | Absolute rate parameters for the reaction of atomic hydrogen with carbonyl sulfide and ethylene episulfide[END_REF][START_REF] Lee | Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide[END_REF][START_REF] Yokota | Reaction of hydrogen atoms with thiirane[END_REF] Thus, the desulfuration reactions of C 2 H 4 S are a convenient source of sulfur-containing radicals for use in laboratory studies of their reactions.

The title reaction O + C 2 H 4 S  products (1) potentially leads to the formation of SO radicals, which are important intermediates in atmospheric and combustion chemistry. SO radicals are involved in the mechanisms of sulfur oxidation, where they are formed, for example, in the reactions of oxygen atoms with OCS and CS 2 and are further oxidized to SO 2 by reactions with O 2 and NO 2 . The title reaction has only been studied in one previous work. Lee et al. [START_REF] Lee | Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide[END_REF] used flash photolysis-resonance fluorescence method to measure its kinetics between 268 K and 424 K. To our knowledge, the yield of SO has never been measured.

The objective of the present work was to determine the rate constant and products of the title reaction over an extended temperature range, T = 220 -950 K, in order to provide a new kinetic and mechanistic data set for use in laboratory studies and, potentially, in theoretical reaction rate calculations.

EXPERIMENTAL

Experiments have been carried out in a discharge-flow apparatus at 2 Torr total pressure of Helium using electron-impact ionization (25-30 eV energy) mass spectrometry to measure the concentrations of the species involved. [START_REF] Bedjanian | Rate constants of the reactions of O( 3 P) atoms with Br 2 and NO 2 over the temperature range 220-950 K[END_REF][START_REF] Morin | Rate Constants of the Reactions of O( 3 P) Atoms with Ethene and Propene over the Temperature Range 230-900 K[END_REF][START_REF] Bedjanian | Reaction of O( 3 P) with C 3 H 6 : yield of the reaction products as a function of temperature[END_REF][START_REF] Morin | Reaction of O( 3 P) with C 2 H 4 : yield of the reaction products as a function of temperature[END_REF] The flow reactor used at low temperatures k 2 = (6.1 ± 0.4)  10 -12 exp((155 ± 18)/T) cm 3 molecule -1 s -1 (T = 220 -950 K), [START_REF] Bedjanian | Rate constants of the reactions of O( 3 P) atoms with Br 2 and NO 2 over the temperature range 220-950 K[END_REF]

(

RESULTS AND DISCUSSION

Rate Constant of Reaction (1)

The rate constant of reaction (1) was determined using two approaches: by absolute and relative-rate measurements. Consistent results were observed with both methods. Diffusion corrections [START_REF] Kaufman | Kinetics of elementary radical reactions in the gas phase[END_REF] (calculated with the diffusion coefficient of C 2 H 4 S in He D 0 = 290 × (T/298) 1.85 Torr cm -2 s -1 ) [START_REF] Tang | Compilation and Evaluation of Gas Phase Diffusion Coefficients of Reactive Trace Gases in the Atmosphere: Volume 1. Inorganic Compounds[END_REF] on the pseudo-first order rate constants,

Absolute measurements. The absolute measurements of

k 1 ' = k 1 ×[O] (obtained
from the slope of the semilogarithmic plots), typically did not exceed a few percent. The consumption of the excess reactant, oxygen atom, in reaction (1) and, mainly, due to the heterogeneous loss (≤ 10 s -1 ) was within a few percent, although reaching up to 15% in a few kinetic runs. In all cases, the average concentration of the oxygen atoms along the reaction The small (close to zero) intercepts in Figure 2 are consistent with the experimental observation of the absence of any consumption of thiirane in the absence of the oxygen atoms in the reactor. This indicates a negligible role of the thermal decomposition of thiirane [START_REF] Chin | Thermal Decomposition of Thiirane and 2-Methylthiirane: An Experimental and Theoretical Study[END_REF] under experimental conditions of the kinetic measurements, even at highest temperatures of the study. ) are presented in Figure 3.

According to equation (I) the slopes of the straight lines in Figure 3 correspond to the k 1 to k 3 ratio. The final k 1 values calculated from k 3 and the slopes are given in Table 2. We estimate the total uncertainty of the k 1 values obtained from relative-rate measurements to be 20%.

The uncertainty is slightly higher than in the direct measurements because of the uncertainty of the rate constant of the reference reaction. The current results are plotted as a function of temperature in Figure 4 together with the measurements of Lee et al. [START_REF] Lee | Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide[END_REF] One can note close agreement between the absolute and relative measurements of this study and with the previous literature determination. [START_REF] Lee | Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide[END_REF] The rate constant of the reaction of O atom with thiirane clearly exhibits non-Arrhenius behavior. Interestingly, weak, negative temperature dependence is observed below 300 K, whereas positive temperature dependence is observed above 380 K. A modified Arrhenius fit to the current results yields the following expression: k 1 = 1.13×10 -12 (T/298) 2.18 exp(733/T) cm 3 molecule -1 s -1 at T = 220 950 K.

We recommend this expression be used in kinetic models. The expression is expected to be accurate within 15 % between 220 K and 950 K.

Products of reaction (1)

To Negative temperature dependence is often associated with the formation of a pre-reactive complex. The switch from negative to positive temperature dependence of the total rate constant upon increase of temperature can be due to different reasons. For example, due to opening of new reaction channels (leading to different reaction products). This is the case for some reactions of unsaturated organic compounds (for example, Br + trans-2-butene, [START_REF] Bedjanian | Low-Pressure Study of the Reactions of Br Atoms with Alkenes. 2. Reactions with Ethene and Trans-2-Butene[END_REF] OH +

  Figure S1, Supporting Information (SI)), T = 220 -325 K, consisted of a halocarbon wax coated Pyrex tube (45 cm length and 2.4 cm i.d.). The temperature in the reactor was controlled by circulating thermostated ethanol. The high temperature reactor, employed at T = 310 -950 K, was an electrically heated Quartz tube (45 cm length and 2.5 cm i.d.) with water-cooled extremities (Figure S2, SI). 10 Oxygen atoms were produced by discharging trace amounts of O 2 in He and were detected at their parent peak, m/z = 16 (O + ): the contribution of O 2 at this mass due to fragmentation of the molecular oxygen in the ion source of the mass spectrometer was properly taken into account. The contribution of O 2 to m/z = 16 could be easily determined by simultaneous detection of the signals from O 2 at m/z = 32 and 16 and was approximately 5 % of the intensity of the parent O 2 peak. The resulting contribution of O 2 to the O atom signal was up to 40%. To ensure the accuracy of the measurements of [O] (directly propagating to the uncertainty on the measurements of the reaction rate constant) two independent procedures were employed. The absolute concentrations of the O atoms were determined from the decrease of the O 2 concentration upon initiation of the microwave discharge ([O 2 ] = 2[O]). In another method, oxygen atoms were titrated with excess NO 2 , O + NO 2  NO + O 2(2)

  k 1 were carried out under pseudo-first order conditions in an excess of oxygen atoms over C 2 H 4 S ([C 2 H 4 S] 0 = (2-3.5)×10 11 molecule cm -3 ). The time dependence of [C 2 H 4 S] and [O] was monitored by changing the position of the movable injector (Figures S1 and S2, SI), and the measured distance between the injector head and the sampling cone of the mass spectrometer was converted into reaction time using the linear flow velocity of the gas mixture in the reactor.

Figure 1

 1 Figure 1 shows typical data obtained in this manner. The linearity of the semilogarithmic plots in Figure 1 clearly illustrates that the C 2 H 4 S decays are first order, ln([C 2 H 4 S] 0 /[C 2 H 4 S]) = k 1 '×t, where k 1 ' = k 1 ×[O].

Figure 1 .

 1 Figure 1. Typical decay profiles of C 2 H 4 S in the presence of different concentrations of O atoms observed at T = 360 K.

Figure 2 .

 2 Figure 2. Dependence of the pseudo-first order rate constant, k 1 ' = k 1 [O], on the concentration of oxygen atoms at different temperatures. Partially shown error bars represent typical uncertainties (≤ 5%) on the determination of k 1 ′.

Figure 3 .

 3 Figure 3. Examples of the dependence of ln([C 2 H 4 S] 0 /[C 2 H 4 S]) on ln(Br 2 ] 0 /[Br 2 ]) observed at different temperatures.

