Reversible photochemistry of pyrene: a light-driven protonation of N,N-dialkylaniline substituted pyrene, uniquely facilitated by chloroform as a proton source, unfolds through intramolecular electron transfer (IET) and radical-mediated hydrogen abstraction reactions.

Abstract

The photophysical and photochemical properties of the organic chromophores are predominantly governed by the aromatic core, electron-donating/withdrawing groups, and inter- and intramolecular electrostatic interactions, while alkyl chains contribute marginally due to their weaker inductive effect. To investigate alkyl chain effects, we synthesized 1-pyrene substituted with N,N-dialkylanilines (C2, C6, C10) via Suzuki coupling. Solvatofluorochromic studies revealed a red-shifted fluorescence with increasing solvent polarity, except in chloroform and methanol, attributed to solute–solvent interaction between solvent proton and the nitrogen atom of N,N-dialkylamine. In chloroform, blue LED irradiation induced light-sensitive transformation, characterized by decreased longer wavelength absorption and emergence of shorter wavelength bands. The resulting photoproduct was identified as a protonated N,N-dialkylaniline, with chloroform serving as a proton source. This process is exclusive to chloroform among the tested chlorinated solvents. Mechanistically, photoexcitation induces single-electron transfer from N,N-dialkylaniline to pyrene, generating radical ions that abstract hydrogen from chloroform. Nanosecond transient absorption spectroscopy confirmed the presence of N,N-dialkylaniline cation and pyrene anion radicals in chloroform but not in THF, highlighting the solvent role in charge separation and stabilizing radical intermediates. The reaction kinetics vary with alkyl chain length, with C6 being the optimal chain length.