Sensitized luminescence in thin layers VI. Benzonaphthofuran—Naphthacene system

Journal of Luminescence 8 (1974) 203-209. © North-Holland Publishing Company

SENSITIZED LUMINESCENCE IN THIN L A Y E R S VI. B E N Z O N A P H T H O F U R A N - N A P H T H A C E N E

SYSTEM

I. HORNYAK Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary

Received 26 April 1973 The fluorescence of benzonaphthofuran was studied in ethanol, in solid state and on Whatman paper. Measurements were carried out at room temperature and 77 °K, respectively. Benzonaphthofuran sensitizes the luminescence of the naphthacene built into its crystal lattice. The phenomenon was applied to the analytical determination of the naphthacene. 1. Introduction The luminescence properties of the benzo (b) naphtho (2,3-d) furan (BNF) have been studied only in a few works. Van Duuren [1 ] mentioned the analytical determination of the heterocycle aromatic compound by spectrofluorimetric method. The phosphorescence of the compound was examined by Zander [2]. Schmillen [3] investigated the lifetime decay of the BNF in binary systems using naphthacene. The solid naphthacene excited by UV light hardly shows fluorescence. However, upon building the BNF into the crystal lattice in a concentration of 10 -6 - 1 0 -5 M the characteristic green-yellow fluorescence of the naphthacene appears. With increasing quantities of naphthacene the fluorescence of the BNF decreases gradually. At a determined concentration it completely disappears and only the sensitized luminescence of the naphthacene may be observed. The aim of this work is to study the properties of BNF by the spectrofluorimetric method and to study the sensitized luminescence of the BNF-naphthacene system on Whatman filter paper and to describe its application for analytical purposes.

2. Experimental The measurements were carried out by a Hitachi-Perkin-Elmer MPF-2A spectrofluorimeter equipped with a 150 W Osram xenon arc lamp and R 106 photomultiplier. For the low temperature measurements the Dewar of the phosphorescence accessory was used. Materials of pa. quality were used for the experiments. The BNF and naphthacene were produced by Ferak, the ethanol and benzene by Reanal. The 203

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L Hornyak, Sensitized luminescence in thin layers. V1

solvents were freshly distilled. A dilute solution of BNF was made and its fluorescence was measured in a quartz cuvette. The excitation wavelength was 280 nm. The spectrum was recorded with 1 nm resolution. The spectra of the solid BNF were measured dropping some quantities of benzene solution on Whatman paper and after evaporating the solvents the fluorescence was measured with 1.5 nm resolution. For excitation, 362 nm wavelength radiation was used. For the measurement of the sensitized luminescence and the analytical determinations, benzene solutions of BNF and naphthacene were made. The concentration of BNF was 5 N 10 - 3 M!~-I ; that of naphthacene varied from 1 0 - 9 - 3 X 10 - 4 M~-1 Whatman paper stripes were immersed into the solutions and after evaporating benzene (about 10 rain) we measured the intensity of fluorescence under the same geometrical and slit conditions. For the excitation of sensitized luminescence light of 362 nm was used. The fluorescence intensity of BNF was measured at 402 n m while that of naphthacene at 493 nm.

3. Results BNF in solution or in solid form shows very intense fluorescence. The compound in ethanol emits a fluorescence spectrum consisting of three bands. The highest intensity is at 354.5 nm and the intensity of other bands gradually decreases toward long waves (fig. 1). The excitation spectrum shows six maxima. The highest intensity is at 333.5 nm. The peaks are shown in table 1. After having cooled the ethanol solution to 77 °K, the shape of both the emission and excitation spectra change. The

Table 1 Excitation and emission maxima of benzonaphthofuran and benzonaphthofuran-naphthacene systems at various temperatures (nm). Benzonaphthofuran In ethanol at 20 °C

BNF-naphthacene In ethanol at 77 °K

On Whatman paper at 20 °C

On Whatman paper at 77 °K

ex.

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ex.

em.

276 284 313.5 325 333.5 350

354.5 372 392

264 275 280 284 308 321 333.5 338 350

352 372 377.5 383.5 388 393 415

314 330 345 362.5 393.5

402 423 448 480

314 330 345 362.5

401 422 446 493.5 528.5 570

I. Hornyak, Sensitized luminescence in thin layers, 1/1

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I. Hornyak, Sensitized luminescence in thin layers. VI

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L Hornyak, Sensitized luminescence in thin layers, VI

wide diffuse bands split and a fine structure of vibration system appears (fig. 2). The maximum positions of emission subbands do not shift, but with the appearance of the new bands the intensity ratios change greatly. In solution the properties of the compounds are characteristic of the molecule, but in solid states the spectroscopic properties mostly belong to the whole crystal lattice. In the crystal there are very strong interactions between molecules and the fluorescence spectrum shifts to long wavelengths. The intensity o f the subbands rises proportionally to that of the main band. The excitation spectrum consists of a wide diffuse band in which some maxima have been seen (fig. 3). Cooling the compound to low temperature, as measured on Whatman paper, the subband intensity of emission spectrum decreases. In the diffuse bands o f the excitation spectrum the maximum positions become more discrete (fig. 4). Benzonaphthofuran is a very good energy donor, exciting the sensitized luminescence of naphthacene. Part of the absorbed energy is transported by radiationless transitions to the naphthacene, embedded into the energy-donor crystal lattice of naphthacene in a small quantity. The very low quantum efficiency increases enormously and so a very low concentration of naphthacene can be detected. The BNF-naphthacene system emits a complex fluorescence spectrum which contains both the characteristic peaks of BNF and ofnaphthacene. At the short wavelength side of the fluorescence spectrum, three bands originate from the BNF and the

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other bands from the naphthacene. The naphthacene quantities of 1 0 - 7 - 1 0 -6 M£-1 hardly influence the fluorescence spectrum of BNF. On increasing the quantity of the naphthacene, the fluorescence of the BNF gradually decreases and that of the naphthacene proportionally increases (fig. 5). If the quantity of the naphthacene approaches the order of the concentration of the BNF, the fluorescence spectrum contains only characteristic naphthacene bands. The phenomenon of sensitized luminescence was applied to the analytical determination of the naphthacene. The fluorescence intensities were measured at the maxima in the spectra of the naphthacene and BNF. The intensity ratio of the spectrum at 493 nm (1493) was plotted against the naphthacene concentration (fig. 6). The curve is linear in the 10 -5 - 3 X 10-4 M~-1 concentration range of naphthacene. This range can be reproduced within the relative error of 1%. For lower concentrations, the curve deviates from the linear one but for analytical purposes can be used. The relative error in this range increases up to 8-10%. The increase of the error is due to the influence of other contaminations, to the great differences of the intensity values, and to the non-uniformity of the samples. The curve is independent of the concentration of BNF. The fluorescence intensity of the benzonaphthofuran-naphthacene system has been measured at liquid nitrogen temperature and found to increase four times.

L Hornyak, Sensitized luminescence in thin layers. VI

References [1] B.L. Van Duuren, Anal. Chem. 32 (1960) 1436. [2] M. Zander, Phosphorimetry (Academic Press, New York, 1968). [3] A. Schmillen, Luminescence of Organic and Inorganic Materials. ed. H.P. KaUman and G.M. Spruch (Wiley, New York, 1962) pp. 30-43.

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