Pierre Cahuc

Thomas Le Barbanchon

LABOR MARKET POLICY EVALUATION IN EQUILIBRIUM: SOME LESSONS OF THE JOB SEARCH AND MATCHING MODEL

Keywords: evaluation, equilibrium effect, labor market policy JEL Classification: J64, J68

We analyze the consequences of counseling provided to job seekers in a standard job search and matching model. It turns out that neglecting equilibrium effects induced by counseling can lead to wrong conclusions. In particular, counseling can increase steady state unemployment although counseled job seekers exit unemployment at a higher rate than the noncounseled. Dynamic analysis shows that permanent and transitory policies can have effects of opposite sign on unemployment.

Introduction

Most policy evaluations are based on comparing the behavior of participants and non participants in the policy. But the di¤erences in outcome between the treatment group and the control group do estimate the policy mean impact only if the outcomes of the control group are not in ‡uenced by the policy, the so-called 'no-interference' [START_REF] Rubin | Baysian Inference for Causal E¤ects; the Role of Randomization[END_REF] or 'stable unit treatment value' [START_REF] Angrist | Identi…cation of Causal E¤ects Using Instrumental Variables[END_REF]) assumption. However, the policy may have equilibrium e¤ects that a¤ect the untreated altogether. For instance, Heckman, Lochner and Taber (1998) strikingly illustrate this point in the context of education policies. This issue, which is discussed in a broader perspective in the survey of [START_REF] Meghir | Dynamic Models for Policy Evaluation[END_REF], is particularly relevant to the evaluation of labor supply based policies (such as increasing incentives or monitoring the unemployed).

First, they generally aim at increasing the overall number of …lled jobs, which depends on the interactions between aggregate labor supply and labor demand. Second, these policies may induce displacement e¤ects: treated persons may crowd out the untreated because they compete for the same jobs.

Although they have been long recognized, these questions have received limited attention to date. [START_REF] Davidson | The Displacement E¤ect of Reemployment Bonus Programs[END_REF] and [START_REF] Calmfors | Active Labor Market Policy and Unemployment -A Framework for the Analysis of Crucial Design Features[END_REF] are early contributions. More recently, Lise, Seitz and Smith (2005) study the equilibrium e¤ects of the Self-Su¢ cient Project incentive program in Canada. They calibrate an equilibrium model of the labor market so that, when used in partial equilibrium, the model matches the e¤ect of the program estimated by direct comparison of treated and untreated. When equilibrium e¤ects are simulated, the impact of the SSP is far lower. In contrast, Albrecht, van den Berg and Vroman (2005) …nd, using a calibrated model, equilibrium e¤ects of a Swedish training program to be stronger than implied by direct comparison. The previous results obtained with calibrated models are not fully backed by the empirical evaluations, conducted by Blundell, Costa Dias, [START_REF] Blundell | Evaluating the employment impact of a mandatory job search program[END_REF], of the New Deal for Young People in the U.K. Based on di¤erent choice of control groups (unemployed in non pilot areas or ineligible unemployed in treated areas), they …nd that direct and equilibrium evaluations provide similar results. Using a job search and matching model with skilled and unskilled workers, [START_REF] Van Der Linden | Equilibrium Evaluation of Active Labor Market Programme Enhancing Matching E¤ectiveness[END_REF] shows that micro and equilibrium evaluations are likely to di¤er widely when job search e¤ort and wages are endogenous. When wages are bargained over, raising the e¤ectiveness of or the access to counseling programs pushes wages upwards and leads to lower search e¤ort among nonparticipants. Induced e¤ects can outweigh positive micro e¤ects on low-skilled employment when the response of wages is taken into account.

The aim of our paper is to analyze the impact of counseling in the standard matching model of the labor market [START_REF] Pissarides | Equilibrium Unemployment Theory[END_REF]. In our speci…cation, counseled unemployed have a constant comparative advantage in the job search. Using this simple model allows us to analyze the consequences of counseling in a dynamic set-up whereas previous studies are limited to the comparison of steady states. More precisely, we shed some light on three important issues:

(i) What is the true impact of the policy when equilibrium e¤ects are taken into account?

The model shows that the true impact of counseling can be very di¤erent from what can be concluded when equilibrium e¤ects are neglected even when the treatment group is small. For instance, we …nd that counseling can increase unemployment when a small proportion of job seekers bene…t from counseling, although counseling improves the e¢ ciency of job search. 1(ii) What is the impact of the generalization of the policy to a large treatment group? The model shows that there is no simple answer. In particular, the relation between the impact of the policy on unemployment and the size of the treatment group is not monotonous. Strikingly, in our framework, unemployment increases with the size of the treatment group when a small share of job seekers are treated but diminishes with the size of the treatment group when a su¢ ciently large share of job seekers are counseled.

(iii) What is the dynamic impact of counseling? Many experimentations made to evaluate labor market policies are transitory. Typically, a group of job seekers is selected to bene…t from counseling (the treatment group) and the control group will never bene…t from counseling. The comparison between the outcomes yields the evaluation of the impact of counseling. Our model allows us to stress that the consequences of permanent and transitory policies can be very different. The di¤erence comes from the reaction of non-counseled job seekers. When the policy is transitory, non-counseled workers do not expect to bene…t from counseling in the future. However, when the policy is permanent, the expectation to bene…t from counseling in the future induces the non-counseled workers to raise in their reservation wage. In our framework, this phenomenon implies that permanent counseling increases unemployment when a small share of job seekers are counseled whereas counseling always decreases unemployment when it is transi-tory. Accordingly, it can be misleading to conclude that a truly successful transitory policy will remain successful when it will become permanent.

The paper is organized as follows. The model is presented in section 1. Section 2 is devoted to the impact of counseling in steady state. Transitory dynamics is analyzed in section 3.

The model

We consider a standard matching model à l a [START_REF] Pissarides | Equilibrium Unemployment Theory[END_REF] with a continuum of in…nitelylived risk neutral workers. The measure of the continuum is normalized to one. There are two goods: a good produced and consumed, which is the numeraire, and labor. There is a common discount rate r; strictly positive. Time is continuous. Workers can be in three di¤erent states:

(1) employed, (2) unemployed and counseled, (3) unemployed and not counseled. Upon entering unemployment, workers are not counseled. They then enter into counseled status at a rate > 0 and they keep on receiving counseling until they …nd a job.

There is an endogenous number of jobs. Each job can be either vacant or …lled. Filled jobs produce y > 0 units of the numeraire good per unit of time, whereas vacant jobs cost c per unit of time. Filled jobs are destroyed with probability > 0 per unit of time.

Vacant jobs and unemployed workers (the only job seekers, by assumption) are brought together in pairs through an imperfect matching process. This process is represented by the customary matching function, which relates total contacts per unit of time to the seekers on each side of the market. Let us denote by u n and u c the number of non-counseled and counseled unemployed workers respectively. In our set-up, the only potential e¤ect of counseling is to increase the arrival rate of job o¤ers to the counseled unemployed workers. Let us normalize to one the number of e¢ ciency units of job search per unit of time of each non-counseled unemployed worker. Counseled unemployed workers are assumed to produce a di¤erent number of e¢ ciency units of search, denoted by 1: In this setting, the number of e¢ ciency units of job search per unit of time amounts to s = u n + u c :

The number of employer-worker contacts per unit of time is given by M (s; v) 0, where v 0 denotes the number of job vacancies and M is the matching function, twice continuously di¤erentiable, increasing, concave in both of its arguments, and linearly homogeneous. Linear homogeneity of the matching function allows us to express the probability per unit of time for a vacant job to meet an unemployed worker as a function of the labor market tightness ratio, = v=s. A vacant job meets on average M (s; v) =v = q ( ) unemployed workers per unit of time, with q 0 ( ) < 0. Similarly, the rate at which counseled and non counseled unemployed job seekers can meet jobs is q ( ) and q ( ) respectively.

Parameter is estimated by econometricians who evaluate the impact of counseling by comparing the exit rate out of unemployment of counseled workers and the exit rate out of unemployment of non-counseled workers assuming that the arrival rate of job o¤ers to the noncounseled workers is not in ‡uenced by counseling. Henceforth, we assume that has been correctly evaluated in this way. The model allows us to analyze the impact of counseling on the non-counseled workers and on labor market equilibrium.

Job creation

Let J c and J n be the present-discounted value of expected pro…t from an occupied job with a counseled worker and a non-counseled worker respectively. Let V denotes the present-discounted value of expected pro…t from a vacant job. V satis…es

rV = c + q( ) [ J c + (1 )J n V ] + _ V ;
where _ V denotes the time derivative of V and = u c u c + u n stands for the probability to meet a counseled worker. The free entry condition for the supply of vacant jobs is V = 0 at any date, implying that

c q( ) = J c + (1 )J n : (1)
Let us denote by w c and w n the wage of a counseled worker and of a non-counseled worker respectively. The asset value of a job …lled with a counseled worker, J c ; satis…es

rJ c = y w c + (V J c ) + _ J c : (2)
Similarly, the asset value of a job …lled with a non-counseled worker, J n ; satis…es

rJ n = y w n + (V J n ) + _ J n : (3)
At this stage, it can be shown that the impact of counseling on the arrival rate of job o¤ers to the non-counseled depends on the wages w c and w n :

The impact of counseling when wages are exogenous

Let us assume for a while that wages w c and w n are exogenous. Then, equations ( 2) and

(3), which de…ne the asset value of …lled jobs, imply that J c = (y w c )=(r + ) and J n = (y w n )=(r + ): Substituting these expressions into the free entry condition (1) yields

c(r + ) q( ) = y [ w c + (1 )w n ] : (4)
It turns out that increases in the share of counseled workers increase the probability that …rms meet counseled workers. Then, equation [START_REF] Blundell | Evaluating the employment impact of a mandatory job search program[END_REF] shows that increases in reduce the labor market tightness (and then the exit rate out of unemployment of the non-counseled, equal to q( )) if the wage of counseled workers is higher than the wage of the non-counseled . In this case, increases in the share of counseled workers raise the proportion of high paid workers.

Then, expected pro…ts decrease and …rms post less job vacancies. If counseled workers get lower wages than non-counseled workers, we get the opposite result: counseling increases labor market tightness. When wages are identical, labor market tightness is independent of the share of counseled workers. This may be the case when there is a minimum wage that is binding for both counseled and non-counseled workers.

The analysis of the case where wages are exogenous allows us to stress the role played by wage adjustment. In our simple search and matching model where workers are ex-ante identical, counseling may have an impact on labor market tightness, and then on the arrival rate of job o¤ers to the non-counseled workers, if it induces wage di¤erentials between the counseled and the non-counseled.

