Spectroscopic properties of pure and Coumarin 153-doped thin films of sol-gel silica xerogels

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1 Journal of Molecular Structure 479 (1999) Spectroscopic properties of pure and Coumarin 153-doped thin films of sol-gel silica xerogels K. Maruszewski a,b, * a Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wrocl aw, Poland b Chemistry Department, Opole University, Opole, Poland Received 12 October 1998; accepted 2 November 1998 Abstract Thin-films doped with Coumarin 153, an important laser dye, have been prepared by the sol-gel method. Spectroscopic properties of the entrapped dye are studied by electronic absorption, diffuse reflectance, electronic excitation, emission and time-resolved emission techniques. The obtained results indicate that the entrapment of the dye results in formation of molecular aggregates having absorption and emission characteristics different from those of the monomeric dye. The blue matrix emission band (at approx. 450 nm) is relatively short-lived while the red band (at approx. 700 nm) displays a very long lifetime. Electronic excitation spectra of the sol-gel thin-films suggest that the two matrix-related luminescence bands possess the same electronic origin Elsevier Science B.V. All rights reserved. Keywords: Sol-gel silica xerogels; Coumarin 153; Doped thin films 1. Introduction * Tel.: ; Fax: ; marusz@highscreen.int.pan.wroc.pl The photochemical properties of coumarin and its derivatives have attracted considerable attention over the past decade. Their most interesting applications vary from promising laser dyes [1 3] (acting in the important blue-green region) to biologically active skin-photosensitizers [4]. Coumarines display a wealth of interesting physicochemical features. The dyes with amino groups free to rotate exhibit strong solvatochromism related to the formation of the twisted intramolecular charge transfer (TICT) state [5]. That feature, along with the relatively large Stokes shifts observed for coumarines, results in their employment as very good optical probes of ultrafast solvent effects [3]. Furthermore, some coumarines exhibit tautomerization equilibria in their lowest excited singlet state leading to the anomalous ph dependence of the fluorescence maximum positions [6]. It has been also observed that absorption and emission features of coumarines depend on pressure [7] and mechanical stress [8]. An interesting property of coumarine is excimer formation via interaction between the electronically excited molecule in the triplet state and the ground-state dye [9,10]. For certain applications, such as lasers based on organic dyes, it is advantageous to immobilize the active molecules in some kind of a solid-state carrier. A variety of matrices have been investigated, including clays [11], porous glasses [12] and polymers [13]. However, each of them possesses various intrinsic limitations [14], which results in on-going research aimed at the development of high-quality /99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (98)

2 54 K. Maruszewski / Journal of Molecular Structure 479 (1999) Entrapment of Coumarin 153 in the sol-gel matrix results in interesting changes in its spectroscopic properties. Significant changes are observed in the absorption spectrum, as compared to the liquid solution data. Furthermore, two very broad matrix-originated luminescence bands have also been observed. The matrix emission occurring at approximately 500 nm has been shown to be relatively short-lived while the band at approximately 700 nm displays a very long lifetime. Electronic excitation spectra of the sol-gel thin-films reveal that the two matrixrelated luminescence bands possess the same electronic origin. Analogous blue and red luminescence features have been also reported for porous silicon [24,25]. 2. Experimental Fig. 1. Electronic absorption spectra of Coumarine 153. Trace A entrapped in sol-gel thin film and Trace B ethanol solution of the dye. support. The sol-gel method [15] provides glasses and porous glass-like xerogels which are promising hosts for organic molecules [14]. For example, rhodamine 6G (R6G) [14,16 18], acridine orange [19] or malachite green [20] have been successfully entrapped in the sol-gel matrix and their physicochemical properties studied. Among other interesting effects, incorporation of organic molecules in the sol-gel carrier typically increases their photostability due to the cage effects [14]. The glassy matrix can also influence significantly aggregation of the entrapped organic molecules [14,17,18,21,22]. In this work we report results of spectroscopic investigations