Figure 4 .

 4 Figure 4. Summary of the measurements of k 1 : AM, absolute measurements; RM, relative rate method. Uncertainties are shown for selected measurements of k 1 and correspond to 15 and 20% for the absolute and relative measurements, respectively.

k 4 = ( 6 .

 46 examine if the change in temperature dependence is caused by a change in the reaction mechanism (new reaction pathways), we measured the branching ratio of the SO + C 2 H 4 forming channel at 298 K and 900 K. The reaction products, SO and C 2 H 4 , were detected upon total consumption of O atoms in an excess of C 2 H 4 S. The determination of their yields consisted in the measurements of the consumed [C 2 H 4 S] and the formed concentrations of the products. The SO radical was monitored at m/z = 48 (SO + ) and at m/z = 64 (SO 2 + ) after being scavenged with an excess of NO 2 ([NO 2 ]  10 14 molecule cm -3 ), added in the end of the reactor 5 cm upstream of the sampling cone, as shown in Figure S3 (SI): SO + NO 2  SO 2 + NO (4) 7±0.2)10 -12 exp((295±10)/T) cm 3 molecule -1 s -1 (T = 220 -960 K). 14 This allowed the determination of the absolute concentrations SO radicals considering their stoichiometric conversion to SO 2 : [SO] = [SO 2 ]. C 2 H 4 was detected at m/z = 26 (C 2 H 2 + ). The reaction time was  0.02 s and [C 2 H 4 S] 0 = (2.5-3.0)×10 13 molecule cm -3 . Under these experimental conditions, O atoms were completely consumed in the reaction with C 2 H 4 S. The experimental data observed at T = 298 and 900 K are shown in Figures 5 and 6.

Figure 5 .

 5 Figure 5. [SO] and [C 2 H 4 ] formed in reaction (1) at 298 K as a function of the consumed concentration of C 2 H 4 S. Error bars are shown for selected points and correspond to typical (5-7)% uncertainties on the determination of the absolute concentrations of the relevant species.

Figure 6 .

 6 Figure 6. [SO] and [C 2 H 4 ] formed in reaction (1) at 900 K as a function of the consumed concentration of C 2 H 4 S. Error bars are shown for selected points and correspond to typical (5-7)% uncertainties on the determination of the absolute concentrations of the relevant species

  [START_REF] Chin | Thermal Decomposition of Thiirane and 2-Methylthiirane: An Experimental and Theoretical Study[END_REF] molecule cm -3 ):zone was used in the calculations. Examples of the second-order plots observed at different temperatures are shown in Figure2. A linear least-squares fit of these data at each temperature gave the bimolecular rate constant. A summary of the absolute measurements of k 1 is given in Table1. The combined uncertainty on k 1 was estimated to be about 15% by

	adding in quadrature statistical error (≤ 2%) and those on the measurements of the absolute
	concentration of O atoms (10%), flows (3 %), pressure (2 %) and temperature (1 %).
	0.11	1.51
	0.19	2.19
	0.51 0.75 1.02	2.82 3.43 3.90

Table 1

 1 Experimental conditions and results of the current absolute measurements of k 1 In the relative-rate measurements, the decays of C 2 H 4 S and the reference compound Br 2 were monitored in the presence of a common reactant (O-Given that there are no rate constant data available for these reactions, we tested the possible impact of this secondary chemistry under the experimental conditions of the relative rate measurements. Br and BrO ( 5×10 11 molecule cm -3 ) were generated in the reaction of O atoms with excess Br 2 in the movable injector and mixed with C 2 H 4 S ( 3×10 11 molecule cm -3 ) in the main reactor. Monitoring of C 2 H 4 S showed that the presence of Br and

	T (K)	[O] (10 13 molecule cm -3 )	k 1 (± 2) (10 -11 cm 3 molecule -1 s -1 )	Reactor surface (a)
	220	0.13-2.64	1.60±0.03	HW
	235	0.12-2.85	1.48±0.02	HW
	250	0.21-3.01	1.46±0.02	HW

a HW: halocarbon wax; Q: quartz.

Relative rate measurements . k 3 = 9.8510 -16 ×T 1.41 exp(543/T) cm 3 molecule -1 s -1 (T = 220 -950 K).

[START_REF] Bedjanian | Rate constants of the reactions of O( 3 P) atoms with Br 2 and NO 2 over the temperature range 220-950 K[END_REF]

The initial concentrations of C 2 H 4 S and Br 2 in these experiments were in the range (2.5 -5) and (2 -4)× 10 11 molecule cm -3 , respectively. The relative consumptions of C 2 H 4 S and Br 2 in the reaction with O-atoms are linked by the ratio of the corresponding rate constants:

Table 2

 2 Results of the current relative measurements of k 1

	T (K)	k 1 /k 3 (±2)	k 1 (10 -11 cm 3 molecule -1 s -1 )
	310	0.690±0.006	1.28
	380	0.741±0.004	1.32
	440	0.773±0.005	1.40
	515	0.816±0.011	1.54
	640	0.913±0.004	1.90
	820	0.971±0.007	2.38
	900	1.077±0.010	2.84

p-cymene [START_REF] Bedjanian | Gas-Phase Reaction of Hydroxyl Radical with p-Cymene over an Extended Temperature Range[END_REF] ), where adduct forming and H-atom abstraction channels dominate at low and high temperatures, respectively. The U-shaped temperature dependence observed [START_REF] Bedjanian | Rate Constant of the Reaction of OH Radicals with HBr over the Temperature Range 235-960 K[END_REF] for the rate constant of the reaction OH + HBr  H 2 O + Br was described by the sum of contributions from complex-(through formation of a collision complex BrH•••OH) and directmode (direct abstraction mechanism) reactions. [START_REF] Ree | Dependence of the Four-Atom Reaction HBr + OH → Br + H 2 O on Temperatures between 20 and 2000 K[END_REF] A mechanism involving competing reactions, proceeding through a pre-reactive H-bonded complex to form CH 3 C(O)O + H 2 O and a direct methyl-H abstraction channel, was proposed to explain a complex temperature dependence of the OH+CH 3 COOH reaction with a sharp minimum of the reaction rate constant at T = 530 K. [START_REF] Khamaganov | Absolute Rate Coefficient of the OH + CH 3 C(O)OH Reaction at T = 287-802 K. The Two Faces of Pre-Reactive H-Bonding[END_REF] Reactions proceeding through the formation of an intermediate complex, separated from the reactants and the reaction products by a loose and a tight (lying below the energy of the reactants) transition states, respectively, are potential candidates for the U-shaped temperature dependence of the reaction rate constant. [START_REF] Mozurkewich | Negative Activation Energies and Curved Arrhenius Plots. 1. Theory of Reactions over Potential Wells[END_REF][START_REF] Benson | Existence of Negative Activation Energies in Simple Bimolecular Metathesis Reactions and Some Observations on Too-Fast Reactions[END_REF][START_REF] Krasnoperov | Modified Transition State Theory and Negative Apparent Activation Energies of Simple Metathesis Reactions: Application to the Reaction CH 3 + HBr → CH 4 + Br[END_REF] Georgievskii and Klippenstein [START_REF] Georgievskii | Strange Kinetics of the C 2 H 6 + CN Reaction Explained[END_REF] in their theoretical calculations, based on the consideration of the two distinct transition state regimes, reproduced the U-shaped (with a strong minimum near 200 K) temperature dependence of the rate constant of the CN+C 2 H 6 reaction in the temperature range (25 -1140) K. It was shown that the low temperature kinetics is governed by the loose outer transition state that leads to the formation of the pre-reactive complex, but as temperature is raised, the properties of the tighter inner transition state become increasingly important. This two transition state model seems highly plausible for the title reaction. Clearly, detailed theoretical calculations are needed to constrain this mechanism; however, this is outside the scope of the present study. The current lowpressure (2 Torr of He) measurements and those of Lee et al. [START_REF] Lee | Absolute rate parameters for the reaction of ground state atomic oxygen with dimethyl sulfide and episulfide[END_REF] at higher pressures (50-100 Torr of Ar) agree within experimental uncertainty, which indicates that reaction (1) is pressure independent. This suggests that no collisionally stabilized adducts are formed, at least at pressures under 100 Torr.

