Regulation of an Ambient‐Light‐Induced Photocyclization Pathway (Norrish–Yang Versus 6π) by Substituent Choice

Abstract

A series of donor/acceptor chromophores 1d-3d containing two types of photoresponsive motifs, namely an electron-deficient BF2-chelated ketone fused with an electron-rich thiophene have been synthesized and their photochromic performance are investigated. The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., under ambient light, methyl-substituted 1d and 2d undergo 6π photocyclization, whereas ethyl-substituted 3d exclusively undergoes Norrish-Yang photocyclization. Single-crystal X-ray analysis of Norrish-Yang cyclization products reveals the formation of four pairs of conformational enantiomers differing in the dihedral angle between benzothiophene and the BF2 core. The UV-vis absorption spectra of 1d-3d cover a broad visible light region (380-572 nm), while theoretical calculations reveal that absorption in this region is dominated by the charge transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient π-conjugated BF2-chelated unit. The spatial separation of the HOMO and LUMO of these dyes promotestriplet state generation and self-photosensitizes intramolecular photocyclization in the visible light region. Three-dimensional time-resolved and steady-state emission spectra of 3d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96%).