<p> One process that has the potential of efficiently harvesting solar energy is singlet fission (SF), a process by which one photon of light can produce two excited states. Investigations of three different series of bistetracene (BT) were used to explore the effect of symmetry on the rate and driving force of SF, as well as the electronic coupling. A dimer and the corresponding monomer were used to build a synthetic infrastructure and explore preliminary photophysics. All of the dimers were connected by one to three norbornyl bridges and exhibit various amounts of electronic coupling.</p><p> The first section of this dissertation will discuss a series of cofacial BT dimers with C_2v symmetry that have one (BT1) to three (BT3) norbornyl bridges linking the two tetracene chromophores. Density functional theory calculations of BT1-BT3 were used to explore the SF driving force and the change in through-space versus through-bond contributions to the electronic coupling. The C_2v symmetry was found to be unprofitable to SF, but vibrations accessible to the ground state would break the C_2v symmetry. The thorough synthetic investigation of the monomeric tetracene-norbornyl bridge was developed to build a synthetic library that aided the synthesis of BT1, the first rigid SF dimer. Preliminary photophysics of BT1 and its monomer will also be described.</p><p> In the second portion of this work, the SF driving force, electronic coupling will be calculated for the second and third series of BT dimers, which are symmetry adaptations of BT1-BT3. In the second series, the orbital overlap of the norbornyl bridge and tetracene arms will be exploited by changing how the bridge and arms are connected to make dimers of C₂ and C_s symmetry. In the final series, a heteroatom substitution of BT1 creates a series of C₂ and C_s dimers that can be built using the synthetic infrastructure developed above. Both symmetry adapted series of BT dimers were found to lead to an increase in electronic coupling, which is expected to be productive for SF.</p>

All organic photovoltaics (OPVs) undergo four major processes to convert sunlight in electrical energy. The first process is the absorbance of sunlight. Due to the limit of available acceptor molecules, the burden of light absorbance weighs heavily on the donor material. This thesis focuses heavily on the development of dyes consisting of donor-acceptor dyads and triads for improved light harvesting in OPVs. Squaraine dyes show impressive light harvesting properties with absorbances in the UV to near IR region with extinction coefficients on the order of 105 M-1 cm-1. Unfortunately, improved light harvesting is not enough to insure optimized OPVs. Energy level tuning to increase VOC and insure efficient exciton dissociation is also required. Functionalizing squaraine dyes with electron donating or electron withdrawing groups allow for the systematic tuning of the HOMO energy levels. This tunability allows for the concurrent optimization of bandgap and VOC. Cyanine dyes have been explored for small molecule OPVs due to their impressive absorbance properties. The absorbance of ketocyanine dyes can be tuned by manipulating the strength of the acceptor moiety. Stronger acceptors are better able to stabilize the negative charge in the charge separated state of the dye. This stabilization allows for a greater contribution from the cyanine structure of the dye, thus red shifting the absorbance. Stronger acceptors also increase the communication between the two amine functionalities as demonstrated by cyclic voltammetry. Block copolymers show impressive morphological control through the tuning of the molecular weight of the blocks as well as the compatibility of the functional groups. This allows for the access of morphologies with small, well ordered, and continuous domains thought to be beneficial in the active layer of OPVs. Unfortunately, block copolymers often show inferior light harvesting compared to their conjugated polymer counterparts. Donor-acceptor systems are explored as sensitizers for block copolymer OPVs. Small molecules without twists or bends or acetylene linkers were found to be most effective for lowering the bandgap and aligning the energy levels.

