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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]

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).)

The orange carotenoid protein (OCP) is a photoactive protein involved in cyanobacterial photoprotection by quenching of the excess of light-harvested energy. The photoactivation mechanism remains elusive, in part due to absence of data pertaining to the timescales over which protein structural changes take place. It also remains unclear whether or not oligomerization of the dark-adapted and light-adapted OCP could play a role in the regulation of its energy-quenching activity. Here, we probed photoinduced structural changes in OCP by a combination of static and time-resolved X-ray scattering and steady-state and transient optical spectroscopy in the visible range. Our results suggest that oligomerization partakes in regulation of the OCP photocycle, with different oligomers slowing down the overall thermal recovery of the dark-adapted state of OCP. They furthermore reveal that upon non-photoproductive excitation a numbed state forms, which remains in a non-photoexcitable structural state for at least ≈0.5 μs after absorption of a first photon., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

3,3-Diphenylbenzo[f]chromene (1) represents an important architectural platform for photochromic systems. Since the practical utility of such chromophores is largely dependent upon the kinetics of coloration and decoloration, elucidating the mechanistic details of these processes is of great value. Toward this end, we studied the photochromic reaction of (3-(2-methoxyphenyl)-3-phenyl-3H-benzo[f]chromene (2) by both time-resolved UV-vis and mid-IR spectroscopies. We found that irradiation of 2 at 365 nm generates long-lived colored transoid-cis isomers with lifetimes of 17.1 s and 17.5 min (at 21 °C) and even longer-lived transoid-trans isomers with a lifetime of 16 h. These experimental results were supplemented with ab initio ground-state and excited-state calculations, and the resulting theoretical interpretation may be useful for the design of new photochromic systems with optimized photofunctionality., (© 2022. The Author(s).)

A substantial number of Orange Carotenoid Protein (OCP) studies have aimed to describe the evolution of singlet excited states leading to the formation of a photoactivated form, OCP R . The most recent one suggests that 3 ps-lived excited states are formed after the sub-100 fs decay of the initial S 2 state. The S* state, which has the longest reported lifetime of a few to tens of picoseconds, is considered to be the precursor of the first red photoproduct P 1 . Here, we report the ultrafast photodynamics of the OCP from Synechocystis PCC 6803 carried out using visible-near infrared femtosecond time-resolved absorption spectroscopy as a function of the excitation pulse power and wavelength. We found that a carotenoid radical cation can form even at relatively low excitation power, obscuring the determination of photoactivation yields for P 1 . Moreover, the comparison of green (540 nm) and blue (470 nm) excitations revealed the existence of an hitherto uncharacterized excited state, denoted as S ∼ , living a few tens of picoseconds and formed only upon 470 nm excitation. Because neither the P 1 quantum yield nor the photoactivation speed over hundreds of seconds vary under green and blue continuous irradiation, this S ∼ species is unlikely to be involved in the photoactivation mechanism leading to OCP R . We also addressed the effect of His-tagging at the N- or C-termini on the excited-state photophysical properties. Differences in spectral signatures and lifetimes of the different excited states were observed at a variance with the usual assumption that His-tagging hardly influences protein dynamics and function. Altogether our results advocate for the careful consideration of the excitation power and His-tag position when comparing the photoactivation of different OCP variants and beg to revisit the notion that S* is the precursor of photoactivated OCP R ., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza- s -indacene) donor and a fullerene C 60 acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C 60 dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C 60 dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C 60 dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C 60 moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C 60 dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

In the photochromic reactions of 3 H -naphthopyrans, two colored isomers TC (transoid- cis ) and TT (transoid- trans ) are formed. In terms of optimized photo-switchable materials, synthetic efforts are nowadays evolving toward developing 3 H -naphthopyran derivatives that would not be able to photoproduce the long-living transoid- trans , TT, photoproduct. The substitution with a methoxy group at position 10 results in significant reduction of the TT isomer formation yield. The TC photophysics responsible for TT suppression were revealed here using a combination of multi-scale time resolved absorption UV-vis spectroscopy and ab initio calculations. The substitution changes the TC excited-state potential energy landscape, the bicycle-pedal isomerization path is favored over the rotation around a single double bond. The bicycle-pedal path is aborted in halfway to TT formation due to S 1 →S 0 internal conversion populating back the TC species in the ground electronic state. This is validated by a shorter TC S 1 state lifetime for methoxy derivative in comparison to that of the parent-unsubstituted compound (0.47 ± 0.05 ps vs. 0.87 ± 0.09 ps) in cyclohexane.

