Chapter 7 Photophysics of Porphyrin-Naphthalenediimide
7.4.2 Transient Absorption Measurements in Toluene
The same TA measurements were also performed in toluene upon
differences are observed: (i) the overall dynamic is slower, and (ii) the features of HGS found in THF are not visible in the TA spectra recorded in toluene independently of the number of attached NDI moieties.
Figure 7.8 Transient absorption spectra recorded with the ZnP-NDI arrays in toluene at selected time delays upon 400 nm excitation.
The temporal evolution of the TA spectra in toluene can be reproduced using target analysis assuming four successive exponential steps (Figure 7.9), except for the sample with one NDI unit. The latter case requires an additional step to properly reproduce the dynamics on the longer timescale (Figure 7.9A).
7.4 Transient Absorption
The resulting SADS of the samples in toluene are similar to those obtained in THF, however with the difference that (i) the processes are slower, that is not surprising by going from a polar to a nonpolar solvent, and that (ii) SADS C, assigned to HGS, is less pronounced in toluene.
Nevertheless, the shape of SADS B is very similar to CSS observed with the compounds in THF, and thus it can be tentatively interpreted likewise.
Additionally, the obtained time constants and SADS point that inverted kinetics in toluene can also be considered.
Figure 7.9 Species-associated difference spectra obtained from target analysis assuming sequential exponential steps of the transient absorption data measured with the ZnP-NDI arrays in toluene upon 400 nm excitation.
-15
Figure 7.10 (A) and (C) Transient absorption spectra at different time delays after 555 nm excitation recorded with the ZnP-NDI arrays in THF; (B) and (D) species-associated difference spectra obtained from target analysis assuming an A → B → C → D → GS scheme.
7.5 Conclusion
Based on the previous considerations and redox potentials reported for similar ZnP-NDI molecules,99, 149 an energy level scheme accounting for CS and CR in the ZnP-NDI arrays can be proposed (Figure 7.11). A strong solvent dependence observed in the TA spectra recorded with the ZnP-NDI arrays is in good agreement with the reported observation of CS occurring in the Marcus normal region and CR in the inverted region, and reveals that CS should be operative in both solvents. Due to the increased stabilization of the CS state in THF the CS process is faster than in toluene. In the same way, lowering of the CSS state should accelerate the CR process if it occurs in the Marcus inverted region. The energy level scheme may vary with changing a number of the attached NDIs if the redox potential of ZnP moiety is changing.
Figure 7.11 Energy levels of the arrays in various solvents.
7.5 Conclusion
Among the numerous artificial analogues of the natural photosynthetic reaction center, many contain porphyrins and naphthalene diimides (NDI) as building blocks. In this chapter, we presented the ultrafast excited-state dynamics of four arrays consisting of zinc tetraphenylporphyrin (ZnP) in a core covalently linked to an increasing number of naphthalenediimide (NDI). The transient absorption spectra are very similar independently of a number of the attached NDIs, and do not present
pronounced features of the charge-separated state, however the overall excited-state dynamics is much faster than one observed with the individual ZnP constituent. This points to an inverted kinetics regime, i.e. charge recombination (CR) is faster than charge separation (CS). Nevertheless, the TA measurements performed in THF reveal the appearance of the band in the 400-450 nm region after a few picoseconds with two attached NDIs, which is characteristic of the vibrationally hot ground state of ZnP. It was found to become more pronounced with increasing number of attached NDIs. As confirmed by global target analysis, this can be explained by an acceleration of CS as the amount of NDIs gets larger. On the other side, contrary to initial expectation, it seems that the acceleration of CS is not fully in line with the increasing number of the attached acceptors. It might be related to the fact that ZnP core loses its electron-donating properties upon substituting on the amino groups of the anilines. For more effective comparison of the investigated compounds it would be better to use a core ZnP without amino groups attached to it. Besides, increasing amount of NDIs seems to affect CS but has no impact on CR. However, the inverted kinetic does not allow to populate CSS enough to be visible in the TA spectra. One of the options to change it could be, for instance, to slow down charge recombination by choosing a weaker electron acceptor. On the other hand, the obtained results give us rather idea of the main timescales of photophysical processes than allow us to make conclusions on its behavior, and should not be over-interpreted. Additional results like quantum calculations, electrochemical measurements, and calculations of the driving forces for charge separation might help better understand the excited-state dynamics of the investigated ZnP-NDI arrays.
