Evaluating the isomeric effects of donors on the structures and photophysical properties of donor–acceptor–π bridge–donor (D1–A–π–D2) prototype fluorophores

Abstract

In order to evaluate the isomeric effects of donors on the structures and photophysical properties of donor–acceptor–π bridge–donor (D1–A–π–D2) prototype fluorophores, three D1–A–π–D2 prototype isomers were designed and synthesized: o-BMNM, m-BMNM, and p-BMNM, where the isomeric biphenyl units functioned as a terminal donor (D2). In addition, the three isomers possessed the same malononitrile acceptors (A) and 6-methoxynaphthalene donors (D1). Molecular structural, computational, and photophysical analyses were conducted to investigate the isomeric effect of a biphenyl donor (D2) on such D1–A–π–D2 prototype isomers. Molecular structural analysis indicated that changing the type of biphenyl donor (D2) would give rise to different molecular geometries, electronic structures and packing modes of the three isomers, which would lead to their different photophysical properties. The three isomers displayed solvatochromic properties and large Stokes shifts. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations indicated that due to the single bond between the 6-methoxynaphthalene donors (D1) and malononitrile acceptors (A), the three isomers displayed the intramolecular charge transfer (TICT) effect, which would endow them with large Stokes shifts.