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In solution, the environment around the solute is determined solely by the solvent molecules, which are present closer to the solute. This interaction between solute and solvent shell is very crucial for equilibrium structure and reactivity of the solute. In the thesis, first we have investigated control of solvent polarity on the excited structure of 2,2,2 trifluroacetophenone and later effect of electronic excitation on the solvent shell organization has been described. It has been reported in literature that the lowest energy triplet configuration of 2,2,2 trifluroacetophenone corresponds to n,π* state. There are some other reports in favor of the probable existence of 2,2,2 trifluroacetophenone in n,π* lowest triplet state. Thus, transient absorption and time resolved resonance Raman spectroscopic methods have been used along with theoretical calculations to investigate the discrepancy in the assignment of the lowest triplet state configuration It has been observed that the lowest triplet state of 2,2,2 trifluroacetophenone is indeed nπ* and there is a solvent polarity induced change in triplet state energy ordering and structure changes. The absorption spectrum of hydrated electron is broad and structureless. So, it was though that the broadening feature could be attributed to homogeneous and inhomogeneous broadening. Transient resonance Raman spectrum of the water bending mode in presence of hydrated electron has been recorded at different excitation wavelengths. Interestingly, it has been observed that, peak position of water bending mode in presence of hydrated electron alters with change of excitation wavelength. A model has been proposed based on the experimental data.

It has long been recognized that course and efficiency of a chemical reaction is largely mediated by the short-lived transient species (excited state or radicals) which are formed as reactive intermediates during a chemical reaction. Subtle changes not only in the bonding and electronic distributions but also in the conformations and geometries of these intermediates have a dramatic influence on the reactivity. A detailed understanding of the structural and dynamical aspects of electronic excited states is therefore essential towards unraveling photoinduced natural processes and for designing novel photonic materials. Time-resolved techniques have been widely used to study the transient species (or intermediates) formed during photochemical and photophysical reactions for better understanding of the reaction mechanism and dynamics. Time-resolved absorption spectroscopy is a promising tool to study the temporal dynamics and the kinetics of photophysical processes. But the absorption spectra of species in solution usually consist of broad spectral band revealing little or no information about the structure of the transient species under investigation. Time-resolved resonance Raman (TR3) spectroscopy, on the other hand, is a potential sensitive modality, which not only allows one to study the dynamics but also provides the vibrational structure of the transient species of interest in microsecond to picosecond time scale. Moreover, by choosing the wavelength of excitation one can selectively probe the particular transient species from a complex molecular system especially a biological molecule. Thioxanthone (TX) is well known for its dramatic solvatochromic behavior and has drawn enormous attention in the recent years. The objective of present thesis has been to understand the solvent-induced structural changes on the lowest excited triplet state of TX and its derivatives. We have primarily employed nanosecond TR3 spectroscopy, a pump-probe technique, to investigate structure of the lowest excited triplet state. Transient absorption experiments have also been carried out to study the excited electronic states. In order to substantiate our experimental findings and also to get more insight into the triplet-state structure, we have performed density functional theory (DFT) calculations. The polarizable continuum solvation model has been employed to account for the solvent effect into the computation. Time dependent (TD) -DFT calculations have also been performed to get the energy and the structure of the excited states. The present thesis has been divided into eight chapters. Chapter 1 gives brief literature review on photochemistry and photophysics of TX and the introduction to the TR3 technique. In this chapter we have briefly introduced key concepts which form the basis of the thesis. Chapter 2 covers the experimental and theoretical methodologies used in the present thesis work. The major components of the TR3 spectrometer as well as the important technical aspect of the TR3 technique have been discussed in detail. In the section of the theoretical method, basic concepts of the computational method, density functional theory and key concepts related to the solvation are briefly discussed. Chapter 3 focuses on a systematic vibrational study of the ground and lowest triplet states of TX. TR3 experiments have been carried out and the observed vibrational frequencies have been assigned. It has been observed that electronic excitation distorts the molecule, enabling the increased electron delocalization in the central ring keeping the ground state symmetry intact. The largest structural reorganization is observed in the central ring of TX, consisting of an oxygen atom. Normal mode analyses show that the normal mode composition is significantly influenced by the electronic excitation. The C=C stretching and C=O stretching modes are coupled to a greater extent in the triplet state as compared to the ground state. In the ground state, the two high-frequency modes can be assigned almost exclusively to the C=O stretching and C=C stretching, whereas in the triplet state, both of these coordinates have comparable contributions to the two totally symmetric modes. Chapter 4 deals with a very unique observation of simultaneous detection of two triplets. This is the first time when two triplet states have been simultaneously deleted using TR3 experiments. We have performed TR3 experiments in wide variety of solvents differing in their polarities and hydrogen atom donor