Doctor Laurent Sulpice
email: laurent.sulpice@chu-rennes.fr

MD.PhD S Garnier

M Rayar

MD B Meunier

MD K Boudjema

Pr Karim Boudjema
email: karim.boudjema@chu-rennes.fr

Biliary cirrhosis and sepsis are two risk factors of failure after surgical repair of major bile duct injury post laparoscopic cholecystectomy

Keywords: Laparoscopic cholecystectomy, Bile duct injury, Repair, Long-term results

Background

Major bile duct injury (MBDI) remains frequent after laparoscopic cholecystectomy (LC) reaching to 0.3 to 0.6% and is associated with a significant mortality rate. The aim of this study was to retrospectively analyze the factors likely to influence the long-term results of surgical repair for MBDI occurring after LC.

Methods

Medical records of patients referred to our referral center from January 1992 to January 2010 for management of bile duct injury following LC were retrospectively analyzed, and patients with MBDI were identified. Clinicopathological factors likely to influence long-term results after surgical repair were assessed by univariate and multivariable analysis.

Results

During the study period, 38 patients were treated for MBDI. These 38 patients underwent Roux-en-Y hepaticojejunostomy (HJ) or HJ revision in 25 (66%) and 13 (34%) cases, respectively. The median follow-up period was 93 (26-204) months. A Clavien-Dindo postoperative morbidity class >3 occurred in 10 (26%) cases and was independently associated with a surgical repair performed during a sepsis period (OR=102.5 IC 95% [7.12; 11352], p<0.007). Long-term results showed that biliary strictures occurred in 5 (13%) cases and were associated with sepsis (p<0.006), liver cirrhosis (p<0.002) and postoperative complications (p<0.012). Multivariate analysis revealed that only liver cirrhosis remained predictive of stricture (OR=26.4, IC95% [2; 1018], p<0,026)

Conclusion

When MBDI occurs following LC, HJ seems to be the optimal treatment but should not be performed during a sepsis period. Long-term results are significantly altered by the presence of a biliary cirrhosis at time of repair.

INTRODUCTION

Cholecystectomy is one of the most frequently performed surgeries, with approximately 50,000 and 750,000 procedures performed annually in France and the United States, respectively [START_REF] Flum | Bile duct injury during cholecystectomy and survival in medicare beneficiaries[END_REF]. The advent of laparoscopy has led to a significant increase in bile duct injury (BDI) rates, now reaching 0.3 to 0.6% [START_REF] Strasberg | An analysis of the problem of biliary injury during laparoscopic cholecystectomy[END_REF][START_REF] Tantia | Iatrogenic biliary injury: 13,305 cholecystectomies experienced by a single surgical team over more than 13 years[END_REF] and mainly occurring during the learning curve [START_REF] Savassi-Rocha | Iatrogenic bile duct injuries[END_REF]. In spite of Strasberg's "critical view of safety", new studies and new recommendations about the management of biliary disease have not yet significantly reduced the rate of BDI [START_REF] Avgerinos | One Thousand Laparoscopic Cholecystectomies in a Single Surgical Unit Using the "Critical View of Safety" Technique[END_REF][START_REF] Strasberg | Avoidance of biliary injury during laparoscopic cholecystectomy[END_REF][START_REF] Callery | Avoiding biliary injury during laparoscopic cholecystectomy: technical considerations[END_REF]. Recently, Fullum et al reported the results of a Nationwide survey of cholecystectomies and showed that the only risk factors of BDI were : i)male gender; ii) age>60 years and iii) academic hospital status. In this series, laparoscopic cholecystectomy was not associated with an increased risk of BDI [START_REF] Fullum | Is laparoscopy a risk factor for bile duct injury during cholecystectomy?[END_REF]. The most important factor mainly correlated for improving the outcomes of cholecystectomies, remains the expertise of the surgeon [START_REF] Andrew | Does concentration of surgical expertise improve outcomes for laparoscopic cholecystectomy? 9 year audit cycle[END_REF][START_REF] Sinha | Epidemiological study of provision of cholecystectomy in England from 2000 to 2009: retrospective analysis of Hospital Episode Statistics[END_REF]. MBDIs correspond to type E of Strasberg's classification and includes all types of sections of the common bile duct or its primary radicles [START_REF] Strasberg | An analysis of the problem of biliary injury during laparoscopic cholecystectomy[END_REF][START_REF] Strasberg | Avoidance of biliary injury during laparoscopic cholecystectomy[END_REF]. Moreover, major BDIs are frequently associated with arterial injuries, i.e., a section of the right branch or the main trunk of the hepatic artery [START_REF] Stewart | Right hepatic artery injury associated with laparoscopic bile duct injury: incidence, mechanism, and consequences[END_REF]. When MBDI occurs, mortality rates can reach 9% versus 0.45% when absent [START_REF] Walsh | Long-term outcome of biliary reconstruction for bile duct injuries from laparoscopic cholecystectomies[END_REF].

Whereas minor BDIs (i.e., leakage from the cystic duct or lateral injury of the common bile duct) can be easily treated endoscopically, MBDIs primarily require surgical repair. The goldstandard surgery is Roux-en-Y hepaticojejunostomy (HJ) as described by Couinaud, combined or not combined with liver resection [START_REF] Bismuth | Postoperative strictures of the bile duct[END_REF][START_REF] Reuver | Referral pattern and timing of repair are risk factors for complications after reconstructive surgery for bile duct injury[END_REF][START_REF] Mercado | Acute bile duct injury[END_REF]. However, the optimal care of major BDIs, including the timing of repair, the type of surgery and the factors effecting worse outcomes, remain widely debated. Moreover, an accurate assessment of the repair results requires a long follow-up because some complications such as anastomotic stricture or secondary biliary cirrhosis may occur several years after surgical repair.

Although numerous studies have reported the incidence and results of major BDIs [START_REF] Perera | Specialist early and immediate repair of post-laparoscopic cholecystectomy bile duct injuries is associated with an improved long-term outcome[END_REF][START_REF] Moraca | Long-term biliary function after reconstruction of major bile duct injuries with Hepaticoduodenostomy or Hepaticojejunostomy[END_REF][START_REF] Sicklick | Surgical management of bile duct injuries sustained during laparoscopic cholecystectomy: perioperative results in 200 patients[END_REF][START_REF] Schmidt | Long-term results and risk factors influencing outcome of major bile duct injuries following cholecystectomy[END_REF][START_REF] Pottakkat | Recurrent bile duct stricture: causes and long-term results of surgical management[END_REF][START_REF] Huang | Long-term results of major bile duct injury associated with laparoscopic cholecystectomy[END_REF], very few have focused on very long-term outcomes. The aim of the present study was to analyze the factors likely to influence very long-term results after primary or secondary surgical repair of post LC MBDI in a tertiary referral center.

METHODS

Patients

The study population included all patients with BDI who were transferred to our institution between January 1992 and January 2010. Only patients with major BDI were analyzed. MBDI corresponds to type E of Strasberg's classification.

The clinical data were retrospectively collected and analyzed after Institutional Review board approval was obtained.

The collected data included age, sex, ASA Score, local sepsis, cholestasis (GGT serum over twice normal rate), presence or lack of jaundice and presence or lack of liver cirrhosis due to biliary obstruction at the time of repair in referral center and vascular injuries associated with BDI. Accordingly, CT scan was performed for each patient in order to search vascular injury before surgery in our center. Delay between the onset of MBDI diagnosis and time of transfer at our referral center was also assessed.

Ongoing sepsis was assessed in the presence of at least one of the following features, independently of previous surgery: leukocytosis >15000/mL, fever > 38.5°C, angiocholitis episodes, peritonitis or intra-abdominal abscess [START_REF] Walsh | Long-term outcome of biliary reconstruction for bile duct injuries from laparoscopic cholecystectomies[END_REF]. The delay between the onset of MBDI diagnosis and surgical management was defined as follows: immediate (perioperative or under 72 h), intermediate (between 72 h to 6 weeks) or late (over 6 weeks) [START_REF] Sahajpal | Bile duct injuries associated with laparoscopic cholecystectomy: timing of repair and long-term outcomes[END_REF].

The number of biliary anastomoses performed at time of repair was sorted into two groups: 1 and more than 1. The level of the anastomoses (on the common bile duct, on the confluence or on the hepatic duct), the use of biliary stenting and the necessity of liver resection were also collected for analysis.

Postoperative morbidity was classified according to the Clavien-Dindo Score (CDS) during the initial hospital stay or within 30 days after surgery if the patient was discharged [START_REF] Dindo | Classification of Surgical Complications[END_REF].

Major postoperative complications were defined by a CDS of 3 or over. Moreover, patient death during the initial hospital stay or within 30 days after surgery if the patient was discharged defined postoperative mortality.

Surgical repair

All patients referred to our center for MBDI repair underwent first HJ or redo HJ with or without associated liver resection. All of the procedures were performed by a senior hepatic surgeon. The enteric loop length was 80 cm, as measured with a ruler. All anastomoses were stented via transjejunal drain using the Völker technique. When liver resection was necessary, it consisted either in segment 4a and partially 5 resection to expose the biliary confluence from the top or a right hepatectomy when BDI had led to right liver atrophy. Systematically, a wedge biopsy of the liver was carried out to evaluate the impact of biliary obstruction.

Long-term follow-up

An update of the clinical, biological and ultrasonography outcome of all patients was performed between December 2011 and March 2012.

Good long-term results were defined by the absence of clinical symptoms, normality of hepatic laboratory tests and absence of dilatation of intrahepatic bile duct on ultrasonography performed at least one time per years after repair.

Statistical analysis

Quantitative variables were expressed as the mean ± standard deviation (SD) or the median value with the 25 th and 75 th percentile and compared using Student's t-test or Wilcoxon test, as appropriate. The qualitative variables were expressed as numbers and percentages and compared with the results of Chi-squared or Fisher's exact tests, as appropriate. All variables with p<0.15 in univariate analysis, as well as variables supposed to have a clinical influence on the complications rate as well as stenosis, were entered into a multiple logistic regression model. A variable selection procedure was performed using the stepwise procedure (backward method) based on minimalizing the AIC (Akaïke Information Criterion).

A p value of < 0.05 was considered to be statistically significant. All statistical analysis were performed using R statistical software, version 2.15.1 (http://www.r-project.org/).

RESULTS

Demographic data and injury characteristics

During the study period, 60 patients were referred to our center for BDI. Among them, 38 patients (15 men, 23 women, median age 61 (range: 33 to 88)) had MBDI. Ten (26.3%) of the 38 had associated right hepatic artery interruption. No patient presented cirrhosis at the time of LC.

