Broadband Transient Fluorescence Reveals a Two-State Picture of Peridinin

Abstract Peridinin is a light-harvesting apocarotenoid whose excited-state relaxation is often described by population transfer among 1Bu, 2Ag, and intramolecular charge-transfer states. Here broadband-detected transient fluorescence reveals a simpler picture for peridinin in ethylene glycol. Following optical excitation, we observe instantaneous blue-green emission from the bright 1Bu state that depopulates with a 150 fs time constant and directly gives rise to weak red emission from the lowest 2Ag state, which decays in 12 ps. The presence of an isosbestic point and the absence of additional fluorescence components indicate direct internal conversion without a resolvable hot S1 or a separate equilibration process involving a charge-transfer state. XMS-RASPT2 and DFT/MRCI calculations reproduce this behavior by showing strong displacement of the 1Bu and 2Ag minima from the Franck–Condon region and weak oscillator strength at the relaxed 2Ag minimum. The results identify peridinin as an emitter from its 1Bu and 2Ag states and revise the interpretation of its charge-transfer-like signatures.