Local properties of adsorption sites probed by the emission of fluorophores of different size and shape

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22 November 1996

CHEMICAL PHYSICS LETTERS

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ELSEVIER

Chemical Physics Letters 262 (1996) 583-588

Local properties of adsorption sites probed by the emission of fluorophores of different size and shape Frank Imans, Lucien Viaene, Mark Van der Auweraer, Frans C. De Schryver LaboraWry for Molecular Dynamics and Spectroscopy, Katholieke Universiteit Leuven, Celestijnenlaan 20OF, BE-3001 Heverlee, Belgium

Received 22 July 1996; in final form 23 September 1996

Abstract

The photophysics of aromatic amines, having a polar excited state, adsorbed on silica is discussed based on the results obtained by stationary and time-resolved fluorescence experiments. The stationary results suggest a correlation between the molecular shape and size and the effective polarity of the adsorption site. Time-resolved fluorescence emission experiments carried out at different emission wavelengths indicate non-exponential fluorescence decays of the adsorbed molecules. These decays could be analysed globally assuming a Gaussian distribution of decay rates. Using the standard deviation of the decay rate as a measure of the width of the distribution of the local properties of the adsorption sites, these data substantiate the interpretation of the stationary fluorescence experiments. 1.

Introduction

The photophysical properties of molecules adsorbed on heterogeneous surfaces have been investigated, using stationary and time resolved fluorescence spectroscopy [1,2]. Several models have been proposed to describe the decay of the excited states of adsorbed molecules [3-6]. These models are based on a distribution of the molecules on the heterogeneous surface. In this Letter a decay law, based on a Gaussian distribution of the decay rates of the excited states of the adsorbed molecules, has been used to characterize the interactions between adsorbates and substrates. Aromatic amines having a polar singlet excited state [7], are an excellent tool for determining the different properties of the adsorption sites. To achieve this goal, the excited state properties of the probe molecules, which have a common photophysically active unit but differ in shape and dimension (Fig. 1), were investigated u s i n g stationary and time-resolved fluorescence spectroscopy.

2. Experimental

The preparation and purification of 5'-[4-[bis(4ethylphenyl)-amino]phenyl]-N,N,N',N'-tetrakis(4-ethylphenyl)-[ 1,1':3', l"-terphenyl]-4,4"-diamine (pEFTP) and N,N-bis-(4-ethylphenyl)-4-aminobiphenyl (modpEFTP) (Fig. 1) have been reported earlier [8,9]. Silica gel (Aldrich, Davisil Grade 634) was used as a solid support. It is a porous solid with an average pore size of 60 A and a surface area of 480 m 2 / g . I t s surface has a large density of SiOH groups and binds a large amount of physically adsorbed water. A solution (solvent: chloroform) of the chromophore was added to a suspension of silica gel in chloroform. The solvent was slowly evaporated at room temperature using a nitrogen flow. The sample was transferred to a vacuum line to remove oxygen and any remaining solvent molecules, and was then

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F. lmans et a l . / Chemical Physics Letters 262 (1996) 583-588

320 and 380 nm were obtained with a SPEX Fluorolog 212 in a front-face geometry. The fluorescence decays were recorded by the single-photon timing technique with apparatus described elsewhere [10]. The sample was excited at 320 nm. The fluorescence decays were fitted to a Gaussian distribution [11,12] of decay times using single-curve and global analysis [13,14] with reference convolution. The goodness of fit was evaluated by visual inspection of the residuals and their autocorrelation function and by the calculation of the statistical parameters X 2 and ZX 2. Using a singlecurve analysis fluorescence decays characterised by values of X 2 and ZX 2 larger than 1.30 or 3.00, respectively, were considered to be unacceptable. For the global analysis the same criteria were used for the individual decays. Furthermore, analyses where Xgz or ZX 2 exceeded 1.30 or 5.00, respectively, were considered as only marginally acceptable even when the statistical parameters of the analysis individual decays were acceptable. POPOP in methylcyclohexane with a decay time of 1.1 ns

modpEFTP

Et X

pEFTP: X=N(O--Et)2

X

X Fig. 1. Structures of modpEFrP and pEFTP.

kept for at least 2 h at 1 × 10 - 3 mbar at room temperature. Finally, the sample was transferred under vacuum from the flask to the sample cuvette and sealed off. As this procedure does not remove the water, it can be assumed that the amount of physically adsorbed water corresponds to that in equilibrium with ambient atmosphere. The concentration of the chromophore on the silica gel was of the order of 10 - 6 m o l / g . Under these conditions, the average distance between the adsorbed molecules is about 30 nm. Corrected fluorescence emission and excitation spectra at different excitation wavelengths between

w a s u s e d as r e f e r e n c e .

