engleski

Aspects of Solar Energy Storage by the Photochemical Valence Izomerization of 9-tert-Butylanthracene

9-tert-Butylanthracene l at 0°C in solution photoisomerizes to its valence isomer l tert – butyl-dibenzo-2, 5-bicyclo [2.2.0] hexadiene (9-tert-butyl-9, 10-Devaranthracene) 2 which, at higher temperatures, reforms slowly the aromatic compound l [l]. The photochemical valence isomerization l → 2 offers a promising potential for the conversion and storage of solar energy because the energy capability is substantially larger than in other types of photochemical reactions. For the exothermicity of the reaction 2 → l a reaction enthalpy ∆ H r = 37 ± ; l kcal/mol was determined by calorimetry in n-hexane, the activation enthalpy to ∆ H≠ = 23.6 ± ; l.4 kcal/mol and the entropy to ∆ S≠ 0.26 ± ; 4.4 e.u. Since l absorbs light from 410 to 300 nm with a fairly large molar extinction coefficient ( 6. x 103 1x mol-1 cm-1 in cyclohexane ) 2 is accessible with tropospheric solar light un – like other Dewar systems known so far. Since for practical applications the use of aqueous solutions offers considerable advantage over organic solvents we studied the raction 1 → 2 and 2 → l in aerated aqueous micellar solutions at room temperature. The triplet state of 9-tert-butylanthracene was detectedby nanosecont laser photolysis as well as inirectly by the measurement of singlet oxygen produced in the reaction between the triplet state of 9 – tert-butylanthracene and molecular oxygen. The quantum yield of formation of the triplet state of l in micellar solutions was determined to be l7 times smaller than the quantum yield of triplet formation of anthracene. Since the fluorescence quantum yield of l at room temperature is only 0.0ll and the photochemical quantum yield for l → 2 only 0.012, a very higl quanum yield for internal converion in l has to be assumed. Since the thermal half – life of 2 at 20 C is 6.5 h a nearly l00% converion of l into 2 with tropospheric solar light can be achieved despite the low quantum yield of the photoreaction l → 2.

photochemical valence izomerization of 9-tert-butylanthracene

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