Color changes caused by conformational polymorphism: Optical-crystallography, single-crystal spectroscopy, and computational chemistry

5-Methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (1) crystallizes as seven conformational polymorphs of red, orange, and yellow colors. Comparable thermodynamic stability of the polymorphs enables the solution of six crystal structures, a number unmatched by current entries in the Cambridge Structural Database, making I well suited for structure-property studies. The different crystal colors of 1 have been studied using optical crystallography, polarized single-crystal absorption spectroscopy and computational methods. Different crystal colors ("color polymorphism") are well explained by the conformational differences between polymorphs, which cause varying degrees of pi-conjugation between the o-nitroaniline chromophore and the thiophene group. The isolation of different conformers in crystal polymorphs permits an evaluation of computational models of electronic structures and transitions through observed spectral properties. The newly matured Time Dependent Density Functional Theory gave the best reproduction of observed wavelengths, oscillator strengths, and directions of electronic transition dipole moments, outperforming Configuration Interaction Singles, and even ZINDO, a semiempirical model specially calibrated on electronic spectra.