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Hybrid organic–inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for th...

Low-dimensional copper halides, such as CsCu2I3, have emerged as promising LED materials featuring strongly Stokes-shifted photoluminescence with high quantum yield. Previous calculations suggest an exciton self-trapping mechanism; however, direct experimental evidence for this process is still lacking. Here, we present femtosecond UV-vis transient absorption experiments of CsCu2I3 thin films. The films were analyzed by SEM, XRD, and 133Cs/63Cu NMR for crystallinity and defects. Unique spectral dynamics is observed. The band gap absorption exhibits a characteristic double-peak structure arising from the 130 meV spin-orbit splitting of the copper d electrons. Emission at the direct band gap disappears because of the formation of the lowest-energy self-trapped exciton state. We determined the time constant of 12 ps for the trapping process of thermally relaxed free excitons, with an energy barrier of at least 60 meV. The data are successfully modeled by global kinetic analysis, providing also accurate time constants for charge carrier cooling processes.

Ultrafast broadband UV–vis transient circular dichroism (TrCD) and transient absorption (TA) spectroscopy are employed to study the dynamics of chiral polymer thin films. Helical cholesteric arrang...

We observed efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene-type chiral additives based on the dibenzo[c,h]acridine motif. Images obtained from circular dichroism (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction-limited resolution. The CD signal shows a characteristic dependence on the film thickness, which supports a supramolecular origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resolution. A systematic variation of the enantiomeric excess of the chiral additive shows that the "Sergeants and Soldiers" concept and "Majority Rules" are not obeyed.

Hydrogenated amorphous silicon (a-Si:H) represents an important material for optoelectronic and photovoltaic applications. Fundamental knowledge regarding its charge carrier dynamics and thermalization behavior after photoexcitation is therefore required. Here, we provide a detailed broadband UV–vis–NIR transient absorption study of 25 nm thin a-Si:H films on glass upon pumping at 390, 400, or 650 nm, covering the wavelength range 300–1600 nm. Photoexcited carriers relax in the extended states within a few hundred femtoseconds and recombine very quickly. In contrast to a previously suggested bimolecular recombination mechanism for carrier densities larger than 5 × 1018 cm–3, we unambiguously identify Auger-type third-order recombination with the rate constant k3 = (2.3 ± 0.6) × 10–29 cm6 s–1, which is extracted from the absorption decay in the NIR region. The released recombination energy leads to a fast lattice heating which is indicated by a fast rise of absorption in the UV–vis region. This spectral fe...

We present a femtosecond pump-probe UV-Vis broadband transient absorption spectroscopy study of two styryl-substituted BODIPY chromophores with different position of the substituent. The α-substituted isomer shows typical BODIPY-type spectral features, such as sharp absorption and emission bands, a small Stokes shift and an excited-state lifetime in the 4 ns range, which only weakly depends on the solvent. In contrast, β-styryl-BODIPY features much broader steady-state absorption and emission spectra and a larger Stokes shift, particularly in polar solvents. Transient absorption spectroscopy including a complete global kinetic analysis reports a substantial decrease in S1 lifetime to 300 ps for polar solvents upon change from α- to β-substitution. In the case of the α-isomer, TD-DFT calculations identify a typical “cyanine-type” electron rearrangement upon S0 → S1 excitation accompanied by a slight reduction in dipole moment. In contrast, the same transition in the β-isomer shows strong intramolecular charge transfer (ICT) character involving a substantial increase in dipole moment. Assuming a simple energy-gap-law argument, the accelerated nonradiative decay of the β-isomer in polar solvents may be linked to the decrease of the S1(ICT)-S0 energy difference. BODIPY dyes with a conjugated substituent in β-position therefore suffer a substantial loss in fluorescence brightness in polar environments compared with their α-substituted counterparts. This might limit their applicability in fluorescence imaging.

Hybrid organic-inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful preparation of (CH

We investigated the excited-state dynamics of bridged and non-bridged chiral helicene-like bidibenzo[c,h]acridines deposited as a thin film. Broadband UV/VIS−NIR transient absorption spectroscopy from the femtosecond to the microsecond time-scale and transient fluorescence measurements were employed to understand the photoinduced processes in these organic thin-film layers. Photoexcitation at 370 nm populates the S1 state, which has a lifetime of 15 and 36 ps for the non-bridged and bridged derivative, respectively. In both cases, the decay is governed by internal conversion (78% and 84%), whereas intersystem crossing represents only a smaller channel (21% and 15%), and the remaining 1% accounting for fluorescence. Fast repopulation of the S0 state leads to the generation of hot ground state molecules (S0*), with characteristic finger-type transient absorption features. The T1 state decays back to S0 by intersystem crossing with a time constant of 775 and 5500 ps, respectively. Even at high excitation fluence, there are no indications for higher-order processes, such as singlet–singlet annihilation or Förster energy transfer. Photoexcitation also launches coherent acoustic phonons in these films, appearing as damped oscillations in the transient absorption kinetics on the red and blue edge of each S0 absorption band, with a period of about 190 ps.