Surgical management prior referral (Fig. 1)

Twelve patients (32%) were immediately referred without any attempt to repair. A previous surgical attempt to repair was performed in 26 cases (68%), including primary repair over a T-Tube, bilio-biliary anastomosis and HJ in 17 (45%), 1 (3%) and 8 (21%) cases, respectively. A second surgical attempt to repair prior to referral was performed in 6 (23%), i.e., HJ in 5 (13%) cases and repair over a T-Tube in 1 (3%) case. The time elapsed between the onset of MBDI and repair before referral was immediate in 15 (39%), intermediate in 7 (18%) cases and late in 4 (11%) patients.

Surgical management and postoperative outcomes after repair in our center

Twenty-five (66%) patients underwent primary Roux-en-Y HJ, and 13 (34%) patients underwent reconstruction of a previous HJ. Median length of hospital stay was 11 days.

In 9 cases (23.7%), liver resection was associated with HJ. Among the 38 cases of the series, 15 (39%) had sepsis, as defined previously, at the time of surgery and 35 (92%) patients had cholestatis. Jaundice was present in 16 (42%) cases. Secondary biliary cirrhosis due to obstruction was found in 7 (18%) cases upon histological examination of the liver specimen.

There was no cirrhosis due to other reason.

Table 2 reports the outcome of the 38 patients after surgical repair in our center. Postoperative biliary leakage occurred in 10 cases (26%). Biliary leakage required redo surgery in 4 (10%) cases and percutaneous drainage in 6 (16%) cases. There was no postoperative mortality.

Risk factors for perioperative major morbidity after repair (Table 2)

Among the 17 variables assessed by univariate analysis, repair performed during a sepsis phase (p<0.001) and the presence of a cirrhotic liver (p<0.06) were related to major morbidity after repair. Multivariable analysis revealed that only the repairs performed during a sepsis period (OR 102.5 IC 95% [7.1; 11352.3], p<0.007) remained a significant independent risk factor for major morbidity.

Long-term results of biliary repair and risk factors for biliary stricture (Table 3)

The median follow-up was 93 months (range: 26 to 204).

During follow-up, the overall mortality was 13% (5 cases). The cause of death was cardiovascular disease in 2 cases at months 60 and 142, respectively. Colon and breast cancer were responsible for death in two cases at 173 and 70 months, respectively. One (3%) patient died due to biliary sepsis on postoperative month 39 after major BDI repair.

Post-repair biliary stricture occurred in 5 (13%) cases (Table 3). The median delay between repair and the onset of stricture was 60 months (range: 9 to 158). These 5 patients were operated during a sepsis period. All 5 patients with stricture required a redo surgery by reconstruction of HJ. Redo surgery was successful in 2 (5%) cases; 1 patient died due to sepsis, and 2 (5%) patients eventually required a liver transplantation for secondary biliary cirrhosis.

At the end of the follow-up, a good result (i.e., laboratory tests within the normal range and absence of dilatation of intrahepatic bile duct) was obtained in 30 (79%) cases. Three patients (8%) still described angiocholitis episodes without visible biliary stricture and were successfully treated with prolonged treatment with quinolones.

Among the 18 factors assessed by univariate analysis, repair performed during a sepsis period (p<0.006), the presence of a cirrhotic liver (p<0.002) and the occurrence of major postoperative complication (p<0.012) were significantly related to biliary stricture.

Multivariable analysis revealed that only cirrhosis (OR 26.4, IC95% [2;1018.14], p<0.026) remained a significant independent risk factor for biliary stricture.

DISCUSSION

To date, MBDIs remain frequent, plateauing at 0.3 -0.6% of all cases of LC [START_REF] Tantia | Iatrogenic biliary injury: 13,305 cholecystectomies experienced by a single surgical team over more than 13 years[END_REF][START_REF] Southern | A prospective analysis of 1518 laparoscopic cholecystectomies[END_REF][START_REF] Roslyn | Open cholecystectomy. A contemporary analysis of 42,474 patients[END_REF]. Most of these patients are referred in tertiary referral centers, either immediately or after having been unsuccessfully reoperated. Analyzing the long-term outcome of 38 patients referred for primary or secondary surgical repair of MBDI showed that reconstruction using RY with or without associated liver resection provided good long-term results in 79% of cases regardless of the number of previously performed surgeries. Surgical repair during a sepsis period or the presence of a biliary cirrhosis were independent risk factors of major postoperative complications or the recurrence of biliary stricture, respectively.

The current report highlights that the factor exerting the most important impact on long-term results was the presence of established liver cirrhosis due to chronic biliary obstruction at the time of surgical repair, a finding that to our knowledge has not been reported to date. These results confirm the work of Lokesh et al. in which the authors showed that the duration of biliary obstruction before repair was an independent factor of secondary stricture [26].

Parallel to this finding, and in line with numerous other reports, we showed that i) performing surgical repair during a sepsis period and the occurrence of major postoperative complications (CDS > 3) were associated with an increased risk of biliary stricture recurrence [START_REF] Walsh | Long-term outcome of biliary reconstruction for bile duct injuries from laparoscopic cholecystectomies[END_REF][START_REF] Schmidt | Long-term results and risk factors influencing outcome of major bile duct injuries following cholecystectomy[END_REF]; ii) the level of the HJ was not a risk factor of anastomotic stricture [START_REF] Moraca | Long-term biliary function after reconstruction of major bile duct injuries with Hepaticoduodenostomy or Hepaticojejunostomy[END_REF][START_REF] Huang | Long-term results of major bile duct injury associated with laparoscopic cholecystectomy[END_REF], whereas others have shown that HJ performed high up to the right-left hepatic duct confluence is associated with an increased rate of failure [START_REF] Mercado | Acute bile duct injury[END_REF]; iii) right hepatic artery obstruction at the time of LC was frequent but had no impact on long-term results after surgical repair, as it has previously been suggested [START_REF] Keleman | Associated vascular injury in patients with bile duct injury during cholecystectomy[END_REF][START_REF] Pulitano | Impact of concomitant arterial injury on the outcome of laparoscopic bile duct injury[END_REF]. This observation is, however, in opposition with the results of Sarno and colleagues [START_REF] Sarno | Cholecystectomyrelated bile duct and vasculobiliary injuries[END_REF], suggesting that patients with concomitant vascular and biliary injuries had worse outcomes.

To date, it is accepted that the safest way to repair MBDIs is Roux-en-Y HJ [START_REF] Reuver | Referral pattern and timing of repair are risk factors for complications after reconstructive surgery for bile duct injury[END_REF][START_REF] Mercado | Acute bile duct injury[END_REF]. However, Moraca et al. reported similar long-term results after hepaticoduodenostomy (HD) or HJ [START_REF] Moraca | Long-term biliary function after reconstruction of major bile duct injuries with Hepaticoduodenostomy or Hepaticojejunostomy[END_REF] and recommended performing HD rather than HJ because the surgery can be completed more quickly, it is easier to perform and secondary endoscopic procedures are still possible when necessary. We believe that HJ remains the only way to divert the biliary tree at the level of confluence or above. In the present study, 9 patients had concomitant liver resection, and in these cases, an HD would not have been possible. Flum et al. [START_REF] Flum | Bile duct injury during cholecystectomy and survival in medicare beneficiaries[END_REF] demonstrated increased mortality when the repairing surgeon was the same as the injuring surgeon, and several studies have confirmed this observation [START_REF] Reuver | Referral pattern and timing of repair are risk factors for complications after reconstructive surgery for bile duct injury[END_REF][START_REF] Perera | Specialist early and immediate repair of post-laparoscopic cholecystectomy bile duct injuries is associated with an improved long-term outcome[END_REF][START_REF] Huang | Long-term results of major bile duct injury associated with laparoscopic cholecystectomy[END_REF][START_REF] Fischer | Timing of referral impacts surgical outcomes in patients undergoing repair of bile duct injuries[END_REF]. The results of the current work reveal no impact of pre-referral repair on outcome. Similarly, the timing of repair remains controversial. Although some studies have suggested a significant correlation between early repair and worse prognosis [START_REF] Walsh | Long-term outcome of biliary reconstruction for bile duct injuries from laparoscopic cholecystectomies[END_REF][START_REF] Reuver | Referral pattern and timing of repair are risk factors for complications after reconstructive surgery for bile duct injury[END_REF][START_REF] Pulitano | Impact of concomitant arterial injury on the outcome of laparoscopic bile duct injury[END_REF], others have found no impact of the time elapsed between injury and repair [START_REF] Perera | Specialist early and immediate repair of post-laparoscopic cholecystectomy bile duct injuries is associated with an improved long-term outcome[END_REF][START_REF] Sicklick | Surgical management of bile duct injuries sustained during laparoscopic cholecystectomy: perioperative results in 200 patients[END_REF]. Recently, Pekolj et al, showed in a large single-center study, that the intraoperative repair of BDI performed during LC by experienced hepatobiliary surgeons either by open or laparoscopic approach appears of paramount importance to assure optimal results [START_REF] Pekolj | Intraoperative management and repair of bile duct injuries sustained during 10,123 laparoscopic cholecystectomies in a high-volume referral center[END_REF]. In fact, these contradictory results suggest that more than the timing of repair, it is the expertise of the surgeon who performs the surgery that may be the key to success [START_REF] Perera | Specialist early and immediate repair of post-laparoscopic cholecystectomy bile duct injuries is associated with an improved long-term outcome[END_REF][START_REF] Schmidt | Long-term results and risk factors influencing outcome of major bile duct injuries following cholecystectomy[END_REF]. The results obtained in the current study emphasize the need to minimize postoperative complications, which may be facilitated by referring patients to tertiary hepato-biliary (HB) centers.

A major focus of interest of the present study lies in the long-term follow-up period. Indeed, failure of HJ may be symptomatic only several years after the initial repair. With a median follow-up period of 93 months, we found that 13% of our patients showed a post-repair stricture These results are in line with those of previous studies that have reported rates of post-repair strictures between 10% to 19% [START_REF] Walsh | Long-term outcome of biliary reconstruction for bile duct injuries from laparoscopic cholecystectomies[END_REF][START_REF] Schmidt | Long-term results and risk factors influencing outcome of major bile duct injuries following cholecystectomy[END_REF]. In these patients, we prioritized surgical repair over endoscopic or radiological approaches with dilatation or stenting of the anastomosis [START_REF] Sicklick | Surgical management of bile duct injuries sustained during laparoscopic cholecystectomy: perioperative results in 200 patients[END_REF][START_REF] Reuver | Survival in bile duct injury patients after laparoscopic cholecystectomy: a multidisciplinary approach of gastroenterologists, radiologists, and surgeons[END_REF]. Despite this aggressive approach, 2 of the 5 patients who underwent anastomotic repair developed secondary biliary cirrhosis and eventually underwent orthotopic liver transplantation (5.3% of the 38 referred cases). The same results were previously reported by Schmidt et al., with a special focus on the delay of occurrence justifying prolonged oversight [START_REF] Schmidt | Long-term results and risk factors influencing outcome of major bile duct injuries following cholecystectomy[END_REF].

