Electroabsorption studies of poly (2,5-pyridinediyl)

The electroabsorption spectrum of poly(2,5-pyridinediyl) has been measured for thin films with unpolarized and polarized light. The spectrum has been successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative. The electroabsorption spectrum is dominated by a Stark redshift of the first allowed optical transition, 1A(g)-->1B(u) at 3.2 eV, and the emergence of a normally one-photon forbidden 2A(g) state at 3.7 eV, which becomes weakly allowed in the presence of the electric field. A small contribution of charge-transfer excitons to the electroabsorption spectrum has been found and assigned to be an interchain n-->pi* transition. From the linear coefficients we calculated the average change of the components of the polarizibility tensor [Delta<(alpha)over left right arrow>] = 4.2 x 10(-37) C m/(V/m) and the average value of the difference between ground- and excited-state dipole moment [Delta p] = 5.7 D. The anisotropy ratio Delta alpha(parallel to)/Delta alpha(perpendicular to), the ratio of the field-induced absorption obtained with light polarized parallel and perpendicular to the applied electric field, has been determined to be 1.6. Furthermore, a Kramers-Kronig analysis has been used to calculate the red and imaginary part of the third-order nonlinear susceptibility chi(3)(-omega;0,0,omega). Results are compared and contrasted with those reported for other conjugated polymers. [S0163-1829(99)08135-7].