In conclusion, the rate constant of the title reaction was measured over a wide temperature range, T = 220 -950 K (for the first time at temperatures above 464 K and below 268 K). Negative temperature dependence is observed below 300 K, whereas positive temperature dependence is observed above 380 K. The SO and C 2 H 4 forming channel of the reaction (1) turned out to be the main, if not the only one, with a branching ratio in the range (0.9 -1.0). The measurements provide a new reliable kinetic and mechanistic dataset for practical use and an experimental basis for theoretical calculations aimed at elucidating the reaction dynamics.

ACKNOWLEDGEMENT

Support from the VOLTAIRE project (ANR-10-LABX-100-01) funded by ANR through the PIA (Programme d'Investissement d'Avenir) is gratefully acknowledged.

SUPPORTING INFORMATION

Additional supporting information may be found online in the Supporting Information section at the end of the article.


Yuri Bedjanian
email: yuri.bedjanian@cnrs-orleans.fr

Experimental Study of the Reaction of O( 3 P) with Carbonyl Sulfide between 220 and 960 K

The reaction of ground-state O atom with carbonyl sulfide is of interest for atmospheric (stratosphere and hot near-source volcanic plume) and combustion chemistry. In the present work, we employed a discharge-flow system combined with modulated molecular beam mass spectrometry technique to measure the rate constant and products of the O+OCS reaction. The overall rate constant was determined either from kinetics of the reaction product, SO radical, formation or under pseudo-first order conditions from the decays of OCS in excess of oxygen atoms: k1 = 1.92×10 -12 ×(T/298) 2.08 ×exp(-1524/T) cm 3 molecule -1 s -1 at T= 220-960 K, with conservative uncertainty of 20 %. The yield of another reaction product, CO2, was found to increase from 3.55 % at T = 455 K to 14.2 % at T = 960 K, resulting in the following Arrhenius expression for the rate constant of the minor (S + CO2 forming) reaction channel: k1b = 4.19×10 - 11 ×exp(-4088/T) cm 3 molecule -1 s -1 at T= 455-960 K (with an uncertainty of 25 %). The kinetic and mechanistic data from the present work are discussed in comparison with previous experimental and computational studies.

INTRODUCTION

Carbonyl sulfide is the most abundant sulfur-containing gas phase compound in the troposphere due to its low reactivity and, as a consequence, reasonably long tropospheric lifetime. [START_REF] Seinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF] OCS is transported from the troposphere or directly injected by volcanic eruptions to the stratosphere, where it undergoes photooxidation (through photolysis and reactions with O atoms and OH radicals) resulting in conversion to sulfate aerosol. [START_REF] Seinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF] In this respect, the information on the kinetics and products of the title reaction O + OCS  products (1) at low stratospheric temperatures is of importance. The high temperature data for O+OCS reaction is of interest for combustion chemistry, and also due to its potential involvement in the hot near-source volcanic plume chemistry. [START_REF] Roberts | Reaction Rates Control High-Temperature Chemistry of Volcanic Gases in Air[END_REF] Reaction 1 has been extensively studied both experimentally [START_REF] Sullivan | Mass Spectrometric Investigation of the Reaction between Oxygen Atoms and Carbonyl Sulfide[END_REF][START_REF] Hoyermann | Bestimmung Der Geschwindigkeit Der Reaktion O + COS → CO + SO. Ber. Bunsenges[END_REF][START_REF] Homann | Geschwindigkeit Von Elementarreaktionen Teil I[END_REF][START_REF] Westenberg | Atom-Molecule Kinetics Using Esr Detection. V. Results for O+OCS, O+CS2, O+NO2, and H+C2H4[END_REF][START_REF] Krezenski | The Reactions of O( 3 P) with Ozone and Carbonyl Sulfide[END_REF][START_REF] Klemm | Absolute Rate Parameters for the Reaction of Ground State Atomic Oxygen with Carbonyl Sulfide[END_REF][START_REF] Wei | Esr Study of the Kinetics of the Reactions of O( 3 P) Atoms with CS2 and OCS[END_REF][START_REF] Manning | The Effect of Infrared Laser Excitation on Reaction Dynamics: O+C2H4 and O+OCS[END_REF][START_REF] Yoshida | Application of Microwave Spectroscopy to Kinetic Study of the Reaction of Carbonyl Sulfide with Atomic Oxygen[END_REF][START_REF] Hsu | Dynamics of Reactions of O( 3 P) Atoms with CS, CS2 and OCS[END_REF][START_REF] Robertshaw | Rate Data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a New Time-Resolved Technique[END_REF][START_REF] Kruger | Shock-Tube Study of the Rate-Constant of the Reaction of Oxygen-Atoms with Carbonylsulfide[END_REF][START_REF] Isshiki | High-Temperature Reactions of O + COS and S + SO2. Abstraction Versus Substitution Channels[END_REF] and theoretically. [START_REF] Saheb | Quantum Chemical and Theoretical Kinetics Studies on the Reaction of Carbonyl Sulfide with H, OH and O( 3 P)[END_REF][START_REF] Chen | Computational Study on the Mechanisms and Rate Constants for the O( 3 P, 1 D) + OCS Reactions[END_REF] Over a dozen available measurements of the reaction rate constant cover a wide temperature range between 239 and 1900 K. [START_REF] Wei | Esr Study of the Kinetics of the Reactions of O( 3 P) Atoms with CS2 and OCS[END_REF][START_REF] Kruger | Shock-Tube Study of the Rate-Constant of the Reaction of Oxygen-Atoms with Carbonylsulfide[END_REF] The previous kinetic data satisfactorily agree with each other (within a factor of 2, depending on temperature), and the general features of the temperature behaviour of the overall reaction rate constant (significant increase with temperature) can be considered as well known. However, the totality of the previous measurements performed by different methods and in different and limited temperature ranges does not allow to establish with precision the temperature dependence of the rate constant in the entire temperature range of the studies. One of the aims of this work was to reconcile previous disparate kinetic data by measuring k1 within a single study and in an extended temperature range T = 220-960 K, which covers almost all previous measurements.

Reaction 1 has two exothermic channels: [START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] O + OCS  SO + CO rH° = -50.8 kcal mol -1 (1a)

 S + CO2 rH° = -53.4 kcal mol -1 (1b)
Currently, there is experimental evidence that SO + CO forming channel is the main reaction pathway. The second, S + CO2 forming, channel of reaction 1 becomes important at high temperatures: with the branching ratio k1b/k1 up to 30% at T = 1500 K. [START_REF] Isshiki | High-Temperature Reactions of O + COS and S + SO2. Abstraction Versus Substitution Channels[END_REF] Another task of this work was to determine the yield of CO2, product of reaction 1b, as a function of temperature.

EXPERIMENTAL

Experiments have been carried out in a discharge-flow apparatus at total pressure of 2 Torr of Helium using an electron-impact ionization (25-30 eV energy) mass spectrometry to measure the concentrations of the species involved. [START_REF] Bedjanian | Rate Constants of the Reactions of O( 3 P) Atoms with Br2 and NO2 over the Temperature Range 220-950 K[END_REF][START_REF] Morin | Rate Constants of the Reactions of O( 3 P) Atoms with Ethene and Propene over the Temperature Range 230-900[END_REF][START_REF] Bedjanian | Reaction of O( 3 P) with C3H6: Yield of the Reaction Products as a Function of Temperature[END_REF][START_REF] Morin | Reaction of O( 3 P) with C2H4: Yield of the Reaction Products as a Function of Temperature[END_REF] The flow reactor used at low temperatures, T = 220 -310 K, consisted of a halocarbon wax coated Pyrex tube (45 cm length and 2.4 cm i.d.).

