J. Phys. Chem. B 1999, 103, 9255-9261
9255
Spectroscopy and Ion-Electron Recombination Kinetics of Radical Ions of Anthracenes and Substituted Anilines on Silica Gel David R. Worrall,† Siaˆ n L. Williams,† Francis Wilkinson,*,† Jill E. Crossley,† Henri Bouas-Laurent,‡ and Jean-Pierre Desvergne‡ Department of Chemistry, Loughborough UniVersity, Loughborough, Leicestershire LE11 3TU, U.K., and Photochimie Organique, CNRS UMR 5802, UniVersite Bordeaux 1, F-33405 Talence Cedex, France ReceiVed: March 17, 1999; In Final Form: June 5, 1999
The spectra of the radical cations produced following laser excitation of a range of anthracene derivatives and amines adsorbed on silica gel are reported and their geminate ion-electron recombination kinetics investigated. Silica gel is shown to be a convenient medium in which to study unstable species such as radical ions which are either very short-lived or inaccessible in a solution environment. Triplet-triplet absorption spectra for dialkyl- and dialkoxy-substituted anthracenes adsorbed on silica gel are consistent with solution phase studies. In contrast to solution phase studies, radical cations from these molecules are readily observed on silica gel and are produced via a multiphoton ionization route. The positions of the radical cation absorption spectra, in contrast to both the ground state and triplet-triplet absorption spectra, are a sensitive function of both substituent and substitution patterns. For substitution across the short molecular axis, electron-donating substituents elicit a hypsochromic spectral shift. In contrast, substitution by electron-donating substituents in the 2,3- or 2,6-positions give rise to bathochromic shifts in the radical cation absorption spectra. Also studied are the kinetics of geminate ion-electron recombination, which when compared with measured oxidation potentials demonstrate that charge transfer energetics are not the only factor controlling the observed recombination rates.
Introduction The photochemistry and photophysics of polynuclear aromatic hydrocarbons adsorbed to insulator surfaces have been the subject of much recent study.1-12 These have shown through the study of triplet-triplet annihilation,1,13,14 excimer formation,7 and energy15,16 and electron transfer2,3,16 that adsorbates can show considerable mobility on such surfaces. We have recently published data regarding the production and decay of both the anthracene triplet state and radical cation on silica gel,1 and we have studied in detail the kinetics of electron transfer from aromatic amines and azulene to the anthracene radical cation2 and the effect of electron diffusion on the observed deactivation rates.3 The triplet state, as in solution, is produced in a monophotonic process by intersystem crossing from the first excited singlet state, while the radical cation is produced via multiphoton ionization.1,2 The decay of the radical cation at early times following laser induced production is dominated by geminate recombination, while at longer times bulk electron diffusion becomes the dominant ion-electron recombination pathway.2 In this paper we present the triplet-triplet and radical cation absorption spectra for a range of anthracene derivatives and amines adsorbed to silica gel following laser photolysis. In common with anthracene itself under such conditions, both triplet-triplet absorption and radical cation absorption is detected. Previous publications1-12,16-33 have shown that supports such as silica gel provide an environment which can stabilize reactive intermediates such as radical cations, thus * To whom correspondence should be addressed. E-mail: F.Wilkinson@ lboro.ac.uk. † Department of Chemistry. ‡ Photochimie Organique.
rendering them amenable to convenient study. In this study we present the first observation and characterization of the absorption spectra of radical cations of a range of symmetrically disubstituted anthracenes and demonstrate correlations between substitution patterns and spectral properties. The spectral properties of the radical cation absorption are a sensitive function of the substitution pattern on the derivative and on the nature of the substituent(s). In many of the studies of electron and energy transfer kinetics on surfaces to date, the issue as to what factors actually control the rates of the observed reactions has not been properly addressed. While in solution the rates of diffusion of small molecules can be considered comparable, on a surface such as silica gel the rate of diffusion will be a function of the heat of adsorption34 which will in turn be determined by the structure of the molecule concerned. Hence it is not straightforward to separate energetic factors from entropic or diffusional constraints. In this study the kinetics of geminate ion-electron recombination for a number of anthracene derivatives and amines when adsorbed to silica gel have been investigated using diffuse reflectance laser flash photolysis, and these rates compared as a function of the measured oxidation potential. We show that there is not a simple correlation between the free energy for the back-electron-transfer process from the surface to the radical cation and the rate of geminate recombination for either the amines or anthracene derivatives, indicating that other factors play an important role in determining the rate of such a process on silica gel. Experimental Section Silica gel (Davisil grade 635, Aldrich) was dried under vacuum while heating to 120 °C for 8 h and was maintained
10.1021/jp9909513 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/18/1999
