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 (ΦF = 0.2) to strong (ΦF = 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 π–π interactions,
thereby resulting in the bright and moderate solid-state emissions
for 1 (ΦF = 0.76) and 2 (ΦF = 0.21), respectively.