Xiyi Chen

Flowing liquid sample jet for resonance Raman and ultrafast optical spectroscopy

A wire-guided, gravity-driven jet apparatus is described that produces optically stable thin films of liquids flowing at rates suitable for high repetition rate spectroscopy. Unlike conventional free-flowing jets, the design works well for low viscosity solvents including water and aqueous solutions of proteins. The construction of the wire guide, jet nozzle, and flow system is described. A stable water film whose thickness can be varied from 6 to 100 μm is demonstrated that has been employed in resonance Raman and femtosecond transient absorption experiments.

Time-resolved scavenging and recombination dynamics from I:e− caged pairs

The competition between geminate recombination of electrons with their parent radicals and electron scavenging with H+ is directly time resolved with ∼100 fs resolution at several acid concentrations. Electrons were produced from iodide photodetachment or two-photon ionization of H2O. With regards to those produced from iodide photodetachment, the separation between primary and secondary I:e− recombination is established using a full numerical solution to the diffusion equation. Electron ejection is found to be short range and a potential well of ∼3kbT depth stabilizing the solvent caged pair is required to yield a satisfactory fit to experiment. From time-resolved scavenging data up to 5 M HCl, it is shown that the electron can be scavenged both inside and outside the caged pair by H+ with nearly equal efficiency. The steady-state scavenging yield as a function of scavenger concentration is then predicted based on the determined time-dependent recombination function. Reassessment of several benchmark sca...

Broadband spectral probing revealing ultrafast photochemical branching after ultraviolet excitation of the aqueous phenolate anion

Electron photodetachment from the aromatic anion phenolate excited into the π-π* singlet excited state (S(1)) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broad-band transient absorption spectra from 300 to 690 nm are recorded. The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S(1) excited state, and confirmation of these assignments is achieved using both KNO(3) as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (∼20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S(1). Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S(1) state and the relaxed S(1) state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions.

Investigation of macrocyclic polymers as artificial light harvesters: subpicosecond energy transfer in poly(9,9-dimethyl-2-vinylfluorene).

The spectroscopy and dynamics of a novel molecular architecture that mimics natural light harvesting have been characterized. The deployment of 9,9-dimethyl-2-fluorenyl (DMF) chromophores in atactic macrocyclic poly(9,9-dimethyl-2-vinylfluorene) is similar to that in the light harvesting antenna LH2 of the purple photosynthetic bacteria. A variety of spectroscopic probes are used to study the dynamics in these novel polymer systems. The number of chromophores is tuned from 12-142 identical chromophore units. Steady-state absorption and emission measurements, time-resolved fluorescence, and ultrafast transient absorption anisotropy techniques provide evidence for distinct differences in the photophysics of matching molecular weight linear and cyclic polymers and of the occurrence of energy transfer in these polymers. There is direct evidence of energy transfer in these macrocycles manifested in the depolarization decay components, which are characterized by two exponentials and are substantially faster than observed for reorientation of the free DMF chromophore. The time constants for the macrocycles are 700-900 fs and 7-8 ps and are size dependent; the biexponential decay arises from conformational and stereochemical disorder and can be well described by a master equation simulation assuming Förster incoherent hopping on model polymer structures. The results suggest energy hopping between adjacent chromophores on a 1 ps time scale. The pathway for energy migration is shown to be primarily between nearest neighbors along the cyclic backbone, but there is a considerable spread in the site-to-site hopping rates. Small cycles adopt a pseudoplanar ring type arrangement of the chromophore transition dipoles as observed in bacterial light harvesting antenna, and it is found that the linear polymers also show similar short-range planarity of transition dipoles. Overall, it is found that such small macrocyclic polymers possess excellent characteristics for light harvesting among identical chromophores and behave as a circular photonic wire.

Spectroscopy of macrocyclic poly (9,9-dimethyl-2-vinylfluorene) as an artificial light harvester

The photophysical properties of macrocyclic poly (9,9-dimethyl-2vinylfluorene) have been characterized by steady state fluorescence and ultrafast spectroscopy. Ultrafast transient absorption anisotropy studies show that energy migration occurs on a picosecond time scale. Three-dimensional crystal structures for the light harvesting antenna complexes of photosynthetic bacteria have recently been resolved [1]. This has led to a more profound understanding of how nature harvests sunlight in an efficient fashion. The antenna complex (LH2) is highly symmetric and the deployment of chromophores is cyclic. It is therefore of interest whether this architecture is important for the very rapid and efficient energy transfer excitation hopping times at room temperature of ~100 fs have been reported [2]. In our investigations, the ring-like structure of LH2 is mimicked in novel two-dimensional macrocyclic polymers, which have recently been synthesized [3]. Such poly-vinyl aromatic polymers can be prepared with 10 – 50 chromophore groups (here the chromophore is 9,9 dimethylfluorene) that are pendent to the polymer backbone. In this report, the steady state and time-resolved spectroscopic properties of the macrocyclic polymers are compared with their linear analogs. A 1kHz Ti:S regenerative amplifier producing 100 fs pulses centered at 800 nm pumps an optical parametric amplifier ( OPA). Sum-frequency gener ation of the OPA signal with residual 800 nm pump followed by frequency doubling is used to produce pump pulses at a wavelength of 300 nm. The variably delayed probe beam is generated from a separate portion of the 800 nm fundamental. The pumpprobe instrument response function is ~170 fs as determined by difference frequency cross-correlation measurements. Both the pump and probe pulses are focused into a 0.5 mm path length CaF 2 flow cell containing poly(9,9-dimethyl-2vinylfluorene), linear or cyclic, dissolved in cyclohexane. The probe is polarized at 45 with respect to the pump in order that parallel and perpendicular transient absorption components may be simultaneously detected. This is achieved by separating the parallel and perpendicular probe fields by a Wollaston prism after the sample and employing two detection photodiodes, which are in turn connected to lock-in amplifiers referenced to a mechanical chopper in the pump path. The two time-resolved signals obtained are then used to calculate the anisotropy, which is fit with a bi-exponential decay function. Absorption and steady-state fluorescence measurements for fluorene, linear and cyclic poly (9,9-dimethyl-2vinylfluorene) with different degrees of polymerization (DP n) have also been recorded. All experiments were performed at room temperature. The absorption spectra of both the linear and cyclic polymers have very similar characteristics. As seen in Figure 1a, the spectra for both systems overlay and, other than a small blue shift, are in fact virtually identical to the chromophore 9,9dimethylfluorene free in solution. This indicates that in the ground state there is minimal electronic interaction between adjacent chromophore molecules as expected for a non-conjugated polymer. The fluorescence spectra for both polymers, however, reveal an enhancement in emission in the cyclic polymers opposed to their linear analogs ( Fig. 1b). Fig. 1. Absorption and fluorescence spectra of macrocyclic polymer and its linear analog Fig. 2. (a) Anisotropy decay for fluorine monomer and linear and cyclic polymers; (b) as a function of DPn in macrocyclic polymers . Figure 2 shows the femtosecond transient absorption anisotropy data for isolated fluorene and the linear and macrocyclic forms of poly (9,9-dimethyl-2vinylfluorene) in cyclohexane. The dependence of the anisotropy decay with size of the macrocyclic polymer (expressed as its degree of polymerization, DP n) is 0 1 2 3 4 5 0.0 0.1 0.2 0.3 0.4 A ∆τ, ps A ni so tr op y linear polymer