This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

Reverse micelles as hosts for proteins and small molecules.

Boron dipyrromethene (Bodipy) is one of the most extensively investigated organic chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochemistry study, photodynamic therapy (PDT), photocatalysis and triplet–triplet annihilation (TTA) upconversion. The recent developments in the study of the production, modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Forster-resonance-energy-transfer (FRET), or the intermolecular charge transfer (ICT). Controlling the triplet excited state will give functional molecules such as activatable PDT reagents or molecular devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) production, photoredox catalytic organic reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.

The triplet excited state of Bodipy: formation, modulation and application

Vibrationally highly excited toluene molecules with 52 000 cm−1 of vibrational energy have been prepared by UV laser excitation of the isomer cycloheptatriene and subsequent isomerization. The collisional loss of energy from the excited toluene molecules has been observed directly by monitoring hot UV absorption spectra. Direct measurements of the average energies 〈ΔE〉 transferred per collision have been made for about 60 different bath gases. For complex bath gases, 〈ΔE〉 values appear to be considerably smaller than those derived from earlier indirect measurements. 〈ΔE〉 was found not (or only slightly) to depend on the excitation energy.

Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene

Preface. Part One: Optical Methods. 1. Developments in Classical Optical Spectroscopy J. Amesz. 2. Linear and Circular Dichroism G. Garab. 3. Fluorescence K. Sauer, M. Debreczeny. 4. Ultrafast Spectroscopy of Photosynthetic Systems R. Jimenez, G.R. Fleming. 5. Data Analysis of Time-Resolved Measurements A.R. Holzwarth. 6. Photosynthetic Thermoluminescence as a Simple Probe of Photosystem II Electron Transport Y. Inoue. 7. Accumulated Photon Echo Measurements of Excited State Dynamics in Pigment-Protein Complexes T.J. Aartsma, R.J.W. Louwe, P. Schellenberg. 8. Spectral Hole Burning: Methods and Applications to Photosynthesis N. Raja, S. Reddy, G.J. Small. 9. Infrared and Fourier-Transform Infrared Spectroscopy W. Mantele. 10. Resonance Raman Studies in Photosynthesis - Chlorophyll and Carotenoid Molecules B. Robert. 11. Stark Spectroscopy of Photosynthetic Systems S.G. Boxer. 12. The Photoacoustic Method in Photosynthesis - Monitoring and Analysis of Phenomena which Lead to Pressure Changes Following Light Excitation S. Malkin. Part Two: Magnetic Resonance. 13. Magnetic Resonance: an Introduction A.J. Hoff. 14. Time-Resolved Electron Paramagnetic Resonance Spectroscopy - Principles and Applications H. Levanon. 15. Electron Spin Echo Methods in Photosynthesis Research R.D. Britt. 16. ENDOR Spectroscopy W. Lubitz, F. Lendzian. 17. Optically Detected Magnetic Resonance (ODMR) of Triplet States in Photosynthesis A.J. Hoff. 18. MagicAngle Spinning Nuclear Magnetic Resonance of Photosynthetic Components H.J.M. de Groot. Part Three: Structure and Oxygen. 19. Structure Determination of Proteins by X-Ray Diffraction M. Schiffer. 20. Electron Microscopy E.J. Boekema, M. Roegner. 21. X-Ray Absorption Spectroscopy:Determination of Transition Metal Site Structures in Photosynthesis V.K. Yachandra, M.P. Klein. 22. Moessbauer Spectroscopy P.G. Debrunner. 23. Characterization of Photosynthetic Supramolecular Assemblies Using Small Angle Neutron Scattering D.M. Tiede, P. Thiyagarajan. 24. Measurements of Photosynthetic Oxygen Evolution H.J. van Gorkom, P. Gast.

Biophysical techniques in photosynthesis

The distribution of solubilisate molecules in micellar assemblies

Photoacoustic measurements of photofragmentation in CH3I

Intersystem crossing in anthracene

Both V-T,R and V-V processes in methane have been studied optoacoustically following excitation of the ν3 level with a He-Ne laser at 2947.9 cm−1. The lifetime of the V-T,R process is 1.55 ± 0.05 μs atm. The rate constants for the fast equilibration between the bending modes is k(ν2 → ν4) = 60 μs−1 atm−1 and k(ν4 → ν2) = 13 μs−1 atm−1. The decay of the ν3 and ν2 stretching modes, which are in very rapid equilibrium, shows a rate constant of 0.23 ns−1 atm−1 and, within experimental error, produces exclusively the ν4 stretching mode. Part of this decay, 4.6%, is by a single-quantum process producing a large amount of translational/rotational energy; the dominant process, 95.4%, is double-quantum through the 2ν4 overtone. Both the yield of the single-quantum process and the exclusive production of the ν4 bending mode from the (ν3, ν2) level are in dispute with current theoretical models.

Vibrational-translational/rotational and vibrational-vibrational processes in methane: Optoacoustic measurements

It is shown how a modulating concentration of radicals, produced photolytically, can be used in an optoacoustic system to give kinetic information. The acoustic signal is the result of the enthalpy change present in the reaction. Results are reported for the rate constants of the reactions X + I2→ XI + I, where X is CH3, C2H5, CD3 and CH2I. Rate constants are also measured for the radical recombination of X with I in the presence of 100 Torr krypton.

Tom F. Hunter

Papers

The distribution of solubilisate molecules in micellar assemblies

Photoacoustic measurements of photofragmentation in CH3I

Intersystem crossing in anthracene

Vibrational-translational/rotational and vibrational-vibrational processes in methane: Optoacoustic measurements

Optoacoustic method of measuring reaction rates of the radicals CH3, CD3, C2H5 and CH2I with I and I2