Chapter 8
Benzodifuran Based Triads *
8.1 Introduction
In this chapter, we continue to investigate photoinduced charge separation in model systems in order to better understand electron transfer in biological and artificial systems and to assist the design of new artificial systems for molecular electronics or solar energy conversion. As we have already seen in the previous chapter, the minimum size of a charge-separating system is that of a two-component donor-acceptor system. In such dyads, light absorption by one of the two moieties triggers intramolecular electron transfer from the donor to the acceptor unit. However, the achievement of long-lived charge-separate state and ability to promote charge transport over long distances are essential to develop the solar energy conversion systems.150-152 One of the most promising approach to obtain charge-separated states of longer lifetime and high-efficiency solar energy conversion is to use molecules composed of an electron donor-acceptor pair separated by a chemical bridge.153-154 In such systems, bridge assists spatial separation of the charges that leads to population of a long-lived charge separated state.
* This chapter is based on a project performed in collaboration with the Hauser group (University of Geneva), manuscript in preparation.
Figure 8.1 Structures of the investigated BDI triads.
8.2 Steady-State Spectroscopy
In this chapter we examine charge transport in triads that are composed of triphenylamine (TPA) as potential electron donor linked by a benzodifuran (BDF) core to anthraquinone (AQ) or tetracyanoanthraquinone (TCAQ) acting as a terminal electron acceptor (Figure 8.1). All compounds were synthesised by Shi-Xia Liu and co-workers in the group of Silvio Decurtins (Department of chemistry and biochemistry, University of Bern).
In recent years BDF showed promising properties in optoelectronic devices such as organic light-emitting diodes (OLEDs)155-156 and solar cells157-160 This is due to the fact that BDF and its derivatives exhibit such interesting properties as high hole and electron mobility,161-165 planar molecular configuration,158, 161 high solubility,165-166 and its ability to be synthesized from furan that is an abundant product converted from renewable resources.158, 167 However, BDF based systems have been rarely investigated before because of the lack of an efficient and convenient synthetic methodology for its preparation.
8.2 Steady-State Spectroscopy
Figure 8.2 shows electronic absorption spectra of the five triads listed in Figure 8.1 in dichloromethane. Except for TPA-BDF-TPA, for all the four triads the lowest energy absorption band can be attributed to an intramolecular charge-transfer (ICT) absorption band (see below). It is at 538, 555, 634, and 640 nm for AQ-BDF-TPA, AQ-BDF-AQ, TCAQ-BDF-TPA, and TCAQ-BDF-TCAQ respectively. These ICT bands are likely due to the charge transfer from BDF to either AQ or TCAQ. In line with the less negative reduction potential of TCAQ in comparison with AQ, the ICT band for the TCAQ containing triads is at lower energy.
Figure 8.2 Absorption spectra of the triads in dichloromethane.
8.3 TPA-BDF-TPA
8.3.1 Steady-State and Time-Resolved Fluorescence Measurements The steady-state absorption and emission spectra of TPA-BDF-TPA in THF are presented in Figure 8.3. The band at ~300 nm probably mainly associated with the π-π* transition of the TPA unit,168 while the structured band between 355 and 510 nm is not straightforward to assign. Absorption features of both units might be expected in this region.165, 169 However density functional theory (DFT) calculations, performed by Dr. Latévi Max Daku Lawson, reveal that optical excitation is rather delocalized over the entire system (Figure 8.4). Both HOMO and LUMO are extended over the triad due to the conjugated double bonds linking the three moieties.
Consequently, we assume the lowest energy transition to be best identified as ππ* transition with at most a very partial charge localisation on BDF in the excited state.
The absorption and emission spectra do apparently not have mirror-image relationship (Figure 8.3). This could be simply due to the contribution of several transitions to the absorption band.
60x103 40 20 0 ε[M-1 cm-1 ]
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wavelength / nm
TPA-BDF-TPA AQ-BDF-TPA AQ-BDF-AQ TCAQ-BDF-TPA TCAQ-BDF-TCAQ
8.3 TPA-BDF-TPA
Figure 8.3 Absorption and fluorescence spectra of TPA-BDF-TPA in THF after 400 nm excitation.
Figure 8.4 Frontier molecular orbitals of TPA-BDF-TPA computed at the B3LYP level of theory.
The fluorescence dynamics of TPA-BDF-TPA in THF on the nanosecond time-scale have been measured by time-correlated single photon counting (TCSPC). The fluorescence dynamics were monitored at 520 nm and 540 nm upon 395 nm excitation. Exponential fit yielded a lifetime of 3.2 ns at both wavelengths (Figure 8.5).
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absorption emission
absorbance fluorescence
Figure 8.5 Fluorescence decays at 520 nm (A) and 540 nm (B) upon 395 nm excitation of TPA-BDF-TPA in THF and best exponential fit.
