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Singlet fission (SF) is a spin-allowed process in which a singlet exciton, 1 (S 1 S 0 ), within an assembly of two or more chromophores spontaneously down-converts into two triplet excitons via a multiexciton correlated triplet pair state, 1 (T 1 T 1 ). To elucidate the involvement of charge transfer (CT) states and vibronic coupling in SF, we performed 2D electronic spectroscopy (2DES) on dilute solutions of a covalently linked, slip-stacked terrylene-3,4:11,12-bis-(dicarboximide) (TDI) dimer. This dimer undergoes efficient SF in nonpolar 1,4-dioxane and symmetry-breaking charge separation in polar dichloromethane. The various 2DES spectral features in 1,4-dioxane show different pump wavelength dependencies, supporting the presence of mixed states with variable 1 (S 1 S 0 ), 1 (T 1 T 1 ) and CT contributions that evolve with time. Analysis of the 2DES spectra in dichloromethane reveals the presence of a state having largely 1 (T 1 T 1 ) character during charge separation. Therefore, the 1 (T 1 T 1 ) multiexciton state plays an important role in the photophysics of this TDI dimer irrespective of solvent polarity.

We have developed a broad bandwidth two-dimensional electronic spectrometer that operates shot-to-shot at repetition rates up to 100 kHz using an acousto-optic pulse shaper. It is called a two-dimensional white-light (2D-WL) spectrometer because the input is white-light supercontinuum. Methods for 100 kHz data collection are studied to understand how laser noise is incorporated into 2D spectra during measurement. At 100 kHz, shot-to-shot scanning of the delays and phases of the pulses in the pulse sequence produces a 2D spectrum 13-times faster and with the same signal-to-noise as using mechanical stages and a chopper. Comparing 100 to 1 kHz repetition rates, data acquisition time is decreased by a factor of 200, which is beyond the improvement expected by the repetition rates alone due to reduction in 1/f noise. These improvements arise because shot-to-shot readout and modulation of the pulse train at 100 kHz enables the electronic coherences to be measured faster than the decay in correlation between laser intensities. Using white light supercontinuum for the pump and probe pulses produces high signal-to-noise spectra on samples with optical densities <0.1 within a few minutes of averaging and an instrument response time of <46 fs thereby demonstrating that that simple broadband continuum sources, although weak, are sufficient to create high quality 2D spectra with >200 nm bandwidth.

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.

The dynamics of electronic transitions in solid-state materials are closely linked to microscopic morphology, but it is challenging to simultaneously characterize their spectral and temporal response with high spatial resolution. We present a time-resolved nonlinear microscopy system using white-light supercontinuum pulses as a broadband light source. This system is capable of correlating nanometer scale sample morphology determined from atomic force topography measurements with broadband transient absorption hyperspectral images and ultrafast 2D white-light spectra, all with a spatial resolution of ≤1 μm. The experimental apparatus is described with a focus on the dispersion management strategies necessary to minimize the duration of optical pulses when implementing an AOM based pulse-shaping system covering a broad-spectral range in the VIS/NIR. Experiments on TIPS–pentacene organic semiconductor microcrystals are used to demonstrate the unique capabilities of this technique.

Polarized two-dimensional white-light (2D-WL) spectra are reported for thin films of semiconducting carbon nanotubes. The orientational responses for 4-point correlation functions are derived for samples that are isotropic in two dimensions. Spectra measured using <-45°,+45°,0°,90°> polarizations eliminate the diagonal peaks in the spectra arising from S1 transitions to uncover cross peaks to a weaker transition that is assigned to radial breathing modes. In nanotubes purified by unwrapping PFO-BPY polymer using metal chelation, an absorption at 1160 nm is observed that is assigned to hole doping that forms trions. The trion peak may have a transition dipole non-parallel to the S1 transitions, and so its cross peak is prominent in polarized 2D WL spectra. Energy transfer of photoexcitons to the trion peak occurs within 1 ps. Identifying and understanding the effects of purification on the electronic structure of thin films of semiconducting carbon nanotubes is important for learning how the inherent photophysics of individual carbon nanotubes translates to coupled nanotube thin film materials.

We report on a new broadband, ultrafast twodimensional white-light (2DWL) spectrometer that utilizes a supercontinuum pump and a supercontinuum probe generated with a ytterbium fiber oscillator and an all-normal dispersion photonic crystal fiber (ANDi PCF). We demonstrate compression of the supercontinuum to sub-20 fs and the ability to collect high quality 2D spectra on films of single-walled carbon nanotubes. Two spectrometer designs are investigated. Supercontinuum from ANDi PCF provides a means to generate broadband pulse sequences for multidimensional spectroscopy without the need for an optical parametric amplifier.

We compare the ultrafast dynamics of singlet fission and charge generation in pentacene films grown on glass and graphene. Pentacene grown on graphene is interesting because it forms large crystals with the long axis of the molecules "lying-down" (parallel to the surface). At low excitation fluence, spectra for pentacene on graphene contain triplet absorptions at 507 and 545 nm and no bleaching at 630 nm, which we show is due to the orientation of the pentacene molecules. We perform the first transient absorption anisotropy measurements on pentacene, observing negative anisotropy of the 507 and 545 nm peaks, consistent with triplet absorption. A broad feature at 853 nm, observed on both glass and graphene, is isotropic, suggesting hole absorption. At high fluence, there are additional features, whose kinetics and anisotropies are not explained by heating, that we assign to charge generation; we propose a polaron pair absorption at 614 nm. The lifetimes are shorter at high fluence for both pentacene on glass and graphene, indicative of triplet-triplet annihilation that likely enhances charge generation. The anisotropy decays more slowly for pentacene on graphene than on glass, in keeping with the smaller domain size observed via atomic force microscopy. Coherent acoustic phonons are observed for pentacene on graphene, which is a consequence of more homogeneous domains. Measuring the ultrafast dynamics of pentacene as a function of molecular orientation, fluence, and polarization provides new insight to previous spectral assignments.