Low-dimensional copper halides, such as CsCu

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs2AgBiBr6, in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs2AgBiBr6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO2. Femtosecond UV–vis–NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron–phonon coupling via Frohlich interactions is observed in terms of pronounced coherent oscillation of a strong A1g optical phonon mode of the double perovskite at 177 cm–1. Similar behavior is also found for the related vacancy-ordered perovskite Cs3Bi2Br9 and the parent compound BiBr3. Excitonic effects and electron–phonon couplin...

We present steady-state and time-resolved spectroscopic investigations combined with detailed kinetic modelling for the triarylamine derivatives X60 and PTAA which serve as hole transport materials (HTMs) in perovskite solar cells and represent model compounds for organic electronic materials. Photoexcitation of the spiro-fluorene-xanthene X60 populates its S1 state which decays via intersystem crossing (ISC) as well as fluorescence and internal conversion on a nanosecond time scale. Photoexcitation of the PTAA polymer leads to the formation of singlet excitons which relax and migrate in the time regime from a few to about a hundred picoseconds and decay to the ground state with a lifetime of ca. 900 ps. Both X60 and PTAA exhibit efficient photoinduced electron injection into mesoporous TiO2 thin films forming radical cations with characteristic spectral bands, as confirmed by spectroelectrochemistry in solution. Photoluminescence (PL) experiments performed for the HTMs on methylammonium lead iodide perovskite deposited on mesoporous TiO2 show that small molecular HTMs, such as X60, quench the PL much better than the PTAA polymer. Ultrafast transient absorption experiments on the other hand suggest that the hole transfer at the interface between the perovskite and these HTMs is very fast, regardless of the type of HTM. It is therefore concluded that small molecular HTMs infiltrate much better into mesoporous structures and therefore more efficiently accept holes from the perovskite on such thin film architectures due to the better interfacial contact compared with their polymer-based counterparts.

We report on an ultrafast transient absorption study of all-trans-3,3′-dihydroxyisorenieratene (“DHIR”) and all-trans-(3R,3′R)-zeaxanthin in organic solvents covering the wavelength range 350–770 nm. The lifetime of the S2 state in both carotenoids is 160–170 fs. Upon internal conversion (IC) non-equilibrated S1 molecules are formed which internally relax on a 300–400 fs time scale. The time constant for IC from S1 depends on the type of terminal substituent: Replacement of the two terminal β-ionone rings of zeaxanthin by two aryl rings in DHIR results in an increase from 9.5 to 10.9 ps in THF. This suggests a mild decrease in the effective conjugation length of DHIR. IC to the ground electronic state prepares vibrationally hot S0* molecules which exhibit characteristic bleach and absorption bands. These are typically denoted as “S* features”. Collisional cooling of S0* happens with a time constant of 15 ps. Based on our results and the findings from previous studies for other carotenoids, such as macro-β-carotenes, β-carotenes and long-chain apocarotenals, we conclude that S0* spectral features are ubiquitous in carotenoid photophysics: They are particularly easy to observe in systems with a very short S1 lifetime and a high quantum yield for IC to the ground electronic state.

The ultrafast charge carrier dynamics of the metal-deficient lead-free perovskite derivative methylammonium antimony iodide ((CH3NH3)3Sb2I9, MASbI) is studied using femtosecond UV–vis–NIR transient absorption spectroscopy. The transient and steady-state absorption spectra show strong excitonic features. This is confirmed by an analysis of the near-band-gap absorption using Elliott’s theory and its reformulation by Tanguy with subsequent modeling of the free-carrier and exciton contributions using the Saha equation. Time scales of the different carrier scattering processes are identified. The kinetics in the UV–vis range is largely independent of pump laser fluence suggesting that the dynamics involves localized excitons, with recombination time constants of 190 ps and >5 ns. Damped oscillations in the kinetics indicate coherent phonon dynamics and strong electron-phonon coupling. Fourier transformation of the time-domain data provides the steady-state Raman spectrum of MASbI which is dominated by vibratio...

