Temperature dependent electron–phonon coupling in chlorin-doped impurity glass and in photosynthetic FMO protein containing bacteriochlorophyll a

Difference fluorescence line-narrowing spectra are studied for evaluating the temperature dependence of the linear electron–phonon and vibronic coupling strengths in the two weakly coupled low-temperature glassy systems: chlorin-doped 1-propanol and the native Fenna–Matthews–Olson (FMO) light-harvesting protein complex folded with bacteriochlophyll a chromophores. Roughly three-fold increases of the electron–phonon coupling strength and constant vibronic couplings were observed in both systems in the available temperature range from 4.5 to 70 K. However, while the chlorin system is amenable to simple modeling of impurity spectra, the FMO shows significant deviations, consistent with the complex (exciton) nature of its lowest-energy singlet electronic transition.