of thin films prepared by the sol-gel method doped with Coumarin 153. This dye molecule, depicted below, belongs to the rigid-skeleton class of coumarines [23]. Samples of the thin silica films doped with the Coumarin 153 laser dye (Lambda Physics) were obtained by the sol-gel method according to the literature procedures [15]. The films were coated on regular silica glass plates by the spin-top method. Subsequently, the samples were dried at 250 C for 5 min. The thickness of the films was determined by means of a profilgraphprofilometer and was equal to approximately 0.2 mm (within an error of 5%). We have investigated several concentrations (0.20%, 0.15% and 0.10% (weight)) of the dye in the sol-gel matrix. The sol-gel thin films without the dopant were obtained analogously. Electronic absorption measurements were carried out on a Cary Varian 2100 spectrophotometer. Diffuse reflectance spectra were obtained on a Carl Zeiss Jena Specord M42 spectrophotometer. Emission spectra were measured using a Jobin Yvon spectrophotometer THR A photomultiplier tube Hamamatsu R928 was used as a detector. Time-resolved emission

3 K. Maruszewski / Journal of Molecular Structure 479 (1999) Results and discussion Fig. 2. Electronic emission spectra of Coumarine 153. Trace A presents data obtained using nm CW excitation for the dye ethanol solution. Traces B and C were obtained with 308 nm excitation for pure sol-gel thin film (Trace B) and the Coumarin 153 doped film (Trace C). Insert shows results of subtraction of spectrum in Trace B from data in Trace C. measurements and lifetime measurements were performed with the Jobin Yvon spectrophotometer equipped with a Stanford Research Systems SR 400 Gated Photon Counter. Emission decay characteristics were observed on a Tektronix T 556 oscilloscope. Pulsed excitation of 308 nm, used for the time-resolved emission measurements, was obtained from a Lambda Physics LPX100 excimer laser. CW excitations of nm, 488 nm and nm were obtained from a Carl-Zeiss Jena ILA 120 Ar ion laser. Electronic excitation measurements were performed on a SLM Aminco SPF-500 spectrofluorimeter. Fitting of the observed electronic decay characteristics was performed by origin 5.0 software package. Fig. 1 presents the diffuse reflectance spectrum of the dye entrapped in the sol-gel thin film (Trace A) and electronic spectrum of Coumarin 153 obtained in ethanol solution (Trace B). Upon entrapment, the broad, symmetrical absorption band observed at 424 nm for the liquid solution of the dye blue-shifts to 418 nm and splits into a few components. The shoulders on the main absorption band can be seen on the red-edge side (at 442 nm and 590 nm) as well as on the blue-edge side (380 nm). The up-shift of the absorption maximum of the dye upon entrapment might be caused by the solvent effects induced by the xerogel matrix. Another possibility is changes of the entrapped dye molecule geometry induced by the rigid environments. Splitting and broadening of absorption bands upon incorporation in solid matrices have been reported for several dyes such as rhodamine 6G [14], acridine orange [19] and malachite green [20], and attributed to the influence of the host on the dye agglomeration equilibria, leading to the appearance of various aggregates. Therefore, the splitting of the absorption band at 418 nm into several overlapping features might be associated with formation of several different agglomerates. The electronic emission spectra of Coumarin 153 are shown in Fig. 2. Trace A presents the spectrum of the dye in ethanol solution. A single, broad band with maximum at 527 nm can be observed upon excitation at nm. Since the Coumarin 153 absorption band shows several overlapping features for the immobilized dye (Fig. 1), emission spectra have also been obtained with CW excitations at 308 nm, 488 nm and 514 nm. While there was no luminescence observed for the 488 nm and 514 nm excitations, the spectra recorded with the 308 nm line are shown in Trace B (pure sol-gel film) and Trace C (dye-doped film). The blue emission from porous and oxidized nanocrystalline silicon and from organosilanes has been previously observed (maximum at 470 nm) and attributed to the luminescence originating from the silanol (xsi OH) groups [24,25]. Such groups are common in sol-gel matrices [15] and, therefore, we can tentatively assign the matrix blue emission (Trace B) to the OH-related centers. However, the red emission at 700 nm, observed in Trace B, is an interesting new feature for the sol-gel obtained

