Pyrenyl-Capped Benzofiurene Derivatives: Synthesis, Characterization, and the Effects of Flexible Side Chains on Modulating the Optoelectronic Properties

A new series of pyrenyl-capped benzofiurene derivatives (PFP-1, PFP-2, PFP-3, and PFP-4) were designed, synthesized, and investigated as model compounds for understanding the effects of flexible side chains on modulating the functional properties of organic semiconductors for optoelectronics. The resulting compounds exhibited high fluorescence yields (changing from 28% for PFP-1 to 46% for PFP-3), good thermal stability (increasing from 439 °C for PFP-4 to 510 °C for PFP-1), and fair glass-transition temperatures (ranging from 84 °C for PFP-4 to 175 °C for PFP-1). According to ultraviolet absorption (UV) and photoluminescence (PL) spectra, the long flexible side chains on diaryl substituents have played an important role on influencing the intermolecular interactions and radiative deactivation decays. Moreover, the flexible side chains on diaryl substituents also influence the process of exciton migration and exciton quenching, further resulting in different photoluminescence quantum yield (PLQY) and transient lifetimes for PFP-X. As evidenced by atomic force microscopy (AFM) images and X-ray diffraction (XRD) patterns, an increase in the lengths of flexible chain substituents can effectively depress the crystalline nature of the rigid conjugated molecular backbone, which can endow the corresponding materials with improved morphology properties. The solution-processed nondoped organic light-emitting diodes (OLEDs) based on PFP-3 showed high efficiency (up to 2.56 cd/A and 8372 cd/m²) and bright blue-light emission with Commission Intermationale de L'Eclairage (CIE) coordinates of (0.15, 0.15). It is worthwhile to mention that the performance of these solution-processed OLEDs is comparable to and even better than that of vacuum-deposited OLEDs. One-dimensional (1D) distributed feedback lasers using PFP-3 as gain media were constructed with a tunable wavelength ranging from 456.0 to 471.4 nm and low pump energy thresholds (0.28 KW/cm²), which is among the best results achieved from small molecular gain media. This study emphasizes that subtle structural alteration even for flexible side chains can significantly affect the corresponding characteristics, which are vital for rational design of the molecular structures for optoelectronic applications.