In conclusion, the treatment of MBDI remains a challenge requiring support at expert HB centers. Achieving repair by HJ provides good long-term results but should not be performed during a sepsis period in order to reduce major postoperative complications and an increased risk of anastomotic stricture. The presence of biliary cirrhosis was determined to be the only independent risk factor for anastomotic stricture in the current study. In these cases, liver transplantation should be discussed at an early stage.

LEGENDS to Tables and Figures

  10 (26%) yrs = years; wks = weeks; 2N = twice normal rate E1: Common hepatic duct division, >2 cm from bifurcation E2: Common hepatic duct division, <2 cm from bifurcation E3: Common bile duct division at bifurcation E4: Hilar stricture, involvement of confluence and loss of communication between right and left hepatic duct E5: Involvement of aberrant right hepatic duct alone or with concomitant stricture of the common hepatic duct

Table 1 .

 1 . Patients' characteristics

Table 2 . Risk factors of major complications after repairTable 3 . Long-term results after hepaticojejunostomy to repair major bile duct injury after laparoscopic cholecystectomy Figure 1. Flow chart of the studyTable 1 . Patients' characteristics

 231

	Parameters	n= 38 (%)
	Age (years)	61±16
	Sex (male/female)	15/23
	ASA Score
	1	12 (32%)
	2	16 (42%)
	3	10 (26%)
	4	0
	Sepsis	15 (39%)
	Bilirubin over 2N	16 (42%)
	Cholestasis over 2N	35 (92%)
	Cirrhosis	7 (18%)
	Strasberg's Classification of BDI
	E1	6 (16%)
	E2	14 (37%)
	E3	6 (16%)
	E4	9 (24%)
	E5	3 (8%)
	Associated Vascular Injury

Table 2 . Risk factors of major complications after repair

 2

The study received no external support LS conceived and designed the study, undertook the analysis and interpretation of data, and wrote and finalized the manuscript. SG collected data, undertook


Stéphane Cordier
email: stephane.cordier@univ-rennes1.fr

Yann Molard

Konstantin A Brylev

Yuri V Mironov

Fabien Grasset

Bruno Fabre

Nikolay G Naumov
email: naumov@niic.nsc.ru

F Gasset

N G Naumov Nikolaev

Advances in the Engineering of Near Infrared Emitting Liquid Crystals and Copolymers, Extended Porous Frameworks, Theranostic Tools and Molecular Junctions Using Tailored Re6 Cluster Building Blocks

Keywords: Rhenium metal atom clusters, Nanoparticles, Supramolecular frameworks, Copolymers and Liquid crystals, Functional surfaces I. Introduction

published or not. The documents may come

The solid state chemistry of metal atom clusters and in particular the solid state chemistry of octahedral metal atom clusters has been extensively studied since the 60's [1][2][3][4][5][6][7][8]. This chemistry has yielded a wide library of compounds with fascinating crystal structures and properties as superconductivity [9], thermoelectricity [10], intercalation/de-intercalation [11], Mott insulating behaviors [12] with potential applications, for instance, for energy conversion or the design of Random Access Memories (RAM). Besides their physical properties in the solid state, that originate from number of valence electrons per cluster available for metal-metal bonding and to the strength of electronic interactions between cluster units, many solid state compounds and in particular octahedral cluster compounds can be used as precursors to afford discrete soluble molecular species for the structuration and the design of molecular assemblies [13], extended molecular solids [14][15][16][17][18][19][20][21][22][23][24][25], supported materials and nanomaterials [21,26,27] that are of interest in the fields of hydrogen storage, sensors, imaging and displays.

Octahedral nanometer-sized metallic clusters of transition elements are associated with halogen or chalcogen ligands to form edge-bridged [(M 6 L i 12 )L a 6 ] n-and face-capped [(M 6 L i 8 )L a 6 ] n-units (a = apical, i = inner). As sketched in Fig. 1, M 6 clusters are bonded to six terminal ligands (L a ) in both kinds of units but they are edge-bridged by twelve inner ligands in the [(M 6 L i 12 )L a 6 ] n-unit (M = Nb or Ta) and face-capped by eight inner ligands in the [(M 6 L i 8 )L a 6 ] n-unit (M = Mo, W or Re). The intrinsic properties of M 6 cluster units depend on both the nature of the metal, the number of valence electron and the nature of L i and L a ligands. Both [(M 6 L i 12 )L a 6 ] n-and [(M 6 L i 8 )L a 6 ] n-exhibit a wide absorption window from UV to visible range [28][29][30][31][32][33][34][35][36][37][38] but only the [(M 6 L i 8 )L a 6 ] n-(M = Re, L i = S, Se or Te; M = Mo or W, L i = Cl, Br or I) cluster units exhibit photoluminescence properties [35,[38][39][40][START_REF] Dorson | Dalton Trans[END_REF][START_REF] Brylev | [END_REF][43][44][45][46][47][48][49]. Both units exhibit reversible electron-oxidation processes and magnetic properties when they carry unpaired valence electrons [30,32,35,39,[50][51][52][53][54][55][56][57][58][59][60][61][62]. Compared with other metal-based solid state complexes, the added value of [(M 6 L i 12 )L a 6 ] n-and [(M 6 L i 8 )L a 6 ] n-is that the inorganic L a ligands can be exchanged by other L' a inorganic ones or functional organic ligands [29,31,[63][64][65][66][67][68]. This substitution can be complete or partial leading to isomeric [(M 6 L i 12 )L a x L' a 6-x ] n-/+ and [(M 6 L i 8 )L a x L' a 6-x ] n-/+ cluster units. Interestingly, the (M 6 L i 8 ) x+ and (M 6 L i 12 ) x+ cores can be viewed as nodes that can be selectively functionalized for their controlled assembling by supramolecular interactions via linker in one, two or three crystallographic directions in extended molecular solid. [(M 6 X i 12 )L a 6 ] n-cluster units (X = halogen and L = H 2 O, N 3 -, CN -and SCN -groups) [69][70][71][72] have been involved in the design of extended frameworks [73,74], deposited as redox-active moieties on gold electrodes [75] or used, in the case of tantalum, for the structural determination of proteins [76,77]. It is worth noting that the chemistry of [(M 6 L i 8 )L a 6 ] n-face-capped units is much more developed than that of [(M 6 X i 12 )L a 6 ] n-edge-bridged ones owing to, probably, a higher chemical stability with higher oxidation potential compared to [(M 6 X i 12 )L a 6 ] n-cluster units but also because such units exhibit additional unique photoluminescence properties, photocatalytic activity [78,79] and capability to generate singlet oxygen [62,80,81]. Many recent works have been reported on the elaboration and design of Mo 6 cluster-based molecular assemblies, nanomaterials and functional surfaces [13,20,21,27,[82][83][84]. This review focuses on the recent advances and perspectives on molecular assemblies, nanomaterials and surfaces using rhenium cluster building blocks which is actually a hot topic of research. The idea to prepare solid state compounds and to use them as precursors in the elaboration of nanomaterials is not straightforward and can be decomposed in three important chronologic steps. The first step started with the pioneering work of P. Batail and A. Perrin with the discovery of the solubility of K[(Re 6 Se i 5 Cl i 3 )Cl a 6 ] in ethanol [85]. This property of Re 6 clusters was subsequently used for the design of charge transfer salts. Then, the second step started with the works of R. H. Holm, J. R. Long, V. E. Fedorov and Y. Sasaki consisting in the functionalization of the Re 6 clusters by donor ligands like CN, phosphine or pyridine derivatives. Indeed, a multiplicity of molecular assemblies and extended supramolecular solids or extended frameworks were synthesized and characterized. The third step started at the beginning of 2000s by the engineering of hybrid molecular compounds and nanomaterials [66,[86][87][88][89][90][91][92] using the Re 6 clusters as building blocks or active moieties for applications ranging from biomedicine and bio-imaging to lighting and displays. This field of research is becoming more and more interdisciplinary from the synthesis of solid state compounds, functionalization of clusters, design of multifunctional materials and evaluation of toxicity. Fifty years after the introduction by F. A. Cotton of the term metal atom cluster, this review reports advances of this interdisciplinary work consisting to use Re 6 cluster-based solid state compounds for the engineering of near infrared emitting liquid crystals and copolymers, extended porous frameworks, theranostic tools and molecular junctions. This constitutes a first step before applications in many fields ranging from telecoms, molecular electronics, displays, lighting and medicine.

II. Art of work: from solid state compounds to functional molecular assemblies

The solid state synthesis and excision reactions to form soluble scaffolds for functional molecular assemblies have been largely described earlier [36,[93][94][95][96], in the following main results are summarized (Table 1). The synthesis methods of the cluster compounds may be divided into two main groups: the high-temperature synthesis, which is carried out in closed systems (usually, in sealed evacuated tubes), and the synthesis in solution.

II.a. Synthesis of cluster precursors by high temperature and solution routes

The high-temperature reactions are extremely important for the synthesis of the (Re 6 Q i 8 ) n+ cluster cores which hitherto have never been obtained in solution. In the solid state, the cluster core is stabilized by halogen and/or chalcogen to form polymeric frameworks as for instance Re 6 Q 8 Br 2 (Q = S, Se) or Re 6 Te 15 . When alkali metal salts are added to the starting mixture, the cluster core is stabilized by halogen to form discrete charged units as found for instance in the double salts Cs 4 [(Re 6 Q i 8 )X a 6 ] CsX (Q = S, Se; X = Br, I). The preparation method for Re 6 cluster-based compounds is based on the assumption that a phase thermodynamically stable under defined experimental conditions (i.e. temperature, pressure) is obtained from starting precursors taken in the appropriate ratio. Simple substances and binary compounds (for example, metal halides, metal chalcogenides, chalcogenides and halides) are used as starting compounds. For instance, polymeric Re 6 Q 8 Br 2 is prepared from elements, addition of excess of CsX loading results in formation of Cs 4 [(Re 6 Q i 8 )X a 6 ] CsX with discrete clusters. In addition to the ratio of the starting compounds in the reaction mixture, the synthesis temperature is the key point of the procedure. The standard procedure for the synthesis of a cluster chalcohalogenide can be described as follows.