Trivalent bismuth compounds featuring octahedral halide coordination, such as BiI3, double-perovskite structures and vacancy-ordered perovskite derivatives, have recently attracted considerable attention as potential environmentally benign thin film materials for solar light-harvesting and optoelectronic applications aiming to replace toxic lead-based perovskites. Yet, their photovoltaic performance still lags considerably behind their lead-counterparts. Improving their performance requires a detailed knowledge of the photoinduced carrier dynamics and kinetics in these systems. In the current study, we present a detailed investigation of layered BiI3. The compound exhibits pronounced exciton dynamics which is associated with a characteristic second-derivative type feature in the transient absorption spectra. Accurate time constants are assigned to carrier cooling and recombination processes. Pronounced coherent oscillations are observed in the kinetic traces indicating strong electron–phonon coupling. Analysis of the coherences recovers the steady-state Raman spectrum of BiI3 with a dominant mode of Ag symmetry at 114 cm−1, associated with the zone center C(Γ) phonon. The line position is well reproduced by DFT calculations. In terms of the carrier lifetime, high absorption coefficient and band-gap position, BiI3 is a promising candidate as a photovoltaic material but challenges posed are the difficulty to split the strongly bound excitons which are confined in the 2D layers and the efficient extraction of electrons and holes at n- and p-selective contacts, beside well-known issues regarding high resistivity and the intrinsic softness of this material.

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 f...

We present an investigation of the ultrafast charge carrier dynamics for the one-third metal-deficient lead-free perovskite (CH3NH3)3Bi2I9 on mesoporous TiO2. Excitation of the perovskite at 400 or 505 nm leads to characteristic second-derivative-type spectral features in the transient absorption spectra, suggesting substantial contributions of bound excitons, in contrast to the widely used lead-based perovskites. The immediate appearance of broad NIR absorption is assigned to TiO2 conduction band electrons formed by instantaneous dissociation of a subpopulation of excitons at the perovskite/TiO2 interface. Excitation with excess energy above the perovskite’s band gap opens up an additional fast (70 fs) exciton dissociation channel with about 26% amplitude. Antisolvent-assisted synthesis of (CH3NH3)3Bi2I9 reduces the crystallite size to about 500 nm but has only a minor effect on the carrier dynamics. The results suggest that photovoltaic applications of this material will likely require bulk-heterojuncti...

Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s−1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack.

We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet-visible-near-infrared (UV-vis-NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV-vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV-vis region.

Organic–inorganic hybrid lead halide perovskites already reach very high power conversion efficiencies above 22% on architectures employing mesoporous TiO2, but the carrier injection processes across the different interfaces are still not fully understood. Here we use ultrafast broadband transient absorption spectroscopy to determine time constants and yields for hole and electron injection. We show that hole transfer from the perovskite valence band (VB) to the hole-transport material (HTM) H101 at the perovskite/HTM interface occurs in less than 500 fs, but is limited by imperfections of the contact layer and poor infiltration of the HTM into the mesoporous structure. Electron injection from the perovskite conduction band (CB) into the CB of mesoporous TiO2 is only a small channel (25%). Electron transport inside mesoporous MAPI/TiO2 architectures therefore mainly occurs via the perovskite. We also show that electron injection from H101 into the perovskite is feasible for excitation at 400 nm resulting in light-harvesting of high-energy photons by the HTM. Accurate absolute NIR absorption coefficients for CB electrons in mesoporous TiO2 are provided.

We present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterise its photophysics in different organic solvents and on mesoporous Al2O3 and TiO2 thin films. Photoexcitation of H101 at 400 nm in organic solvents populates the S1 state which shows intramolecular relaxation on a picosecond time scale. Branching from the relaxed S1 state leads to population of the T1 triplet state and the ground electronic state S0. Triplet formation is induced by the internal heavy-atom effect of the sulphur atom, and the triplet yield decreases substantially with solvent polarity. On mesoporous Al2O3, intermolecular exciton-splitting is observed leading to the formation of a radical cation – radical anion pair (H101(S1) + H101(S0) → H101˙+ + H101˙−) followed by exciton recombination. On mesoporous TiO2, efficient electron injection is observed in addition to exciton-splitting. Complementary spectroelectrochemistry experiments enable a full spectral characterisation of the cation species H101˙+ and H1012+. Extensive DFT/TDDFT calculations successfully assign the spectral features of all experimentally observed species. Implications for the function of H101 in photovoltaic devices are discussed.

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.

