Optically Controlled Molecular Switching of an Indolobenzoxazine-Type Photochromic Compound

Singlet fission in naturally-organized carotenoid molecules

We have investigated the photophysics of aggregated lutein/violaxanthin in daffodil chromoplasts. We reveal the presence of three carotenoid aggregate species, the main one composed of a mixture of lutein/violaxanthin absorbing at 481 nm, and two secondary populations of aggregated carotenoids absorbing circa 500 and 402 nm. The major population exhibits an efficient singlet fission process, generating μs-lived triplet states on an ultrafast timescale. The structural organization of aggregated lutein/violaxanthin in daffodil chromoplasts produces well-defined electronic levels that permit the energetic pathways to be disentangled unequivocally, allowing us to propose a consistent mechanism for singlet fission in carotenoid aggregates. Transient absorption measurements on this system reveal for the first time an entangled triplet signature for carotenoid aggregates, and its evolution into dissociated triplet states. A clear picture of the carotenoid singlet fission pathway is obtained, which is usually blurred due to the intrinsic disorder of carotenoid aggregates.

Surface Plasmon-Enhanced Switching Kinetics of Molecular Photochromic Films on Gold Nanohole Arrays

Diarylethene molecules are discussed as possible optical switches, which can reversibly transition between completely conjugated (closed) and nonconjugated (open) forms with different electrical conductance and optical absorbance, by exposure to UV and visible light. However, in general the opening reaction exhibits much lower quantum yield than the closing process, hindering their usage in optoelectronic devices. To enhance the opening process, which is supported by visible light, we employ the plasmonic field enhancement of gold films perforated with nanoholes. We show that gold nanohole arrays reveal strong optical transmission in the visible range (∼60%) and pronounced enhancement of field intensities, resulting in around 50% faster switching kinetics of the molecular species in comparison with quartz substrates. The experimental UV-vis measurements are verified with finite-difference time-domain simulation that confirm the obtained results. Thus, we propose gold nanohole arrays as transparent and conductive plasmonic material that accelerates visible-light-triggered chemical reactions including molecular switching.

Lycopene crystalloids exhibit singlet exciton fission in tomatoes

Transient absorption studies conducted on in vitro lycopene aggregates, as well as on lycopene crystalloids inside tomato chromoplasts, reveal the appearance of a long-lived excited state, which we unambiguously identified as lycopene triplet. These triplet states must be generated by singlet exciton fission, which occurs from the lycopene 2Ag state. This is the first time the singlet fission process has ever been shown to occur in a biological material. We propose that the formation of carotenoid assemblies in chromoplasts may constitute a photoprotective process during chromoplast maturation, in addition to their function in signaling processes.

Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Redefining the established understanding of excitation dynamics of photochromic oxazines

Since the introduction of photochromic indolo-benzoxazines, difference absorption of these photoexcited compounds has been assigned to the ground state of the ring-opened isomer. This assignment relies on the alleged resemblance of the spectra of photo- and chemically induced forms. In this paper, we expose the issue of the discrepancy between the absorption spectra of photoproducts and the corresponding chemically opened forms. As a result, a substantial change in the current explanation of photodynamics of photochromic oxazines is proposed. The spectral features earlier ascribed to the photoproduct are suggested to arise due to the absorption of the triplet state. This hypothesis was tested and confirmed in acetonitrile by measuring the effect of oxygen quenching of photoproduct states. In view of this interpretation, light-induced ring opening does not occur in indolo-benzoxazines dissolved in acetonitrile, and, consequently these molecules should no longer be regarded as molecular switches. On the other hand, we show that methanol solution under UV light does produce small amounts of ring-opened form of the molecule.