Dual luminescence properties of differently benzo-fused N-phenylphenanthridinones

Abstract

The photophysics of some newly prepared N-arylphenanthridinone derivatives have been investigated. It has been demonstrated how the luminescence properties are influenced by the size of the aromatic ring system. It has been shown that the replacement of the phenyl group in N-phenyphenanthridinone (PP) by an α-naphthyl or β-naphthyl group (αNP and βNP, respectively), influences the fluorescence spectra very differently. For αNP, the long-wavelength (LW) emission, which is well observable in case of PP, disappears, while for βNP, the intensity of LW emission increases compared to the short-wavelength (SW) fluorescence. The rotation of the α-naphthyl group to the coplanar geometry, which is a requirement of the formation of the LW state, is strongly hindered, resulting in the lack of LW emission. In respect of steric hindrance, the β-napthyl group is similar to phenyl, however, it decreases the energy of the LW state more as a consequence of its better electron donating character and the more extended conjugation of the coplanar system. This causes the increase of the LW/SW fluorescence ratio. The benzo-fusing on the phenanthridinone moiety results in a 6–7 kcal mol⁻¹ decrease in the SW singlet energy, however, surprisingly the LW state energy also decreases in almost the same manner. The phenomenon shows that the entire benzo-phenanthridinone group is strongly involved in both transitions. As a consequence, the benzo-fused N-aryl derivatives also show dual luminescence. The dipole moments of the LW state of βNP and βNBiP (6-naphthalen-2-yl-6H-benzo[i]phenanthridin-5-one) proved to be bigger by 30 and 50% than that of the SW state, respectively. MO calculation indicates that in the SW → LW reaction not only the size but the direction of the excited state dipole also changes significantly. In apolar solvents, the dominant deactivation process of the examined molecules is intersystem crossing. In polar solvents, where the LW emission energy is smaller, internal conversion becomes more significant than the other processes, resulting in low fluorescence yield.