Phenyl-substituted 2,2′:6′,2″-terpyridine as a new series of fluorescent compounds—their photophysical properties and fluorescence tuning

Several phenyl-substituted 2,2′:6′,2″-terpyridines (tpy) were synthesized and it was found that 4′-phenyl tpy (ptp, 3) exhibited the most effective fluorescence, whose quantum yield was up to 0.64 in cyclohexane. For further study on tuning the fluorescence properties of ptp, different substituents were introduced into the p-position of the phenyl group. While Br- 10, Cl- 11, and CH3-ptp 12 showed their absorption and fluorescence in the same region as 3, those of NH2- 14 and Me2N-ptp 15 were observed at much longer wavelengths. In addition, fluorescence maxima of 14 and 15 showed large (>130 nm) solvent dependence. The difference between ground and excited state dipole moment (Δμ) for 15 was estimated to be 15.2 D by the Lippert–Mataga equation, indicating the intramolecular charge transfer (ICT) process. Semi-empirical MO calculation (MOPAC/AM1) demonstrated that the HOMO-1, HOMO and LUMO of 3, 10–12 were mainly localized on the phenyl (πph), tpy (πtpy) and tpy (π*tpy) part, respectively, indicating that the lowest energy absorption band of 3, 10–12 was the local excitation (πtpy–π*tpy). In the case of 14 and 15, which have an electron-donating substituent, πph instead of πtpy became the HOMO. Thus, the lowest energy absorption of 14 and 15 was an ICT transition (πph–π*tpy), and a large red shift of the fluorescence occurred. In these compounds, the energy level of πph is controlled without affecting that of πtpy and π*tpy, suggesting a novel approach for tuning the color of fluorescence.