Search

Transient infrared and visible absorption measurements along with density functional theory (DFT) calculations on carotenoids 8'-apo-beta-caroten-8'-al (I) and 7',7'-dicyano-7'-apo-beta-carotene (II) were used to explore the nature of a long-lived species observed in transient infrared absorption measurements following two-photon excitation (Pang et al. J. Phys. Chem. B 2009, 113, 13806). The long-lived species of I has a very strong infrared absorption around 1510 cm(-1) and a visible transient absorption band centered at 760 nm. The long-lived species appears on two different time scales of approximately 16 and 140-270 ps. The longer rise component is absent in nonpolar solvents. DFT calculations using the B3LYP functional and the 6-31G(d) basis set were used to investigate the ground-state potential-energy surface of I and II including its conformational isomers, a pi-diradical "kinked" structure, and cation and neutral radicals. From the simulated infrared spectra of all the structures considered, we found a close match in the cation radical spectrum to the experimental infrared spectrum of the long-lived species. However, the visible absorption band does not match that of the monomeric cation radical. On the basis of our experimental and theoretical results, we propose a charge-transfer complex between a carotenoid and a solvent molecule for the origin of the long-lived species formed from the direct two-photon excitation of the S(1) state.

We present infrared transient absorption measurements of the substituted apocarotenoids 8'-apo-beta-caroten-8'-al (I) and 7',7'-dicyano-7'-apo-beta-carotene (II) in the S(1) excited states by one-photon excitation (1PE) and two-photon excitation (2PE). 1PE populates the higher S(2) state, which converts to the S(1) state via rapid internal conversion, and 2PE populates the S(1) state directly. The 1PE-prepared population returns to the ground state on a approximately 19 ps time scale in I and a approximately 2.0 ps time scale in II. Distinct vibrational spectra and dynamics are observed from the 2PE-prepared S(1) state for both I and II: the symmetric C=C stretching vibration around 1500 cm(-1) in the S(1) state is several-fold increased in strength over other C=C stretching modes compared to the 1PE case, and long-lived absorptions are observed even after all the excited-state populations have decayed. The lifetimes of the S(1) state prepared by 2PE are slightly shorter (approximately 17 ps for I and approximately 1.7 ps for II) than the 1PE values. It is proposed that 1PE and 2PE lead to the population of different conformational minima of the S(1) potential surface. These two minima do not communicate on the time scale of the S(1) lifetime and have different relaxation channels on the ground-state surface.

Femtosecond mid-infrared spectroscopy was applied to study the vibrational spectra and dynamics in the electronic excited states of semiconducting single-walled carbon nanotubes (SWNTs). The experiments were performed by exciting SWNTs dispersed individually in polymethylmethacrylate and polyvinyl alcohol polymer films with 40 fs laser pulses at 800 nm, and the resulting responses were monitored with broadband mid-infrared pulses ranging from 1510 to 1670 cm(-1). The structured spectra observed show vibrational bands with up-shifted frequencies by approximately 10-50 cm(-1) with respect to their ground-state counterparts. The observation provides direct evidence for the theoretically predicted lattice distortions in the electronic excited state. Analysis of the kinetics probed in the mid- and near-infrared regions provide an estimate of the time scales for the vibrational relaxation.

Time resolved visible pump, infrared probe transient absorption measurements of the solutes 4-dicyanomethylene-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM) and its isotopomer DCM-d6 are employed to probe the dynamics of charge transfer state formation in dimethyl sulfoxide (DMSO) and acetonitrile (MeCN). We observe a two stage charge transfer (CT): the first step is an instrument-response-limited charge separation to the dicyanomethylene group, and the second involves a structural evolution of the dimethylamino group. Theoretical calculations and isotopic substitution indicate that the observed vibration is due to the dimethylamino group twisting out of plane, stabilizing the charge separation.

The effect of nonlocal interactions on the local structural propensities of folded dendrimers was evaluated in this work by comparing, under identical conditions, the conformational properties of isomeric dendrimers differing in their global packing efficiency. Accordingly, a modular synthesis of two series of dendrimers up to the third generation was developed to provide efficient access to isomeric dendrimers displaying different levels of overall compaction. Dendrimer compaction levels were adjusted by connecting the folded dendrons to 1,3,5-benzenetricarbonyl chloride, as the central core, via either a 2- or a 4-aminobenzamide linkage to induce relatively "compacted" or "expanded" conformations, respectively. The hydrodynamic volumes of the dendrimers were measured by time-resolved fluorescence anisotropy (TRFA) measurements as a function of the dendrimer series, generation level, and solvent. Packing efficiencies (compaction levels) were estimated by the ratio (V(h)/V(vw)) of the experimental hydrodynamic volume (V(h)) to the calculated van der Waals volume (V(vw)). The extent and stability of local helical bias was measured using circular dichroism and correlated with the packing efficiency (V(h)/V(vw)). These studies suggested that compaction plays an extremely important role in determining the secondary structural preferences of the dendrimers; however, the nature of compaction was more important than the extent of compaction.