Dual fluorescence and intramolecular charge transfer with N-phenylphenanthridinones

The photophysical behavior of four N-phenylphenanthridinones is investigate d as a function of temperature in n-hexane and acetonitrile by using photos tationary 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 fr om a locally excited (LE) state, similar to that of N-methylphenanthridinon e (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-m ethoxyphenyl)phenanthridinone (MOPP), which consists of an LE emission band in the same spectral region as that of MP and a strongly red-shifted intra molecular charge transfer (ICT) band. X-ray crystallography reveals that in PP the phenyl/phenanthridinone dihedral angle equals 80.6 degrees. From th e absence of dual emission in the case of DMPP, it is concluded that the IC T 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 d etermined. These dipole moments are considerably larger values than those o f their LE states: 7.8 D (MOPP) and 7.4 D (PP). The observation of double-e xponential LE fluorescence decays for PP in n-hexane and acetonitrile and f or MOPP in n-hexane indicates that the ICT reaction is reversible. From mea surements of the fluorescence decays as function of temperature, the activa tion energies and preexponential factors of the reversible LE <----> ICT re action are determined, giving ICT stabilization enthalpies for PP of -0.9 k cal/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 mai n deactivation channel of the ICT state, whereas with MOPP IC is a more eff ective ICT deactivation process than ISC, especially in acetonitrile in whi ch t(0)' (28 ps at 20 degreesC) is completely dominated by IC. The IC is as sumed to occur via a conical intersection, brought about by the planarizati on of the strongly twisted LE state during the ICT reaction of PP and MOPP.