Langmuir 1992,8, 501-507 501

Photophysical and Photochemical Studies of N,N,N’,N’-Tetramethylbenzidine on y-Alumina

Surapol Pankasem and J. Kerry Thomas*

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556

Received July 29, 1991. In Final Form: October 18, 1991

Fluorescence and photoionization studies of NJVP,N’-tetramethylbenzidine, TMB, adsorbed on y- alumina are reported. Both steady-state and time-resolved studies indicate that there are a variety of surface active sites for TMB on the y-alumina. The physisorption sites which are characterized by surface hydroxyl groups dominate at low pretreatment temperatures, the cation sites or the Lewis acid sites dominate at high pretreatment temperatures, and the charge-transfer complex sites which are a combination of the physisorption sites and the cation sites dominate at intermediate temperatures. The mechanism of photoionization of TMB on y-alumina is monophotonic. Both the singlet and the triplet excited states of TMB are quenched by nitrous oxide. The cation radical of TMB adsorbed on y-alumina exhibits near-IR emission bands with maxima at 1100 and 1250 nm.

Introduction Photochemistry in heterogeneous systems has become

a focus of attention of photochemists for decades,lp2 with metal oxides such as silica and alumina receiving much interest re~ently.~-T Although there have been a number of publications reporting on photochemistry of organic molecules on alumina, most studies have concentrated mainly on polyaromatic hydrocarbons.

In two earlier papers, fluorescence probing and diffuse reflectance laser-flash photolysis were used to investigate photophysical and photochemical behavior of pyrene, pyrene derivatives, and 1,l’-binaphthyl on y-alumina.81~ The results have revealed characteristics of the adsorption sites and the relative mobilities of adsorbed species a t these adsorption sites. In this paper, an attempt is made to extend the studies to the photochemical behavior of aromatic amines. N,N,”,”-Tetramethylbemidine, TMB, was chosen due to its low ionization potential (6.1-6.8 eV)l0

and easy production of its cation radical, and the cation radicals of TMB have been investigated extensively in a variety of systems including micelles,11-15 c1ays,16 cyclo- ~~ ~

(1) Thomas, J. K. In The Chemistry of Excitation at Znterjaces; ACS Monograph Series 181; American Chemical Society: Washington, DC, 1984. --- -.

(2) Thomas, J. K. J . Phys. Chem. 1987, 91, 267. (3) Bauer,R. K.; Borenstein, R.;deMayo, P.; Odaka, K.;Rafalska,M.;

Ware, W. R.; Wu, K. C. J . Am. Chem. SOC. 1982, 104,4635. (4)Bauer, R. K.; de Mayo, P.; Ware, W. R.; Wu, K. C. J. Phys. Chem.

(5) de Mayo, P.; Nakamura, A.; Tsang, P. N. K.; Wong, S. K. J. Am.

(6) Beck, G.; Thomas, J. K. Chem. Phys. Lett. 1983, 94, 553. (7) Oelkrug, D.; Flemming, W.; Fiillemann, R.; Gtinther, R.; Honnen,

(8) Pankasem, S.; Thomas, J. K. J . Phys. Chem. 1991,95,7385. (9) Pankasem, S.; Thomas, J. K. J . Phys. Chem. 1991, 95,6990. (10) Fulton, A.; Lyons, L. E. A u t . J . Chem. 1968, 21, 873. (11) Nakajima, A.; Akamutau, H.Bull. Chem. SOC. Jpn. 1969,42,3030. (12) Alkaitis, S. A,; Gritzel, M. J . Am. Chem. SOC. 1976, 98, 3549. (13) Narayana, P. A.; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC. 1981,

1982,86,3781.

Chem. SOC. 1982,104,6824.

W.; Uhl, S. Pure Appl. Chem. 1986,58, 1207.

103,3603.

dextrins,17 and polymer lattices.18-20 Previous techniques employed have been resonance Raman spectroscopy, electron spin resonance, electron spin-echo modulation, and optical spectroscopy.

In this study, the photophysical and photochemical characterization of TMB adsorbed on y-alumina is pre- sented. Short-lived excited and ionic transients of the adsorbed species, the singlet excited state, the triplet, and the cation radical, will be discussed in detail as well as their reactions with coadsorbed nitrous oxide. In addition to photophysical and photochemical studies in the UV- vis region, the near-IR emission of TMB cation radical will also be reported.

Experimental Section Materials. NJV,”,”-Tetramethylbemidine (Aldrich Chem-

ical Co.) was recrystallized twice from benzene. yAlumina was obtained from La Roche Chemicals, and has a BrunauerEm- mett-Teller (BET) surface area of 200 m2/g (nitrogen adsorption). Specific properties of this alumina at various pretreatment tem- peratures are described else~here.~,~ Cyclohexane (Aldrich, HPLC grade) was used as received.

Sample Preparation. y-Alumina was activated at particular temperature in an oven for 20 h, followed by coolingin a desiccator. Predetermined amounts of TMB solutions in cyclohexane were then added to 0.5 g of activated alumina, and the solvent was then removed under vacuum. After the samples were dried, they were kept under vacuum for at least 2 h to eliminate the oxygen. The monolayer capacities of alumina surfaces were determined by adsorption from liquid cyclohexane. These adsorption isotherms exhibit saturation at about 30 mg of TMB/g of alumina depending on the pretreatment temperature of alumina.

Methods. Diffuse reflectance spectra were obtained with a Perkin-Elmer 552 UV-vis spectrophotometer. Steady-state emission spectra were obtained with a SLM spectrofluorometer, model SPF 500C. The diffuse reflectance laser photolysis experiments were performed with a 337.1-nm laser pulse (&mJ pulse width 6 ns) from a Lamda-Physik XlOO Nz laser. The

(14) Narayana, P. A.; Li, A. S. W.; Kevan, L. J . Am. Chem. SOC. 1982,

(15) Wolff, T.; Kevan, L. J . Phys. Chem. 1989, 93, 2065. (16) Kovar, L.; Dellaguardia, R.; Thomas, J. K. J. Phys. Chem. 1985,

(17) Hashimoto, S.; Thomas, J. K. J . Phys. Chem. 1984,88, 4044. (18) Baglioni, P.; Rivara-Minten, E.; Kevan, L. J . Phys. Chem. 1989,

(19) Tsuchida, A.; Nakao, M.; Yoshida, M.; Yamamoto, M.; Wada, Y.

(20) Rivara-Minten, E.; Baglioni, P.; Kevan, L. J . Phys. Chem. 1988,

104,6502.

88, 3595.

93, 1570.

Polym. Bull. (Berlin) 1988,20, 297.

92, 2613.

IE) 1992 Amerirnn Chemirnl SnriPt.v

Langmuir, Vol. 8, No. 2, 1992 Pankasem and Thomas

acid sites on the surface. During dehydration, OH groups which have low acidity combine with hydrogen atoms from the neighboring sites with stronger acidity, forming water molecules. This process creates an anion vacancy (Lewis acid site) which exposes coordinatively unsaturated (CUS) aluminum cations and a cation vacancy (CUS oxygen).

In earlier work,8 the fluorescence probing of y-alumina with pyrene and its derivatives has demonstrated that there exist a variety of surface active sites, namely, physi- sorption sites, charge-transfer complex sites, and cation sites. Physisorption sites, where adsorbed molecules interact with the surface through hydroxyl groups, dom- inate on alumina surfaces of low pretreatment tempera- tures. The cation sites or the Lewis acid sites, which are produced from surface dehydroxylation at high pretreat- ment temperatures, are responsible for cation radical formation. The charge-transfer complex sites, which are a combination of the physisorption sites and the Lewis acid sites, are present at intermediate pretreatment tem- peratures.

502

4 -

0.05 - 0.04 - 0.03 - 0.02 - 0.01 -

240 280 320 360 400 44U 4 d O 520 560

Wavelength, nm

Figure 1. Diffuse reflectance spectra for TMB adsorbed on aluminaat two different pretreatment temperatures: (A) 140 O C

and (B) 750 "C. Loading 4 X lo-' mol/g.

time-resolved fluorescence experiments were performed with a 337-nm laser pulse (PRA LN 100 NZ laser, 100-J pulse width 1 ns). The details of apparatus and procedures for the time- resolved fluorescence studies and the diffuse reflectance laser photolysis are described previ~usly.~J' Steady-state photolysis experiments were performed with an Oriel 100-W Xe lamp. A UV clear filter (Kopp CS 5-96) was used to cut off the stray of light of wavelength below 330 nm, and diffuse reflectance spectra of the samples were taken immediately after irradiation. For near-IR emission experiments, the details of detector unit

and experimental setup have been described elsewhere.21*z2 The electron spin resonance (ESR) spectrum was recorded at

room temperature on a Varian E-line Century Series ESR spectrometer.

Data Treatment. The diffuse reflectance spectra of adsorbed species in this study will be presented in the form of 1 -RT, where RT is the relative reflectance defined by the ratio of the reflectance of the sample to the reflectance of the background. In particular cases where the data needed to be treated quantitatively, the Kubelka-Munk function was calculated by using MgO as a

Results and Discussion Nature of Adsorption Sites for TMB on Alumina.

The characteristic diffuse reflectance spectra of TMB ad- sorbed on y-alumina surfaces (4 X mol/g which cor- responds to 3.0 % monolayer coverage) are shown in Figure 1. On alumina of low pretreatment temperature Ta (Ta = 130 "C), TMB exhibits two distinct absorption bands, one at short wavelength with a peak maximum at 310 nm and the other one at longer wavelength with a maximum at 470 nm. The short wavelength absorption band is identified as the physisorbed TMB, in agreement with that.of TMB in polar solvents such as methanol and water." The structured band at long wavelengths is identified as the cation radical of TMB similar to that found in photolyzed micellar solutions.12 As with other metal oxides, the surface of y-alumina is

generally covered with a number of hydroxyl layers. It is known that several types of hydroxyl sites are present on the surface of y - a l ~ m i n a . ~ ~ + ~ ~ Pretreatment or heating alumina a t high temperatures leads to dehydroxylation of the surface hydroxyl groups and the formation of Lewis

standard.s~9s23

(21) Iu, K. K.; Thomas, J. K. J. Am. Chem. SOC. 1990,112,3319. (22) Pankasem, S.; Iu, K. K.; Thomas, J. K. J. Photochem. Photobiol.

A, in press. (23) (a) Kortiim, G. In Reflectance Spectroscopy; Springer-Verlag:

Berlin, 1969. (b) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectros- copy; Interscience: New York, 1966.

(24) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960.64, 1526. (25) Knozinger, H.; Ratnasamy, P. Catal. Reo-Sci. Eng. 1981, 17, 31.

Low Temp High Temp

fp@&@ OH OH

physisorpdon Sites Charge-Transfer Complex Cation Sites Sites

+represents an AI% on the layer below the surface

The physisorption between polyaromatic hydrocarbons and surfaces of metal oxides has been described as bonding between surface hydroxyl groups and the .rr-electron system of polyaromatic hydrocarbon^.^^^^^ However, the physi- sorption between TMB and surface hydroxyl groups may occur via hydrogen bonding between nonbonding electrons from nitrogen atoms and hydroxyl groups. Lewis acid sites (CUS cations) are strong electron acceptors which may accept nonbonding electrons from the nitrogen atom of a TMB molecule, thus forming a TMB cation radical (TMB+). The structured absorption band at 470 nm of TMB on alumina is identical to that reported for TMB+ in other systems, for example, in the irradiated micellar solution of sodium dodecyl sulfate.12 Later cation fluo- rescence data will be given which further support the assignment of the 470-nm absorption to the TMB cation. Cation radicals of pyrene were observed only with pyrene on alumina at high pretreatment temperatures (Ta > 450 "C), but the cation radicals of TMB were observed on alumina even a t Ta = 130 "C. This is a consequence of the lower ionization potential of TMB (6.1-6.8 eV) compared to that of pyrene (7.55 eV).28929 The increase of the TMB cation radical (470-nm absorption band) with increasing pretreatment temperature is attributed to the larger number of the Lewis acid sites produced on higher Ta surfaces.25

Oelkrug et al. studied acridine adsorbed on alumina and silica.30 They proposed that, in addition to hydrogen bonding between the hydroxyl groups and the acridine nitrogen atom, on alumina a t low pretreatment temper- atures protonated acridine is formed from chemical

(26) Anderson, J. H.; Lombardi, J.; Hair, M. L. J. Colloid Interface Sci. 1975, 50, 519.

(27) Pohle, W. J. Chem. SOC., Faraday Trans. 1 1982, 78,2101. (28) Birks. J. B.: Slifkin. M. A. Nature 1961.191.761. (29) Briegleb, G:; Czekdla, J. Z. Elektrochem. 1969, 63, 6. (30) Oelkrug, D.; Uhl, L.; Wilkinson, F.; Willsher, C. J. J. Phys. Chem.

1989,93,4551.

N,N,N',N'-Tetramethylbenzidine on yrllumina

33806

Langmuir, Vol. 8, No. 2, 1992 503

10 I 1

V Figure 2. ESR spectrum of TMB+ on alumina at T. = 140 O C

(loading 4 X 10-7 mol/g), recorded at 9.5 GHz and at room tem- perature.

bonding via the nitrogen atom. At high pretreatment tem- peratures a u-bond between the nitrogen atom and the Lewis acid surface sites is formed. In the present study, the formation TMB cations from the interaction between TMB and the strong Lewis acid sites (cation sites) is observed instead of formation of a u-bond between the nitrogen atom and the Lewis acid site. This indicates that bonding between the Lewis acid and the nitrogen atom of TMB is not a simple covalent a-bond, but complete electron transfer. The protonated TMB, TMBH+, has been reported in several systems.16 It generally exhibits absorption and emission bands with maxima around 310 and 400 nm, respectively. Due to the similarity between the absorption band and the emission bands of TMB and TMBH+, the existence of the protonated TMB on alumina surfaces cannot be identified unambiguously. However, the results from the fluorescence probing of alumina with l-aminopyrene show no evidence of proton transfer from the alumina surface to the amino group for alumina a t low pretreatment temperatures.8 I t is therefore inferred that proton transfer from alumina to TMB is not significant in our studies.

Other evidence for formation of TMB cation radicals on y-alumina is provided by ESR studies. The ESR spectrum of TMB+ adsorbed on alumina a t Ta = 140 OC is shown in Figure 2. The loss of hyperfine splittings in the spectrum compared to those exhibited in anionic and cationic micelles13J4 can be explained in terms of a cation with restricted rotational freedom in which the unpaired electron has strong anisotropic interaction with a certain part of the molecule. This phenomenon has been observed for several solid surfaces.31

Singlet Excited State of TMB. The fluorescence of TMB in various media (from hydrocarbons to polar solventa, micelles, and acids) has been reported, with a singlet lifetime of the excited TMB varying from 1 to 10 11s.l' Both steady-state and time-resolved studies of the singlet excited state of TMB on alumina provide infor- mation about the mechanism of the reaction between the singlet excited state of TMB and N2O.

1. Steady-State Fluorescence. The 310-nm excita- tion of TMB adsorbed on alumina (4 X lo-' mol/g) a t Ta = 140 OC gives rise to a broad fluorescence band with a maximum a t 400 nm, as shown in Figure 3. The intensity of this fluorescence band decreases with increasing pres- sure of N20, and the Stern-Volmer plots of N20 fluores- cence quenching at various temperatures are shown in Figure 4. At room temperature (22 "C), a linear rela- tionship is found a t small pressures of N20, but the quench- ing efficiency becomes smaller a t higher pressures. This

. . . . . . . . . . . . . . . . . . . . . 1 320 340 360 380 420 440 460 UIO 5W 520

Wavelength, nm

Figure 3. Fluorescence for TMB adsorbed on alumina at T, = 140 "C (loading 4 X lo-? mol/g) at various concentrations of ad- sorbed NzO from top to bottom, 0,18,37,53,67,87,109,143, and 260 mbar.

0 2 2 %

0 0 10 20 30

[N20IbUIk, 10-3 molldm3

Figure 4. Stern-Volmer plots of quenching of lTMB* (loading 4 X 10-7 mol/g, excitation 310 nm) by N20 observed at 375 nm at various temperatures.

indicates that the mechanism of the quenching is due to adsorption of the quencher on the surface (discussed next), similar to the oxygen quenching of pyrene and 9,lO-diphen- ylanthracene on the silica surfaces.32

If the time-limiting factor is diffusion of N2O in the gas phase to the surface, then the quenching efficiency would increase with temperature in the same fashion as oxygen quenching of the singlet excited state of pyrene on alumina.8 A decrease of the quenching efficiency with increasing temperature, which is shown in Figure 4, supports the idea that quenching takes place via the interaction between adsorbed N2O and the singlet excited state of adsorbed TMB. Unlike 0 2 , which does not significantly adsorb on alumina surfaces (no adsorption was found up to 250 mbar of 0 2 ) , N20 adsorbs well on alumina. A typical adsorption isotherm for NzO on alumina (Ta = 130 "C) at 22 "C is shown in Figure 5.

2. Time-Resolved Fluorescence Studies. The ki- netics of fluorescence decay of TMB adsorbed on alumina dues not follow a single exponential which is a consequence of a differentiation of adsorption sites for TMB on y-

(32) Krasnansky, R.; Koike, K.; Thomas, J. K. J . Phys. Chem. 1990,

(33) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J. Am. 94,4521.

Chem. SOC. 1985,107, 1854. (31) Hall, W. K. J. Catal. 1962, 1, 53.

504 Langmuir, Vol. 8, No. 2, 1992 Pankasem and Thomas

N E

P

f

P

m e 51 1

0 100 zoo

[N 2 OI,,,, L mbar Figure 5. Adsorption isotherm for NzO on alumina (T, = 140 "C) at 23 OC.

alumina. The Gaussian distribution kinetic model, which was developed by A l b e r ~ ~ ~ on the basis of a Gaussian distribution in the free energy of activation or in the logarithm of the measured rate constant, In k, was then used to obtain the average decay rate constant of lTMB*. A number of studies have adopted this model to success- fully analyze decays of the transients in various hetero- geneous systems: the singlet excited state of pyrene and 1,l'-diphenylanthracene on silica gels,32 (SCN)2'- in aque ous suspensions of titanium dioxide,34 the singlet and the cation radical of pyrene on y-alumina,a9 and electron transfer in polyelectrolytes.35

The existence of a distribution of first-order kinetic processes in the fluorescence decay profiles of TMB ad- sorbed on alumina surfaces reflects a distribution of fluorophores on various adsorption sites. These adsorption sites are represented by a combination of physisorption sites and the charge-transfer complex sites as described earlier.8~~ The average lifetime of TMB on alumina at T, = 130 "C in vacuum was found to be 3.9 ns.

The plot of the observed fluorescence decay rate constants Roba, which were calculated from the Gaussian distribution model, for TMB adsorbed on alumina at T, = 130 O C , as a function of adsorbed NzO is shown in Figure 6. The quenching rate constant of lTMB* is calculated to be 7.79 X 1015 m2mol-l 5-1, which is an order of magnitude less than that of oxygen quenching of 1,l'-diphenylan- thracene on silica (6.0 X 10l6 m2 mol-l ~ ~ 1 . 3 ~ The TMB number is also in agreement with the result from steady- state studies.

Photoionization of Adsorbed TMB. The photoion- ization of TMB in homogeneous systems such as micelles and polymer matrices has been investigated exten- sively.12*20 The nature of photoionization in the above systems can be either one- or two-photon. In the earlier work, the photoionization of pyrene adsorbed on alumina has been described as a biphotonic process. Due to the smaller ionization potential of TMB compared to pyrene, the nature of the photoionization of adsorbed TMB could be different from that of adsorbed pyrene.

1. Steady-State Photolysis. Figure 7a shows the diffuse reflectance spectra of TMB adsorbed on alumina at Ta = 130 "C in vacuum, at various times of irradiation.

(34) Draper, R. B.; Fox, M. A. J . Phys. Chem. 1990,94, 4628. (35) Wolszczak, M.; Thomas, J. K. Radiat. Phys. Chem. 1991,38,155. (36) Krasnansky, P. Ph.D. Dissertation, University of Notre Dame,

IN, 1990.

4.25

r

v) m z

lUO

- 3.25 U P

2.25 0 2 4 6 6 1 0 1 2

[N2OIads, lVg mollm2

Figure 6. Observed rate constant for lTMB* on alumina at T, = 140 "C versus adsorbed concentration of NzO.

0.19 0.18

0.17 0.16 0.11 0.14

0.13 0.12

0.11

0.1 0.w 0.M

0.07 0.01

aw 0.04 0.03 0.01 0.01

0 2.0 uo uo am 40(1 uo rsa uo 5ca

Wavelength, nm

0.18 - 0.16

0.11

0.11 P\ aw \ \

0.01 :k 0.01

a u ) a m ~ m H o 4 D o u o r s a u o

Wavelength, nm n

Figure 7. (a) Diffuse reflectance spectra of TMB adsorbed on alumina at T. = 140 "C in vacuum under various irradiation times: (A) 0, (B) 20, and (C) 140 8. (b) Same aa (a) but in the presence of 25 mbar of NzO: (A) 0, (B) 30, and (C) 170 8.

A growth in the structured absorption band a t 470 nm was observed upon irradiation, while the magnitude of the absorption band at 310 nm decreases with irradiation time. The process which takes place under irradiation is therefore attributed to the photoionization:

hu

TMB(ads) -TMB+(ads) + e-(ads) (1) In vacuum, the cation radical yield, which is determined from the magnitude of the 470-nm band, increases with irradiation time; however, in the presence of N20 (25 mbar), Figure 7b, the rate of cation radical formation is smaller

N,N,N',N'-Tetramethylbenzidine on r-Alumina Langmuir, Vol. 8, No. 2, 1992 505

0 100 200

l i m e , sec Figure 8. Steady-state photoionization of TMB on alumina at Ta = 140 O C (loading 4 X mol/g and observed at 470 nm): (0) in vacuum, (0) in the presence of 25 mbar of NzO.

than that in vacuum (Figure 7b). Figure 8 also shows the relative rate of photoionization of TMB as a function of time in vacuum and in the presence of N2O. It was also observed that, in the presence of N20, the absorption band at low wavelengths (-300 nm) is shifted about 15 nm to the blue and the absorption band at high wavelengths (-470 nm) becomes broader. A similar result was also reported in a pulse radiolysis study of TMB in aqueous solutions in the presence of N20 in which an adduct between TMB and hydroxyl radical has been proposed.37 On the other hand, N2O does not affect the photoioniza- tion of TMB adsorbed on alumina at high Ta (Ta = 750 "C) where irradiation of TMB adsorbed on this surface in the presence of N2O does not show any significant difference from that in vacuum. The reaction of N2O with TMB adsorbed on physisorption sites (hydroxyl groups on alumina at low Ta) is different from that where TMB is adsorbed on charge-transfer complex sites (on alumina a t high Ta). This will be discussed in detail later.

2. Diffuse Reflectance Laser Flash Photolysis of Adsorbed TMB. The photolysis of TMB adsorbed on alumina, using a 337.1-nm pulsed laser, produces transients whose absorption spectra are presented in Figure 9a,b. The transient absorption spectrum in vacuum, Figure 9a, shows a broad band with the maximum around 470 nm and long tails extending to both the UV and the red wavelength regions. Figure 9b illustrates the transient absorption spectrum in the presence of oxygen (10 mbar) where a vibronic structured band is observed. The resemblance of this spectrum to the spectrum of TMB adsorbed on alumina at high Ta (see Figure 1) suggests that this is the spectrum of the cation radical of TMB. A decay profile of the transient monitored at 470 nm, which is shown in Figure 10A, is characterized by a fast decay, followed by a decay process with a longer lifetime which changes very little on the experimental time scale. These characteristics of the decay curve may be intepreted as either a decay of a single species at two different environments or the concomitance of decays of two transient species. In the presence of oxygen the rapidly decaying decay portion disappears while the long lifetime decay portion does not change (Figure 10B). A comparison of the behavior of TMB in micellar solutions, where oxygen quenches the triplet of TMB efficiently but does not react with the cation of TMB,12 suggests that the latter case is

(37) RaO, P. S.; Hayon, E. J. Phys. Chem. 1975, 79, 1063.

0.11

0.1

nag

0.08

0.07

4 o'08 0.05

0.04

0.03

0.02

0.01

0 YO sm 420 .(y1 xa am m m 6~

Wavelength, nm

0.06

1 b o.06 4 h

YO 380 420 460 500 540 510 620 660

Wavelength, nm

0.0s IC

m a o o u o ~ y x ) y o y o u o u o

Wavelength, nm Figure 9. (a) Transient diffuse reflectance spectra of TMB ad- sorbed on alumina at Ta = 140 O C in vacuum at various times after the laser pulse: (0) 0, (+) 0.5, ( 0 ) 1, (A) 3 ms. (b) Same as (a) but in the presence of 25 mbar of 02, and at 0 (0) and 3 ms (+) after the pulse. (c) Diffuse reflectance spectra of the cation radical (0) and the triplet (+) of TMB on alumina at Ta = 140 "C. operative, with the fast decay being assigned to the TMB triplet state and the long-lived species to the cation radical of TMB.

The diffuse reflectance spectrum of the triplet of TMB on y-alumina, which is obtained by subtracting the spectrum of the cation radical from the transient spectrum in the vacuum, is shown in Figure 9c along with the spectrum of the cation radical. The spectrum exhibits a rather broad absorption band with a maximum around 460 nm.

3. Reaction of TMB Triplets with Nitrous Oxide. Nitrous oxide quenches the triplet of adsorbed TMB. The

506 Langmuir, Vol. 8, No. 2, 1992

$ ' O D t I

2 -

Pankasem and Thomas

2 -

0 00 , I I I

0 1 2 3 4 5

TIME ( m i 11 i s e c o n d s )

Figure 10. Decays of the transients of TMB adsorbed on alumina observed at 470 nm: (A) Tu = 140 "C in vacuum, (B) Tu = 140 " C with 10 mbar of 02, and (C) Tu = 750 O C in vacuum.

o

x 0

o o o

2 0 . 2 5

U U

0.00 . ,001 ,008 ,012 ,016 .OP

TIME ( m i 11 i s e c o n d s )

[N 201 a d s , i o - 1 o mol/m2

Figure 11. (a) Decays of the triplet state of TMB adsorbed on alumina at T, = 140 "C at various concentrations of adsorbed NtO: from top to bottom, 0, 1.04, 2.09, 3.13, 4.17, 5.22,6.26 X 10-lo mol/m2. (b) Observed rate constant of the triplet TMB versus concentration of adsorbed N20.

time-resolved decay profiles of triplet TMB with various bulk pressures of NzO are shown in Figure l la . Due to the much longer lifetime of the cation radical (50 ms) compared to that of the triplet (7.1 ps), the decay rate constant was analyzed by using the absorption at the long times as the base line. The plots of decay rate constant versus pressure of N20 are presented in Figure llb. The bimolecular quenching rate constants are calculated to be 8.09 X 1014 m2 mol-'sC1.

4. TMB Cation Radicals. In order to elucidate the mechanism of photoionization, a dependency of the cation

o

x 0

o o o . o o ' . * *

0 Triplet 0..

0 .

Cation

" , . , . . 0 20 40 60 80 100

Relative Laser Intensity

Figure 12. Dependency of the triplet (e) and the cation radical yield (0) on the laser pulse (337.1 nm) intensity on alumina at Tu = 140 " C (loading 4 X lo-' mol/g, and excitation 337.1 nm).

0.045 -

R 0.04

3u1 380 410 460 5W 540 580 620

Wavelength, nm

0

Figure 13. Transient reflectance spectra of TMB adsorbed on alumina at T, = 140 "C in the presence of 25 mbar of NzO at various times after pulse: (0) 0 ms, (+) 0.5 ms, and (A) 3 ms.

radical yield on laser pulse intensity was investigated. A relative yield of the cation radical was determined from the absorption at 470 nm in the presence of 25 mbar of 02 where the triplet state is quenched completely. The yield of the triplet was taken from the difference between the absorption in the vacuum and the absorption of the cation radical (in the presence of 02). A linear relationship between the cation radical yield and the relative incident light intensity, as shown in Figure 12, indicates that the mechanism of photoionization of TMB adsorbed on alumina is monophotonic, and similar to those established in methanol and in sodium dodecyl sulfate (SDS) micellar solution.12 The dependence of the triplet yield on the incident light intensity which is almost identical to that of the cation also supports a monophotonic mechanism. Furthermore, it was found that the cation radical yield does not change with oxygen, thus eliminating photolysis of intermediates and supporting a monophotonic process.

Figure 13 shows the transient absorption spectrum of TMB adsorbed on alumina at T, = 130 "C in the presence of 25 mbar of NzO where the triplet of TMB is completely quenched. Although the observed absorption band, which exhibits an absorption maxima at 465 nm, is similar to that of the cation radical of TMB observed in the presence of 02 (Figure 9b), certain differences exist between these spectra. Firstly, the magnitude of the absorption band at 470 nm in the presence of NzO (0.04) is less than that in the presence of 0 2 (0.055). Secondly, the spectrum in the

N,N,N',N'-Tetramethylbenzidine on y-Alumina

presence of N2O is broader than that in the presence of 0 2 , and similar to that found in the steady-state photol- ysis. In oxygenated systems the triplet of TMB is quenched completely by oxygen, and the transient spec- trum then belongs to the cation radical. These differences suggest that there may be some reaction between the triplet

An early photolysis study of N,N,",N'-tetramethyl- p-phenylenediamine (TMPD) in cyclohexane showed that the triplet of TMPD reacted with N2O to form a complex which exhibited a broad absorption band below 500 nm.38 It was also found that the singlet of TMPD reacts with N2O. In a similar way, the broad absorption band of pho- tolyzed TMB on alumina in the presence of N20 can be explained in terms of a complex between the TMB triplet and N20.

On alumina a t Ta = 750 "C, no significant effect of N2O was observed, which is similar to that found in the steady- state photolysis. A lack of reactivity of N20 toward TMB adsorbed on this surface, where the Lewis acid sites dominate, indicates that the photophysical and photo- chemical behavior of TMB adsorbed on these Lewis acid sites differ from those adsorbed on the physisorption sites. Decay of the transient species observed at 470 nm for TMB adsorbed on alumina at Ta = 750 OC (Figure 1OC) shows that amuch smaller amount of the triplet TMB is produced on this surface compared to the surface at Ta = 130 OC. This further supports the concept that the broad absorp- tion band found in TMB-N20 on the low Ta surface is a complex between the triplet of TMB and NzO, as the triplet of TMB is not formed on the high Ta surface.

