Torsional disorder and planarization dynamics: 9,10-bis(phenylethynyl)anthracene as a case study

Conjugated molecules with phenylethynyl building blocks are usually characterised by torsional disorder at room temperature. They are much more rigid in the electronic excited state due to conjugation. As a consequence, the electronic absorption and emission spectra do not present a mirror-image relationship. Here, we investigate how torsional disorder affects the excited state dynamics of 9,10-bis(phenylethynyl)anthracene in solvents of different viscosities and in polymers, using both stationary and ultrafast electronic spectroscopies. Temperature-dependent measurements reveal inhomogeneous broadening of the absorption spectrum at room temperature. This is confirmed by ultrafast spectroscopic measurements at different excitation wavelengths. Red-edge irradiation excites planar molecules that return to the ground state without significant structural dynamics. In this case, however, re-equilibration of the torsional disorder in the ground state can be observed. Higher-energy irradiation excites torsionally disordered molecules, which then planarise, leading to important spectral dynamics. The latter is found to occur partially via viscosity-independent inertial motion, whereas it is purely diffusive in the ground state. This dissimilarity is explained in terms of the steepness of the potential along the torsional coordinate.

Beyond the Threshold: A Study of Chalcogenophene-Based Two-Photon Initiators

A series of nine soluble, symmetric chalcogenophenes bearing hexyl-substituted triphenylamines, indolocarbazoles, or phenylcarbazoles was designed and synthesized as potential two-photon absorption (2PA) initiators. A detailed photophysical analysis of these molecules revealed good 2PA properties of the series and, in particular, a strong influence of selenium on the 2PA cross sections, rendering these materials especially promising new 2PA photoinitiators. Structuring and threshold tests proved the efficiency and broad spectral versatility of two selenium-containing lead compounds as well as their applicability in an acrylate resin formulation. A comparison with commercial photoinitiators Irg369 and BAPO as well as sensitizer ITX showed that the newly designed selenium-based materials TPA-S and TPA-BBS outperform these traditional initiators by far both in terms of reactivity and dose. Moreover, by increasing the ultralow concentration of TPA-BBS, a further reduction of the polymerization threshold can be achieved, revealing the great potential of this series for application in two-photon polymerization (2PP) systems where only low laser power is available.

Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving force range of more than 1.3 eV. The intrinsic ET rate constants, k₀, deduced from the quenching dynamics in the static regime, are of the order of 10¹²-10¹³ M^-1 s^-1, i.e., at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveals that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Propyl acetate/butyronitrile mixture is ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions

Characterization of propyl acetate/butyronitrile (PA/BuCN) mixtures by various spectroscopic techniques is described. The neat solvents have identical viscosities and refractive indices but their dielectric constants differ significantly. Detailed solvatochromic and titration data show that the mixtures do not exhibit specific solute-solvent interactions or significant dielectric enrichment effects. Therefore, the mixtures are ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions. Dynamic Stokes shift experiments performed on two push-pull probes demonstrate that the solvation dynamics are significantly decelerated in the mixtures as compared to the neat solvents. Therefore, the mixtures allow for varying both the extent and time scale of the dielectric stabilization in a predictable manner.

Picosecond Switchable Azo Dyes

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Ultrafast dynamics of the antibiotic Rifampicin in solution

Ultrafast time-resolved studies demonstrate that intra- and intermolecular H-bonds with water molecules act synergistically to stabilize the active zwitterionic form of Rifampicin, an effective antibiotic against mycobacterial infections.

Excited-state dynamics of a molecular dyad with two orthogonally-oriented fluorophores

The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin-orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state.

Energy transfer and charge separation dynamics in photoexcited pyrene-bodipy molecular dyads

The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2,6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2-dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the pyrene (Py) subunit leads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5-5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with ability to undergo CS.

Probe dependence on polar solvation dynamics from fs broadband fluorescence

Solvation dynamics is remarkably independent of the probe as long as specific interactions remain similar.

Excited state structural evolution during charge-transfer reactions in betaine-30

Ultrafast photo-induced charge-transfer reactions are fundamental to a number of photovoltaic and photocatalytic devices, yet the multidimensional nature of the reaction coordinate makes these processes difficult to model theoretically.

