Concentration and Solvent Effects on the Excited State Dynamics of

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Concentration and Solvent Effects on the Excited State Dynamics of the Solar Cell Dye D149 - The Special Role of Protons Ahmed M. El-Zohry, and Burkhard Zietz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400782g • Publication Date (Web): 11 Mar 2013 Downloaded from http://pubs.acs.org on March 11, 2013

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The Journal of Physical Chemistry

Concentration and Solvent Effects on the Excited State Dynamics of the Solar Cell Dye D149 - The Special Role of Protons Ahmed M. El-Zohry and Burkhard Zietz* Department of Chemistry - Ångström Laboratories, Box 523, SE-751 20 Uppsala, Sweden *[email protected]

Abstract D149 is one of the best-performing metal-free, organic dyes for dye-sensitised solar cells. Excited state lifetimes strongly depend on solvent and have previously been reported to be between 100 and 700 ps, without any mechanistic explanation being given. We have earlier shown that photoisomerisation is one of several deactivation processes. Here, we report that lifetimes in certain solvents depend on concentration, even in very dilute (nanomolar) solutions. A detailed investigation of the concentration dependence enables us to assign a second, faster deactivation channel besides isomerisation that reduces lifetimes further: a ground-state, hydrogen-bonded 1:1 complex of D149 with acids or interaction with protic solvents leads to excited state quenching, most probably through excited state proton transfer. This includes self-quenching caused by D149's own carboxylic group through intermolecular interaction, accounting for the concentration-dependent lifetimes. We are now able to dissect the complex excited state behaviour into its components, allowing us to attribute rate constants to the isomerisation and the excited-state proton transfer process. We are also able to explain the excited state of D149 in a wide range of environmental conditions, in the presence of acids/bases, at different concentrations as well as with varying temperatures. Furthermore, we determine the barrier for isomerisation, a thermally activated process. The consequences of these effects on solar cells are discussed. Also we show that ultrafast techniques like femtosecond pumpprobe and upconversion inherently do not provide the required responsiveness for work with the concentration ranges required here, whereas single photon counting with its ultimate sensitivity is able to resolve the underlying processes.

Keywords Indoline dye, D149, D102, photo-isomerisation, dye-sensitised solar cells, DSSC, DSC, excited state proton transfer, ESPT, concentration dependent lifetimes, self-quenching

1. Introduction Dye-sensitised solar cells1,2 (DSSCs) convert sunlight into electricity using small dye molecules adsorbed on the surface of a transparent semiconductor, such as TiO2 or ZnO. The highest results have been achieved with metal-organic absorbers, exceeding 12 % conversion efficiency3. Organic 1

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dyes have slowly been catching up with the classical ruthenium dyes that have long set the standard for highly efficient cells. Indoline derivatives4,5 are an interesting class of compounds, giving up to 9.5% efficiency,6 the record for organic dyes at the time. They have meanwhile been slightly exceeded by a triphenylamine dye reaching 10.0-10.3%.7 Metal-free, pure organic dyes can be easily synthesised in large amounts once a suitable synthetic route is available and do not require any poisonous or expensive precious metals. This makes them ideal candidates for large-scale production of cheap solar cells, given that other obstacles can be overcome. To design perfect dyes, undesirable competitive routes need to be excluded or limited, requiring an in-depth understanding of the excited state photochemistry and -physics, strengthening the desire to fully elucidate the behaviour after photoexcitation. Dyes for DSSCs typically feature an electron-rich centre which serves as electron donor and an acceptor unit that either acts itself as anchor or is close to the anchoring group. Upon absorption of light, i.e. electronic excitation, electron density is moved from the donor to the acceptor unit and therefore closer towards the semiconductor surface, facilitating electron injection. D149 (see Figure 1 for structure) is a donor-acceptor dye with an electron-rich indoline group as a donor moiety and two rhodanine rings that serve as acceptors. D149 is one of the most efficient pure-organic dyes and has given a conS

N O N

S

S

tigated theoretically with time-dependent density functional theory (TD-DFT)8,9 as well as experimentally with time-

N COOH O

Figure 1 Structure of D149

version efficiency of 9%.6 Its excited state had been inves-

resolved methods.9-12 The nature of the S1 state was shown to be of charge-transfer character with a main contribution from the HOMO→LUMO, while the S2 state was dominat-

ed by HOMO–1→LUMO transition. The HOMO is delocalised over the central part of the molecule, including the indoline nitrogen, while the LUMO has large contributions around the rhodanine rings. This leads to an effective charge shift towards the anchoring group, enabling efficient electron injection into the semiconductor. The same study8 also estimated the (in vacuo) radiative lifetime using the DFT-calculated oscillator strength and the Einstein transition probability formula and obtained a value of 3.2 ns. On the experimental side, a fluorescence upconversion study showed as longest decay times 630 ps (46% amplitude) for toluene and 220 ps (33%) for acetonitrile, with faster components due to solvent (and possible intramolecular) relaxation. However, the accuracy of these values has been questioned10,12 due to the short measurement range used in the experiments. A rigorous pump-probe study gave values of 280±10 ps for acetonitrile. We have previously determined the decay time of D149 in C6H6 and in toluene to 310±10 ps.12 We also observed a concentration dependence for sev2


