No abstract

We present here the first room-temperature 2D electronic spectroscopy study of energy transfer in the plant light-harvesting complex II, LHCII. Two-dimensional electronic spectroscopy has been used to study energy transfer dynamics in LHCII trimers from the chlorophyll b Qy band to the chlorophyll a Qy band. Observing cross-peak regions corresponding to couplings between different excitonic states reveals partially resolved fine structure at the exciton level that cannot be isolated by pump-probe or linear spectroscopy measurements alone. Global analysis of the data has been performed to identify the pathways and time constants of energy transfer. The measured waiting time (Tw) dependent 2D spectra are found to be composed of 2D decay-associated spectra with three timescales (0.3 ps, 2.3 ps and >20 ps). Direct and multistep cascading pathways from the high-energy chlorophyll b states to the lowest-energy chlorophyll a states have been resolved occurring on time scales of hundreds of femtoseconds to picoseconds.

Ultrafast two-dimensional electronic spectroscopy has been used to study the spectral diffusion of the Qy transition of chlorophyll a in methanol. The two time frequency-fluctuation correlation function (FFCF) of the transition has been measured using the center line slope method, together with optimized fitting of the linear spectrum. The FFCF was measured to decay over four time scales. The three fastest time constants of which were measured to be ∼65 fs, ∼0.5 ps, and ∼7 ps. These are assigned as the inertial component of solvation and spectrally diffusive solvation processes respectively. The fourth time constant (>1 ns) may be attributed to the chromophore structural inhomogeneity.

Following excitation of the A state nu(2) (')=4 mode in ammonia, we show how the time scale to dissociation of the N-H bond depends on the internal energy imparted to the NH(2) photofragment. Using a combination of femtosecond pump/probe spectroscopy and velocity map ion imaging techniques, the time and energy resolved H-atom elimination can be directly related to the nonadiabatic nature of the photodissociation for high kinetic energy H atoms with evidence for adiabatic dynamics to dissociation giving the lowest energy H atoms. Extrapolation of the time scales for dissociation versus internal energy of the NH(2) photofragment implies that dissociation to the vibrationless ground state of NH(2) occurs in <50 fs, in very good agreement with frequency resolved measurements. The anisotropy of the H fragments with the highest kinetic energies seems to also suggest that the NH(2) partner fragment comes off with very low rotational excitation.

The photoresistive properties of DNA bases, amino acids and corresponding subunits have received considerable attention through spectroscopic studies in recent years. One photoresistive property implicates the participation of (1)πσ* states, allowing electronically excited states to evolve either back to the electronic ground state or undergo direct dissociation along a heteroatom-hydride (X-H) coordinate. To this effect, time-resolved velocity map imaging (TR-VMI) studies of imidazole (a subunit of both adenine and histidine) and methylated derivatives thereof have been undertaken, with the goal of understanding the effects of increasing molecular complexity, through methylation, on the dynamics following photoexcitation at 200 nm. The results of these measurements clearly show that H-atom elimination along the N-H coordinate results in a bimodal distribution in the total kinetic energy release (TKER) spectra in both imidazole and it's methylated derivatives: 2-methyl, 4-methyl and 2,4-dimethylimidazole. The associated time constants for H-atoms eliminated with both high and low kinetic energies are all less than 500 fs. A noticeable increase in the time constants for the methylated derivatives is also observed. This could be attributed to either: ring methylation hindering in-plane and out-of-plane ring distortions which have been implicated as mediating excited state dynamics of these molecules or; an increase in the density of vibrational states at 200 nm causing an increased sampling of orthogonal modes, as opposed to modes which drive any dynamics that cause subsequent H-atom elimination. The results of these findings once again serve to illustrate the seemingly ubiquitous nature of (1)πσ* states in the photoexcited state dynamics of biomolecules and their subunits.