Photophysical properties of functionalized anthradithiophene (ADT) and pentacene (Pn) derivatives, as well as energy and charge transfer properties of donor-acceptor (D/A) pairs of these derivatives, are presented. The molecules studied were imaged on the single-molecule level in a polymeric and in a functionalized benzothiophene (BTBTB) crystalline host using room-temperature wide-field epifluorescence microscopy. The BTBTB host imposed orientational constraints on the guest molecules, depending on their functionalization. Flexibility of functionalization of both guest (ADT, Pn) and host (BTBTB) molecules can be used for systematic studies of nanoscale morphology and photophysics of D/A organic semiconductor bulk heterojunctions using single-molecule fluorescence microscopy. (C) 2015 Elsevier B.V. All rights reserved.