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eral solvents, most visibly for acetonitrile, but also for benzene. Fakis et al. had measured two different concentrations in acetonitrile (10 µM and 100 µM) which, as we will show below, are both too large to show major concentration dependence, as they both fall in the high-concentration range. 1.1. Isomerisation The estimated radiative lifetime of D149 of 3.2 ns is much larger than the measured lifetimes of hundreds of picoseconds. Neither the transient absorption study10 nor the upconversion studies9,11 tried to give mechanistic explanations for the reduced lifetimes of D149 compared to the theoretical and more typical lifetime value of a few nanoseconds. We have shown12 that the experimentally determined natural lifetime of ca. 2.5 ns can in fact be approached when embedding D149 in a plastic matrix where large-scale motion is blocked or limited. In solution, on the contrary, where the molecule is free to rotate, our previous study showed that isomerisation around the central double bond (linking the indoline phenyl ring to the rhodanine units) takes place upon excitation into the S2 band.12 In general, excited state isomerisation can occur on the femtosecond scale, as is the case in e.g. retinal13 or bilirubin.14 The lifetimes observed for D149 suggest an activated process including a barrier. This should manifest itself by a reduced lifetime with increasing temperature (more thermal energy to cross the barrier). We had reported a temperature dependence of the lifetimes of D149 in acetonitrile.12 As the measured decays often followed bi- or multiexponential laws, it was not possible at that time to determine a barrier for the isomerisation. 1.2. Aggregation Previous fluorescence upconversion results11 of D149 in a plastic matrix showed fast decay components of 11 ps (41%) and 210 ps (59%) that were quite certainly affected by the high concentration of dye used in the experiments. We have shown that the fast processes observed at higher concentrations are slowed down at least one order of magnitude when using very dilute films. The same effect of aggregation was also seen in experiments involving mesoporous, noninjecting films of ZrO2. The upconversion study,11 which did not use any co-adsorbent and Al2O3 as noninjecting, high-band gap semiconductor, measured a main decay parameter of 2.3 ps (57%). This can again be attributed to aggregation of D149.15 The effects of aggregation are also seen in complete solar cells, which is the reason why a coadsorbent was used to minimise these effects. 1.3. Role of Protons While the effect of aggregation is only observable in solids (matrix or mesoporous films), isomerisation is a competitive pathway for excited molecules in solution and will lead to a reduction of observed lifetimes. However, the measured results for a wide range of solvents cannot be explained merely by isomerisation competing with the natural lifetime, i.e. fluorescence. As Lohse et al. already pointed out,10 no direct connection between lifetime and solvent polarity was observed: MeOH and acetonitrile have about the same polarity, but quite different lifetimes of 99 and 280 ps, respectively. Ethanol, despite being less polar than acetonitrile, gave a shorter lifetime of 178 ps. 3



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Also, the concentration dependence of lifetimes requires a different explanation. We will show that protons play a special role in the excited state deactivation of D149 and related dyes. These can either come from protic solvents (such as alcohols) or from acids, including D149 itself. This explains the observed concentration dependence of lifetimes down to the nanomolar range. We will discuss the role of protons for D149, including the situation in complete solar cells. Having separated the role of protons from the process of isomerisation, we can attribute rates to each of these processes and also determine a barrier for the activated isomerisation process.

2. Materials and Methods 2.1. Chemicals D149 and D102 were obtained as a kind gift from Masakazu Takata, Mitsubishi Paper Mills and used as received. Identity and purity, including isomeric purity, were confirmed by means of NMR spectroscopy. The solvents, benzene (Merck, p.a.), acetonitrile and CHCl3 (both Sigma-Aldrich, spectrophotometric grade), methanol (MeOH, Sigma-Aldrich, Chromasolve), tetrahydrofuran (THF, Riedel de Haën, p.a.), 2-propanol (i-PrOH, Fluka analytical), 3-methoxyproponitrile (>99%, Fluka) and dimethylsulphoxid (DMSO, anhydrous, 99.9%, Aldrich) where used without further purification. 1,8-diazabicyclo[5.4.0]undec-7-ene (DABCU, puriss., ≥99%) and chloroacetic acid (puriss., ≥99%) were purchased from Fluka. 2.2. Steady-State Spectroscopy Absorption spectra were measured on a Varian Cary 5000, emission measurements were performed using a Horiba Jobin Yvon Fluorolog and automatically corrected for wavelength dependent instrument sensitivity. Measurements were carried out in a 1 cm cuvette, and at right angle to the excitation in case of emission measurements. 2.3. Time-Correlated Single Photon Counting (TC-SPC) A detailed description of the experimental set-up has been given recently.12 Briefly, tThe sample was excited with a picosecond diode laser (Edinburgh Instruments, EPL405) at 404.6 nm (77.1 ps pulses). Certain measurements were done with 470 nm excitation (EPL-470). As practically the same results were obtained, we used 404.6 nm excitation because of the shorter instrument response function (IRF) due to shorter excitation pulses. The laser's pulse energy was ca. 15 pJ and was attenuated (often more than one order of magnitude) to the desired count rate of 1% or less of the excitation frequency. This ensures that the results are free from pulse pile-up,16,17 which otherwise might affect especially concentration dependent measurements. Measurements where done in reverse mode at 5-10 MHz and under magic angle polarisation. A cut-off filter, OG590, was used to block stray excitation light. A dilute solution of Ludox was used to record the instrument response function without any filter. No monochromator was used, i.e. all wavelengths transmitted by the cut-off filter were collected, increasing the instrumental sensitivity. 4


