We present photophysical properties of functionalized anthradithiophene (ADT) and pentacene (Pn) derivatives, studied both at the ensemble (single crystals and thin films) and at the single molecule level using a combination of time-resolved photocurrent and photoluminescence measurements, ultrafast spectroscopy, numerical modeling, and single molecule fluorescence spectroscopy (SMFS).
At the ensemble level, we quantified (i) various pathways of photoexcitation relaxation in fluorinated ADT crystals and films, depending on the molecular packing and (ii) charge transfer (CT) and its contribution to time-resolved photocurrent in donor-acceptor blends containing a fluorinated ADT derivative as the donor and a variety of acceptor molecules. Acceptor molecules with various energies and molecular packing motifs were selected. Most efficient CT was observed in D/A blends with the similar donor and acceptor packing motifs (2D brick-work -stacking); however the CT state was highly emissive and did not significantly contribute to the photocurrent. In contrast, addition of PCBM improved crystallinity of the ADT donor domains, resulting in up to a factor of 4 enhancement in ultrafast charge carrier separation efficiency.
The same ADT and Pn derivatives used in our device studies were fluorescent and photostable enough to be imaged on the single molecule level in a variety of hosts using SMFS in a wide-field geometry at 532 nm and 633 nm excitation, respectively, at room temperature in air. Total numbers of photons emitted by the molecule reached ~106, depending on the derivative and on the host, approaching values comparable to those for standard SMFS fluorophores. The molecules functionalized with larger-size side groups exhibited up to a factor of ~2 higher photostability as compared to those with small side groups, indicative of a protective function the side groups can perform against photo-oxidation. Forster resonant energy transfer (FRET) with the FRET radius of 4.2-4.8 nm was observed in donor-acceptor pairs of the molecules with a fluorinated ADT donor and various acceptors. Molecular packing of guest molecules into host matrices was investigated by polarization-dependent SMFS, which revealed that the crystalline functionalized benzothiophene (BTBTB) host imposed orientational constraints on the guest Pn molecules, so that the transition dipole moment of the guest molecules was oriented almost perpendicular to the substrate; such constraints were absent in polymer hosts. Differences in molecular orientation of BTBTB-embedded guest molecules functionalized with different side groups were also observed. These studies lay foundations for monitoring formation of organic semiconductor bulk heterojunctions and for systematic studies of effects of local nanoenvironment on the molecular photophysics and intermolecular interactions at nanoscales using SFMS.