Theoretical studies of charge-transfer complexes of I2 with pyrazoles, and implications on the dye-sensitized solar cell performance

The charge-transfer 1:1 n–σ* complexation between pyrazole derivatives and I2 was theoretically investigated by the ab initio molecular orbital (MO) method at the second-order Møller–Plesset (MP2, full)/LANL2DZ* level. The nitrogen atom at position 2 in the pyrazole ring and the amino nitrogen atom of the substituent were preferred over the other n-donor sites of the substituents as binding sites for I2 molecules. By comparing pyrazoles complexes binding to I2 via the nitrogen atom at position 2 in the pyrazole ring, it was determined that the more electron donicity the substituent has at positions 3 and 5 in the pyrazole ring, the more stable the intermolecular charge-transfer complex formed. However, pyrazoles with an electron-accepting substituent at position 4 formed less favorable complexes with I2 even if they were substituted by electron-donating groups at positions 3 and/or 5. These pyrazoles were also examined as additives in an I−/I3− electrolyte solution in a Ru(II) bipyridine dye-sensitized solar cell, and the resultant open-circuit photovoltage (Voc) values were correlated with the intermolecular charge-transfer properties determined by theoretical calculations. The optimized geometries, Mulliken population analyses, and natural bond orbital (NBO) analyses indicated that a Voc is higher when the I2 and pyrazoles form a more favorable intermolecular charge-transfer complex in the I−/I3− redox electrolytic solution of dye-sensitized solar cell. The Voc enhancing mechanism by the pyrazoles–I2 complexation is discussed.