Formatted: Superscript

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2.4. Data Analysis Decay-curves obtained by single photon counting were analysed by iterative re-convolution using an exponential decay model with 1, 2 or 3 components in the SpectraSolve Program. The instrument response function (IRF) was free to move relative to the decay during analysis, the time-scale presented in the graphs is arbitrarily chosen and does not affect the fitting. Concentration dependent decays in acetonitrile were analysed by global fitting assuming a gauss function as IRF in IgorPro 6.

3. Results and Discussion 3.1. Concentration dependence Steady-state absorption and emission spectra of 20

kCounts

15

10

D149 in acetonitrile: 143 nM 1.4 µM 2.74 µM 5.23 µM 7.51 µM 11.5 µM τ1 = ∼50 ps 16.4 µM 32 µM τ2 = 325 ps 143 nM IRF τ3 = 650 ps

D149 in a wide range of solvents have been

τ4 = 5.2 ns

a biexponential decay. The faster of the ob-

5

reported previously.9,10,12 In previous lifetime measurements of D149 in acetonitrile12 we had seen the existence of more than one lifetime, i.e.

32 µM

served components roughly matched the report-

0

0.0

0.5

1.0

1.5

2.0

2.5

ed ca. 280 ps, while the second component was

3.0

Time (ns)

substantially longer (ca. 650 ps). A more thorough analysis revealed that the amplitudes of

20

the two components depended on concentration

D149 in MeOH 100 nM: (τ = 100 ps) 100 µM: (τ = 100 ps) IRF

15

kCounts

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and the slower one only reached appreciable weight at very low concentrations, with samples

10

as dilute as a few nM investigated. Only a small 5

shift of ca. 4 nm was observed in the absorption spectra (see supporting information, inset in

0 -200

0

200

400

600

800

1000

Figure S1), excluding the formation of aggre-

Time(ps)

gates. On the other hand, a systematic decrease Figure 2 Fluorescence decay of D149 in acetonitrile (top) and methanol (bottom) at different concentrations

in relative quantum yield could be seen in the emission as the concentration increased (Figure

S1). A graph of the time-resolved fluorescence decay in acetonitrile at varying concentrations is given in Figure 2 (top). The decays were analysed by global fitting using two major and two minor components. Besides the major lifetimes of 325 ps and 650 ps, a short component of tens of ps and a long component of 5.2 ns were required to give good agreement with the measured data. The short component can be related to the 19 ps obtained by Lohse et al.10 and is too fast to be fully resolved in our experiments. The longer component, carrying only a small amplitude, is likely to be due to tiny amounts of impurities that can have relatively high weight due to the extremely low concentra5



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tion (nM) used in the experiments. Correspondingly, the longer component was practically absent in the micromolar range and reached values of a few percent amplitude in the nanomolar range. From Figure 2 (top) it is clearly seen that higher concentrations lead to a faster decay, i.e. increased (reduced) weight of the faster, 325 ps (slower, 650 ps) component with higher concentration. Previous studies of the excited state of D149 in acetonitrile did not observe the slower of the two components due to the high concentrations involved in the experiments. The pump-probe study10 did not specify the concentration used. Based on the cuvette's optical path length of 400 µm and observed absorption changes of up to 160 mOD for the ground state bleach, the lowest possible concentration would be 60 µM. Taking further into account photoselection, i.e. only a subpopulation of the molecules with the right orientation can be selected with linearly polarised light, and the fact that usually only a small portion of the molecules is excited due to limited laser intensity, a realistic concentration would be ca. 200 µM, corresponding to an absorption of ca 0.5, a typical value for pump-probe measurements. This concentration is around 10 times as high as the highest one used in our single-photon counting experiments (see Figure 2). It is therefore apparent that the slower component would be impossible or very hard to detect. Similar arguments hold for the upconversion study,11 where a 100 µM solution was used, i.e. three times the highest concentration seen in Figure 2, and therefore missing the slow decay contribution. A comparative measurement at 10µM was carried out in the study, but the decay was only measured up to 95 ps, too short to reveal a minor lifetime component of 650 ps. This demonstrates the limitations of the ultrafast femtosecond methods (transient absorption and fluorescence upconversion) due to their reduced sensitivity. In contrast, time-correlated single-photon counting and to a large extend streak camera measurements are sensitive enough to allow for the investigation of very dilute samples. Interestingly, concentration dependence is not observed in all solvents: in methanol, no change in lifetime was observed over a three-order-of-magnitude range of concentrations, see Figure 2 (bottom). These measurements gave virtually the same value as already obtained earlier.10,12 To elucidate the underlying cause for the variation of lifetimes, we first examined if the results can be explained by dimerisation. D149 contains a carboxylic group, these groups are known to lead to dimer formation.18,19 We therefore tried to analyse the relative amplitudes of the short and long lifetimes obtained in the fitting by a scheme of monomers and dimers, however no satisfactory results could be obtained. Furthermore, the very low concentrations used would have pointed towards an extremely high dimerisation constant on the order of 105 M-1. Rubio et al.20 determined and summarised dimerisation constants for a range of aliphatic carboxylic acids in nonpolar solvents and obtained values ranging from ca. 5 M−1 (for propionic acid in nitrobenzene) up to 4 650 M−1 (propionic acid in cyclohexane). Dimerisation via carboxylic dimer hydrogen bonds is therefore unlikely to cause the observed concentration dependence. As an alternative, dynamic quenching is another typical cause leading to concentration dependent fluorescence.17 However, even this can be excluded 6


