Stark effect on the S-1 <- S-0 transition of cis-free base isobacteriochlorin in an n-octane single crystal at 5 K

High-resolution Stark effect measurements on the S-1<-- S-0(pi pi(*)) origi n of cis-free base isobacteriochlorin in single crystals of n-octane at 5 K are reported. The spectral splittings of the band at 15 822 cm(-1) are lin early dependent on the applied electric field. The change in dipole moment (Delta mu) was found to be 1.32 D and is parallel to the crystal's b-axis. This band blueshifted as the polarity of the solvents at room temperature w as increased, which implies that Delta mu is negative. To rationalize the l arge magnitudes and opposite signs of the cis and trans tautomers' Delta mu values, we propose a simple valence-bond model based on ionic resonance st ructures. (C) 1999 American Institute of Physics. [S0021-9606(99)01032-6].