Quantum yield in blue-emitting anthracene derivatives: vibronic coupling density and transition dipole moment density

A theoretical design principle for enhancement of the quantum yield of light-emitting molecules is desired. For the establishment of the principle, we focused on the S1 states of blue-emitting anthracene derivatives: 2-methyl-9,10-di(2'-naphthyl)anthracene (MADN), 4,9,10-bis(3',5'-diphenylphenyl)anthracene (MAM), 9-(3',5'-diphenylphenyl)-10-(3'',5''-diphenylbiphenyl-4''-yl) anthracene (MAT), and 9,10-bis(3''',5'''-diphenylbiphenyl-4'-yl) anthracene (TAT) [Kim et al., J. Mater. Chem., 2008, 18, 3376]. The vibronic coupling constants and transition dipole moments were calculated and analyzed by using the concepts of vibronic coupling density (VCD) and transition dipole moment density (TDMD), respectively. It is found that the driving force of the internal conversions and vibrational relaxations originate mainly from the anthracenylene group. On the other hand, fluorescence enhancement results from the large torsional distortion of the side groups in the S1 state. The torsional distortion is caused by the diagonal vibronic coupling for the lowest-frequency mode in the Franck-Condon (FC) S1 state, which originates from a small portion of the electron density difference on the side groups. These findings lead to the following design principles for anthracene derivatives with a high quantum yield: (1) reduction in the electron density difference and overlap density between the S0 and S1 states in the anthracenylene group to suppress vibrational relaxation and radiationless transitions, respectively; (2) increase in the overlap density in the side group to enhance the fluorescence.