Evaluating the Solvent Stark Effect from Temperature-Dependent Solvatochromic Shifts of Anthracene.

The solvent Stark effect on the spectral shifts of anthracene is studied with temperature-dependent solvatochromic measurements. The Stark contribution ΔvStark to the absorption shift Δvp in polar solvents is measured to be ΔvStark =(53±35) cm-1 , in reasonable agreement with dielectric continuum theory estimate of 28 cm-1 , whereas the major shift Δvp ∼300 cm-1 presumably originates from the solute quadrupole. We pay attention to the accurate correction of Δvp for the nonpolar contribution that is crucial when the shifts are modest in magnitude.

Insights into Charge Transfer at an Atomically Precise Nanocluster/Semiconductor Interface.

The deposition of an atomically precise nanocluster, for example, Ag44 (SR)30 , onto a large-band-gap semiconductor such as TiO2 allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H2 generation, with the H2 production rate reaching 7.4 mmol h-1 gcatalyst -1 . This is five times higher than that of TiO2 modified with Ag nanoparticles and even comparable to that of TiO2 modified with Pt nanoparticles under similar conditions. Energy band alignment and transient absorption spectroscopy reveal that the role of the metal clusters is different from that of both organometallic complexes and plasmonic nanoparticles: A type II heterojunction charge-transfer route is achieved under UV/Vis irradiation, with the cluster serving as a small-band-gap semiconductor. This results in the clusters acting as co-catalysts rather than merely photosensitizers.

Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck-Condon Region.

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S1FC and S2FC are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S1 state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S3FC to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S1. The polarity of S1 is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.

ß-Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy.

ß-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S1 excited-state absorption, a picosecond process is found that is absent when the C9 -methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S1 potential energy surface, involving dihedral angles between C6 and C9 . In Raman studies, electronic states S2 or S1 were selected through resonance conditions. We observe a broad vibrational band at 1770 cm(-1) in S2 already. With 200 fs it decays and transforms into the well-known S1 Raman line for an asymmetric C=C stretching mode. Low-frequency activity (<800 cm(-1) ) in S2 and S1 is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between ß-carotene and nearby n-hexane molecules.

Time-Resolved Photochemistry of Stiffened Stilbenes.

Broadband transient absorption spectroscopy is used to study the photoisomerization of stiffened stilbenes in solution, specifically E/ Z mixtures of bis(benzocyclobutylidene) (t4, c4) and ( E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t6). Upon excitation to S1, all evolve to perpendicular molecular conformation P, followed by decay to S0, while the spectra and the kinetic behavior crucially depend on the size of the stiffening ring. In 4, contrary to all previously studied stilbenes, the trans and cis absorption and excited-state spectra are nearly indistinguishable, while the corresponding isomerization times are comparable: τi = 166 ps for t4 and τi = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile, respectively. Faster isomerization in polar solvents agrees with the zwitterionic character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S1 → P evolution being directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P state is remarkably long-lived, τP = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S1 energy surface.

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy.

We monitor the time-dependent Stokes shift (TDSS) of fluorescence from the zwitterionic probe N-methyl-6-oxyquinolinium betaine in water. A spectral relaxation time τsolv = 0.57 ps (at 20.5 °C) is attributed to a solvation process involving water in the hydration layer. In this article we show that a tertiary butyl group, when attached to the chromophore, slows the dynamics to τsolv = 0.76 ps and increases the corresponding activation energy by 5 kJ/mol. In a companion paper (10.1021/acs.jpcb.7b05039), simulations suggest that the observed slow-down indicates coupling of solute vibrations to hydration water. Thus, a new angle on a thoroughly researched topic, solvation dynamics, has been opened.

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy.

In the photoisomerization path of stilbene, a perpendicular state P on the S1 potential energy surface is expected just before internal conversion through a conical intersection S1/S0. For decades the observation of P was thwarted by a short lifetime τP in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of trans-1,1'-dicyanostilbene, P is populated significantly (τP = 27 ps in n-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm-1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.

Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S2 State of All-Trans ß-Carotene.

Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans ß-carotene in n-hexane. The short-lived excited electronic state S2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S2 state, exploiting resonance from an Sx ← S2 transition, and (ii) by FSR without actinic excitation, using S2 ↔ S0 resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ((5)) and χ((3)), respectively. Both methods show low-frequency bands of the S2 state at 200, 400, and ∼600 cm(-1), which are reported for the first time. With (ii) the intensities of low-frequency vibrational resonances in S2 are larger compared to those in S0, implying strong anharmonicities/mode mixing in the excited state. In principle, for short-lived electronic states, the χ((3)) method should allow the best characterization of low-frequency modes.

Fluorescence following excited-state protonation of riboflavin at N(5).

Excited-state protonation of riboflavin in the oxidized form is studied in water. In the -1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H(+) concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule.