Enhancement of fluorescence in anthracene by chlorination: Vibronic coupling and transition dipole moment density analysis

- Vibronic coupling analyses for chlorination effect in the S1 state of anthracene A.
- Vibrational relaxation originates from the sides of A; chlorinations suppress this.
- High symmetry plays an important role in suppressing the internal conversion.
- Delocalized overlap density along to C9-C10 atoms increases transition dipole moment.
- The findings lead to a design principle for a molecule with a high quantum yield.

The vibronic coupling constants and transition dipole moments for the Franck-Condon and adiabatic S1 states of anthracene, 9-chloroanthracene, and 9,10-dichloroanthracene were calculated and analyzed by using the concept of vibronic coupling density (VCD). The transition dipole moments are also analyzed on the basis of the transition dipole moment density (TDMD). The VCD analyses indicate that the vibronic couplings in the Franck-Condon S1 state come from the side rings of anthracene, and introduction of chlorine atoms reduces the vibronic couplings in the side regions and the reorganization energy. The TDMD analyses indicate that the chlorination enhances the transition dipole moment and that the contribution of the chlorine atom to the transition dipole moment is the largest. Finally, we derived a design principle for anthracene derivatives with a high quantum yield: the same long acceptors should be introduced into the two central carbon atoms in the anthracene's central ring for the derivative to keep the point group to be D2h.