Photoinduced Dynamics of 13,13'-Diphenylpropyl-ß-carotene.

Carotenoids are ubiquitous pigment systems in nature which are relevant to a range of processes, such as photosynthesis, but the detailed influence of substitutions at the polyene backbone on their photophysics is still underexplored. Here, we present a detailed experimental and theoretical investigation of the carotenoid 13,13'-diphenylpropyl-ß-carotene using ultrafast transient absorption spectroscopy and steady-state absorption experiments in n-hexane and n-hexadecane, complemented by DFT/TDDFT calculations. In spite of their bulkiness and their potential capability to "fold back" onto the polyene system, which could result in π-stacking effects, the phenylpropyl residues have only a minor impact on the photophysical properties compared with the parent compound ß-carotene. Ultrafast spectroscopy finds lifetimes of 200-300 fs for the S2 state and 8.3-9.5 ps for the S1 state. Intramolecular vibrational redistribution with time constants in the range 0.6-1.4 ps is observed in terms of a spectral narrowing of the S1 spectrum over time. We also find clear indications of the presence of vibrationally hot molecules in the ground electronic state (S0*). The DFT/TDDFT calculations confirm that the propyl spacer electronically decouples the phenyl and polyene π-systems and that the substituents in the 13 and 13' positions point away from the polyene system.

Ultrafast Excited-State Dynamics of all-trans-Capsanthin in Organic Solvents.

The C40 carotenoid capsanthin is of photophysical interest because it belongs to the family of terminally carbonyl-substituted apocarotenes. These have the potential to exhibit intramolecular charge transfer (ICT) character in the excited state. We studied its ultrafast dynamics in different solvents using broadband transient absorption spectroscopy in the 260-1600 nm range. Photoexcitation initially populated the S2(11Bu+) state which decayed to the S1(21Ag-) state in 120-150 fs. The lifetime of the S1 state decreased from 10.3 ps (isooctane) to 9.1 ps (methanol). Together with the absence of stimulated emission this suggested a weak ICT character of S1. A steric influence of the five-membered ring in capsanthin was identified based on a comparison with the sister apocarotenone citranaxanthin which features methyl substitution at the keto group. While both apocarotenones exhibit the same S1 lifetime in isooctane, the decrease in lifetime in polar solvents is weaker for capsanthin because presumably the five-membered ring sterically perturbs stabilization of ICT character by the solvent. For the S1 state of capsanthin we further observed intramolecular vibrational redistribution and collisional energy transfer to the solvent with time constants of 340-420 fs and 8.5-9.1 ps, respectively. No evidence for excited-state photoisomerization of capsanthin was found.

Ultrafast electron and hole transfer dynamics of a solar cell dye containing hole acceptors on mesoporous TiO2 and Al2O3.

The stability of dye cations against recombination with conduction band electrons in mesoporous TiO2 electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO2 conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO2 surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a "non-injecting" Al2O3 surface generating a radical cation-radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S1/ICT.

Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films.

We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous Al2O3 and TiO2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramolecular charge transfer character ("S1/ICT"). On Al2O3 and in the three organic solvents, the dynamics are fully explained in terms of S1/ICT stabilisation (by reorientation of adjacent solvent or D49 molecules and collisional cooling), intramolecular vibrational redistribution and S1/ICT → S0 electronic decay. A substantial decrease of the S1/ICT lifetime is observed with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addition of the nonpolar co-adsorbent deoxycholic acid (DCA) to the Al2O3 surface leads to a substantial increase of the S1/ICT lifetime. DCA spacers reduce the local polarity around the dye molecules, thus interrupting D49 "self-solvation". These results are in contrast to a recent experimental study for the indoline dye D131 on Al2O3, where charge transfer from electronically excited D131 to adjacent dye molecules was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 molecules and also not for electron injection from D49 into Al2O3 trap states. Charge separation is only observed for D49 bound to TiO2 thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49˙+e-] complex and a transient Stark effect signalling the formation of mobile electrons upon dissociation of the complex.

A comprehensive picture of the ultrafast excited-state dynamics of retinal.

All-trans retinal is the chromophore of microbial rhodopsins initiating energy conversion and cellular signalling by subpicosecond photoinduced switching. Here, we provide detailed UV-Vis transient absorption experiments to disentangle the complex photochemistry of this polyene, which is governed by its terminal aldehyde group. After photoexcitation to the S2((1)Bu(+)) state, the system exhibits polarity-dependent branching, populating separate S1((1)Ag(-)) and intramolecular charge transfer (ICT) species. In all solvents, population of a singlet nπ* state from S1 is observed which represents the precursor of the T1 triplet state. While triplet formation dominates in nonpolar solvents (67% quantum yield), it is dramatically reduced in polar solvents (4%). The channel closes completely upon replacing the aldehyde by a carboxyl group, due to an energetic up-shift of (1)nπ*. In that case, internal conversion via the ICT species becomes the main pathway, with preferential formation of the initially excited isomer.

Charge carrier dynamics of methylammonium lead iodide: from PbI2-rich to low-dimensional broadly emitting perovskites.

