Unraveling the structural, optoelectronic and vibrational properties of L-phenylalanine L-phenylalaninium malonate crystal: An experimental and theoretical approach

A combined experimental and computational investigation was conducted on the crystal L-phenylalanine L-phenylalaninium malonate (LPPMA) to explore its structural, electronic, vibrational, and thermal properties. X-ray diffraction analysis confirmed that LPPMA crystallizes in the monoclinic P2₁ space group, with zwitterionic and cationic phenylalanine molecules and malonate anions forming hydrogen-bonded layers and orthogonal hydrophobic stacking. Molecular and solid-state (periodic) DFT calculations were also performed. The LDA-D band structure revealed a quasi-degenerate fundamental electronic gap of 4.14 eV, with the highest occupied and lowest unoccupied crystal orbitals (HOCO and LUCO) localized on Phe and Phe⁺ units, respectively. Complementary hybrid functional calculations using HSE06 yielded an increased band gap of 5.48 eV, providing a more accurate upper-bound estimate in agreement with the material's insulating character. TD-DFT/B3LYP molecular calculations predicted a strong vertical excitation at 5.69 eV, in close agreement with the most intense experimental UV–vis absorption band at 5.99 eV. Temperature-dependent Raman spectroscopy (298–468 K) demonstrated redshifts and progressive intensity loss of vibrational modes, culminating in irreversible spectral suppression above 468 K. DFPT simulations supported vibrational assignments and elucidated the role of supramolecular interactions in stabilizing the crystal structure under thermal stress. Thermogravimetric and thermal analyses identified a five-step thermal degradation process, associated with the sequential release of NH₃, CO₂, and CH₂ fragments.