8.3.2 Transient Absorption Measurements
The transient absorption (TA) absorption spectra recorded with TPA-BDF-TPA in THF (Figure 8.6A) do not show significant changes within the 0-1.5 ns time window. During the first ~ 5 ps after excitation, the intensity of the negative band at 490 and 535 nm decreases. These bands can be ascribed to the stimulated emission (SE) as they coincide with the stimulated emission spectrum of TPA-BDF-TPA, calculated by multiplying its spontaneous fluorescence intensity by λ4.170 After this decay, the TA intensity remains almost constant over the whole experimental time window.
The time dependence of the TA intensity could be reproduced using target analysis with a 𝐴!!𝐵!!𝐶 scheme and yielded the time constants and species-associated difference spectra (SADS) shown in Figure 8.6B. These spectra are similar and should correspond to the same species. Species A and B can be assigned to the unrelaxed and relaxed S1 excited state, respectively, whereas the 3.9 ps time constant could be ascribed to vibrational/solvent
8.4 AQ-BDF-AQ
Figure 8.6 (A) Transient absorption spectra at different time delays after 400 nm excitation of TPA-BDF-TPA in THF; (B) species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps.
8.4 AQ-BDF-AQ
8.4.1 Steady-State Measurements
The absorption spectrum of AQ-BDF-AQ is presented in Figure 8.7.
The lowest energy absorption is around 555 nm, at much lower in energy than any of the absorption bands of the constituting moieties. Furthermore, the intense luminescence of BDF is totally quenched in this triad.165 As mentioned above, the lowest energy optical transition not present in the spectra of the isolated moieties corresponds to the ICT from BDF to AQ.
This interpretation is in line with the DFT calculations (Figure 8.8) where the HOMO is fully localised on the BDF unit and the LUMO on the AQ
Figure 8.7 Absorption spectrum of AQ-BDF-AQ in THF.
Figure 8.8 Frontier molecular orbitals of AQ-BDF-AQ computed at the B3LYP level of theory.
8.4.2 Transient Absorption Measurements
The TA spectra recorded with AQ-BDF-AQ in THF (Figure 8.9A) show a negative band due to the ground-state bleach and a broad positive band above 470 nm. During the first 2 ps, the band maximum at 625 nm shifts to ca. 615 nm. Afterwards, the intensity of all the TA bands decreases on the ~20 ps timescale. From about 40 ps, the TA spectra remain unchanged and consist of a residual bleach and a weak absorption above 550 nm.
The sum of three exponential functions is enough to reproduce the time evolution of the TA intensity. The SADS obtained from target analysis assuming a 𝐴!!𝐵 !!𝐶 !! 𝐷 scheme with τ1 < τ2 are depicted in Figure 8.9B.
The spectra associated with A and B do not differ much and thus A and B could be ascribed to the unrelaxed and relaxed excited state, respectively.
However, this does not explain the very short lifetime of B. Alternatively, A and B could be the unrelaxed and relaxed product of a charge separation
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AQ-BDF-AQ_THF
absorbance
8.4 AQ-BDF-AQ
(CS) that is faster than the IRF. In this case, τ2 would correspond to a charge recombination (CR). A third possibility is that CS is not ultrafast but slower than CR. To explore this possibility, target analysis of the TA data with inverted kinetics, i.e. with τ2 < τ1, was performed (Figure 8.9C). In this case, τ2 was kept fixed at 1.8 ps.
Figure 8.9 (A) Transient absorption spectra at different time delays after 400 nm excitation of AQ-BDF-AQ in THF; (B) and (C) species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps with different initial conditions.
The A and B SADS are now totally different. Species A would correspond to the locally excited (LE) state and B to the CS state. It is important to note that the absence of strong features related to the CS state in
8
the TA spectra is due to the fact that CR occurs faster than CS. Because of this, the CS state population does not accumulate enough to be clearly visible.
We will see below with the compounds containing TCAQ that this interpretation is plausible. The species C still needs to be interpreted. One possibility would be a triplet state, either populated directly from the LE state (intersystem crossing (ISC) in AQ is ultrafast)171 or upon CR. From the amplitude of the bleach, the efficiency of this process is at most 20%.
Confirmation of the assignment of A to the LE state would require measurements in an apolar solvent, where CS is not operative or much slower. However the solubility of AQ-BDF-AQ in cyclohexane is too low to perform TA measurements.
8.5 AQ-BDF-TPA
8.5.1 Steady-State Measurements
The absorption spectrum in THF (Figure 8.10) of this triad is similar to the one of AQ-BDF-AQ with the lowest energy absorption band at
~540 nm. The absorption band corresponds to the BDF to AQ ICT transition, however with the HOMO having substantial electron density also on the TPA moiety (Figure 8.11). The intense band at 430 nm corresponds to the ππ* transition involving orbitals delocalised over BDF and TPA.
