ELECTRONIC-TRANSITION MOMENT DIRECTIONS AND IDENTIFICATION OF LOW-ENERGY N-PI-ASTERISK STATES IN WEAKLY PERTURBED PURINE CHROMOPHORES

Measurements of UV linear dichroism on purine and three methyl derivatives partially oriented in poly(vinyl alcohol) matrix gave direct evidence for the assignment of the first singlet npi* state. Intensity distributions and moment directions for the first three pi --> pi* transitions were also determined. The pi --> pi* transitions in purine were found to be polarized at (angles, relative to the pseudo-symmetry long axis, counted positive in the N7 direction): -31-degrees +/- 5-degrees (II at 265 nm), +38-degrees +/- 5-degrees (III at 244 nm), and +36-degrees +/- 10-degrees (IV at 214 nm). The transition energies and moment directions were not markedly perturbed by methyl substitution at the sixth, seventh, or ninth position. Therefore, these methyl substituents could be used as orientational perturbers to resolve a sign ambiguity problem regarding transition moment directions. The orientation were determined by infrared dichroic measurements using both in-plane and out-of-plane polarized vibrational transitions. In addition, the phosphorescence spectra were studied, including phosphorescence anisotropy, phosphorescence lifetimes, and quantum yields, for the purines in an organic glass at 80 K. Based on these measurements, the lowest triplet state is concluded to have effectively pipi* character, and its emission allowedness appears to originate from spin-orbit interactions primarily with singlet sigmapi* states but also with singlet pipi* states via vibronic mixing. The phosphorescence emission spectra of purine and 6-methylpurine are complex, compared to 7-methylpurine and 9-methylpurine, with emission wavelength-dependent lifetimes and excitation spectra. This is ascribed to a prototropic tautomeric equilibrium between the 7H and 9H forms of purine and 6-methylpurine, a ground-state heterogeneity that we believe has caused confusion in earlier studies and, e.g., led to an assignment of the phosphorescence origin of purine.