First, a quartz tube is loaded with starting compounds and then evacuated and sealed. If necessary, the tube is cooled down by liquid nitrogen prior to evacuation in order to avoid the loss of volatile components (e.g. Br 2 or I 2 ). Afterwards, the sealed tube is placed into a furnace and heated at temperature ranging from 500°C to 900°C depending on the chemical system. The reaction time varies from several tens of hours up to several days, in order to complete the reaction. Such high temperature reactions produce exclusively valence electron-precise counts irrespectively to stoichiometry or reaction conditions. The most widely used starting precursors are Re 6 Q 8 Br 2 (Q = S, Se), Re

II.b. Solution chemistry

The solubility of [(Re 6 Q i 8 )X a 6 ] n-cluster units is governed by the charge of the units and the nature of counter cations. [(Re 6 Q i 8 )X a 6 ] n-(X = halogen) cluster units associated with alkaline earth ions are soluble in alcohol and/or in water. The functionalization by organic moieties requires the solubilization of Re 6 clusters in organic media. Indeed, after dissolution of inorganic precursors in aqueous solutions, the precipitation of [(Re 6 Q i 8 )X a 6 ] n- units with tetrabutylammonium ((n-C 4 H 9 ) 4 N) + organic cations affords ((n-C 4 H 9 ) 4 N) n [(Re 6 Q i 8 )X a 6 ] (n = 4 for S, n = 3 for Se) precursors, from which a wide variety of hybrid [(Re 6 Q i 8 )L a 6 ] 2+ units based on a (Re 6 Q i 8 ) 2+ inorganic cluster core and functional organic moieties utilized as apical ligands can be prepared (L = N-and P-donor ligands). Indeed, the more straightforward functionalization method is the direct reaction of (TBA) 3 [(Re 6 Q i 8 )Cl a 6 ] (TBA = tetrabutyl ammonium) with pyridine and functional pyridine to form bi-and trifunctional species [37,101]. It has even been demonstrated that monofunctional [(Re 6 Q i 8 )(Cl a 5 (4-phenylpyridine) a ] 3-species is obtained by reaction between (TBA) 4 [(Re 6 Q i 8 )Cl a 6 ] and 4-phenyl-pyridine in a MeCN forced by photoirradiation of the solution [102]. In order to obtain higher substituted cluster units, the use of phosphine derivatives is preferred. Usually reactions with phosphine are more complete [66,86,87,94,103], hexasubstituted [(Re 6 Q i 8 )L a 6 ] 2+ functional units were prepared from [(Re 6 Q i 8 )I a 6 ] 3-units (Q = S, Se) after the prior removal of iodine apical ligands from the cluster core by reaction with silver salt in order to facilitate the apical ligand substitution. Partially substituted [(Re 6 Q i 8 )(PEt 3 ) a 6-x I a x ] (2-x)+ units were synthesized by the direct reaction of [(Re 6 Q i 8 )I a 6 ] 3-units with PEt 3 , leading to a set of [(Re 6 Q i 8 )(PEt 3 ) a 6-x )I a x ] (2-x)+ anionic units with different x values and possible isomeric units. The Re-P bonds being very strong, functional [(Re 6 Q i 8 )(PEt 3 ) a

6-

x )L a x ] 2+ units were obtained by reactions of [(Re 6 Q i 8 )(PEt 3 ) a 6-x I a x ] (2-x)+ with various L in the presence of Ag + in order to remove the apical iodine atoms without destroying the Re-PPh 3 bond. Beyond different types of functional Re 6 supramolecular units, this technique afforded original supramolecular network based on hydrogen-bonding by grafting, for instance, isonicotinamide on the cluster. It turns out that the lower reactivity of the (Re 6 S i 8 ) 2+/3+ cluster cores compared to that of (Re 6 Se i 8 ) 2+/3+ leads to a smaller set of supramolecular assemblies with sulfur than with selenium. All these derivatives have been widely studied experimentally and theoretically for their spectroscopic and photophysical properties [38][39][40][START_REF] Dorson | Dalton Trans[END_REF][START_REF] Brylev | [END_REF][104][105][106][107][108].

II.c. Mixing Solid state and solution chemistries, design of functional Re6 cluster units containing CN and OH apical ligands: [(Re6Q i 8)(OH) a 6-x(CN) a x] 4-

In a similar way to what has been observed with Re-P bonds for phosphine ligands, the Re-CN bond is very stable at room temperature. [(Re 6 Q i 8 )L a 6 ] n-(L = CN -, OH -, NCS -, N 3 -) cluster units associated with alkaline earth ions are readily soluble in water [48,49,[109][110][111][112][113][114][115][116][117][118][119][120]. Thanks to the bidentate nature of CN -group, the [(Re 6 Q i 8 )(CN) a 6 ] n-(Q = S, Se, Te) cluster units have been widely used for the elaboration of low dimensional cluster framework by co-crystallization via CN with transition metal cations. The hydroxo species are interesting since the OH groups react with acid to form water molecules [49,117]. They also react with carboxylic acids to form corresponding carboxylate cluster complexes [START_REF] Dorson | Dalton Trans[END_REF][START_REF] Brylev | [END_REF]88,121] ] 0 is soluble in organic solvents and that the trans-isomer can be separated from the cis-isomer by two successive recrystallizations in CH 2 Cl 2 -ether mixtures [START_REF] Dorson | Dalton Trans[END_REF]. The added value of all these species is that they are not air sensitive and that they can be produced in a large scale. In the following, we will focus on new perspectives based on recent work dealing on in-situ synthesis of supramolecular compounds using organic melts and on hydroxo, cyano and cyano-hydro complexes as building blocks for the elaboration of supramolecular frameworks, nanoparticles, liquid crystals and functional surfaces.

[(Re6Q8)X6] 3- (Q = S, X = Cl, Br or I; Q = Se, X = I) [(Re6Q8)X6] 4- oxidation of [(Re6Q8X6]
[(Re6Q8)(NCS)6] 4- (Q = S or Se) 1) [TBA]3[(Re6S8)Cl6] and KSCN 2) [(Re6Se8)(MeCN)6] 2+ and SCN 3) Cs2[(Re6Se8)Br4] and KSCN 1) 200 º C, 1 h 2) reflux of a chlorobenzene-DMF (2:1 v/v) solution under Ar, 12 h 3) reaction in a mechanochemical reactor at room temperature, 20h [113] [(Re6S8)(PEt3)xX6-x] x-4 (X = Cl or Br) [(Re6S8)Cl6] 4-or [(Re6S8)Br6]

III. Description of Photophysical Properties

As described above, during the last two decades, a large number of hexarhenium chalcogenide cluster complexes with various apical ligand environments have been synthesized and their structures and properties were evaluated. Following are summarized some important features about electronic structures, redox and luminescence properties.

Electronic structures and redox properties

The electronic structures of [(M 6 L i 8 )L a 6 ] clusters were studied by several authors using quantum chemical calculations. The molecular diagram of [(M 6 L i 8 )L a 6 ] unitsconsidering an O h symmetryshows a set of twelve metal-metal bonding molecular orbitals (a 1g , t 1u , t 2g , t 2u , and e g ). The population of these twelve bonding molecular orbitals (MO) corresponding to metal-metal bonds in a metal cluster Re 6 leads to an optimum number of valence electrons, the so-called valence electron concentration (VEC) of 24. This corresponds to a rhenium ion with a d 4 . This oxidation provides compounds with paramagnetic cluster. Further oxidation of Re 6 cluster was demonstrated in electrochemical experiments but up to now 22 electrons moieties were not isolated as individual phases.

Luminescence properties

By many examples, it was shown, that complexes of the general formula [(Re 6 Q i 8 )L a 6 ] 4-(Q = S or Se; L = halogen ion, CN -, NCS -, N 3 -, OH -/H 2 O, anions of carboxylic acids, pyridine and phosphine derivatives, etc.) with the (Re 6 Q i 8 ) 2+ core having 24 valence electrons in the solid state and in solutions emit luminescence in visible and near-infrared regions (NIR) upon ultraviolet or blue light excitation with emission lifetimes in the microsecond range [38-42, 44, 48, 49, 81, 86, 88, 101, 102, 113, 117, 119, 121, 127, 128]. The spectroscopic and photophysical properties of these complexes have been extensively studied experimentally (see references above) and by theoretical calculations [34, 38, 42, 48, 104-108, 115, 119, 129]. On the basis of the fairly long emission lifetimes and the insensitive nature of the emission spectrum to the type of solvents employed, it has been proposed that the emitting excited state of the hexarhenium (III) complex is a spin-triplet type and involves orbitals that are primarily localized on the (Re 6 Q i 8 ) 2+ core. In addition to these experimental observations, theoretical studies of the excited state have demonstrated that the lowest-energy unoccupied molecular orbitals (LUMOs) are primarily localized on the (Re 6 Q i 8 ) 2+ core [38, 42, 44, properties (emission maximum wavelengths λ em , quantum yields  em and lifetimes  em ) of hexarhenium cluster complexes allowed to reveal some dependencies and make important conclusions, namely: 1) as a rule, the (Re 6 Se i 8 ) 2+ -based complexes show a stronger emission than those based on (Re 6 S i 8 ) 2+ possessing the same apical ligand environment [38,40,127]; 2) influence of the nature of apical ligands:

-in the series [(Re 6 S i 8 )X a 6 ] 4-(X = Cl -, Br -or I -) the emission maximum shifts to a longer wave-length while  em and  em decrease as the terminal halide ions become heavier [39];

-in CH 2 Cl 2 solutions complexes with oxygen-or nitrogen-based organic ligands (DMSO, DMF, pyridine, acetonitrile) as well as the chalcocyanides [(Re 6 Q i 8 )(CN) a 6 ] 4-deliver maximal quantum yields and the longest lifetimes and tend to the higher-energy emission, while clusters having six soft, polarizable ligands, such as iodide or bromide, emit with less intensity, shorter lifetimes, and redder luminescence. Phosphine ligation (PEt 3 ) also diminishes  em [38,127]; -in-depth study of the series of complexes with different N-heteroaromatic ligands demonstrated that the emission is effectively quenched as the result of (Re 6 Q i 8 ) 2+ -to-ligand charge transfer (MLCT) if the ligand (such as 4,4'-bipyridine or pyrazine) has free nitrogen sites [44,