9256 J. Phys. Chem. B, Vol. 103, No. 43, 1999 under vacuum for a further 16 h. The anthracene derivative was dissolved in n-hexane (spectrophotometric grade, Aldrich), previously dried by reflux over calcium hydride, and added to the dried silica gel. After equilibration for 3 h, the solvent was removed under vacuum and the sample dried to 5 × 10-5 mbar. Samples were then sealed within glass cuvettes under vacuum. Ground-state diffuse reflectance spectra were recorded using a Perkin-Elmer Lambda Bio 40 spectrophotometer equipped with a Spectralon integrating sphere. Fluorescence spectra were recorded using a Spex FluoroMAX spectrofluorimeter using front surface geometry. Diffuse reflectance laser flash photolysis studies were carried out exciting with the third or fourth harmonic (355 nm, 8 ns fwhm, 20 mJ per pulse; 266 nm, 8ns fwhm, 30 mJ per pulse) of an HY200 Nd:YAG laser (Lumonics). The analyzing source is a 275 W xenon arc lamp, detection of transient diffuse reflectance changes being with an R928 photomultiplier (Hamamatsu) via an f/3.4 grating monochromator (Applied Photophysics). Digitization was achieved with a 2432A digital oscilloscope (Tektronix) or a 9845 transient digitizing card (EG&G). Time-resolved transient emission measurements were performed using a gated intensified photodiode array system (EG&G Princeton Applied Research) described in detail elsewhere.3 Anthracene (Anth) was scintillation grade (Sigma) and used as received. 9-Methylanthracene (9-Me) was supplied by Eastman Kodak and used as received. 9-Carboxylic acid anthracene (9-COOH), 9-phenylanthracene, 9-cyanoanthracene (9-CN), and 9-methoxyanthracene were reagent grade (Aldrich) and used as received. 9,10-didecyloxyanthracene (9,10-DDO), 9,10-dipropylanthracene (9,10-DP), 9,10-dimethylanthracene (9,10-DMe), 2,3-didecyloxyanthracene (2,3-DDO), 2,3-dimethoxyanthracene, 2,6-didecyloxyanthracene (2,6-DDO), 1,5-didecyloxyanthracene (1,5-DDO), and 1,5-didecylanthracene (1,5-DD) were prepared according to the method published elsewhere.35 N,N,N′,N′-Tetramethyl-1,4-phenylenediamine (TMPD), N,Ndimethylaniline (DMA), triphenylamine (TPA), N,N-dibutylaniline (DBA), and N-methylaniline (NMA) were supplied by Aldrich and used as received. Cyclic voltammetry was used to determine the oxidation potentials of both the anthracene derivatives and the amines. Measurements were carried out using an EG&G Princeton Applied Research model 173 potentiostat with a Princeton Applied Research model 179 digital coulometer interfaced using a Maclab/4e data recording and analysis system. The oxidation potentials (Ep) were measured against a saturated calomel electrode (SCE) using a platinum wire electrode as the working electrode and a platinum gauze as the counter electrode, with a voltage scan rate of 100 mV s-1. The amines and anthracenes were dissolved to a concentration of approximately 1mM in 0.1 M tetra-n-butylammonium tetrafluoroborate in acetonitrile, where solubility permitted. Ep values on the positive voltage sweep are reported since in all instances the oxidation was not fully reversible, indicative of radical reaction prior to the reductive sweep. Ferrocene was used as a standard compound and gave a value for (Epa + Epc)/2 under these conditions of 0.43 ( 0.01V versus SCE. Results Ground state absorption spectra of the anthracene derivatives studied here have been reported previously in solution and in thin polymer films,36 where the polarizations of the La, Lb, and Bb transitions have been elucidated. The ground state spectra of the derivatives are not substantially changed on adsorption to silica gel, and as is the case with anthracene1 the spectra
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Figure 1. Transient difference spectra showing triplet-triplet absorption for (a) anthracene (ground-state concentration 1.7 µmol g-1). (b) 9-carboxylic acid anthracene (1.7 µmol g-1). (c) 9-phenylanthracene (1.7 µmol g-1). (d) 2,3-didecyloxyanthracene (1.5 µmol g-1) adsorbed on silica gel.
TABLE 1: Quantum Yields of Fluorescence OF and of Triplet State Production OT in Fluid Solution. See References for Details compound
φT
φF
anthracene37 9-methylanthracene37 9-phenylanthracene37 9-carboxylic acid anthracene38 9,10-dimethylanthracene37 9,10-dipropylanthracene 9,10-didecyloxyanthracene35,36 9-cyanoanthracene37 2,3-didecyloxyanthracene35,36 2,6-didecyloxyanthracene36 1,5-didecyloxyanthracene35,36
0.71 0.67 0.51 0.03
0.30 0.33 0.49 0.04 0.89
0.14 0.04 0.72 0 0.77
0.86 0.93 0.19 1.00 0.28
show that the environment encountered by the molecules on the surface of silica gel is very similar to that of a polar solvent. (a) Anthracene Triplet-Triplet Absorption Spectra. We have shown in a previous publication1 that the triplet state of anthracene is populated by a monophotonic process and the radical cation is formed by multiphoton ionization; consequently, at low laser fluences the transient absorption is dominated by triplet-triplet absorption. Comparison of triplet-triplet absorption intensities following excitation with low laser fluences reveals that the relative magnitudes of the triplet-triplet absorptions are consistent with the relative values of the triplet quantum yields φT determined in solution35-37 and given in Table 1. Triplet-triplet absorption spectra are shown in Figure
Ions of Anthracenes and Amines
J. Phys. Chem. B, Vol. 103, No. 43, 1999 9257
Figure 2. Transient difference spectra showing radical cation absorption for anthracene (ground-state concentration 1.7 µmol g-1), 9-cyanoanthracene (1.9 µmol g-1), 2,3-didecyloxyanthracene (2,3-DDO) (0.9 µmol g-1), 2,6-didecyloxyanthracene (2,6-DDO) (1.9 µmol g-1), 1,5-didecyloxyanthracene (1,5-DDO) (0.7 µmol g-1), and 9,10-didecyloxyanthracene (9,10-DDO) (1.5 µmol g-1) adsorbed on silica gel.