abilities. The transient Raman signal has been observed from both n - π∗ and π - π∗ triplet states simultaneously. The population ratio of the two triplet states has been found to be dependent on the solvent polarity. Additionally, the excitation wavelength study has revealed that the relative ratios of the transient Raman peaks (assigned to two different triplet states) change with the excitation wavelength. Our claim of simultaneous detection of two triplets has been reconfirmed by triplet quenching experiments carried out at different temperature. It has also been observed that the CO bond length is very sensitive to the solvent polarity and specific interactions play an important role in determining the structure of lowest triplet-state. In Chapter 5, we focus on the understanding of the effect of chlorine substitution on the lowest excited triplet state of TX. TR3 spectroscopy has been used as an experimental tool to study the vibrational structure of 2-chlorothioxanthone (CTX). TR3 results indicate the coexistence of two lowest triplet states in the thermal equilibrium akin to the parent compound. The above observation has been further substantiated by probe wavelength dependent study. The configuration of the T1 state has been assigned to π – π∗, whereas the T2 state has been ascribed as n - π∗. The population ratio of 3n - π∗ to 3 π - π ∗ triplet states has been found to be more for CTX as compared to TX which has been substantiated by the flash photolysis experiments. Chapter 6 highlights the influence of solvent effect on lowest triplet state structure of CTX. Transient absorption spectroscopy has been employed to understand the triplet state electronic structure; whereas solvent induced changes in the structure of the lowest triplet state have been studied using TR3 spectroscopy. Time-resolved absorption measurements show that solvent polarity has dramatic dependence on the wavelength of T1 - Tn absorption maximum. A good correlation between the wavelength of T1 - Tn absorption maximum and ET(30) value of the solvent is observed. TR3 experiments carried out in solvents of varying polarities indicate that the contribution of n - π∗ character to the lowest excited triplet state increases with the increase in the solvent polarity. Both transient absorption and TR3 studies reveal that specific solvent effect is more pronounced in comparison to the nonspecific solvent effect. Chapter 7 of the thesis deals with the study on the triplet state structure and solvent effect on 2-trifluoromethyl Thioxanthone. Flash photolysis in tandem with TR3 spectroscopy has been employed to understand both the electronic and the vibrational structures of this pharmaceutically important thioxanthone derivative. Experiments have also been carried out in solvents of varying polarities to study solvent-induced changes in the triplet-state electronic spectra. We have observed the coexistence of two lowest triplet states alike the parent compound. The T1 state has been assigned to π - π∗ state, whereas n - π∗ configuration has been attributed to the T2 state. The wavelength of triplet-triplet absorption maximum of the lowest triplet state has been found to be sensitive to the solvent polarity and good correlation has been observed with the ET(30) value. The transient Raman results indicate that the CF3 substitution leads to increase in the population ratio of n - π∗ and π - π ∗ triplet states. Finally, Chapter 8 contains overall summary of the thesis and future directions of the present investigation.

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Benzophenone (BP) and substituted BPs constitute a major class of aromatic ketones and are of potential interest in various areas of excited state solution phase photochemistry and photobiology. High triplet state energy, faster rate of intersystem crossing (ISC) and higher triplet state quantum yield enables BP systems as potential photosensitizers via triplet energy transfer mechanism. The short lived triplet state of BP systems are highly reactive and acts as potential electron acceptor and interesting photochemical behavior have been observed for photoinduced electron transfer reactions in various solvent media, in particular for donor-bridgeacceptor (D-B-A) family. Though detailed spectroscopic studies of BP and substituted BP are documented, not much attention are given to its heterocyclic analogue. Substitution of aromatic ring carbon with one or more heteroatom (N and S) results in drastical change in photochemical properties and excited state reactivity. In solution phase and in nanosecond time domain heteroaromatic ketones form the triplet excited state that upon subsequent photoreactions, leads to formation of short lived species viz. radicals, ions and radical ions. Therefore exploring the trends in excited state reactivity with the variation with functional group and ring substitution and solvent medium is of considerable interest. The complete reaction mechanism of a photoreaction can be understood by studying reactivity of various short lived intermediates formed. In solution phase, the reactivity of a certain species or rate of a chemical reaction can be well understood by correlating to its structure. This approach requires accurate reproducible techniques for the excited state structural determination. Wide range of time resolved (TR) spectroscopies spanning over whole electromagnetic spectrum have been developed over decades and successfully applied to study excited state phenomena. In a typical two beam experiment, the pump pulse excites the molecular system to higher electronic state and the probe pulse records the spectrum of intermediate species at variable delay time with respect to the pump. The data from different TR techniques used to be complementary in nature and the combination helps in a deeper understanding of excited state reaction mechanism. Though time resolved absorption (TRA) is the most popular and oldest technique to study the excited state photoreactions, no structural information and the poor spectral resolution of the broad and overlapping absorption bands are the limitations towards predicting the reactive intermediates with accuracy. However time resolved resonance Raman (TR3) spectroscopy is a very sensitive