Determination of triplet formation quantum yields of dyes on particulate solids by laser-induced optoacoustic

EUGENIA TOMASINI; ENRIQUE SAN ROMÁN; SILVIA E. BRASLAVSKY

Cubatao, Brasil

Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008

Cepema

Determination of triplet formation quantum yields of dyes on particulate solids by laser-induced optoacoustic spectroscopy Eugenia Tomasini,a Enrique San Romána and Silvia E. Braslavskyb a INQUIMAE / DQIAyQF, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pab. II, C1428EHA Buenos Aires, Argentina, e-mail: eugenia@qi.fcen.uba.ar bMax-Planck-Institut für Bioanorganische Chemie, Postfach 101365, D 45413 Mülheim an der Ruhr, Germany             At present, there are no methods available for the determination of absolute quantum yields for the formation of triplet states, FT, and singlet molecular oxygen in light-scattering systems, either in the solid state or in suspension. We applied recently laser-induced optoacoustic spectroscopy (LIOAS) to the determination of fluorescence quantum yields of dyes in optically thick layers of microparticles or nanoparticle aggregates.[1] Results were consistent with those obtained on grounds of diffuse reflectance measurements, according to a technique also developed in our laboratory.[2] We report now on the application of LIOAS under excitation at 532 nm with a Nd:YAG laser to the determination of FT for Rose Bengal (RB) and hypericin (HY) attached to microcrystalline cellulose particles using brilliant blue G (BBG) as the calorimetric reference.             Extension of the already developed theoretical framework1 shows that, under specific conditions, an equation similar to that found for solution studies applies:                       (1) where H is the first maximum of the optoacoustic signal, E is the laser pulse energy, R is the sample reflectance, is the observed fluorescence quantum yield (affected by reabsorption and reemission of radiation), n0 is the excitation frequency, <nF> is the average fluorescence frequency, ET is the triplet energy, and h is the Planck constant.             As deduced from eq. (1) and shown in the figure, H/E is proportional to (1 – R) for BBG. The linear behavior found for RB with the exception of an outlying value (open circle) is consistent with a low value (0.04 - 0.08, experimental) and a nearly constant FT, which may be calculated according to eq. (1) as 0.71 ± 0.08 for ET = 14200 cm-1 (frozen glass).[3] The result is in excellent agreement with the value found in methanol, FT = 0.76.[4] Experiments performed with HY did not show the expected results, as sample and reference yield almost indistinguishable signals, though FF » 0.2 in organic polar solvents and a broad range of FT values between 0.3 and 0.7 have been reported.[5] Experiments are currently under way to certify these results and test the method using other dyes. [1] E. Tomasini, E. San Román, S. Braslavsky. Enviado a Langmuir [2] M. Mirenda, M.G. Lagorio, E. San Román, Langmuir 20 (2004) 3690-3697 [3] C.R. Lambert, I. Kochevar, Photochem. Photobiol. 66 (1997) 12-15 [4] K. Gollnik, G.O. Schenck, Pure Appl. Chem. 9 (1964) 507-525 [5] A. Losi, Photochem. Photobiol. 65 (1997) 791-801