Wage bargaining

Let us now suppose that wages are bargained over. One needs to de…ne the workers' returns when employed and unemployed in order to derive the outcome of the wage bargaining. The present-discounted value of the expected income stream of, respectively, a counseled and a noncounseled unemployed, is denoted by U c and U n : The present-discounted value of the expected income stream of employees who found a job while counseled is denoted by W c . The present discounted value of the employees who obtained a job without being counseled is denoted by W n : All unemployed workers enjoy some instantaneous return z which includes unemployment bene…ts and the imputed return of leisure. The non-counseled exit unemployment at rate q( ) and enter into counseling at rate : The counseled exit unemployment at rate q( ): Hence U n ; U c ; W n and W c satisfy

rU n = z + (U c U n ) + q( )(W n U n ) + _ U n ; (5)
rU c = z + q( )(W c U c ) + _ U c ; (6)
rW n = w n + (U n W n ) + _ W n ; (7)
rW c = w c + (U n W c ) + _ W c : (8)
We assume that the wage bargaining outcome yields a share of the surplus of the job to the worker. The surplus of a job …lled by a previously counseled worker is

S c = W c U c + J c V:
The surplus of a job …lled by a worker who did not bene…t from counseling is

S n = W n U n + J n V:
The surplus sharing rule reads

W i U i = S i ; J i V = (1 )S i ; i = c; n: (9)
The outcome of the wage bargaining being de…ned, it becomes possible to derive the set of equations that de…nes the value of endogenous variables in equilibrium.

Labor market equilibrium

Using the sharing rule, the de…nitions of the surpluses and equations ( 5) through [START_REF] Heckman | General Equilibrium Treatment E¤ects: A Study of Tuition Policy[END_REF] we can write

(r + )S c _ S c = y z q( ) S c ; ( 10
) (r + )S n _ S n = y z q( ) S n ; (11)
where

= W c W n > 0 satis…es (r + ) _ = q( ) ( S c S n ): (12)
Equations ( 10), ( 11) and ( 12) comprise four unknow variables: S c ; S n ; and : Using the free entry condition (1) together with the sharing rule [START_REF] Heckman | Evaluating theWelfare State[END_REF], we obtain a relation between the labor market tightness and the surpluses which involves two more unknown u n and u c :

c q( )(1 ) = u n u c + u n S n + u c u c + u n S c : (13)
Then, the relations between the labor market tightness and the unemployment rates are derived from the law of motion of u n and u c ; which read

_ u n = (1 u n u c ) u n q( )u n (14)
_ u c = u n q( )u c : (15)
Finally, the system of six equations from ( 10) to (15) comprises six unknown variables S n ; S c ; ; ; u n ; u c : Although this system of six equations cannot be simpli…ed into a system with a smaller dimension, it allows us to examine some consequences of counseling on the arrival rate of job o¤ers to the non-counseled. This can be conveniently done by looking at the case where there is a pool of counseled workers and where the entry rate into counseling, ; equals zero. In that case, the number of counseled workers decreases at rate q( ): Equations ( 10) and [START_REF] Meghir | Dynamic Models for Policy Evaluation[END_REF] show that the surplus of jobs …lled with counseled workers is smaller than the surplus of jobs …lled with non-counseled workers. 2 The surplus of job …lled with counseled workers is smaller because the reservation wage of counseled workers is higher than the reservation wage of non-counseled workers. Therefore, …rms make less pro…ts when they are matched with counseled workers rather than with non-counseled. Hence, as shown by equation [START_REF] Meghir | Dynamic Models for Policy Evaluation[END_REF], the labor market tightness is reduced when more workers are counseled. This induces less job o¤ers to the non-counseled. This phenomenon is ampli…ed when the entry rate into counseling, , is positive, because the perspective to be counseled in the future pushes upward the reservation wage of non-counseled workers, which is detrimental to job creation.

Policy evaluation in steady state

In this section we calibrate the model and we analyze the equilibrium e¤ect of counseling in steady state. 2 When > 1; Sn is necessarily larger than Sc. Suppose that this is not the case, so that Sc Sn;then Sc > Sn. From equations [START_REF] Lise | Equilibrium Policy Experiments and the Evaluation of Social Programs[END_REF], [START_REF] Meghir | Dynamic Models for Policy Evaluation[END_REF] and [START_REF] Pissarides | Equilibrium Unemployment Theory[END_REF], we obtain the folowing expession:

(r + )(Sc Sn) = r + r + q( ) ( Sc Sn)
which implies that Sc < Sn; which is incompatible with the assumption that Sc Sn: This enables us to conclude that Sc < Sn.

Calibration

The frequency of the model is monthly. The 3 month interest rate is set to 1.2 percent, which makes the monthly discount factor equal to 0.996. We need to specify the matching function:

q ( ) = q 0 We choose a conservative value for the elasticity = 0:5. The bargaining power is set equal to to ensure that the Hosios condition is ful…lled. We want to reproduce features of the French labour market (means are taken from 2000 to 2007, which correponds to the last business cycle). The instantaneous return of unemployment, z; is equal to 60 percent of the productivity y; which value is normalized to one. This implies a replacement ratio (z over w) slightly above 60 percent since wages take values around 0:96 in equilibrium. The mean unemployment duration, measured in the Labor Force Survey ("Enquête emploi") between 2004 and 2005, is 1:07 year. The monthly exit rate out of unemployment consistent with this mean unemployment duration is e ss = 7:80 percent. The overall unemployment rate averaged 9:5 percent over the same time period.

To compute the baseline equilibrium, we assume that there is no counseling so that = u c = 0 and u n = 0:095. The separation rate is thus = 0:8 percent. The cost of posting a vacancy is set to be roughly one third of a period production c = 0:3y.

The value of parameter q 0 of the matching function is determined by the following relation

q 0 = c y z
(e ss ) =(1 ) (r + + e ss ) 1 1 1 :

Policy experiment

In this subsection, we look at the consequences of the introduction of a counseling policy that improves the e¢ ciency of the search activity of counseled workers. We assume that non-counseled workers produce one unit of search per unit of time, so that their arrival rate of job o¤ers amounts to q( ): In line with the estimations of Crepon et al. (2005), we assume that the counseled produce = 1:2 unit of search per unit of time, so that their arrival rate of job o¤ers is 1:2 q( ): evaluation, relying on a simple comparison of the outcomes of participants and non participants that neglects equilibrium e¤ects, can lead to the wrong conclusion that counseling decreases unemployment, especially when the share of counseled workers is small. Obviously, the negative impact of counseling on unemployment comes from its e¤ect on the arrival of job o¤ers to the non-counseled. Figure 2 shows that the arrival rate of job o¤ers to the non-counseled decreases with the share of counseled workers. The drop in the baseline arrival rate of job o¤ers, q( ); is the result of two e¤ects. First, there is a decrease in pro…tability due to the new composition of the unemployed population. Because the counseled get higher wages than the non-counseled, 3 c(r + )q 0 ( ) (q( )) 2 @ @ = @ @ (w c w n ) @w c @ + (1 ) @w n @ :

The impact of counseling on unemployment

The …rst term of the right-hand side corresponds to the composition e¤ect. The second e¤ect, which shows up in the second term, comes from the adjustment of wages (see Figure 3). The wage of non-counseled workers is pushed upward by counseling because non-counseled workers anticipate that they may bene…t from counseling in the future. In contrast, the wage of counseled workers diminishes with the entry rate into counseling because the opportunity cost of accepting job o¤ers is higher when few job seekers are counselled. More precisely, counseling creates an opportunity cost of accepting job o¤ers: counseled job seekers who …nd jobs can loose them and will then have to wait a while before bene…ting from counseling again. This opportunity cost is higher when the probability to be counseled again, after the accepted job is lost, is lower.

Thefore, the opportunity cost to accept a job, and then the negotiated wage, is higher when the entry rate into counseling is smaller.

Finally, the composition and the wage e¤ects result in a negative impact of counseling on the baseline arrival rate of job o¤ers, q( ); as shown by Figure 2.

The decline in the baseline arrival rate of job o¤ers induced by counseling tends to drive the unemployment rate upwards. This e¤ect competes with the direct e¤ect of counseling which makes counseled job seekers leaving unemployment faster. When the share of counseled workers is small, the …rst e¤ect dominates: the share of non-counseled workers who are adversely a¤ected is large and counseled workers gets very high wages. When the share of counseled workers is large, the second e¤ect dominates: even if counseled workers are numerous, they get lower wages than when they are fewer.

Welfare analysis

Our model allows us to yield a welfare analysis. In our context, where workers are risk neutral, instantaneous social welfare is merely equal to the production of employees, (1 u)y, plus the production of the unemployed, uz; minus the cost of job vacancies, cv: Assuming that counseling is provided at zero cost, we …nd that counseling more unemployed always enhances social welfare, as shown by reduces the baseline arrival rate of job o¤ers and then their probability to …nd a job. Actually, non-counseled workers bene…t from counseling because the drop in the baseline arrival rate of job o¤ers induced by counseling is compensated by the possibility to be counseled in the future.

Counseled unemployed job seekers, like the non-counseled, are always better o¤ in a world with counseling than in a world without. However, their welfare, U c ; decreases with the share of counseled workers.

Evaluation errors

Our model allows us to shed light on the size of the evaluation errors coming from ignoring equilibrium e¤ects. Standard evaluations, relying on a simple comparison of the outcome of the treated and the non treated, can lead to wrong results if the policy induces equilibrium e¤ects which change the baseline arrival rate of job o¤ers q( ): The error comes from the choice of wrong counterfactuals when evaluating the impact of the policy: standard evaluations assume that the counterfactual arrival rates of job o¤ers to the non treated in the absence of the policy are the same as those observed by the econometrician in the presence of the policy.

In our model, the exit rate out of unemployment of counseled job seekers amounts to q( ):

Non treated individuals exit unemployment at rate, q( ): The e¤ect of the treatment on the treated is usually de…ned as the ratio between these two exit rates, that is : However, this approach yields a naive evaluation of the e¤ects of the treatment to the extent that it does not account for equilibrium e¤ects which may change the value of the arrival rate of job o¤ers to the non counseled job seekers. To account for such e¤ects one needs to know the exit rate out of unemployment in the absence of counseling, that we denote by 0 q( 0 ): Then, the e¤ect of the treatment on the treated accounting for equilibrium e¤ects is de…ned as q( )= 0 q( 0 ):

The error induced by the ignorance of equilibrium e¤ects, expressed in percentage of the naive evaluation , is thus [ q( ) 0 q( 0 )] = 0 q( 0 ): Figure 5 shows that the naive evaluation leads to an over estimation of the 'true'e¤ect. The absolute error increases with the share of counseled workers. It is equal to 4 percent when the share of counseled workers amounts to 20 percent and goes up to 9 percent when the share goes to one.

