Absolute Quantum Efficiencies

Abstract

Recent developments in several areas of chemistry, laser technology, photodetector instrumentation, and calorimetry are surveyed, and their probable impact on the measurement of quantum yields is assessed. Chemical developments include: (a) synthesis and design of new luminescent molecules that could possibly serve as standards, (b) application of improved separation techniques to provide samples of extreme purity, and (c) advances in photochemistry that portend the development of wide-range chemical actinometers. The potential use of lasers in quantum-yield measurements and their advantages over conventional sources for application in both optical and calorimetric techniques are pointed out. New methods of quantum-yield measurements, based on the novel characteristics of laser pump sources, are suggested, including the feasibility of measuring yields under time-resolved conditions and of employing internal standards. The possible lifting of wavelength restrictions on both laser sources and detector devices and the implications of these developments for extending the spectral range of quantum-yield measurements are discussed. The current status of calorimetry for determining yields is surveyed, and the impact of recent technology on the feasibility of developing calorimetric methods competitive with optical methods is assessed.

Keywords: Calorimetry in quantum yields; laser; photodetectors in quantum yields; quantum efficiencies; use in quantum yields.

Figure 1.

Principal pathways of degradation of an excited organic molecule. →, radiative decay; , radiationless decay. kf = radiative rate constant for depopulating S₁; $k_{q_f}$= radiationless rate constant for depopulating S₁; k_is = radiationless rate constant for converting from S₁ to T₁; k_p = radiative rate constant for depopulating T₁; $k_{q_p}$= radiationless rate constant for depopulating T₁. $k_f/\left(k_f+k_{q_f}+k_{is}\right)={\varphi }_f$(quantum yield of fluorescence from S₁). $k_p/\left(k_p+k_{q_p}\right)={\varphi }_p$(quantum yield of phosphorescence from T₁). $k_{is}/\left(k_{is}+k_f+k_{q_f}\right)={\varphi }_{is}$ (quantum yield of intersystem crossing).

Figure 2.

Relative quantum yield (a) and absorption spectrum (b, c) of tris(2,2′-bipyridine)ruthenium(II) chloride in methanol at room temperature: (a) 0.2 g/5 ml in a 1-cm cell, (b, c) 6.7 × 10⁻⁵ M in a 1-cm cell. The first and last yield points are less accurate than the others. [Ref: J. N. Demas and G. A. Crosby, J. Amer. Chem. Soc. 93, 2841 (1971).]

Figure 3.

Relative quantum yields (a, b) and absorption spectra (c) or tris(1,10-phenanthroline)osmium(II) iodide in ethanol-methanol (4:1, v/v): (a) 4.5 × 10⁻⁷ M in a 1.76-cm cell at room temperature; (b) 4.5 × 10⁻⁷ M in a 1.76-cm cell at 77 K (glass); (c) - - -, 9.0 × 10⁻⁶ M in a 10-cm cell at room temperature; ———, 7.12 × 10⁻⁵ M and 1.42 × 10⁻⁵ M in 1.76-cm cells at 77 K (glass). [Ref: J. N. Demas and G. A. Crosby, J. Amer. Chem. Soc. 93, 2841 (1971).]

Figure 4.

Absorption (———) and luminescence (- - -) spectra of phenyl-substituted iridium(III) complexes in ethanol-methanol glass (4:1, v/v) at 77 K. (a) cis-dichlorobis(4,4′-diphenyl-2,2′-bipyridine)-iridium(III) chloride, (b) cis-dichlorobis(4,7-diphenyl-1,10-phenanthroline)iridium(III) chloride. [Ref: R. J. Watts and G. A. Crosby, J. Amer. Chem. Soc. 94, 2606 (1972).]

Figure 5.

Temperature dependence of quantum yields. ×, cis-dichlorobis(4,4′-diphenyl-2,2′-bipyridine)iridium(III) chloride. ○, cis-dichlorobis(4,7-diphenyl-1,10-phenanthroline)iridium(III) chloride. Both molecules were dissolved in poly(methylmethacrylate) and irradiated with ~ 365 nm radiation.

Figure 6.

Conduction calorimeter.

Figure 7.

Tian-Calvet conduction microcalorimeter modified for quantum-yield determination.

Figure 8.

Thermogram from Tian-Calvet microcalorimeter.

Figure 9.

Heat flow compensation calorimeter.

Figure 10.

Flash calorimeter.