This suggests that TMB adsorbed on the Lewis acid sites, via a charge-transfer interaction, tends to undergo photoionization, while TMB adsorbed on physisorption sites tends to undergo intersystem crossing to the triplet state which can interact with N2O. Scheme I illustrates the proposed mechanism for these processes under pho- tolysis.

of TMB and N2O.

Scheme I

20 -

10'

Langmuir, Vol. 8, No. 2, 1992 507

30 -

hv TMB - 'TMB* - TMB + hv'

NzO physhrption sites - unknown

N20 - ~ M B * - complex

TMB k T M B + + e-(ads)

The photoinduced cation radicals are separated from electrons by small distances compared to the mean distances between the adsorbates. Consequently, the cations decay through a geminate ion pair recombination leading to the original neutral molecule^.^ It is noted that the decay of the cation radical of TMB is quite different from that of pyrene reported earlierVg Pyrene cation radicals decay via a Gaussian distribution kinetic type with a y parameter of 4.5 which indicates that these radicals are produced at the adsorption sites that are very different from each other. On the other hand, TMB cation radicals decay monoexponentially which indicates that the vari- ation of adsorption sites responsible for the cation for- mation is very small (y parameter - 0). This suggests that the cation radicals of TMB are produced from uniform adsorption sites which could be the surface Lewis acid sites.

hwie acid sitaa

_ _ _ _ _ _ _ ~ _ _ _ _ _ ~ ~ ~ ~~ ~

(38) Richard, J. T.; Thomas, J. K. J. Chem. SOC., Faraday Trans. 1 1970,66, 621.

0 0

0

v - 1000 1100 1200 1300 1400

Wavelength (nm) Figure 14. Near-IR fluorescence spectrum of TMB+ on alumina at T, = 750 O C (loading 4 X lo-' mol/g and excitation 450 nm).

Near-IR Emission of TMB Cation Radical on y- Alumina. The absorption spectrum of the TMB cation radical is characterized by two structured bands. In addition to a structured band in the visible region which exhibits the maximum at 475 nm, the spectrum in the near-IR region shows an absorption band witha maximum at 1010 nm and a small shoulder at 880 nm. Assignments for these absorption bands are described e l ~ e w h e r e . ~ ~ ~ ~

According to Kasha's rule, the emitting electronic level of a given multiplicity is the lowest excited level of that mult ipl i~i ty .~~ Because of the existence of the low-energy absorption bands (visible and near-IR), a visible emission of TMB+ is not expected. A study of near-IR fluorescence of TMB+ indicated that excitation of the radical cation of TMB in a micellar solution of sodium dodecyl sulfate by the 450-nm laser pulse leads to an emission spectrum which exhibits a peak maximum a t 1100 nm with a shoulder a t 1250 nm.22 The spectrum also exhibits agoodminor image with the absorption band at 1010 nm.

The near-IR emission spectrum of the radical cation of TMB produced on the surface of y-alumina is shown in Figure 14. The spectrum exhibits two emission bands with maxima at 1110 and 1270 nm which are similar to that described previously for TMB+ in micellar solution.22

Conclusions The photolysis of TMB adsorbed on different alumina

surfaces in the presence and the absence of N2O has been investigated. Transient species including the TMB sin- glet excited state, the triplet state, and the cation radical were observed. It was found that the singlet excited states and the triplets were produced at the physisorption sites, while the cation radicals were produced at the Lewis acid sites. The mechanism of photoionization is monopho- tonic in nature.

On alumina surfaces at low T,, coadsorbed N2O reacts with both the singlet and the triplet TMB. The latter leads to formation of a complex which is characterized by a broad absorption band between 400 and 500 nm.

Acknowledgment. This work was supported by the Environmental Protection Agency via Grant No. EPA- R-815953-01-0. Dr. Yun Mao is thanked for technical assistance with ESR measurements.

Registry No. TMB, 366-29-0; TMB+, 21296-82-2;N20,10024- 97-2; 02, 7782-44-7; alumina, 1344-28-1.

(39) Buntix, G.; Poizat, 0. J. Chem. Phys. 1989, 91, 2153. (40) Guichard, V.; Bourkba, A,; Poizat, 0.; Buntix, G. J. Phys. Chem.

(41) Kasha, M. Discuss. Faraday SOC. 1950, 9, 4. 1989, 93, 4429.