A combined electrical and time-resolved optical investigation of the perovskite formamidinium tin iodide (FASnI3) and its phenylethylammonium (PEA) derivative PEA0.08FA0.92SnI3, which recently achieved a power conversion efficiency of 9%, is presented to study the specifics of contact characteristics and charge carrier dynamics. Microstructured gold electrode arrays were used to investigate the charge transport across a metal-perovskite interface and through micrometers of the perovskite films. Symmetrical contact configuration enabled detailed polarization studies. Hysteresis in the current-voltage characteristics and a corresponding current-time behavior indicated limitations by charge transfer in the contacts. Hysteresis was less pronounced in PEA0.08FA0.92SnI3 compared with FASnI3. This is explained by a 2-dimensional interlayer at the contacts, which leads to decreased field-induced migration of ions at the contact. Carrier recombination in the bulk of FASnI3 films, however, was only slightly modified by the presence of PEA. Femtosecond broadband transient absorption experiments up to 1.5 ns provided rate constants for the Auger and bimolecular recombination processes in FASnI3 of k3 = 1 × 10−29 cm6 s−1 and k2 = 3.1 × 10−10 cm3 s−1, respectively. In PEA0.08FA0.92SnI3, no significant differences in k2 and an only slightly increased k3 = 2 × 10−29 cm6 s−1 were measured. In extension to previous photoluminescence studies, we found efficient cooling of hot carriers by coupling to optical phonons (τcop = 0.5 ps), which is even faster than in lead perovskites. The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support (Nos. SCHL 340/21-3, OU 58/10-3, LE 926/11-3, and RTG 2204). We are also grateful to R. Kentsch for complementary steady-state absorption and PL measurements, M. Morgenroth for assistance during the transient absorption experiments, H. Hemmelmann for measuring XRD at grazing incidence, and ZfM/LaMa (JLU Gieβen) for support in SEM and photolithography.

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 cm3 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 cm3 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.

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.

Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a “non-injecting” mesoporous thin film with large band-gap, consisting of Al2O3 nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250–350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0 → S1 absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+ [N(CN)2] – show that D149 is ideally suited for solar cell applications because of its long S1 lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential.

We investigated the excited state dynamics of selected all-trans C40 xanthophyll carotenoids by ultrafast pump-supercontinuum probe (PSCP) spectroscopy in the spectral range 350–770 nm upon photoexcitation by a femtosecond laser (ca. 80 fs pulse length) near 500 nm in acetone. The following carotenoids were selected considering a systematic variation of the number and position of carbonyl (CO) and hydroxyl (OH) functional groups on the β-ionone rings and the change of the effective conjugation length of the polyene system: β-cryptoxanthin (1), echinenone (2), 3-hydroxyechinenone (3), 3′-hydroxyechinenone (4), canthaxanthin (5), adonirubin (6) and astaxanthin (7). The carotenoids featuring an increasing conjugation length, for example, (1), (3) and (6), showed a systematically shorter S1 lifetime of 8.7, 6.2 and 4.7 ps, respectively. Carotenoids with OH groups adjacent to a CO group showed slightly broader steady-state and transient absorption bands, but the influence on the S1 lifetime was minor compared to the case of CO substitution. Moreover, in all cases we observed clearly visible “S* signals”, namely a longer-lived characteristic S0 → S2 red-edge absorption and a negative signal in the S0 → S2 bleach region. This spectral signature is assigned to highly vibrationally excited molecules in the ground electronic state S0 * which are generated by internal conversion from S1. The S0 * species cool with a time constant of 9.2–9.6 ps in the case of the keto-substituted carotenoids (2)–(7), whereas we obtain a larger value of 13.2 ps for (1) which does not have a keto group. Our findings are supported by results from a global kinetic analysis procedure.

Es wurde eine hocheffiziente Palladium-katalysierte vierfache Tandem-Dominoreaktion entwickelt, die zwei Carbopalladierungen und zwei C-H-Aktivierungen umfasst. Dabei wurden die helical-chiralen tetrasubstituierten Alkene 3 und 6 ausgehend von den Substraten 1 und 4 erhalten. Die fur den Aufbau der Ausgangsverbindung erforderliche Sonogashira-Reaktion lasst sich zudem im Sinne einer sechsfachen Tandem-Dominoreaktion integrieren. Es wurden 20 Verbindungen mit unterschiedlichen Substitutionsmustern in Ausbeuten von bis zu 97 % synthetisiert, und die helicale Chiralitat der Verbindungen wurde durch Rontgenstrukturanalyse nachgewiesen. Photophysikalische Untersuchungen zeigen einen sehr schnellen lichtinduzierten Wechsel der stereochemischen Konfiguration.

