Ultrafast Spectroscopies of Nitrophenols and Nitrophenolates in Solution: From Electronic Dynamics and Vibrational Structures to Photochemical and Environmental Implications

• Published in: Molecules (Basel, Switzerland) Vol. 28; no. 2; p. 601
• Main Authors: Bailey-Darland, Sullivan, Krueger, Taylor D, Fang, Chong
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 06.01.2023; MDPI
• Subjects: Charge transfer; Chemistry; Chromophores; Dynamic structural analysis; Excitation; femtosecond stimulated Raman; Hydrocarbons; Light; Light effects; molecular rotor; Nitrophenol; nitrophenol compounds; Nitrous acid; Organic chemistry; Oxidative stress; Peak frequency; Phenolic compounds; Phenols; Photochemicals; Photodegradation; photophysics and photochemistry; Protonation; Raman spectroscopy; Solvation; Spectroscopy; Substitutes; ultrafast spectroscopy; Waterways; molecular rotor; femtosecond stimulated Raman; nitrophenol compounds; photophysics and photochemistry; charge transfer; ultrafast spectroscopy
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules28020601

Nitrophenols are a group of small organic molecules with significant environmental implications from the atmosphere to waterways. In this work, we investigate a series of nitrophenols and nitrophenolates, with the contrasting -, -, and -substituted nitro group to the phenolic hydroxy or phenolate oxygen site (2/3/4NP or NP ), implementing a suite of steady-state and time-resolved spectroscopic techniques that include UV/Visible spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy with probe-dependent and global analysis, and femtosecond stimulated Raman spectroscopy (FSRS), aided by quantum calculations. The excitation-dependent (400 and 267 nm) electronic dynamics in water and methanol, for six protonated or deprotonated nitrophenol molecules (three regioisomers in each set), enable a systematic investigation of the excited-state dynamics of these functional "nanomachines" that can undergo nitro-group twisting (as a rotor), excited-state intramolecular or intermolecular proton transfer (donor-acceptor, ESIPT, or ESPT), solvation, and cooling (chromophore) events on molecular timescales. In particular, the -substituted compound 3NP or 3NP exhibits the strongest charge-transfer character with FSRS signatures (e.g., C-N peak frequency), and thus, does not favor nitroaromatic twist in the excited state, while the -substituted compound 2NP can undergo ESIPT in water and likely generate nitrous acid (HONO) after 267 nm excitation. The delineated mechanistic insights into the nitro-substituent-location-, protonation-, solvent-, and excitation-wavelength-dependent effects on nitrophenols, in conjunction with the ultraviolet-light-induced degradation of 2NP in water, substantiates an appealing discovery loop to characterize and engineer functional molecules for environmental applications.