Charge Transfer-Induced Torsional Dynamics in the Excited State of 2,6-Bis(diphenylamino)anthraquinone

Abstract

Intramolecular charge transfer (ICT) in a multibranched pushpull chromophore is a key photophysical process which is attracting attention due to its relevance to the development of highly efficient organic light-emitting diodes, but the excited-state dynamics of multibranched pushpull chromophores is still unclear. Here, using femtosecond transient absorption spectroscopy and singular value decomposition analysis, we studied the excited state dynamics of 2,6-bis(diphenylamino)anthraquinone (DPA-AQ-DPA), which contains two diphenylamino (DPA) groups as electron-donors (D) and anthraquinone (AQ) as an electron-acceptor (A) and is a candidate for an efficient red TADF (thermally activated delayed fluorescence) emitter. The emission of DPA-AQ-DPA exhibits large Stokes shifts with increasing solvent polarity, indicating that the emission can be attributed to an ICT process. The charge separated (CS) state formed by ICT undergoes torsional dynamics, involving twisting between D and A, resulting in the formation of a twisted charge separated state (CStwisting). This twisting reaction between D and A is accelerated in high-polarity solvents compared with that in low-polarity solvents. Such faster CT-induced torsional dynamics in high-polarity solvents is explained in terms of the localization of ICT on one of two ICT branches, suggesting that DPA-AQ-DPA in localized CStwisting formed in high-polarity solvents has two different dihedral angles between a single A group and two D groups. On the other hand, with increasing solvent polarity, the CS and CStwisting states of DPA-AQ-DPA become stabilized, making their energy levels considerably lower than that of (3)(pi,pi*), consequently blocking the formation of the triplet excited state and TADF in a high-polarity solvent such as acetonitrile. By contrast, the energy levels of CS and CStwisting states in a low-polarity solvent, such as diethyl ether, are higher than that of (3)(pi,pi*), allowing for deactivation into (3)DPA-AQ-DPA* through intersystem crossing. This result indicates that the energy levels of CS and CStwisting states can be adjusted by controlling aspects of the local environment, such as solvents, so that intersystem crossing can be either inhibited or promoted. In other words, the energy gap (Delta E-ST) between the lowest singlet and triplet excited states for DPA-AQ-DPA can be regulated by changing the solvent polarity.