The temperature in the reactor was controlled by circulating thermostated ethanol. High temperature reactor, employed at T = 299 -960 K, was an electrically heated Quartz tube (45 cm length and 2.5 cm i.d.) with water-cooled extremities (Figure S1, Supporting Information). [START_REF] Morin | Experimental Study of the Reactions of OH Radicals with Propane, n-Pentane, and n-Heptane over a Wide Temperature Range[END_REF] Oxygen atoms were produced by discharging a trace amounts of O2 in He and were detected at their parent peak, m/z = 16 (O + ): the contribution of O2 at this mass due to fragmentation of the molecular oxygen in the ion source of the mass spectrometer was properly taken into account. In the procedure used for kinetic measurements, the uncertainty on the measurements of the absolute concentrations of oxygen atoms propagated directly to the uncertainty on the measurements of the reaction rate constant. For this reason, two independent methods were used to ensure the accurate measurements of the absolute concentrations of O atoms. In the first one, the O concentrations were determined from the decrease in the O2 concentration upon initiation of the microwave discharge ([O2] = 2[O]). In another method, oxygen atoms were titrated with an excess of NO2,

O + NO2  NO + O2 (2)
k2 = (6.1 ± 0.4)  10 -12 exp((155 ± 18)/T) cm 3 molecule -1 s -1 (T = 220 -950 K), [START_REF] Bedjanian | Rate Constants of the Reactions of O( 3 P) Atoms with Br2 and NO2 over the Temperature Range 220-950 K[END_REF] and their absolute concentrations could be determined from the consumed fraction of NO2 and concentrations of NO and O2 formed:

[O] = [NO2] = [NO] = [O2].
Both methods gave almost identical (within 10%) results.

All stable species (OCS, C2H4S, SO2, O2, CO2, NO2, NO and Br2) were detected by mass spectrometry at their parent peaks. Their absolute concentrations were derived from measured flow rates of their manometrically prepared calibrated gas mixtures.

He (carrier gas) and O2 were taken directly from high-pressure tanks and had stated purities better than 99.99% (Alphagaz). SO2, NO2, NO were Alphagaz products (> 99.9%). The purities of OCS (15% in He, Messer), C2H4S (Merck) and Br2 (Aldrich) were 99.99, 98 and 99.99%, respectively.

RESULTS AND DISCUSSION

Rate constant of reaction 1.

The rate constant of the title reaction was determined using two different methods. At low temperatures, under conditions where reaction 1 is relatively slow and consumption of the reactants is insignificant, the rate constant was determined by monitoring the kinetics of the reaction product, SO radical. The second approach used at higher temperatures, consisted in a direct monitoring of the OCS decays in excess of oxygen atoms. molecule cm -3 ), which led to a rapid conversion of the SO radicals, formed in reaction 1a, to SO2 through reaction 3:

SO + NO2  SO2 + NO (3)
k3 = (6.7±0.2)10 -12 exp((295±10)/T) cm 3 molecule -1 s -1 (T = 220 -960 K). [START_REF] Bedjanian | Rate Constants for the Reaction of SO with NO2 over the Temperature Range 220-960 K[END_REF] The chemical conversion of SO to stable species SO2 allowed the determination of the absolute concentrations of the SO radicals. Thus, the Figures and expressions showing Information). Most of the experiments were carried out with fixed oxygen atom and varied OCS concentrations (Table 1). Figure S2 All the data obtained in this way, as well as initial concentrations of the reactants used in these experiments, are given in Table 1. The combined uncertainty on the measurements of k1a was estimated to be of nearly 20 %, including statistical error and those on the measurements of the flows, pressure, temperature and absolute concentrations of the three species involved. S3 (Supporting Information).

These concentration-time profiles yield k1' = k1[O], the pseudo-first-order rate constant. It is worth noting that consumption of oxygen atom was negligible (no more than a few percent) in all the experiments due to its high excess over OCS and relatively low rate of heterogeneous loss on the wall of the flow reactor (kw = (0.8 -2.0) s -1 , increasing with temperature). Figure 3 shows the pseudo-first order rate constants, k1' = k1[O], plotted against the concentration of oxygen atoms. The slopes of the straight lines in Figure 3 (linear fit to the experimental data)

provide the values of k1 at respective temperatures, shown in Table 1. the data of the present study obtained from the OCS and SO kinetics correspond to the total rate constant and to that of channel 1a, respectively. However, as will be shown below, at T < 410 K (highest temperature of the k1a measurements) the contribution of the second reaction channel (1b) to the overall rate constant is less than 2.5 %, and k1, in this temperature range, can be considered identical to k1a.

The present value of the rate constant at T = 299 K, k1 = (1.16±0.23)×10 -14 cm 3 molecule - 1 s -1 , is in good agreement with previous room temperature measurements (T = 296-300 K, not shown in Figure 4) of Sullivan and Warneck, (0.91±0.14)×10 -14 , [START_REF] Sullivan | Mass Spectrometric Investigation of the Reaction between Oxygen Atoms and Carbonyl Sulfide[END_REF] Manning et al., (1.39±0.14) ×10 -14 , 10 and Yoshida and Saito, (1.18±0.12)×10 -14 cm 3 molecule -1 s -1 . 11 Somewhat higher value, k1 = (1.7±0.2)×10 -14 cm 3 molecule -1 s -1 at T = 296, was reported by Robertshaw and Smith. [START_REF] Robertshaw | Rate Data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a New Time-Resolved Technique[END_REF] The use of two different approaches in k1 measurements in the present work allowed to determine the absolute values of the rate constant varying by a factor of 5000. The combination of the relatively precise measurements (within 15-20%) and a wide temperature range (220 -960 K) allowed to discern a weak curvature of the Arrhenius plot at T > 600 K.

This has been difficult to do in previous disparate studies conducted over limited temperature ranges. The three-parameter fit to the present data for k1 (solid line in Figure 4) yields the following modified Arrhenius expression: k1 = 1.92×10 -12 ×(T/298) 2.08 ×exp(-1524/T) cm 3 molecule -1 s -1 . In a theoretical study of the O+OCS reaction, Saheb et al. [START_REF] Saheb | Quantum Chemical and Theoretical Kinetics Studies on the Reaction of Carbonyl Sulfide with H, OH and O( 3 P)[END_REF] calculated the following non-Arrhenius expressions for two reaction channels: k1a = 1.14×10 -12 ×T 0.6 ×exp(-2430/T) and k1b = IUPAC evaluation [START_REF] Atkinson | Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume I -Gas Phase Reactions of Ox, HOx, NOx and SOx Species[END_REF] Saheb et al. [START_REF] Saheb | Quantum Chemical and Theoretical Kinetics Studies on the Reaction of Carbonyl Sulfide with H, OH and O( 3 P)[END_REF] / Calculation Chen et al. [START_REF] Chen | Computational Study on the Mechanisms and Rate Constants for the O( 3 P, 1 D) + OCS Reactions[END_REF] / Calculation 5.83×10 -14 ×T 0.87 ×exp(-3600/T) cm 3 molecule -1 s -1 at T = (200 -2500) K. In another, more recent theoretical study, Chen et al. [START_REF] Chen | Computational Study on the Mechanisms and Rate Constants for the O( 3 P, 1 D) + OCS Reactions[END_REF] predicted k1a = 6.37×10 -11 exp(-2461/T), k1b = 1.08×10 -10 exp(-4615/T) and k1 = k1a + k1b = 9.2×10 -11 exp(-2607/T) cm 3 molecule -1 s -1 at T = (300 -1600) K. The resulting total rate constants, k1 = k1a + k1b, from both computational studies are shown in Figure 5. The calculated values of k1 are fairly close to the experiment; however, do not adequately reflect the curvature of the Arrhenius plot.

Products of reaction 1.

In accordance with available experimental and theoretical data for the reaction products (although very scarce), one would expect that in the temperature range of this work, channel 1a of the O+OCS reaction would be the dominant one with the branching ratio more than 80 %. We have verified this point by measuring the yield of SO, product of reaction 1a, at T = 700 K. To avoid measuring absolute concentrations and thus improve the precision of the measurements, we used a relative method to determine the k1a/k1 ratio.

