Dual fluorescence and intramolecular charge transfer with N-phenylphenanthridinones

The photophysical behavior of four N-phenylphenanthridinones is investigated as a function of temperature in n-hexane and acetonitrile by using photostationary acid time-resolved measurements. The fluorescence spectrum of N-(p-trifluoromethylphenyl)phenanthridinone and the sterically hindered N-(2,6-6-dimethylphenyl)-phenanthridinone (DMPP) consists of a single emission from a locally excited (LE) state, similar to that of N-methylphenanthridinone (MP), In these compounds, intersystem crossing (ISC) to the triplet state is the dominant deactivation process of the LE state at room temperature. Dual fluorescence is observed with N-phenylphenanthridinone (PP) and N-(p-methoxyphenyl)phenanthridinone (MOPP), which consists of an LE emission band in the same spectral region as that of MP and a strongly red-shifted intramolecular charge transfer (ICT) band. X-ray crystallography reveals that in PP the phenyl/phenanthridinone dihedral angle equals 80.6 degrees. From the absence of dual emission in the case of DMPP, it is concluded that the ICT state is considerably more planar than the LE state. From solvatochromic measurements, the ICT dipole moments of MOPP (13.7 D) and PP (10.6 D) are determined. These dipole moments are considerably larger values than those of their LE states: 7.8 D (MOPP) and 7.4 D (PP). The observation of double-exponential LE fluorescence decays for PP in n-hexane and acetonitrile and for MOPP in n-hexane indicates that the ICT reaction is reversible. From measurements of the fluorescence decays as function of temperature, the activation energies and preexponential factors of the reversible LE <----> ICT reaction are determined, giving ICT stabilization enthalpies for PP of -0.9 kcal/mol in n-hexane and -1.5 kcal/mol in acetonitrile and of -2.4 kcal/mol for MOPP in n-hexane. The ICT lifetime tau (0)' of PP and MOPP is unusually short (subnanosecond). For PP in n-hexane and acetonitrile, ISC is the main deactivation channel of the ICT state, whereas with MOPP IC is a more effective ICT deactivation process than ISC, especially in acetonitrile in which t(0)' (28 ps at 20 degreesC) is completely dominated by IC. The IC is assumed to occur via a conical intersection, brought about by the planarization of the strongly twisted LE state during the ICT reaction of PP and MOPP.