Theoretical insights into the central “acceptor” bridge function on the whole visible light and near-infrared emission in tetraphenylpyrazine-based luminogens

Abstract

Tetraphenylpyrazine (TPP) is a conspicuous heterocyclic fluorophore with an aggregation-enhanced emission (AEE) character. In general, the TPP building block is considered either a molecular rotor or an electron acceptor in the construction of its derivatives. Therefore, a faint-blue emission is expected in its solution. Herein, we concentrated on a series of unusual DTPP-A type analogues, where two TPP units (DTPP) were connected using the electron-accepting group (A), i.e., benzodiazine (BQ), benzothiadiazole (BZ), thienyldioxide (TO), naphthothiadiazole (NZ) and benzobisthiadiazole (B2Z). The excited-state properties of two experimentally available compounds (DTPP-BZ and DTPP-TO) and three theoretically designed molecules (DTPP-BQ, DTPP-NZ and DTPP-B2Z) were studied by employing time-dependent density functional theory coupled with thermal vibration correlation function formalism. The tetrahydrofuran solvent environment was mimicked through the polarizable continuum model. It was found that the TPP components turned into electronic donors in these DTPP-A type systems, and the emission color widened from blue to red and even to near-infrared upon increasing the electron-accepting ability of the central bridge. The excitons localized on the central “acceptor” bridge offer sufficient orbital overlaps and contribute to the effective radiative transition. The rotation behaviors of the phenyls in the TPP components are also restricted upon such linkage, which could reduce the relaxation energy and give rise to a much slower non-radiative decay process. Thus, the emission of the bridged systems is boosted in solution though TPP itself is an AEE element. The present study provides a valuable strategy for the molecular design of heterocycle-containing emitters with whole visible light and near-infrared emission.