16768
J. Phys. Chem. 1995,99, 16768-16775
Photochemistry on Surfaces. Intermolecular Energy and Electron Transfer Processes between Excited Ru(bpy)32+and H-Aggregates of Cresyl Violet on Si02 and Sn02 Colloids Di Liu, Gordon L. Hug, and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: June 23, 1995; In Final Form: September 7, 1 9 9 9
Cresyl violet, a cationic dye ( C V ) , forms H-aggregates on the negatively charged Si02 and SnO2 colloids. + , are These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, R ~ ( b p y ) 3 ~we able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal Si02 and by the dye-aggregates proceeds SnO2 suspensions. On Si02 surfaces, the excited state quenching of R~(bpy)3~+ via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surfacepromoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO2 colloids undergo photosensitized reduction since the excited sensitizer, R~(bpy)3~+*, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described.
Introduction Organic dyes play an important role in many practical applications. Their strong absorption in the visible region and ability to sensitize large band gap semiconductor materials have made them useful for photography and light energy conversion devices.'-6 Of particular interest are molecular aggregates of dyes, which are widely used in color photography since they are capable of sensitizing silver bromide nanocry~tallites.~-'~ Despite their practical importance, little effort has been made to investigate the excited state behavior and electron transfer reactions of dye-aggregates.' In our previous study" we have shown that thiazine and oxazine dyes readily form H-aggregates on negatively charged SnO2 nanocrystallites. Because of the strong electrostatic interaction between a cationic dye and a negatively charged oxide surface, the dye aggregation can be seen even at very low dye concentrations ( -= 1O-s M). These dye-aggregates are electrochemically active and are capable of extending the photoresponse of large band gap semiconductor materials. Cresyl violet in its monomeric form is highly fluorescent (@ = 0.5) with strong absorption in the visible The photosensitizing properties of the cresyl violet monomer have been studied by several researchers!.23 We have now undertaken a detailed photochemical investigation of H-aggregates of cresyl violet adsorbed onto Si02 and SnO2 colloids. As discussed in our previous review,' support materials can be broadly classified into two categories: (i) nonreactive surfaces, such as silica or alumina, that provide an ordered twodimensional environment for effecting and controlling photochemical processes more efficiently than can be attained in homogeneous solutions (see, for example, Scheme 1, top left) and (ii) reactive sur3caces, such as SnO2 or TiO2, that directly participate in photochemical reactions by quenching the excited state of the adsorbed molecules (see, for example, Scheme 1, top right). We have now made use of the intrinsic properties of oxide surfaces to investigate the triplet excited state and reduced forms of the cresyl violet aggregate, (CV+)2, on Si02 and SnO2 ~
~~~
* Author
to whom correspondence should be addressed (Email: Kamat. 1 @nd.edu). Abstract published in Advance ACS Ahsrrucrs. October 15, 1995. @
OO22-3654/95/2099- 16768$09.00/0
SCHEME 1: Idealized Illustration of Energy and Electron Transfer Processes between Molecules Adsorbed on Colloidal Particle9
si02
Sn02
Cresyl Violet, Cv+
* The top-left diagram shows the surface-promoted energy transfer between a sensitizer and a dye-aggregate on SiOz, and the top-right diagram shows the surface-mediated photosensitized reduction of a dyeaggregate on Sn02 . The structure of the cresyl violet monomer ( C V ) is also shown.
colloids. H-aggregates of cresyl violet do not produce longlived excited states with direct excitation. However, coadsorption of a photosensitizer (ruthenium" trisbipyridyl complex), R~(bpy)3~+, on the colloid surface opens up the possibility of initiating triplet energy transfer (on Si02 surfaces) and electron transfer (on SnO2 surfaces). Optical excitation of Ru(bpy)32+ results in the population of a relatively long-lived metal-to-ligand charge-transfer (MLCT)excited state (z= 580 ns in an aqueous solution at room temperature), which is simultaneously more reducing and more oxidizing than the precursor ground ~ t a t e . ~ ~ - ~ This long lifetime of the MLCT excited state allows a number of interesting energy transfer or redox processes to occur following excitation. Photophysical and photochemical properties of R~(bpy)3~+ adsorbed onto inorganic oxides, such as Si02,31-45 and Ti02 have been extensively studied. Recently, ruthenium complexes have also been employed to sensitize SnO2 colloid^^*-^^ and nanocrystalline thin fiims.6~~63 The surface photochemical processes that elucidate triplet excited state behavior and photosensitized reduction of cresyl violet aggregates in colloidal Si02 and SnO2 suspensions are reported here. 0 1995 American Chemical Society
Photochemistry on Surfaces
J. Phys. Chem., Vol. 99, No. 45, 1995 16769
