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

Abstract

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.

Impact of non-symmetric 2,9,10-aryl substitution on charge transport and optical properties of anthracene derivatives

“…The energy of T 4 state was higher than S 1 for all compounds except the mentioned 4a and 4b compounds where T 4 was situated below S 1 . The calculated oscillator strength of S 0 →S 1 transition was found to increase for compounds 1-4a/b (from 0.125 to 0.232 and from 0.097 to 0.229 for a compounds and b compounds, respectively) due to the conjugation extension [32] while a bit lower values were observed for compounds 5a/b and 6a/b (~0.2 and 0.16, respectively).…”

“…However the additional π-conjugation extension towards the long axis of the molecule results in the reorganization of singlet and triplet energy levels when the T n level emerges below S 1 leading to the efficient ISC recombination and low Φ F [26]. Despite the importance of the intersystem crossing, recent studies have suggested that the vibrational relaxation may also be the important pathway for the non-radiative decay in anthracene derivatives [31,32]. Modification of anthracene core with various alkyl groups allows to obtain amorphous thin films, however those anthracene derivatives like 9,10-di(2-naphthyl)anthracene posses low hole drift mobility (μ h ) [33][34][35] of about 10 -7 -10 -5 cm 2 /Vs.…”

“…On the other hand, few recent studies suggested significant role of the electron-vibron coupling facilitating non-radiative deactivation in 9,10-chloro substituted [31] and 9,10-aryl substituted [32] anthracenes.…”

Fluorescence via Reverse Intersystem Crossing from Higher Triplet States in a Bisanthracene Derivative

“…However, this design principle also has certain drawbacks: (1) the small overlap between the HOMO and LUMO leads to suppression of the oscillator strength 9, 10 , and (2) TADF OLEDs exhibit broad emission wavelengths because of charge-transfer (CT) excitation. In order to overcome these drawbacks, Sato et al .…”

“…and Sato et al . have already designed and proposed iST molecules based on anthracene 9, 10 and perylene derivatives 11 , respectively.…”