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

Abstract

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 ortho-, meta-, and para-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 meta-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 ortho-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.

The Role of Aqueous Solvation on the Intersystem Crossing of Nitrophenols

“…In aqueous solution, a bathochromic shift is observed in the maxima of the absorption spectra analogous to experimentally measured absorption spectra (SI Figure S1). Nevertheless, both spectra computed with QM/MM and full TDDFT are red-shifted as compared to an experimentally measured absorption spectrum in aqueous solution . The difference between QM/MM and full TDDFT solvation is small, with an additional 9 nm redshift for the latter.…”

“…Most previous 4-NP studies were performed in solution and elaborated on the solvatochromic shift and pH dependence. − Experimentally measured absorption spectra in aqueous solution located a maximum at 3.88 eV, 3.90 eV, or 3.91 eV. , Figure a summarizes the 4-NP absorption spectra calculated in this work. Whereas the absorption peaks at 3.81 eV in full TDDFT solvation, a reduced redshift as compared to the gas phase is obtained with QM/MM (maximum at 4.1 eV).…”

“…Experimentally, the excited state dynamics of nitrophenols was found to significantly depend on the solvent and corresponding potential proton transfer processes . Ernst et al did not clearly observe a solvent effect for 2-NP, but they acknowledge the limited experimental resolution and noise from excited state absorption .…”

Ultrafast Excited-State Proton Transfer in 4-Nitrocatechol: Implications for the Photochemistry of Nitrophenols

“…All calculations were performed at T = 300 K. Values for Δ E and λ were obtained by geometry optimization of the TDDFT excited states in which the Tamm–Dancoff approximation (TDA) needed to be used to overcome issues with imaginary roots in full TDDFT optimizations. Though TDDFT has been shown to work well with small molecules, it is recognized that the accuracy of TDDFT in determining the geometries of excited states is less accurate than coupled-cluster singles and doubles (CCSD). , It is also recognized that full TDDFT often underestimates the stability of charge-transfer (CT) states, which are present in nitroaromatics, ,, though the use of TDA may help in this regard. , If CT states are present in 4NC, then the incurred errors will be directly dependent on the amount of CT character for each state …”

“…The photochemistry of nitrophenols and other nitroaromatics is commonly assumed to proceed through the excited triplet state manifold, − based on the notion that they would exhibit similar photochemical characteristics to nitrobenzene . The earliest studies of nitrobenzene identified photoreduction products from photochemistry conducted in alcohol solutions, namely, 2-propanol, with evidence of hydrogen abstraction from 2-propanol as the likely reaction pathway. − Similar conclusions have been extrapolated to many other nitrobenzene derivatives. − No studies, however, have been able to definitively prove that the degradation of 4NC occurs via a direct reaction of its triplet state, leaving a possibility that photodegradation could occur through some other pathway such as charge or proton transfer in the excited singlet state manifold …”