SYNTHESIS AND PHOTOPHYSICAL STUDIES OF HOMOLEPTIC COPPER(I) COMPLEXES

ABSTRACT

In this work, homoleptic copper(I) complexes with organic ligands were obtained via a mechanochemical route. This method was found to be a better choice for preparing pure coordination complexes with high yield. The complexes show metal-to-ligand charge transfer (MLCT) bands in the UV-vis spectrum. Using sterically bulky ligand found to support further MLCT transitions. Raman spectroscopy analysis showed that coordination takes place through the nitrogen atom. Fourier transform infrared spectroscopy studies also confirm the bonding character. Diffuse reflectance studies confirmed their photoelectronic properties.

АННОТАЦИЯ

В этой работе гомолептические комплексы меди(I) с органическими лигандами были получены механохимическим путем. Было обнаружено, что этот метод является лучшим выбором для приготовления чистых координационных комплексов с высоким выходом. Комплексы показывают полосы переноса заряда от металла к лиганду (MLCT) в УФ-видимом спектре. Использование стерически объемного лиганда, как обнаружено, поддерживает дальнейшие переходы MLCT. Анализ спектроскопии комбинационного рассеяния показал, что координация происходит через атом азота. Исследования инфракрасной спектроскопии с преобразованием Фурье также подтверждают характер связи. Исследования диффузного отражения подтвердили их фотоэлектронные свойства.

Keywords: copper(I) complexes, homoleptic, mechanochemical synthesis, optical spectroscopy, structure.

Ключевые слова: комплексы меди(I), гомолептические, механохимический синтез, оптическая спектроскопия, структура.

Introduction.

For many years transition metal complexes have attracted great attention because of their numerous applications. One of the most explored domains is photovoltaics. Utilization of charge-transfer dyes first commenced when Brian O'Regan and Michael Grätzel reported their revolutionary work in 1991[1].  The general mechanisms for light-to-electrical power conversion in dye sensitizer solar cells us as follows (i) light is absorbed by a sensitizer to form a molecular excited state; (ii) the excited state may inject an electron into the semiconductor thus causing charge separation; (iii) the oxidized sensitizer is ‘‘regenerated’’ by an external electron donor [2,3]. Once the electron has performed useful work in the external circuit, it returns to a counter electrode where it reduces the oxidized electron donor. Hence the solar cell is termed “regenerative” as all oxidation chemistry at the dye-sensitized electrode is reversed at a dark counter electrode such that no net chemistry occurs [4]. 

Coordination chemistry of copper, particularly Cu(I) is very diverse [5-7]. A tetrahedral environment is favored for Cu(I), it forms stable complexes with different ligands. Copper possesses d10 electron configuration in copper(I) complexes. An entirely filled outer electronic shell causes a symmetrical distribution of the electronic density, which favors a tetrahedral arrangement of ligands around the central ion. Structural nature of the complex attributes to and even reveals the overall photochemical and electrochemical properties. Several publications have attempted to explain structural changes and photochemical properties through excitation and relaxation processes combining ultrafast spectroscopic experiments and in theoretical way [8].

In this work two homoleptic complexes of copper(I) were prepared using a mechanochemical route. Photophysical properties explored using optical and structural methods.

Scheme 1. Structure of 3,4,7,8-tetramethyl-1,10-phenanthroline.

Experimental.

Tetrakis(acetonytril)copper(I) hexafluorophosphate [Cu(CH₃CN)₄]PF₆, 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen), 2,2ˈ-biquinoline were purchased from Haihang Industry Co., Ltd (PRC). All solvents were analytical grade and used without any further purification.

FTIR spectra were acquired on spectrophotometer IRAffinity-1S (Shimadzu, Japan) in ATR mode. Raman spectra were recorded on InSpect confocal Raman microscope (Renishaw, UK). UV-vis and DRS spectra were obtained using UV-2600 (Shimadzu, Japan) in integrating sphere.

Synthesis of complexes

Tetrakis(acetonitrile)copper(I) hexafluorophosphate (0.08 mmol, 29.8 mg)) and 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) (0.16 mmol, 33 mg) were taken and placed in an agate mortar. The precursor mixture was initially colorless, but after 10 min mixing it turned orange, after 20 min it turned light red, and after 30 min it turned dark red. This color remained unchanged after 40–50 min.

Scheme 2. Synthesis of a homoleptic complex [Cu(tmphen)₂]PF₆.

Tetrakis(acetonitrile)copper(I) hexafluorophosphate (0.07 mmol, 26.1 mg)) and 2,2ˈ-bixiquinoline (biq) (0.14 mmol, 35.9 mg) were taken and placed in an agate mortar. Initially, the precursor mixture was colorless, but after 10 minutes it turned light purple and after 20 minutes it turned dark purple. This color remained unchanged after 30, 40, and 50 minutes.

Scheme 2. Synthesis of a homoleptic complex [Cu(biq)₂]PF₆.

Results and discussion.

IR and Raman spectra of the complex [Cu(biq)₂]PF₆ given below (Fig. 1 and Fig. 2 respectively).