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based on the low concentration, making diffusion-limited reaction rates far too slow compared to the short, sub-nanoseconds lifetimes. 3.2. Effect of acid/base and isotope effect To test for other possible mechanisms enabling quenching, we have focused on the role that protons seem to play in excited state quenching; protic solvents (MeOH, EtOH) showed faster decays than nonprotic (acetonitrile), independent of their 20000

polarity. Therefore, we examined the behaviour D149 in: Acetonitrile only Acetonitrile + DABCU (τ = 720 ps) Acetonitrile + Water (τ = ~200 ps)

Counts

15000

of D149 with active protons scavenged by the organic base DABCU. The result in Figure 3

10000

shows an obvious increase in the lifetime. The decay in the presence of base could be fitted

5000

with a monoexponential function, whereas neu0 0

2000

4000

tral acetonitrile required a biexponential decay.

Time(ps)

On the contrary, adding small amounts of water Figure 3 Fluorescence decay of D149 in neutral acetonitrile (black curve), with addition of base (red) and water (blue).

as a very strongly protic solvent leads to an increased quenching and shorter (monoexpo-

20000

nential) lifetime of ca. 200 ps (Figure 3).

15000

Counts

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To further confirm the direct role that protons

MeOD (τ = 180 ps) MeOH (τ = 100 ps) MeOH+DABCU (τ = 250 ps)

play in deactivating excited D149, we compared

10000

protic methanol with its deuterated counterpart (see Figure 4) and found a clear increase of life-

5000

time in the case of MeOD with an isotope effect

IRF 0 0

500

1000

1500

Time( ps)

was observed with CD3CN). This demonstrates

Figure 4 Fluorescence decay of D149 in methanol (red), deuterated methanol (black) and MeOH in the presence of base (DABCU, blue)

τ(MeOD)/τ(MeOH) of 1.8 (no isotope effect

that protons are directly involved in the excited state deactivation mechanism of D149. A possible reaction is excited state proton transfer,

where the solvent protonates the excited D149, which as a result returns to the ground state as a protonated species (D149H+). While likely, excited state proton transfer is not the only mechanism by which protons can lead to deactivation of excited states. For example, Masuda et al. have observed a significant isotope effect of the solvent on the radiationless relaxation of osmium complexes where no proton transfer was supposed to be involved. Rather, energy transfer to water molecules penetrating into the ligand sphere, possibly through OH vibrations, was assumed to contribute to the relaxation.21

7


Formatted: Position: Vertical: 3.58", Relative to: Paragraph


Having established the importance of protons in the quenching of D149 and taking into account that D149 carries a strongly protic carboxylic group, we sought to test if the self-quenching observed as a concentration dependent lifetime can be mimicked by small (µM) concentrations of carboxylic acids. Using very dilute (ca. 50 nM) solutions of D149 and adding optically inactive acids should lead to a similar behaviour as in higher concentrations of D149, where we expect the carboxylic group of one molecule to quench another D149 molecule. From the results with chloroacetic acid, depicted in Figure 5, three important points can 20

kCounts

be learned. First, lifetimes decrease as more

D149 in acetontrile (50 nM) + CH2ClCOOH:

acid is added, confirming the effect of quench-

0M 3.32 µM 8.26 µM 14.8 µM 29.1 µM 47.6 µM 76.9 µM 90.9 µM 104 µM 118 µM D149 34 µM

15

10

0 µM 5

ing. Second, and remarkably, higher concentrations of acid do not lead to a corresponding linear decrease in lifetimes; instead the values level off above ca. 50 µM. This is seen more

118 µM 0

clearly in Figure 6, where the total fluorescence 8

9

10

11

12

13

intensity (taken as the integral of the time-

Time (ns)

Figure 5 Fluorescence decay of D149 (50 nM) in acetoni-

resolved data) is plotted against the concentra-

trile with different concentrations of chloroacetic acid. Also

tion of chloroacetic acid. Clearly, values flatten

given is a higher concentration trace (34 µM) of D149.

out at higher concentrations. This is in line with the observed value of ca. 280 ps measured pre-

2.8

Total Fluorescence Intensity (MCounts)

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viously for D149 in acetonitrile, which shows