Q i 8) (CN) a 4 (OH) a 2-x (H 2 O) a x ]
x-4 are mainly defined by the CN -ligands and not appreciably affected by the OH -or H 2 O ligand(s) [49,119]; 3) the quantum yield and lifetimes significantly depend on a solvent while the emission maximum remains the same. In particular, a complex in a deaerated aqueous solution shows much lower  em and  em values than in a deaerated organic solvent that can be explained by interaction of water molecules with the inner chalcogenide ligands providing more efficient vibrational decay pathway of the excited state [40]; 4) the dependences of photophysical properties on both the inner and apical ligands were discussed in terms of the energy gap law, by which the natural logarithm of the nonradiative decay rate constant (k nr ) of a cluster is inversely proportional to the emission maximum energy. Since it was found that the emission lifetime of the cluster is determined mainly by the k nr value, the energy gap dependence of k nr demonstrates that the emission lifetime of a cluster can be predicted approximately from the emission energy [38,40,113, 127]; 5) solid-state emission lifetimes and quantum yields usually occupy a similar range at room temperature [38,[START_REF] Brylev | [END_REF]44,48,88,113,119,121]. Low-temperature luminescence experiments indicate a progressive red shift of the emission maximum upon cooling, with an increase in the excited-state lifetime and the emission quantum yield [38,44,89,113]; 6) the long lifetimes, large Stokes shifts, and excited-state quenching by O 2 indicate the spin-triplet nature of the luminescent excited state of the cluster complexes, i.e. the change in spin multiplicity is involved in the electronic transitions [38,39,86,89]. Note that to date there is only one described example of a luminescent (Re 6 Te i 8 ) 2+ -based complex, namely the photoluminescence of the complex [(Re 6 Te i 8 )(CN) a 6 ] 4-has been studied in acetonitrile and aqueous solutions and it was shown, that the telluride cluster shows much weaker and shortlived emission comparing to [(Re 6 S i 8 )(CN) a 6 ] 4-and [(Re 6 Se i 8 )(CN) a 6 ] 4- [40]. The lack of data on luminescence of telluride hexarhenium cluster complexes with other apical ligands is a consequence of the fact that the tellurocyanide complex is still the unique representative of soluble (Re 6 Te i 8 ) 2+ -based complex. The luminescence properties of Re 6 cluster complexes provided an incentive to establish their potential applications [61,81,88,90,132]. Synthetic flexibility of the apical ligand environment affords a range of clusters for developing innovative luminescent materials. The quenching of the phosphorescence by O 2 suggests applications to optical sensor technology and singlet oxygen generation that can be used, for example, in the photodynamic therapy of cancer.

IV. Molten salt approach: Toward the one pot synthesis of supramolecular frameworks

It was found that reactions between chalcohalogenide hexarhenium cluster complexes and some molten organic compounds which can coordinate to rhenium lead to substitution of some apical halogen atoms by the organic units [43,129,[133][134][135][136]. Such approach allows to produce cluster complexes with functional organic ligand environment in high yield. The reactions in a molten organic medium may result in two types of cluster complexes: partially substituted neutral complexes [43,129,135,136] or hexasubstituted cluster cations [133,134]. However, so far there is no evidence on the nature of the driving force at the origin of formation of neutral (partially substituted) or cationic (hexasubstituted) complexes. 3c,d), that were separated by column chromatography. The interesting feature of these structures is the ligand ordering in mixed chalcobromide cluster cores (Re 6 Q i 7 Br i ) 3+ and (Re 6 S i 6 Br i 2 ) 4+ . Note that the neutral complexes with mixed-ligand apical ligand environment were obtained in reactions with a large excess of EPh 3 . It was expected that the reactions result in the substitution of all terminal bromide ligands in the starting cluster compounds by EPh 3 ligands similarly to reactions with an excess of molten 3,5-dimethylpyrazole described above. However, complexes obtained in the reactions with EPh 3 have only four, three, or even two terminal EPh 3 ligands depending on the composition (and consequently, the charge) of cluster core. Such results were explained by the preferred formation of neutral molecular compounds in this system. No remarkable interactions were found between the molecular complexes in all these crystal structures of cluster complexes with EPh 3 ligands. By the example of reaction with molten pyrazine it was shown that functional Re 6 cluster units can be obtained by a direct interaction of cluster chalcohalogenides with molten organic ligands based on organic ring containing nitrogen atoms [43]. Such one pot synthesis enables an easier preparation of functional units and do not need a preliminary functionalization of the cluster precursor before self-assembling from solutions as shown in the strategies developed by R. H. Holm, Z. Zheng and T. Yoshimura who oxidized parent chalcohalogenide cluster complexes [37,96,101,103]. It turns out that the number of substituted apical ligands depends on the charge of the unit in the starting precursor. The easy formation of the stable CsBr inorganic salt in the organic melt must be the key point of this reaction. The limiting factor of the technique is the thermal stability of organic ligands that can decompose before the exchange occurs. Among organic ligands stable at mild temperatures, bidentate linkers such as, for instance, bipyridine or pyrazine constitute relevant candidates to build coordination polymers with low dimensionalities or supramolecular frameworks. One of the actual hot topics is the research of porous neutral supramolecular frameworks. A possible strategy is the reaction of cluster unit leading to more than two substituted apical sites. It is clear that substitution of one halide ligand by a functional organic moiety can lead to the formation of dimers, while two substituted complexes should lead to the formation of chains. Thus, we can suppose that cluster units with charges of 3-or 4-should lead to the formation of supramolecular frameworks by relevant choice of ligands. In this frame, reactions of [(Re 6 Q i 7 Br i )Br a 6 ] 3-with molten pyrazine were investigated. After the optimization procedure, it turned out that reactions at 140 °C for 2 days in sealed glass tubes afforded very interesting supramolecular compounds built up from octahedral rhenium cluster complexes fac-[(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ] (Q = S, Se) (Fig. 4a). Despite their bidentate nature, each pyrazine is bonded to only one Re 6 cluster. Indeed, this finding evidenced that the idea to link cluster units together via simple bidentate ligands containing aromatic groups is counter balanced by the formation of strong supramolecular interactions (i.e. hydrogen bonding and π-π stacking). But at the end, fortunately the structures of fac-[(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ]•xH 2 O have porous structures containing hollow tubular channels (Fig. 4b). The (Re 6 Q i 7 Br i ) 3+ cluster core is built up from a Re 6 cluster lying in a cube of ligands formed by seven Q's and one Br. It is worth noting that, in the structures of starting salts Cs 3 [(Re 6 Q i 7 Br i )Br a 6 ]•H 2 O the seven Q's and Br are randomly distributed on the eight inner positions of the anionic cluster complex. The presence of only one Br atom among the inner ligands generates the local C 3V symmetry for the [(Re 6 Q i 7 Br) i Br 6 ] 3-unit and does not enable the formation of isomers. Thus, owing to the orientational disorder, the average apparent symmetry of the [(Re 6 Q i 7 Br i )Br •xH 2 O hybrid structure is driven by the selective substitution of three apical bromines by three pyrazine groups. One of the explanations of this selective ligand coordination is the specific distribution of electron density on the rhenium atoms in the starting precursor. Indeed, owing to the metal-to-ligand charge transfers, charge balances, and the stronger Re-Q i bond compared to the Re-Br i one, the three rhenium atoms bonded to four μ 3 -chalcogen ligands have a more positive charge than the three rhenium atoms bonded to three μ 3 -chalcogen and one μ 3 -halogen atom. The result is that the three Re-Br a bonds for which Re atoms are exclusively bonded to inner μ 3 -S are weaker than the three opposite Re-Br a ones favoring their substitution by the neutral pyrazine ligands. This affords the fac isomer of a neutral trisubstituted hybrid unit with an ambivalent inorganic/organic character. The self-assembly of the latter building blocks leads to the formation of a unique structure in which the blocks are held together by halogen/chalcogen van der Waals contacts found in pure inorganic solids along with π-π stacking and hydrogen bonds found in organic solids. Each cluster interacts with three adjacent ones through π-π stacking interactions as well as C-H• • •N bonds between pyrazine rings, building a zigzag hexagonal layer of clusters. The cluster layers fit together according to an AA′A sequence. Interactions between layers occur via (i) van der Waals contacts between μ 3 -Q i and μ 3 -Br i along the c axis; (ii) van der Waals contacts between μ 3 -Q and μ 3 -Q and between μ 3 -Br and μ 3 -Br of three adjacent cluster units lying in the same plane but belonging to two successive layers; and (iii) N-H• • • N, C-H• • • Br a , and C-H• • • Br i bonds. Note that the inner chalcogen atoms are not involved in C-H• • • Q i nor N-H• • • S i bonds, whereas all bromine ligands are involved in hydrogen bonds. Indeed, the formation of a robust hydrogen network involving μ 3 -Br i must be a driving force that contributes to the Q/Br ordering. The AA′A packing of layers generates large tubular channels parallel to the c axis that are arranged according to a honeycomb disposition imposed by the trigonal symmetry of the fac-[(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ] cluster unit. The effective aperture of the channel is found roughly equal to 6.10 and 6.43 Å for Q = S and Se, respectively. The total free volume of the unit cell was found to be 43 %, whereas the free volume available for solvent molecules is 29 %. This strategy offers now new perspectives in the design of porous framework containing metal atom clusters. The choice of the precursor determines the level of substitution and the choice of ligand determines the type of supramolecular interactions whilst their size should enable to control the aperture of the pores.

