On the Ruhemann's Purple electronic spectrum: the role of torsion angle and coordination with Zn(II).

The Ruhemann's Purple (RP) is a chromophore formed from ninhydrin reaction with amines mainly applied to develop latent fingerprints in forensic science. Further enhancement of RP photoluminescence is achieved thought post-treatment with transition metal salt (commonly ZnCl2 alcohol solution), due to the coordination of RP with the metal ion. The literature attributes this improvement to the RP planarity increase in the complex, suggesting this characteristic as important to the post-treatment success. In the present work, the experimentally reported ZnRP complexes and the isolated RP ligand were investigated at the ωB97XD/6-311++G(d,p)/LanL2DZ/IEFPCM(MeOH) level, and their geometrical parameters, complexation energies, and spectroscopic properties (harmonic vibrations and electronic spectrum) were evaluated and agree with the experimental data. The systematic variation of the RP planarity (tilt angle) showed that the lowest energy electronic transition is quite sensitive to RP of this torsion angle. However, the RP planarity increase corresponds to approximately 30% of the total band shift achieved in the complex formation. The results reinforce the importance of the metal ion in the photoluminesce property changing and draw attention to improvements on transition metal complexes design in the post-treatment of developed latent fingermarks.