Two noncovalent nanohybrids between cationic porphyrin (free-base TMPyP and zinc(II) ZnTMPyP) bearing cationic ( N -methylpyridyl) groups and graphene oxide (GO) were constructed with the aim of generating a photocatalyst active for rhodamine B (RhB) degradation. The obtained materials were thoroughly characterized by steady-state and time-resolved absorption and emission methods, which indicated that metalation of the porphyrin with Zn(II) increases the affinity of the porphyrin toward the GO surface. Photocurrent experiment together with femtosecond transient absorption spectroscopy clearly showed the existence of electron transfer from the photoexcited porphyrin to GO. Both hybrid materials demonstrated higher photocatalytic activity toward RhB degradation as compared to GO; however, ZnTMPyP-GO exhibited more efficient performance (19% of RhB decomposition after 2 h of irradiation). Our data indicate that the presence of Zn(II) in the core of the porphyrin can promote charge separation in the ZnTMPyP-GO composites. The higher degradation rate seen with ZnTMPyP-GO as compared to the TMPyP-GO assemblies highlights the beneficial role of Zn(II)-metalation of the porphyrin ring., Competing Interests: The authors declare no competing financial interest.

The present study explores the influence of graphene oxide (GO) on deactivation pathways of the excited states of zinc 5,10,15,20-tetrakis(4-(hydroxyphenyl))porphyrin (ZnTPPH). The interaction of light with free ZnTPPH molecules and with ZnTPPH molecules adsorbed on graphene oxide sheets was probed via UV-vis spectroscopy, fluorescence spectroscopy, femtosecond pump-probe technique and nanosecond flash photolysis. Formation of the ground-state ZnTPPH-GO complex in solution was monitored by the red-shift of the porphyrin Soret absorption band. It was found that Stern-Volmer fluorescence quenching can be described in terms of two different quenching regimes depending on the GO concentration. In addition, our comprehensive analysis of the steady-state and time-resolved emission experiments led to the conclusion that the observed quenching was entirely attributable to a static mechanism. Laser flash photolysis showed that the triplet lifetime of the ZnTPPH increased in the presence of GO from 174 μs to 292 μs, which is related to the decrease in the rate constant of a radiationless decay mechanism involving rotation of the peripheral hydroxyphenyl rings of the porphyrin. Femtosecond transient absorption spectroscopy demonstrated the presence of a fast photoinduced electron transfer from the singlet excited state of ZnTPPH to the GO sheets, as indicated by the formation of a porphyrin radical cation. Quantum chemical calculations were used to gain deeper insights into the nature of the electronically excited states in the free ZnTPPH as well as in the ZnTPPH-GO complex.

Structural details on the species involved in the photochromic reaction of 3H-naphthopyrans in solution have been formerly determined using NMR spectroscopy. Herein we show that at room temperature time-resolved FT-IR spectroscopy is a simple and efficient tool for structural characterization of colored species generated upon continuous UV light irradiation of the model compound 3H-naphthopyran: 3,3-diphenyl-3H-naphtho[2,1-b]pyran. In solution and in the polymer matrix phase, a colored species transoid-cis is formed after a single-photon excitation process, while transoid-trans is a secondary long-lived photoproduct generated after two-step excitation involving two photons. Understanding the reaction mechanism leading to long-lived colored species can help with the design of new 3H-naphthopyran derivatives structurally optimized for making a photochromic reaction free from transoid-trans products, which is often important for applications. Ab initio calculations show that photoinduced ring-opening followed by isomerization occurs on a multidimensional potential-energy surface. The barriers separating the considered isomeric forms, both in the ground and in the excited state, help to interpret the step-by-step dynamics of the photoprocesses. The system is composed of a variety of ground state equilibrium forms. Each of them is characterized by fast excited-state deactivation pathways which may drive the system through different conical intersection regions.

