The role of higher excited states in the photophysics and photochemistry of a BODIPY-phenol derivative.

The photophysics and photochemistry of the BODIPY phenol derivative 1 were investigated using steady-state and time-resolved spectroscopies, combined with nonadiabatic dynamics simulations. Upon visible excitation (λ > 350 nm), rapid (sub-100 fs) internal conversion to S1 leads to the characteristic BODIPY emission around 510 nm without detectable photochemistry. In contrast, UV excitation (λ < 350 nm) opens additional relaxation pathways, producing weak fluorescence in the 350-400 nm region together with photo-deamination that is presumed to occur on an ultrafast timescale. Simulations identify the high-energy emission as originating from a higher-lying charge-transfer state with phenyl-to-BODIPY (CT_PH → BP) character, where partial population trapping occurs due to restricted access to conical intersections and a relatively large interband energy gap. In contrast, photo-deamination proceeds through higher charge-transfer states with BODIPY-to-phenyl (CT_BP → PH) character and involves ultrafast C-N bond cleavage occurring before intramolecular vibrational energy redistribution. These results rationalize the excitation-wavelength-dependent photophysics and photochemistry of BODIPY-phenol derivatives and provide guidelines for future modifications.