Interplay of micro- and macroscopic solvation in spectral responses of PACO in 1,4-dioxane–water mixtures: A combined experimental and quantum chemical analysis

• Spectral response of PACO is studied in 1,4-dioxane–water mixtures.
• PACO–nH2O (n=1,2,3n=1,2,3) molecular clusters formation are investigated in the ground state.
• Dielectric enrichment occurs in the excited state.
• Resonance assisted hydrogen bonding between PACO and water has been observed.
• Crossing between S1 and S2 states occurs in PACO–3H2O microcluster.

In this paper, we report the spectral signatures of 3-(phenylamino)-2-cyclohexen-1-one (PACO) in a series of 1,4-dioxane–water mixtures obtained with steady-state and time-resolved spectroscopic techniques. PACO appears to sense changes in local solvent structures when water is added gradually to pure 1,4-dioxane. For water mole fraction (χw) < 0.85, the emission primarily originates from PACO solvated by pseudopolar 1,4-dioxane molecules. For χw ≥ 0.85 dual emission as observed in bulk water takes over. Time-resolved emission spectroscopy suggests progressive formation of hydrogen bonded PACO–water clusters as χw increases. Quantum chemical calculations (DFT) predict H-bonding lengths and angles in free PACO and PACO–nH2O (n=1,2,3n=1,2,3) molecular clusters that seem to be consistent with the trend suggested by a resonance assisted hydrogen bonding (RAHB) descriptor (Q). The hydrogen bonding energies echo the same pattern only in the gas phase. A crossing between the first (S1) and the second excited (S2) states takes place in the PACO–3H2O microcluster. Calculations with long-range corrected DFT functional, CAM-B3LYP also corroborate the role of RAHB in the aforesaid clusters. The results presented are expected to shed light on the pathways of many photophysical processes that are influenced by either H-bonding or RAHB process.

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