Dual-Phase Photochromism: Synthesis, Spectroscopy, Theoretical Insights, and Functional Application

Advanced reversible photochromism in the dual phases (solid and liquid phases of the same material) is vital for practical device integration. In this study, a nonphotochromic Schiff base ligand, rhodamine B-3,5-ditert-butylsalicylaldehyde hydrazone (L-2), and its Zn complex (L-2-Zn Complex) are synthesized to achieve dual-phase photochromism. Job’s plot (1:1 stoichiometry), isosbestic point (progress of the reaction), ESI-MS (mass of the product), and Benesi–Hildebrand analysis (binding constant = 4.81 × 104 M–1) confirm the complex formation. The complex exhibits a reversible color change from pale greenish-yellow to deep purple-pink under UV light (photoconversion) and reverts in the dark (photoreversion) or upon heating (thermoreversion), both in solution and on solid support. Poloxamer-407 acts as the most efficient solid support through the formation of a SCM (soft coordination microenvironment). The SCM is a significant strategy for a solid-state photochromic system. The polymer-supported dual-phase photochromic material becomes easy processable, quicker responsive, better fatigue durable, and multiwavelength (UV + vis) excitable (i.e., dual mode photochromism, DMP). Thus, it makes a significant advancement over single-phase metal-guided photochromism. UV–vis spectra reveal an absorption band at 553 nm due to photocoloration. The initial (pale greenish-yellow) form emits a strong emission at 526 nm (greenish-yellow), while the final (purple pink) form exhibits weak split bands at 522 and 580 nm. Both photoconversion and thermoreversion follow first-order kinetics with a rate constant of 1.04 × 10–3 s–1 and 3.49 × 10–4 s–1 at 20 °C, respectively. Thermal bleaching accelerates with increasing temperature and becomes optimum at 50 °C. Spectrokinetic, thermodynamic, and DFT analyses confirm a unique dual-phase metal-guided photochromic system with potential for advanced optical applications in photopatterning and photoinking.