Experimental Section Materials. SnO2 colloidal suspension (18%) was obtained from Alfa Chemicals and used without further purification. Si02 colloidal aqueous suspension (14.5 %) was obtained from NALCO Chemical Company. The particle diameter of these colloids is in the range 30-50 A. All the colloidal concentrations indicated in this study are expressed as particle concentrations. (Our estimate of particle concentration is based on the assumption that the colloidal particles are spherical in size with density values of 2.2 and 6.95 g/cm3 for Si02 and SnO2, respectively.) R ~ ( b p y ) 3 ~was + obtained from Sterm Chemicals and cresyl violet was obtained from Exciton. All other chemicals and solvents were analytical reagents of the highest available purity. In order to estimate the negative charge on each particle, we employed a laser flash photolysis method as described in our earlier studies.38 By employing an anionic dye as triplet sensitizer (Rose Bengal) and cationic quencher (methyl viologen), we monitored the triplet lifetime of the sensitizer at different concentrations of negatively charged Si02 (or Sn02) colloids. In the absence of Si02 colloids, the triplet lifetime is significantly lower since methyl viologen quenches the excited triplet via diffusion kinetics. Upon addition of Si02 collords, the cationic quencher is adsorbed onto the colloids, and thus, the effective solution concentration of the quencher decreases. This results in an increase in the lifetime of the sensitizer triplet. The point at which the addition of colloid makes no further increase in triplet lifetime is considered to be the electrostatic neutralization point. By employing this method, we estimate the average negative charges accumulated on each Si02 and SnO2 colloidal particle (pH 9) to be 32 and 44, respectively. Absorption spectra were recorded with a Perkin-Elmer 3840 diode array spectrophotometer. Emission spectra were recorded with a SLM-8 100 spectrofluorometer system. Laser Flash Photolysis Experiments. Nanosecond laser flash photolysis experiments were performed with a Quanta Ray Model CDR-1 Nd:YAG system using a 355 nm (third harmonic) The laser laser pulse (-6 ns laser pulse width) for excitation.@' output was suitably attenuated to less than 10 mJ/pulse and defocused to minimize the multiphoton process. The experiments were performed in a rectangular quartz cell of 6 mm path length with a right angle configuration between the direction of laser excitation and the direction of analyzing light. The photomultiplier output was digitized with a Tektronix 7912 AD programmable digitizer. A typical experiment consisted of a series of five replicate shots per single measurement. The average signal was processed with an LSI-11 microprocessor interfaced to a VAX computer. Picosecond laser flash photolysis experiments were performed with 355 nm laser pulses from a mode-locked, Q-switched Quantel YG-501 DP Nd:YAG laser system (output 2-3 mJ/ pulse, pulse width 18 ps). The white continuum picosecond probe pulse was generated by passing the fundamental output through a D20/H20 solution. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced to an IBM-AT computer. The details of the experimental setup and its operation are described e l ~ e w h e r e . ~Time ~ . ~ ~zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported in this study have an experimental error of &5%.
-
-
0.6
t Wavelength (nm)
Figure 1. Absorption spectra of 10 pM CV' in (a) aqueous solution, (b) 4.5 pM colloidal SiO2, and (c) 4.5 pM colloidal SnOz suspension.
Results and Discussion Aggregation of Cresyl Violet on Si02 and SnO2 Colloids. The aggregate formation of cresyl violet can be observed upon addition of small amounts of Si02 or SnO2 colloidal suspension to the aqueous solution of the dye. Figure 1 shows the absorption changes observed upon adsorption of the dye onto the negatively charged colloid surface. The monomeric dye has a characteristic absorption maximum at 585 nm in aqueous solution. In the colloidal Si02 and SnOz suspension a blue shift in the absorption maximum is seen with a maximum at 520 nm. The appearance of the 520 nm band is at the expense of the monomeric dye, as evidenced by the decreased absorption at 585 nm. The blue shift in the absorption maximum, as well as broadness of the absorption band, is indicative of the fact that these aggregates are H-type. I t should be noted that such aggregates also coexist with monomers in concentrated aqueous dye solutions and exhibit a broad shoulder around 520 nm. The equilibrium between the dye and the colloid surface can be expressed as
-
+ (s~o,)(cv+)~-(s~o,)
(la)
+
(1b)
~CV+ or
~ C V + (s~o,)
Q
(cv+),-(sno2>
The strong electrostatic interaction between cationic dye molecules and negatively charged Si02 or Sn02 colloids in the present experiments led to close packing of dye molecules on the colloid surface. The quadratic dependence of the 520 nm absorption on the dye concentration suggests that these dyeaggregates consist of dimer units. Such surface-promoted aggregation has also been observed in the dimerization of 1,4anthracence sulfonate on alumina-coated silica3' and cyanine dyes on s i l i ~ a . ~ . ' ~ . ~ ' . ~ ~ Emission Measurements. Cresyl violet, CVt, in its monomeric form is a strongly fluorescing dye with an emission maximum at 630 nm in aqueous solutions.'8,20s21 Figure 2 shows the fluorescence spectra of cresyl violet in water at different concentrations of SnO2 colloids. With increasing Sn02 concentration, the fluorescence intensity of CV+ decreases and disappears when the particle concentration of SnO2 is greater than 0.2 pM. The excitation was at 530 nm, which is an isosbestic point for the adsorption of the monomeric and aggregate forms of the dye. Similar behavior was also observed in the presence of Si02 colloids, thus confirming our observation that the cationic dyes strongly interact with the colloid surface.
Liu et al.
16770 J. Phys. Chem., Vol. 99, No. 45, 1995
+
Ru(bpy),*+* k,,R ~ ( b p y ) ~ ~heat + Ru(bpy)
