Highly Emissive Whole Rainbow Fluorophores Consisting of 1,4-Bis(2-phenylethynyl)benzene Core Skeleton: Design, Synthesis, and Light-Emitting Characteristics.

To create the whole-rainbow-fluorophores (WRF) having the small Δλem (the difference of λem between a given fluorophore and nearest neighboring fluorophore having the same core skeleton) values (<20 nm) in full visible region (λem: 400-650 nm), the high log ε (>4.5), and the high Φf (>0.6), we investigated molecular design, synthesis, and light-emitting characteristics of the π-conjugated molecules (D/A-BPBs) consisting of 1,4-bis(phenylethynyl)benzene (BPB) modified by donor groups (OMe, SMe, NMe2, and NPh2) and an acceptor group (CN). As a result, synthesized 20 D/A-BPBs (1a-5d) were found to be the desired WRF. To get the intense red fluorophore (Φf > 0.7, λem > 610 nm), we synthesized new compounds (5e-5i) and elucidated their photophysical properties in CHCl3 solution. As a result, 5h, in which a 4-cyanophenyl group is introduced to the para-position of two benzene rings in the terminal NPh2 group of 5d, was found to be the desired intense red fluorophore (log ε = 4.56, Φf = 0.76, λem = 611 nm). The intramolecular charge-transfer nature of the S1 state of WRF (1a-5d) was elucidated by the positive linear relationship between optical transition energy (νem) from the S1 state to the S0 state and HOMO(D)-LUMO(A) difference, and the molecular orbitals calculated with the DFT method. It is demonstrated that our concept (Φf = 1/(exp(-Aπ) + 1)) connected with the relationship between Φf and magnitude (Aπ) of π conjugation length in the S1 state can be applied to WRF (1a-5d). It is suggested that the prediction of Φf from a structural model can be achieved by the equation Φf = 1/(exp(-((ν̃a - ν̃f)(1/2) × a(3/2)) + 1), where ν̃a and ν̃f are the wavenumber (cm(-1)) of absorption and fluorescence peaks, respectively, and a is the calculated molecular radius. From the viewpoint of application of WRF to various functional materials, the light-emitting characteristics of 1a-5i in doped polymer films were examined. It was demonstrated that 1a-5i dispersed in two kinds of polymer film (PST and PMMA) emit light at the whole visible region and have the small Δλem values (<20 nm) and the high Φf values (>0.6). Therefore, the present D/A-BPBs can be said to be the desired WRF even in the doped polymer film.