3. Results and discussion

3.1. Stationary fluorescence The

fluorescence

spectrum

of the

1.2 modpEFTP 1

4

"~ 0.8 ..~ "~0.6 ~0.4 0.2 0 350

400

450

500 wavelength (nm)

I 550

600

Fig. 2. Normalised emission spectra of biphenyl and polyphenyl adsorbed on silica gel (Aexc = 320 nm).

adsorbed

F. lmans et al./ Chemical Physics Letters 262 (1996) 583-588

maximum in acetonitrile, while the emission maximum of adsorbed pEFI'P resembles the one in THF. Silica has, as have other solid surfaces, a heterogeneous surface which results in a variety of adsorption sites all with different size and polarity. Smaller holes, with a higher silanol density, are more polar than larger holes. Although the mean pore size of the surface is 60 A, which exceeds the dimensions of the adsorbed molecules, the smaller molecules can adsorb into smaller holes. This results in a bathochromic shift of the emission maximum because of the higher polarity, and, hence, a better stabilisation of the singlet excited state. It can, however, not completely be excluded that this different behaviour of the pEFTP and the model compound could be related to a larger torsional mobility of the latter leading to a better excited state intramolecular relaxation.

Table 1 Fluorescence m a x i m a of the chromophores adsorbed on silica gel and in solution, with several excitation wavelengths

modpEPTP pEF'fP

Aexc = 320 nm

Aexc = 320 nm (solvents)

Aexc = 350 nm

425 420

437 (ACN) 420 (THF)

431 430 a

585

o

ACN: acetonitrile, THF: tetrahydrofuran a)tex c = 380 nm.

molecule consists of a broad structureless band (Fig. 2). The emission maxima do not depend significantly on the excitation wavelengths and are compared with the values found for pEFTP and modpEFTP in solution (Table 1). However, determination of the polarity of the silica surface by means of spectroscopic methods is not straightforward because of the heterogeneity in polarity of the sites. The photophysical behaviour will be different in the adsorbed state compared with a dilute solution for molecules with different shapes and sizes but with the same photophysically active unit. In solution, the solvent dependence of the fluorescence spectra suggests the formation of a polar excited state. The bathochromic shift upon increasing solvent polarity is more pronounced for pEFTP than for modpEFTP. Upon adsorption on silica however the model compound (modpEFTP) emits at longer wavelengths than pEFTP. The emission maximum of modpEFTP adsorbed on SiO 2 is approximately the same as the

3 . 2 . Time resolved measurements

Upon laser excitation, non-exponential fluorescence decays were observed (Fig. 3) and the decays become longer at longer emission wavelengths (Tables 2 and 3). This suggests that the molecules are adsorbed in sites having a broad distribution in polarity. Molecules adsorbed into more polar sites emit at longer wavelengths and are characterised by a longer decay time. The latter was also observed for pEFTP

45000 40000

~

35000

~

reference ,ll compound '

fluorescence decay, full line has been calculated according to

0000 25000

8 20000 15000 10000 ooo 0 0

50

100

150

200

250

time channel Fig. 3. Fluorescence decay of pEFTP adsorbed on silica gel, analysed according to Eq. (5). Aexc = 320 nm, )tem = 430 nm, 147 ps per channel k 0 = 0.1093 ns ~, tx = 0.2662 n s - i, X 2 = 1.034, Zx2 = 0.495.

F. lmans et al./ Chemical Physics"Letters262 (1996) 583-588

586

Table 2 Results of the global analysis for modpEFTP adsorbed on silica gel hem(nm) ko(ns -I)

~(ns -1)

~ + k o(ns - j )

~(ns t)

445a 465 b 485c 505a

0.2474 0.2015 0.1258 0.0319

0.3500 0.3041 0.2284 0.1345

0.2835 0.2835 0.2835 0.2835

0.1026 0.1026 0.1026 0.1026

f(t) =f(0)e-kot[

Z=~c

e-ZtY'(Z)dZ.

(3)

~Z=0

Assuming that X and therefore also Z depend on a large number of random variables [21], Y ( X ) or Y ' ( Z ) will be normally distributed

Y'(Z)-

Aexc = 320 nm. a h, 2 = 1.117, Z~ = 1.483. X 2 = 1.130, Zx2 = 1.362. c X 2 = 1.024, Z~ = 0.287. a X2 = 1.141, Zx2 = 1.310. and modpEFTP in solvents with different polarity. For other transients of adsorbed molecules (i.e. triplet excited state and radicals), non-exponential decays were also observed [15-17]. Several models for analysing these non-exponential decays have been proposed [4,18,19]. Beside the fitting according to these models, the fluorescence decay can be analysed as a bi-exponential decay [20]. This has only a physical meaning, though, if there are two different types of adsorption sites on the solid surface or when there is an overlap in emission between monomers and dimers or when the formation of excimers occurs. The non-exponential fluorescence decays could also be related to a continuous distribution of decay rates. As the average polarity that the molecules experience is related to the presence of several randomly oriented polar groups, one could assume, in a first approximation, that the decay rates, are Gaussian distributed. When it is assumed that the fluorescence decay rate constant k ( X ) depends on a parameter X, the fluorescence decay of the ensemble of molecules will be given by

f ( t ) = f ( 0 ) e - k ° t f k i ~ ' S f e - k~X),y [ k ( X )] d k ( X ) ,

(1) where Y ( X ) is the probability that the variable X amounts to X and k 0 is the rate constant of the decay of the excited state when k ( X ) equals zero. Making a first-order Maclaurin expansion of k ( X ) , k ( X ) can be approximated by k(X) = aX= Z.

Expressing Eq. (1) as a function of Z yields

(2)

1 o.(2~r},/2exp

,

(4)

w h e r e / z and o- correspond to the average value and the standard deviation of the distribution of Z, respectively. Eq. (4) is also a plausible distribution function when it is assumed that the free energy content o f the different sites is developed in a Taylor series around X ( t z ) the value of X for which it is minimal. In this case a Boltzmann expression for the probability o f the different sites (Y'(Z)) will be p r o p o r t i o n a l to e x p { - f l [ X - X ( t x ) ] 2 / k T } or exp{-[-½(Z-ix)2/o'2kT] with fl equal to a/2o-2kT. Combination of Eq. (3) and (4) leads to

[ t20- 2 - 2 ~ t f(t) =/(0)e-k,"exp[

×erfc

-~

"

(5)

Eq. (5) differs from the expression proposed by Albery et al. in the cut-off rate constant k 0, which has been introduced here. This avoids a finite number of molecules having a physically unacceptable decay rate equal to zero. Table 3 Results of the global analysis for pEFTP adsorbed on silica gel hem (nm) k0 (ns- i ) ~ (ns- J) p, + ko (ns- l) o- (ns- i) 410 a 430 b 450 c 470 ~ 490 e 510 f

0.1093 0.1093 0.1093 0.1093 0.1093 0.1093

0.3036 0.2662 0.2255 0.1798 0.1588 0.1227

Aexc = 320 nm. a X2 = 1.054, Z~ = 0.794. b X 2 = 1.034, Zxz= 0.495. c X 2 = 1.156, Zx2= 2.280. a X 2 = 1.010, Zx2=0.146. eX2=1.076, Zxz=I.106. fX 2 = 1.112, Z~ = 1.639.

0.4129 0.3755 0.3348 0.2891 0.2681 0.2320

0.1912 0.1912 0.1912 0.1912 0.1912 0.1912

F. Imans et al. / Chemical Physics Letters 262 (1996) 583-588 2.5

587

pEFTP, ~-¢m:450 nm, = 0.1912 ns q

1.5

modpEFTP, ker~: 445 rim,

•~

// J

=

.

"

0.5

0

~ 0

0.2

I 0.4

0,6

0.8

1

1.2

k 0 ( a s "l)

Fig. 4. Spread of the distribution of decay rates of adsorbed modpEFTP ( aem = 445 nm) and adsorbed p E F r P (Aem = 450 nm).

The fluorescence decays were recorded at several emission wavelengths. A single-curve analysis of the fluorescence decays using a bi-exponential decay law and Eq. (5) yielded acceptable statistical parameters. Performing a global analysis, however, resulted in unacceptable fits of the bi-exponential model, whereas Eq. (5) yielded acceptable statistical parameters for the fit when k 0 and tr were treated as global parameters (Table 2 and 3), This means that k 0 and or are independent of the emission wavelengths. The obtained average decay times resembled those found in solution [7] and in a polymer matrix [22]. As the molecules situated in a more polar environment emit at longer wavelength together with a decreased average decay rate (p. + k0), this trend can be correlated with the change in the non-radiative decay rate as a function of polarity. In solution, it was found that the non-radiative decays of pED-TP and modpEFTP were slowed down by increasing solvent polarity [7]. Energy transfer between molecules in sites with different polarity could in principle influence the decay. The large average distance between the molecules (30 nm), however, together with the large Stokes shift, indicative of a small R 0 value, makes energy transfer unlikely. For pEFTP, the larger compound, or seems to be smaller than the value found for modpEFTP (Tables 2 and 3). Furthermore, the larger molecules have

more difficulties adsorbing into the smallest sites with the higher polarity. It can be concluded that the larger the adsorbed molecules, the smaller the spread (Fig. 4) for a given pore size. The silica surface is porous and contains holes of several sizes with difo ferent polarity, with a mean pore size of 60 A. The smaller holes are more stabilising due to the higher silanol density and thereby expose a more pronounced hydrogen bonding [23]. MOdpEFTP can penetrate even the smaller holes, so the energy of the polar excited state has a broader distribution, which results in a larger spread of the fluorescence decay rates. These results confirm the conclusions from the stationary fluorescence measurements,

4. Conclusions

The stationary fluorescence measurements show a difference in the excited state behaviour of molecules of different shape and size but with the same photophysically active unit. The decay of the singlet excited state is non-exponential and was globally analysed based on a Gaussian distribution model. The excited state properties of modpEFTP show a large spread because of its higher capability of penetrating the smaller pores of the silica surface, whereas the energy levels of p E F r P show, due to its larger volume, a narrower distribution.

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F. lmans et a l . / Chemical Physics" Letters 262 (1996) 583-588

Acknowledgements FI thanks the IWT for financial support. MVDA is 'Onderzoeksleider' of the F.K.F.O. The continuing support of the Fonds voor Kollektief Fundamenteel Onderzoek, the Nationale Loterij and the Ministry of Science Programming through IUAP-III-040 and IUAP-II-16 is gratefully acknowledged.

[10] [11] [12] [13] [14] [15]

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