Ultrafast chemical reactions and energy transfers investigated by time-resolved electronic and vibrational spectroscopy

Abstract

Photo-induced chemical reaction dynamics of chromophores has been of great interest in chemistry and related fields for decades. Understanding the dynamics of molecules including excited state proton transfer, charge transfer (CT), energy transfer provides important keys to the chemical reactions in various chemical and biological systems. This can also be beneficial in many applications including the artificial photosynthesis, dye-sensitized solar cell, or molecular optoelectronics. To investigate these photochemical reactions rapidly occurring on picosecond to femtosecond time scales, a femtosecond time-resolved spectroscopic method have been widely used. In my dissertation, time-resolved electronic and vibrational spectroscopy including transient absorption, femtosecond stimulated Raman, impulsive stimulated Raman were used to seek detailed understanding of the excited-state dynamics of chromophores with the structural changes. In Chapter 1 and Chapter 2, the overall introduction and experimental setups used in this dissertation were described. With a transient absorption setup, the excited-state intramolecular proton transfer (ESIPT) process of 1,2-dihydroxyanthraquinone (alizarin), the photo-induced electron transfers from the S2 state of carotenoid molecules to TiO2 semiconductor nanoparticles, and the energy transfer process in porphyrin-peptoid complexes were explored. In Chapter 3, the population dynamics (87 ps) and vibrational relaxation (0.35 and 8.3 ps) during the ESIPT process of alizarin in ethanol solution were described. The ESIPT reaction of alizarin seems to be blocked by the strong hydrogen bond formation between water and alizarin. Even though the ultrafast ESIPT of time constant (<100 fs) was not observed due to the overlapped several contributions in transient absorption signal, the structural dynamics of alizarin accompanied by the ESIPT process will be further described by femtosecond stimulated Raman measurements. In Chapter 4, the ultrafast excited-state dynamics including the electron and recombination dynamics between ACOA and TiO2 nanoparticles were described. The electron injection of 240 fs from ACOA into TiO2 nanoparticles with quantum yields of ~33 % were observed with no dependence on excitation wavelengths. In Chapter 5, high efficiencies (96.3-97.6 %) through the energy transfer rate (66 ps-1 ~ 101 ps-1) for five porphyrin-peptoid complexes were investigated depending on the relative spatial arrangements between the zinc porphyrin (donor) and free base porphyrin (acceptor) with the transient absorption measurements. Two porphyrins were synthesized at the defined residues of peptoids with varying the distances and orientations. With the control of spatial arrangements of porphyrins, the precisely tuneable energy transfer efficiency of porphyrin-peptoid conjugates were demonstrated. In Chapter 6-8, the structural changes of push-pull emitters including DCM and nitroaromatic molecules have been investigated by using the femtosecond stimulated Raman spectroscopy with high temporal (<50 fs) and high spectral resolution (<10 cm-1). Many TDDFT simulations estimated twisted intramolecular charge transferred states of push-pull dyes with rotation of electron donating or withdrawing groups. With the experimental evidence, the structures of push-pull emitters during the ICT process were investigated. For 4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl)-4H-pyran (DCM), the ICT formation with ~1 ps time constant followed by the vibrational relaxations of 4~7 ps was observed in dimethyl sulfoxide (highly polar) solution from the population changes, frequency shifts, and bandwidth changes of the major vibrational modes including ν(C≡N), ν(C=C/C-C), and δ(CH3) (Chapter 6). In chloroform (slightly polar) solution, CT state simultaneously populated with the locally excited (LE) state decays to the LE state with a fast (~300 fs) lifetime followed by the vibrational relaxation in the LE state (4~7 ps). The time-dependent density functional theory (TDDFT) simulations were performed with comparison of two distinct Raman spectra of DCM in the LE state and CT state, and a twisted geometry of the dimethylamino group of DCM during the ICT process was proposed. In Chapter 7, the accompanied structural changes of nitroaromatic molecules including 4-dimethylamino-4'-nitrobiphenyl (DNBP) and 4-dimethylamino-4′-nitrostilbene (DMANS) during the ICT process were explored. During the ICT process of nitroaromatic molecules, the vibrational resonance at stretching vibrational modes (ν8a) between two phenyl rings observed in the region of 1550-1650 cm-1 were broken with the twisted biphenyl group. Based on the TDDFT simulations, a twisted geometry of the nitrophenyl group of nitroaromatic molecules during the ICT process was proposed. In Chapter 8, LD 688 dye which has similar molecular structure of DCM with restricted rotation around dimethylamino group was investigated. Upon photoexcitation, the ICT with the 1.0 ps time constant followed by ~5 ps vibrational relaxation in the product CT state was observed, which are evidenced in several vibrational modes including δ(CH/CH3), ν(C=C), and ν(C≡N) as the changes in the Raman intensity and frequency shift. Based on the experimental results and the TDDFT simulations, a twisted molecular geometry with rotated julolidine group of LD 688 during the ICT process was suggested. In Chapter 9, the setup of time-resolved impulsive stimulated Raman spectroscopy (ISRS) has been described, in which the frequency-domain vibrational spectra are obtained by a fast Fourier transform of the temporal coherent nuclear wavepacket motions which are impulsively excited by ultrashort Raman pump. In order to observe the fingerprint region over the 3000 cm-1 of molecules, ultrashort (<20 fs) and broadband pulses were generated from a noncollinear optically parametric amplifier (NOPA). To get the ISRS signal with high signal to noise ratio, the detection noise level was significantly improved as <25 μOD per 1000 pulses by minimizing the fluctuation level with the reference detector. With this technique, several preliminary results on including the protonation dynamics of a photoacid HPTS and the solvation dynamics of dimethyl sulfoxide during the ESIPT process of alizarin will be introduced.