The analysis of the IR and Raman spectra shows the following results: the aromatic ring C–H stretching vibration bands are very weak in intensity, appearing as a band around 3000 cm^-1. The stretching vibration of the methyl group at carbons 3,4,7,8 of the ring appears as a weak band at a frequency of 2915 cm^-1. A narrow sharp band with very strong intensity was seen at 731 cm^-1 and is a signal of the resonance vibration of the deformation vibration of the ring C–H bond. The C=N stretching vibration band in the ring appears at 1618 cm^-1 and the C=C stretching vibration band at 1542 cm^-1. The signals located around 826 cm^-1 are attributed to the stretching vibrations of the hexafluorophosphate anion. The shifts in the vibrational bands of the [Cu(tmphen)₂]PF₆ complex indicate that a coordination bond is formed between the nitrogen atom (donor) and the copper(I) ion (acceptor) during complex formation, i.e., while the C=N bond valence vibration frequency shifts from 1609 cm^-1 to 1618 cm^-1 (due to the metal withdrawing electron cloud density from the phenanthroline nitrogen), the C=C stretching vibration frequency of the phenanthroline aromatic ring shifts from 1510 cm^-1 to 1542 cm^-1 (due to changes in delocalization).1652 cm^-1.

UV-vis spectrum of [Cu(tmphen)₂]PF₆ in different solvents is given below (Fig. 3). 

From the above spectrum, it can be seen that the complex [Cu(tmphen)₂]PF₆ exhibits a characteristic absorption band in the spectrum due to its good solubility in acetonitrile, dimethylformamide, methanol and dimethylsulfoxide. The absorption maxima are located at 430, 434, 428 and 435 nm, respectively. The bands in the ultraviolet region are considered to be the absorption bands of the corresponding ligands or the copper(I) precursor since the complex is insoluble in the same solvent. Such intense bands in the visible region are of MLCT nature and arise from the electron transfer from the d-orbital of copper to the π* orbital of the tetramethylphenanthroline ligand. Ligand-centered LC bands are not visible in all spectra.

Above are the band gap energies estimated using the diffuse reflectance spectrum of the complex [Cu(tmphen)₂]PF₆ (Fig. 4.). It can be seen that the complex [Cu(tmphen)₂]PF₆. has a true semiconductor nature. Calculations have shown that the band gap in this complex is 2.76 eV. This indicates that the complex absorbs more light and performs better charge injection.

The IR spectra of the complex [Cu(biq)₂]PF₆ and its corresponding precursors are presented below (Fig. 5). 

The analysis of the IR spectra shows the following results: the aromatic ring C–H stretching vibration bands are very weak in intensity, appearing as weak bands around 3048 cm^-1. A narrow sharp band of moderate intensity was seen at 731 cm^-1 and is a signal of the resonance vibration of the deformation vibration of the ring C–H bond. The C=N stretching vibration band in the ring appears at 1618 cm^-1 and the C=C stretching vibration band at 1593 cm^-1. The signals located around 837 cm^-1 are attributed to the stretching vibrations of the hexafluorophosphate anion. The shifts in the vibrational bands of the [Cu(biq)₂]PF₆ complex indicate that a coordination bond is formed between the nitrogen atom (donor) and the copper(I) ion (acceptor) during complex formation, i.e., while the C=N bond stretching vibration frequency shifts from 1610 cm^-1 to 1618 cm^-1 (due to the metal absorbing the electron cloud density from the quinoline nitrogen), the quinoline aromatic ring C=C stretching vibration frequency shifts from 1600 cm^-1 to 1593 cm^-1 (due to changes in delocalization).

The Raman spectra of the complex [Cu(biq)₂]PF₆ can be analyzed as follows. The band corresponding to the C=N bond is intensely visible at a frequency of 1590 cm^-1. The second band with high intensity is also due to the symmetric vibration of the C=C bond at 1460, 1425 cm^-1. A new vibration of the metal-ligand bond is observed at 415 cm^-1.

The absorption spectra of the biquinoline complex of copper(I) in various solvents are presented in Figure 7.

From the above spectrum, the absorption bands characteristic of the complex can be seen in the spectrum, since the complex [Cu(biq)₂]PF₆ is well soluble in tetrachloromethane, acetonitrile, dimethylsulfoxide, methanol and dimethylformamide. The absorption maxima are located at 554, 546, 552, 544 and 550 nm, respectively. The bands in the ultraviolet region are the absorption bands of their respective ligands or copper(I) precursors, since the complex is insoluble in the same solvent. Such intense bands in the visible region are of MLCT nature and arise from the electron transfer from the copper d-orbital to the π* orbital of the biquinoline ligand. Ligand-centered LC bands are not visible in all spectra.

From Fig. 8, it can be seen that the complex [Cu(biq)₂]PF₆ has a proper semiconductor nature. Calculations have shown that the band gap in this complex is 2.81 eV. This indicates that the HOM1 complex absorbs more light and performs better charge injection.

Conclusion

The UV-vis spectroscopy of complexes in different solvents showed their stability in both polar and nonpolar media. DRS studies confirmed that both complexes exhibit good semiconductor characteristics and have high molar absorptivity. Both sterically hindered organic ligands enhance MLCT transitions. FTIR and Raman spectral studies confirmed metal-ligand bond formation via nitrogen atoms in the ring. DRS studies showed that the complexes have a direct band gap nature.

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