Tetracene Diacid Aggregates for Directing Energy Flow toward Triplet Pairs.

A comprehensive investigation of the solution-phase photophysics of tetracene bis-carboxylic acid [ ()] and its related methyl ester [ ()], a non-hydrogen-bonding counterpart, reveals the role of the carboxylic acid moiety in driving molecular aggregation and concomitant excited-state behavior. Low-concentration solutions of exhibit similar properties to the popular 5,12-bis((triisopropylsilyl)ethynl)tetracene, but as the concentration increases, evidence for aggregates that form excimers and a new mixed-state species with charge-transfer (CT) and correlated triplet pair (TT) character is revealed by transient absorption and fluorescence experiments. Aggregates of evolve further with concentration toward an additional phase that is dominated by the mixed CT/TT state which is the only state present in aggregates and can be modulated with the solvent polarity. Computational modeling finds that cofacial arrangement of and subunits is the most stable aggregate structure and this agrees with results from H NMR spectroscopy. The calculated spectra of these cofacial dimers replicate the observed broadening in ground-state absorption as well as accurately predict the formation of a near-UV transition associated with a CT between molecular subunits that is unique to the specific aggregate structure. Taken together, the results suggest that the hydrogen bonding between molecules and the associated disruption of hydrogen bonding with solvent produce a regime of dimer-like behavior, absent in , that favors excimers rather than CT/TT mixed states. The control of aggregate size and structure using distinct functional groups, solute concentration, and solvent in tetracene promises new avenues for its use in light-harvesting schemes.