Another error can be made when simulating the consequence of the spread of the policy to all workers. Looking at this error is important to the extent that some policy makers think that policies should …rst be evaluated at a small scale before being generalized if their evaluations are favorable. This idea is right only if equilibrium e¤ects are properly taken into account. Ignoring such e¤ects can lead to false conclusions, because it is wrong to simulate the impact of the generalization of counseling to all job seekers with the assumption that the arrival of job o¤ers remains unchanged. We can shed light on this type of error by looking at the di¤erence between the true value of the unemployment rate, denoted by u ; and the value of the unemployment rate, denoted by ũ; computed when it is assumed that the baseline arrival rate remains unchanged, equal to 0 q( 0 ): Figure 6 plots the true unemployment rate, u , (continuous line) and the unemployment rate computed without accounting for equilibrium e¤ects, ũ.

The evaluation error made when equilibrium e¤ects are neglected also biases cost-bene…t analysis based on the welfare criterion presented above. As previoulsy assumed, ignoring general equilibrium e¤ects boils down to keeping labor market tightness equal to its value in the absence of counseling. In that case, a naive prediction of the unemployment rates for a given treatment intensity can be computed as in the previous paragraph (ũ n ( ); ũc ( ); ũ( )). Given 0 0.2 0.4 0.6 0.8 these predictions, one can compute the naive net welfare gain from the policy !. In the naive cost-bene…t analysis, one compares the naive net welfare gain per counseled worker to the unit cost of counseling. 4 Figure 7 shows that the maximal unit cost of counseling that makes counseling pro…table is relatively small, equal to 0:066 (recall that the productivity of employees is normalized to 1). The naive maximal cost above which it is not worth counseling is always higher than the true maximal cost, that accounts for equilibrium e¤ects. The naive evaluation over-estimates the e¢ ciency of counseling because it neglects the negative impact of counseling on the baseline arrival rate of job o¤ers.

Policy evaluation and dynamic adjustment

Up to now, we analyzed the impact of counseling on labor market equilibrium in steady state. It is also important to keep in mind that most labor market policies induce dynamic adjustements that take time. Our model allows us to study the dynamic path of the endogenous variables.

We consider three policy experiments that di¤er by the proportion of people being counseled.

In the baseline scenario the entry rate into counseling, ; is equal to 5 percent. There is also a 'light'scenario, where is equal to 1 percent, and an 'intensive'scenario, with an entry rate into counseling equal to 20 percent. We also consider two versions of these policy experiments.

In the …rst, the policy is permanent: the entry rate into counseling remains constant over time from time t = 0: In the second, it is transitory: some workers enter into counseling at time t = 0 only. Then, these workers remain counseled until they …nd a job and other workers cannot bene…t from counseling. 5 As in the previous section, in all the simulations, the counseled have a comparative advantage which increases their relative probability of …nding a job by 20 percent ( = 1:2).

Permanent policy

In the baseline scenario the entry rate into counseling, ; is equal to 5 percent, which entails that 36 percent of the unemployed are counseled in steady state. In the 'light'scenario, where equals 1 percent, it turns out that 5:2 percent of the unemployed are counseled in steady state.

In the 'intensive' scenario, with an entry rate into counseling equal to 20 percent, 69 percent of the unemployed are counseled in steady state. Figure 8 shows the dynamics of the share of counseled workers for the three cases.

Figure 9 shows that the baseline arrival rate of job o¤ers decreases monotically with time.

The baseline arrival rate of job o¤ers adjusts more rapidly to its steady state value when the entry rate into counseling is bigger. However, in all cases considered here, the arrival rate of job o¤ers is very close to its steady state value after one year.

Figure 10 shows that the unemployment rate dynamics is not always monotonic. There is an overshooting of the unemployment rate when the share of counseled job seekers is su¢ ciently large. This phenomenon is the consequence of the interaction between the positive impact of counseling on the entry rate into employment of counseled job seekers and the equilibrium e¤ects, which reduce the entry rate into employment of the non-counseled. When the entry rate into counseling is large enough, the drop in the baseline arrival rate of job o¤ers, induced by equilibrium e¤ects, dominates at the beginning, which induces an increase in the unemployment rate. Then, as time elapses, there are more and more counseled workers whose exit rate out of unemployment is relatively high.

Figure 10 leads us to stress that it is important to account for the dynamics of the unemployment rate when evaluating the equilibrium e¤ects of counseling. A priori, it could be possible to estimate the equilibrium e¤ects of counseling by gathering data on similar employment pools in which there are di¤erent proportions of counseled individuals. However, this strategy can lead to very di¤erent conclusions according to the time horizon at which the evaluation is done. In the baseline scenario, where the entry rate into counseling amounts to 5 percent, the evaluation of the equilibrium e¤ects 6 months after the introduction of the policy leads to the conclusion that they increase signi…cantly unemployment. However, there are no signi…cant e¤ects on the unemployment rate beyond two years.

We also compute the dynamics of the evaluation error [ t q( t ) 0 q( 0 )] = 0 q( 0 ). The true treatment e¤ect on the treated is over estimated by the naive evaluation. As shown by …gure 9, the size of the error increases with time because it takes time to increase the number of counseled workers and then to get sizeable equilibrium e¤ects.

Transitory policy

Now, we look at situations where some workers bene…t from counseling at date zero and remain counseled until they …nd a job. The other job seekers, who do not bene…t from counseling at date zero, are never counseled. Figure 12 displays the evolution of the share of counseled workers over time. Figure 13 shows the corresponding evolution of the unemployment rate. It turns out that counseling always decreases the unemployment rate, contrary to the case where the policy is permanent. The di¤erence between the two cases comes from the role of the expectations of noncounseled workers. When the policy is permanent, non-counseled workers anticipate that they will bene…t from counseling in the future. Therefore, their reservation wage and then their bargained wage increase (as shown in Figure 3 above). When the policy is transitory, non-counseled job seekers know that they will never bene…t from counseling. Therefore, their reservation wage does not increase. Actually, their reservation wage decreases because the baseline arrival rate of job o¤ers, q( ), drops when some workers are counseled, as shown by Figure 14. Note that there is a spike in the job o¤er arrival rate at the time of the policy shock. This is due to the assumption made in the discrete time version of the model presented in appendix. At date zero, there is no counseled worker ready to be hired since vacant jobs posted at date t are matched with workers unemployed at date t 1 (recall that unemployment is a predetermined variable). Moreover, at date zero, non-counseled job seekers reduce their reservation wage because they anticipate that the baseline arrival rate of job o¤ers is going to decrease in the next future. The combination of this two phenomena increases the value of job vacancies, and then job creation at date zero. At date one, vacant jobs meet counseled job seekers whose reservation wage is higher. This is detrimental to job creation, as shown by Figure 14.

The comparison of the impact of transitory and permanent policies highlights the role of anticipations. When the policy is permanent, it turns out that a non negligeable share of its impact on the unemployment rate is induced by the reaction of non-counseled job seekers. The raise in their reservation wage, and then in their bargained wage, induced by the expectation to bene…t from counseling in the future dampens job creation. This phenomenon implies that permanent counseling increases unemployment when a small share of job seekers are counseled whereas counseling always decreases unemployment when it is transitory. Accordingly, it can be misleading to conclude that a truly successful transitory policy will remain successful when it will become permanent.

Conclusion

Our paper stresses that it is worth accounting for equilibrium e¤ects to provide a proper evaluation of counseling policies. Neglecting such e¤ects could lead to the conclusion that counseling reduces steady state unemployment although its true e¤ect could be opposite. A striking result obtained in the paper is that this type of error can arise when the size of the treatment group is small. It also turns out that it can be wrong to conclude that a truly successful transitory policy remains successful when it becomes permanent. This result is important to the extent that many policy evaluations rely on the experimentation of policies on a temporary basis. Typically, a policy is evaluated during a transitory period. Then, it is often considered that this evaluation provides relevant information to evaluate the e¤ect of the policy that will be implemented permanently. Our analysis shows that this is not always the case.

Figure 1

 1 Figure1displays the relation between the unemployment rate and the share of counseled workers in steady state. It is striking that steady state unemployment increases with the share of counseled workers when this share is small, below 10 percent. This result shows that a naive

Figure 1 :

 1 Figure 1: The relation between the unemployment rate (y-axis) and the share of counseled workers (x-axis).

Figure 2 :

 2 Figure 2: The relation between the arrival rate of job o¤ers to non-counseled workers (y-axis) and the share of counseled workers (x-axis).

Figure 4 .

 4 It is worth noting that social welfare increases with the share of counseled workers even when counseling increases unemployment. We also …nd that the welfare of noncounseled unemployed, U n ; increases with the share of counseled workers, although counseling

Figure 3 :Figure 4 :

 34 Figure 3: The relation between the wage of counseled workers (broken line) and the wage of non-counseled workers (continuous line) (y-axis) and the share of counseled workers (x-axis).

Figure 5 :

 5 Figure 5: The relation between the error (in percentage of the naive evaluation ) in the evaluation of the e¤ect of counseling on the exit rate out of unemployment of counseled workers (y-axis) and the share of counseled workers (x-axis).

Figure 6 :

 6 Figure 6: The relation between the unemployment rate (true unemployment rate: continuous line, equilibrium unemployment rate computed without accounting for equilibrium e¤ects: broken line) and the share of counseled workers (x-axis).

Figure 7 :

 7 Figure 7: The relation between the maximal pro…table counseling cost (true : continuous line, computed without accounting for equilibrium e¤ects: broken line) and the share of counseled workers (x-axis).

Figure 8 :

 8 Figure 8: The evolution of the share of counseled workers (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Figure 9 :

 9 Figure9: The evolution of the baseline arrival rate of job o¤ers, q( ); (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Figure 10 :Figure 11 :

 1011 Figure 10: The evolution of the unemployment rate (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Figure 12 :

 12 Figure 12: The evolution of the share of counseled workers (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Figure 13 :

 13 Figure 13: The evolution of the unemployment rate (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Figure 14 :

 14 Figure14: The evolution of the baseline arrival rate of job o¤ers, q( ); (y-axis) over time (x-axis, in month) for = 0:01 (continuous line), = 0:05 (crosses) and = 0:2 (circles).