technique to obtain vibrational structural information of short lived intermediates. The position and intensity of highly resolved Raman bands provide information about the structural and kinetics parameters respectively. From a set of Raman spectra along various delay time, structure of multiple intermediates evolved for parallel photoreactions can be predicted accurately. We have employed TRA, TR3 and density functional theoretical (DFT) calculation to address few fundamental questions about effect of solvent and ring substitution on the excited state structure and energetics of heterocyclic ketones, hence the reactivity. Comparing the experimental findings with the theoretical output not only makes the data more accurate but also several additional conclusions can be drawn that could not be performed only with the experimental modality. In chapter 1 of the thesis, we have presented a general summary of photophysical phenomena and measured properties and parameters of heterocyclic ketones. Typical photoreactions involving various related aromatic ketones obtained from literature are discussed. This is followed by a brief account of theory of resonance Raman spectroscopy and density functional theoretical calculation. The objectives of the present investigation are highlighted. The detailed assembly of experimental techniques employed for present investigation is discussed in chapter 2. The lasers, spectrometers, collection optics, detection systems and data collection and analysis procedures are briefly illustrated for individual set up. The theory of methods of DFT calculations is also discussed. The effect of substitution of N atom in the aromatic rings on excited state structure and reactivity (hydrogen abstraction reaction) for isomeric (2, 3, 4) benzoylpyridines (BzPy) in various solvents is studied using the above experimental and theoretical methodologies and is presented in Chapter 3. In neutral solvents viz. acetonitrile and carbon tetrachloride the photogenerated lowest triplet state (T1) is observed to be formed that follow monoexponetial decay. In the presence of hydrogen donating solvents like methanol and isopropanol the triplet state is found to undergo hydrogen abstraction reaction to form a ketyl radical and solvent radical. The lifetime and absorption and Raman features of triplet state and ketyl radicals are entirely different from each other and lack any overlapping characteristics. The observed enhanced reactivity of BzPy in comparison to BP is believed to be because of the introduction of the N hetero atom in one of the phenyl ring. From the theoretical data, it was clear that more planarity is attained in case of BzPy as compared to BP and contributes to the enhanced reactivity. The spin density calculation shows that one third of the spin is localized in the phenyl ring in case of BP. The total spin density on Phenyl ring is 0.62 and on carbonyl group is 1.45. In case of BzPy the spin density on phenyl ring is 0.45 and on carbonyl group is 1.59. This indicates that in the excited state the spin is localized more on the carbonyl group. Also from charge density calculation using DFT it is clear that in the triplet state of BzPy the oxygen atom of C=O group is more positive than in case of BP which makes it more electrophilic. Among the three isomeric BzPy the trend in charge density is dependent on the position of nitrogen and found to be in the order of 2-BzPy>3-BzPy>4-BzPy. This can be explained on the basis of -I and –M effect of N atom and the extent depends on its position. So the trend for case of photoreduction follows the order 2-BzPy>3-BzPy>4-BzPy. The hydrogen abstraction reaction used to be considerably fast that produces a substrate ketyl radical and solvent radical (donor radical). These radicals further can dimerise to form various photoproducts viz. Pinacols or can form a stable complex between them. The fate of the radicals formed as a result of hydrogen abstraction of 4-BzPy and the accurate characterization of the adduct is explained in Chapter 4. In the present case the cross coupling reaction of the radicals is observed at longer delay time to form a light absorbing transient (LAT) which is the dominant pathway over other parallel reactions. The exact position of the donor radical in the complex is predicted by correlating the experimental Raman bands and theoretically obtained structural parameters and vibrational frequency. The adduct formed as a result of cross coupling reaction was identified as p-LAT, 2-[4-(hydroxylpyridylmethylene)cyclohexa-2,5dienyl]propan-2-ol. In case of benzoylthiophenes (BzTh), the effect of substitution of S atom on the excited state structure and reactivity towards various hydrogen donors viz. phenol and indole in different solvents are presented in Chapter 5. The difference in rate and mechanism of photoreaction for both the hydrogen donors are compared. For TPK the T1 state is of ππ* character and the T2 state is of nπ* character as is confirmed by flash photolysis and low temperature phosphorescence spectra in EPA matrix. The CO bond length for the triplet state species is more than that of ground state. In case of the ππ* triplet prominent structural changes in thienyl ring are observed and the phenyl ring remains much unaltered. The reaction of the triplet state species with phenol in two different solvents shows a relatively faster rate of reaction. If only ππ* triplet has been taking part in reaction, it might have resulted in slow reaction rate. Because the reaction rate is fairly high, It is concluded that not only ππ* triplet is involved in reaction but there is a contribution from the little higher energy T2 state having nπ* character. The reactivity trends towards hydrogen transfer reaction for three isomeric dithienyl ketones with respect to the position of heteroatoms in the ring are presented in Chapter 6. Energetically close lying (ππ* and nπ*) triplet states are observed to undergo state switching with the change in position of heteroatom in the ring and thus define the characteristics of the triplet state and plays important role in predicting the reactivity trend. Brief summary of the present investigation along with important possible extensions of the present work in described in Chapter 7.

