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The work outline in this dissertation, we report the possible mechanisms for the photogeneration of carbon and nitrogen reactive intermediates by studying their excited state reactivity at room temperature and cryogenic temperatures (4 K – 77 K) in both solution and solid state.The feasibility of high energy photochemical pathways and the verifications of intermediates to validate the reaction mechanisms are estimated using DFT, Gaussian 09 computational platform. In Chapter 02, we look at the α-phenyl substituted isoxazole in cryogenic matrices with the focus of capturing azirines from vinylnitrenes. We demonstrated that conjugation of the α-phenyl group to the vinylnitrene moiety renders it more flexible and lower the rotational barrier than vinylnitrenes with a-methyl substituents, and therefore facilitate intersystem crossing of vinylnitrene to azirine, rather than ketenimine. In Chapter 03, we rationalizes and provides physical insight into vinylnitrene stability at room temperature and low temperature. We theorize that the unique reactivity of triplet vinylnitrene is due to their flexibility which is a reflection of their 1,3-biradical character. Moreover, triplet vinylnitrenes can be stabilized by limiting the flexibility of the built in sensitizer and the vinyl C=C bond at room temperature and low temperature.In Chapter 04, we explore the thermal and photochemical reactions of (CH3)3Ga and O3 to form metal oxides, using a combination of matrix isolation, infrared spectroscopy and theoretical calculations. The potential mechanisms for the formation of intermediates were discussed.In Chapter 05, we report the photochemistry of β γ-vinylarylketone with a built-in triplet sensitizer. This work demonstrated that intramolecular sensitization of a simple alkenes can be used to form triplet 1,2-biradicals with the two radical centers localized on the adjacent carbon atoms, which can potentially be used for cis-trans isomerization of simple alkenes in applications.In Chapter 06, we investigated the thermal and photochemical reactions of (CH3)3Al and O3 using a combination of matrix isolation, infrared spectroscopy and theoretical calculations. The potential mechanisms for the formation of intermediates were discussed in this chapter which has an important impact on semiconducting metal oxide thin film formation.In Chapter 07, we conducted a theoretical study on the carbon centered radical reaction with molecular oxygen to investigate the factors determining the radical reactivity with oxygen. Herein we demonstrate that the calculated transition state barrier energy can be used to predict which radical will not react with oxygen and which will react. Further, the difference in transition state barrier energy can be explained by focusing on the charge distribution in the transition structure. In Chapter 08, we were interested in studying the photoreactivity of azidovinylketones using a built in triplet sensitizer and characterize the intermediates formed upon excitation at cryogenic temperature. In Chapter 09, we report the photoreactivity of a-diazido acetophenone derivatives containing an intramolecular triplet sensitizer. We demonstrated that germinal diazido alkane undergo alpha-cleavage at room temperature and at cryogenic temperature it undergo different reactivity resulting the formation of alkylnitrenes.