Experiments consisted in a consecutive titration of the initial concentration of O-atoms with OCS and thiirane accompanied with a direct detection of the reaction product, SO. Thereby the yield of SO in reaction 1 was measured relatively to that in the reaction of oxygen atoms with C2H4S, where the yield of SO is 100%:

O + C2H4S  SO + C2H4 (4)
k4 = 1.34  10 -11 exp(-4/T) cm 3 molecule -1 s -1 (T = 268 -424 K). [START_REF] Lee | Absolute Rate Parameters for the Reaction of Ground State Atomic Oxygen with Dimethyl Sulfide and Episulfide[END_REF] The observed data are shown in Figure 6. The slope of the line in Figure 6 corresponds to the branching ratio for the SO forming channel of reaction 1: k1a/k1 = 0.922 ± 0.008 (2). It can be noted that the CO2 yield (k1b/k1) at T = 700 K is in good agreement with the results for the SO yield (k1a/k1) discussed above. Isshiki et al. [START_REF] Isshiki | High-Temperature Reactions of O + COS and S + SO2. Abstraction Versus Substitution Channels[END_REF] have determined the k1b to k1 ratio by analysis of time profiles of the S atom upon photolysis of COS and SO2 mixtures diluted in Ar behind reflected shock waves. Their results are quite consistent with the extrapolation (which is, of course, conditional) of our data to the corresponding temperatures. As for the theoretical calculations of k1b/k1 presented in Figure 8, one can note that those of Saheb et al. [START_REF] Saheb | Quantum Chemical and Theoretical Kinetics Studies on the Reaction of Carbonyl Sulfide with H, OH and O( 3 P)[END_REF] describe quite well the temperature behavior observed for k1b/k1 in the present work, although in absolute value they are shifted down by a factor of 1.6. at T = 455 -960 K and with 2 statistical uncertainties. This expression can be used to derive k1a by subtracting k1b from the experimental data for k1 (Table 1). The fit of three-parameter modified Arrhenius expression to obtained in this way (k1 -k1b) results in k1a = 2.35×10 -12 ×(T/298) 1.84 ×exp(-1585/T) cm 3 molecule -1 s -1 .

It can be noted that in the temperature range of the study, the sum of the two partial rate constants, k1 = k1a + k1b, provides the values of k1 similar (within 0.6 %) to those calculated )

3. 1 . 1 .

 11 Kinetics of SO production in reaction 1. In this series of experiments, the rate constant of reaction 1 was determined in the temperature range 220 -410 K from the kinetics of the reaction product, SO, formation under conditions where consumption of the reactants (OCS and O-atom) was insignificant (less than 10 % at all temperatures of the study). Under such experimental conditions where the concentrations of O-atom and OCS can be considered as constant, the [SO] buildup is governed by zeroth order kinetics, d[SO]/dt = k1a×[O]×[OCS]= constant, and linear increase of the concentration of the SO radicals with reaction time is expected. Experiments were carried out in the presence of NO2 which was added in the end of the reactor, 5 cm upstream of the sampling cone of the mass spectrometer ([NO2] = (5 -8)×10[START_REF] Robertshaw | Rate Data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a New Time-Resolved Technique[END_REF]

  [SO] in this section actually mean the measured [SO2], which is equal to [SO]. Examples of the SO kinetics are shown in Figure 1.

Figure 1 .

 1 Figure 1. Kinetics of SO radical production in reaction 1 observed at T = 310 K with [O] = 1.0×10 14 molecule cm -3 and different initial concentrations of OCS. Continuous lines represent linear fit to the experimental data.

  displays the experimental data demonstrating that variation of [O] had no impact on the results of the measurements. The slopes of the observed linear dependences of d[SO]/dt on [O]×[OCS] provide the values of k1a at respective temperatures.

Figure 2 .

 2 Figure 2. Examples of the dependence of the rate of SO production in reaction 1 on the product of the concentrations of OCS and O-atom. Partially shown error bars represent the typical 10% uncertainties on the measurements of the respective parameters.

Figure 3 .

 3 Figure 3. Pseudo-first order rate constant, k 1 ' = k 1 [O], as a function of the O-atom concentration at different temperatures.

3. 1 . 3

 13 Temperature dependence of k 1 . All the kinetic data from the present work are shown in Figure 4 in line with previous temperature dependent measurements. Before discussing the temperature dependence of the rate constant of reaction 1, it should be noted that

Figure 4 .

 4 Figure 4. Summary of the temperature dependent measurements of the rate constant of the reaction O+OCS. Uncertainty on the values of k 1 from the present study (nearly 15-20 %) corresponds to the size of the symbols. Solid line: three-parameter fit to the present data; dashed lines: deviation from the three-parameter fit by a factor of 1.35.

Figure 5

 5 Figure 5 demonstrates the available experimental temperature dependent k1 data in comparison with current recommendations and theoretical calculations of the rate constant.Current recommendation of the NASA Panel for data evaluation, k1 = 2.110 -11 exp(-2200/T) cm 3 molecule -1 s -1 at T = (239 -808) K,[START_REF] Burkholder | Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies[END_REF] adequately represents the experimental data at low temperatures, however underestimates k1 at T > 600 K. Similar expression for k1 is recommended by the IUPAC Subcommittee on kinetic data evaluation: k1 = 1.610 -11 exp(-2150/T) cm 3 molecule -1 s -1 at T = (230 -500) K.[START_REF] Atkinson | Evaluated Kinetic and Photochemical Data for Atmospheric Chemistry: Volume I -Gas Phase Reactions of Ox, HOx, NOx and SOx Species[END_REF]

Figure 5 .

 5 Figure 5. Available experimental temperature dependent data for k 1 in comparison with current recommendations and computational studies.

Figure 6 .

 6 Figure 6. Concentration of SO radicals formed in reaction of O atoms with OCS as a function of the concentration of SO formed in reaction O+C 2 H 4 upon consumption of the same concentration of oxygen atoms. The main part of the experiments on the measurements of the reaction products in the present study was focused on the determination of the branching ratio for the minor channel of reaction 1, forming S and CO2, and consisted in the determination of the CO2 yield. Experiments were conducted in an excess of oxygen atoms ([O] = (0.35 -1.25)×10 14 molecule cm -3 ) over OCS in order, among other things, to minimize the contribution of OCS to the mass spectrometric signal of CO2 due to dissociative ionization of OCS in the ion source of the mass spectrometer (m/z = 44, CS + ). Reaction time was in the range (0.025-0.070) s and consumption of OCS was between 65 and 97 %, depending on concentration of oxygen atoms and temperature in the reactor. The concentration of CO2 was measured as a function of [OCS] consumed. Some typical experimental data are shown in Figure 7. All the results obtained for the yield of CO2 (corresponding to k1b/k1 ratio) from slopes of the straight lines like those in Figure 7 are presented in Table S1 and in Figure 8 in comparison with existing experimental

Figure 7 .

 7 Figure 7. Concentration of CO 2 formed in reaction of O atoms with OCS as a function of the consumed concentration of OCS.

3 )Figure 8 .

 38 Figure 8. Branching ratio for the S+CO 2 forming channel 1b of the reaction O+OCS as a function of temperature.

15 Figure 9 .

 159 Figure 9. Temperature dependence of k 1b observed in the present work and in previous studies.

Table 1 . Experimental Conditions and Results of the Measurements of the Rate Constant of Reaction 1

 1 Units of 10 13 molecule cm -3 . c k 1a in case of SO kinetics, k 1 in case of OCS kinetics; units of 10 -13 cm 3 molecule -1 s -1 ; statistical 2 uncertainty is ≤ 2%; total estimated uncertainty is 15-20 %.

	T (K) a	[O] b	[OCS] b	k c	Reactor surface d	method e
	220	15.9	0.343-7.53	0.00974	HW	SO kinetics
	235	4.87	0.236-7.80	0.0197	HW	SO kinetics
	250	7.69	0.173-4.24	0.0290	HW	SO kinetics

a 8-12 kinetic runs at each temperature. b d HW: halocarbon wax; Q: quartz. e k 1 derived from kinetics of SO formation (SO kinetics) or OCS consumption (OCS kinetics).

3.1.2. Kinetics of OCS consumption in excess of O atoms. The measurements were carried out under pseudo-first order conditions with [O] >> [OCS] (see Table 1). The

concentrations of both reactants, OCS and oxygen atoms, at various reaction times were monitored by mass spectrometry. Examples of the observed exponential decays of [OCS], in accordance with d[OCS]/dt = -k1[O][OCS], are shown in Figure

This work / 455-960 Isshiki et al. 15 / 1120-1540 Isshiki et al. 15 / 670-2000 / Calculation Saheb et al. 16 / 200-2500 / Calculation Chen et al. 17 / 300-1600 / Calculation

CONCLUSIONS

In this study, a discharge flow reactor combined with an electron impact ionization mass spectrometer is employed to measure the total rate constant of the reaction of O( 3 P) atoms with carbonyl sulfide at as well as the branching ratio for the minor, S+CO2 forming, channel of the reaction. Analysis of the previous data shows that the modified Arrhenius expression k1 = 1.92×10 -12 ×(T/298) 2.08 ×exp(-1524/T) cm 3 molecule -1 s -1 obtained at T = 220 -960 K can be recommended to calculate k1 at temperatures up to 2000 K. The measurements of CO2 yield resulted in a first determination of the rate constant of the corresponding reaction pathway, k1b = 4.19×10 -11 ×exp(-4088/T) cm 3 molecule -1 s -1 at T= 455-960 K. Comparison of the current data with computational studies shows that the latter describe the reaction quite adequately. The present kinetic data for k1 and k1b provide an experimental dataset for use in atmospheric chemistry and combustion modelling and in refining theoretical models. S1); rate of SO production in reaction 1 as a function of the product of the concentrations of OCS and O (Figure S2); example of concentration-time profiles of OCS in reaction with excess O (Figure S3); branching ratio for the S + CO2 forming channel of reaction 1 (Table S1).

