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Radiationless processes measured by laser-induced optoacoustic spectroscopy (LIOAS)
375
NEWS AND VIEWS
Radiationless spectroscopy H. HEIHOFF
processes (LIOAS)
measured
by laser-induced
optoacoustic
and S. E. BRASLAVSKY
Max-Planck-Institut
fiir Strahlenchemie,
D-4330
Miilheim/Ruhr
(F.R.G.)
Laser-induced optoacoustic spectroscopy (LIOAS) is a photothermal technique that permits kinetic (yields and lifetimes) and calorimetric measurements on photoproduced short-lived species. LIOAS detects radiationless deactivation processes (i.e. heat), thus being complementary to other detection techniques, e.g. fluorescence. LIOAS allows measurements on systems that have low yields for radiative processes, are opaque or turbid, or in which transients have similar optical properties to those of the ground state. We have developed a fast time-resolved pressure detection system with a bandwidth up to about 35 MHz as LIOAS detector (Fig. 1). It consists of a high polymer piezoelectric PVF, (P-polyvinylidene difluoride) foil of thickness 9 - 45 pm [l]. In order to improve the sensitivity, an impedance conversion of the signals is performed. This allows the measurement of the pressure on the real time scale, with fastest detectable signal risetimes of about 30 ns (Fig. 1). The instrumental time resolution 7, (the acoustic transit time of the pressure wave across the laser beam diameter) determines the upper limit of fast (prompt) heat in the time scale from about 50 ns to about 2 @s (depending on the excitation beam diameter). The signal amplitudes arising from a sample are calibrated using a reference sample that delivers all the absorbed
3 0
alumlnlum
6
Y I-
Steelspr,ng* o-
2
3
L Time,
Fig. 1. PVFz optoacoustic spectroscopy ent instrumental time res6lutions [ 2 1.
5
6
ps
detector, and signals from a reference with differ-
376
NEWS AND VIEWS
energy as fast heat (i.e. it shows no other deactivation processes like luminescence or photochemistry, and the radiationless processes are faster than 7,) ]3,41. The radiationless decay of a transient (with lifetime rr > 7,) leads to a different ratio (a) of fast and slow heat depending on 7, and rr, as compared with a reference. The ratio cyis proportional to the quantum yield of production and the internal energy content of the transient, which acts as energystoring species for times longer than T,. The measurement of ~1as a function of the instrumental time resolution 7, yields the lifetime of the transient. In addition, the decay of a transient leads to a time-delayed heat and pressure evolution. The changes in the signal form arising from the decay of a transient enable the determination of the transient lifetime by signal deconvolution [ 51. LIOAS was successfully applied to the plant pigment phytochrome. We could determine for the first time the quantum yield for the production of the first transients I&, during the photoconversion P, + Pti, areact > 0.5 [2]. This determination could not be obtained by other methods such as fluorescence or flash photolysis. With the same method we determined for the first time the lifetime of a photoisomer produced after excitation of the tetrapyrrole phycocyanobiline dimethyl ester, a model compound for the phytochrome chromophore. The isomer lifetime is about 170 + 30 ns. 1 L. Bui, H. J. Shaw and L. T. Zitelli, Experimental broadband ultrasonic transducers using PVFz piezoelectric film, Electron. Lett., 12 (1976) 393 - 394. 2 K. Heihoff, S. E. Braslavsky and K. Schaffner, Study of 124-kilodalton oat phytochrome photoconversions in vitro with laser-induced optoacoustic spectroscopy, Biochemistry, 26 (1987) 1422 - 1427. 3 S. E. Braslavsky, R. M. Ellul, R. G. Weiss, H. Al-Ekabi and K. Schaffner, Photoprocesses in biliverdin dimethyl ester in ethanol studied by laser-induced optoacoustic spectroscopy (lioas), Tetrahedron, 39 (1983) 1909 - 1913. 4 S. E. Braslavsky, Time-resolved photoacoustic and photothermal methods. Application to phytochrome and tetrapyrroles, Photobiochem. Photobiophys. Suppl., (1987) 83 - 91. 5 K. Heihoff and S. E. Braslavsky, Triplet lifetime determination by laser-induced optoacoustic spectroscopy. Benzophenone/iodide revisited, Chem. Phys. Lett., 131 (1986) 183 - 188.
What makes a good photosensitizer for photodynamic therapy? CHARLES J. GOMER Clayton Foundation for Ocular Oncology, Childrens Hospital of Los Angeles, of Medicine, 4650 Sunset Boulevard, Los Angeles, CA 90027 (U.S.A.)
