Photoluminescence and electroluminescence of methoxy and carboethoxy derivatives of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline

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Abstract

A series of methoxy (MO) and carboethoxy (CE) derivatives of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline ([DPPQ]) are characterized by spectroscopic methods. All dyes show the photoluminescent spectra which are highly solvatochromic. In the case of 6MO[DPPQ] and 6CE[DPPQ] the emission bands are broad and shifted to the red with increasing of solvent polarity whereas the dyes 6MO1pMO[DPPQ] and 6MO13pMO[DPPQ] exhibit a reverse solvatochromism. The large difference between the excited- and state-dipole moments indicates a strong electron transfer effects in all dyes. The EL spectra are obtained for the fabricated OLEDs with a general structure of EL device ITO/PVK:6X[DPPQ]/Ca/Al. The blue emission originating from PVK host matrix appears to be quenched in EL spectra of doped PVK matrix giving rise to emission in blue, blue-green or green spectral regions. The obtained results demonstrate that a series of newly synthesized methoxy and carboethoxy [DPPQ]-derivatives may be considered as promising materials for electroluminescent applications.

Introduction

Organic light emitting diodes (OLEDs) are electronic devices made of an one or a series of organic thin films placed between two conductors. Electroluminescence (EL) in the organic polymer film occurs via the recombination of excitonic electron–hole pairs. In the case when a host polymer matrix is doped by a guest organic dye (luminophore) such excitations may be transferred to a dopant molecules giving light emission in a certain spectral range. Among the organic luminophores pyrazoloquinoline (PQ, 1H-pyrazolo[3,4-b]quinoline) and its derivatives have recently emerged as ones of the most promising dopants for efficient EL applications [1], [2], [3]. PQ emits the light in the blue spectral region, but by introducing different types of moieties into the basic pattern one may shift the EL emission to a blue-green or green regions [4]. Green photoluminescence (PL) and EL have been recently also reported by Gondek et al. [5], [6]. The interest in molecular organic materials is thereby driven by the need to find new luminophores with desirable properties, including color gamut and high efficiency. Basic characteristics of newly synthesized materials may be obtained through studying of their electronic properties by means of optical absorption, PL and EL technique.

In the present paper we introduce several recently synthesized derivatives of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline ([DPPQ]) representing a subclass of PQ. In particular, molecules of several methoxy and carboethoxy [DPPQ]-derivatives will be in a focus of our studies. The general structure of these compounds may be given by the scheme:

where R1and R2 is H or OCH3 and R3 is H, OCH3 or COOC2H5. The equilibrium conformation of the organic compounds, as obtained from the quantum chemical modeling for the isolated molecule in vacuo (T=0 K) [7], are presented in Fig. 1, in particular, 6-methoxy-1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline (6MO[DPPQ]) [Fig. 1A], 6-methoxy-1,3-(p-methoxyphenyl)-1H-pyrazolo[3,4-b]quinoline (6MO13pMO[DPPQ]) [Fig. 1B], 6-methoxy-1-(p-methoxyphenyl)-1H-pyrazolo[3,4-b]quinoline (6MO1pMO[DPPQ]) [Fig. 1C] and 6-carboethoxy-1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline (6CE[DPPQ]) [Fig. 1D]. In our recent paper [7] we reported the results of the experimental investigations and quantum chemical calculations of the UV/Vis optical absorption spectra of methoxy and carboethoxy [DPPQ]-derivatives. A common feature of their equilibrium molecular conformation is two non-equivalent phenyl rings Ph1 and Ph2 connected to pyrazoloquinoline group by C–C or C–N bonds, respectively. The aromatic ring Ph1 shares with pyrazoloquinoline group the same plane, whereas Ph2 group is tilted out of this plane on a certain angle. A rotation of the aromatic group Ph1 is accompanied by equivalent deep minima of the total molecular energy (an energy barrier ΔE35 kcal/mol) corresponding to a planar conformation. On the other hand the phenyl group Ph2 is characterized by two angular equilibrium positions at ϕ1eq69° and ϕ2eq109° separated by a small energy barrier ΔE of about 0.12 kcal/mol. Since ΔERT, it can be easily overcome giving a large libration angular amplitude (up to ±45°) of the Ph2 ring at room temperature. Nevertheless, it was shown [7], [8] that the phenyl dynamics itself appears to be not so important neither in the spectral position of the absorption thresholds nor in a broadening of most absorbtion bands of a large number of 1,3-diphenyl derivatives of PQ. The broadening of absorption pattern and the band shifts may be obtained within molecular dynamics (MD) simulations. The combination of quantum chemical PM3 method with the MD simulations exhibits in most cases the best agreement with the experimental data.

The present paper is an extension of the spectroscopic studies on the PL and EL properties of methoxy and carboethoxy [DPPQ] derivatives. The PL spectra of these materials are measured in a several organic solvents of different polarity. The EL spectra are recorded for OLEDs based on doped poly(N-vinylcarbazole) (PVK) matrixes. We present also the fragments of the optical absorption spectra, which seem to be important in a general discussion of PL and EL properties as well as the luminance–current–voltage dependencies to give a more precise insight into the characterization of the fabricated EL devices.

Section snippets

Experimental

Synthesis of the methoxy and carboethoxy DPPQ derivatives used in the study is described in our previous article [7]. The optical absorption and PL spectra have been measured in organic solvents (Merck) of spectroscopic grade without their further purification. The optical absorption spectra were recorded in tetrahydrofuran (THF) solution (mass concentration of about 0.1%) using Shimadzu UV–vis 2101 scanning spectrophotometer in the range of 200–600 nm. The measurements were performed by means

Results and discussion

Fig. 2 shows normalized steady-state optical absorption and PL spectra (both recorded in THF solution) of 6X[DPPQ] and EL spectra of ITO/PVK:6X[DPPQ]/Ca/Al devices (X=MeO, CE, MO13pMO, MO1pMO). The positions of the absorption and photoemission maxima are collected in Table 1.

The optical absorption spectra of 6MO[DPPQ](Fig. 2a) and 6CE[DPPQ] (Fig. 2b) are characterized by three to four relatively intense broad bands in the spectral region of 230–600 nm [7]. Fig. 2 shows the region of the first

Conclusion

A series of methoxy and carboethoxy [DPPQ]-derivatives have been characterized by spectroscopic methods. The optical absorption and PL are measured in the organic solutions of different polarity. All 6X[DPPQ] derivatives show the PL spectra which are highly solvatochromic. In the case of 6MO[DPPQ] and 6CE[DPPQ] the emission bands are broad and shifted to the red with increasing of solvent polarity. The large difference between the excited- and state-dipole moments indicates on strong electron

Acknowledgement

This work was supported by KBN grant no. 3T11B 074 26.

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