triboluminescence in n-alkyl and n-alkyl-3-substituted carbazole crystals

Volume 125, number 4 CHEMICAL PHYSICS LETTERS 11 April 1986

TRIBOLUMINESCENCE IN N-ALKYL AND N-ALKYL-3-SUBSTITUTED CARBAZOLE CRYSTALS

Noboru KITAMURA, Onusa SARAVARI I, Haeng-Boo KIM and Shigeo TAZUKE

Research Laboratory of Resources Utilization, Tokyo Institute of Technology,

4259 Nagatsuta, Midon-ku, Yokohama 227. Japan

Received 4 October 1985; in final form 27 December 1985

The triboluminescence (TL) of crystals of four carbazole derivatives was investigated by means of a multichannel

plate/photodiode array detector system. 3,9_diethylcarbazole and N-isopropyl-3-vinylcarbazole were demonstrated for the first

time to be triboluminescenc. The TL active carbazole crystals belong to polar space groups, while the non-TL active crystals

belong to non-polar ones. This suggests that TL activity in carbazole derivatives is closely related to crystal structure, in

particular, to the space group polarity and thus to the generation of piezo- or pyro-electric charges.

1. Introduction

Recent spectroscopic studies on triboluminescence (TL), the luminescence caused by application of mechanical energy to a solid, have shown a number of TL active inorganic and organic compounds as well as their excited-state origins of the luminescence [ 11. The two most common origins of TL are luminescence from the molecules comprising the crystal and luminescence from adsorbed molecular nitrogen. Although a detailed assignment of the excited-state origins and spectral features of TL is now possible owing to ad- vances in TL spectroscopy, the mechanisms of exci- tation bringing about TL are poorly understood.

In 1981, we discovered the unique character of N-ethyl-3-vinylcarbazole (NE3VCz) for the first time; it is triboluminescent, tribopolymerizable, and also photopolymerizable in the solid state [2]. These phenomena are characteristic of this particular carbazole derivative and are extremely sensitive to the crystal structure. More recently, Nowak et al. [3] demonstrated that N-isopropylcarbazole (NIPCz) showed strong triboluminescence with vibronic fine structure around 385,412,435 and 465 nm. The tri-

’ Present address: Department of Material Science, Faculty of

Science, Chulalongkorn University, Bangkok, Thailand.

boluminescence spectrum of NIPCz, however, does not agree with the reported photoluminescence spectrum of N-alkylcarbazole in the solid state [4 1. Fur- thermore, it is well known that commercially available carbazole or carbazole derivatives contain anthracene as the main impurity which cannot be re- moved by any method of purification [4]. Thus, we suspect that the TL spectrum by Nowak et al. is due to fluorescence from anthracene and not due to pure TL of NIPCz. Our preliminary report on TL of NE3VCz also seems to be influenced by anthracene fluorescence [2].

In order to obtain the true TL spectra of carbazole derivatives, we synthesized carbazole free from anthracene impurity and N-alkyl and N-alkyl3_sub- stituted carbazoles. In this Letter, we report the TL and photoluminescence (PL) of several carbazole derivatives thus synthesized and discuss the mecha- nism of TL.

2. Experimental

The structures and abbreviations of the carbazole derivatives used in this study are shown in fig. 1. An- thracene-free carbazole was synthesized by conden- sation of phenylhydrazine and cyclohexahone in glacial

360 0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)


&CH=,, &CH2CH3

NIP3VCz DECz -

Fig. 1. Structures and abbreviations of carbazole derivatives. NIPCz; N-isopropylcarbazole, NE3VCz; Nethyl3-vinylcarbazole, NIP3VCz; N-isopropyl-3+inylcarbazole, DECz; 3,9- diethylcarbazole. .

acetic acid for 3 h followed by dehydrogenation over Pd/C [5]. N-alkyl3-substituted carbazoles were prepared according to the literature [2] ; further details will be described elsewhere [6]. NIPCz represents anthracene-free N-isopropylcarbazole prepared from carbazole as described above. NIP&(s) was obtained by a reaction of commercial carbazole (Tokyo Kasei, guaranteed reagent) with isopropyl bromide. NIPCz(c) was purchased from Kanto Chemicals (guaranteed reagent) and was used as supplied. All the compounds except for NICPz(c) were purified by repeated recrys- tallization and were identified by NMR and elemen- tal analysis.

