Determination of the ground and excited state dipole moments of free base chlorin and isobacteriochlorin

The ground state dipole moment of free base chlorin (H(2)Ch) was determined by dielectric constant measurements of H(2)Ch/benzene solutions (mu(g) = 2 .93 +/- 0.08 D). This was combined with the previously measured change in d ipole moment (Delta mu(ge) = - 0.255 +/- 0.005 D) associated with the S-1 < -- S-0 (pi pi*) transition to calculate the excited-state dipole moment (mu (e) = 2 68 +/- 0.08 D) of H(2)Ch. These values were used with solvatochromi c shift data to find the cube of the cavity radius, a(3), for the H(2)Ch mo lecule; the assumption was made that the cis and trans tautomers of free ba se isobacteriochlorin (H(2)iBC) have this same a(3). This number ((4.49 +/- 0.05) x 10(-23) cm(3)) was combined with the spectral shifts and measured Delta mu(ge) vectors to determine the ground and excited state dipole momen ts of cis-H(2)iBC (mu(g) = 0.35 +/- 0.01 D; mu(e), = -0.97 +/- 0.01 D) and trans-H(2)iBC (mu(g) = 2.16 +/- 0.05 D, mu(e) = 3.51 +/- 0.05 D). Our solva tochromic shift data also indicated that when inferring the sign of a Stark effect determined change in dipole moment, only solvents with low polariza bilities (nonaromatic) should be used.