A highly efficient palladium-catalyzed fourfold tandem-domino reaction consisting of two carbopalladation and two C-H-activation steps was developed for the synthesis of two types of tetrasubstituted alkenes 3 and 6 with intrinsic helical chirality starting from substrates 1 and 4, respectively. A sixfold tandem-domino reaction was also developed by including a Sonogashira reaction. 20 compounds with different substitution patterns were prepared with yields of up to 97 %. Structure elucidation by X-ray crystallography confirmed helical chirality of the two alkene moieties. Photophysical investigations of some of the compounds showed pronounced switching properties through light-controlled changes of their stereochemical configuration.

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 of0.08, 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 and1000 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.

Trivalent bismuth compounds featuring octahedral halide coordination, such as BiI3, double-perovskite structures and vacancy-ordered perovskite derivatives, have recently attracted considerable attention as potential environmentally benign thin film materials for solar light-harvesting and optoelectronic applications aiming to replace toxic lead-based perovskites. Yet, their photovoltaic performance still lags considerably behind their lead-counterparts. Improving their performance requires a detailed knowledge of the photoinduced carrier dynamics and kinetics in these systems. In the current study, we present a detailed investigation of layered BiI3. The compound exhibits pronounced exciton dynamics which is associated with a characteristic second-derivative type feature in the transient absorption spectra. Accurate time constants are assigned to carrier cooling and recombination processes. Pronounced coherent oscillations are observed in the kinetic traces indicating strong electron-phonon coupling. Analysis of the coherences recovers the steady-state Raman spectrum of BiI3 with a dominant mode of Ag symmetry at 114 cm-1, associated with the zone center C(Γ) phonon. The line position is well reproduced by DFT calculations. In terms of the carrier lifetime, high absorption coefficient and band-gap position, BiI3 is a promising candidate as a photovoltaic material but challenges posed are the difficulty to split the strongly bound excitons which are confined in the 2D layers and the efficient extraction of electrons and holes at n- and p-selective contacts, beside well-known issues regarding high resistivity and the intrinsic softness of this material.

We present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterise its photophysics in different organic solvents and on mesoporous Al

Organic-inorganic hybrid lead halide perovskites already reach very high power conversion efficiencies above 22% on architectures employing mesoporous TiO

Nonreactive dynamics of molecules in ionic liquids (ILs) have been investigated in various environments, such as in neat aprotic and protic ILs, as well as in binary mixtures of ILs with polar organic solvents and CO 2 . Transient broadband absorption and fluorescence spectroscopic techniques on timescales from femtoseconds to milliseconds have been applied to elucidate the importance of different types of solute–solvent interactions in these media. Molecular probes such a 12′-apo-β-carotenoic-12′-acid and Coumarin C153 have been used as reporter molecules for ionic liquid solvation dynamics, micropolarity, hydrogen-bonding and collisional relaxation. With Coumarin C153 in ILs, the relationship between the complete solvation response and microwave-terahertz absorption measurements of the neat liquid was explored quantitatively. A reduced value for the conductivity plays a key role in such solvation experiments. Aprotic ILs exhibit a micropolarity larger than expected from their static dielectric constants, whereas the local polarity of protic alkylammonium formate ILs appears to be similar to that of alcohols having comparable dielectric constants. In IL mixtures with organic solvents or CO 2 , there is no indication for differences in the bulk composition and the local composition around the molecular probe. Collisional relaxation processes in ILs take place on similar timescales as in dipolar organic solvents.

We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical density ΔOD was probed over the range 260–640 nm (1.94–4.77 eV), reaching down into the region of graphene’s Fano resonance, previously not investigated in femtosecond broadband pump–probe experiments. Time constants of 160 fs and 4 ps were found and assigned to carrier–optical phonon scattering and slower phonon relaxation processes, respectively. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was observed below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time constant of 35 ps for monolayer and time constants of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. ...

During the last decade, ionic liquids (ILs) have revealed promising properties and applications in many research fields, including biotechnology and biological sciences. The focus of this contribution is to give a critical review of the phenomena observed and current knowledge of the interactions occurring on a molecular basis. As opposed to the huge advances made in understanding the properties of proteins in ILs, complementary investigations dealing with interactions between ILs and peptides or oligopeptides are underrepresented and are mostly only of phenomenological nature. However, the field has received more attention in the last few years. This Review features a meta-analysis of the available data and findings and should, therefore, provide a basis for a scientifically profound understanding of the nature and mechanisms of interactions between ILs and structured or nonstructured peptides. Fundamental aspects of the interactions between different peptides/oligopeptides and ILs are complemented by sections on the experimental (spectroscopy, structural biology) and theoretical (computational chemistry) possibilities to explain the phenomena reported so far in the literature. In effect, this should lead to the development of novel applications and support the understanding of IL-solute interactions in general.

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 Al

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.

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.