4 56 K. Maruszewski / Journal of Molecular Structure 479 (1999) Fig. 3. Electronic emission spectra of Coumarine 153 obtained using 308 nm pulse excitation. Trace A presents spectrum obtained with delay ˆ 0 and gate ˆ 100 ns. Trace B presents spectrum obtained with delay ˆ 0 and gate ˆ 40 ms. Trace C presents spectrum obtained with delay ˆ 150 ms and gate ˆ 600 ms. Trace D presents spectrum obtained with delay ˆ 5 ms and gate ˆ 5 ms. Inserts present semilogarithmic plots of the emission intensity decays versus time measured at 514 nm and 700 nm. materials. Similar luminescence has been observed for porous silicon and there exist several possible explanations of this phenomenon such as quantumconfinement effects, siloxen formation or alloying effects [24,25]. The blue-side band is much stronger for the dyedoped thin film (Trace C). In order to extract the emission spectrum of the entrapped Coumarin 153, the spectrum in Trace B (pure matrix) was subtracted from the spectrum of the dye in the sol-gel film (Trace C). The result is shown in the insert to Fig. 2. As it can be seen, the Coumarin 153 absorption is blue-shifted compared to the free dye (Trace A). Furthermore, the single band at 527 nm observed for the ethanol solution of the dye splits into two components at 471 nm and 495 nm in the solid matrix. This observation is in agreement with possible agglomeration of the entrapped dye. More insight into the nature of the spectra obtained with 308 nm excitation can be gained from the timeresolved emission measurements of the doped xerogels, performed in pulse experiments (Fig. 3). The lifetime of the excited state of Coumarin 153 in liquid solvents at room temperature is of the order of a few nanoseconds [2]. Therefore, it is not expected to achieve temporal resolution of the dye emission due to the limitations of our experimental set-up (signals longer than ca. 50 ns). Is has to be stressed that timeresolved measurements with this excitation were performed for plain sol-gel films as well as for all the dopant concentrations under investigation (i.e. 0.20%, 0.15% and 0.10%). In all those cases (including the pure sol-gel films) the obtained timeresolved spectral profiles were virtually identical. Thus, the spectra presented in Fig. 3 originate from the matrix only. Trace A shows the spectrum obtained immediately after a laser pulse (delay ˆ 0 s) and with a very short integration time (gate ˆ 100 ns). Extending the recording period (gate ˆ 40 ms) and keeping the delay equal to 0 s resulted in the spectrum shown in Trace B. Trace C presents luminescence recorded with a delay equal to 150 ms and the gate having temporal width of 600 ms. Finally, Trace D was obtained with a delay equal 5 ms and the gate also set for 5 ms. As it can be seen, the maxima of the blue-side emission bands show temporal progression from approx. 450 nm (Trace A) to 490 nm (Trace B) and to approx. 515 nm for the spectrum shown in Trace C. Thus, the very broad luminescence feature observed in Trace B of Fig. 2 (CW excitation) is, in fact, a composition of several components of the xerogel blue emission possessing different lifetimes. It is interesting to note that the luminescence band at 700 nm (Fig. 2, Traces B and C) was absent from the spectra presented in Traces A and B. However, it can be observed in Traces C and D of Fig. 3. We have obtained emission decay curves for the emission bands excited with the 308 nm pulse excitation and observed at 514 nm and 700 nm. The measurements were performed at room temperature and the results are presented in the inserts to Fig. 3 in the form of semilogarithmic plots. It is important to state that in

5 K. Maruszewski / Journal of Molecular Structure 479 (1999) Fig. 4. Electronic excitation spectra of Coumarin 153 entrapped in sol-gel thin film. Trace A presents spectrum with emission intensity monitored at 410 nm, Trace B was obtained at 514 nm and Trace C at 700 nm. both cases the onset of the signals were below the instrumental resolution (e.g. 50 ns). Thus, at this time-scale, no delayed emission phenomena can be invoked to explain the apparent increase of the luminescence signal at 700 nm with time. However, the extremely long lifetime of the red-side emission (milliseconds) could account for the relatively large temporally-integrated intensity (Fig. 2, Traces B and C) of the intrinsically weak emission feature. Thus, the apparent absence of the 700 nm band in Traces A and B might be due to the presence of the much stronger blue-side matrix emission band. The luminescence decay characteristics for bands at 514 nm and 700 nm (Fig. 3, inserts) were fitted with mono-exponential functions, yielding unsatisfactory fits. However, in both cases excellent fits (correlation factors 0.997) were