Ultrafast photoisomerization reactions generally start at a higher excited state with excess of internal vibrational energy and occur via conical intersections. This leads to ultrafast dynamics which are difficult to investigate with a single transient absorption spectroscopy technique, be it in the ultraviolet-visible (UV-vis) or infrared (IR) domain. On one hand, the information available in the UV-vis domain is limited as only slight spectral changes are observed for different isomers. On the other hand, the interpretation of vibrational spectra is strongly hindered by intramolecular relaxation and vibrational cooling. These limitations can be circumvented by fusing UV-vis and IR transient absorption spectroscopy data in a multiset multivariate curve resolution analysis. We apply this approach to describe the spectrodynamics of the ultrafast cis-trans photoisomerization around the C-N double bond observed for aromatic Schiff bases. Twisted intermediate states could be elucidated, and isomerization was shown to occur through a continuous complete rotation. More broadly, data fusion can be used to rationalize a vast range of ultrafast photoisomerization processes of interest in photochemistry.

Instantaneous coloration with large absorbance and quick color fading in the dark are desired properties for thermally reversible photochromic compounds. In the case of naphthopyran derivatives, which have been employed to commercial ophthalmic lenses, the quick color fading has been recently achieved by suppression of the generation of the transoid-trans (TT) form by steric hindrance of bulky substituents. However, there are still open questions whether the steric hindrance decreases the photochromic reaction efficiency, which is a crucial problem for industrial applications. Herein, we apply a wide range of electronic and vibrational time-resolved spectroscopies and reveal that the photochromic reaction yields of the naphthopyrans with bulky substituents are almost comparable (∼0.7) to that of nonsubstituted naphthopyran. The suppression of the formation of the TT form and the effect of solvent polarity on the photodynamics are systematically investigated. These findings are important for fundamental photochemistry and developing naphthopyran-based optimal photofunctional materials.

Betanin is the best known natural dye belonging to the betacyanin family. In this work, efficient singlet oxygen quenching by betanin in deuterated water with the rate constant 1.20 ± 0.15 × 10(8) M(-1) s(-1) is reported, deduced from the (1)O2 phosphorescence decays measured as a function of betanin concentration. The quenching occurs by a chemical mechanism, as confirmed by the analysis of the transient absorption kinetics at the probe λ ∼ 535 nm, by comparison of the initial triplet signal amplitude of perinaphthenone acting as the (1)O2 photosensitizer with the final bleaching signal of betanin. The main betanin oxidation product is 2-decarboxy-2,3-dehydrobetanin, with its formation observed as the transient absorption signal at λ ∼ 445 nm. LC-MS/MS analysis of the photolyzed solutions supports the product identification as 2-decarboxy-2,3-dehydrobetanin, based on the molecular ion [M](+) observed at m/z 505. Isobetanin also undergoes a similar photooxidation reaction.

The photophysical properties of betanin in aqueous and alcoholic solutions were determined at room temperature using ultrafast UV-vis-NIR transient absorption spectroscopy (λexc = 535 nm). Its S1 → Sn (n > 1) absorption bands appear with maxima at about λ ∼ 450 and 1220 nm. The short betanin S1 state lifetime (6.4 ps in water) is mainly determined by the efficient S1 → S0 radiationless relaxation, probably requiring a strong change in geometry, since the S1 lifetime grows to 27 ps in the more viscous ethylene glycol. The fluorescence quantum yield is very low (Φf ∼ 0.0007 in water), therefore this deactivation path is of minor importance. Other processes, such as S1 → T1 intersystem crossing or photoproduct formation, are virtually absent, since full S0 ← S1 ground state recovery is observed within tens of picoseconds after photoexcitation. The observed fast light-to-heat conversion in the absence of triplet excited state formation supports the idea that betanin is a photoprotector in vivo.