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Differently coloured polymorphs of 4-[(2-nitrophenyl)azo]phenol
A V Yatsenko1, K A Paseshnichenko
1Chemistry Department, Moscow State University, 119899 Moscow, Russia. yatsenko@biocryst.phys.msu.su
Summary
The color difference between brown-yellow and orange crystal forms of 4-[(2-nitrophenyl)diazenyl]phenol is due to mixed intramolecular and intermolecular electronic excitations, not molecular geometry. This finding aids in understanding polymorphism and color variation in organic compounds.
Area of Science:
- Crystallography and Solid-State Chemistry
- Computational Chemistry
- Spectroscopy
Background:
- Polymorphism, the ability of a solid material to exist in more than one crystal structure, can significantly impact material properties.
- The compound 4-[(2-nitrophenyl)diazenyl]phenol exhibits distinct brown-yellow and orange polymorphs, necessitating an investigation into the origin of their color difference.
Purpose of the Study:
- To determine the crystal structures of the brown-yellow and orange polymorphs of 4-[(2-nitrophenyl)diazenyl]phenol.
- To record and analyze their visible reflection spectra.
- To elucidate the electronic origins of the observed color variation using computational methods.
Main Methods:
- Single-crystal X-ray diffraction for structure determination.
- Visible reflection spectroscopy for spectral analysis.
- Ab initio and semi-empirical (AM1, INDO-CISD) quantum chemical calculations for electronic structure analysis.
Main Results:
- Crystal structures of both polymorphs were solved, revealing stacking arrangements with interstack hydrogen bonds.
- Visible reflection spectra were recorded for both forms.
- Computational analysis ruled out molecular geometry and intermolecular electrostatic interactions as the primary cause of color difference.
Conclusions:
- The color variation between the polymorphs is most likely attributed to the mixing of intramolecular n --> pi* and intermolecular charge-transfer excitations.
- This study provides insights into structure-property relationships in organic crystalline materials and the factors influencing color in polymorphs.