36 CHAPTER EXPERIMENTAL AND CALCULATION METHODS 37 2-1 Measurements 2-1-1 Ultraviolet -visible absorption spectroscopy (UV-vis) UV-Vis spectrometer measures the absorption of light in the visible and “near” ultraviolet region (i.e., in the range of 250-800 nm). As a whole, ultraviolet radiation is absorbed by a chromophore rather than the molecules. When absorption occurs, electronic transition of molecule takes place. It is thus particularly suitable for the study of electronic structure of conjugated polymers, which contain extended πconjugated chain and exhibit unusual color changes. 2-1-2 Photoluminescence spectroscopy (PL) During the process of absorbing ultraviolet or visible electromagnetic radiation, molecules are elevated to an excited electronic state. Some molecules will emit some of this excess energy as light of a wavelength different from that of the absorbed radiation. This process is called fluorescence (photoluminscence), which can be considered as a deexcitation process that occurs after excitation by photons. 2-1-3 Differential scanning calorimetry (DSC) DSC measures the heat flow of a sample with increasing temperature, and is a determinant of exo/endothermic transitions such as glass transition temperature (Tg), crystallization temperature and melting point (m.p.). 38 2-2 Semiempirical molecular orbital calculation Various semiempirical methods could be employed in calculating the desired parameters. In these methods, calculations are greatly simplified by approximating certain integrals with parametric functions or by neglecting some of these integrals. The numerical parameters are usually obtained through fitting calculated properties to known experimental data, such as heats of formation and ionization potentials, of a selected set of compounds. As only a limited set of molecules, for which precisely known experimental data exist, is chosen for parametrization, the semiempirical approach is therefore not expected to give good fits in all instances. Nonetheless, semiempirical methods are widely used because the computational time required for such methods is relatively short. Various semiempirical SCF methods with different parametrization schemes have been developed over the past years. AM1 (Austin Model 1) was introduced in 1985 with the aim of overcoming some common defects in MNDO (Modified Neglect of Diatomic Overlap), such as overestimation of energies for sterically crowded molecules and inadequate description of hydrogen bonding. The MNDO method tends to overestimate the repulsion between atoms that are separated by a distance comparable to the νan der Waals distance. By introducing additional radical Gaussian terms to the core-core repulsion terms, the inadequacies in MNDO could be reduced. 39 Figure 2.1 shows the flow chart for a typical semiempirical geometry optimization process. The Fock matrix is generated from empirical parameters. Read Input Geometry Assign Parameters Calculate Initial Guess of MOs Solve SCF Equations No SCF Achieved ? Yes Calculate Energy Gradients Calculate New Geometry No Optimized ? Yes Calculate Other Properties Requested Print Output Figure 2.1 Flow chart for a typical semiempirical geometry optimization [Dewar, M. J. S.; Zoebisch, E. G.; Healey, E. F.; Steward, J. J. P., J. Am. Chem. Soc., 1985, 107, 3902.] . electronic transition of molecule takes place. It is thus particularly suitable for the study of electronic structure of conjugated polymers, which contain extended π- conjugated chain and exhibit unusual. 36 CHAPTER 2 EXPERIMENTAL AND CALCULATION METHODS 37 2- 1 Measurements 2- 1-1 Ultraviolet -visible absorption spectroscopy (UV-vis) UV-Vis spectrometer measures the absorption of light. the aim of overcoming some common defects in MNDO (Modified Neglect of Diatomic Overlap), such as overestimation of energies for sterically crowded molecules and inadequate description of hydrogen