Coumarins, as a family of molecules, exhibit a wide range of fluorescence emission properties. In many cases, this fluorescence is extremely sensitive to the local environment of the molecule, especially the local polarity and microviscosity. In addition, coumarins show a wide range of size, shape, and hydrophobicity. These properties make them especially useful as fluorescent probes of heterogeneous environments, such as supramolecular host cavities, micelles, polymers and solids. This article will review the use of coumarins to probe such heterogeneous systems using fluorescence spectroscopy.

The maximum entropy method and a new more robust exponential series method are described and tested for the recovery of underlying fluorescence lifetime distributions derived from digital fluorescence data. Both real and simulated data are used in this study. Except for the resolution of single-lifetime systems, it is found that the two methods are similar in their resolving powers. The problem of differentiating between the case of a continuous distribution and three discrete components is discussed. The effects of radio-frequency noise, background subtraction, time base, and data precision on the recovery of distributions are described.

No abstract

48) (a) Saxe, P.; Yamaguchi, Y.; Schaefer, H. F. 111 J. Chem. Phys. 1982, 77,5647. (b) Osamura, Y.; Yamaguchi, Y.; Saxe, P.; Vincent, M. A.; Gaw, J. F.; Schaefer, H. F. 111 Chem. Phys. 1982, 72, 131. (49) (a) Osamura, Y.; Yamaguchi, Y.; Saxe, P.; Fox, D. J.; Vincent, M. A.; Schaefer, H. F. J. Mol. Strucr. 19113, 103, 183. (b) Handy, N. C.; Schaefer, H. F. J. Chem. Phys. 1984, 81, 5031. (50) (a) Rendell, A. P.; r#, T. J. J. Ch" Phys. 1991,946219. (b) Lee, (51) PSI 1.0, 1989, PSITECH, Inc., Watkinsvilk, GA. (52) TITAN is a set of electronic structure programs written by T. J. Lee, (53) Allen, W. D. Program INTDER, Stanford University, Stanford, CA. T. J.; Rendell. A. P. J. Chem. Phys. 1991,91, 6229.The UV-visible absorption spectra, S2 lifetimes, and S2 -SO fluorescence quantum yields of azulene, azulene-d8, 1,3-dichloroazulene, 1,3-dibromoazulene, 4,6,8-trimethylazulene, and 1,4-dimethyl-7-isopropylazulene (guaiazulene) have each been measured accurately in six solvents. The S2-Sl electronic energy spacings of each solute vary by ca. 500 cm-' in these solvents. The variations in the S2 nonradiative relaxation rates with electronic energy spacing are interpreted within the framework of the energy gap law of radiationless transition theory. S2 --S1 internal conversion dominates the nonradiative decay in azulene and azulene-d8, but intersystem crossing (likely S2 --TJ is important in the halogenated derivatives.The alkyl-substituted compounds exhibit anomalous behavior and demonstrate that factors other than the electronic energy spacing are involved in determining the rates of their radiationless relaxation. Previous energy gap law correlations based on data from a series of structurally different compounds must be reinterpreted.Azulene (AZ) and guaiazulene (GAZ, 1 ,Cdimethyl-7-isopropyldene), both from Aldrich, were used a received. Samples of highly purified AZ and azulene-d8 were kindly supplied by J h . B. Nickel. 1,3-Dichloroazulene (DCAZ) and 1 , 3 d i b r o m d e n e 0 1992 American Chemical Society Decay of the S2 States of Azulene

Laser flash photolysis with time-resolved infrared detection of transients (LFP-TRIR) has been used to study the IR spectroscopy and reactivity of a number of substituted ketenes, prepared by the 308-nm photolysis of α-diazocarbonyl precursors in acetonitrile solution at room temperature. The correlation of the experimental ketene asymmetric stretching frequency to the Swain−Lupton field (F) and resonance (R) effect substituent parameters was unsatisfactory, whereas the correlation to the inductive substituent parameter (σI) of Charton gave excellent results. This suggests that the asymmetric stretching frequency of substituted ketenes depends mainly on the inductive (i.e., field) effect of the substituents. The mechanism and kinetics of the reactions of these ketenes with various amines in acetonitrile were also studied. An intermediate species identified as either zwitterionic ylide or amide enol formed in the nucleophilic addition of the secondary amine to the Cα of the ketene is observed by TRIR. The decay of this species is assisted by the amine and is concomitant with the formation of an amide, the final product of the reaction. Our kinetic data on ketene amine reactions show a general trend, indicating a much higher reactivity (ca. 3 orders of magnitude difference in the corresponding rate constants) of secondary amines compared with that of tertiary amines. Secondary diethylamine shows reactivity similar to those observed for primary amines, while secondary piperidine seems to be, in general, somewhat more reactive. The observed trend is rationalized in terms of the steric effects exerted by both amine and ketene substituents. Our data on para-substituted phenyl ketenes provide support for the negative charge development on the ketene moiety in the transition state, with electron-withdrawing substituents accelerating and electron-releasing substituents slowing down the addition reaction.