Energy transfer and spatial scrambling of an exciton in a conjugated dendrimer.

Photoexcitation of multichromophoric light harvesting molecules induces a number of intramolecular electronic energy relaxation and redistribution pathways that can ultimately lead to ultrafast exciton self-trapping on a single chromophore unit. We investigate the photoinduced processes that take place on a phenylene-ethynylene dendrimer, consisting of nine equivalent linear chromophore units or branches. meta-Substituted links between branches break the conjugation giving rise to weak couplings between them and to localized excitations. Our nonadiabatic excited-state molecular dynamics simulations reveal that the ultrafast internal conversion process to the lowest excited state is accompanied by an inner → outer inter-branch migration of the exciton due to the entropic bias associated with energetically equivalent conjugated segments. The electronic energy redistribution among chromophore units occurs through several possible pathways in which through-bond transport and through-space exciton hopping mechanisms can be distinguished. Besides, triple bond excitations coincide with the localization of the electronic transition densities, suggesting that the intramolecular energy redistribution is a concerted electronic and vibrational energy transfer process.

Ultrafast electronic energy relaxation in a conjugated dendrimer leading to inter-branch energy redistribution.

Dendrimers are arrays of coupled chromophores, where the energy of each unit depends on its structure and conformation. The light harvesting and energy funneling properties are strongly dependent on their highly branched conjugated architecture. Herein, the photoexcitation and subsequent ultrafast electronic energy relaxation and redistribution of a first generation dendrimer (1) are analyzed combining theoretical and experimental studies. Dendrimer 1 consists of three linear phenylene-ethynylene (PE) units, or branches, attached in the meta position to a central group opening up the possibility of inter-branch energy transfer. Excited state dynamics are explored using both time-resolved spectroscopy and non-adiabatic excited state molecular dynamics simulations. Our results indicate a subpicosecond loss of anisotropy due to an initial excitation into several states with different spatial localizations, followed by exciton self-trapping on different units. This exciton hops between branches. The absence of an energy gradient leads to an ultrafast energy redistribution among isoenergetic chromophore units. At long times we observe similar probabilities for each branch to retain significant contributions of the transition density of the lowest electronic excited-state. The observed unpolarized emission is attributed to the contraction of the electronic wavefunction onto a single branch with frequent interbranch hops, and not to its delocalization over the whole dendrimer.

Exploring the role of phase modulation on photoluminescence yield.

We report an investigation to elucidate the mechanisms of control in phase-sensitive experiments in two molecular systems. A first inspection of optimization procedures yields the same experimental result: increase in the emission efficiency upon excitation by a phase modulated pulse in a two-photon transition. More detailed studies, which include power dependence, spectral response, one and two color pump-probe and pump-pump experiments show that while for one chromophore phase modulation leads to spectral matching between the two-photon cross section and the second order power spectrum for the other it provides a tool to manipulate the wavepacket dynamics in the excited state.

Broadband femtosecond transient infrared spectroscopy using a 256 x 256 element indium antimonide focal-plane detector.

Application and characterization of large-format IR focal-plane arrays as detectors for ultrafast, high-resolution IR spectroscopy are discussed. We also present generation of broadband IR probe-reference pulses by use of collinear non-phase-matched geometry and shot-to-shot dual-track normalization to obtain transient spectra from broadly absorbing hydrogen-bonded systems. As much as 400-cm(-1)-wide coverage with 15-cm(-1) FWHM spectral resolution and +/-6.4 x 10(-4)(DOD = 3 x 10(-4)) baseline standard deviation (+/-1sigmas) is demonstrated near 2.9 microm.

Mercury cadmium telluride focal-plane array detection for mid-infrared Fourier-transform spectroscopic imaging.

By combining step-scan Fourier-transform Michelson interferometry, an infrared microscope, and mercury cadmium telluride focal-plane array image detection we have constructed a mid-infrared spectroscopic imaging system that simultaneously records high-fidelity images and spectra of materials from 3500 to 900 cm(-1) (2.8 to 11 microm) at a variety of spectral resolutions. The fidelity of the spectral images is determined by the pixel number density of the focal-plane array. Step-scan imaging principles and instrument design details are outlined. Spatial resolution measurements and infrared chemical imaging examples are presented, and the results are discussed with respect to implications for chemical analysis of biosystems and composite materials.