Influence Of Molecular Vibration And Solvent Environment On Optical Properties Of Molecular Materials: A Theoretical Study

Since the first observation of second-harmonic phenomenon in quartz induced by a rubylaser in 1961 by Franken et al, nonlinear optics has quickly developed as a very importantbranch of modern optics. Recently, nonlinear optical technology has demonstrated a variety ofapplications in many different fields, such as, modern laser technology, opticalcommunication, data storage, optical information processing and dynamical therapy etc.. Oneof the main tasks in the field is to find new nonlinear optical materials with even higherefficiency. Molecular materials have attracted great attention since they can be designed andmodified at molecular level to achieve the best nonlinear optical response coefficient andother photoelectric properties. The rapid development of modern quantum chemical theory atab-initio level has also led to the rapid development of theoretical study on nonlinear opticalproperties of molecular materials. Theoretical studies not only can forecast the nonlinearoptical properties of the already existed molecular materials, but also can easily design manynew molecular materials with exceptional properties and provide guide lines for experimentalsyntheses.It is well-known that the movement of a molecule consists of movement of its electronsas well as its nuclei. When molecular materials are exposed to a laser electromagnetic field,molecular vibrations can have strong effects on their nonlinear optical properties. Thus, it isvery important to include vibration effects when evaluate the nonlinear optical properties ofmolecular materials theoretically.Most of the experimental measurements on nonlinear optical properties of molecularmaterials are carried out in liquid phase or in solution. However, theoretical studies areusually performed on a single molecule in the gas phase where weak interaction betweenmolecules occurs. In order to compare the theoretical results with the experimental ones, oneneeds to include the effect of the solvent environment on the nonlinear optical properties inthe theoretical model.The work in this thesis is based on the ab-initio quantum chemical theories. Theinfluence of molecular vibration and solvent environment on optical properties of molecularmaterials has been study theoretically with the use of different theoretical models andapproximations. All quantum chemical calculations on electronic properties are performedwith Gaussian and Dalton program packages. While the vibrational or vibronic properties ofmolecular materials are calculated with my own computational codes. All derivatives ofelectronic properties on vibrational normal coordinates are calculated numerically. This thesiscontains mainly two parts: the study of the influence of molecular vibration and solventenvironment on linear polarizability and nonlinear hyperpolarizabilities of molecularmaterials, and the study of the influence of vibronic coupling and solvent environment onone-photon absorption profiles of molecular materials. The main results obtained in this thesisare as follows.I. Solvent effects on nonlinear optical properties of para-nitroaniline.First, we take para-nitroaniline as a model system for one-dimension push-pullconjugated molecules to test the validity of two-state model at density functional theory level.Our results agree very well with that from response theory at the same computational level,which shows that with two-state model one can get reliable results for the first order nonlinearhyperpolarizability of similar conjugated molecules and can correctly describe the dispersionrelation of the first order nonlinear hyperpolarizability.Then, we use two hybrid density functionals (B3LYP and B3PW91) and three solventmodels (polarized continuum model, super-molecular model and semi-continuum model) tostudy the solvent effects on nonlinear optical properties of para-nitroaniline. We take threepolarized solvents: acetone, methanol and acetonitrile as examples. Two hybrid densityfunctionals give similar results. The results show that both the long-distance interaction (suchas interaction due to the polarity of solvent) and the short-distance interaction (such ashydrogen-bonding between solute and solvent molecules) are important to the nonlinearoptical properties of para-nitroaniline. The results from the semi-continuum model designedin this work agree very well with the experimental results, which show that semi-continuummodel is an excellent solvent model.II. Vibrational contributions to nonlinear optical properties of methanol, ethanol andpropanolStatic and dynamic vibrational contributions to linear polarizability and