The Curious Case of Dual Emission in 9,10-Bis(phenylethynyl)anthracene.

The photophysics of molecular crystals are governed by the interplay of molecular packing, electronic coupling, and lattice disorder. 9,10-Bis(phenylethynyl)anthracene (BPEA) is a benchmark system for singlet fission and solid-state triplet-triplet annihilation (TTA), yet its optical spectra display long-standing anomalies, including dual absorption and emission features that defy conventional excitonic interpretations. Here, we resolve these puzzles using steady-state and time-resolved spectroscopy combined with exciton-charge-transfer (CT) vibronic modeling, molecular dynamics simulations, and first-principles electronic structure calculations. We show that the characteristic double-band absorption of crystalline BPEA arises from electronic mixing between Frenkel excitons and low-lying CT states rather than polymorphism or conventional H-aggregate behavior. In contrast, the anomalous low-energy emission originates from structural defects associated with X-shaped BPEA dimers, whose stabilized CT character yields emissive states decoupled from the bulk exciton manifold. These trap states act as inherent dopants to suppress singlet fission while enhancing CT-triplet-pair mixing, creating efficient TTA hotspots and directly linking packing defects to increased upconversion efficiency.