2.6

2.4

D149 in acetonitrile + CH2ClCOOH

little concentration dependence once a high

Fit: D149 + HA ⇄ D149⋅HA K ≈ 66 000 M-1 Fit: D149 + 2HA ⇄ D149⋅2HA

concentration regime is reached. At the same

2.2

time, some dependence on concentration is pre-

2.0

sent, explaining differences between different

1.8

values measured by different groups. Third, the

1.6 0

50

100

150

200

[CH2ClCOOH] (µM)

nism, which can be ruled out due to the low

Figure 6 Total fluorescence vs. acid concentration. The lines give the fit for different equilibria (see text).

quenching doesn't follow a dynamic mecha-

concentration as discussed above. Neither is a pure static quenching observed, in which the

ground state complex is non-fluorescent (which would lead to reduced intensity of fluorescence, but no change in lifetimes). We can also exclude the presence of aggregates of the type that is responsible for the quenching in e.g. a plastic matrix as we have reported on previously.12 Instead, the case is an intermediate, in which a ground state complex is formed, but quenching occurs within tens or hundreds of picoseconds. Assuming some ground-state interaction between the quencher (chloroacetic acid) and D149, we have tested for different ground state equilibria between them. Let us assume a case of a 1:1 complex (D149 + HA ⇆ D149⋅HA) and describe the fluorescence by the cor8


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responding fraction of free and bound D149: I = ID149⋅[D149]/c0 + Icomplex⋅[D149⋅HA]/c0, where ID149 and Icomplex represent the fluorescence from the free (no quencher added) and complexed D149, respectively and c0 is the sum of the free and complexed D149. Based on these assumptions we would expect a behaviour of the fluorescence at any acid concentration [HA] of the type: I(HA) = (ID149Icomplex)/(K·[HA] + 1) + Icomplex, with K being the association constant between D149 and quencher HA. The corresponding fit (seen in Figure 6 as a red line) gives excellent agreement with the data, while the corresponding case for a equilibrium of the type D149 + 2HA ⇆ D149⋅2HA (shown as blue line) does not match equally well. We can therefore attribute the quenching to a 1:1 ground state complex between D149 and chloroacetic acid with an obtained complexation constant of ca. 66 000 M-1. We would like to stress that the extreme dilution (50 nM) corresponding to an absorption of 0.000 34/mm. (i.e. 340 µOD/mm) inhibits both femtosecond transient absorption and fluorescence upconversion from detecting reasonable signals. Time-correlated single photon counting, on the other hand, gave superb signal/noise. None of the decay seen in Figure 5 took more than 3 minutes to record, and signal to noise could easily have been improved but was deemed sufficient, as is confirmed in the fitting of Figure 6, clearly allowing to differentiate between two important cases of binding. It should be noted, though, that the high sensitivity is achieved, among others, at the price of lost wavelength information (little intraband variation has been seen in our previous streak camera measurements on D14912). The upconversion method on the other hand always selects a narrow wavelength range, yielding additional information, but does not allow for broadband detection (reducing the crystal thickness used for upconversion will increase acceptance bandwidth, but this will lower overall conversion efficiency). 3.3. Mechanism of proton-mediated quenching The deactivation process resulting from protons can probably be attributed to an excited state proton transfer. There are two principle mechanisms that can explain the decay time of 100(s) of ps involving proton transfer. Either proton transfer is very fast, subpicosecond, especially if a pre-formed ground state complex exists or in the case of intramolecular proton transfer.22 The protonated, excited molecule (or tautomer in the case of intramolecular transfer) will then show red-shifted fluorescence and typically have a different lifetime than the unprotonated species. On the other hand, proton transfer itself may be rate-limiting due to solvent orientation, diffusion or molecular reorganisation. The observed lifetime can then be associated with processes that lead to proton transfer, while the protonated species decays very fast back to the ground state and will never reach detectable concentrations. For D149, so far no clear sign of the formation of a protonated species has been observed. Decay parameters of tens of picoseconds were observed by both Lohse et al. (19 ps in acetonitrile, 30 ps in THF) as well as by Fakis et al. (40 ps in toluene, 23 ps in acetonitrile). These have tentatively been 9