TL and PL spectra were measured by a Spectro- metric multichannel analyzer (SMA, Tokyo Instru- ments Inc./Princeton Instruments Inc.) consisting of an IRYd 12 multichannel plate/photodiode array detector, an HR-320 monochromator, an ST-l 10 SMA controller, and a Multi 16-11 microcomputer (Mitsubishi Electric Co.). Emission spectra were not corrected for the instrumental response. For the TL experiments, the crystals were rubbed in a glass vessel in front of a monochromator slit under aerobic conditions. For the PL experiments, the crystals in the same glass vessel were irradiated by a mercury lamp. The spectra were calibrated using a mercury lamp.

3. Results

Figs. 2a, 3a and 3b show TL spectra of anthra-

h(nm)

Fig. 2. Tribo- and photo-luminescence spectra of N-isopropylcarbazole crystals. Solid and broken lines represent TL and PL spectra, respectively. (a) NIPCz, (b) NIPCz(s), (c) NIPCz(c), (d) reproduced from ref. [ 31.

cene-free NIPCz, NE3VCz and NIP3VCz, respectively, together with the corresponding PL spectra measured under the same experimental conditions. For comparison, the TL and PL spectra of NIPCz(s) and NIPCz(c) are shown in figs. 2b and 2c, respectively. DECz showed a blue luminescence similar to other TL active carbazole crystals under mechanical stress. However, the TL intensity was so weak that no reli-

h(nm)

Fig. 3. Tribo- and photo-luminescence spectra of (a) NE3VCz and (b) NIP3VCz. Solid and broken lines represent TL and PL spectra, respectively.

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able spectrum could be recorded. NIPCz and NE3VCz have already been reported to be triboluminescent [2,3], while in the present study NIP3VCz and DECz were found for the first time to be triboluminescent.

4. Discussion

The TL spectra of anthracene-free NIPCz, NE3VCz and NIP3VCz are almost identical with the corresponding PL spectra except for their peak- ing wavelengths. Since the PL spectra of these crystals have been assigned to monomer fluorescence [7], the present TL can be assigned to monomer fluorescence on the basis of the similarities between the TL and PL spectra.

The TL spectra of NE3VCz and NIP3VCz shift to the red in comparison with the PL spectra. From high- pressure studies of PL for aromatic hydrocarbon crystals, it is well established that the PL spectrum shifts to the red with increasing hydrostatic pressure [8]. The origin of the red-shift is supposed to be either a broadening of the vibrational potential well or an in- crease in the equilibrium nuclear distances in the excited state with pressure change. Furthermore, changes in the vibronic intensities of the TL in comparison with those for PL are common features of TL, as reported by Zink [ 11. It has been suggested that changes in the vibronic intensities of the TL are related to changes in the Franck-Condon factors [ 11. In the present study, we confirmed a pressure-induced red- shift of PL (x3 nm) in NE3VCz crystals under pres- sures of O-8 kg/cm2. It is noteworthy that a hydrostatic pressure as small as 8 kg/cm2 induces a significant change in the PL spectrum. In the actual TL ex- periment, the pressure applied to a unit area of crystal surface or edge may be much greater than 8 kg/ cm2, so that the present large red-shifts of TL for NE3VCz and NIP3VCz are explicable on the basis of the pressure effects mentioned above.

Neither of the TL and PL spectra of NIPCz(s) and NIPCz(c) (figs. 2b and 2c, respectively) agree with those of anthracene-free NIPCz (fig. 2a). Recently, Nowak et al. reported the TL spectrum of N-isopropylcarbazole (they did not mention the source of the chemical but used it after purification by multiple sublimation and zone refining) [3]. The TL spectrum was reproduced as shown in fig. 2d. The TL spectrum

