Photochemistry of phenanthroline-containing spirooxazines in a low-temperature methanol matrix

Both stationary and laser flash photolysis were applied to study the photochemistry of a closed A-form of phenanthroline-containing spirooxazines in a methanol matrix (77 K). One of spirooxazines was nonsubstituted (SPO1) and the second one (SPO2) had a long alkyl substituent, n-C16H33, in the indoline part of the molecule. For SPO1, excitation of the A-form causes absorption of the B-form (600 nm) during the action of a laser pulse. Kinetics of a further increase in the concentration of B-form is determined by the inhomogeneity of a rigid matrix and is described by a set of exponents with characteristic times from hundreds of nanoseconds to tens of milliseconds. Absorption of the B-form is accompanied by absorption in the region of 400-750 nm, which belongs to X-isomer (an intermediate form of spirooxazine). The disappearance of X-isomer was described by a set of exponential functions with the characteristic times similar to those typical of the B-form appearance. For SPO1, only the B-form is accumulated during the stationary photolysis (X-isomer is completely transformed into the B-form). For SPO2, a range of characteristic times moves toward long times so that the X-isomer absorption is observed even upon stationary photolysis. Thus, a long X-isomer lifetime indicates that the B-form originates from the ground state of this intermediate. The existence of a long substituent substantially increases the time of X → B transformation in a frozen matrix. The appearance of a narrow absorption band at 600 nm immediately after the laser pulse was assigned to the formation of only one of the possible isomers of B-form upon X → B transformation.