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- Title
Regulation of an Ambient‐Light‐Induced Photocyclization Pathway (Norrish–Yang Versus 6π) by Substituent Choice.
- Authors
Liu, Hui; Gao, Hu; Wang, Sisi; Yao, Shengxin; Wu, Fan; Zhao, Yue; Chan, Kin Shing; Shen, Zhen
- Abstract
Photocyclization, irrespective of whether multiple steps (e.g. Norrish–Yang cyclization) or a single concerted step (e.g. 6π 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 chromophores 1 d–3 d containing 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‐substituted 1 d and 2 d undergo reversible 6π photocyclization, whereas ethyl‐substituted 3 d exclusively 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 BF2 core, namely (±)N‐3 d@68°, (±)N‐3 d@‐77°, (±)N‐3 d@‐78°, and (±)N‐3 d@‐102°. The UV/Vis absorption spectra of 1 d–3 d cover 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 π‐conjugated BF2‐chelated unit and the n→π* and π→π* 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 of 3 d show that the Norrish–Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).
- Subjects
PHOTOCYCLIZATION; PHOTOCHROMISM; PHOTOINDUCED electron transfer; ORGANIC cyclic compounds; DIHEDRAL angles; ABSORPTION spectra
- Publication
Chemistry - A European Journal, 2020, Vol 26, Issue 54, p12418
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.202000990