Synthesis and Optical Properties of Donor-Acceptor-Type 1,3,5,9-Tetraarylpyrenes: Controlling Intramolecular Charge-Transfer Pathways by the Change of π-Conjugation Directions for Emission Color Modulations.

Abstract

In dipolar organic π-conjugated molecules, variable photophysical properties can be realized through efficient excited-state intramolecular charge transfer (ICT), which essentially depends on the π-conjugation patterns. Herein, we report a controllable regioselective strategy for synthesis and optical properties of two donor–acceptor (DA)-type 1,3,5,9-tetraarylpyrenes (i.e., 1,3-A/5,9-D (4b) and 1,3-D/5,9-A (4c)) by covalently integrating two phenyl rings and two p-OMe/CHO-substituted phenyl units into the 2-tert-butylpyrene building block, in which the two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and the two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively. Density functional theory calculations on their frontier molecular orbitals and UV–vis absorption of S0 → S1 transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through a variety of substitution patterns, implying that the dissimilar ground-state and excited-state electronic structures exist in each molecule. Their single-crystal X-ray analysis reveal their highly twisted conformations that are beneficial for inhibiting the π-aggregations, which are strikingly different from the normal 1,3,5,9-tetraphenylpyrenes (4a) and related 1,3,6,8-tetraarylpyrenes. Indeed, experimental investigations on their optical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deep-blue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the thin film and in the doped poly(methyl methacrylate) film were obtained, suggesting them as promising emitting materials for the fabrication of organic light-emitting diodes.