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assigned to the vibrational relaxation of "hot" S1 molecules. However, an involvement of protons cannot be ruled out and the observed signals could correspond to protonated D149 in the excited state. In any case, is clear that a ground state complex has to be present before excitation to explain the observed (self-)quenching, as diffusion within the lifetime of the excited state is unreasonable at these low concentrations. We have tried to identify possible sites that are involved in ground state hydrogen bonding. To reduce the complexity, we have measured the concentration dependence of the lifetime for D102 in acetonitrile, a simpler indoline dye5 in which the distal rhodanine ring is replaced by a sulphur atom. The same behaviour as for D149 was observed: reduced lifetimes with higher concentration (see Figure S2). We can therefore exclude any important role of the second rhodanine ring. Further experiments to identify the nature of a possible proton acceptor are currently going on in our laboratory by comparison with indoline dyes that do not contain rhodanines. A partial explanation of the results may be a hydrogen-bonding donor attached to the indole nitrogen, the electron-richest part of the molecule and probably the most basic site. Upon excitation, charge density is removed from the central indoline part and transferred to the rhodanine acceptor, breaking the hydrogen bond. The hydrogen bond donor can diffuse and can reach the now electron-rich rhodanine, to which excited state proton transfer takes place (likely the carbonyl group or nitrogen), leading to the observed quenching. Excited state proton transfer has been demonstrated to carbonyl groups (in a coumarin dye23 and an anthraquinone22) and to nitrogens (such as quinoline24 and acridine25), where these systems act as photobases. The distance between the indole nitrogen and the carbonyl oxygen is d=8.5 Å.26 Assuming a diffusion constant of D=2.2·10-10 m2/s, (according to D=kBT/(6πηr) based on a radius of r=10 Å for D149 and ηCH3CN=0.37mPa·s27) the time constant for diffusion (d2/6D) can be estimated as ca. 200 ps.28 This is reasonably close to the ca. 325 ps measured in acetonitrile, considering that the molecular shape is rather far from spherical. Also, during the excited state not only the formerly hydrogen-bond donating molecule, but also the excited molecule itself would diffuse, leading to a different diffusion time than the one estimated. While the above points towards a possible role of diffusion, the results in e.g. MeOH cannot (only) be explained by diffusion. Here, although hydrogen binding is likely to take place in the ground state, no diffusion is required for interaction with the carbonyl group or nitrogen of the rhodanine group, since there are many more solvent molecules surrounding the excited D149 molecule. As a result, a much faster deactivation would be expected. Therefore, another factor playing in must be the strength of the protic properties of the solvent: lifetimes increase systematically in the series H2O < MeOH < EtOH < i-PrOH. We have even tested for the possibility of ground state protonation of D149 by acids as an alternative to hydrogen binding. Results of a titration with the very strong, non-carboxylic triflic acid lead to the complete disappearance of the charge-transfer band around 540 nm as well as the S2 band around 390 nm (see Figure S6 in the supporting information). Instead, a new band appeared with a 10


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maximum at 435 nm. We can therefore rule out any involvement of the protonated form of D149 in the ground state. At the same time, further evidenced for the fact that ground state coupling between the solvent and D149 is involved is given by Figure 7 (top), in which the lifetime of D149 in a wide range of solvents is plotted against the hydrogen bond donor strength of the solvent, as given in the α-parameter of the linear solvation energy relationship by Kamlet and Taft.29 As an alternative parameter, the acity according to Swain et al.30 is used, with a corresponding plot given in Figure 7 (bottom). Both plots show a similar behaviour. Protic solvents like water and alcohols lead to strongly reduced lifetimes, while in non-protic ones longer-lived excited states are seen. Some lifetime values, given in red in the graph, can be attributed to self-quenching, and the actual lifetimes should be taken from very dilute solutions (see Figure S5 for the concentration dependence of D149 in DMSO). For benzene, those values could only be estimated as 900

cant contribution of the shorter lifetime, howev-

THF CH CHCl Cl2 3 2 CH3CN

700 600

Lifetime τ (ps)

even nanomolar solutions still showed signifi-

DMSO C6H6

800

er with a clearly increasing longer component.

i-PrOH

Acetone 500 400

DMSO

300

C6H6 Toluene

In the case of water, the lifetime is estimated CH3CN

from experiments with mixed H2O/methanol, as

200

EtOH

D149 is insoluble in water. The values for ace-

MeOH

100

H2O 0

tone and ethanol were taken from reference 10. 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Hydrogen bond donor strength α

There is uncertainty about the α for chloroform (a value of 0.44 with high uncertainty is given

900

DMSO

in the summarising reference cited above). In

800

C6H6 THF

700 600

Lifetime τ (ps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH2Cl2CHCl3 CH3CN

the original publication,31 only a single value of i-PrOH

Acetone

0.23 is obtained and no meaningful average

500

DMSO

400

C6H6 Toluene

300

could be calculated. Further it is noted that this

CH3CN

200

value may be affected by the 0.5 % ethanol

EtOH MeOH

100

H2O 0 0.0

0.2

0.4

0.6

0.8

used as a stabiliser in most commercial chloroform. A value of 0.2 is given in a review by

1.0

Acity

Marcus32 and used here. From the plot it is obFigure 7 Plot of D149's lifetime in different solvents vs. the hydrogen bond donor strength α (top) and vs. acity (bottom). Red values are high concentration values.

vious that a strong relationship exists between the hydrogen bond donor strength of the solvent, alternatively its acity, and the lifetime of

D149. One reason for the lack of quantitative agreement between the strength of hydrogen bonding and lifetime is the fact that the observed lifetime still involves isomerisation, not just the radiative decay of the dye. Therefore, more efficient isomerisation, as e.g. in a less viscous solvent, will lead

