Stark effect on the 580 nm S1←S transition of isobacteriochlorin using photochemical hole-burning spectroscopy

Abstract

High resolution excitation spectra of free base isobacteriochlorin in an n-octane crystal at 4.2 K have been obtained. In the 580 nm region (the 0–0 band of the lowest-lying singlet transition of the trans tautomer) five lines were observed. Using a tunable dye laser two pairs of these lines could be photochemically interconverted, while one line did not burn. With still narrower laser excitation, holes were burned in the four convertible lines which were limited in width by the laser output (0.07 cm−1). Electric fields were applied along each of the crystal axes and Stark splittings of the spectral lines and holes were observed. The splittings are linear over the range 1.21 to 50.0 kV/cm. The magnitude of the total dipole moment change (Δμ) for all lines was found to be 1.64 D; this is much larger (∼7×) than the value measured in free base chlorin. Also, the direction of this vector is a function of the position of the tautomers inner protons; it rotates 90° in going from one trans tautomer to the other. The direction cosines of the Δμ̄’s indicate that free base isobacteriochlorin’s orientation in the n-octane crystal is similar to that of free base chlorin, but with the second exterior reduced double bond rotated about 25° out of the alkane chain plane. Furthermore, the burning pattern of the 580 nm 0–0 bands verifies the earlier hypothesis that these are due to the trans tautomer.