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Photoinduced electron and energy transfer pathways are elucidated in a panchromatic light-absorbing, covalently linked triad (BODIPY-C60-distyryl BODIPY, noted as BDP-C60-DSBDP) along with the two reference dyads: BODIPY-C60 (BDP-C60) and distyryl-BODIPY-C60 (DSBDP-C60). The flexible linker between the BODIPY and C60 units leads to different possible conformations with varying donor–acceptor distances. Ultrafast transient absorption along with the time-resolved emission spectroscopies revealed the occurrence of different photoinduced electron/energy transfer processes in these conformers. Photoexcitation of the BODIPY units in the two reference dyads leads to one electron and two energy transfer steps from BODIPY to C60. However, in the BDP-C60-DSBDP triad, additional energy transfer processes from BDP to DSBDP were evidenced upon photoexcitation of the BDP unit. The singlet excited state of DSBDP in the triad then follows the same relaxation route as that of the DSBDP-C60 dyad. The intricate photophysics, particularly the formation of radical ion pairs in these flexible covalently linked donor–acceptor systems contribute to our fundamental understanding of electron and energy transfer mechanisms, which is important to build donor–acceptor assemblies for energy applications. [ABSTRACT FROM AUTHOR]

The primary physical and chemical processes in the photochemistry of 1-(trideuteromethyl)-2,3,4-trideutero ( 1) and 1-acetoxy-2-methoxy-( 2) 9,10-anthraquinones were studied using femtosecond transient absorption spectroscopy and computational chemistry. Excitation of 1 and 2 at 270 nm results in the population of a set of highly excited singlet states which decay within the laser pulse by internal conversion and vibrational energy redistribution. The transient absorption spectra of the lowest singlet and triplet excited states of substituted anthraquinones 1 and 2 as well as the triplet excited and ground states of the products were detected. The assignments of the transient absorption spectra were performed on the basis of quantum chemical calculations of the electronic absorption spectra of the intermediates. Time-dependent density functional theory or CASSCF/CASPT2 procedure were used to calculate the spectroscopic properties of the intermediates. [ABSTRACT FROM AUTHOR]

Abstract: The T1 state of thiocoumarin has been investigated by transient absorption, resonance Raman scattering, and ab initio calculations. For comparison, the ground state molecule has also been examined by Raman spectroscopy and ab initio calculations. This analysis shows that the benzo moiety is only slightly modified in the T1 state compared to the ground state and keeps its aromatic character. In contrast, stronger distortions arise in the pyranthione moiety, resulting essentially from a lowering of the π electron density on the CC bond in position β to the CS bond, concomitant with an increase of the π density on the CC bond adjacent to the CS bond. However, the CS bond appears not significantly modified on going from S0 to T1, demonstrating definitely the ππ* nature of the T1 state. The planar conformation of the ground state is preserved in the triplet state. [Copyright &y& Elsevier]

We have studied for the first time the photophysics of the thioketone, 4H-1-benzopyrane-4-thione (BPT), in water and deuterated water, by using femtosecond fluorescence up-conversion and transient absorption spectroscopies. We propose that the S2-state deactivation occurs in the hydrogen-bonded BPT–water complex. The lifetime of the complex in the S2 state is found to be as short as 1.0 ± 0.1 ps in H2O, and 2.2 ± 0.1 ps in D2O. [Copyright &y& Elsevier]

Reorientational dynamics of 4H-1-benzopyrane-4-thione (BPT) in the S2 state, in hydrocarbon solution, is studied from the anisotropy decay of fluorescence upconversion and pump–probe absorption transients with femtosecond time resolution. The anisotropy decay is found to be monoexponential in n-hexane, n-heptane, n-octane, n-decane, n-dodecane and cyclohexane. The dependence of the rotational time constant, τor, on the solvent viscosity is approximated by a linear function with a slope of 2.6 ± 1 ps/cP indicating slip conditions. The decay of orientational anisotropy is observed for the first time for a molecule in the S2 state. [Copyright &y& Elsevier]

Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile. [Copyright &y& Elsevier]

