Studies On Ultrafast Dynamics Of Quaterthiophene And Anthraquinone Derivatives Compounds By Transient Absorption Spectroscopy

In the course of history, people have been pursuing the true essence of phenomena in nature. Many scientists focus on a variety of events for their development time scales to reveal their evolutions. With the development of science and technology, people tend to be more and more interested to study the fundamental reaction, such as fundamental process of photochemistry reaction in the atmosphere, the energy capture and energy transfer in photosynthesis, the photodamage and photoprotection in DNA. Upon photon excitation, molecules undergo many processes, such as fluorescence emission, internal conversion, intersystem crossing, charge transfer, proton transfer, intermolecular energy relaxation, many of which are in ultrafast time scales. The emergence of femtosecond laser and the development of femtosecond time-resolved technology provide favorable technical support to explore the ultra photophysical and photochemical reactions in molecules. Transient absorption spectroscopy is an ideal experimental method for investigating the ultrafast dynamics in solution. It can effectively detect the evolution of the excited states and obtain the dynamics of the molecules. In this thesis, the ultrafast dynamic processes of quarterthiophene and anthraquinone derivatives compounds are investigated by the transient absorption spectroscopy technology combined with quantum chemistry calculations. The thesis mainly consists of three parts:The first part is ultrafast geometry relaxation and intersystem crossing of quaterthiophene in 1,4-dioxane. Upon excitation at 400 nmto the S1 state, the signals of stimulated emission, excited state absorption and triplet-triplet absorption were observed. The data reveal that the excitation to the Si state results in an approximately 70 ps conformational relaxation from a twisted to a more rigid planar structure. Two triplet-triplet absorption bands centered at 563 nm and 600 nm show a direct dynamical conversion. The intersystem crossing is determined to be-398 ps. The high triplet yield is measured as-0.7 via the efficient intersystem crossing. Two triplet states are proven to energetically exist below the S1 state by quantum calculations, and the energy gap between the S1 and T2 states becomes smaller during the geometrical relaxation of the S1 state. It suggests a favourable situation for the effective ISC process and a high triplet quantum yield.The second part is the excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution. Upon excitation at 400 nm to the S1 state, stimulated emission and excited state absorption signals were abserved. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S2 and S1 states are reversal and a conical intersection is demonstrated to exist along the proton transfer coordinate. The transfer to the lowest excited state of tautomer is dominated due to the consistency of orbits. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with-200 fs and the following intermolecular energy relaxation with-16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. Finally, the proton transfer goes back to the ground state of the normal-form from the tautomer-form. Based on the novel experimental and computational data, a six-state model is proposed for 1-hydroxyanthraquinone to describe the dynamics of excited state intramolecular proton transfer.The third part is the investigation on the excited state intramolecular charge transfer dynamics of 1-aminoanthraquinone. Upon excitation at 480 nm to the S1 state, stimulated emission and excited state absorption signals were observed. The optically bright S1 state has the character of intramolecular charge transfer because of the large dipole moment change during excitation.The S1 state relaxes with the twisting of the-NH2 group. This twisting enhances the character of intramolecular charge transfer of the S1 state and forms the twisted intramolecular charge transfer state. The crossing of the energies of the S1 and T2 states suggests an intersystem crossing process between the singlet and triplet states at 40° twisted angle. It takes 5 ps for forming the twisted intramolecular charge transfer state via a barrier at 60° twisted angle on the S1 potential energy surface. The twisted intramolecular charge transfer state goes to the hot So state via ultrafast internal conversion for the small energy gap between the So and S1 states. Based on the computational results, a twisted intramolecular charge transfer mechanism is proposed to describe the dynamics of excited state intramolecular charge transfer.