Our results rely ont the adjustment of wages as in the paper of[START_REF] Van Der Linden | Equilibrium Evaluation of Active Labor Market Programme Enhancing Matching E¤ectiveness[END_REF]. However, Van der Linden assumes that wages are collectively bargained over, whereas we assume an individual bargaining framework where counseled and non-counseled workers can get di¤erent wages.

Recall that the counseled get higher wages because counseling enhances their exit rate out of unemployment and then their reservation wage.

The unit cost of counseling is de…ned as the cost per counseled worker during one unit of time.

The simulations are made with the software Dynare (http://www.cepremap.cnrs.fr/dynare/) which necessitates to write the model in discrete time. The discrete time version of the model is presented in appendix.

A Appendix: The model in discrete time

The aim of this appendix is to present the discrete time version of the continuous time model presented in the text. Unemployment rates are predetermined. During period t, matching involve the unemployed populations inherited from the previous period with the job vacancies posted in period t. To make clear that unemployment rates are predetermined, we index them by t 1. The timing of events within each period is the following, production takes place, …rms post vacant jobs, jobs and unemployed workers are matched, jobs are destroyed at rate and, …nally, payments are made. The assumptions about timing allow us to write the system of six equations from [START_REF] Lise | Equilibrium Policy Experiments and the Evaluation of Social Programs[END_REF] to (15) that de…nes the equilibrium value of (S n ; S c ; ; ; u n ; u c ) as follows

Because agents are risk-neutral per period social welfare can be written as the production net of the cost of vacant jobs. Thus, we de…ne the period welfare as:

! t = y(1 u n;t 1 u c;t 1 ) + z(u n;t 1 + u c;t 1 ) c t (u n;t 1 + u c;t 1 ):

The discounted present value of intertemporal welfare, t ; writes:


Dr Haidong Li

Prof Jonathan Daniel

Jean-Baptiste Verlhac

Prof. N. Mireille Blanchard-Desce
email: mireille.blanchard-desce@u-bordeaux.fr

Neso Sojic
email: neso.sojic@u-bordeaux.fr

Dr J.-B Verlhac

Bright Electrogenerated Chemiluminescence of a Bis-donor Quadrupolar Spirofluorene Dye and its Nanoparticles

Keywords: spirofluorene dye, fluorescent organic nanoparticles, luminescence, electrochemistry, electrogenerated chemiluminescence

A series of symmetric fluorescent dyes built from a spirofluorene core, bearing electroactive end-groups and having different conjugated linkers were prepared in view of their use as building blocks for the preparation of electrochemiluminescent (ECL) dyes and nanoparticles. Their electrochemical, spectroelectrochemical and ECL properties were first investigated in solution and structure/activity relationships were derived. The electrochemical and ECL properties show drastic variation that could be tuned contingent on the nature of the π-conjugated system, end-groups and of the core. Among the series, the highly fluorescent dye based on a spirofluorene core and triphenylamine end-groups connected via thiophene moieties (1) shows the most promising and intriguing properties. 1 is reversibly oxidized in three well-separated steps and a very intense and large ECL signal is generated. Its ECL efficiency is 4.5time greater than that of the reference compound, Ru(bpy)3 2+ . This remarkably high efficiency is due to the very good stability of the higher oxidized states and it makes 1 a very bright organic ECL luminophore. In addition, thanks to its molecular structure, this dye retains fluorescence when nano-precipitated in water, leading to Fluorescent Organic Nanoparticles (FONs). Redox behaviour of these FONs shows oxidation waves consistent with the initial molecular species. Finally, ECL from FONs made of 1 was recorded in water and strong ECL nano-emitters are thus obtained.

Introduction

Thiophenes and their functional derivatives based materials have been widely investigated in the fields of electroluminescence devices, [1] chemosensors, [2] and biosensors. [3] In particularly, oligo/polythiophenes have been advanced to be the most frequently investigated structures. [4] Thiophene studies have been developed and well established for a long time. [START_REF] Perepichka | Handbook of thiophene-based materials: applications in organic electronics and photonics[END_REF] Thiophenes are ideal platforms in transition metal-catalyzed crosscoupling reactions, which have provided numerous methods for the synthesis of most functionalized thiophene derivatives in the past decades. [START_REF] Diederich | Metal-catalyzed cross-coupling reactions[END_REF] Thiophene-based materials have many outstanding chemical and physical properties, such as their unique variety of electronic properties and easy structure variation. [START_REF] Bäuerle | [END_REF] Recently, N-perarylated aromatic amines have received intense interest due to their use in organic synthesis and structure studies. [8] Especially, thiophene derivatives functionalized with N-perarylated aromatic amino groups can be used in holetransporting materials for optical and microelectronic devices, like OLEDs, [9] OFETs, [10] solar cell [11] and photocopiers. [12] Many thiophene derivatives with N-perarylated aromatic amino moieties have been successfully prepared to investigate the interaction of both components. [4c, 13] Electrogenerated chemiluminescence (ECL) is a type of chemiluminescence involving the in situ generation of an excited state at the electrode surface by electron-transfer reactions. [START_REF] Bard | Electrogenerated chemiluminescence[END_REF] ECL mechanisms have been established in two dominant pathways. In the annihilation pathway, an oxidized and a reduced species of the luminophore are electrochemically produced at the electrode surface by applying an alternating pulsed potential. Then, these two species diffuse and undergo an annihilation reaction to generate an electronically excited state. It relaxes to the ground state and emits a photon. [15] An alternative mode with more complex kinetic pathways is based on the use of sacrificial co-reactants (e.g. tri-n-propylamine, [16] oxalate, [17] benzoyl peroxide) [18] capable to react with the oxidized or reduced luminophore to generate the desired excited state, which makes ECL very useful in practical applications. Since by essence ECL does not require any external light source for the excitation, it is a very sensitive technique which is based on orthogonal electrochemical addressing and optical detection modalities. Due to its interdisciplinary nature and also to its remarkable characteristics, ECL has attracted large interests in different fields such as photochemistry, electrochemistry, analytical chemistry, etc. [19] ECL has shown great achievements in fundamental research but also for real applications, mainly in the diagnostic market with immunoassays. [20] The model systems are mainly based on organic dyes such as aromatic heterocycles and organometallic compounds such as tris-cyclometalated ruthenium and iridium complexes. [21] To increase the sensitivity and the analytical performances of ECL, many efforts are currently focused on the development of novel luminophores and ECL nano-emitters with tunable wavelengths. [13a, 21a, 22] For example, a remarkable example of self-enhanced ECL based on an iridium-based complex has been reported by Ding and co-workers where the ECL efficiency is increased after each oxidation steps. [22j] Kim's group described enhanced ECL of pyrene derivatives attributed to the stability of the cation and anion radicals due to peripheral multidonors. [22n] In the present work, we exploit a conceptually-similar strategy with spirofluorene-based molecules where the higher oxidized states are stabilized and thus lead to a very intense and large ECL signal.

Scheme 1. Chemical structures of the dyes 1, 2, 3, 4 and 5.

Recently, the design and synthesis of a new fluorescent dye (Scheme 1) having a quadrupolar scheme (1) for the preparation of ultra-bright FONs was published by Blanchard-Desce et al. [23] This dye is a push-push derivative based on a spirofluorene core and terminated by triphenylamine moieties as electron donating end-groups. This dye shows interesting properties, including strong absorption in the violet-blue visible region and intense blue-green emission in solution and the ability to retain fluorescence when aggregated as organic nanoparticles leading to FONs with huge brightness and good photostability. Moreover, when the dye aggregates into nanoparticles in water, its photophysical properties are strongly affected by the molecular confinement. A hypochromic and hypsochromic shift as well as a broadening of the low-energy absorption band is observed which reveals excitonic coupling between dye subunits within FONs. A bathochromic shift of the fluorescence emission spectrum is also observed due to excitonic splitting. As compared to dye 1 dissolved in organic solutions, FONs made from dye 1 show somewhat reduced fluorescence quantum yield (from about 0.7 in organic solutions to 0.15 for FONs in water) which is observed as a result of both the decrease of the radiative rate (in relation with the red-shifted emission and excitonic coupling) and major increase in the non-radiative decay rate (most probably due to efficient vibrational deactivation processes favored by water molecules H-bonded to the nanoparticles surfaces or electron transfer reactions). Yet due to the molecular confinement, huge brightness are obtained (typically 2.2 10 8 M -1 cm -1 for FONs having a radius of about 22 nm) making FONs [1] of major interest for bioimaging purposes. [23] Due to the presence of their electron-donating end-groups, we thus posited that such dyes could also generate interesting ECL properties both as individual dyes and FONs. In that line of thinking, it has already been reported that diarylamino derivatives are able to give stable oxidized species with good ECL capability in organic solvent. [24] Even if the fluorene core is a good fluorescence emitting center, the stability of the oxidized cation radical is poor making it an inefficient ECL luminophore. However, its stability may be improved by modifying it with appropriate redox groups such as thiophene. [25] The choice of suitable π-spacers and terminal groups is crucial since they can directly influence the energies of the highest occupied molecular orbital (HOMO), LUMO, absorption spectrum, and the charge separation upon photoexcitation. For these reasons, we investigated the electrochemical properties of the dye 1 dissolved in organic solution and aggregated as FONs in buffered water. For comparative studies, and in order to gain deeper insight into the origin of these luminescence processes and in the role of the electronic structure of the dye (core, -conjugated system, end-groups), we also investigated related dyes having a similar core but bearing opposite end-groups (i.e. electron accepting groups instead of electron donating endgroups: dye 2) or without end-groups (3). An extended version of 1 having elongated -conjugated system was also investigated (4) as well as a more compact version (without thiophene moieties in the -conjugated system) built from a truxene (i.e. with three fused fluorene units) core having different symmetry (5). We thus synthesized the series of new dyes (Scheme 2) and analyzed their electrochemical and ECL properties in organic solvent. All these features could be of interest for biosensing if combined with the benefits of nano-confinement.

Scheme 2. Synthesis of the dyes 1, 2, 3, [26] 4, and 5.