Femtosecond transient absorption has been used to study the mechanism and dynamics of the quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT), in the S2 state, by saturated hydrocarbons (n-hexane, dodecane, hexadecane, cyclohexane, cyclohexane-d12). The results demonstrate the involvement of the hydrocarbon C–H bonds in the quenching process. Two quenching mechanisms are proposed: efficient H-atom abstraction followed by ultrafast back hydrogen transfer, or ‘aborted hydrogen abstraction’. In the latter case, the progress along the reaction path to a conical intersection is assumed to deactivate the S2 state through transition to the S1 state energy surface. [Copyright &y& Elsevier]

Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet 1 (π,π*) excited state deactivation pathway through nonadiabatic transition to the 1 (n,π*) excited- state, and subsequent deactivation to the ground electronic state (S 0 ), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited-state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T 1 state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C (2) -O distance elongates by 0.16 Å in the 1 (n,π*) state, with respect to the ground state. Intermediate-state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques., (© 2024. The Author(s).)

Two BODIPY-C 60 -peptide assemblies were synthesized by CuAAC reactions of BODIPY-C 60 dyads and a helical peptide functionalized with a terminal alkyne group and an azide group, respectively. The helical peptide within these assemblies was functionalized at its other end by a disulfide group, allowing formation of self-assembled monolayers (SAMs) on gold surfaces. Characterizations of these SAMs, as well as those of reference molecules (BODIPY-C 60 -alkyl, C 60 -peptide and BODIPY-peptide), were carried out by PM-IRRAS and cyclic voltammetry. BODIPY-C 60 -peptide SAMs are more densely packed than BODIPY-C 60 -alkyl and BODIPY-peptide based SAMs. These findings were attributed to the rigid peptide helical conformation along with peptide-peptide and C 60 -C 60 interactions within the monolayers. However, less dense monolayers were obtained with the target assemblies compared to the C 60 -peptide, as the BODIPY entity likely disrupts organization within the monolayers. Finally, electron transfer kinetics measurements by ultra-fast electrochemistry experiments demonstrated that the helical peptide is a better electron mediator in comparison to alkyl chains. This property was exploited along with those of the BODIPY-C 60 dyads in a photo-current generation experiment by converting the resulting excited and/or charge separated states from photo-illumination of the dyad into electrical energy., (© 2024 The Authors. ChemPlusChem published by Wiley-VCH GmbH.)

In photochemistry the excited-state intramolecular proton transfer process (ESIPT) is often observed as a highly efficient singlet excited state depletion pathway, which in the presence of a strong intramolecular hydrogen bond may proceed on a subpicosecond time scale. The present work describes the suppression of unwanted transoid-trans isomer formation in photochromic 3H-naphthopyran derivatives by the introduction of a 5-hydroxy substituent. According to time-resolved spectroscopy experiments and excited-state ab initio calculations, transoid-cis → transoid-trans photoisomerization is reduced by a competitive ESIPT channel in nonpolar solvent (cyclohexane). Upon specific solute-solvent interactions (methanol, acetonitrile) the intramolecular hydrogen bond in the transoid-cis form is perturbed, favoring the internal conversion S 1  → S 0 process as photostabilizing channel., (© 2022. The Author(s).)

The orange carotenoid protein (OCP) is a photoactive protein involved in cyanobacterial photoprotection. Here, we report on the functional, spectral and structural characteristics of the peculiar Planktothrix PCC7805 OCP (Plankto-OCP). We show that this OCP variant is characterized by higher photoactivation and recovery rates, and a stronger energy-quenching activity, compared to other OCP studied thus far. We characterize the effect of the functionalizing carotenoid and of his-tagging on these reactions, and identify the time scales on which these modifications affect photoactivation. The presence of a his-tag at the C-terminus has a large influence on photoactivation, thermal recovery and PBS-fluorescence quenching, and likewise for the nature of the carotenoid that additionally affects the yield and characteristics of excited states and the ns-s dynamics of photoactivated OCP. By solving the structures of Plankto-OCP in the ECN- and CAN-functionalized states, each in two closely-related crystal forms, we further unveil the molecular breathing motions that animate Plankto-OCP at the monomer and dimer levels. We finally discuss the structural changes that could explain the peculiar properties of Plankto-OCP., (Copyright © 2022. Published by Elsevier B.V.)

