Single-Molecule Level Insight into Nanoscale Environment-Dependent Photophysics in Blends

Organic semiconductors have attracted considerable attention due to their applications in low-cost (opto)­electronic devices. Many successful organic materials utilize blends of several types of molecules that contribute different functions (e.g., serving as donors and acceptors in solar cells). In blends, the local environment, which is inherently heterogeneous, strongly influences the (opto)­electronic performance and photostability. We use functionalized fluorinated pentacene (F8 TCHS-Pn) molecules as single-molecule probes of the nanoscale environment in blends containing donor and acceptor molecules incorporated into a polymer (PMMA) matrix. Single F8 TCHS-Pn donor (D) molecules were imaged in PMMA in the presence of functionalized indenofluorene (TIPS-IF) or PCBM acceptor (A) molecules using wide-field fluorescence microscopy at various concentrations. Long-lived dark states attributed to a reversible formation of an endoperoxide (TCHS-EPO) were observed, and the EPO formation and reversal processes, which evolved upon acceptor addition, were quantified. Our study provides a nanoscale-level insight into how the presence of acceptor molecules alters the photophysics of the donor molecules dispersed in the polymer. Kinetics of the F8 TCHS-Pn photo-oxidation reaction and its reversal in such blends are determined by a fine balance of the acceptor-modified morphology (which in our case speeds up the photo-oxidation and slows down its reversal) and singlet oxygen quenching by acceptors (which prevents repeated photo-oxidation/reversal events)