The steady-state measurements were performed in apolar and polar solvents respectively (Figure 8.10). It was found that the fluorescence appears only in cyclohexane, while in THF it is suppressed pointing to an ultrafast non-radiative decay process of the excited state in the polar solvent.
The fully quenched BDF luminescence in THF could be due to both oxidative and reductive quenching which are thermodynamically possible and could be kinetically competitive. But based on our investigations on the TPA-BDF-TPA and AQ-BDF-AQ, it is more plausible that the oxidative quenching dominates. In other words, the initial electron transfer is rather expected from BDF to AQ and is supported by DFT calculations (Figure 8.11).
8.5 AQ-BDF-TPA
Figure 8.10 Absorption and fluorescence spectra of AQ-BDF-TPA in (A) cyclohexane and (B) THF.
Figure 8.11 Frontier molecular orbitals of AQ-BDF-TPA computed at the B3LYP level of theory.
8.5.2 Transient Absorption Measurements in cyclohexane
The solubility of AQ-BDF-TPA in cyclohexane is just enough for TA measurements. The TA spectra are similar to that of species A obtained with AQ-BDF-AQ in THF (Figure 8.9) and do not show significant changes over the whole experimental time window (Figure 8.12A). These data could be analysed using an exponential fit with a lifetime of >2 ns (Figure 8.12B).
It can be assigned to the decay of the S1 (LE) state. These results indicate
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B
absorption absorption emission
A
absorbance fluorescence
that no fast non-radiative process such as CS or ISC is occurring in cyclohexane and in good agreement with the steady-state fluorescence measurement (Figure 8.10A).
Figure 8.12 (A) Transient absorption spectra at different time delays after 400 nm excitation of AQ-BDF-TPA in cyclohexane; (B) species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps.
8.5.3 Transient Absorption Measurements in THF
The TA spectra recorded with AQ-BDF-TPA in THF are very similar to those measured with AQ-BDF-AQ in THF (Figure 8.13A and 8.9A respectively). During the first 2 ps, the band at 625 nm transforms slightly to a new band with maxima at 620 nm and 685 nm, while the bleach at 445 nm remains almost constant. Afterwards, all transient bands decays within a few tens of picoseconds to a weak spectrum that remains constant over the whole experimental time window. The latter resembles that found with AQ-BDF-AQ in THF but its amplitude is substantially weaker.
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B 1.5
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0.5 ps 1.5 ns 100 ps abs em
A
∆A x 103
8.5 AQ-BDF-TPA
Figure 8.13 (A) Transient absorption spectra at different time delays after 400 nm excitation of AQ-BDF-TPA in THF; (B) and (C) species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps with different initial conditions.
Target analysis was performed using an scheme with 𝐴!!𝐵!!𝐶
analysis assuming inverted kinetics (τ1 > τ2) was also carried out (Figure 8.13C). Now A and B have very different spectra as one can expect for LE and CS states. This inverted kinetic hypothesis is further supported by the results obtained with TCAQ instead of AQ.
The population of long-lived species C is much smaller than with AQ-BDF-AQ. Moreover, this species is apparently not present in
cyclohexane. Therefore, this species/state is probably not the triplet state populated by ultrafast ISC from the LE (S1) state. It could still be the triplet state but resulting from CR. However, this process is spin forbidden and should not compete with CR to the ground state. Another possibility could be a product arising from a follow-up reaction from the CS state.
8.6 TCAQ-BDF-TCAQ
8.6.1 Steady-State Measurements
Apart from spectral red shift, the steady-state absorption spectrum of TCAQ-BDF-TCAQ in THF is very similar to that measured AQ-BDF-AQ, the only difference being a red shift (Figure 8.14). TCAQ is considered as a better electron acceptor than AQ, it means that its reduction will occur at less negative potential and can explain the observed red shift.
Figure 8.14 Absorption spectrum of TCAQ-BDF-TCAQ in THF.
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TCAQ-BDF-TCAQ in THF
absorbance
8.6 TCAQ-BDF-TCAQ
8.6.2 Transient Absorption Measurements
The early TA spectra recorded with TCAQ-BDF-TCAQ in THF exhibit two positive bands, one around 400 nm and the other around 600 nm, whose amplitude decreases considerably during the first picoseconds and which evolves to a spectrum with maxima at ~525 nm and above 700 nm difference absorption spectra obtained from target analysis assuming sequential exponential steps.
The first two processes have similar time constants and the SADS are quite similar. Species A and B are probably the unrelaxed and relaxed LE states, respectively, whereas C is the CS state, which recombines in 2.5 ps to the ground state. Given the timescale of these processes, the target scheme used is too simplistic as it assumes equilibrium dynamics. This is of course
6
not the case as CS and relaxation of the LE state occur in parallel. Thus the
not the case as CS and relaxation of the LE state occur in parallel. Thus the