Time-resolved thermodynamic profile upon photoexcitation of a nitrospiropyran in cycloalkanes and of the corresponding merocyanine in aqueous solutions

The photoconversion of 2',3'-dihydro-6-nitro-1',3',3'-trimethylspiro[2H-1-b enzopyran-2,2'-indole] (Sp) to its open merocyanine form (Me) in a series o f aerated cycloalkanes (cyclopentane, cyclohexane, and trans- and cis-decal in) and of the protonated merocyanine (McH(+)) to Sp in aqueous solution we re studied by laser-induced optoacoustic spectroscopy (LIOAS). The + (11 +/ - 2) ml mol(-1) expansion determined for the ring closure is due to deproto nation of McH(+) plus the reaction of the ejected proton with the monoanion of malonic acid (added to stabilize Mc), an intrinsic expansion and a smal l electrostriction term. The energy difference between Sp and initial McH() is (282 +/- 110) kJ mol(-1). An intrinsic contraction of - (47 +/- 15) ml mol(-1) occurs upon ring opening, forming triplet (3)Mc in the cycloalkane s, whereas no volume change was detected for the (3)Mc to Mc relaxation. El ectrostriction decreases the (3)Mc energy, (165 +/- 18) kJ mol(-1), to 135 kJ mol(-1). The difference in the values of the ring-opening (Sp to Me) rea ction enthalpy in cycloalkanes as derived from the temperature dependence o f the Sp reversible arrow Me equilibrium, (29 +/- 8) kJ mol(-1), and from t he LIOAS data, - (9 +/- 25) kJ mol(-1), is due to the formation of Mc-Sp ag gregates during steady-state measurements. The Sp-sensitized singlet molecu lar oxygen, O-2((1)Delta (g)), was taken as a, quantum yield (average Phi ( Delta) = 0.58 +/- 0.03) derived from the near-IR emission of O-2((1)Delta ( g)), measure of Me production in the cycloalkanes, These solvents, albeit t roublesome in their handling, provide an additional series for the determin ation of structural volume changes in nonaqueous media. besides the alkanes already used.