Nonlinear optical studies of molecular orientations and conformations

The orientation and conformation of molecules are of great interest in the fields of chemistry and industry, and also in our everyday lives. They impact several properties of molecules and often define whether they are suitable for a given application or not. In this thesis, we studied the effects of the molecular conformation by coherent second-harmonic generation (SHG) and the orientation of molecules on surfaces and in solution by coherent and incoherent SHG (or hyper-Rayleigh scattering). Second-order nonlinear optical methods were chosen because of their inherent sensitivity to symmetry.
Chapters 2 and 3 are devoted to the effects of the molecular conformation in solution. Both chapters deal with the conformation of poly(thiophene)s, that was investigated with hyper-Rayleigh scattering (HRS). This technique was used to determine the first hyperpolarizability of the polymers, a parameter that yields information on molecular structure and conformation.
In chapter 2, HRS was used to probe the effect of an increasing chain length, of a series of regioregular poly(3-hexylthiophene), on their nonlinear optical scattering potential. We showed that, despite their atypical molecular structure (they lack the typical acceptor-conjugated bridge-donor structure that is usually required for SHG), the poly(thiophene)s were able to generate significant HRS. In addition, we showed the first hyperpolarizability is dominated by electronic effects, and scales with the conjugation length.
.In chapter 3, inspired by the results from the fully regioregular poly(thiophene) investigated in chapter 2, the effects of adding
defects in the conjugation path was studied. For this purpose a series of poly((S)-3,7- dimethyloctylthiophene)s with variable amounts of regioregularity was used. The dimethyloctyl side-groups, bulkier than the formerly used hexyl side-groups, were used to efficiently induce conjugation breakage in the conjugation path. It was expected, and confirmed, that adding these conjugation breakages diminished the first hyperpolarizability values.

In chapter 4, we used coherent second-order generation, to probe the conformational change of a surface-tethered polymer in-situ during polymerization. Surface-tethered polymers have a wide range of applications that exploit changes in molecular conformation, but appropriate characterization of these samples is generally difficult. Using a polymer modified with chromophore side-groups, we probed the changes in side-group orientation, in-situ as well as ex-situ during polymerization, and we were able to identify conformational transitions during polymerization.
In chapter 5, HRS was used to probe the effects of ultrasound irradiation on a solution of a dipolar chromophore. Initially this research was aimed at studying ultrasonically induced birefringence, caused by the molecular orientation, using a nonlinear optical technique. Although we were unable to use HRS for this purpose, ultrasound irradiation was found to cause a dramatic signal intensity increase of multiphoton fluorescence as well as HRS. Therefore, the results could be of interest in ultrasound-modulated techniques currently under investigation for applications in medical research, which are hampered by low modulation-efficiencies.