Time-Resolved Thermodynamic Profile upon Photoexcitation of a Nitrospiropyran in Cycloalkanes and of the Corresponding Merocyanine in Aqueous Solutions

Abstract

Dedicated to Professor Andre M. Braun on the occasion of his 60th birthdayThe photoconversion of 2',3'-dihydro-6-nitro-1',3',3'-trimethylspiro[2H-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 AE 2) ml mol À1 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 AE 110) kJ mol À1 . An intrinsic contraction of À (47 AE 15) ml mol À1 occurs upon ring opening, forming triplet 3 Mc in the cycloalkanes, whereas no volume change was detected for the 3 Mc to Mc relaxation. Electrostriction decreases the 3 Mc energy, (165 AE 18) kJ mol À1 , to 135 kJ mol À1 . 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 AE 8) kJ mol À1 , and from the LIOAS data, À (9 AE 25) kJ mol À1 , is due to the formation of Mc-Sp aggregates during steady-state measurements. The Sp-sensitized singlet molecular oxygen, O 2 ( 1 D g ), quantum yield (average F D 0.58 AE 0.03) derived from the near-IR emission of O 2 ( 1 D g ), 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.Introduction. ± The knowledge of the structural volume changes in photochemical reactions [1] is not just of academic interest, but also represents a challenge regarding the possibility of creating materials displaying a reversible volume response towards light [2]. Our major interest in this area is the relationship between the structural volume changes and the type of photoreaction, in order to understand the origin of the molecular structural changes upon excitation of biological photoreceptors [3].A quantitative correlation was found between the reaction structural volume changes determined by laser-induced optoacoustic spectroscopy (LIOAS) [4] and the entropy changes of the reaction for intramolecular photoinduced electron-transfer reactions of Ru II bipyridyl cyano complexes as well as for photoinduced intermolecular electron-transfer reactions between Ru(bpy) 3 2 and the methyl viologen cation, in both cases in aqueous solution containing various monovalent salts. Based on the constancy of the reaction free energy, the changes were attributed to an entropy-enthalpy

Isomerization of spirobenzopyrans bearing electron-donating and electron-withdrawing groups in acidic aqueous solutions

“…Previous studies have demonstrated that the McH irradiated with visible-light usually underwent the ring closure and proton dissociation to give the corresponding Sp. [28][29][30] Fig. 3 represents UV-visible spectra of polymers 6a-e recorded before and after bluelight-irradiation in 2 mM HCl at 0.5 1C.…”

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

“…[41][42][43] Cyclic alkanes have been used recently and provide an additional series for the determination of structural volume changes in non-aqueous media. 44 Very recently, a homologous series of aprotic, polar nitrile solvents, consisting of acetonitrile, propionitrile, butyronitrile, and valeronitrile, has been also used. 45 This procedure is valid provided that no significant perturbations are introduced in the photophysics and photochemistry of the solute, and the reaction enthalpy and volume are not changed in the solvent series.…”

“…The photoconversion of a nitrospiropyrane to its open merocyanine form in a series of aerated cycloalkanes (cyclopentane, cyclohexane, and cis-and trans-decalin) and of the protonated merocyanine to nitrospiropyrane in aqueous solution were studied by laser-induced optoacoustic spectroscopy. 44 The primary photochemistry of urocanic acid (UCA), one of the primary UV-absorbing chromophores in the stratum corneum of human skin, has been studied by time-resolved PAS. 230 UCA is formed initially as the trans isomer, t-UCA.…”

Volume of reaction, energetics, and kinetics of tetrahydrothiophene displacement of solvent from Mo(CO)5(alkane)

“…5 Recent studies indicate that volumes determined by LIOAS are free of electrostriction contributions. 21 The volume of reaction formally includes the difference in volumes of the reactants and products, and changes in solvent interactions with the reactants and products. The derivation of eqn.…”