In this dissertation, we report the photochemical mechanistic investigations on triplet sensitized and unsensitized organic azides and isoxazoles. Photochemical decomposition of the parent compounds generates carbon and nitrogen centered reactive intermediates leading to remarkable applications. Furthermore, reported synthetically modified photoremovable protecting group (PRPG) system shows significantly high quantum yield and broaden the applicability of the benzophenone based PRPGs. Detection of reactive intermediates were done at room temperature and cryogenic temperatures. Required photon energy for the photochemical processes were provided using various UV light sources depending on the experimental procedures. Femto to nano second transient UV /Vis and resonance Raman has been used in aiding the characterization of the intermediates at room temperature. Detection of cryogenic temperature intermediates were done using Electron spin resonance (ESR), FT-IR matrix isolation, fluorescence, phosphorescence and UV/Vis spectroscopy techniques. Gaussian 09 computations have used to predict favorability of reaction pathways and molecular properties.Our investigation reported in this dissertation outlines three different radical types,1) Triplet 1,4-biradicalsa) Generation of irreversible imine biradical by expelling of a N2(g) by an excited state intermediateb) Applicability of irreversible imine biradical intermediate in high quantum yield applications2) Triplet nitrenesa) Generation Feasibility of triplet aryl nitrene from unsensitized precursor and sensitized precursor at ambient and cryogenic temperatures b) Revelation of factors affecting the lifetime of triplet vinylnitrenes, rigidity and the ring strain approach 3) Singlet alkyl nitrenesHow the immediate molecular environment determines the singlet photoproduct formationIn chapter 02, we explore photoreactivity of methyl benzophenone with azido modification to study the behavior of benzophenone chromophore and azido chromophore at room temperature/cryogenic temperature. In chapter 03, we report the high quantum yield photorelease of azidomethyl benzophenone based Photoremovable protecting group (PRPG). With azido functionalization at 2’methyl position allows formed 1,4 biradical undergo expulsion of N2 to give rise to the irreversible imine biradical that make the photorelease reaction much efficient. Further, the azidomethyl substituted butyrophenone ester which is an acetophenone based counterpart of the benzophenone based PRPG was identified as an unsuccessful PRPG candidate. In chapter 04, we investigated two different photoprecursors that could generate same aryl nitrene upon photolysis. Compound 2-azidobenzphenone which includes inbuilt triplet sensitizer found to be generating triplet aryl nitrene in room and cryogenic temperature conditions. The 3-phenylbenzisoxazole which does not carry a triplet sensitizer found to be generating singlet nitrene at room temperature and at cryogenic temperature we detected the formation of triplet aryl nitrene. In Chapter 05, we considered vinylnitrene stability at room and low temperature using 5,6-dihydro-3-phenyl-4-H-cyclohex[c]isoxazole. Based on the experimental and computational evidences, we theorize that the unique reactivity of undetected triplet vinylnitreneTN is due to flexibility which reflects their 1,3-biradical character. In chapter 06, we rationalized the factors that determining photoproduct formation from singlet alkyl nitrenes based on immediate environment of the nitrene. In our present molecule, we have both ring and migratable phenyl group and photolysis results indicated that at room temperature, migration product is favored over ring expansion product.

Time resolved spectroscopic techniques including Ultrafast Transient Absorption, Laser flash photolysis with UV-visible and infrared detection and modern quantum chemical calculations were used to directly observe and identify reactive intermediates involved in the photolysis of azido and diazo compounds. In this dissertation the photolysis of 8-azidoadenosine and its derivatives were studied in water and in buffer solutions. Using ultrafast spectroscopic techniques it was shown that the corresponding singlet nitrene undergoes a hydrogen shift to form a closed azaquinodimethane faster then it can relax to a triplet nitrene or react with biological target molecules in a photoaffinity labeling experiment. The spectroscopic characteristics of azaquinodimethane were determined using nanosecond laser flash photolysis in the UV-visible and IR regions. The lifetime of this species was estimated to be several minutes in the absence of good nucleophiles, and it is the species which forms adducts in cross-linking and photoaffinity labeling experiments via selective reactions with nucleophiles. Quantum chemical calculations were utilized to identify reactive intermediates produced upon photolysis. Implications of the results of this work on the interpretation of results obtained in photoaffinity labeling and cross-linking experiments are discussed. Ultrafast spectroscopy and nanosecond time resolved Laser Flash Photolysis methods were used to study the photochemistry of 3,5-dichloro 2-biphenyl azide in different solvents. The corresponding singlet nitrene was detected and corresponding spectroscopic characteristics were determined. The photochemistry of diphenyldiazomethane in series of alcohols solvent reveals the fastest known proton transfer reaction to a carbon atom. Studies performed using ultrafast transient absorption spectroscopy revealed the formation and decay of the benzhydryl carbocation. It was shown that benzhydryl carbocation is a short lived intermediate in neat alcohol solvents. Rates of protonation were measured directly and shown to correlate with solvation times in alcohols from methanol to 2-propanol. The rate-limiting step in the overall O-H insertion reaction was determined from a study of the kinetic isotope effect. Transient absorption actinometry measurements were performed on the femtosecond time scale and the contribution of the protonation channel to the O-H insertion reaction was estimated for several alcohols.