Results and Discussion

Electrochemistry

The electrochemical properties of all the spirofluorene-based and truxene-based dyes were investigated by using cyclic voltammetry (Figure 1) and the results are gathered in Table 1. These experiments were performed on platinum working electrode in dichloromethane (DCM) that contained 1 mM of each dye and 0.1 M n-tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. As shown in Figure 1, the voltamogramms of 1 exhibits three reversible oxidation waves, at E1,ox = 0.89 V, E2,ox = 1.24 V, and E3,ox = 1.50 V vs. SCE. The peak current of the first oxidation wave is near twice that of the other two waves. This is probably related to a 2electron transfer reaction occurring in the first wave and monoelectron transfer reactions in the next two waves. The first oxidation wave can be attributed to the dication radical formation through the oxidation of the two triphenylamine moieties simultaneously. [4c, 13a, 25] This 2-electron wave which generates triphenylaminium dication derivatives indicates that the electronic communication is negligible between the two terminal triphenylamine groups. [13a, 25a] The influence of the scan rate for oxidation waves was varied in a broad range from 0.05 to 1 V/s (Figure S1) and the study of the current peak shows a linear evolution with respect to the square root of scan rate (Figure S2). This demonstrates a diffusion controlled process on the Pt surface in this potential window for all three well-separated anodic steps. The next two reversible anodic 1-electron waves are consistent with the oxidation of the central spirofluorene core [4c, 13a, 25b] and the cation 1 4+ is stable in solution during at least the time scale of the voltammetric experiments. We have not observed the reduction of 1 in the explored potential window accessible in DCM. However, with the goal of obtaining bright ECL luminophores, the reversible oxidation behavior should allow the ECL generation following the oxidative-reduction pathway with tri-npropylamine (TPrA) as a co-reactant. [15a, 27] Further electrochemical experiments were performed on the dyes 2 and 3 to investigate the influence of the end-groups on the electrochemical properties. We recorded the cyclic voltammogram of the compound 3 which is an intermediate in the synthesis of 1. The cyclic voltammograms in Figure S3 present the irreversible oxidation waves of the thiophene moieties and the first anodic peak potential at 1.26 V vs. SCE. We also tested the dye 2 which is similar to 1 but instead of an electron-donor group (i.e. triphenylamine), the dye is terminated by an aldehyde function which is known to be a good electron-withdrawing group. [28] By changing the nature of the end-group from a donor to an acceptor, one should reasonably expect a shift of both the reduction and the oxidation waves to occur at more positive (or less negative) values. As depicted on figure S4, the cyclic voltammogram of the dye 3 in DCM shows an irreversible oxidation at high potential (Eox = 1.6 V vs. SCE). The results obtained for both 2 and 3 are comparable to the wave observed at 1.31 V vs. SCE for the electropolymerization of thiophene moieties. [29] After only a single scan, polymer films were observed on the Pt sur-face. This kind of electropolymerization is not of interest in present study, but provides the evidence that the two end thiophene moieties exhibit high electrochemical and chemical reactivity in 3 and also 2. Eventually, the drastic change in oxidation behaviors between 1 and both precursors shows that the substitution with the terminal electron-donor triphenylamine moiety in the alpha position prevents its oxidation through these positions and subsequent polymerization. Similarly to 1, dye 4 which presents an alkene connector in between the thiophene moiety and the terminal donating groups (i.e. dibutyl-amino groups) shows a first 2-electron reversible oxidation wave and a one-electron oxidation wave. The first two oxidation potentials (i.e. E1,ox and E2,ox) of 4 are shifted to less anodic potentials by about 300 mV in comparison to 1 (Table 1). Indeed, the butyl chain is known to be a stronger electron-donor moieties compared to the diphenyl. [28] This donor effect may also be reinforced by the alkene function placed between the thiophene groups and the terminal ones. It makes the molecule much easier to oxidize and can stabilize the radical cation by charge delocalization. The subsequent oxidation processes are more complex and reveal the presence of features that are characteristic of irreversibility. They consist of a shoulder at 1.18 V before a mainly irreversible wave at 1.27 V vs. SCE. The irreversibility of 4 for potentials above E2,ox remains so far unclear but it points out to the instability of the corresponding electrogenerated radicals, most probably of the spirofluorene core due to the presence of the alkene moiety which may be responsible for isomerization processes. Whatever, this characteristic may provide some insights into the comparative studies of the ECL efficiency for the different compounds, as detailed below in the ECL section. The electrochemical properties of the electron-donating diphenylamine groups were also examined by studying a different new compound, 5, which mainly consists of three diphenylamine moieties grafted on a truxene core. Interestingly, the cyclic voltammogram of 5 shows three reversible oxidation waves: the first one occurs at 0.83 V vs. SCE and the last two are partially overlapped. Considering the chemical structure, these three waves are consistent with the oxidation of the terminal diphenylamine moieties. The fact that they are partially separated indicates that the electronic communication between the terminal diphenylamine groups is not negligible, in contrast to what as observed for 1. The reversibility points out also to a good stability of the trication 5 3+ . In fact, the reversibility dominates the entire oxidation processes of the analyzed molecules, especially for 1. As already mentioned, the stability of the electrogenerated luminophore radicals is of fundamental importance to produce strong ECL intensity. [22n] Table 1. Electrochemical and photophysical properties of 1, 4 and 5 in DCM.

Compounds

E1,ox (a) E2,ox (a) E3,ox (a) λmax Abs λmax FL λmax ECL EFL (b) ΔG° (c) ΦFL (d) ΦECL (e) [ (g) 1.15 (g) 360 388 - (h) 3.20 0.40 0.40 0.05

[a] Oxidation potentials calculated as the mean value of the redox peaks. [b] EFL, energy of the excited singlet state estimated from fluorescence emission maximum (Figure S5) with Eq. 10. [c] The values of ΔG° were calculated by using Eq. 9. [d] Fluorescence quantum yields in DCM were determined using fluorescein for 1 and using quinine bisulfate for 4 and 5. [e] The ECL efficiency was calculated by comparison with Ru(bpy)3 2+ as a reference (ΦECL = 1). [f] Anodic wave not well-defined with a loss of reversibility. [g] Partially overlapped anodic waves. [h] ECL signal too weak to allow recording the ECL spectrum.

Spectroelectrochemistry

The in situ spectroelectrochemistry studies of 1 was performed in solution to evaluate the spectral changes between their neutral and first oxidized state (i.e. E1,ox). This is of importance for studying in more details the oxidation process and the stability of the radicals which is essential for the ECL efficiency. It allows also assessing their suitability as electrofluorochromic and electrochromic materials. [30] The dye was dissolved in 0.1 M TBAPF6 solution in DCM and the absorption and emission spectroelectrochemical studies were done using a Pt mesh working electrode. Upon the first oxidation potential Eapp = 0.8 V vs. Ag/Ag + , the evolution of the UV-vis and fluorescence spectra with time were collected and depicted on Figure 2. Interestingly, the monitoring of the absorption revealed a nice modification of the electronic transition when the first oxidized state is reached. As shown on Figure 2a, the periphery-to-core intramolecular charge transfer (ICT) band [31] of 1 in DCM at 415 nm vanished upon oxidation and a new red-shifted absorption band appeared at 610 nm in the meantime. The evolution of the fluorescence band at 496 nm is a good mirror image of the absorption one at 415 nm. This observation may be attributed to the reversal of ICT (i.e. core-to-periphery ICT transition) due to the strong electronwithdrawing character of triphenylaminium end-groups generated after loss of electrons from the triphenylamine moieties. [30b] After approaching the equilibrium, an open circuit potential (E = 0 V vs. Ag/Ag + ) was applied to the system. The absorbance recovered with time until the intensity and shape of the curve were as same as the initial state. It confirms the reversibility observed by cyclic voltammetry for the oxidation process. Similar determination was done through fluorescence. The fluorescence intensity showed reversible variation during the oxidation and recovery processes by exciting at 415 nm (Figure 2b). We also excited 1 at the new absorption band that appeared at 600 nm. However, it did not lead to any fluorescence emission during the oxidation process (Figure S6). This is consistent with an excited state having quinoidal character (in relation with strong core-to-periphery ICT) resulting in very low fluorescence quantum yield. Therefore, the absorption and fluorescence properties of 1 also are controlled by these terminal electron-donating triphenylamine groups. Besides, thanks to this totally reversible oxidation behavior, a different method to determine the half-wave potential E1/2 and the number n of electrons for the first oxidation process was applied based on the spectroelectrochemistry studies discussed above. [32] A series of oxidation potential was applied to the thin layer cell in DCM, and individual UV-vis or fluorescence spectra were recorded after applying an anodic potential during the time necessary to obtain equilibrium. As shown on Figure 2, the absorbance and the fluorescence decay in a similar way when imposing more anodic potentials. The absorbance (λmax Abs = 415 nm) and fluorescence (λmax Fl = 496 nm) variations can be related to the ratio of the oxidized state to reduced state at different holding potentials by the following equations:

[R] / [O] = [A2 -A3] / [A1 -A2] (1) Eapp = E1/2 -(0.0591 / n) log([R] / [O]) (2)
where A2 is the absorbance measured at a wavelength at a given potential, and A1 and A3 are the absorbance values of the totally reduced and oxidized species, respectively. According to the Nernst equation, a plot of Eapp vs. log

([R] / [O]
) is linear with a slope 0.0591 / n V and an intercept of E1/2. The potentialresolved UV-vis spectra were shown in Figure 3a where the inset plot represents the profile of Eapp vs.

log([R] / [O]

). The linear fitting gives a slope of -0.028 and an intercept of 0.70 V (Figure 3a, inset), indicating that the number of electrons transferred is 2 and the E1/2 is 0.70 V for the first oxidation reaction. We performed a similar treatment of the potential-resolved fluorescence data with A1, A2 and A3 having the same meaning than above, except that they correspond to the fluorescence intensities measured at 496 nm (Figure 3b). For the fluorescence, a slope of -0.030 and E1/2 = 0.71 V were found (Figure 3b, inset). These results confirmed a two-electron transfer reaction occurring at the first oxidation wave and the stability of the oxidized resulting dications for several minutes. It also demonstrates that the two following oxidation waves correspond to a one-electron transfer reaction. ECL of the luminophores ECL of spirofluorene derivatives was generated essentially via the annihilation mechanism or via the reductive-oxidation pathway by applying a cathodic potential in presence of benzoyl peroxide as a cathodic co-reactant. [4c, 18, 25, 33] In the present work, we exploited the good stability of the oxidized forms of 1 and ECL of the luminophores was thus tested by an anodic sacrificial co-reactant pathway. In this case, TPrA was used as a coreactant in order to follow the "oxidative-reduction" pathway and the excited state is generated after the simultaneous oxidation of TPrA and of the tested luminophore (Figure 4). For this, we imposed anodic potentials in TPrA solution by scanning the potential up to 1.7 V vs. SCE. ECL intensity / a.u.