Known methods for production of phenoxyl radicals involve either bimolecular oxidations or photolysis of parent phenols. We have described the synthesis and characterization of three photochemical and thermal unimolecular leaving groups. These leaving groups are: aryloxy oxalyl tert-butylperoxides (AOB's), aryloxy oxalyl chlorides (AOC's), and diaryl oxalates (DAO's), of which the AOB and DAO are the most efficient and stable. The quantum efficiency of a typical AOB system is 0.42, while that of an analogous DAO system is 0.10. A photolysis time of $\sim$3.5 h yields $\sim$35% of radicals using the AOB method. Using these leaving groups, phenoxyl radicals can be obtained in good yield in solution and in the solid state. We have attempted to apply these leaving groups toward production of conjugated, non-Kekule phenoxy-based biradicals. Our results indicate production of a large amount of monoradical, which probably obscures any biradicals produced. In addition to producing radicals, photolysis of diaryl oxalates in the solid state yields up to three types of triplet phenoxy-phenoxy radical pairs, which we have termed the initial, wide, and narrow radical pairs. The initial radical pair has been observed only from photolysis of di(2,6-di-tert-butyl-4-methoxyphenyl) oxalate, and has $\vert$D/hc$\vert$ = 0.0108 cm$\sp{-1}$, with a corresponding interelectronic separation of 6.2A. We have observed the formation of the wide radical pair primarily through the use of short photolysis times; thermal annealing of the initial radical pair also leads to its observation in the case described above. Wide radical pairs are observed from photolysis of di(2,6-di-tert-butyl-4-methoxyphenyl) oxalate, 2,6-di-tert-butylphenyl phenyl oxalate, and di(2,4,6-tri-tert-butylphenyl) oxalate. These radical pairs have $\vert$D/hc$\vert$ = 0.0099 to 0.0124 cm$\sp{-1}$, corresponding to interelectronic separations of 6.0-6.4A. In the absence of oxygen, the wide radical pairs are stable at room temperature for from 2 h to $>$1 week. Narrow radical pairs are formed upon extended photolysis of all DAO's, and exhibit $\vert$D/hc$\vert$ = 0.0034 to 0.0070 cm$\sp{-1}$, corresponding to interelectronic distances of 7.2-9.2A.

In an attempt to ascertain the relationship between the structure and the electronic spin ground state of open-shell species, various model $\pi$-conjugated dinitrene systems of the general structure:N-Ph-X-Ph-N: were synthesized. Variable low temperature electron spin resonance (ESR) spectroscopy was used to determine the ground state multiplicity of the dinitrenes and to determine the degree of exchange coupling between the unpaired electrons on each nitrogen. The experimentally determined ground state spin multiplicity could be predicted using Borden and Davidson's disjointness criterion, even for heterocyclic non-alternant ring systems. Parity-based predictions did not provide such consistent agreement with the experimental results as the latter method. Many similarities between the ESR spectra of the quintet dinitrenes are proposed to arise from different conformers of the same quintet state. Analysis of two conformationally restrained dinitrenes tend to support this view, and imply that geometric considerations are of importance in the design of an organic ferromagnet.

A series of bis(arylnitrene) compounds was generated for studies by ESR spectroscopy at cryogenic temperature. Zfs $\vert$D/$h$c$\vert$ and $\vert$E/$h$c$\vert$ were determined and temperature dependent studies were done on some of the bis(arylnitrene)s. Bis(p-phenylnitrene) compounds were generated, and their ESR spectra showed mononitrene signals and triplet dinitrene signals including $\Delta$m$\sb{\rm s}$ = 2 transitions. Zfs $\vert$D/$hc\vert$ of mononitrenes and triplet dinitrenes decreased as conjugation between phenylnitrenes increased. Zfs $\vert$E/$h$c$\vert$ was estimated to be zero except for 1,4-phenyldinitrene. The temperature dependent studies concluded that all bis(p-phenylnitrene)s have ground state singlets. Triplet zero field splitting observed for bis(p-phenylnitrene)s are due mostly to one-center interaction of $\sigma$ and $\pi$-electrons of the nitrenes. Three biphenyldinitrenes were generated: the 4,4$\sp\prime$-isomer, 3,4$\sp\prime$-isomer, and 3,3$\sp\prime$-isomer. The 4,4$\sp\prime$-isomer showed an ESR spectrum typical of a triplet dinitrene, and a temperature study showed a singlet ground state. The 3,4$\sp\prime$-isomer showed a complex ESR spectrum typical of a quintet state, and a temperature study showed a ground state quintet. The 3,3$\sp\prime$-isomer showed only a mononitrene signal in the ESR spectrum, so it is presumed to be a disjoint compound with almost no interaction between nitrene sites. Three phenylether dinitrene compounds were generated for comparison with the biphenyl compounds. The 4,4$\sp\prime$-isomer had only a mononitrene ESR signal, so a ground state singlet is assumed due to a superexchange mechanism between the paired electrons of oxygen and the $\pi$-electrons of the nitrenes. The 3,4$\sp\prime$-isomer showed very weak quintet signals, so a ground state quintet was assumed. The 3,3$\sp\prime$-isomer showed a very weak quintet signal, and a temperature study showed this to be an excited quintet state. Bis(4$\sp\prime$-nitrenophenyl)-1,3-butadiyne was generated for a study of the effect of acetylene as a coupling spacer. Its ESR spectra showed a strong mononitrene signal only and very weak signals presumed to be a quintet state. The spectrum suggests that this acetylene is a twisted structure where any coupling between nitrenes was changed from linear behavior.

