Modulating stepwise photochromism in platinum(II) complexes with dual dithienylethene-acetylides by a progressive red shift of ring-closure absorption.

To modulate stepwise photochromism by shifting ring-closure absorption of the dithienylethene (DTE) moiety, trans-Pt(PEt3)2(C≡C-DTE)2 [C≡C-DTE = L1o (1oo), L2o (2oo), L3o (3oo), and L4o (4oo)] and cis-Pt(PEt3)2(L4o)2 (5oo) with two identical DTE-acetylides were elaborately designed. With the gradual red shift of ring-closure absorption for L1c (441 nm) → L2c (510 nm) → L3c (556 nm) → L4c (602 nm), stepwise photochromism is increasingly facilitated in trans-Pt(PEt3)2(C≡C-DTE)2 following 1oo → 2oo → 3oo → 4oo. The conversion percentage of singly ring-closed 2co-4co to dually ring-closed 2cc-4cc at the photostationary state is progressively increased in the order 1cc (0%) → 2cc (18%) → 3cc (67%) → 4cc (100%). Compared with trans-arranged 4oo, stepwise photochromism in the corresponding cis-counterpart 5oo is less pronounced, ascribed to either direct conversion of 5oo to 5cc or rapid conversion of 5co to 5cc. The progressively facile stepwise photocyclization following 2oo → 3oo → 4oo is reasonably interpreted by gradually enhanced transition character involving LUMO+1, which is the only unoccupied frontier orbital responsible for further photocyclization of singly ring-closed 2co-4co.