To what extent can charge localization influence electron injection efficiency at graphene–porphyrin interfaces?

With careful control of the charge localization of the TMPyP cavity using β-cyclodextrin as an external cage, we successfully improved the interfacial-electron injection efficiency from cationic TMPyP to GC by 120% compared to TMPyP alone.

Structure–property relationship of isomeric diphenylethenyl-disubstituted dimethoxycarbazoles

Isomeric 3,6-dimethoxy- and 2,7-dimethoxycarbazoles containing diphenylethenyl moieties were synthesized by condensation of the appropriate dimethoxycarbazoles with diphenylacetaldehyde.

On the relation of energy and electron transfer in multidimensional chromophores based on polychlorinated triphenylmethyl radicals and triarylamines

Chromophores with many donors and acceptors show electron transfer which is identical to energy transfer.

Excited-state dynamics of 3-hydroxyflavone anion in alcohols

The electronic absorption spectrum of 3-hydroxyflavone (3HF) in various solvents exhibits a long-wavelength (LW) band, whose origin has been debated. The excited-state dynamics of neutral and basic solutions of 3HF in alcohols upon excitation in this LW band has been investigated using a combination of fluorescence up-conversion and transient electronic and vibrational absorption spectroscopies. The ensemble of results reveals that, in neutral solutions, LW excitation results in the population of two excited species with similar fluorescence spectra but very different lifetimes, namely 40-100 ps and 2-3 ns, depending on the solvent. In basic solutions, the relative concentrations of these species change considerably in favor of that with the short-lived excited state. On the basis of the spectroscopic data and quantum chemistry calculations, the short lifetime is attributed to the excited state of 3HF anion, whereas the long one is tentatively assigned to an excited hydrogen-bonded complex with the solvent. Excited-state intermolecular proton transfer from the solvent to the anion yielding the tautomeric form of 3HF is not operative, as the excited anion decays to the ground state via an efficient nonradiative transition.

Excitation Wavelength Dependence of the Dynamics of Bimolecular Photoinduced Electron Transfer Reactions

The dynamics of photoinduced electron transfer between polar acceptors and donors has been investigated in apolar solvents using femtosecond-resolved fluorescence spectroscopy. It was found to be ultrafast and to continuously accelerate by varying the excitation wavelength from the maximum to the red edge of the absorption band of the acceptor, the overall difference being as large as a factor 4-5. This violation of the Kasha-Vavilov rule is explained by a correlation between the composition of the acceptor environment and its transition energy, that is, the more donors around an acceptor, the longer its absorption wavelength, and the faster the quenching. Because of preferential solvation, this dependence is already observed at low quencher concentrations. This effect, which requires quenching to be faster than the fluctuations of the environment composition, should be quite general for photoinduced charge transfer processes in low-polarity, viscous, or rigid media, such as those used in organic optoelectronic devices.

Ultrafast long-distance excitation energy transport in donor-bridge-acceptor systems

The excited-state dynamics of two energy donor-bridge-acceptor (D-B-A) systems consisting of a zinc tetraphenylporphyrin (ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-p-phenyleneethynylene units with different substituents has been investigated using ultrafast spectroscopy. These systems differ by the location of the lowest singlet excited state of the bridge, just above or below the S(2) porphyrin states. In the first case, Soret band excitation of the porphyrins is followed by internal conversion to the local S(1) state of both molecules and by a S(1) energy transfer from the ZnP to the FbP end on the 10 ns time scale, as expected for a center-to-center distance of about 4.7 nm. On the other hand, if the bridge is excited, the energy is efficiently transferred within 1 ps to both porphyrin ends. Selective bridge excitation is not possible with the second system, because of the overlap of the absorption bands. However, the time-resolved spectroscopic data suggest a reversible conversion between the D*(S(2))-B-A and D-B*(S(1))-A states as well as a transition from the D-B*(S(1))-A to the D-B-A* states on the picosecond time scale. This implies that the local S(2) energy of the ZnP end can be transported stepwise to the FbP end, i.e., over about 4.7 nm, within 1 ps with an efficiency of more than 0.2.