E / V vs. SCE ECL signal of 1 starts to increase at 0.9 V, reaches a maximum at 1.29 V and then decreases until 1.7 V vs. SCE (Figure 4). ECL emission was very intense and extended over a large potential range compared to the reference compound, Ru(bpy)3 2+ (Figures S7-S9). In addition, it is noteworthy that ECL is generated just after the first 2-electron oxidation wave corresponding to the simultaneous oxidation of the two terminal triphenylamine moieties, but it starts before the second oxidation wave (Figure S10). Since the TPrA co-reactant is already oxidized at 0.8 V vs. SCE (Figures S7-S9), it is not the limiting step in the ECL process. In a similar way, ECL signal of 4 increases at 0.75 V vs. SCE and is composed of two overlapping peaks with a maximum at 1.1 V vs. SCE. Afterwards, it decays very rapidly. The ECL emission occurring at lower anodic potential is consistent with the shift of the first oxidation wave to less anodic potential for 4 in comparison to 1. For both compounds, ECL is thus generated after the two-electron oxidation of the end-capping donor groups, but 200 mV before the second oxidation wave (Figure S10). The different shapes of the ECL responses displayed on Figure 4 can be evidenced by considering the width-at-halfmaximum of the ECL peak which is 450 mV, 380 mV and 180 mV for 1, 4 and Ru(bpy)3 2+ , respectively. The ECL signal extends over a much larger potential range for both spirofluorene dyes in comparison with Ru(bpy)3 2+ . There is thus a clear correlation between the oxidation behavior and both the position and the extension of the potential domain where ECL is generated. An explanation for this intriguing behavior could be that the third oxidation wave contributes partially to the ECL process giving high ECL intensity due to the remarkable stability of the corresponding cationic species. Ding et al. reported the selfenhanced ECL of an iridium-based complex where the ECL efficiency is increased after each oxidation steps. [22j] But, in their case, self-enhancement of ECL involved the oxidation of dimethylamino groups on the bipyridine ligands of the iridium complex which act as self-coreactant. [22j] The cascade of reactions leading in fine to the ECL emission could be described by the following mechanism:

1 -2 e -→ 1 2+ (3) 1 2+ -e -→ 1 3+ (4) TPrA -e -→ TPrA •+ (5) TPrA •+ → TPrA • + H + (6) 1 3+ + TPrA • → 1 2+ * + Im + (7) 1 2+ * → 1 2+ + hν (8)
where Im + is the iminium product.

Both TPrA and 1 (or 4) are oxidized at the electrode surface. Then the cation radical TPrA •+ undergoes a deprotonation step which produces a strong reducing radical TPrA • . This intermediate reacts with the oxidized 1 3+ organic dye to generate its excited state 1 2+ *. Finally, it relaxes to the ground state and emits light. [16,34] The calculation of the ECL efficiency ΦECL is detailed in the Supplementary Information section and the corresponding results are collected in Table 1. ECL signal is recorded by scanning the potential of a glassy carbon electrode from 0 to +1.5 V vs. SCE in a DCM solution containing 0.1 M TPrA and 0.1 M TBAPF6 (Figures S7-S9). The ECL efficiency ΦECL depends mainly on the generation of the excited state (Eq. 7) and subsequent radiative emission to return to the ground state (Eq. 8). Therefore, it is governed by two main contributions that are the thermodynamics of reaction 7 (ΔG°) and the photophysics of the radiative de-excitation (ΦFL). The free energy ΔG° may be calculated from:

ΔG° = E°red(TPrA • ) -E2,ox + EFL (9) EFL = h.c / λmax FL = 1239.81 / λmax FL (10)
where E°red(TPrA • ), the reduction potential of the TPrA neutral radical, has previously been found to have a value of -2.1 V. [16,35] Since ECL is mainly emitted at the level of the second oxidation wave, we employed the corresponding potential E2,ox in Eq. 9. Considering the values of E2,ox and of EFL in Table 1, one can see clearly that reaction 7 provides enough energy for the compounds 1 and 4 to populate the excited state, while it is not energy sufficient for 5. This explains why 5 gives very little or no ECL. 1 showed very bright ECL and its efficiency value is 4.5time greater than the reference compound, Ru(bpy)3 2+ . The value obtained is remarkably high as compared to those observed in other organic ECL dyes and fluorene compounds. [4c, 18, 25, 33, 36] From an energetic point of view, the reaction between the dication radical (1 2+ ) and TPrA • would also be exergonic enough to populate the first singlet excited state. Indeed, if we use the value of E1,ox in Eq. 9, the available energy is sufficient by about 0.14 eV and 0.08 eV for 1 and 4, respectively, to populate the excited state and to generate ECL. It means that there is another photochemical process which quenches the ECL emission until both donor groups are oxidized. Since the fluorescence quantum yield are relatively similar for 1 and 4 (0.71 and 0.67, respectively), the important difference for the ECL efficiency between both dyes may be due to the more exergonic reaction generating the excited state and to the better stability of the oxidized cation radicals. Reaction 7 is actually energy sufficient by about 0.49 eV and 0.42 eV according to Eq. 9 for 1 and 4, respectively. This slightly higher available energy cannot justify a so important difference for the efficiency of the ECL process. But, the better stability of the oxidized forms [22n] explains this remarkable characteristic, as seen in the voltammetric studies.

Thanks to their strong ECL emission, ECL spectra for 1 and 4 were recorded and shown on Figure 5. ECL signal of 5 was too low to allow recording its ECL spectrum. By comparison with the corresponding fluorescence spectra, a red-shift was observed in ECL spectra, 7 nm for 1 and 3 nm for 4, respectively. Such a red-shift is common in ECL and caused by the inner filter effect due to the high concentration of chromophores used in ECL experiments. [25b, 37] Nonetheless, these results confirmed that the same excited states were produced by photochemical and electrochemical excitation.

Electrochemistry and ECL of the FONs

Fluorescent Organic NPs, and more particularly fluorene-based NPs, [13a, 37a, 38] have been the subject of relatively few reports describing their electrochemical and ECL properties compared to inorganic NPs. [22a, 22c, 22d, 39] To investigate their characteristics in comparison with the molecular compounds, FONs were prepared in aqueous solution using the nano-precipitation method [START_REF] Masuhara | Single Organic Nanoparticles[END_REF] according to a reported procedure. [23] More precisely, FONs were obtained by fast addition of a minute amount of a stock solution of the corresponding dye in tetrahydrofuran (THF) into a large amount of water under sonication. Instantly after addition, colored non-turbid solutions are obtained that show intense absorption in the near-UV violet region, indicating the formation of NPs. Furthermore, under UV irradiation, FONs suspensions in water emit strong fluorescence in the visible region (Figure S5), thus attesting the formation of FONs. FONs [1] emit in the green region while FONs [4] emit in the yellow region leading to ultra-bright nanoparticles (Table 2). The elongated -conjugated system of dye 4 is responsible for the redshifted absorption and emission of FONs [4] as compared to those made from dye 1. Both FONs exhibit good colloidal and structural stability in water as evidenced by monitoring over time the evolution of the absorption and fluorescence spectra (Figure S13) The FONs size and morphology (spherical shape) was characterized by Transmission Electron Microscopy (Table 2).

The good colloidal stability of the FONs is further attested by their large negative zeta potentials. We note that the hydrated radius values determined by Dynamic Light Scattering in water are somewhat larger than those measured for "naked" nanoparticles by TEM. This difference can be ascribed to the highly negative surface potentials of both FONS responsible for the presence of a shell of H-bonded water molecules. [23] The thickest bound water shell is obtained for the FONs having the more negative potential (i.e. FONS [1]). We observe that the fluorescence quantum yields of the FONs are significantly reduced in comparison to that of individual dyes in DCM solution (see Ta-bles 1-2). This can be related to both decrease of the radiative decay rate (in relation with excitonic coupling between dye subunits within FONs) and increase in non-radiative decay rate. This can be related to the presence of water molecules strongly bonded to the FONs surface which provide efficient nonradiative vibrational decay channels. Yet both FONs retain fluorescence and due to their huge absorption coefficient (Table 2), they show giant brightness, typically orders of magnitude larger than those of QDs or even dye-doped silica nanoparticles. [START_REF] Bonacchi | [END_REF] Since FONs [1] and [4] are stable in water, the electrochemical characterization of these FONs was performed in 100 mM PBS solution (pH 7.4). Differential Pulse Voltammetry (DPV) of FONs [1] dispersed in buffer solution shows two oxidation peaks at 0.99 V and 1.20 V vs. SCE and one small shoulder at 1.33 V vs. SCE (Figure 6b). This last one is not well-defined because water oxidation starts in the same potential region. As already mentioned, 1 exhibits three oxidation waves, at E1,ox = 0.89 V, E2,ox = 1.24 V, and E3,ox = 1.50 V vs. SCE. So there is a good agreement for 1 between its molecular and NPs forms. In addition, the current of the first anodic peak is near twice that of the second one for FONs [1]. It corresponds thus to sequential 2-electron and 1-electron transfer reactions for the first two peaks, as demonstrated by cyclic voltammetry and spectroelectrochemistry. It is more difficult to determine for the third peak due to its shape. In any case, the DPV results show that both the triphenylamine terminal moieties and the spirofluorene core are electrochemically-accessible in the FONs [1]. DPV of FONs [4] exhibits only a one small oxidation wave which starts at 0.9 V and finishes at 1.3 V vs. SCE (Figure S11a). Yet, although DPV is a differential technique allowing to resolve peak current better than cyclic voltammetry, it is difficult to identify specific peaks and to attribute them in that particular case. Then ECL emission was generated in the buffer solution containing both the FONs and TPrA when the electrode potential was scanned from 0 to 1.5 V vs. SCE. As detailed in the cascade of reactions 3-8, the oxidized NPs reacted with the strong reducing radical TPrA • to populate the excited state and eventually emit the ECL signal. As shown on Figure 6c for FONs [1], ECL intensity started to increase at 0.95 V and reached a plat-eau up to 1.5 V vs. SCE. FONs made from 1 thus behave in water as 1 in organic solution and generate ECL after the oxidation of the triphenylamine terminal moieties. It means that FONs 2+ react with the neutral TPrA • radical to generate the excited state in the NPs. For FONs made from dye 4, after a similar initial increase, the ECL intensity decreases very rapidly (Figure S11b). This behavior is consistent with the ECL study of the molecular species where ECL of 4 decreases rapidly whereas it lasted over a larger potential range for 1.