V. Hybrid organic-inorganic nanocomposites and functional surfaces

V.a. Hybrid copolymers

Despite visible emission and interesting properties for displays and lighting, the direct use of solid state compounds for applications is limited because of their ceramic nature (brittle, low plasticity). One solution to overcome this drawback is the sintering processing under inert atmosphere however such method drastically limits the size of the targeted devices. Another strategy is the use of hybrid organic-inorganic technologies that associate an organic matrix with photochemically active and reinforcing inorganic moieties. Hybrid materials obtained by this way have enhanced properties and combine the processability of the organic matrix with the intrinsic properties of the inorganic moieties. For luminescent cluster containing hybrid copolymers, it is important to find the best cluster/polymer weight ratio in order to optimize the optical properties without altering significantly the host matrix processability. Recently, luminescent clusters containing co-polymers have demonstrated their high applicative potential in technologies needing deep red emission. For instance, it has been showed that the luminescence of the trivalent Er(III) luminescence used in C-band telecom technologies is sensitized by metal atom clusters [137]. A major difficulty in the elaboration of hybrid polymers, is to avoid the segregation of the organic and inorganic phases. One possible strategy to tackle this problem is to maximize the interactions between both parts of the material by modifying the inorganic moieties with polymerizable ligands and copolymerize them with organic monomers. Two examples have been described up to now in which materials have been obtained by copolymerization of functionalized Re 6 clusters with an organic matrix. The first one is a copolymer resulting from the copolymerization of [(Re 6 Se i 8 )(PEt 3 ) a 5 (4-vinylpyridine) a ] 2+ with styrene [138]. Although a mono functionalized cluster seems the best way to introduce clusters without altering significantly the mechanical properties of the organic host because monofunctionalization does not imply cross linking, the major drawbacks of this technique are that it requests the synthesis of non-stable Re 6 intermediates, tedious purifications of pentafunctionalized building blocks leading finally to an overall synthetic scheme with poor yield. Moreover, optical properties of such copolymer were not reported. This fact could be eventually explained by the presence of five phosphine ligands around the Re 6 core that have the unfortunate ability to drastically decrease the luminescence ability of the metallic Re 6 core. To prepare new luminescent materials directed towards optical applications, poly-(methyl methacrylate) (PMMA) is a relevant matrix because of its excellent optical properties (i.e., transparency from the near-UV to the near-IR regions), good mechanical and electrical properties, thermal stability, water resistance, and easy shaping. The embedding of Re 6 clusters was performed by a prior functionalization with methacrylate (MA) by reaction of [(Re 6 Q i 8 )(tbp) a 4 (OH) a 2 ] 0 cluster units with methacrylic acid to form [(Re 6 Q i 8 )(tbp) a 4 (MA) a 2 ]. Copolymers with different cluster content were then prepared to optimize the synergy between the processability and transparency of the organic matrix and the high luminescence of the Re 6 cluster core. Several pellets were obtained by a radical polymerization process for 24 h at 60 °C from a mixture of cluster precursors (from 0.025 to 0.1 wt%), methyl methacrylate and AIBN (azobisisobutyronitrile) as initiator (Fig. 5a). Owing to the low solubility of [(Re 6 Q i 8 )(tbp) a 4 (MA) a 2 ] in neat MMA, the maximum amount of the cluster in the pellet was limited to 0.1 wt%. Higher cluster content resulting in the loss of pellets transparency. As the Re 6 cluster/MMA ratio was kept very low, the processability of the hybrid copolymer appeared very close to that of neat PMMA with an enhanced thermal stability as demonstrated by TGA analysis. Co-polymers emit bright-red phosphorescence (Fig. 5b) upon excitation anywhere in the absorption band of the cluster (300-550 nm). Luminescence spectrum of copolymers revealed the typical spectra of the cluster core with a broad and structureless band that extended from 600 nm to more than 950 nm, with the maximum wavelength λ em centered at around 710 nm. Absolute emission quantum yield was measured to be about 0.07. It is worth noting that this value is one of the highest photoluminescence quantum yields reported so far for red-NIR luminescent polymers doped with inorganic compounds and does not degrade after several months of ageing. Encapsulation of clusters in this particular polymeric matrix prevents (or minimize) quenching of photoluminescence by oxygen as it is observed in solution. Beyond the possibility to be shaped in bulk form by usual melting processes of polymers technologies, these hybrid materials are soluble in organic solvents which allows their processing into thin films with techniques used for large surfaces preparation like casting, dip coating, or spin coating. Compare to other λ ex = 365 nm red photobleaching proof emitters, Re 6 cluster are not sensitive to their coordination environment as found for rare earths ions and they do not exhibit toxic behavior like quantum dots.

V.b. Liquid crystals

Metal-containing liquid crystals, the so-called metallomesogens, combine the unique properties of anisotropic fluids with the specific properties of metals (e.g. geometry of coordination, optic, electronic, magnetic). Clustomesogens associate mesomorphism with the unique properties of metal atom clusters [139,140]. The added value of clustomesogens, compared to metallomesogens is that their luminescence properties are not influenced by their supramolecular organization and only poorly by their surrounding ligands. This peculiar point is of interest since known properties can be directly imported in the desired hybrid matrices. The development of such a class of nanomaterials should offer great potential in the design of new electricity-to-light energy conversion systems, optically based sensors, and displays. Two approaches have been developed to obtain clustomesogens: a covalent approach and an ionic one.

The covalent approach is based on the covalent anchoring of promesogenic moieties containing organic ligands onto the clusters which leads to hybrid building blocks able of self-assembling. The covalent approach is well developed in the case of molybdenum. Some preliminary results have been reported in the case of Re 6 clusters. The strategy was to functionalize [(Re 6 Q i 8 )(tbp) a 4 (OH) a 2 ] 0 cluster by two gallate where methoxy groups have been replaced by an octyloxy chains. Luminescence experiments (λ ex = 450 nm) showed a intense emission spectrum centered at 720 nm. Irradiation at λ ex = 405 nm of trans-functionalized clusters induced a bright red luminescence. A promising birefringent texture could be observed at 25 °C by polarised optical microscopy. However, compounds decompose around 250 °C before reaching the clearing temperature.

The ionic approach is based on the pairing via Coulomb interactions of anionic clusters with promesogenic moieties containing organic cations [61]. Indeed, [(Re 6 Se i 8 )(CN) a 6 ] n-(n = 3, 4) cluster units were paired with appropriate amphiphilic ammonium cations (Fig. 6). These inorganic units are particularly relevant because, thanks to cyano ligands that prevent any ligands exchange, they are chemically very stable and exhibit a low reversible one electron oxidation potential. The latter enables to switch reversibly from an orange colored form (n = 4, VEC = 24) with red-NIR luminescence properties, to a magnetic green colored species (n = 3, VEC = 23). The oxidation-reduction process can be easily implemented by both electrochemical and chemical oxidations. In this frame (Kat) properties. Reprinted from [61] with permission from ACS.

The reaction pairing simply consists in a precipitation by addition of a hot ethanol solution of KatBr in a water solution of [(Re 6 Se i 8 )(CN) a 6 ] n-. Both hybrid compounds exhibit a lyotropic behavior with organic solvent such as chloroform: (Kat) ] self-organizes smoothly in a layered phase of Smectic A type between 40°C and 140°C. The intrinsic properties of the molecular inorganic cluster are preserved within the mesomorphic material. It is therefore possible to switch reversibly this LC material from a green-colored magnetic form to a bright red-NIR luminescent one. These compounds, because of their low clearing temperature, can be integrated and annealed directly in devices. Yet that the concept of such class of material is demonstrated, the endless range of possibilities offered by organic chemists to design particular mesogenic cations allowing a fine-tuning of the final material mesomorphic behavior, associated with the versatility of the inorganic clusters (charge, functions) obtained via high temperature solid-state synthesis, open new and fascinating perspectives in the design of easy processable functional material containing transition metal clusters.

V.c. Functional surfaces

One successful strategy to design functional devices consists in the immobilization onto surfaces of nanometerscaled building blocks endowed with specific properties. Many approaches can be developed. Indeed, for optical or catalytic properties and even hydrogen storage, metal atom clusters can be immobilized onto a surface like graphene or incorporated in metal organic frameworks. Photocatalytic properties are thus driven by irradiation and depend on the used wavelength. In order to obtain electrically addressable and switchable functional devices (e.g. optical or magnetic arrays, biochemical sensors with electrical detection, or hybrid junctions for charge storage components), active building blocks must be integrated on to conducting surfaces. Many research work has been devoted to the design of molecular memory cells or junctions consisting in the immobilization of redoxactive molecules (e.g., metal-complexed porphyrins, ferrocene, etc.) including metal atom clusters on conducting surfaces. The strategy that has been developed for the anchoring of Mo 6 X i 8 L a 6 cluster unit on surfaces was based on the synthesis and manipulation of [(Mo 6 X i 8 )(CF 3 SO 3 ) a 6 ] 2-unstable intermediates. The surface grafting consisted in the reaction of [(Mo 6 X i 8 )(CF 3 SO 3 ) a 6 ] 2-with a pyridine-terminated alkyl monolayer-modified silicon semiconducting surface which had been previously prepared from an oxide free, hydrogen-terminated silicon (Si-H). Thanks to the lability of the CF 3 SO 3 -groups, the cluster was immobilized through a strong covalent bond between the cluster and the nitrogen atom from the pyridine unit endcapping the alkyl chain. After the anchoring l step, the modified surface was then dipped into neat pyridine in order to replace the remaining CF 3 SO 3 -groups by pyridine rings and obtain a stable surface and not reacting with water vapour. For the immobilization of Re 6 clusters and thanks to stability and the reactivity of the [(Re 6 Q i 8 )(tbp) a 4 (OH) a 2 ] 0 with carboxylic acids, a more straightforward strategy was developed. Indeed, Re 6 -functionalized surfaces were obtained by simple acido-basic reaction of [(Re 6 Q i 8 )(tbp) a 4 (OH) a 2 ] 0 with a Si-H surface modified by an alkyl monolayer terminated by carboxylic groups (Fig. 7). After reaction in refluxing chlorobenzene, carboxylate groups were formed which led to the anchoring of the cluster on the surface through strong covalent Re 6 -O bonds. This simple water elimination reaction enabled the formation of a stable organic chain/cluster interface. This grafting technique greatly simplifies the cluster immobilization procedure as compared with that used previously for [(Mo 6 X i 8 )L a 6 ] cluster units. Interestingly, the surface coverage of the metal cluster could be finely controlled by preparing mixed monolayers containing reactive COOH-terminated alkyl chains diluted with inert alkyl chains with a predefined surface chain ratio. Using mixed dodecyl/undecanoic acid monolayers, the surface coverage of [(Re 6 Q i 8 )(tbp) a 4 (OH) a 2 ] 0 cluster units could be varied from 1 × 10 13 to 6 × 10 13 cm -2 [141]. Different surface characterization techniques, such as scanning tunneling microscopy (STM, Fig 7), spectroscopic ellipsometry, and quantitative X-ray photoemission spectroscopy (XPS), were used to demonstrate the covalent immobilization of Re 6 clusters. Furthermore, the cluster integrity within the monolayer was confirmed by its vibrational Raman signature. Charge transport characteristics were evaluated by measuring the current intensity flowing through the functional monolayer upon the application of a voltage between a soft top contact (mercury droplet) and the underlying silicon surface. The as formed Hg/(Re 6 Se i 8 )-OML-Si(111) molecular junctions (OML = organic chain monolayer) with the highest (Re 6 Se i 8 ) molecular coverage showed current-voltage (I -V) characteristics different from those observed for classical Schottky diodes. The following three main features were observed: (i) a small rectification factor; (ii) a decrease in the conductance G(V) by more than one decade. The observation of the signature of the metal cluster immobilization in G(V) characteristics required a molecular coverage on the order of 4 × 10 13 cm -2 for Re 6 -cluster-based junctions. Moreover, from scanning tunneling spectroscopy (STS), it has been found that the HOMO-LUMO energy gap increased from roughly 2 eV to 4 eV after the immobilization step. Therefore, this grafting strategy is particular attractive for controlling the electronic properties of the metal cluster-terminated molecular junctions. Moreover, through a judicious choice of functional ligands substituting the immobilized cluster units, it becomes also possible to develop redox-active interfaces endowed with versatile magnetic, luminescent and catalytic properties.

