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Spectroscopic Studies of Nile Red in Organic Solvents and Polymers

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Spectroscopic Studies of Nile Red in Organic Solvents and Polymers ,loerllof AND Pl-loitll:lltg.IX~ A: CHI'~[ISTiiY ELSEVIER Journal of Photochemistry and Photobiology A: Chemistry 93 ( 199b.~ 57-64 Spectroscopic studies of nile red in organic solvents and polymers Ashim Kumar Dutta, Kenji Kamada, Koji Ohta * Photonic Chemistry Section, Department of Optical Materiolv, Osaka National Research h~stitute, AIST. Ikeda, Osaka 563, Japan Received 20 March 1995; accepted 14 June 1995 Abstract We have studied tile spectroscopic properties of nile red (NR), a highly fluorescent laser dye, ill organic solvents, binary solvent mixtures and polymers. Spectroscopic studies reveal remarkable changes in the absorption and emission band positions and intensities as a function of the polarity of the medium, Such large dmnges have been attributed to the twisted intramolecular charge transfer (TICT) state of the molecule in polar ntedium. Experimental results show that the molecule is sensitive to the polarity of its microenvironment and is an excellent probe for systems presenting restricted geometries. We have incorporated NR into thin films of poly ( melhyl methacrylate) (PMMA) and poly ( vinyl alcohol) (PVA); it is Ibund that the micropolarity in PVA is greater than that in PMMA; in pv ~., the micropolarity corresponds to that of a binary mixture of acetonitrile and water, whereas in PMMA, the micropolarity corresponds ctosely to that of pure acetonitrile. Keywords: Nile red; Twisted intratnolecular charge transfer (TICT) states; Steady state fluorescence; Time-resolved fluorescence; Aggregation-induced dual fluorescence 1. Introduction films [ 13,14], microparticles [ 15,16] and zeolites [ 17,18], have provided information on the role of the topology of these systems in controlling the photophysical properties of Organic polymers have largely replaced conventional the probe molecules. materials due to their high durability, transparency and pro- More interesting results are expected for molecules which cessability, even at the molecular level, which makes them are themselves very flexible and sense the microenvironment extremely suitable for optoelectronic and microelectronic in which they are located. Such possibilities are offered by devices. Moreover, the non-linear optical response of organic molecules which undergo eonfigurational changes when sub- materials doped in polymers and their direct application in jected to pressure or when encapsulated in pores or vessels information processing systems have made the study of dyes where free movement is restricted. Molecules exhibiting doped in polymers of interest in recent years. Although studies on organic fluorescent dyes incorporated twisted intramolecular charge transfer (TICT) properties into polymers and glasses have been performed, little infor- [ 19-21 ] are ideal for such purposes and can be profitably mation is available on the nature of the interaction between utilized to sense the effects of changes in the mierocavity due the dye and its microenvironment. Various factors, such as to external influences such as temperature, pressure, etc. the pore size, polarity, hydrogen bonding and other specific [ 22]. Recently, several aminobenzoic acid derivatives [ 23 ] interactions, may be responsible for changes in the photo- exhibiting TICT characteristics have been employed to study physical behaviour of dye molecules doped into such the effects of polarity and free volume in polymers. systems. Recent studies [1-31 have identified certain fluo- In this work, we report the spectroscopic properties of nile rophores which specifically sense a particular aspect of the red (NR), a highly fluorescent laser dye, in various solvents microenvironment, but are unaffected by other factors. Spec- and polymers. The strong dependence of the absorption and troscopic studies of such specific dye molecules and investi- emission band maxima on the polarity of the solvent indicates gations of the energy and electron transfer processes in that the molecule is highly sensitive to the polarity of the various systems presenting restricted geometries, namely medium in which it is located. The spectroscopic studies cyclodextrins [4], micelles [5,6], silica gels [7,81, clays suggest that the polarity dependence of the NR molecules [9,101, polymer latexes [ I 1,121, Langmuir-Blodgett (LB) may be explained in terms of TICT processes; this is possible due to the presence of the flexible diethylamino end group * Corresponding author. attached to the otherwtse rigid structure of the molecule, I 0 ! 0-6030/96/$ ! 5.00 © ! 996 Elsevier Science S.A. All rights reserved SSD! I 0 ! 0-603 0 ( 95 ) 04140-0 58 A.K. Durra et al. / Journal of Photochemistry and Photobiology A: Chemistry 93 (1996) 57-64 Moreover, we h~ve established that NR may be used as a probe molecule to estimate the micropolarity in polymer thin films using steady state luminescence spectroscopy. 0.06 i/I'II~'~'"" "~tl W "ll 0.04 2. Experimental details NR (Aldrich Chemical Co., USA) was used as received. 