Supporting Information. Diagram of the flow reactor (Figure

ACKNOWLEDGEMENT


Yoshimichi Shimomura

Prof. Dr Kazunobu Igawa

Dr Shunsuke Sasaki

Dr Noritaka Sakakibara

Prof. Dr Raita Goseki

Prof. Dr Gen-Ichi Konishi
email: konishi.g.aa@m.titech.ac.jp

Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission

come

Introduction

Recently, ultrabright solid-state fluorescence materials containing organic dyes have been applied in organic light-emitting devices (OLEDs), [1] nano particle, [2] , circularly polarized luminescence, [3] bio-imaging, [4] and lasers [5] due to their high degrees of freedom in design and excellent processability. Optimizations of the luminous efficiencies, emission colors, crystallinities, and molecular orientations of the solid-state fluorescent dyes are required in designing advanced OLEDs. The current remaining issues associated with these devices are as follows. (1) Developing fluorescent dyes that exhibit high luminescence efficiencies without concentration quenching in their solid states is essential. [6] J-aggregates are fluorescent dyes that exhibit strong solid-state luminescence; however, the fluorescence wavelengths of their aggregated forms are different with respect to the monomer emissions. [7] The simple system involves external shielding by bulky substituents (site isolation effect), [8] or by preventing association via hydrogen bonding or ion-pair formation. [9] Another type of system, viz. aggregation-induced emission (AIE) luminogens, has been investigated. [10] However, in these systems, the dye molecules are large. Small dye molecules and crystals with small free volumes are desired for application in electronic devices. [11] (2) Predicting fluorescence spectra in the solid state using those of dilute solutions or distributed matrices is challenging. Numerous organic dyes form aggregates of ≥2 molecules in the excited state, [12] and the fluorescence wavelengths often change due to intermolecular electronic interactions. Therefore, monomeric emissive dyes with high quantum yields exhibit advantages in the field of material design.

(3) To obtain monomeric emission, controlling the dye arrangement is essential. It is still underdeveloped to the methodology of arranging molecular π-skeletons appropriately such that dyes exhibit monomeric emission without bulky substituents (without increased free volume). Therefore, novel functional groups attached to the π-electron framework are necessary to control the crystalline system. In addition, introducing a new functional group into an existing π-electronic compound may afford a different crystalline structure. Increased diversity in crystal engineering is important for the development of material design strategies.

In this study, we prepare highly fluorescent monomeric emissive dyes, bridged distyrylbenzenes (DSBs) [di-bridged distyrylbenzenes]: DBDB [7]s, with high dye density per volume crystalline structures, by introducing flexible cyclic systems into the π-conjugated systems. We propose a new crystal engineering method to control luminescence and electronic properties by introducing simple alkylene bridges to π-electron compounds. We designed DBDB [7]s with reference to a well-known fluorescent dye, viz. DSB. [13,[START_REF] Delori | Other examples of highly emissive DSB derivatives in the solid state exist, based on cocrystal strategies[END_REF] The DBDB [7]s contain two seven-membered rings with C=C bonds bridged loosely by propylene chains, which are much smaller functional groups than those studied in previous reports (Figure 1). This structural design suppressed intermolecular electronic interactions without distancing the chromophores via steric hindrance. It is in stark contrast with DSB exhibiting H-aggregates [15] -DBDB [7]s rarely alter the emission wavelength. Moreover, the DBDB [7]s exhibit AIE properties, controllable fluorescence emission, and non-mechanochromism.

Results and Discussion

In this study, we classified DSB analogs into two categories as a function of their photophysical properties; Category A is represented by DBDB [7] and structural homologues, while Category B includes not only DSB but also other all related dyes exhibited similar photophysical properties. DSB, PPB, and DSDMB were previously reported. [15][16][17] We classified the novel compound DBDB[6] to Category B due to its similarity with DSBs in their photophysical properties. We synthesized DSBs and bridged DSBs (DBDB[6] and DBDB[7]s), and the detailed synthetic methods [18] are described in the Supporting Information (ESI). Previously, we proposed a bridged-stilbene concept to design AIEgen. [19] Namely, the flexible ring structure mechanically controlled the accessibility to the configuration taken during the quenching process of the excited-state stilbene. Compared to the melting point of 4-phenylstilbene, that of bridged 4-phenylstilbene (8-([1,1'-biphenyl]-4-yl)-6,7-dihydro-5H-benzo [7]annulene) [19] decreased by 120 °C without significant increase of free volume within the crystal. That fact inspired the design of new dyes that we present herein. A similar is known for curved nanographene [20] and carbon nanotubes, [21] whose solubility was improved by introducing a seven-membered ring.

Photophysical properties

The photophysical properties, i.e., the molar absorption coefficient (ε), the maximum absorption wavelength (λ abs ), the wavelength of maximum fluorescence emission (λ fl ), and quantum yield (Φ fl ) in both tetrahydrofuran (THF) and the solid-state are listed in Table 1. DSBs were already known, [15][16][17] however, a systematic study of the photophysical properties of DSBs in both dilute solution and the solid state has not been reported. Therefore, we newly synthesized DSBs. For example, DSB was measured λ abs , λ fl , and Φ in dilute dichloromethane and in a single crystal, PPB was measured λ abs and λ fl in dilute THF and in aggregates, and DSDMB was only measured λ abs in dilute chloroform.

The dilute solutions of DSB, DSDMB, and DBDB[6] in THF exhibit similar λ abs of ~350 nm and Φ THF of ~0.90. Meanwhile, PPB, which contains methyl groups at the C=C bonds, exhibits a ~40 nm blue-shifted λ abs of 314 nm and nearly quenched fluorescence with Φ THF of 0.01. DBDB [7] with a flexible propylene bridge also exhibits a blue-shifted λ abs (320 nm) and a low Φ THF (0.11). Moreover, introducing methyl groups at the central benzene or bridging C=C bonds results in a further blue-shift in λ abs (DBDMDB [7]: 273 nm, DBαMDB[7]: 284 nm), with Φ THF values of <1%. [START_REF]The non-radiative decays of the DBDB[7] molecules occur via a conical intersection (CI)[END_REF] ε depends on the flatness and decreases in the following order: structures without cyclic moieties > sixmembered-ring > seven-membered-ring > methyl-substituted C=C. These results are due to the elevation in π-conjugation length and the destabilization of the Franck-Condon (FC) energy at the excited state. All compounds, except DBαMDB [7], 5DBαMDB [7], and t BuDBαMDB [7] (no emission; we call these three compounds DBαMDB [7]s in this report), exhibit similar fluorescence maxima λ fl, THF (392-426 nm). It implies that the nature of their excited states is similar at the lowest singlet excited state (S 1min ), regardless of the bridged structure or introduction of the methyl group at the C=C bonds.

We calculated the radiative and non-radiative transition rates (k r and k nr , respectively) in the THF solutions of DSB, DSDMB, DBDB[6], and DBDB [7]. In the case of DSB, fluorescence lifetimes (τ) of both solution and solid have two components. However, since the contribution of one lifetime component was far more important than the other, that value was used to approximate the rate constant. All k r values are similar (5.2 × 10 8 -7.1 × 10 8 s -1 ), but k nr of DBDB [7] is more than 50-fold larger than those of the other compounds (Table 1). The fluorescence quenching of the DBDB [7]s in solution occurs via pyramidal structures due to cleavage of π-conjugation of the C=C bonds, similar to the situation in stilbene derivatives. [19,23] In addition, the conjugation lengths shorten, and the FC energies are destabilized. .31 ns - 1 between dilute THF solutions and the solid states, whereas that of DBDB [7] only differ by 0.05 ns -1 . We analyzed these differences between the fluorescence properties in solution and the solid state in the two categories shown in Figure 1.

