| Abstract: | The photoconversion of 2′,3′‐dihydro‐6‐nitro‐1′,3′,3′‐trimethylspiro2H‐1‐benzopyran‐2,2′‐indole] ( Sp ) to its open merocyanine form ( Mc ) in a series of aerated cycloalkanes (cyclopentane, cyclohexane, and trans‐ and cis‐decalin) and of the protonated merocyanine ( McH + ) to Sp in aqueous solution were studied by laser‐induced optoacoustic spectroscopy (LIOAS). The +(11±2) ml mol?1 expansion determined for the ring closure is due to deprotonation of McH + plus the reaction of the ejected proton with the monoanion of malonic acid (added to stabilize Mc ), an intrinsic expansion and a small 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 3Mc in the cycloalkanes, whereas no volume change was detected for the 3Mc to Mc relaxation. Electrostriction decreases the 3Mc energy, (165±18) kJ mol?1, to 135 kJ mol?1. The difference in the values of the ring‐opening ( Sp to Mc ) reaction enthalpy in cycloalkanes as derived from the temperature dependence of the Sp ? Mc equilibrium, (29±8) kJ mol?1, and from the LIOAS data, ?(9±25) kJ mol?1, is due to the formation of Mc‐Sp aggregates during steady‐state measurements. The Sp ‐sensitized singlet molecular oxygen, O2(1Δg), quantum yield (average ΦΔ=0.58±0.03) derived from the near‐IR emission of O2(1Δg), was taken as a measure of Mc production in the cycloalkanes. These solvents, albeit troublesome in their handling, provide an additional series for the determination of structural volume changes in nonaqueous media, besides the alkanes already used. |
