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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.

In photosynthesis, collective pigment excitations – excitons – facilitate chemical reactions for sustainable biological function. Here, the effect of hydrostatic pressure – an important thermodynamic stress factor – on optical spectral properties of excitons in the cyclic light-harvesting 2 pigment-protein complex from photosynthetic bacterium Rhodoblastus acidophilus was first studied. The high pressure-induced modifications of absorption, fluorescence and polarized fluorescence excitation spectra were theoretically analyzed in terms of the disordered molecular exciton model. We uniquely show that the observed shift of the spectra under pressure is largely governed by the pressure-induced rise of two factors: the exciton displacement energy and the exciton coupling energy, which increases the spread of the exciton state manifold. A significant increase of static energy disorder revealed by model calculations suggests that high-pressure compressing of the complex is actually accompanied by altered spatial orientations and conformations of the protein-embedded pigment molecules.

By applying an advanced hole-burning fluorescence line-narrowing technique, an unexpectedly strong (several-fold) increase of the electron–phonon coupling strength on excitation wavelength through the inhomogenously broadened absorption origin band was first observed for low-temperature organic glass samples doped with biologically relevant chlorin and chlorophyll a molecules. The dependence, which suggests a direct correlation between solvent shift and the electron–phonon coupling, demonstrates great sensitivity of the zero-phonon transitions localized on single chromophores as optical probes of the structure and dynamics of local subnanoscopic environments.