VI. Toward theranostic tools based on silica nanoparticles

[(Re 6 Q i 8 )L a 6 ] clusters units emit in the red and near-infrared (NIR) regions, which constitute a very interesting emitting window for biotechnology since it corresponds to the window of minimum absorption of tissues [142]. Moreover, because they combine the important properties of perfectly reproducible nanosize, photostability, and NIR emission with other properties like the generation of singlet oxygen and do not show acute toxic effects, Re 6 clusters may offer complementary assets to the luminophores nowadays developed for bioimaging and biolabeling technologies. They are mainly based on organic dyes, inorganic quantum dots, or lanthanide based nanocrystals. Several recent results evidence the increasing interest of rhenium atom clusters in theranostic [81,132,143,144]. Beyond interesting labelling properties and thanks to the generation of oxygen singlet, the Re 6 clusters are relevant candidates for photodynamic therapy. Moreover, it has been demonstrated that the [(Re 6 Se i 8 )I a 6 ] 3-cluster anionic units in certain conditions selectively increase tumor cell death, leaving nontumoral cells unaffected. Indeed, Re 6 clusters may be useful for cancer diagnostics, localization of tumors, and may enable the observation of tumor regression through phosphorescence [132]. Last but not least, the transport and delivering of [(Re 6 Q i 8 )(OH) a 6 ] 4-cluster units can be achieved by sugar-decorated dendritic nanocarriers and the cellular uptake can be considerably improved by the functionalization with an amphiphilic diblock copolymer [89]. Indeed different systems can be imagined and designed and not only for theranostic. In this frame, the versatility of silica nanoparticles is very relevant for the elaboration of multifunctional systems and in particular nanoparticles. Because they can support very different kind of functional nanoobjects, they found numerous applications in photonics, catalysis or biotechnologies [145,146]. Beyond their non-toxicity, their surface can be functionalized by molecules of interest in biology [147]. Luminescent cluster-based monodispersed nanoparticles prepared by water in oil microemulsion or Stöber processes have been reported [46,90,[148][149][150][151]. Additional properties have been obtained by the inclusion of maghemite at the core of the nanoparticle or by the growth of gold nanoparticles at the surface of silica [152,153] ]@SiO 2 nanoparticles were prepared through a microemulsion process adapted from a previously reported synthesis for the encapsulation of hexamolybdenum clusters in silica nanoparticles (Fig. 8). STEM pictures of K4[(Re6S i 8)(OH) a 6]@SiO2 nanoparticles with discrete cluster units (bright spots). Reprinted from [90] with permission from ACS.

To generate the microemulsion, heptane (oil phase) was first mixed with Brij30 (surfactant), and then an aqueous ammonia solution and an aqueous cluster sol were slowly added to the mixture. The aqueous cluster sol was obtained simply by solubilization of the cluster precursor in water. Once the microemulsion became a clear, stable solution, the silica precursor (tetraethoxysilane, TEOS) was added for the synthesis of the silica shell that occurred directly inside the aqueous nanodroplets already containing the cluster units. After 3 days, the microemulsion was destabilized with ethanol and the nanoparticles were recovered after washing. Resulting nanoparticles could be either redispersed in solution (water or ethanol) or dried in ambient air. The A 4 [(Re 6 Q i 8 )L a 6 ]@SiO 2 nanoparticles showed that they all exhibited a perfectly spherical, monodisperse morphology with an average diameter of 30 nm for the synthesis conditions. The encapsulation of the [(Re 6 Q i 8 )(OH) a 6 ] 4-(Q = Se or S) and [(Re 6 S i 8 )(CN) a 4 (OH) a 2 ] 4-cluster units in the silica matrix resulted in a slight shift of the emission maxima after embedding, but the shape of the emission spectrum remained unchanged (Figure 5a-c). The emission maximum shifting, ranging from 10 to 20 nm, to higher wavelengths between solidstate precursors and the nanoparticles must be due to some perturbations of the molecular orbital diagram of [(Re 6 Q i 8 )L a 6 ] 4-units after embedding. One could suspect a slight decrease in the energy gap between excited and fundamental states. Note that the use of K ]@SiO 2 meaning that during the reaction, apical bromine atoms are substituted by OH groups. By analogy with the work on the encapsulation of [(Mo 6 Br i 8 )Br a 6 ] 2-units in silica where the substitution of bromine apical bromine by hydroxo groups occur, two kinds of interactions occur with silica. The first one is an interaction by the formation of hydrogen bonds between OH of the cluster and the silanol groups of the silica and the second one is an interaction by covalent bond between cluster units and the silica matrix. These Re 6 cluster-based nanoparticles have the luminescent properties of the cluster precursor. It means that the luminescent properties of the cluster from the core of the nanoparticles are not quenched by oxygen. Moreover, since the luminescence intensity of nanoparticles decreases until a minimum in the presence of oxygen, it means that the silica is porous and clusters near to the surface of silica are quenched by oxygen and thus generate singlet oxygen. Based on these results and the fact that Re 6 clusters are not significantly toxic and that they exhibit antitumoral properties, Re 6 cluster based silica nanoparticles should be relevant tools for theranostic applications.

VII. Conclusions

Fifty years after the definition of the term "metal atom cluster" by F. A. Cotton, this review showed the step by step evolution of the rhenium metal atom cluster chemistry toward an interdisciplinary field of research from the synthesis of cluster by solid state chemistry, their functionalization by solution chemistry or in organic melts to their use as building block for the elaboration of molecular assemblies, extended molecular frameworks and nanomaterials. It showed in particular all the different strategies that have been used in order to take advantage of their unique properties originating from metal-metal bonding to obtain nanostructured and processable functional materials. We thus believe that the three first steps of the development of metal atom cluster chemistry and in particular the third step dealing with functional materials described in this review will be followed by a fourth step that should be focused on the development and commercialization of devices and nanomaterials built up from transition metal atom clusters in the fields of lighting, display, green chemistry or health. It should not only concern rhenium chemistry but all nanosized transition metal atom clusters.

Fig. 1

 1 Fig. 1 Representation of the edge-bridged [(M6L i 12)L a 6] n-(a) and face-capped [(M6L i 8)L a 6] n-(b) units

[

4 (

 4 )(PEt3)xI6-x] x-4 and Ag + reaction in a mixture of CH2Cl2 with MeCN, DMF, DMSO or pyridine, respectively, )(OH)6] 4- (Q = S or Se) Re6Q8Br2 and KOH or CsOH 280 º C (for Q = S) or 200-230 º C (for Q = Se)Q = S or Se, n = 0-6) [(Re6Q8)(OH)6] 4-and H + reaction in an aqueous solution at room temperature [117] [(Re6Q8)F6-n(H2O)n] n-4 (Q = S or Se, n = 3 or 4) [(Re6S8)(OH)6] 4-or [(Re6Se8)(OH)6] 3-, F -and H2O reaction with molten KHF2 (270-300 º C, 2 h) followed by recrystallization from an aqueous solution )(CN)4(OH)2] 4-[(Re6Q8)(OH)6] 4-and CN - reflux of an aqueous solution, 2 h [119]

Fig. 2

 2 Fig. 2 Structure of cationic cluster complexes [(Re6Q i 8)(3,5-Me2PzH) a 6] 2+ (a) and [(Re6Q i 7O i )(3,5-Me2PzH) a 6] 2+ (b)

Fig. 3

 3 Fig. 3 Structure of neutral cluster complexes trans-[(Re6Q i 8)(EPh3) a 4Br a 2] (a), fac-[(Re6Q i 7Br i )(EPh3) a 3Br a 3] (b), cis-[(Re6S i 6Br i 2)(PPh3) a 2Br a 4] (c) and trans-[(Re6S i 6Br i 2)(PPh3) a 2Br a 4] (d)

a 6 ] 3 -

 63 unit in Cs 3 [(Re 6 Q i 7 Br i )Br a 6 ]•H 2 O deduced from X-ray single crystal diffraction analysis is close to O h . In fac-[(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ]•xH 2 O, the [(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ] 0 complex is not submitted to an orientational disorder, and the local C 3V of the unit symmetry is preserved. The ordering of μ 3 -Br atoms in the fac-[(Re 6 Q i 7 Br i )(pyz) a 3 Br a 3 ]

Fig. 4

 4 Fig. 4 Structure of neutral cluster complexes fac-[(Re6Q i 7Br i )(pyz) a 3Br a 3] (a) and projection of structure of fac-[(Re6Q i 7Br i )(pyz) a 3Br a 3]•xH2O along the c direction (b).

Fig. 5 a

 5 Fig. 5 a) Synthetic pathway for the preparation of the Re6-doped PMMA hybrid material. b) Digital photographs of the PMMA and Re6-PMMA pellets obtained by bulk synthesis. Top: under normal daylight; bottom: under UV irradiation at λex = 365 nm. The weight percentage of clusters increases from left to right: 0, 0.025, 0.05, and 0.1 wt%, respectively. Reprinted from [88] with permission from John Wiley and Sons.

Fig. 6

 6 Fig. 6 The pairing of [(Re6Se i 8)(CN) a 6] 3-with Kat leads to the formation of smectic liquid crystal phase with a paramagnetic behavior and the [(Re6Se i 8)(CN) a 6] 4-with Kat leads to the formation of diamagnetic luminescent phase with liquid crystal

Fig. 7

 7 Fig. 7 Left, (a) Representation of the trans-[(Re6Se i 8)(TBP) a 4(OH) a 2] cluster unit. Selenium, rhenium from the Re6 clusters, and nitrogen atoms are represented as yellow, grey, and blue balls, respectively. (b) Representation of the same units according to a different orientation. (c) Schematic representation of the (Re6Se i 8) 2+ cluster immobilization on Si(111) through an acido-basic reaction with carboxylic acid headgroups endcapping an alkyl chain. Right, STM image of a cluster functionalized surface evidencing the homogeneous distribution of clusters onto the surface. Reprinted from [141] with permission from ACS.