11



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Page 12 of 19

to lower lifetimes and vice versa. This can be seen for DMSO and i-PrOH, both more viscous solvents that also have longer lifetimes than corresponding to their acity/α-value. As mentioned above and seen in Figure 2, lifetimes did not vary with changes in concentration for methanol. Here, interaction with the solvent's hydrogens/protons is stronger than interaction with another D149's carboxylic group (taking into account the much higher solvent concentration). In the protic solvents, the protons lead to a quenching of the excited state. In the case of CHCl3, which is able to form hydrogen bonds,33 and also in CH2Cl2, lifetimes are independent on concentration (see Figure S3), as the interaction with the solvent is stronger than with another D149 molecule. Because chloroform does not act as a protic solvent it does not quench the excited state and gives a long lifetime of 700 ps. In contrast, a clear concentration dependence is observed in CH3CN (Figure 2) and DMSO (Figure S4), intermediate cases where interaction between solvent and solute is strong enough at ca. nM concentrations. In benzene, toluene and CCl4 (τ = 260 ps) the solvent's properties are too weak to efficiently compete with the strong interaction that takes place between the D149 molecules; therefore the longer lifetime is only seen in extremely dilute solutions. 3.4. Temperature dependence Following the above, we are able to attribute two photochemical reaction pathways for D149: proton-mediated

quenching including

self-

quenching on one hand and isomerisation on the other. As organic bases such as DABCU hinder the former, the latter can be probed in the absence of other pathways (besides radiative relaxation). While photoisomerisation in many cases is an ultrafast (femtosecond) process, the Figure 8 Kinetics of D149 in acetonitrile in the presence of

rates observed here clearly suggest a substantial

DABCU at different temperatures, ranging from 3 to 59 °C.

barrier. Given a natural lifetime of 2.5 ns as

The inset shows an Arrhenius plot.

measured in solid matrix12 and using the room

temperature value of 700 ps in basic acetonitrile, a rate close to 1 ns-1 is obtained. Monitoring the fluorescence decay in the absence of active protons at different temperatures allows us therefore to determine the barrier for isomerisation. Results from a measurement of D149 in acetonitrile in the presence of DABCU are shown in Figure 8. In contrast to neutral acetonitrile solutions, all kinetic curves could be fitted with monoexponential decays yielding one lifetime per temperature. These were then fitted to the Arrhenius equation k = A ⋅ e − E a / RT , where k is the experimentally determined rate, A is the pre-exponential factor, Ea is the activation energy and R and T are the gas constant and temperature, respectively. From the fit, an activation energy of ca. 13 kJ/mol was obtained.

12


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For solvents where protons are active in the photochemistry of D149, the question arises if the proton-quenching mechanism also is a thermally activated process. We measured therefore the excited state decay of D149 in methanol in the same temperature range as for acetonitrile. As can be seen in Figure 9, a small but systematic variation of the kinetic curves is present. The data could be fitted with single exponential functions. From our previous study, it is known that D149 isomerises not only in acetonitrile, but also in methanol. Therefore, we can assume that Figure 9 Decay of D149 in methanol in the temperature

the observed decay follows a rate law of the

range 3 to 59°C. The inset shows an Arrhenius plot.

following form: kobs = kproton + kisom + kfluo, i.e. a

competition between three paths for a single population of D149. The strongly reduced temperature dependence compared to basic acetonitrile strongly suggests that the dominating component (kproton) is independent of temperature and only a smaller contribution (kisom) has thermal dependence. Fitting the observed rates in methanol with a model k obs (T ) = k + A ⋅ e − E a / RT , i.e. assuming a temperature independent contribution (proton quenching plus radiative decay) and one that is thermally activated (isomerisation) gave good results, see inset in Figure 9. The barrier determined this way is 19 kJ/mol. In an alternative analysis, we calculated the rate for proton-mediated quenching accordCH 3CN MeOH ing to k obs = k proton + k isom + k fluo , i.e. the rate for isomerisation was taken from the measurement

in basic acetonitrile. The obtained values for the rate related to protons varied from (114 ps)-1 to (120 ps)-1. When now limiting k in k obs (T ) = k + A ⋅ e − E a / RT to this range during the fit, an activation barrier of 15 kJ/mol was obtained, very close to the 13 kJ/mol obtained for acetonitrile. Because kproton is much larger than kisom in MeOH, it is obvious that any determination of the barrier will be subject to large errors, therefore the obtained values of 13 and 15 (19) kJ/mol seem reasonably close. As a result, we can assume that the proton-induced deactivation has no or only a very small barrier in the temperature range investigated. A diffusion process would express itself as a much stronger temperature dependence as the diffusion itself (D=kBT/(6πηr)) as well as viscosity change with temperature (ηMeOH = 0.82 mPa⋅s at 0 °C, 0.403 mPa⋅s at 50 °C).34 Therefore, an acceleration by a factor of 2.4 would be expected between 0 and 50°C in methanol. This strongly reduces the probability of diffusion being the reason for this lifetime component.

3.5. Role of Protons and Isomerisation in Solar Cells Dyes containing carboxylic anchor groups will release protons upon adsorbing to TiO2.35 These protons can bind to the semiconductor surface, assisting electron injection and re-enforce binding of the dye. On the other hand, the positive charge leads to a shift to more positive values of the con-

