Abstract
In this thesis a number of compounds are examined in the context of their intended purposes as electron donating polymers for polymer organic photovoltaics (OPV) (Chapters 3 and 4) or as an organic nonlinear optical (NLO) chromophore (Chapter 5). Both of these emerging fields use solution-processable organic compounds to fulfill roles that have traditionally been filled by crystalline inorganics.
For the OPV polymer portion of this thesis, the study of several different donor-acceptor copolymers is outlined. Recently the use of donor-acceptor copolymers has rapidly accelerated gains in efficiency in polymer OPV cells. Compared to homopolymers, however, relatively little is known about the excited states that help govern their performance in OPV devices.
The first group of copolymers studied included poly[N-1-octylnonyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT), poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2’,1’,3’-benzothiadiazole)] (PCBT) and poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2’,1’,3’-benzoselenadiazole)] (PCBSe). These copolymers are composed of alternating units of carbazole, which acts as an electron donor, and benzothiadiazole, benzoselenadiazole or dithiophene benzothiadiazole, which act as electron acceptors. The use of resonance Raman (RR) spectroscopy shows experimental evidence for the charge transfer nature of the lowest energy transition of each of these polymers. Previously, the description of the excited state orbital distribution has only been achieved computationally. Calculation of the equilibrium excited structure of the monomer unit of PCDTBT is also consistent with the charge transfer description, showing increased geometric distortion of the acceptor unit relative to the donor unit. The calculation of oligomer units of up to 11 nm sets this study apart from many studies of shorter oligomers and it is found that significant amplitude of the excited state orbitals can extend as much as 10 nm, even in a narrow π* band. This delocalization was also found to be dependent on the DFT method that is selected.
The next copolymer studied, poly5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo [c]-1,2,5-thiadiazole (PTBT), is made up of alternating units of thiophene and benzothiadiazole. Electronic absorption spectra of this copolymer show vibronic character and asymmetry which has been cited as evidence for charge delocalization in conjugated polymers. The vibronic character in solution phase electronic absorption spectra was shown to be the result of a temperature dependent and concentration independent equilibrium between an ordered and disordered phase. The type of structure in the ordered phase was studied using temperature dependent UV-Vis and emission, DFT and electron microscopy. Resonance Raman spectra of PTBT do not show the same enhancement of the acceptor based modes that was noted for the carbazole based polymers. This is surprising as the absorption energy is relatively low, which is expected to be the result of a charge transfer. DFT calculations correlate well with RR data and show minimal orbital localization in the excitedstate. DFT calculations also show more extensive delocalization in PTBT than in the carbazole-based polymers as evidenced by extended molecular orbitals and a broad π* bandwidth.
The calculation of second-order nonlinear optical and geometric properties of highly solvatochromic compounds is a challenge for computational methods. In light of this, we have used spectroscopic methods in various solvents and DFT calculations in various strength solvent fields to extensively study the structure and electronics of a merocyanine dye, 2-(3-cyano-5,5- dimethyl-4-(3-(1-octadecylpyridin-4(1H)-ylidene)prop-1-enyl)furan-2(5H) -ylidene)malononitrile (pyr3pi). Experiments and calculations in several solvents show that solvent field computations significantly underestimate the dielectric effects of solvent. This is also reflected in hyperpolarizability for which experiments show a maximum in low polarity solvents while the calculated hyperpolarizability is maximized in high polarity solvent fields. A method to alleviate the underestimation of solvent effects by calibration with Raman data is suggested.
Finally, it is known that intermolecular interactions are important in both polymer OPVs and organic NLO materials. With the aim of investigating intermolecular interactions we developed two separate Raman spectrometers to measure vibrational bands with low Raman shift. When using a 785 nm excitation to a collect Raman spectra on the compounds mentioned previously it was found that fluorescence dominated the signal. However, using another OPV polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), it was shown that low-frequency Raman could be an effective tool to track the effects of annealing in a conjugated polymer.