USC School
There is considerable activity related to the development of new photosensitizers for use in the field of photodynamic therapy (PDT). Interest in new photosensitizers is due in part to the encouraging initial clinical results
NEWS AND VIEWS
Radiationless spectroscopy H. HEIHOFF
processes (LIOAS)
measured
by laser-induced
optoacoustic
and S. E. BRASLAVSKY
Max-Planck-Institut
fiir Strahlenchemie,
D-4330
Miilheim/Ruhr
(F.R.G.)
Laser-induced optoacoustic spectroscopy (LIOAS) is a photothermal technique that permits kinetic (yields and lifetimes) and calorimetric measurements on photoproduced short-lived species. LIOAS detects radiationless deactivation processes (i.e. heat), thus being complementary to other detection techniques, e.g. fluorescence. LIOAS allows measurements on systems that have low yields for radiative processes, are opaque or turbid, or in which transients have similar optical properties to those of the ground state. We have developed a fast time-resolved pressure detection system with a bandwidth up to about 35 MHz as LIOAS detector (Fig. 1). It consists of a high polymer piezoelectric PVF, (P-polyvinylidene difluoride) foil of thickness 9 - 45 pm [l]. In order to improve the sensitivity, an impedance conversion of the signals is performed. This allows the measurement of the pressure on the real time scale, with fastest detectable signal risetimes of about 30 ns (Fig. 1). The instrumental time resolution 7, (the acoustic transit time of the pressure wave across the laser beam diameter) determines the upper limit of fast (prompt) heat in the time scale from about 50 ns to about 2 @s (depending on the excitation beam diameter). The signal amplitudes arising from a sample are calibrated using a reference sample that delivers all the absorbed
3 0
alumlnlum
6
Y I-
Steelspr,ng* o-
2
3
L Time,
Fig. 1. PVFz optoacoustic spectroscopy ent instrumental time res6lutions [ 2 1.
5
6
ps
detector, and signals from a reference with differ-
376
NEWS AND VIEWS
energy as fast heat (i.e. it shows no other deactivation processes like luminescence or photochemistry, and the radiationless processes are faster than 7,) ]3,41. The radiationless decay of a transient (with lifetime rr > 7,) leads to a different ratio (a) of fast and slow heat depending on 7, and rr, as compared with a reference. The ratio cyis proportional to the quantum yield of production and the internal energy content of the transient, which acts as energystoring species for times longer than T,. The measurement of ~1as a function of the instrumental time resolution 7, yields the lifetime of the transient. In addition, the decay of a transient leads to a time-delayed heat and pressure evolution. The changes in the signal form arising from the decay of a transient enable the determination of the transient lifetime by signal deconvolution [ 51. LIOAS was successfully applied to the plant pigment phytochrome. We could determine for the first time the quantum yield for the production of the first transients I&, during the photoconversion P, + Pti, areact > 0.5 [2]. This determination could not be obtained by other methods such as fluorescence or flash photolysis. With the same method we determined for the first time the lifetime of a photoisomer produced after excitation of the tetrapyrrole phycocyanobiline dimethyl ester, a model compound for the phytochrome chromophore. The isomer lifetime is about 170 + 30 ns. 1 L. Bui, H. J. Shaw and L. T. Zitelli, Experimental broadband ultrasonic transducers using PVFz piezoelectric film, Electron. Lett., 12 (1976) 393 - 394. 2 K. Heihoff, S. E. Braslavsky and K. Schaffner, Study of 124-kilodalton oat phytochrome photoconversions in vitro with laser-induced optoacoustic spectroscopy, Biochemistry, 26 (1987) 1422 - 1427. 3 S. E. Braslavsky, R. M. Ellul, R. G. Weiss, H. Al-Ekabi and K. Schaffner, Photoprocesses in biliverdin dimethyl ester in ethanol studied by laser-induced optoacoustic spectroscopy (lioas), Tetrahedron, 39 (1983) 1909 - 1913. 4 S. E. Braslavsky, Time-resolved photoacoustic and photothermal methods. Application to phytochrome and tetrapyrroles, Photobiochem. Photobiophys. Suppl., (1987) 83 - 91. 5 K. Heihoff and S. E. Braslavsky, Triplet lifetime determination by laser-induced optoacoustic spectroscopy. Benzophenone/iodide revisited, Chem. Phys. Lett., 131 (1986) 183 - 188.
What makes a good photosensitizer for photodynamic therapy? CHARLES J. GOMER Clayton Foundation for Ocular Oncology, Childrens Hospital of Los Angeles, of Medicine, 4650 Sunset Boulevard, Los Angeles, CA 90027 (U.S.A.)
USC School
There is considerable activity related to the development of new photosensitizers for use in the field of photodynamic therapy (PDT). Interest in new photosensitizers is due in part to the encouraging initial clinical results