Time-resolved photoacoustic calorimetry

In this chapter, we describe how time-resolved photoacoustic calorimetry (PAC) can be used to obtain both the energetics and kinetics of carbenes in solution.7-9 PAC measures the magnitude and temporal profile of volume changes in solution following deposition of energy. These time-resolved volume changes can be directly related to carbene reaction enthalpies. We will first discuss the principles of this photoacoustic technique and then how it has been... 

J. E. Rudzki, J. L. Goodman, K. S. Peters. Simultaneous Determination of Photoreaction Dynamics and Energetics Using Pulsed, Time-Resolved Photoacoustic Calorimetry. J.Am. Chem. Soc. 1985, 107, 7849-7854. 

K. S. Peters. Time-Resolved Photoacoustic Calorimetry From Carbenes to Proteins. Angew. Chem. Int. Ed Engl. 1994, 33, 294-302. 

R. M. Borges dos Santos, V S. F. Muralha, C. F. Correia, R. C. Guedes, B. 1. Costa Cabral, J. A. Martinho Simoes. S-HBond Dissociation Enthalpies in Thiophe-nols A Time-Resolved Photoacoustic Calorimetry and Quantum Chemistry Study. J. Phys. Chem. A 2002,106, 9883-9889. 

As defined by K. S. Peters and G. J.. Snyder (University of Colorado), time-resolved photoacoustic calorimetry is an experimental technique... 

Peters. K.S. and G.J. Snyder, Time-Resolved Photoacoustic Calorimetry Probing the Energetics and Dynamics of Fast Chemical and Biochemical Reactions, Science. 1053 (August 26, 1988). 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

Adam, W., Platsch, H., Sendelbach, J., Wirz, J., Determination of the Triplet Lifetimes of 1,3 Cyclopentanediyl Biradicals Derived from the Photodenitrogenation of Azoalkanes with Time resolved Photoacoustic Calorimetry, J. Org. Chem. 1993, 58, 1477 1482. 

Due to its thermal instability, the ring structure of trans-cycloheptene (39a) has only been characterized in trans-2-cycloheptenone (41) by its IR spectrum and by trapping experiments (47). To date, no direct structural evidence has been found for trans-cyclohexene (43a). It seems likely, however, that trans-l-phenylcyclohexene (43b) is the species detected by pulsed time-resolved photoacoustic calorimetry (cf. Section III. B) (48). 

Volume changes related to photophysical properties can also be deduced from time-resolved photoacoustical calorimetry (PAG) studies [28]. This technique has been used with the Ru(II) complexes Ru(bpy), ds-Ru(bpy)2(CN)2 and Ru(bpy)(CN)4" [29, 30]. Analysis of such data was used to calculate enthalpy and volume changes induced by the formation and decay of MLCT excited states in aqueous solutions. For Ru(bpy) formation of the MLCT state is associated with a modest contraction (AVmlct = —3.6 + 0.3 cm mol ), which was rationalized in terms of slight decreases ( 0.001 A) in Ru-N bond lengths in the ES [29]. The AV for decay is the same magnitude, but of opposite sign (-F3.4 0.4 cm mol" )... 

Pulsed, time-resolved photoacoustic calorimetry employs a pulsed uv laser source to induce a photochemical reaction in the molecule of... 

The ratio of the emission quantum yield and lifetime of the triplet state emission (phosphorescence) yields the product of the intersystem crossing efficiency and radiative decay rate constant. Determination of intersystem crossing efficiencies is generally not straightforward and often techniques other than emission spectroscopy, such as time-resolved photoacoustic calorimetry, are used. 

Time-resolved photoacoustic calorimetry (PAC) is a technique complementary to flash photolysis. In flash photolysis we stndy transient species throngh their radiative processes (light absorption or emission) in the wavelength region analysed. In contrast, in photoacoustic calorimetry we study the non-radiative transitions (vibrational relaxation, physical quenching of excited states or chemical reactions), again normally induced by excitation... 

Goodman, J.L., Peters, K.S., Misawa, H., and Caldwell, R.A., Use of pulsed time-resolved photoacoustic calorimetry to determine the strain energy of tra s-l-phenylcyclohexene and the energy of the relaxed 1-phenylcyclohexene triplet, /. Am. Chem. Soc., 108, 6803-6805,1986. 

Schuster, D. I., Heibel, G. E., Caldwell, R. A., and Tang, W., Determination of triplet excitation energies of cyclic enones by time-resolved photoacoustic calorimetry, Photochem. PhotobioL, 52, 645, 1990. 

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