The ECL signal of FONs [1] was strong enough to allow recording the ECL spectrum (Figure 6d). Comparison with the corresponding fluorescence spectrum clearly shows the similarity between photoluminescence and ECL spectra (Figure 6d), confirming that the same excited state is reached via both excitation methods. Unfortunately, ECL intensity of FONs [4] is too weak to allow acquiring an ECL spectrum for these FONs. This emphasizes that the choice of suitable dye subunits is critical to achieve stable and ultrabrigth FONs also capable to generate ECL in water. As such, the fully organic nanoparticles easily prepared from the quadrupolar spirofluorene dye having triphenylamine terminal moieties 1 offer major promises for bioimaging and sensing purposes. (a) [nm]

dh (b) [nm]

ζ (c) [mV]

λmax Abs [nm] εmax [10 9 M -1 cm -1 ] λmax FL [nm]
Φf (d) εmax Φf (e) [10 8 M -1 cm -1 ] λmax ECL

Conclusions

We synthesized and characterized a series of blue to yellow emitting symmetric luminophores built from a spirofluorene core connected to terminal donor or acceptor groups via -conjugated linkers having thiophene moieties. Only quadrupolar derivatives having donor end-groups (luminophores 1 and 4) show reversible oxidation waves. In contrast, a related dye based on the direct grafting of three diarylamino onto a truxene core shows three successive one-electron oxidation steps showing that the nature of the core and conjugated system influence the communication between the electroactive donor peripheral moieties. Interestingly only quadrupolar derivatives having donor endgroups exhibit intense ECL. The most efficient ECL is achieved with green-emitting 1 which presents three reversible oxidation waves with highly stable oxidized states (1 2+ , 1 3+ , 1 4+ ). The π conjugation with the thiophene direct connector allows the charge delocalization and the stabilization of the electrogenerated cation radicals. The stability of the oxidized radicals gives remarkably high ECL efficiency for this luminophore using TPrA as a co-reactant. It is 4.5-time greater than that of the model Ru(bpy)3 2+ compound with a very intense and large ECL signal which may involve the cumulative contribution of the higher oxidized states. In addition, we found that ECL is only emitted after the oxidation of the end-capping (triarylamino) donors. Redox and ECL behaviors of the FONs are consistent with the initial molecular species. The molecular confinement of these luminophores within fluorescent nanoparticles does not change the electrochemical and ECL properties of the FONs which remain very bright nano-emitters. In conclusion, the investigated molecules demonstrate the possibility to design highly efficient green-emitting molecular species and organic NPs for ECL applications.

Experimental section

Materials

Compounds A, [42] 1 [23] and D [43] were prepared according to literature procedures.

General methods

Commercially available reagents were used without further purification. Dry solvents were distilled from the appropriate drying reagents prior to use. All air-or water-sensitive reactions were carried out under argon. Reactions were monitored by performing thin-layer chromatography on Merck silica gel precoated aluminium sheets (60F-254). Column chromatography was performed using Fluka silica gel Si 60 (40-63 µm, 230-400 mesh). Melting points were determined using a Kofler bench. IR spectrum of 2 was recorded on a Perkin Elmer Spectrum 100 Optica. 1 H and 13 C NMR spectra were recorded on a Bruker Avance I 300 spectrometer at 300 MHz and 75 MHz respectively or on a Bruker Avance II 400 MHz at 400 MHz and 100 MHz respectively. 1 H chemical shifts () are reported in ppm relative to TMS as internal standard, coupling constants (J) are given in Hertz, 13 (26.81 g, 477.81 mmol) in water (26.8 mL), then stirred at 75°C for 30 min. A solution of 2,7diiodofluorene (4.00 g, 9.57 mmol) in toluene (26.0 mL) was added and the reaction mixture was stirred at 75°C for 2 h. After addition of water, extraction with CH2Cl2, drying over MgSO4, the organic layer was filtered through a short pad of silica and evaporated. The crude compound was recrystallized by slow cooling of a saturated solution in hot heptane to yield 3.62 g (80%) of B as pale cream crystals; 1 H NMR (CDCl3, 300 MHz):  = 7.73 (d, J = 1.5 Hz, 2H), 7.64 (dd, J = 1.5 Hz, J = 7.9 Hz, 2H), 7.40 (d, J = 7.9 Hz, 2H), 2.07 (m, 8H); 13 C NMR (CDCl3, 75 MHz):  = 156. 1, 138.2, 136.0, 132.3, 121.5, 93.4, 57.8, 39.7, 27.1; HRMS (ESI/ASAP) calcd.for C17H14I2 (M + ) m/z 471.9185; found 471.9192; m.p. = 172°C; Rf = 0.6 (DCM/heptane; 1:9).

2',7'-Bis(5-formyl-thiophen-2-yl)spiro[cyclopentane-1,9'-fluorene]

(2): Air was removed from a solution of 2 (512 mg, 1.08 mmol), 5-formyl-2-thienylboronic acid (634 mg, 4.06 mmol), PdCl2dppf (14 mg, 0.02 mmol) and K2CO3 (1460 mg, 10.56 mmol) in 30 mL of toluene/MeOH (2:1) by blowing argon for 30 min. Thereafter, the mixture was stirred at 75°C for 20 h. The resulting mixture was poured into water (30 mL) and Et2O (30 mL). The organic layer was dried over Na2SO4, filtered trought a short pad of silica (using Et2O as eluent) and the solvent was evaporated. The resulting powder was recrystallized by slow cooling of a saturated solution in hot heptane to yield 449 mg (94%) of 2 as yellow cristals. 1H NMR (CDCl3, 300 MHz): = 9.91 (s, 2H), 7.77 (d, J = 3.9 Hz, 2H), 7.76 (d, J = 7.9 Hz, 2H), 7.73 (d, J = 1.7 Hz, 2H), 7.69 (dd, J = 7.9 Hz, J = 1.7 Hz, 2H), 7.47 (d, J = 3.9 Hz, 2H), 2.20 (m, 8H); 13C NMR (CDCl3, 75 MHz):

= 182. 7, 155.7, 154.7, 142.3, 140.0, 137.4, 132.8, 125.6, 124.0, 121.0, 50.0, 39.8, 27; FTIR (cm-1) H 4.58; found C 73.24, H 4.47; m.p. > 265°C.

2',7'-Di(2-thienyl)spiro[cyclopentane-1,9'-fluorene] (3):

A solution of 2-thienyl magnesium bromide in THF (0.3 M, 50.0 mL) was added dropwise on a mixture of B (1.55 g, 3.28 mmol) and PdCl2dppf (0.09 g, 0.11 mmol) and the reaction mixture was then refluxed for 48 h. After addition of ice, extraction with Et2O, drying over MgSO4, and filtered through a short pad of celite, the solvent was evaporated. The crude compound was recrystallized by slow cooling of a saturated solution in hot heptane to yield 0.94 g (75%) of 3 as pale crystals. 1 H NMR (CDCl3, 300 MHz)): 7.68 (d, J = 7.9Hz, 2H), 7.66 (d, J = 1.6 Hz, 2H), 7.60 (dd, J = 7.9 Hz, J = 1.6 Hz, 2H), 7.37 (dd, J = 3.6 Hz, J = 1.1 Hz, 2H), 7.30 (dd, J = 5.1 Hz, J = 1.1 Hz, 2H), 7.11 (dd, J = 5.1 Hz, J = 3.6 Hz, 2H), 2.19 (m, 8H); 13 C NMR (CDCl3, 75 MHz):  = 155. 4, 145.2, 138.7, 133.9, 128.2, 125.1, 124.7, 123.1, 120.6, 120.1, 57.9, 40.0, 27.2; m.p. = 196°C.

2',7'-Bis(5-iodo-thiophen-2-yl)spiro[cyclopentane-1,9'-fluorene] (C):

In a round flask, was added N-iodosuccinimide (369 mg, 1.64 mmol) to a solution of 3 (300 mg, 0.78 mmol) in 4 mL of CHCl3/AcOHgl (4:1). Then the solution was quickly warmed to 90°C (the oil bath was heated at this temperature) for 5 min exactly. After, CHCl3 was evaporated and the crude was triturated in glacial acetic acid and filtered. The precipitate was washed with ethanol to give 506 mg (quant.) of product C as a yellow powder. 1 H NMR (CDCl3, 400 MHz):  = 7.66 (dd, J = 7.9 Hz, J = 0.5 Hz, 2H), 7.55 (s, 2H), 7.50 (dd, J = 7.9 Hz, J = 1.6 Hz, 2H), 7.26 (d, J = 3.8 Hz, 2H), 7.02 (d, J = 3.8 Hz, 2H), 2.17 (m, 8H); 13 C NMR (CDCl3, 100 MHz,):  = 155. 5, 151.1, 139.0, 138.1, 133.2, 125.0, 124.6, 120.4, 120.3, 72.2, 57.9, 40.0, 27.2; HRMS (FD) calcd. for C25H18I2S2 (M + ) m/z 635.89393; found 635.89411.

