Student Project Abstracts 2005 - Pluto - University of Washington

CHARACTERIZATION OF THE MOLECULAR PARAMETERS DETERMINING CHARGE-TRANSPORT IN A SERIES OF SUBSTITUTED OLIGOACENESeter Lee, Yang, Parr (B3LYP) 12 functional. The results of the reorganizationenergy calculations for system (a) are shown in chart 1below. The LanL2dz basis set 13,14 was used during the calculations,unless otherwise specified, because the 6-31G** basis set 15 is notavailable for tellurium, but would be preferred. Therefore, chart 1contains values calculated with the 6-31G** basis set for sulfur andselenium as a comparison with the LanL2dz basis set.The results of the reorganization energy calculations for system(b) are shown in chart 2 below, calculated using the 6-31G**basis set.Chart 2. The Reorganization Energy for System (b)Calculated with B3LYP/6-31G**For Hole-transportChart 1. Reorganization Energy of System (a)Calculated with B3LYP/LanL2dzFor Hole-transportThe results on chart 1 show as a general trend, for system (a),that the reorganization energy, for hole-transport, decreases movingalong the system from the sulfur analog to the tellurium analog.This is attributed to a smaller geometry relaxation for the telluriumanalog than that of the sulfur analog because the bond-lengthchange of the carbon-carbon bonds becomes smaller moving towardthe tellurium analog (0.10A to 0.068A) showing that largerportion of the positive charge in the cation state is being locatedon the chalcogen atom. A population analysis confirms that moreof the cation charge is located on the larger chalcogen atoms (0.18electron charge on S in BDT-syn compared to 0.28 electron chargeon Te in BDTe-syn) but that the geometry change associated withthe larger population is minimal (0.090A in BDT-syn compared to0.084A in BDTe-syn).Chart 1 also displays an isomer dependence in the system,where the syn isomer has a lower reorganization energy. This trendis also caused by the change in cationic character on the chalcogenatom in the syn isomer compared to the anti isomer. The bondlengthchange of the chalcogen-carbon bonds was larger in the synisomer than that of the anti isomer (0.09A for the syn isomer and0.08A for the anti isomer of BDT) while the bond length change ofthe carbon-carbon bonds was much larger in the anti isomer thanthe syn isomer (0.10A for the syn isomer and 0.15A for the antiisomer of BDT). A population analysis also confirms the correlationbetween reorganization energy and amount of charge of thechalcogen atoms (0.18 electron charge in BDT-syn compared to0.17 electron charge in BDT-anti).The results from chart 2 show that the isomer dependencebecomes negligible at long molecular lengths. The reorganizationenergy for the syn isomer stays approximately constant along theseries which is due to the overall change in geometry being thesame. This may be attributed to more of the cation charge beinglocalized on the chalcogen atom at small molecular lengths butbecomes less of a factor at longer molecular lengths.The transfer integral, which is the dominant constituent of theexponential prefactor in equation 2, may be approximated as halfthe electronic coupling between the donor and acceptor. However,a direct calculation of the transfer integral for a particularsystem is dependent on bulk order, which is difficult to determine.A general trend may be extracted by changing the dimerorientation during the electron splitting calculations, which ispresented in chart 3 below, where the dimer distance is increased.The calculations were performed on Ampac 16 using the AM1 17semi-empirical method.Chart 3. Transfer Integral for System (a)Calculated with AM1For Hole-transport104 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005