10 September 1999 Ž . Chemical Physics Letters 310 1999 485–494 www.elsevier.nlrlocatercplett Electronic spectral shifts, reorganization energies, and local density augmentation of Coumarin 153 in supercritical solvents R. Biswas, J.E. Lewis, M. Maroncelli ) Department of Chemistry, 152 DaÕey Laboratory, The PennsylÕania State UniÕersity, UniÕersity Park, PA 16802, USA Received 25 May 1999; in final form 29 June 1999 Abstract Ž . Solvent reorganization energies and local densities of Coumarin 153 C153 in supercritical C H , CO , and CHF 2 6 2 3 Ž . y1 T s5 K are measured using fluorescence spectroscopy. Reorganization energies are 320 "70 and 770 "70 cm in CO c 2 Ž . and CHF , respectively – nearly independent of density 0.3 Frrr F2.1 . The spectral shifts imply similar effective local 3 c densities in all three solvents. These effective densities can exceed 3–5 times the bulk density. In CO and CHF , the 2 3 maximum density augmentation calculated from emission shifts is ;20% greater than that calculated from excitation shifts. No such difference is found in C H . q 1999 Elsevier Science B.V. All rights reserved. 2 6 1. Introduction Supercritical solvents, solvents at temperatures Ž . slightly above their critical points T , have become c a topic of intense current interest 1 . The unique feature of fluids in this region of the phase diagram is that their high compressibility allows for large and continuous variation of density with only modest changes in applied pressure. This tunability enables selection or variation of solubilities, solvation ener- getics, and transport rates so as to optimize condi- tions for a number of practical applications. This same attribute also renders supercritical solvation ) Corresponding author. Fax: q1-814-863-5319; e-mail: mpm@chem.psu.edu 1 A thematic issue of Chemical ReÕiews on supercritical fluids wx edited by R. Noyori 1 provides an excellent overview of much of the recent work in this area. distinct from solvation in typical liquid solvents. In particular, as a result of attractive solute–solvent interactions, solvent densities in the vicinity of a solute often appear to be much higher than that of the bulk fluid. Since local solvent density presum- ably dictates the behavior of many of the variables one would like to control, this phenomenon of ‘lo- cal density augmentation’ has received considerable attention from both experimental and theoretical w x perspectives 2,3 . However, a fully quantitative understanding of how solute and solvent properties determine local density augmentation in supercriti- cal fluids has yet to be achieved. The present Letter describes experimental measurements we hope will be of use in this context. We report fluorescence measurements of excitation and emission shifts of Ž the polarity probe Coumarin 153 hereafter desig- . nated C153 in the supercritical solvents C H , CO , 2 6 2 Ž . and CHF at a reduced temperature T s TrT of 3 r c 1.02. From these measurements we deduce the den- 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00838-6