obtained by assuming twoexponential fitting functions. The lifetimes of the two components observed at 514 nm are 419 ns and 4.64 ms, respectively. The relative initial intensities of the two luminescence components, obtained from the fitting procedure, are 4.4 (419 ns): 1 (4.64 ms). Analogously for the 700 nm emission feature, the lifetimes of the two components are: ms and ms and the intensity ratios are 2.0 (77.17 ms): 1 ( ms). Furthermore, we have obtained emission spectra for the sol-gel entrapped Coumarin 153 at various laser powers and the relative intensity ratios of the matrix luminescence bands at 500 nm and 700 nm were virtually independent of the excitation intensity. Electronic excitation spectra of the xerogel-immobilized Coumarin 153 are presented in Fig. 4. Trace A shows excitation data obtained when luminescence was monitored at 410 nm, Trace B was obtained at 514 nm and Trace C at 700 nm. As it can be seen, Traces A and C are dominated by a strong excitation feature at 380 nm. Since these emission-monitoring wavelengths are outside of the Coumarin 153 absorption envelope (Fig. 2) this characteristic, narrow excitation feature might be assigned to the matrix absorption. This observation suggests that both luminescence features related to the sol-gel matrix (at approx. 500 nm and 700 nm) seem to have the same electronic origin. In the case of the excitation spectrum observed at 514 nm (Trace B) the observed broad band is expected to be a sum of the sol-gel matrix and Coumarin 153 absorption features. Thus, the red-side edge of this feature (ca. 425 nm) could be assigned to the dye electronic transitions and the blueside edge (ca. 360 nm) to the matrix absorption envelope (Fig. 1, Trace A). 4. Conclusions Thin-films containing Coumarin 153 have been prepared by the sol-gel method and investigated by several spectroscopic techniques. The electronic absorption spectra of the entrapped dye differ from those obtained from analogous liquid solutions. The observed changes indicate aggregation of the dye molecules in the solid matrix. Emission spectra of the doped thin-films obtained with the pulse 308 nm excitation reveal two bands (at 500 nm and 700 nm) originating from the sol-gel matrix itself. Combination of the time-resolved emission measurements and excitation spectroscopy reveals that the two

6 58 K. Maruszewski / Journal of Molecular Structure 479 (1999) matrix-related luminescence features possess the same electronic origin but occur on significantly different time scales. While the blue-side band is relatively short-lived, the red-side band displays at room temperature a very long lifetime (of the order of milliseconds). References [1] G. Jones II, W.R. Jackson, Chem. Phys. Letters 72 (1980) 391. [2] G. Jones II, W.R. Jackson, C.-Y. Choi, W.R. Bergmark, J. Chem. Phys. 89 (1985) 294. [3] P.K. McCarthy, G.J. Blanchard, J. Phys. Chem. 97 (1993) [4] W.W. Mantulin, P.-S. Song, J. Am. Chem. Soc. 95 (1973) [5] Z.R. Grabowski, J. Dobkowski, Pure and Appl. Chem. 55 (1983) 245. [6] S.G. Schulman, L.S. Rosenberg, J. Phys. Chem. 83 (1979) 447. [7] A.P.G. Kutty, S. Venkateswaran, S.N. Vaidya, V.B. Kartha, J. Phys. Chem. 97 (1993) [8] J.I. Zink, W. Klimt, J. Am. Chem. Soc. 96 (1974) [9] D.R. Graber, M.W. Grimes, A. Haug, J. Chem. Phys. 30 (1969) [10] E.T. Harrigan, A. Chakrabarti, N. Hirota, J. Am. Chem. Soc. 98 (1976) [11] Z. Grauer, D. Avnir, S. Yariv, J. Phys. Chem. 88 (1984) [12] S. Wolfgang, H.D. Gafney, J. Phys. Chem. 87 (1983) [13] R. Reisfeld, C.K. Jørgensen, Structure & Bonding 49 (1982) 1. [14] K. Maruszewski, D. Andrzejewski, W. Strȩk, J. Luminesc., (1997) 226; D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) [15] C.J. Brinker, G.W. Scherer, Sol-Gel Science, Academic Press, New York, [16] D. Avnir, V.R. Kaufman, R. Reisfeld, J. Non-Cryst. Solids 74 (1985) 359. [17] Y. Takahashi, T. Yamanaka, K. Uchida, J. Luminesc. 62 (1994) 299. [18] Y. Takahashi, T. Kitamura, M. Nogami, T. Yamanaka, J. Luminesc (1994) 451. [19] S.V. Gaponenko, I.N. Germanenko, A.P. Stupak, M. Eyal, D. Brusilovsky, R. Reisfeld, S. Graham, C. Klingshirn, Appl. Phys. B. 58 (1994) 283. [20] V. Chernyak, R. Reisfeld, Chem. Phys. Letters 181 (1991) 39. [21] T. Yamanaka, Y. Takahashi, T. Kitamura, K. Uchida, J. Luminesc (1991) 256. [22] T. Kitamura, Y. Takahashi, T. Yamanaka, J. Luminesc (1991) 373. [23] Y. Jiang, P.K. McCarthy, G.J. Blanchard, Chem. Phys. 183 (1994) 249. [24] M. Rückschloss, Th. Wirschem, H. Tamura, G. Ruhl, J. Oswald, S. Vepřek, J. Luminesc. 63 (1993) 279. [25] S.V. Gaponenko, I.N. Germanenko, E.P. Stupak, A.P. Stupak, V.P. Bondarenko, A.M. Dorofeev, Appl. Phys. Lett. 64 (1994) 85.