Photocyclization, irrespective of whether multiple steps (e.g., Norrish-Yang cyclization) or a single concerted step (e.g., 6 pi photocyclization) are involved, is an intramolecular photochemical process resulting in the formation of one new single bond to afford a ring system. In particular, visible-light-induced photocyclization offers a green and sustainable route to organic cyclic compounds that are difficult to access by thermal reactions. Herein, we describe the ambient light-induced intramolecular photocyclization of a series of donor/acceptor chromophores1 d-3 dcontaining two types of photoresponsive motifs, namely an electron-deficient BF2-chelated ketone fused with an electron-rich thiophene, and probe the solution-phase and solid-state photochromic performance of these compounds. 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-substituted1 dand2 dundergo reversible 6 pi photocyclization, whereas ethyl-substituted3 dexclusively undergoes irreversible 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 BF(2)core, namely (+/-)N-3 d@68 degrees, (+/-)N-3 d@-77 degrees, (+/-)N-3 d@-78 degrees, and (+/-)N-3 d@-102 degrees. The UV/Vis absorption spectra of1 d-3 dcover a broad visible-light region (380-572 nm), while DFT and TD-DFT 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 pi-conjugated BF2-chelated unit and the n ->pi* and pi ->pi* transitions within the latter unit. The spatial separation of the HOMO and LUMO of these dyes promotes triplet-state generation and self-photosensitizes intramolecular photocyclization in the visible-light region. Three-dimensional time-resolved and steady-state emission spectra of3 dshow that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).
