6-Methylisoxanthopterin (6-MI) is a pteridine-based guanine analog that has a red-shifted absorption and high fluorescence quantum yield. Its Watson Crick base-pairing and base stacking properties are similar to guanine. The fluorescence quantum yield of 6-MI is sensitive to its nearest neighbors and base stacking, making it a very useful real-time probe of DNA structure. The fundamental photophysics underlying this fluorescence quenching by base stacking is not well understood. We have explored the excited-state electronic structure of the 6-MI in frozen 77 K LiCl glasses using Stark spectroscopy. These measurements yielded the direction and degree of charge redistribution for the S-0 -> S-1, transition as manifested in the difference dipole moment, Delta(mu) over right arrow (01), and difference static polarizability, Tr Delta alpha TDDFT (time-dependent density functional theory) was employed to calculate the transition energy, oscillator strength, and the dipole moments of the ground and lowest optically bright excited state of 6-MI (S-0 -> S-1). The direction of Delta(mu) over right arrow (01), was assigned in the molecular frame based on the Stark data and calculations. These results suggest that the C4=O and C2-NH2 groups are electron-deficient in the excited state, a very different outcome compared with guanine. This implies that Watson-Crick hydrogen bonding in 6-MI may be modulated by absorption of a photon so as to strengthen base pairing, if only transiently. Solvatochromism was also obtained for the absorption and emission spectra of 6-MI in various solvents and compared with the Stark spectroscopic results using both the Lippert-Mataga and Bakhshiev models.