0.02' i The purity of the sample was checked using thin layer chro- | e'~ matography and absorption and emission spectroscopy. All 0.(30' m solvents used were of spectroscopic grade (Dojin Chemical Co.. Japan). The polymers poly(methyl methacrylate) 006 (PMMA) and poly(vinyl alcohol) (PVA) were obtained from Mitsubishi Rayon Co. (Japan) and Nakarai Chemicals 004 Ltd. (Japan) respectively and were used after purification. The polymers were purified by dissolution in appropriate solvents and reprecipitation from methanol. Doping of the 0 0~ polymers was achieved by mixing appropriate amounts of the dye in a common solvent (stirring well for a prolonged 000 period oftim¢ to ensure homogeneous mixing), and thin films 400 4~0 50~ 5S0 600 ~5( were obtained by the spin coating method using a spin coater wavelength (nm) unit (Mikasa IH-DXi I, Japan). The films wcrc allowed to Fig. I. (a) Absorption (broken line) and emission (full line) Sl~'clra of NR dry in a vacuum oven for about 24 h and then washed with (2,51 x 10 °~ M) in hexane, (b) Absorption (broken line) and emission ethanol to remove NR sticking to the surface of the films. (full line) specln~ of NR ( !,88 × 10 "~ M) in carbon tetrachloride, Washing was continued until the last traces of the dye had disappeared, which was ascertained from the absorption spec- range 40(b6~ nm is broad and diffuse with two b~nd max- tra of the washings. Ethanol was used to wash PMMA films ima, one at 488 nm and the other at 508 nm, The steady state and chloroform was used to wash PVA films to ensure min- room temperature emission spectrum in the spectral region imal damage, Finally, the films were dried in a vacuum oven 500-800 nm is also broad with two distinct band maxima at at a temperature of approximately 80 °C for a period of about 525 and 570 nm. The substantial overlap between the absorp- S days to ensure no trace of the solvents remained, The tion and emission spectra of NR in hexane indicates that there absoq)tion and emission spectra of the films were recorded is no or little distortion of the molecules in the excited state using a Shimadzu 2200 UV abmrption spectrophotometer in this s~dvenl, Fig. I (b) shows the absorption and emission and a Hitachi Fo3010 fluorescence spectrophotometer respec- spectra of NR in carbon tetrachloride at room temperature. tively, The emission from the films deposited on quartz plates The absorption spectrum in the region 400-600 nm shows w~,; reco~ed by placing the films in suitable holders which two distinct bands at 500 and 522 nm and the emission spec- were inclined at an angle of45 ° to the incident beam. Suitable trum in the region 50(b-800 nm shows similar bands at 555 narrow bandpass filters were used to eliminate contributions and 585 nm, The red shift of the absorption and emission from the scattering of the incident beam from the surface of bands in carbon tctrachioride relative to those in bexane may the films. Fluores~nce lifetime measurements were per- b¢ attributed to the slightly greater polarity of carbon tetra- f~ on a Horiba NEAS 1100 nanosecond spectrofluoro- chloride than hexane. The red shift of the bands and their meter using a pulsed hydrogen lamp with a pumping comparatively smaller overlap in carbon telrachloride suggest freq~y of 10 Hz and a full width at half-maximum that the molecules arc strongly sensitive to the polarity of (I~,YHM) of about 0,5 ns, The fluorescence decay profiles their microcnvironment. To investigate the origin of the two were analysed using single- or multi-exponential software bands in the absorption and emission spectra, the excitation programs obtained from Horiba. spectra and lifetimes monitored at the two emission band maxima were recorded. The identical nature of the excitation spectra and lifetimes (data not shown) monitored at the emis- & Pmdts arm discussion sion band maxima clearly indicates that the bands originate from the same electronic state and ate probably vibronic. 3, l, Spcczroscopic $t~ies of NR in hexane and carbon tet~hloride 3.2. Spectroscopic studies of NR in polar solvents absot~on and emission spectra of NR in hexane and In order to study the role of the polarity in modifying both carbon tetrachloride are shown in Figs. I(a) and I(b). In the ground and excited states of the molecule, we investigated bexane (Fig. I(a)), the absorption profile in the spectral the absorption and emission characteristics of NR in various A.K. Durra et al. / Journal of Photochemistry and Phntobiology A: Chemistry 93 (1996) 57.-64 59 rhodamine derivatives [33-36] show similar behaviour in polar solvents. Such observations have been accounted for in terms of the TICT process [ |9-21,29-36]. According to this model, on electronic excitation, the molecule initially forms a moderately non-polar state with a geometry similar to that t in the ground state. The transfer of an electron from an elec- tron donor to an electron accepter group results in a twisted •~ 4 -5 L configuration of the molecule, in which the donor and accep- 500 ter groups are oriented almost perpendicular to each other.
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