Fig. 8

 8 Fig. 8 Top, aqueous solution of [(Re6S i 8)(OH) a 6] 4-clusters (a) before and (b) after embedding in silica nanoparticles, under daylight (left picture) and under UV irradiation at 365 nm (right picture). Down-left, FE-SEM pictures of (a) K4[(Re6S i 8)(OH) a 6]@SiO2 (inset: schematic representation of dispersed clusters inside the nanoparticles). Down-right, ADF-

  6 Te 15 , Cs 4 [(Re 6 Q i 8 )X a 6 ] CsX (Q = S, Se; X = Br, I). The subsequent reaction of Re 6 Q 8 Br 2 and Re 6 Te 15 with KCN, KOH or KNCS yields hexasubstituted units with the [(Re 6 Q i 8 )L a 6 ] 4-formula (L = CN, OH or NCS). Let us note that the reaction of Re 6 Se 8 Br 2 with molten KOH at 280 ºC results in the formation of a (Re 6 Se i 4 O i 4 ) 2+ cluster core with ordered ligands. Four positions of one face of the Se 4 O 4 cube inscribing the Re 6 cluster are occupied exclusively by Se atoms and the four O atoms lie in the opposite face [97]. Cyanocomplexes are formed by reactions of ReS 2 (or ReSe 2 ) with KCN leading to the formation of two unusual chalcocyanide compounds, K 4 [(Re 6 S i

						8 )S a-a	4/2 (CN) a	2 ] and K 4 [(Re 6 S i 8 )(Se 2 ) a-a 2/2 (CN) a	4 ], that contain
	bridging ligands S 2-and (Se 2 ) 2-together with terminal CN -ligands [98]. A chain polymer Cs 4 [(Re 6 S i 8 )S a-
	a	2/2 (CN) a	4 ] which is the next member of a series M 4 [(Re 6 S i 8 )S a-a	(6-2x)/2 (CN) a	2x ], where M is an alkali metal, was
	obtained with high yield by high temperature reaction of ReS 2 with KCN in presence of CsCl excess [99]. Four
	mixed ligand octahedral cluster complexes K[(Re 6 S i 8 )F a 3 (H 2 O) a	3 ]•7H 2 O, H 3 O[(Re 6 Se i 8 )F a	3 (H 2 O) a	3 ]•7H 2 O and
	[(Re 6 Q i 8 )F a	2 (H 2 O) a	4 ]•12H 2 O (Q = S or Se), have been synthesized using the reaction of corresponding rhenium
	chalcobromide complexes K 4 [(Re 6 S i 8 )Br a	6 ]•2H 2 O and Cs 3 [(Re 6 Se i 8 )Br a	6 ]•H 2 O with molten KHF 2 and further
	crystallization form aqueous solutions under different conditions [100].

  -contains six OH groups capable to react with an acid, the possibility to block apical sites with CN groups will enable to tailor building blocks with controlled number and positions of reactive OH sites. As stressed above, the solid state reaction of Re 6 Q 8 Br 2

						. Indeed, based on the fact that the
	[(Re 6 Q i 8 )(CN) a 6 ] 4-cluster unit is chemically inert and that [(Re 6 Q i 8 )(OH) a 6 ] 4with KCN or KOH [110, 115, 122] affords hexasubstituted units with the [(Re 6 Q i 8 )L a	6 ] 4-formula and solid state
	reaction of [(Re 6 S i 8 )(CN) 4 S a-a 2/2 ] 4-with KOH affords trans-[(Re 6 S i 8 )(CN) a	4 (OH) a	2 ] 4-anionic cluster unit [118].
	Protonation of [(Re 6 Q i 8 )(OH) a	6 ] 4-cluster units leads to the formation of [(Re 6 Q i 8 )(H 2 O) a	2 (OH) a	4 ] 2-and
	[(Re 6 Q i 8 )(H 2 O) a	4 (OH) a	2 ] 0 species with low solubility owing to strong hydrogen bonding between clusters [117].
	It is worth noting that compounds based on [(Re 6 Q i 8 )(H 2 O) a	2 (OH) a	4 ] 2-units like K 2 [(Re 6 S i 8 )(H 2 O) a	2 (OH) a	4 ] 2H 2 O
	exhibit extended 3D hydrogen-bonding networks [117]. Protonation of the [(Re 6 S i 8 )(CN) a	6 ] 4-cluster complex
	was not observed under similar experimental conditions up to pH 1. On the other hand, the
	K 4 [(Re 6 S i 8 )(CN) a	4 (OH) a	2 ] is very soluble in water at pH > 10.5, a pH increase in range 5.5 < pH < 9 enables the
	formation of [(Re 6 S i 8 )(CN) a	4 (OH) a (H 2 O)] 3-and finally [(Re 6 S i 8 )(CN) a	4 (H 2 O) a	2 ] 2-at pH < 3.5 [49]. The latter
	constitute thus useful building blocks for the elaboration of monofunctional and trans-bifunctional molecular
	assemblies by reaction with functional acids in aqueous solutions. A relevant strategy has been developed to
	obtain organic solvent soluble [(Re 6 Q i 8 )L a	6 ] clusters in high yield without the use of tetrabutyl ammonium salts
	as developed in the case of rhenium chalcohalogenides. It consists in the reaction between [(Re 6 Q i 8 )(OH) a	6 ] 4-
	with an excess of tert-butylpyridine (tbp) in water leading to the precipitation of the neutral
	[(Re 6 Q i 8 )(tbp) a 4 (OH) a	2 ] 0 cluster units. It turns out that [(Re 6 Q i 8 )(tbp) a	4 (OH) a	2

Table 1

 1 Synthetic approaches to the common (Re6Q8) 2+ -cluster-core-based precursors used for preparation of cluster containing functional materials and hybrid materials

	Complex	Starting materials	Reaction conditions	Reference
	[(Re6S8)Cl6] 4-	1) Re, S, ReCl5 and MCl	1) 900 º C, ~116 h	[123, 124]
		(M = Tl or Cs)	2) 850 º C, 7 days
		2) Re, S, S2Cl2 and KCl

  electron configuration, found in [(Re 6 Q i 8 )Y a 6 ] 4-(Q = S or Se; Y = halogen, OH or CN) with a (Re 6 Q i 8 ) 2+ cluster core. One-electron oxidation can be easily performed in solutions of molecular rhenium complexes. This corresponds to the removal of one electron from the HOMO level leading to a VEC = 23. Redox potentials depend on both the nature inner and apical ligands and vary from 0.

				065 V for
	[(Re 6 Te i 8 )(CN) a	6 ] 4-to 1.01 V for [(Re 6 Se i 8 )(-dpph) a	2 I a	4 ] 2+ moieties [65]

  2+ is the strongest luminophore with the emission quantum yield and lifetime in CH 2 Cl 2 being 0.238 and 22.4 µs, respectively[127]. Systematical studies of spectroscopic and photophysical

		101, 108, 130, 131]. Among the known Re 6
	clusters, [(Re 6 Se i 8 )(DMSO) a	6 ]

  ] x-4 through the protonation of the apical hydroxo ligands accompanied by red shift of λ em and irregular changes in  em and  em values[117]. Whereas, spectroscopic -are completely insensitive to pH of their aqueous solutions, demonstrating that the luminescence properties of trans-[(Re 6

	-the acidification of an aqueous solution with [(Re 6 Q i 8 )(OH) a	6 ] 4-(Q = S or Se) ions leads to transformation
	of the anionic cluster to [(Re 6 Q i 8 )(H 2 O) a 6-x and photophysical properties of cyanohydroxo complexes trans-[(Re 6 Q i x (OH) a 8 )(CN) a	4 (OH) a 2 ] 4-and hexacyano
	ones [(Re 6 Q i 8 )(CN) a	6 ] 4
				101];
	-mixed-ligand cyanohydroxo complexes trans-[(Re 6 Q i 8 )(CN) a	4 (OH) a	2 ] 4-(Q = S or Se) in aqueous solutions
	exhibit the emission spectra and photophysical characteristics appreciably different from hexahydroxo
	clusters [(Re 6 Q i 8 )(OH) a	6 ] 4-and at the same time very similar to the corresponding hexacyano ones
	[(Re 6 Q i 8 )(CN) a 6 ] 4-[49, 119];

  3 [(Re 6 Se i 8 )(CN) a 6 ] shows a texture of nematic type containing broad schlieren features

	along with large homeotropic areas, while (Kat) 4 [(Re 6 Se i 8 )(CN) a	6 ] exhibits a marbled texture also of nematic
	type. From the thermotropic liquid crystal point of view, (Kat) 4 [(Re 6 Se i 8 )(CN) a	6 ] does not exhibit clear
	mesomorphic properties but (Kat) 3 [(Re 6 Se i 8 )(CN) a	6
	Illustration of
	the lamellar
	packing of 1 in
	the smectic A
	liquid crystal
	phase

  . Re 6 -cluster based silica nanoparticles have been obtained by embedding cationic phosphine-terminated rhenium chalcogenide clusters [(Re 6 Se i 8 )(Et 3 P) a 5 I a ]I, [(Re 6 S i 8 )(Et 3 P) a 5 Br a ]Br, [(Re 6 Se i 8 )(Bu 3 P) a 5 I a ]I, and [(Re 6 S i 8 )(Bu 3 P) a 5 Br a ]Br in silica nanospheres in a one-pot, base-catalyzed hydrolysis in acetonitrile [121]. On the basis of their ability to generate singlet oxygen, they are potential candidates for photodynamic therapy and for other applications. In such nanoparticles, it appears that interactions between cationic cluster units (i.e. [(Re 6 Se i 8 )(Et 3 P) a 5 I a ] + , [(Re 6 S i 8 )(Et 3 P) a 5 Br a ] + , [(Re 6 Se i 8 )(Bu 3 P) a 5 I a ] + , and [(Re 6 S i 8 )(Bu 3 P) a 5 Br a ] + ) and the anionic deprotonated silanol groups occur. Another strategy is the direct synthesis of Re 6 cluster-silica nanoparticles directly from water soluble precursors: K 4 [(Re 6 Q i 8 )(OH) a 6 ] (Q = S or Se), Cs 4 [(Re 6 S i 8 )Br a 6 ], and Cs 1.68 K 2.32 [(Re 6 S i 8 )(CN) a 4 (OH) a 2 ] [90]. Indeed, the A 4 [(Re 6 Q i 8 )L a 6

Acknowledgement.

We thank the France-Russia programs PICS (2011-2013 N°5822) and International Associate Laboratory (2014-2017) as well as the IDEMAT chemistry project in the frame of the Franc Siberian Center of Training and Research. We acknowledge A. Perrin, C. Perrin and V. E. Fedorov for helpful discussions. We are grateful to T. Aubert, M. A. Shestopalov, A. Y. Ledneva, O. A. Efremova and F. Dorson who made a part of their PhD work on this topic of research.