Hugh D. Bunows and A. Correia Cardow Universidade de Coimbra, 3049 Coimbra, Portugal Following light absorption, excited molecules can decay by re-emitting light as luminescence, losing their excess energy thermally in radiationless decay, or converting to some new chemical species: A+hu-A* A" - A A' - A
A*-B
+ hv + heat
light absorption luminescence radiationless decay photochemical change
The relative efficiencies of the various processes depend on a number of structural and molecular factors. but with the majority of molecules radiationless relaxatibn is the predominant decav route of the excited state. This leads to heating of the smnple; the technique of photoacoustic spectroscopvmakes use of this Drocess todetermine the electronic speEtia of opaque mate;ials.' The radiationless processes are normally subdivided into changes of electronic state (internal conversion, intersystem crossing) and vibrational relaxati~n.~ We have previously described a simple demonstration of radiationless decay, where beakers containing water and a solution of methylene blue are irradiated and the temperature difference noted.3 We report here a modification that probably is more palatable. Red wine gets its colour, predominantly, from anthocyanin, tannins, and related pigments,' and absorbs in much of the blue and green regions of the spectrum, while white wine only absorbs weakly in the visible. Absorption spectra of Portuguese red and white wines are shown in the figure. On irradiation with a suitable light source, the red wine will absorb more radiation than the white, which should lead to a difference in temperature due to the radiationless decay. This can be shown very simply. Red and white wine (-25 mL) are placed in two 50-mL beakers, which are placed under a 60 W tungsten lamp (such
'Mei, E. H.; Eyring, E. M. J. Chem. Educ. 1981, 58, 812 and referencestherein. See, for example, Turro, N. J. Modem Molecular Spectroscopy; Benjarnin/Cummings: Menlo Park. 1978. Burrows. H. D.; Grap Miguel, M.; Correia Cardoso, A. ERA. News 1987,29,39. Amerine. M. A. In Kirk-Ofhmer Encyclopedia of Chemical Tech nology, 3rd ed.; Mark, H. F.; Othmer, D. F.; Overberger, C. G.; Seaborg, G. T.. Eds.; Wiley-Interscience: New York, 1984;Vol. 24, p 562. Johnson. H. The World Atlas of Wine, 3rd ed.; Beaziey: London. 1985
'
as a desk lamp) in positions where they receive roughly the same amount of light. The temperature of the wine is noted before irradiation, and the difference in temperature is noted after 15 min irradiation using a thermometer reading to 0.1 " C . In both cases the temperature increases, hut the red wine is seen to he hotter. We have found a dieital electronic thermometer to he particularly suitahle for the demonstration and haw obrninrd trmuerature dit't'erencrs heiween 0.5 and 1 "C. Students can then be presented with a more detailed picture of radiative and radiationless processes in terms of the familiar Jablonski diagram. The demonstration can, obviously, be made more theatrical, for example by using wine glasses instead of beakers, or by consuming the test solutions (note: white wine should always be drunk before red wine). In addition the effect can be put to practical use. Although the temperature a t which wine should he served is often a matter of cultural preference," the bouquet of many red wines is enhanced if they are sliahtlv .. , above room tem~erature.A simole wav of effectine rhi> i < 11, hold a glass of wine nrar a lamp for a few minutes. We are arateful to rhc, E:unmean Photorhemiral Association for tKe digital thermometer. The supply of the wine samples was our pleasure.
Absorption spectra of Portuguese red (solidline) and white (broken line)wines from the regions of D50 and Bairrada, respectively. Samples were diluted with distilled water (0.5 mL wine in 3 mL solution)and spectra recorded in I-cm cells
Volume 64
Number 12 December 1987
995