Category A

In general, efficient solid-state fluorescence usually arises from aggregates, such as J-aggregates [7] or excimers [24] or with monomeric emission. [6] In DBDB [7], DBDMDB [7], and DBαMDB [7], the change in fluorescence wavelength is short (wavelength shift of <10 nm) (Figure 3(a)), and the quantum yield is >0.8 higher in the solid state than in solution. Although new band structures of these dyes were observed, the changes were small; therefore, the fluorescence properties are comparable to monomeric luminescence.

The DBDB [7], DBDMDB [7], and DBαMDB [7] agglomeration experiments were performed to better understand the photophysical characteristics of these molecules and to determine whether their photophysical properties in the agglomerated state and the solid state differ. The agglomeration study of DBDB [7], DBDMDB [7], and DBαMDB [7] in THF/water reveals that λ abs , λ fl , and ε in a 90% water suspension are similar to those in dilute THF solutions. However, for the aggregates in 70% or 80% water suspensions, large blue-shifts in λ fl (particularly DBαMDB [7]) and decreases in ε are observed possibly due to variations in the molecular conformations from those in dilute THF solutions or the solid states (Figures S11-13, Table S1-3).

Photophysical measurement of ground DBαMDB [7] was performed (Figure S6). λ fl and Φ solid are similar before and after grinding, revealing that DBαMDB [7] is a non-mechanochromic dye. [25] Entry ε

[M cm -2 ]
λabs j [nm] λfl, (THF) a [nm] λfl (powder) [ 5DBαMDB [7] exhibits similar emission behaviors to those of DBαMDB [7], whereas t BuDBαMDB [7], with a bulky substituent, exhibits a high Φ solid value (0.84), although λ fl(solid) is approximately 40 nm shorter than those of the other molecules due to conformational change. tBuDBαMDB [7] in the solid state and DBαMDB [7] in a 70%-80% water suspension exhibit identical conformations, as their λfl values are similar. Steric hindrance due to the bulky substituents at the axial positions in tBuDBαMDB [7] results in a larger distance between the π-electron systems in the core than in the other DBDB [7]s. The introduction of substituents on the axis of the π-electron framework enables control over the crystal structure and fluorescence wavelength.

Although the conjugation lengths are shortened by the introduction of flexible bridges, it is apparent that this design strategy is advantageous for the development of AIEgens because DBDB [7]s exhibited almost no emission in THF and strong fluorescence in the solid state.

Category B

In a previous study, [15,16] DSB and PPB were found to form Haggregates in the solid state. Compared to PPB in the solution, that in the solid-state exhibits a longer λ fl(solid) (Figure 3(b)). Furthermore, solid-state PPB exhibits a higher Φ solid value.

Single-crystal X-ray structure analysis

We analyzed the relationships between the photophysical properties and crystal structure.

Category A

The torsion angles between the C=C bonds and central and outer benzene rings are φ i and φ o , respectively, shown in Figure 5.

DSB exhibits almost parallel central and outer benzene rings. Conversely, DBDB [7] and DBDMDB [7] exhibit large, twisted structures. The difference between φ i and φ o in DBDB [7] is only 7°-9°; thus, the central and outer benzene rings are twisted to the same degree with respect to the double bond. DBDB [7] exhibits two configurations of the bridged structure, and the two crystal structures are enantiomeric. In DBDMDB [7], φ i is approximately 74° larger than φ o , indicating that the central benzene ring is further twisted with respect to the double bond in relation to the outer benzene ring. DBαMDB [7] is dissolved in a hexane/dichloromethane mixture, and then the solution is gradually evaporated under atmospheric conditions to yield two types of single crystals: a cross type, with twisted central and outer benzene rings, and a parallel type, with almost no twisting (Figure 5). In both structures, the magnitudes of φ o and φ i are similar, and the central and outer benzene rings are twisted equally with respect to the bridging double bond. The λ fl and Φ values of the two single crystals of the DBαMDB[7]cross/parallel are shown in Figure 6.

In DBDB [7], DBDMDB [7], and DBαMDB [7], the benzene rings of adjacent overlapping molecules are almost orthogonal; thus, the electronic interactions are negligible. DBDB [7], DBDMDB [7],

and DBαMDB [7] exhibit monomeric emissions in their solid states. The molecular structure is likely similar to that in a dilute THF solution, as their fluorescence properties in dilute THF solutions and the solid states are similar. In the case of DBαMDB [7], the molecular conformation may be controlled via recrystallization, and the conjugate length and luminescence properties of the chromophore may thus be manipulated. Moreover, the λfl values of the DBαMDB [7] aggregate in a 70% water suspension and DBαMDB[7]-parallel crystal are similar, indicating that these conformations are identical.

Finally, in terms of the molecular densities and occupancies, no difference is observed between DSBs and DBDB[7]s (Table S6). Introducing a flexible bridging structure into the π-framework results in a different crystal form in relation to that of the π-framework without increasing the free volume.

The molecular structure of 5DBαMDB [7] was similar to that of DBαMDB[7]-cross, and that of t BuDBαMDB [7] was identical to that of DBαMDB[7]-parallel (Figure S20). In t BuDBαMDB [7], the distance between the benzene rings of adjacent molecules that overlapped in phase was approximately 1.1 Å longer than that in DBαMDB[7]-parallel. We could not analyze the fluorescence properties of the DBαMDB [7]s in detail because they exhibited no fluorescence in dilute THF solutions (AIE). However, they should exhibit monomeric fluorescence in their solid states, based on their Φ solid and τ solid values and crystal structures. [START_REF]Determining whether organic dyes strictly exhibit monomeric emission is difficult; however, luminescence spectrum changes caused by an intermolecular electronic interaction should be observable[END_REF] Category ). The electronic interaction of PPB is small because it exhibits a d chr value larger than those of the other compounds, and α and β are >70°, suggesting large dihedral angles between adjacent molecules. Thus, the quantum yield of PPB is higher than those of the other DSBs. Conversely, DSDMB adopts a faceto-face zigzag column structure. In DSDMB, the d chr value is 3.7 Å, which is the same as that of PPB, but the electronic interaction is greater than that within PPB due to the face-to-face structure, which may result in the decrease in Φ solid . DBDB [7] has unique crystal structures differing from those of any DSB derivatives, and exhibits many excellent characteristics. The introduction of flexible cyclic structures into π-cores contributes to diversity in crystal chemistry.

Theoretical study

We calculated the transition from the ground to the excited state of a single molecule with the same configuration in a single crystal using time-dependent self-consistent field theory at the ωB97XD/6-311G (d,p) level of theory. The calculated absorption wavelengths in the solid states are similar to the experimental values (Table S10). It is challenging to assess precisely the absorption wavelengths and the molecular packing structures in (a) Arrangements of adjacent molecules within layers in the solid states of DSB, [START_REF] Delori | Other examples of highly emissive DSB derivatives in the solid state exist, based on cocrystal strategies[END_REF] PPB, [15] and DBDB the solid states using broad diffuse-reflection spectra or this calculation. Therefore herein, we discuss the qualitative trend of their photophysical properties. The excitation levels of all compounds exhibit S o → S 1 transitions, and highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transitions are dominant (Table S10). The electron clouds at the HOMO/LUMO and energy levels are shown in Figures S31 andS32 and Table S11. In DBDMDB [7], the πconjugation is localized and not extended. In contrast, in the other compounds, the π-conjugation is extended over the entire molecule. Because φ i of DBDMDB [7] is >90°, the conjugation may be extended only externally. Considering the HOMO-LUMO energy gaps of all compounds, using DSB as a reference, the HOMOs are within ±0. These results conflict with the little electronic interaction exhibited by the DBDB [7]s, which are supported by our photophysical study. However, the NBO analysis evaluates the states of bonding orbitals isolated from the stabilizing structure of the π-conjugated system. Therefore, the DBDB[7]s are unique crystal structures with few electronic interactions, despite the fairly close adjacent π-conjugated planes.

