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.
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