Fig. 1. Study of the photocycle of QuasArs.

a Excitation spectra of QuasAr2 compared between one-photon (green) and two-photon (purple) excitation. The two-photon excitation spectra data points are depicted with purple dots, and the resulting average is indicated with a solid line for visual guidance. The corresponding excitation wavelengths for two-photon spectra are noted below. b The QuasAr2 absorption spectra change after illumination with LED. The black curve shows IDS D590 absorption. The red curve shows a mixture of P580 and P400 photoproduct absorption spectra after sample illumination for 60 s with 625 nm. The blue curve shows a dark state-like recovered D590’ photoproduct absorption after subsequent 60 s 400 nm illumination. c Schematic representation of photoproduct formation after illumination. d–f The fs-to-µs transient absorption spectra of QuasAr2 obtained with fs pump-probe spectroscopy are shown in the middle row, where d reconstructed 3D plot of TAS, e evolution-associated difference spectra (EADS), and f scaled-up EADS. g The reaction coordinate of the excited-state dynamics is shown. Upon light excitation the QuasAr2 is brought to the excited-state S₁, followed by Franck–Condon decay in τ₀. The excited state decays with two-time constants—τ₁ and τ₂. Minor (~4%) fraction of the excited molecules undergo isomerization and contribute to the formation of a photointermediate I640, which transitions into photointermediate I620 with τ₃ and then decays with τ₄. After excitation, most of them relax to the initial ground state. The excited state is long-lived and results in slow decaying SE. h Comparison of normalized excited-state decay between QuasAr1 and QuasAr2. The subpanel indicates the fluorescence QY determined in this study of the QuasArs, using the same excitation wavelengths as for pump-probe experiments (620 nm). The spectra of the purified sample were recorded in room temperature (RT) and buffer solution at pH 8. Source data are provided in the Source Data file (a–h).