Conclusion

We successfully synthesized bridged DSBs (DBDB [7] S7) and good processability because they can adopt many conformations owing to their flexible cyclic structures; and (c) nonmechanochromism. Non-mechanochromic dyes have no limitations in terms of the processing methods and can be used in materials and for device applications; they can perform at a certain level independent of the environment. These unique properties will contribute to future material design, e.g., a desired luminescence color can be generated by arranging multiple dyes without them interacting with each other. The crystal structure obtained by introducing flexible cyclic structures into π-cores will depend on the selected π-electron molecules. This strategy dramatically expands the diversity of crystal structure fabrication. We will introduce alkylene bridges into various skeletons and develop crystal chemistry in the future. The flexible-alkylenebridge concept should reveal novel material functions of hardly soluble rigid-rod π-conjugated molecules and polymers. [START_REF] Ding | [END_REF] In these rod-shaped molecules, there is a trade-off between improving the processability and molecular alignment. This issue is an obstacle to improving device performance, and we are currently attempting to resolve this problem.

Figure 1 .

 1 Figure 1. Solid-state fluorescence of previously reported organic dyes (Ref. 8a, 8b) and the organic dye in this work. Functional groups are shown in red.

Figure 2 .

 2 Figure 2. Structures of the DSBs, DBDB[6], and DBDB[7]s classified into two categories.

Table 1 .

 1 Photophysical properties of DSBs, DBDB[6], and DBDB[7]s in THF and their solid states. Excitation wavelength: a) max λabs, b) 370 nm, c) 400 nm, d) 330 nm, e) 300 nm, f) 379 nm, and g) 269 nm. (h) Only τ2 is shown due to 91% contributing rate. τ1 = 0.74 ns (9%). (i) Only τ1 is shown due to 93% contributing rate. τ2 = 3.92 ns (7%). (j) Two components. DSDMB: 0.82 ns (63%) and 1.67 ns (37%); t BuDBαMDB[7]: 0.13 ns (32%) and 1.74 ns (68%). (k) As we did not have the proper wavelength cutting filter or LED laser, DBDMDB[7] could not be measured and the result of t BuDBαMDB[7] may be incorrect. (l) Calculated from τ2. (m) Calculated from τ1. (n) Dilute THF solution. The absorption spectra of the solid states were measured by the diffuse-reflection method (see Figure S3). That fact explains the low values of the ε, λ abs , and Φ THF values of the DBDB[7]s and PPB decrease. The photophysical properties of DSDMB and DBDB[6] are similar to those of DSB, as the planarity of the molecules is preserved. The quantum yields of DSB, DSDMB, and DBDB[6] are higher in dilute solutions than in the solid state, conforming to typical dyes exhibiting concentration quenching. In contrast, PPB and the DBDB[7]s, which exhibit weak fluorescence in dilute solutions, display solid-state Φs values that are >0.80 higher than those in solution. The λ fl-solid values of DSBs are red-shifted by approximately 50 nm in relation to those of λ fl-THF , whereas those of the DBDB[7]s, except t BuDBαMDB[7], are approximately 400 nm. The λ fl-solid value of t BuDBαMDB[7] is 365 nm, which is 40 nm shorter than those of the other DBDB[7]s. The fluorescence lifetimes of the solids (τ solid ) for all compounds, except DBDB[6] (3.92 ns), are ≤2.0 ns. Bridged DSB derivatives may not form excimers. The k r values of DSB and DBDB[6] differ by >0

Figure 3 .

 3 Figure 3. Fluorescence spectra of (a) DBDB[7]B, DBDMDB[7], and (b) DSBs in dilute THF (dashed line) and in the solid states (solid line).

Figure 4 .

 4 Figure 4. Single molecular and crystal structures of DBDB[7], DBDMDB[7], DBαMDB[7]-cross, and DBαMDB[7]-parallel. The central benzene groups are shown in red, and the bridging groups are shown in blue.

DSDMB

  and DBDB[6] are also likely to form H-aggregates as inferred from λ fl and Φ fl values comparable with DSBs, and the crystal structures of DSDMB and DBDB[6] are similar to that of DSB (see Figures S21, S22, S24). Introducing methyl groups at the lateral positions of DSB (DSDMB) or DSB containing a highly planar six-membered-ring core (DBDB[6]) renders monomeric emission unfavorable in the solid state and solution.

Figure 5 .

 5 Figure 5. (a) Definitions of φo and φi. (b) Torsion angles of DBDB[7], DBDMDB[7], DBαMDB[7]-cross and DBαMDB[7]-parallel.

B

  Each molecule is highly planar, and DSB, PPB, and DBDB[6] adopt edge-to-face herringbone structures. The intermolecular distances between the chromophores (d chr , Figure 7(c)) of DSB and DBDB[6] are 3.6 Å, whereas that of PPB is 3.7 Å. We define the tilt angle between the central/outer benzenes of adjacent molecules as α/β (Figure 7(a)). The magnitude of α is DBDB[6] > PPB > DSB, whereas that of β is PPB > DSB > DBDB[6] (Figure 6(b)

Figure 6 .

 6 Figure 6. Fluorescence spectra, λfl, and Φ of DBαMDB[7]-cross and DBαMDB[7]-parallel.

Figure 7 .

 7 Figure 7. (a) Arrangements of adjacent molecules within layers in the solid states of DSB,[START_REF] Delori | Other examples of highly emissive DSB derivatives in the solid state exist, based on cocrystal strategies[END_REF] PPB,[15] and DBDB[6]. The central and outer benzene rings are shown in red and blue, respectively. (b) Values of α and β for each compound. (c) Definition of dchr.

  Figure 7. (a) Arrangements of adjacent molecules within layers in the solid states of DSB,[START_REF] Delori | Other examples of highly emissive DSB derivatives in the solid state exist, based on cocrystal strategies[END_REF] PPB,[15] and DBDB[6]. The central and outer benzene rings are shown in red and blue, respectively. (b) Values of α and β for each compound. (c) Definition of dchr.

  4 eV, but the LUMOs differ by up to 1.5 eV. The HOMO energy levels of DBαMDB[7]-cross and DBαMDB[7]-parallel, in particular, are similar, although their molecular structures differ considerably. We calculated the overlaps of the natural bond orbitals (NBOs) between adjacent molecules of the DSBs, DBDB[7], DBDMDB[7], and DBαMDB[7]-cross/parallel in single crystals at the ωB97X/6-311G (2d,2p) level of theory. (Figure S33-S39) We analyzed only the overlaps of occupied and unoccupied orbitals of C=C bonds between adjacent molecules because we focused on the molecular light-emitting behaviors in the solid state. The overlap of the NBOs of DSDMB is observed, but in DBDMDB[7], with the introduction of the bridging structure, NBO overlap is not observed. The overlap of the NBOs of DBαMDB[7]parallel is 0.49 kcal/mol smaller than that of PPB. Conversely, that of DBDB[7] is 0.79 kcal/mol larger than that of DSB, and that of DBαMDB[7]-cross is 0.63 kcal/mol larger than that of PPB.

  s), which exhibited crystal structures with compact packing and solid-state monomeric fluorescence spectra similar to those in dilute solutions. Unlike conventional design methods that use bulky functional groups or capsules to isolate dyes, introducing small functional groups (flexible propylene bridges) into a π-conjugated platform could yield monomeric emission in the solid state. The π-distortion and appropriate steric hindrance due to the flexible propylene bridges enabled the DBDB[7]s to form compact crystal structures with few intermolecular electronic interactions. The DBDB[7]s also displayed the following valuable features: (a) AIE character (Φ solid > 0.80); (b) low melting points (Table

Acknowledgements

The authors thank Mr. Masato Koizumi (Materials Analysis Division, Tokyo Institute of Technology) for the measurements of HRMS spectra. This research was supported in part by MEXT/JSPS KAKENHI grants 18H02045 and 17H05145 (to G. K.), 21K05165 (to R. G.) and JSPS fellows 21J01295 (to N. S.), by The Toshiaki Ogasawara Memorial Foundation (to G. K.) and The Murata Science Foundation (to G. K.). This research was supported by the Research Program of "Network Joint Research Center for Materials and Devices". Keywords: solid state fluorescence • highly fluorescent dyes • organic light-emitting devices (OLED) • crystal engineering • distyrylbenzene

Entry for the Table of Contents

Flexible alkylene bridges can realize monomeric emission in the solid state without bulky substituents, and have many potentials such as expanding the diversity of crystal structure fabrication.