Aggregate-Induced Self-Assembly and Ultrafast Dynamics of Light-Harvesting D-A-A Polymorphs.

Organic dipolar molecules are an emerging class of light harvesters useful in electronic applications and have captured new urgency with the design and synthesis of new molecular structures for device testing. However, research has not evolved beyond the cyclical thin film preparation-device testing-chemical structural modification approach. Without an understanding of polymorphism, molecular photophysics at the interface or metastable morphologies that regulate charge carrier dynamics, it is not obvious a priori if a new molecular structure will produce a suitable thin film morphology for superior device performance without developing structure-function relationships that consider morphology and photophysics. Dipolar, light harvesting molecules are synthesized with a covalent, para-functionalized triphenylamine donor (D) and acceptor (A) in π-conjugated structures, D-A1 and D-A1 -A2 , that have previously achieved 9.6% power conversion efficiency in thermally evaporated organic solar cell devices with C70 . Solution processing and morphological manipulation are hypothesized to reduce ultrafast radiative charge recombination, unique to dipolar structures, that prevents full charge separation to the fullerene. The photophysics of the D-A interface using femtosecond transient absorption spectroscopy is explained, and microscopy data reveal a newly discovered, supramolecular amorphous polymer metastable state presented as a transient absorption assisted strategy for photofunctional polymorph design.

Ultrafast Spectroscopic Dynamics of Quinacrine-Riboflavin Binding Protein Interactions.

Redox active cofactors play a dynamic role inside protein binding active sites because the amino acids responsible for binding participate in electron transfer (ET) reactions. Here, we use femtosecond transient absorption (FsTA) spectroscopy to examine the ultrafast ET between quinacrine (Qc), an antimalarial drug with potential anticancer activity, and riboflavin binding protein (RfBP) with a known Kd = 264 nM. Steady-state absorption reveals a ∼ 10 nm red-shift in the ground state when QcH32+ is titrated with RfBP, and a Stern-Volmer analysis shows ∼84% quenching and a blue-shift of the QcH32+ photoluminescence to form a 1:1 binding ratio of the QcH32+-RfBP complex. Upon selective photoexcitation of QcH32+ in the QcH32+-RfBP complex, we observe charge separation in 7 ps to form 1[QcH3_red•+-RfBP•+], which persists for 138 ps. The FsTA spectra show the spectroscopic identification of QcH3_red•+, determined from spectroelectrochemical measurements in DMSO. We correlate our results to literature and report lifetimes that are 10-20× slower than the natural riboflavin, Rf-RfBP, complex and are oxygen independent. Driving force (ΔG) calculations, corrected for estimated dielectric constants for protein hydrophobic pockets, and Marcus theory depict a favorable one-electron ET process between QcH32+ and nearby redox active tyrosine (Tyr) or tryptophan (Trp) residues.