Photoinduced intramolecular charge transfer to meta position of benzene ring in 6-aminophthalides.

Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.