Luminescence and light scattering

MARTÍN MIRENDA; ENRIQUE SAN ROMÁN

Granada, España.

Simposio; XX IUPAC Symposium on Photochemistry; 2004

IUPAC

Luminescence and light scattering Martín Mirenda and Enrique San Román Instituto de Química Física de los Materia­les, Medio Ambiente y Energía (INQUIMAE), Ciudad Universitaria, Pab. II, 1428 Buenos Aires, Argentina esr@qi.fcen.uba.ar Determination of luminescence spectra and quantum yields in powered solids or turbid suspensions is complicated by the simultaneous occurrence of light scattering. The problem has attracted the attention of photochemists and spectroscopists since a long time and different methods have been developed and applied to inorganic phosphors, colored solids and dyes adsorbed on particulate substrates.1 After the development of theories for light dispersion,2 the calculation of inner filter effects has been also addressed.3 Since several years we are involved in this kind of measurements on systems composed of dyes adsorbed on particulate materials. Our work led to the development of a model to account for dye aggregation and luminescence reabsorption and reemission based on the Kubelka-Munk theory of light dispersion4 and, more recently, to the determination of absolute fluorescence quantum yields.5 To evaluate luminescence quantum yields the number of photons emitted from the sample with the number of photons scattered by the sample and a suitable blank have to be measured. This may be done in a number of ways using a fluorescence spectrometer with or without an integrating sphere or a reflectance spectrometer. In all cases the photodetector responsivity as a function of wavelength must be known. Emission and scattering can be separated using an emission monochromator. Otherwise, luminescence can be blocked partially with optical filters or totally with a suitable quencher. This approach may be used in general for highly luminescent samples. When dealing with scarcely luminescent samples a suitable reference must be used. The angular distribution of luminescence and scattering is relevant when comparing sample and blank. Reabsorption of luminescence becomes a serious problem is Stokes shifts are small and dye concentrations high. For low luminescent samples, only reabsorption has to be taken into account but, on dealing with highly luminescent ones a series of consecutive reabsorption and reemission steps must be considered. In all cases models including scattering and luminescence must be applied. Simple, continuum models are based on a two-flux scheme according to the Kubelka-Munk theory or more elaborate models considering a three dimensional flux scheme. In the last case, approximations restrict application to systems having low luminescence quantum yields. Eventually, electromagnetic or stochastic models can be applied. Different approaches are compared and advantages and limitations of the various methods and models will be discussed on grounds of experimental evidence, instrumental requirements and theoretical considerations. REFERENCES (1)     See for example Liu, Y.S.L; de Mayo, P: Ware, W.R. J. Phys. Chem. 1993, 97, 5995 (2)     a) Wendlandt, W.W.; Hecht, H.G. Reflectance Spectroscopy; Wiley, New York, 1966. b) Kortüm, G. Reflectance Spectroscopy, Springer-Verlag; New York, 1969 (3)     See for example Gade, R.; Kaden, U., J. Chem. Soc. Faraday Trans., 1990, 86, 3707 (4)     Lagorio, M.G.; Dicelio, L.E.; Litter,M. I.; San Román, E. J. Chem. Soc., Faraday Trans. 1998, 94, 419 (5)     Mirenda, M.; Lagorio, M. G.; San Román, E., “Photophysics on Surfaces: Determination of Absolute Fluorescence Quantum Yields from Reflectance Spectra”, submitted