Small push-pull diacetylenes as emergent fluorophores

Organic fluorophores containing acetylene spacers have gained significant current interest because of their wide-spread applications in optoelectronics. In this present review, we summarize our recent photophysical understanding on small organic dyes which contain a diacetylene conduit. Diphenylbutadiynes with push-pull substituents exhibited emissions from both locally excited (LE) and intramolecular (ICT) states in non-aqueous media. The LE emission was confined to the normal mode involving C≡C stretching of the diacetylene moiety. Quantum chemical calculations showed that the fluorophores are twisted and the diyne moiety deviates from the usual linearity (sp) in the ICT excited state. The diphenylbutadiynes with an acceptor group at meta position of the phenyl ring produced the lowest absorption energy compared to their ortho and para isomers, which was related to the ‘meta effect’ well-known in organic photochemical reaction. The ‘meta effect’ was not perceived when the chain length (n) was increased (n≥4). Butadiyne bridged pyrene-phenyl hybrid derivatives showed the LE emission originating from the pyrene core, which was in contrast to the observation of the LE emission in the diphenylbutadiynes. The dyes showed aggregate emission in mixed-aqueous solvents. In addition to the solution state emission, the fluorophores exhibited emission in solid powder form and showed reversible fluorescence switching in the solids. The solid state emission of the derivatives was either blue (excitonic coupling) or red (excimeric coupling) shifted with regard to the solution phase emission spectrum, depending on the size of the peripheral aromatic moiety. The diynes containing donor and acceptor peripheries displayed single component white light emission, which was exploited to polar aprotic vapor detection in a polymer film matrix. The photophysical outcomes of the diacetylenic dyes make them a promising class of important π-conjugated organic fluorophores.