the first andsecond nonlinear hyperpolarizabilities of methanol, ethanol and propanol have beencalculated. Both pure vibrational contribution and zero-point vibrational averagingcontribution have been determined. The pure vibrational contribution is quite important atstatic limit, while in optical region it varies for different nonlinear optical processes. Thezero-point vibrational averaging contribution is a quite large correction to the electronicnonlinear optical (NLO) properties, especially for the second hyperpolarizability at finitefundamental frequencies. Vibrational analysis shows that the swinging modes at around 300cm-1 and the C-H stretching modes at around 3160 cm-1 often give large contributions.III. Solvent effects on vibronic one-photon absorption profile of dioxaborineheterocycles.We have calculated the vibronic profiles of one-photon absorption spectra of dioxaborineheterocycles (molecular A) in dichloromethane solution using different methods. BothFranck-Condon and Herzberg-Teller contributions to the spectra are analyzed. The polarizedcontinuum model has been applied to simulate the solvent effect. It is found that severaltypical DFT methods fail to provide the correct description for the highest occupied molecularorbital of molecule A in the gas phase. With the inclusion of solvent effect, B3LYP methodreproduces the spectral profile of experimental one-photon absorption spectrum for moleculeA in dichloromethane solution. The vibronic profile of one-photon absorption is dominated byFranck-Condon contribution. The maximum of Herzberg-Teller profile of the first excitedstate shows an interesting blueshift with respect to that of Franck-Condon profile, whichagrees well with the measured energy difference between two-and one-photon absorptions ofthe first excited state. The vibrational analysis reveals that the C–C stretching modescontribute the most to the vibronic profile.IV. Density functional theory study on vibronic structure of S0→S1 absorption spectrumin free-base porphinThe vibronic structure in S0→S1 absorption spectrum of free-base porphin is investigatedby hybrid density functional theory and linear coupling model. Both Franck–Condoncontribution and Herzberg–Teller contribution are calculated and studied. In our study offree-base porphin the Herzberg–Teller contributions are much more important than that fromFranck–Condon. The calculated absorption spectrum indicates a very good agreement withthe experimental high-resolution counterparts over the entire energy range of the S0→S1transition (540-660 nm). This corresponds to the so-called Q xsystem. In the framework oflinear coupling model the absorption and emission spectra should have mirror symmetryrelationship, which is confirmed to a great extent by selectively recorded fluorescence andabsorption spectra from experiment. From our calculations it follows that the maximum ofthe 0-1 band in absorption at 581 nm is connected with the fundamentals ω 94 (ag)=1654.8cm-1 and ω 92 ( b1g)=1640.7 cm-1. The red wing of the intense 0-1 band in the low resolutionabsorption spectrum consists of a large number of less intense fundamentals of a g and b1 gsymmetry in the range 1500—700 cm-1.This thesis contains nine chapters in total. The first chapter gives a brief introduction oflinear and nonlinear optics, the definition of molecular materials and their advantages innonlinear optics. The importance of considering influence of molecular vibration and solventenvironment in theoretical modelling is also stressed. The second chapter introduces the basicab-initio quantum chemical methods, which mainly focuses on Hartree-Fock approximationand density functional theory. In the third chapter, some commonly used methods to calculatelinear polarizability and nonlinear hyperpolarizabilities are introduced. A method to calculatevibrational contributions of linear polarizability and nonlinear hyperpolarizabilities is alsobriefly described. In the fourth chapter, the solvent effects on nonlinear optical properties ofpara-nitroaniline are studied by choosing three solvent models. In chapter five, vibrationalcontributions to nonlinear optical properties of methanol, ethanol and propanol are calculatedand analyzed. Chapter six describes a method to calculate vibronic absorption profiles usingthe so-called linear-coupling model. In chapter seven, vibronic one-photon absorption profileof a newly synthesized molecular material in dichloromethane solvent is calculated withseveral methods. In chapter eight, vibronic structure in S0→S1 absorption spectrum offree-base porphin is investigated using hybrid density functional theory. The last chaptersummaries the work in this thesis and states what can be done in future.