A study of the adsorption of block copolymers with respect to such variables as adsorption rate, concentration, molecular weight, relative block sizes, architecture, adsorption solvent, and temperature was conducted for the adsorption block copolymers to silanol surfaces. Poly(styrene-b-isoprene) and poly(styrene-b-1,2-butadiene) with precise molecular weight and narrow polydispersity were synthesized by anionic polymerization techniques. These block copolymers were then specifically modified to incorporate an organic moiety (sticky foot) which would promote adsorption. Hydrosilylation to incorporate trimethoxysilanes into the diene block was unsuccessful due to low reaction yields and crosslinking. Hydroboration/oxidation to incorporate alcohols into the diene block was used to prepare polymers for adsorption studies due to the high reaction yields with no crosslinking. Adsorption studies examining the effects of molecular weight, number of adsorbing segments, time, concentration, polymer architecture, and adsorption solvent were conducted for the adsorption to aerosil 130. Adsorptions were analyzed by UV spectroscopy and thermal gravimetric analysis. Adsorption studies examining the effects of molecular weight, relative block sizes, and temperature were conducted for the adsorption to glass slides. Adsorptions were analyzed by water contact angle and X-ray photoelectron spectroscopy. The competitive polymer adsorptions between two different polymers adsorbing to aerosil 130 were studied. Simultaneous adsorptions for two polymers with respect to concentration were examined for three separate sets of polymers. The polymers were adsorbed to aerosil 130 and analyzed by gel permeation chromatography. Sequential competitive adsorptions for these same polymer sets were also conducted using the same substrate and analysis technique. The effect of surface affinity on polymer adsorption was examined by adsorbing one polymer sample to a series of different surfaces. Poly(chlorotrifluoroethylene) polymers with carboxylic acid, alcohol, amine, and ethyl ester surfaces as well as glass slide and aminated glass slide surfaces were prepared and used for adsorptions. These adsorptions were analyzed by X-ray photoelectron spectroscopy.

Single molecule studies on CdSe quantum dots functionalized with oligo-phenylene vinylene ligands (CdSe-OPV) provide evidence of strong electronic communication that facilitate charge and energy transport between the OPV ligands and the CdSe quantum dot core. This electronic interaction greatly modify, the photoluminescence properties of both bulk and single CdSe-OPV nanostructure thin film samples. Size-correlated wide-field fluorescence imaging show that blinking suppression in single CdSe-OPV is linked to the degree of OPV coverage (inferred from AFM height scans) on the quantum dot surface. The effect of the complex electronic environment presented by photoexcited OPV ligands on the excited state property of CdSe-OPV is measured with single photon counting and photon-pair correlation spectroscopy techniques. Time-tagged-time-resolved (TTTR) single photon counting measurements from individual CdSe-OPV nanostructures, show excited state lifetimes an order of magnitude shorter relative to conventional ZnS/CdSe quantum dots. Second-order intensity correlation measurements g(2)(τ) from individual CdSe-OPV nanostructures point to a weak multi-excitonic character with a strong wavelength dependent modulation depth. By tuning in and out of the absorption of the OPV ligands we observe changes in modulation depth from g(2) (0) ≈ 0.2 to 0.05 under 405 and 514 nm excitation respectively. Defocused images and polarization anisotropy measurements also reveal a well-defined linear dipole emission pattern in single CdSe-OPV nanostructures. These results provide new insights into to the mechanism behind the electronic interactions in composite quantum dot/conjugated organic composite systems at the single molecule level. The observed intensity flickering , blinking suppression and associated lifetime/count rate and antibunching behaviour is well explained by a Stark interaction model. Charge transfer from photo-excitation of the OPV ligands to the surface of the CdSe quantum dot core, mixes electron/holes states and lifts the degeneracy in the band edge bright exciton state, which induces a well define linear dipole behaviour in single CdSe-OPV nanostructures. The shift in the electron energies also affects Auger assisted hole trapping rates, suppress access to dark states and reduce the excited state lifetime.