Disordered light-harvesting aggregates can host functional vibronic couplings at room temperature

Photosynthesis relies on a network of chlorophyll-like molecules which together lead to efficient long-range energy transfer. Evidence at cryogenic temperatures suggests that mechanistic details of energy/charge transfer must invoke coupled vibrational-electronic (vibronic) states. Whether these survive at physiological temperature in large photosynthetic aggregates is an open question. Parallel research on artificial templates has relied on cyanines which are unlike chlorophylls. Here we report time-resolved optical spectra of porphyrin nanotubes where we selectively probe coupled Q_x - Q_y states through polarization control. Early time cross peaks between electronic states, their rapid broadening and survival of quantum beats with anisotropic beating amplitude conclusively demonstrate that overlapping vibrational-electronic bands of photosynthetic aggregates indeed host functional vibronic couplings at room temperature. Calculations reveal that disorder is the vital ingredient that dramatically enhances intraband vibronic couplings across the entire Q band. The parameter regime where energetic disorder is of the order of dense Raman-active vibrations with weak reorganization energies may be the key design principle.