Switching off the charge transfer and closing the S1-T1 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 σ 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 S1-T1 seam between the vertically excited Franck-Condon (FC) point and the S1 equilibrium geometry favors intersystem crossing (ISC). The T1 state of the protonated form remains well below S1 (1.5 eV) because of favorable exchange interactions, whereas the T1 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 S1 and T1 states. The S1 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.