Al ³⁺ induced planarization, conformational arrest and metallochromic shift in a pyrimidine dione dye: Insight from integrated hybrid quantum-classical calculations

In order to explore the possibilities of simulating metallochromism by modern molecular modeling, we apply a sequential hybrid quantum-classical approach to a prototype metallochromic system - the Al ³⁺ ion and pyrimidinedione (PY) dye complex. The complex shows several structural features with relevance for the metallochromism: the PY dye exhibits conformers with dynamical transitions between twisted structures, which are inhibited by the addition of the metal ion leading to planarization and a conformational arrest: the Al ³⁺ ion behaves like a structure-modifier for both intra and intermolecular degrees of freedom and with respect to the intermolecular solvation shell structure. The sequential approach that we have employed uses DFT/MM molecular dynamics for structure modeling and TDDFT/PCM for property modeling. The computed metallochromic shift between PY and the Al(PY) ³⁺ complex in DMSO solvent is obtained in excellent agreement with experiment. The results infer optimism for future use of such modeling techniques to design metallochromic indicators.