1 for those compounds where triplet quantum yields are sufficiently high for spectra to be measured. Clearly on the basis of these spectra there is little influence of substitution pattern on the position of the maximum in the triplet-triplet absorption spectrum.
TABLE 2: Radical Cation Absorption Maxima Observed Following Laser Excitation of the Anthracene Derivatives on Silica Gel
(b) Anthracene Radical Cation Absorption Spectra. Excitation of the samples at higher laser fluences results in the production of both triplet-triplet absorption (where quantum yields are high) and absorption due to the radical cation. However, the rate of relaxation of the triplet state is very much faster than that of the radical cation,1 and therefore at longer times, the transient difference spectrum is exclusively due to the radical cation absorption. This absorption is assigned as due to the radical cation on the basis of electron transfer studies involving aromatic amines, on which basis we have previously assigned the anthracene radical cation absorption on silica gel.2 Most of the spectra presented here represent the first observation of radical cation absorption spectra for symmetrically disubstituted anthracenes. The radical cation transient difference spectra are shown in Figure 2; these may be treated as reflectance spectra above 400 nm where there is no contribution to the spectrum from ground state depletion. Clearly in contrast to the triplet-triplet absorption spectra, the radical cation absorption
anthracene 9-phenylanthracene 9-carboxylic acid anthracene 9-methylanthracene 9,10-dimethylanthracene 9,10-dipropylanthracene 9,10-didecyloxyanthracene 9-cyanoanthracene 1,5-didecylanthracene 1,5-didecyloxyanthracene 2,3-didecyloxyanthracene 2,6-didecyloxyanthracene
compound
surface loading/ µmol g-1 λ1/nm λ2/nm 1.7 1.7 1.7 1.7 2.0 1.5 1.5 1.9 1.0 0.7 1.5 0.9
330 320 320 330 320 310 360 330 330 395 345
425 420 425 420 420 420 420 450 410 410 470 440
λ3/nm 715 715 710 685 655 655 560 (broad) 770 620 570 (broad) 725 750
spectra are profoundly affected by the substitution pattern. The maxima in the radical cation absorption spectra are summarized in Table 2. Figure 3 shows the effect of changing the nature of the substituent in the 9,10-position on the radical cation spectrum. Electron donating substituents have the effect of shifting the spectrum to shorter wavelengths, with a broadening of the
9258 J. Phys. Chem. B, Vol. 103, No. 43, 1999
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Figure 3. Transient difference spectra showing radical cation absorption for 9-carboxylic acid anthracene (ground-state concentration 1.7 µmol g-1), 9,10-dimethylanthracene (2.0 µmol g-1), 9-cyanoanthracene (1.9 µmol g-1), and 9,10-didecyloxyanthracene (1.5 µmol g-1) adsorbed on silica gel. The peak of the anthracene radical cation transient difference spectrum is shown as a dotted line for comparison.
absorption bands. The addition of an electron-withdrawing group in the 9-position has the opposite effect; 9-cyano substitution shifts the absorption spectrum to longer wavelengths. Substitution in the 1,5-position gives a similar effect to substitution in the 9,10-position, while 2,3- and 2,6-substitution does not follow this same pattern (Table 2); the shift of absorption with substitution is dependent upon the molecular axis substituted. For the ground state of anthracene and its derivatives studied here, the Bb transition is known to be long-axis polarized, and for all the derivatives studied here the Lb transition (long axis polarized) is at higher energy than the La transition (short axis polarized), although both occur in a similar spectral region.36 The observation that the spectral shift observed for the radical cation is a sensitive function of both the nature of the substituent and the axis on which the substituent appears may suggest that the transition moments giving rise to the observed radical cation absorptions are strongly polarized along particular molecular axes; further discussion of this phenomenon is beyond the scope of this paper but is the subject of ongoing investigations. (c) Radical Cation-Electron Recombination Kinetics of Anthracene Derivatives. As shown in previous publications,1,2 anthracene and substituted anthracenes readily form radical cations on the surface of silica gel as a result of multiphoton ionization. Previously2 we have shown that for low (
