Rosindone revisited: a computational and photophysical study of 7-phenylbenzo[a]phenazine-5(7H)-one (PBP)

A reasonable gram quantity of the crystalline red dye 7-phenylbenzo[a]phenazine-5(7H)-one (PBP) was synthesised by the condensation of N-phenylbezene-1,2-diamine with 2-hydroxynaphthalene-1,4-dione in acetic acid (58% yield). The molecular structure of the dye, as determined by single-crystal X-ray crystallography, reveals a near planar phenazinone-like core, and an N-phenyl group twisted out of this plane by around 85°. The CO bond length of 1.241(2) Å is consistent with double bond character, which supports minor ground state zwitterionic character to the compound. The wavelength maximum for the observed partially structured low-energy absorption band is relatively insensitive to changes in the solvent polarity and polarizability. TD-DFT calculations predict that the long wavelength absorption envelope originates from localised π-π* transitions with no contribution from an n-π* state. The fluorescence quantum yield and singlet lifetime of the dye in MeCN are 0.12 and 3.0 ns, respectively. Fluorescence maxima are slightly sensitive to the solvent and changes in the Stokes shifts for a small series of alkanols were fitted to the Lippert-Mataga equation to afford a change in dipole moment of 8 ± 2 D. Calculations also reveal that full rotation of the N-phenyl group is severely restricted in the ground state (ΔE_GS = 29 kcal mol^-1) and in the first-excited singlet state (ΔE_ES = 34 kcal mol^-1). The rocking back and forth of the phenyl group distorts the phenazinone-like backbone as it becomes co-planar and a minor solvent viscosity effect was observed in hydrogen bonding alkanol solvents.