Substitution pattern on anthrol carbaldehydes: excited state intramolecular proton transfer (ESIPT) with a lack of phototautomer fluorescence

Photophysical properties and excited state intramolecular proton transfer (ESIPT) reactivity for anthrol carbaldehydes 1–5 have been investigated computationally and experimentally by steady-state and time-resolved fluorescence and laser flash photolysis (LFP). 1,2-Disubstituted anthrol carbaldehydes 1 and 2 are not ESIPT reactive, contrary to naphthol analogues. The main deactivation channels from S1 for 1 and 2 are fluorescence (ΦF = 0.1–0.2) and intersystem crossing (ISC) to almost isoenergetic T2 states. The triplet states from 1 and 2 were detected by LFP (in N2-purged CH3CN, τ = 15 ± 2 μs for 1, and τ = 5.5 ± 0.1 μs for 2). In contrast, 2,3-disubstituted anthrols 3–5 undergo efficient barrierless ultrafast ESIPT. However, the typical dual emission from locally excited states and ESIPT tautomers were not observed since ESIPT proceeds via a conical intersection with S0 delivering the keto-tautomer in the hot ground state. Therefore, anthrols 3–5 are about ten times less fluorescent compared to 1 and 2, and the emission for 3–5 originates from less-populated conformers that cannot undergo ESIPT. Keto-tautomers for 3–5 were detected in CH3CN by LFP (λmax = 370 nm, τ = 30–40 ns). The difference in ESIPT reactivity for 1–3 was fully disclosed by calculations at ADC(2)/aug-cc-pVDZ level of theory, and particularly, by calculation of charge redistribution upon excitation to S1. Only 2,3-disubstituted anthrols exhibit polarization in S1 that increases the electron density on the carbonyl and decreases this density on the phenolic OH, setting the stage for ultrafast ESIPT.