Photophysics of hydroxy-nitrobenzoic acid UV-MALDI matrices investigated via spectroscopic, photoacoustic, and luminescence techniques.

The photophysical behavior of novel UV-MALDI matrices belonging to the family of hydroxy-nitrobenzoic acids (HNBAs) was investigated in both acetonitrile and methanol solutions, as well as in the solid state, using spectroscopic, luminescence, and photoacoustic techniques. The study focused on the following positional isomers: 2-hydroxy-3-nitrobenzoic acid, 2-hydroxy-4-nitrobenzoic acid, 2-hydroxy-5-nitrobenzoic acid, 3-hydroxy-2-nitrobenzoic acid, 3-hydroxy-4-nitrobenzoic acid, 3-hydroxy-5-nitrobenzoic acid, 4-hydroxy-3-nitrobenzoic acid, and 5-hydroxy-2-nitrobenzoic acid. Most isomers exhibited strong solid-state absorption at the excitation wavelengths commonly used in UV-MALDI technique (>90% at 337 and 355 nm), high photostability, no detectable luminescence, and highly efficient excitation energy dissipation as prompt heat, resembling the behavior of calorimetric references under both N2- and air-saturated conditions, suggesting a potential role in facilitating desorption/ionization processes in UV-MALDI. These findings provide valuable insight into the performance of HNBAs as matrix candidates.