Photophysical studies on a BODIPY-fullerene-distyryl BODIPY triad (BDP-C 60 -DSBDP) and its reference dyads (BODIPY-fullerene; BDP-C 60 and distyryl BODIPY-fullerene; DSBDP-C 60 ) are presented herein. In the triad, the association of the two chromophore units linked by a fullerene moiety leads to strong near UV-Visible light absorption from 300 to 700 nm. The triplet-excited state was observed upon visible excitation in all these assemblies, and shown to be localized on the C 60 or BODIPY moieties. Using quantitative nanosecond transient absorption, we provide a complete investigation on the lifetime and formation quantum yield of the triplet-excited state. In the BDP-C 60 dyad, the triplet excited state of C 60 (τ = 7 ± 1 μs) was obtained with a quantum yield of 40 ± 8%. For the DSBDP-C 60 dyad and BDP-C 60 -DSBDP triad, a longer-lived triplet excited state with a lifetime of around 250 ± 20 μs centered on the DSBDP moiety was formed, with respective quantum yields of 37 ± 8 and 20 ± 4%. Triplet-triplet annihilation up-conversion is characterized in the BDP-C 60 dyad and the bichromophoric triad in the presence of perylene and DSBDP-monomer as respective annihilators. The photo-induced formation of a long-lived 3 DSBDP* in the triad coupled with panchromatic light absorption offers potential applications as a heavy-atom-free organic triplet photosensitizer., (© 2022. The Author(s), under exclusive licence to European Photochemistry Association, European Society for Photobiology.)

In all published photoactivation mechanisms of orange carotenoid protein (OCP), absorption of a single photon by the orange dark state starts a cascade of red-shifted OCP ground-state intermediates that subsequently decay within hundreds of milliseconds, resulting in the formation of the final red form OCP R , which is the biologically active form that plays a key role in cyanobacteria photoprotection. A major challenge in deducing the photoactivation mechanism is to create a uniform description explaining both single-pulse excitation experiments, involving single-photon absorption, and continuous light irradiation experiments, where the red-shifted OCP intermediate species may undergo re-excitation. We thus investigated photoactivation of Synechocystis OCP using stationary irradiation light with a biologically relevant photon flux density coupled with nanosecond laser pulse excitation. The kinetics of photoactivation upon continuous and nanosecond pulse irradiation light show that the OCP R formation quantum yield increases with photon flux density; thus, a simple single-photon model cannot describe the data recorded for OCP in vitro . The results strongly suggest a consecutive absorption of two photons involving a red intermediate with ≈100 millisecond lifetime. This intermediate is required in the photoactivation mechanism and formation of the red active form OCP R ., Competing Interests: The authors declare no competing interests., (© 2022 The Authors.)

Two graphene oxide nanoassemblies using 5-(4-(aminophenyl)-10,15,20-triphenylporphyrin (TPPNH 2 ) were fabricated by two synthetic methods: covalent (GO-CONHTPP) and noncovalent bonding. GO-CONHTPP was achieved through amide formation at the periphery of GO sheets and the hybrid material was fully characterized by FTIR, XPS, Raman spectroscopy, and SEM. Spectroscopic measurements together with theoretical calculations demonstrated that assembling TPPNH 2 on the GO surface in DMF-H 2 O (1:2, v/v) via non-covalent interactions causes changes in the absorption spectra of porphyrin, as well as efficient quenching of its emission. Interestingly, covalent binding to GO does not affect notably neither the porphyrin absorption nor its fluorescence. Theoretical calculations indicates that close proximity and π-π-stacking of the porphyrin molecule with the GO sheet is possible only for the non-covalent functionalization. Femtosecond pump-probe experiments revealed that only the non-covalent assembly of TPPNH 2 and GO enhances the efficiency of the photoinduced electron transfer from porphyrin to GO. In contrast to the non-covalent hybrid, the covalent GO-CONHTPP material can generate singlet oxygen with quantum yields efficiency (ΦΔ = 0.20) comparable to that of free TPPNH 2 (ΦΔ = 0.26), indicating the possible use of covalent hybrid materials in photodynamic/photothermal therapy. The spectroscopic studies combined with detailed quantum-chemical analysis provide invaluable information that can guide the fabrication of hybrid materials with desired properties for specific applications., (© 2022. The Author(s).)