Optical Spectra of Oligofurans. A Theoretical Approach to the Transition Energies, Reorganization Energies, and the Vibronic Activity

There is experimental evidence of high vibronic activity that accompanies the strongly allowed transition between the ground state and the lowest electronic singlet excited state of oligofurans that contain 2,3, and 4 furan rings. The absorption and emission spectra of the three lowest oligofurans measured in liquid nitrogen temperature show distinct fine structures that are reproduced using the projection-based model of vibronic coupling (with Dushinsky rotation included) parameterized utilizing either DFT (with several different exchange-correlation functionals) or ab initio (CC2) quantum chemistry calculations. Using as reference the experimental data concerning the electronic absorption and fluorescence for the 8 lowest oligofurans we first analyze the performance of the exchange-correlation functionals for the electronic transition energies and the reorganization energies. Subsequently, we use the best functionals alongside the CC2 method to explore how the reorganization energies are distributed among the totally symmetric vibrations, identify the normal modes that dominate in the fine structures present in the absorption and emission bands, and trace their evolution with the increasing number of rings in the oligofuran series. Confrontation of the simulated spectra with the experiment allows for verification of the performance of the selected DFT functionals and the CC2 method.

Density Functional Theory Study of the Solvent Effects on Electronic Transition Energies of Porphyrins

We have calculated the solvent effects on the ground state and the lowest triplet state absorption spectra of meso-tetraphenylporphyrin (TPP), meso-tetrakis(p-sulfonatophenyl)porphyrin (TSPP) and their diprotonated forms (H4TPP and H4TSPP) in thirty-nine different solvent using time-dependent-DFT density functional theory (TD-DFT) coupled with CPCM method. The results of the calculations show that the Q-bands and Soret-bands (or B-bands) in the absorption spectra of these compounds substantially change as function of solvent dielectric constant (ε) up to 20.493 (acetone), but become stabile in high polar solvents with dielectric constants ε > 20. The relative shifts in the B-bands are more significant than that in the Q-bands. The magnitude of the shifts in the spectral position of the Q and B bands are in the following order: H4TSPP > H4TPP > TPP > TSPP for the B-bands and H4TSPP > H4TPP > TSPP > TPP for the Q-bands. We also have determined that the energy-gaps between the B/Q-bands and their nearest triplet states are also solvent dependent for ε < ~ 20.493.

Color tuning of chlorophyll a and b pigments revealed from gas-phase spectroscopy

New perspectives of light harvesting: impacts of dimerization and axial ligation on electronic transition energies of chlorophylls in vacuo.

Exciton Absorption and Luminescence in i-Motif DNA

We have studied the excited-state dynamics for the i-motif form of cytosine chains (dC)10, using the ultrafast fluorescence up-conversion technique. We have also calculated vertical electronic transition energies and determined the nature of the corresponding excited states in a model tetramer i-motif structure. Quantum chemical calculations of the excitation spectrum of a tetramer i-motif structure predict a significant (0.3 eV) red shift of the lowest-energy transition in the i-motif form relative to its absorption maximum, which agrees with the experimental absorption spectrum. The lowest excitonic state in i-(dC)10 is responsible for a 2 ps red-shifted emission at 370 nm observed in the decay-associated spectra obtained on the femtosecond time-scale. This delocalized (excitonic) excited state is likely a precursor to a long-lived excimer state observed in previous studies. Another fast 310 fs component at 330 nm is assigned to a monomer-like locally excited state. Both emissive states form within less than the available time resolution of the instrument (100 fs). This work contributes to the understanding of excited-state dynamics of DNA within the first few picoseconds, which is the most interesting time range with respect to unraveling the photodamage mechanism, including the formation of the most dangerous DNA lesions such as cyclobutane pyrimidine dimers.

Photomodulated Rayleigh Scattering from Single Semiconductor Nanowires

ABSTRACTWe demonstrate the newly developed technique Photomodulated Rayleigh Scattering spectroscopy in order to probe the electronic band structure of single semiconductor nanowires. We show that both the electronic transition energies and nanowire diameter can be measured simultaneously and with high accuracy in a single non-destructive measurement. We demonstrate our results for zincblende GaAs as well as wurtzite InP nanowires where we probed the band gaps and transition energies at both room and low temperatures. This technique should advance the study of optical properties of single nanowires as well as other types of nanostructures.