New insights on persistent nonphotochemical hole burning and its application to photosynthetic complexes

Nonphotochemical hole burning (NPHB) at low temperatures of the electronic absorption bands of molecular chromophores imbedded in amorphous solids (glasses and polymers) and in proteins is a striking manifestation of configurational tunneling triggered by electronic excitation. The current mechanism of NPHB has it due to a hierarchy of relaxation events that begin in the outer shell and involve the intrinsic two-level systems (TLSint) of the glass and terminate in the inner shell where the rate determining step involving the extrinsic TLS (TLSext) occurs. The TLS correspond to asymmetric intermolecular double well potentials. The TLSext are associated with the chromophore and the inner shell of solvent molecules. The TLSint are intimately associated with the excess free volume of glasses. Their tunneling leads to diffusion of excess free volume. Results for Al-phthalocyanine tetrasulfonate (APT) in hyperquenched glassy water (HGW) and ethanol (HGE) films that provide strong support for the critical role of excess free volume are discussed. Hole spectra of APT/HGW obtained over eight decades of burn fluence reveal that the current mechanism needs to be modified to include multilevel extrinsic systems (MLSext) in order to explain why the antihole ("photoproduct" absorption) lies to the blue of the burn frequency for sufficiently high burn fluences, an intriguing up-conversion process. The spectra also reveal, for the first time, that the zero-phonon hole (ZPH) profile is non-lorentzian. This is shown to be a natural consequence of the interplay between the three distributions that result in dispersive hole growth kinetics. They are associated with the tunnel parameter lambda of the TLSext, the angle alpha between the laser polarization and transition dipole, and off-resonant absorption of the zero-phonon line (the omega distribution). Theoretical simulations of hole growth data for APT/HGW obtained over six decades of burn fluence show that the lambda distribution is of primary importance, describing well the first 80% of the saturated burn. The paper ends with an application of NPHB combined with high pressure and external electric (Stark) fields to the critically important "red" antenna states of photosystem I. The addition of pressure and Stark fields enhances the already impressive selectivity of NPHB, The results show that the linear pressure shift, permanent dipole moment change, and linear electron-phonon coupling are correlated. Of particular importance is that these properties can be used to identify states which involve interacting chlorophyll molecules that possess significant charge transfer character because of electron-exchange coupling. The results also show that the site distribution functions of the antenna states are largely uncorrelated, consistent with the findings for previously studied complexes. This is important because the absence of correlation means that the electronic energy gaps of donor and acceptor states are distributed which, in turn, means that the kinetics can be dispersive under certain conditions.