2',7'-Bis[5-[(1E)-2-[4-(N,N'-dibutylamino)phenyl]ethenyl]-thiophen-2yl

]spiro[cyclo-pentane-1,9'-fluorene] (4): tBuOK (149 mg, 1.33 mmol) was added to a solution of 2 (149 mg, 0.34 mmol) and 4-(N,N'dibutylaminobenzyl)triphenylphosphonium iodide (413 mg, 0.68 mmol) in CH2Cl2 (4 mL). The mixture was stirred at r.t. for 24 h. The resulting mixture was filtered through a short pad of celite and evaporated. The crude product was purified by column chromatography (heptane/CH2Cl2, 2:1) to yield 202 mg (71%) of 4 as a yellow powder. 1 H NMR (CDCl3, 200 MHz):  7.69 (d, J = 7.9 Hz, 2H), 7.66 (d, J = 1.7 Hz, 2H), 7.60 (dd, J = 7.9 Hz, J = 1.7 Hz, 2H), 7.28 (d, J = 3.8 Hz, 2H), 6.97 (d, J = 3.8 Hz, 2H), 6.87-7.00 (AB, JAB = 16.0 Hz, 4H), 6.62-7.32 (AA'XX', JAX = 8.9 Hz, 8H), 3.30 (dd, J = 7.9 Hz, J = 7.3 Hz, 8H), 2.20 (m, 8H), 1.57 (m, 8H), 1.38 (m, 8H), 0.96 (t, J = 7.3 Hz, 12H); 13 C NMR (CDCl3, 75 MHz): 155.4, 148.0, 143. 7, 142.1, 138.5, 133.9, 128.9, 127.8, 125.8, 124.6, 124.2, 123.5, 120.0, 117.2, 111.8, 57.7, 50.9, 40.1, 29.6, 27.5, 20.5, 14.2; HRMS calcd. for C57H66N2S2 (M + ) m/z 842.4661; found 842.4667; elemental analysis calcd. (%) for C57H66N2S2 (843.29 g/mol): C 81. 19, H 7.89, N 3.32, S 7.60; found C 81.11, H 7.83, N 3.32, S 7.57; m.p. = 181°C; Rf = 0.6 (heptane/CH2Cl2, 2:1). 2,7,12-tri(diphenylamine)-5,5',10,10',15,15'-hexabutyltruxene (5): A mixture of 2,7,12 triiodo-5,5',10,10',15,15'-hexabutyltruxene (2.86 g, 2.70 mmol), diphenylamine (2.4 g, 14.18 mmol), potassium carbonate (5.60 g, 40.50 mmol), copper powder (0.10 g, 1.56 mmol) and 18-crown-6 (0.04 g, 0.15 mmol) in o-dichlorobenzene (25 mL) under Ar was heated to reflux for 60 h. After cooling the reaction mixture was diluted with CH2Cl2 (100 mL) and filtered over a Celite pad. The solvents were evaporated to dryness and the residue was dissolved in petroleum ether for flash chromatrography. The suspension was poured on a small silica pad (10 cm) and rinsed with petroleum ether in order to remove remaining odichlorobenzene. Then the small column was eluted with CH2Cl2 and the solvent was evaporated. In order to remove unreacted diphenylamine (same Rf as compound 5) the solid was recrystallized in a 1:1 heptane/ethyl acetate mixture (100 mL). 2.56 g of 5 were obtained as a offwhite powder (77%). 1 H NMR (CDCl3, 300 MHz):  8.35 (d, J = 8.7 Hz, 3H), 7.31 (m, 12H), 7.21 (m, 15H), 7.05 (m, 9H), 2.81 (m, 6H), 1.89 (m, 6H), 0.96 (m, 12H), 0.62 (m, 12H), 0.57 (t, J = 7.2 Hz, 18H); 13 C NMR (CDCl3, 75 MHz): 155.0, 147.9, 146.0, 143.3, 137.9, 135.3, 129.2, 125.3, 124.1, 122.5, 121.9, 117.7, 55.3, 36.3, 26.6, 22.9, 14.0; HRMS calcd. for C87H93N3 (M + ) m/z 1179.73695; found 1179.73884; elemental analysis calcd (%) for C87H93N3 (1180.72 g/mol): C 88.50, H 7.94, N 3.56; found C 88.31, H 8.07, N 3.39; m.p. = 263°C.

Photophysical characterization

All photophysical properties were analyzed with freshly prepared air equilibrated solutions at room temperature (293 K). UV/Vis absorption spectra were recorded using a Jasco V-570 spectrophotometer. Steadystate fluorescence measurements were performed on dilute solutions (optical density < 0.1) contained in standard 1 cm quartz cuvettes using a Horiba (FluoroLog or FluoroMax) spectrometer in photon-counting mode.

Fully corrected emission spectra were obtained for each compound at ex = abs max with an optical density at ex ≤ 0.1 to minimize internal absorption. Fluorescence quantum yields were measured according to literature procedures [44] using fluorescein in 0.1 M NaOH (f = 0.9), quinine bisulfate in 0.5 M H2SO4 (f = 0.54) or cresyl violet in MeOH (f = 0.54).

The molar decadic absorption coefficients of FONs [1] and [4] in water were calculated from the FONs concentrations derived from the total dyes concentrations in water and the average number of dye subunits per FONs (N = 29 000 for 1 and N = 29 700 for 4). The average number of dye subunits per FONs was estimated based on the FONS diameter determined via TEM.

Preparation of the FONs [1] and [4]

The nanoparticles were prepared using the nano-precipitation method which consists of an instant addition of a solution of 1 or 4 in THF (1 mM) in distilled water under sonication at 10 W for 3 min (using a VibraCell 1300 W sonicator probe of 6 mm of diameter). The total amount of THF in water is set to be lower than 1%. Quickly after the addition, the solution evolved from colorless to yellow.

Characterization of the FONs [1] and [4]

TEM imaging was carried out using a HITACHI H7650. Copper grids coated with a carbon membrane were pre-treated using the Glow discharge technique to yield to positively charged grids thus helping the interaction between the FONs and the grid. One droplet of the aqueous FONs suspension was deposited on the grid followed, once the excess of liquid was drown-off with paper, by a staining procedure using uranyl acetate. DLS experiments were performed using a SZ-100Z Horiba instrument operating at 173°. Hydrodynamic diameters (dh) were calculated from diffusion coefficients using the Stokes-Einstein equation. All correlogram analyses were performed using the software supplied by the manufacturer and are represented in scattering light intensity. Zetapotential analysis was performed with the SZ-100Z Horiba instrument. Several measurements were realized for each sample according to a predefined operating procedure.

Electrochemical characterization

Cyclic voltammetric experiments were performed with a PGSTAT30 Autolab potentiostat connected to a conventional three-electrode cell, consisting in a silver-wire quasi-reference electrode, a platinum-wire counter electrode, and a platinum working electrode. Potential calibration of the silver-wire was performed using a standard Fc/Fc + redox couple (taken as 0.342V vs. SCE [45] ) after the measurements. For the study of FONs [1], a glassy carbon (GC) electrode and a Ag/AgCl electrode were used as working electrode and reference electrode, respectively. But the potential was presented by referring to SCE. Prior to measurement, both Pt disk and GC electrode were polished with alumina slurry of different size, rinsed thoroughly with Milli-Q water between each polishing step, and sonicated in water and ethanol respectively, followed by a final rinse with acetone and dried with N2 stream. Spectroelectrochemical spectra were recorded on the Cary 100scan UVvisible spectrophotometer and Cary Eclipse spectrophotometer with a µ-Autolab type III potentiostat. Herein, spectroelectrochemical cell was constructed by loading a transparent Pt grid working electrode into 1mm path length thin layer channel in the quartz cuvette and mounting the other two electrodes above the channel.

ECL intensity was measured using a Hamamatsu photomultiplier tube R5070 with a Hamamatsu C9525 high voltage power supply. The PMT detector was held at -750 V and placed a few millimeters in front of the working electrode. The output signal was amplified by a Keithley 6485 Picoammeter before acquisition via the second input channel of the PGSTAT30 AUTOLAB potentiostat. ECL spectra were recorded using a Princeton Instruments Acton SpectraPro 2300i after the CCD camera cooled to -115°C with liquid N2. The electrochemical cell is built with a glass slide on the bottom in order to record the ECL signal. The optical fiber connected to the device is placed close to this glass slide in front of the working electrode.

Figure 1 .

 1 Figure 1. Cyclic voltammograms of (a) 1mM solution of 1, (b) 1 mM solution of 4, and (c) 1 mM solution of 5 in DCM with 0.1 M TBAPF6 as supporting electrolyte. A Pt disk and a Ag wire were used as working electrode and pseudoreference electrode, respectively. The potential was calibrated with Fc/Fc + (0.342V vs. SCE). Scan rate: 100 mV/s.

Figure 2 .

 2 Figure 2. Time-resolved (a) UV-vis and (b) fluorescence spectra (Φex = 415 nm) for 1 under oxidation at 0.8 V vs. Ag/Ag + and electrochemical restoration by applying E = 0 V vs. Ag/Ag + in DCM solution containing 0.1 M TBAPF6.

Figure 3 .

 3 Figure 3. Evolution of the (a) UV-vis and (b) fluorescence spectra (λex = 415nm) for 1 when applying different potentials during the first oxidation process in DCM solution containing 0.1 M TBAPF6. Inset figures are the plot of Eapp vs. log([Red]/[Ox]).

Figure 4 .

 4 Figure 4. Comparison of the ECL signals for (a) 1 (blue line), (b) 4 (green line) and the reference Ru(bpy)3 2+ compound (red line). ECL was recorded with a glassy carbon electrode in DCM solution containing 1 mM luminophore, 0.1 M TBAPF6 and 100 mM TPrA. Scan rate: 100 mV/s.

Figure 5 .

 5 Figure 5. Normalized fluorescence (black line) and ECL spectra (red line) for (a) 1 and (b) 4 in DCM. ECL spectra were recorded in DCM solution containing 0.1 M TBAPF6 with 100 mM TPrA as a co-reactant.

Figure 6 .

 6 Figure 6. (a) TEM image of FONs [1] dispersed in water. (b) DPV of 0.4 nM FONs [1] in 100 mM PBS (pH 7.4) solution. (c) Voltammetric and ECL curves of 0.4 nM FONs [1] in 100 mM PBS (pH 7.4) containing 100 mM TPrA. Scan rate: 100 mV/s. (d) Fluorescence and ECL spectra of FONs [1] in 100 mM PBS (pH 7.4) solution with 100 mM TPrA.

  Diameter of the FONs determined by TEM; (b) Hydrodynamic diameter derived from DLS experiment; (c) Zeta potential; (d) Fluorescence quantum yield were determined using fluorescein* or cresyl violet **; (e) Brightness.

  C chemical shifts are given relative to the central peak of CDCl3 at 77.0 ppm. High-and low-resolution mass spectra measurements were performed at the Centre Régional de Mesures de l'Ouest (C.R.M.P.O., Rennes) by using QTOF II or MicroTOF-Q II for B, 2 and 4 and at CESAMO (Bordeaux, France) for 3 and 5. Elemental analyses were performed at CESAMO (Bordeaux, France) or at I.C.S.N-C.N.R.S. (Gif-sur-Yvette, France). Synthesis 2',7'-Diiodospiro[cyclopentane-1,9'-fluorene] (B): A mixture of nBu4NBr (0.21 g, 0.65 mmol) in 1,4-dibromobutane (12.0 mL, 100.60 mmol) was mixed to a solution of KOH

Table 2 .

 2 Structural and luminescence (fluorescence and ECL) characteristics of FONs

	dTEM

Acknowledgements H. L. acknowledges the China Scholarship Council for his PhD fellowship. M.B.-D. acknowledges financial support from the Conseil Régional d'Aquitaine (Chair of Excellence to M.B.-D. and fellowship to J.D.). This study was achieved within the context of the Laboratory of Excellence TRAIL ANR-10-LABX-57. TEM microscopy was done in the Bordeaux Imaging Center (UMS 3420 CNRS -University of Bordeaux / Inserm US4). The help of Sabrina Lacomme is acknowledged.