Switching Off the Charge Transfer and Closing the S-1-T-1 ISC Channel in Excited States of Quinolizinium Derivatives: A Theoretical Study

Excited states of benzo[b]quinolizinium (BQ) derivatives that show efficient pH-responsive fluorescence switching properties were studied quantum-chemically by employing the CASSCF/CASPT2 and TD-DFT methods. Protonation of aminophenyl-BQ at the electron-donor amine moiety converts the nitrogen lone pair into a sigma bond and the HOMO into a lower-lying orbital that is no longer involved in the excitation, thereby rationalizing the suppression of the charge transfer. An S-1-T-1 seam between the vertically excited Franck-Condon (FC) point and the S-1 equilibrium geometry favors intersystem crossing (ISC). The T-1 state of the protonated form remains well below S-1 (1.5 eV) because of favorable exchange interactions, whereas the T-1 state of the unprotonated form does not experience any analogous stabilization because of the difference in the spatial domains of the singly occupied orbitals in the S-1 and T-1 states. The S-1 surface from the FC point until the equilibrium geometry for the protonated species is energetically downhill. Calculations on models and available experimental data suggest design principles for similarly functioning pH-responsive species, namely, an amine lone pair as the electron donor and a cationic ring of moderate size as the electron acceptor that are structurally separated by virtue of a spacer.