We provide an investigation of the charge carrier dynamics of the (MAI)(x)(PbI2)(1-x) system in the range x = 0.32-0.90 following the recently published "pseudobinary phase-composition processing diagram" of Song et al. (Chem. Mater., 2015, 27, 4612). The dynamics were studied using ultrafast pump-supercontinuum probe spectroscopy over the pump fluence range 2-50 µJ cm(-2), allowing for a wide variation of the initial carrier density. At high MAI excess (x = 0.90), low-dimensional perovskites (LDPs) are formed, and their luminescence spectra are significantly blue-shifted by ca. 50 nm and broadened compared to the 3D perovskite. The shift is due to quantum confinement effects, and the inhomogeneous broadening arises from different low-dimensional structures (predominantly 2D, but presumably also 1D and 0D). Accurate transient carrier temperatures are extracted from the transient absorption spectra. The regimes of carrier-carrier, carrier-optical phonon and acoustic phonon scattering are clearly distinguished. Perovskites with mole fractions x ≤ 0.71 exhibit extremely fast carrier cooling (ca. 300 fs) at low fluence of 2 µJ cm(-2), however cooling slows down significantly at high fluence of 50 µJ cm(-2) due to the "hot phonon effect" (ca. 2.8 ps). A kinetic analysis of the electron-hole recombination dynamics provides second-order recombination rate constants k2 which decrease from 5.3 to 1.5 × 10(-9) cm(3) s(-1) in the range x = 0.32-0.71. In contrast, recombination in the LDPs (x = 0.90) is more than one order of magnitude faster, 6.4 × 10(-8) cm(3) s(-1), which is related to the confined perovskite structure. Recombination in these LDPs should be however still slow enough for their potential application as efficient broadband emitters or solar light-harvesting materials.

Ultrafast photoinduced dynamics of the organolead trihalide perovskite CH3NH3PbI3 on mesoporous TiO2 scaffolds in the 320-920 nm range.

We present femtosecond broadband transient absorption experiments for the investigation of the carrier dynamics in the organolead trihalide perovskite CH3NH3PbI3. The perovskite was prepared on a mesoporous TiO2 scaffold either by 1-step deposition from solution or by 2-step methods employing deposition of lead iodide followed by an on-surface reaction with methylammonium iodide. The thin films were characterized by XRD and FTIR chemical mapping. After pumping with an ultrashort laser pulse at 400 or 500 nm, the dynamics were monitored by a broadband supercontinuum reaching from the near IR (920 nm) into the UV. Specifically, the usage of quartz substrates and thin perovskite/TiO2 films enabled us to cover the spectral development down to 320 nm. The charge carrier dynamics were largely independent from the specific route of perovskite preparation: initial ultrafast carrier relaxation steps with time constants τCC and τCOP of <0.08, 0.2 and 2.6 ps were assigned to carrier-carrier and carrier-optical phonon scattering. Pronounced sub-band-gap absorption was found in the near IR at early times. Transient carrier temperatures were extracted from a Boltzmann fit to the blue wing of the photobleach band in the time range 0.2-700 ps, allowing us to distinguish between the decay of acoustic phonons (τAP = 50 and >1000 ps) and Auger recombination (τAR = 9, 75 and 450 ps). Carrier relaxation was accompanied by formation of an absorption band around 550 nm, with a characteristic structure assignable to a transient Stark effect, i.e. a red-shift of the perovskite spectrum due to the appearance of a directed electric field in the material and possibly additional influence of lattice heating. We observed a substantial Stokes shift between the relaxed photobleach and photoluminescence bands. Contributions of unreacted PbI2 to the transient absorption features appear to be negligible.

Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent.

We investigated photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the organic solvent acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim](+)[B(CN)4](-) using transient absorption spectroscopy, spectroelectrochemistry, and DFT/TDDFT calculations. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochemistry, where the E6˙(+) radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the positive charge. After photoexcitation of E6 at 520 nm (S0 → S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is observed (time constants ca. 0.8 and 12 ps) which is related to a changing electric field generated by the E6˙(+) radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent molecules. However, pronounced differences are observed on longer timescales. In contact with solvent, one observes an increase in the E6˙(+) absorption band above 600 nm with a time constant of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time constants of ca. 100 µs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Additional experiments were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 molecules in S1 quickly decays to S0 (time constants 1 and 35 ps).

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile.

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 µs, 170 µs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.

Ultrafast photoinduced dynamics of the 3,6-diaminoacridinium derivative ATTO 465 in solution.

The excited state dynamics of the dye ATTO 465, a well-known fluorescence marker for biological applications, have been characterized in various solvents including THF, ethanol, methanol, water and the highly polar protic ionic liquid 2-hydroxyethylammonium formate (2-OH-EAF) by combining results from time-correlated single-photon counting (TCSPC) and ultrafast pump-supercontinuum probe (PSCP) spectroscopy as well as steady-state absorption and fluorescence. In water, 2-OH-EAF and two fluorinated alcohols, there is a pronounced blue-shift and broadening of the S(0) → S(1) absorption band and also a larger Stokes shift than in the other solvents, indicating a particular influence of hydrogen-bonding interactions. S(1) lifetimes from TCSPC at 25 °C range from 3.3 ns to 5.6 ns. An unusual increase in the S(1) lifetime with temperature is observed for ethanol and methanol, however water behaves in the opposite way. The behavior can be tentatively explained by a solvent- and temperature-dependent "proximity effect", where coupling of the close-lying S(1) and S(2) states influences the intramolecular relaxation rate of the dye. In addition, temperature-dependent complex equilibria of ATTO 465 with solvent molecules may influence the measured lifetimes. Several excited-state absorption (ESA) transitions are identified in the PSCP spectra, which are in good agreement with the position of the UV bands in the steady-state absorption spectra. Small shifts of the stimulated emission and ESA bands are consistent with solvation dynamics in the excited electronic state. An additional ~16 ps component in water, visible over the entire spectral range, is tentatively ascribed to a fast IC channel which is accessed by a fraction of ATTO 465 molecules.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films.

The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO(2) and ZrO(2) thin films, all in contact with air, using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from ≤70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149˙(+) radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149˙(+) in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149˙(+)-electron complex to isolated D149˙(+) and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149˙(+) by interactions with ZnO. TiO(2) exhibits similar dynamics to ZnO. In the case of ZrO(2), electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149˙(+) in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D(0)→ D(1) absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149˙(+) spectrum employing the MPW1K functional.