13



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duction band, thereby lowering the open-circuit voltage (VOC). The effects that protons play in assisting electron injection is demonstrated e.g. by very recent work where the injection from N3 was monitored with femtosecond pump-probe spectroscopy.36 Complete cells filled with acetonitrile gave ultrafast injection (predominantly subpicosecond) as reported previously for the same system.37 Using electrolyte optimised for high efficiency and containing a base, in stark contrast, injection is slowed down significantly and around 60% of the injection happens on the 11 ps and 310 ps timescale. This behaviour has even been rationalised by DFT calculations,38 showing that protons sitting on the TiO2 surface considerably enhance the coupling between the dyes and TiO2, enabling adiabatic injection. It is therefore interesting to mention that the association of protons in the ground state that we expect to exist for D149 may also have effects on the distribution of protons in the solar cell. At the moment it is not possible to ascribe if the binding will be positive in the sense that injection is enhanced or will be negative in that VOC is lowered. Specifically for D149, however, protons will play an additional role when acting as quenchers to the excited state of D149. This is, possibly among others, a likely reason why the optimised conditions as worked out for D149 contain the base 4-tert-butyl pyridine in high concentrations of 0.5 M.4,39 In the same study mentioned above36 two injection times of ca. 360 fs and 33 ps with about equal amplitude are measured for D149 when using electrolyte optimised for maximum efficiency. The newly revealed slow injection of ca. 30 ps is less than ten times faster than the lifetime of D149 in acetonitrile due to selfquenching (τ = 325 ps). The authors saw that absorption changes due to electron injection reached a plateau after ca. 200 ps, which was explained by the ca. 200 ps lifetime in acetonitrile.10,11 As a consequence, injection may be limited by proton-mediated quenching of the excited state. The solvent used for the electrolyte in the work was 3-methoxyproponitrile; we have tested if lifetimes change at higher concentrations and even here found a concentration dependence (see Figure S4). We have previously established that D149 undergoes photoisomerisation when excited to the S2 state.12 Due to the competitive processes of proton-mediated quenching, it was at that time not possible to attribute a rate to the process of photoisomerisation. Having dissected the effect of protons from the isomerisation, a rate of ca. 1 ns-1 is found for the isomerisation process. This is rather slow not only in comparison to other photoisomerisation processes, such as isomerisation in bilirubin14,40 or retinal,13 but also in comparison with the commonly assumed injection rates of most DSSC dyes. While many studies found very fast injection ≤100 fs,37,41,42 the above mentioned pump-probe study considered the effects of electrolyte and light intensity, showing that under more realistic conditions of using complete cells under intensities close to AM 1.5, injection is slowed down by orders of magnitude.36 Based on the values of injection for D149 of 360 fs (50%) and 30 ps (50%), it is not clear if two populations with different injection efficiency exist (as e.g. a tightly coupled and a less tightly coupled one), or if injection intrinsically is of multiexponential character. In the former case, isomerisation becomes a minor but realistic competitive path for 50% of the molecules with slower 14


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injection time. When taking the faster of the injection times of 360 fs, competition will only limit the efficiency by 0.03%. At the same time, it would take 3 000 excitation for a molecule to undergo isomerisation. Based on a simple calculation by Grätzel,43 dye molecules are excited on a rate of ca. 1 s-1 under full sunlight. As a consequence, on average it takes less than one hour before dye molecules will have converted into isomers and only half a minute when considering the 30 ps component. This should be compared to the thousands of hours for which cells are being tested in longterm studies. Neglected in the above calculation are the back isomerisation and the wavelength dependence, but it becomes clear that side reactions with low efficiency can play a role in DSSCs if the products can accumulate. As nothing is known about the injection efficiency of the isomer at the moment, it is unclear what the effect on the performance of the cell will be.

4. Conclusions Fast lifetimes of hundreds of picoseconds for the excited state of D149 in a wide range of solvents can be explained by protons interacting with the excited molecule, most likely by excited state proton transfer, while other mechanisms cannot be ruled out at the moment. Upon electronic excitation, the pre-formed hydrogen-bonded ground state complex decays and the protons lead to a reduction in excited state lifetimes. A major, second deactivation mechanism is double bond isomerisation, with an attributed lifetime of ca. 1 ns. Our results give a mechanistic explanation of the decay of D149 in neutral, acidic and basic conditions as well as in protic solvent and at different concentrations. The specific role that protons play is demonstrated, among others by isotope effect and importantly explains the observed concentration dependence of excited state lifetimes in concentrations as low as nanomolar. Understanding the above effects can increase the knowledge of processes inside solar cells and lead to improved strategies for dye design. We have also shown that the highly sensitive experimental method of single-photon counting is able to resolve phenomena that are impossible to monitor with pump-probe and upconversion techniques, stressing the importance of judicious choice of experimental techniques.

5. Acknowledgements We are grateful to Masakazu Takata (Mitsubishi Paper Mills Ltd.) for kindly sending a sample of D-dyes. Leif Hammarström and Todd Markle are acknowledged for helpful discussion and Roland Stenutz for useful information about α-parameter and acity.

6. Supporting Information Available Normalised absorption and fluorescence spectra (relative fluorescence quantum yield) for D149 at different concentrations (Figure S1); fluorescence decay of D102 in acetonitrile (Figure S2); fluorescence decay of D149 at different concentrations in CHCl3, 3-methoxypropionitrile and DMSO 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Figures S3, S4 and S5, resp.); titration of D149 in acetonitrile with triflic acid (Figure S6). This information is available free of charge via the Internet at http://pubs.acs.org.

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8. Graphical Table of Contents

19


Concentration and Solvent Effects on the Excited State Dynamics of

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