Diarylamino- and Diarylboryl-Substituted Donor-Acceptor Pyrene Derivatives: Influence of Substitution Pattern on Their Photophysical Properties

Dianisylamino donor (D) and dimesitylboryl acceptor (A) substituents were introduced at the 1,6- and 2,7-positions of pyrene to demonstrate that the substitution patterns influence the photophysical properties. The different pictures in orbital interactions between the pyrene core and the D-A substituents led to the outcome that 1,6-substituted pyrene derivative 1 had stronger electron-donating and electron accepting properties in conjunction with a small HOMO- LUMO gap, as compared to the 2,7-substituted derivative. For these pyrene derivatives, modest (Phi(F) = 0.2) to strong (Phi(F) = 1.0) fluorescence was detected in degassed organic solvents; 1 exhibited a typical intramolecular charge transfer (ICT) emission obeying energy-gap law, while 2 displayed a moderate inverse energy-gap law, originating from the different substitution patterns. Although theoretical calculations predicted that both 1 and 2 adopt highly twisted ICT excited states (TICT excited states) even in the gas phase, but practically, it was suggested that the observed photophysical properties could be determined by the extent of twist angle of the TICT-like excited state in accordance with the solvent polarity. Moreover, the bulky D-A substituents inhibit the intermolecular direct pi-pi interactions, thereby resulting in the bright and moderate solid-state emissions for 1 (Phi(F) = 0.76) and 2 (Phi(F) = 0.21), respectively.