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by Nowak et al. is clearly different from our result as shown in fig. 2a. As has been reported by Kato et al. [4], commercially available carbazole or carbazole derivatives contain a significant amount of anthracene (more than 10m3 mole/mole), which cannot be re- moved by the usual methods of purification including zone melting. Anthracene-free NIPCz shows fluorescence around 370 nm with no distinguishable vibronic structure, while the PL of anthracene crystals exhibits sharp vibronic structure with maxima around 425, 445, and 472 nm. The fluorescence spectrum of the anthracene (guest)-N-ethylcarbazole (host) mixed crystal reported in ref. [4] resembles closely that of the TL of N-isopropylcarbazole obtained by Nowak et al. Thus the TL and PL spectra in figs. 2b (NIPCz(s)), 2c (NIPCz(c)), and 2d (Nowak et al.) are not ascribable to luminescence from the excited carbazole group but to the superimposed spectrum of carbazole and anthracene fluorescence. Since the singlet energy transfer from N-alkylcarbazole (S1 = 100 kcal/mol [9]) to anthracene (S1 = 76.3 kcal/mol [lo]) is exothermic and, therefore, an efficient pro- cess, contamination of the TL by anthracene fluorescence is highly likely.

On the other hand, it has been suggested that TL activity is closely related to the space group polarity of the crystals [3]. Our preliminary study on X-ray crystallography indicates that there is an excellent correlation between TL activity and space group polarity for various carbazole derivatives [7,11]. Name- ly, all TL active crystals belong to polar space groups (i.e. NIPCz orthorhombic, Iba2; NE3VCz and DECz trigonal, R3c t) while non-TL crystals such as N- ethylcarbazole, N-methyl-3vinylcarbazole, N-benzyl- 3vinylcarbazole belong to non-polar ones. In crystals of the former category, externally applied mechanical energy generates piezo- or pyroelectric charges on the crystal surface along the direction of polarity, which results in the creation of a high electric field. This electric field is large enough to produce electronical- ly excited states by certain mechanisms involving elec- tron impact or recombination of charge carriers.

Further experimental details including X-ray crystallography of the present carbazole derivatives will be reported in a forthcoming paper [6].

* For NIP3VCq we failed to grow a single crystal suitable for X-ray crystallography.

Volume 125, numba 4 CHEMICAL PHYSICS LETTERS 11 April 1986

Acknowledgement

The authors are greatly indebted to Professor Y. Sasada and Dr. A. Takenaka of the Laboratory of Chemistry for Natural Products at the Tokyo In- stitute of Technology for their guidance and colla- boration in the X-ray crystallographic study. ST and NK also thank Tokyo Instruments Inc. for permitting us to use their SMA system.

References

[l] J.I. Zink, Accounts Chem. Res. ll(l978) 289. [2] T. Inoue and S. Tazuke, Chem. Letters (1981) 589;

S. Tazuke, 0. Supakorn and T. Inoue, J. Polym. Sci. Polym. Chem. Ed. 20 (1982) 2239.

[ 31 R. Nowak, A. fiajewska and M. Samoc, Chem. Phys. Letters 94 (1983) 270.

[4] K. Kato, M. Yokoyama, K. Okamoto, S. Kusabayashl, H. Mikawa, K. Yoshihara and S. Nagakura, Mol. Cryst. Liquid Cryst. 28 (1974) 37.

[S] C.V. Rogers and B.B. Corson, J. Am. Chem. Sot. 69 (1947) 2910; E.C. Homing, M.G. Homing and G.N. Walker, J. Am. Chem. Sot. 70 (1948) 3935; T.S. Stevens and S.H. Tucker, J. Chem. Sot. (1923) 2145.

[6] 0. Saravari, N. Kitamura and S. Tazuke, to be published. [7] 0. Saravari, MC. Thesis, Tokyo Institute of Technology

(1984). [S] H.W. Offen, in: Organic molecular photophysics, Vol. 1,

ed. J.B. Birks (WiIey, New York, 1973). [9] G.E. Johnson, J. Chem. Phys. 62 (1975) 4697.

[lo] S.L. Murov, Handbook of photochemistry (Dekker, New York, 1973).

[ 111 0. Saravari, N. Kitamura, S. Tazuke, A. Takenaka and Y. Sasada, Acta Cryst. C40 (1984) 1617.

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triboluminescence in n-alkyl and n-alkyl-3-substituted carbazole crystals

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