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Das Ziel der vorliegenden Arbeit ist die bessere Verarbeitung von Polyanilin (PANI), da dies bisher ein großer Nachteil unter leitfähigen Polymeren war. Dazu wird ein sperriges Tensid und Dotand, Dodecylbenzensulfonsäure (DBSA) verwendet. Zur Synthese der PANI kommen zwei verschiedene Methoden zur Anwendung, die in dieser Dissertation in zwei Kapiteln beschrieben werden. Im ersten Teil wurden in einem kleinen Reaktionsvolumen (250 mL) PANI-DBSA-Suspensionen synthetisiert, wobei mit einem binären Gemisch aus 2-Propanol und Wasser als Lösungsmittel gearbeitet wird um die Löslichkeit zu unterstützen. Die micellenunterstützte Synthese produziert grüne Dispersionen, welche nach länger als einem Jahr noch keine sichtbare Ausscheidung zeigen. Eine detaillierte spectroelektrochemische Untersuchung der PANI-DBSA-Nanokolloide wurde durchgeführt und gibt eine bessere Erklärung der Charge-Transfer-Prozesse zwischen PANI-Kolloiden und Elektrodenoberfläche. In einem alkalischen Medium ist das UV-Vis-Spektrum von der Beweglichkeit der Anionen und von einem elektrokinetischen Phänomen abhängig. Um den „metal-to-insulator”-Übergang zwischen PANI-Kolloiden, welcher durch pH-Wert-Änderung des Mediums geschehen kann, zu zeigen, wurden UV-Vis- und pre-resonanz-Raman-Spektroskopie verwendet. Im zweiten Teil der Dissertation wird zur Polymerisation von Anilin sowie seinen Copolymeren mit o-Toluidin eine neue Technik der Polymerisation beschrieben, welche durch inverse Emulsion erfolgt. Diese benutzt Benzoylperoxid, ein ungewöhnlicheres organisches Oxidationsmittel. Die erhaltenen PANI sind in gebräuchlichen organischen Lösungsmitteln, wie in Chloroform, vollständig löslich. Mit einer klar-transparenten, grünen Lösung von PANI können metallische Oberflächen oder Glas leicht tropfenbeschichtet werden. Zyklische Voltammetrie und spektroelektrochemische Verfahren kamen zum Einsatz, um die Elektroaktivität, das UV-Vis-Verhalten und die „metal-to-insulator”-Übergänge der chemisch synthetisierten PANI als Funktion des verwendeten Elektrodenpotentials zu untersuchen. Die elektrische Leitfähigkeit der Materialien ist relativ hoch (R = 10 ). SEM-Untersuchungen zeigen, dass die Menge des zugesetzten DBSA die Morphologie des Polymers stark beeinflusst. Aus in situ UV-Vis-spektroskopischen Messungen lässt sich eine gute elektrochromische Reversibilität des Polymers erkennen. DBSA kann Poly(o-toluidin) (POT) effektiv dotieren, auch wenn von der Methylgruppe eine sterische Hinderung ausgeht. Die spektroskopischen Untersuchungen, wie UV-Vis, FT-IR, Raman-Spektroskopie und zyklische Voltammetrie, zeigen deutlich, dass wirkliche Copolymere gebildet werden und die Möglichkeit von Kompositen nicht in Betracht kommt. Das entstandene Poly(anilin-co-o-toluidin) (PAT) ist in schwach polaren Lösungsmitteln wie Chloroform löslich. Wie erwartet, sind die elektrischen Leitfähigkeiten der Copolymere viel kleiner als die Leitfähigkeit von PANI-DBSA.

This thesis deals with the study of microbial pigment violacein in the real sample of lyophilized microorganisms. The sample was investigated by using methods of vibrational spectroscopy with focusing on the applicability of surface enhanced and resonance micro-Raman spectroscopy. For this purpose several different systems for enhancing Raman intensity together with the set of excitation lasers emitting in the visible light region were used. The conclusion of this thesis are the recommendations connected with the appropriateness of using each amplifying systems and excitation wavelengths for the successful identification of violacein pigment in the sample. Powered by TCPDF (www.tcpdf.org)

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