Fluorescence Anisotropy Detection of Barrier Crossing and Ultrafast Conformational Dynamics in the S₂ State of β-Carotene

Abstract

Carotenoids are usually only weakly fluorescent despite being very strong absorbers in the mid-visible region because their first two excited singlet states, S 1 and S 2 , have very short lifetimes. To probe the structural mechanisms that promote the nonradiative decay of the S 2 state to the S 1 state, we have carried out a series of fluorescence lineshape and anisotropy measurements with a prototype carotenoid, β-carotene, in four aprotic solvents. The anisotropy values observed in the fluorescence emission bands originating from the S 2 and S 1 states reveal that the large internal rotations of the emission transition dipole moment, as much as 50°relative to that of the absorption transition dipole moment, are initiated during ultrafast evolution on the S 2 state potential energy surface and persist upon nonradiative decay to the S 1 state. Electronic structure calculations of the orientation of the transition dipole moment account for the anisotropy results in terms of torsional and pyramidal distortions near the center of the isoprenoid backbone. The excitation wavelength dependence of the fluorescence anisotropy indicates that these out-of-plane conformational motions are initiated by passage over a low-activation energy barrier from the Franck−Condon S 2 structure. This conclusion is consistent with detection over the 80−200 K range of a broad, red-shifted fluorescence band from a dynamic intermediate evolving on a steep gradient of the S 2 state potential energy surface after crossing the activation barrier. The temperature dependence of the oscillator strength and anisotropy indicate that nonadiabatic passage from S 2 through a conical intersection seam to S 1 is promoted by the out-of-plane motions of the isoprenoid backbone with strong hindrance by solvent friction.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots

“…Linear absorption spectra of the QD preparations were recorded at room temperature (23 °C) with a Shimadzu UV-2600 spectrometer. The room-temperature photoluminescence spectrum was recorded with a home-built fluorescence spectrometer employing a broadband visible LED and a compact double monochromator as an excitation source and a spectrograph and a CCD camera as the detector.…”

Spectral Signatures of Canthaxanthin Translocation in the Orange Carotenoid Protein

“…Protein solvation effects were treated using the Poisson–Boltzmann implicit solvent method implemented in QSite. This combination of two different levels of DFT and/or basis sets, one for ground state optimization and the other for excited state energies, has been successfully used before, particularly in the context of carotenoids. − The incorporation of solvent (via Poisson–Boltzmann in this case) is essential to reproduce the experimental values of UV–vis absorption. Such a substantial solvent effect has been pointed out recently by Guberman-Pfeffer et al.…”

Synthesis of Multifunctional Charge-Transfer Agents: Toward Single-Walled Carbon Nanotubes with Defined Covalent Functionality and Preserved π System

“…Therefore, in a rare exception from Kasha's rule, 63 AxM de-excite with the emission of photons from the higher excited states (S 2 , S 3 , or S 4 ) to the ground state S 0 as reported in literature for some πconjugated compounds. 64,65 The calculations also showed (Figure S30) that in A2M an additional barrier has to be overcome for the higher states to go to the nonfluorescent S 1 state, which explains why A2M has the strongest emission intensity.…”