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 a nd it was found that 4'-phenyl tpy (ptp, 3) exhibited the most effective fl uorescence, 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 reg ion as 3, those of NH2- 14 and Me2N-ptp 15 were observed at much longer wav elengths. In addition, fluorescence maxima of 14 and 15 showed large (> 130 nm) solvent dependence. The difference between ground and excited state di pole moment (Delta mu) for 15 was estimated to be 15.2 D by the Lippert-Mat aga equation, indicating the intramolecular charge transfer (ICT) process. Semi-empirical MO calculation (MOPAC/AM1) demonstrated that the HOMO-1, HOM O and LUMO of 3, 10-12 were mainly localized on the phenyl (pi (ph)), tpy ( pi (tpy)) and tpy (pi*(tpy)) part, respectively, indicating that the lowest energy absorption band of 3, 10-12 was the local excitation (pi (tpy)-pi*( tpy)). In the case of 14 and 15, which have an electron-donating substituen t, pi (ph) instead of pi (tpy) became the HOMO. Thus, the lowest energy abs orption of 14 and 15 was an ICT transition (pi (ph)-pi*(tpy)), and a large red shift of the fluorescence occurred. In these compounds, the energy leve l of pi (ph) is controlled without affecting that of pi (tpy) and pi*(tpy), suggesting a novel approach for tuning the color of fluorescence.