Supporting Information Influence of solvent on electronic structure and the photochemistry of nitrophenols

Previous studies have suggested that the photochemistry of nitroaromatics in organic solvents can vary significantly from the photochemistry in aqueous solutions. This work compares the photodegradation of 2-nitrophenol (2NP), 4-nitrophenol...


Photochemical Irradiation Source
. Spectral flux density of the irradiation source used to initiate photochemistry in these experiments is displayed in red. The dark blue trace shows the 24-h averaged flux for Los Angeles (1 July 2022) simulated with the National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet and Visible (TUV) calculator, used to estimate atmospheric lifetimes. The pale blue trace shows the maximum flux within the 24 h window. We used the 24-h average flux for the results reported in this paper, which is appropriate for molecules that have lifetimes exceeding 1 day. We note that for molecules with lifetimes <1 day, their actual ambient lifetime will be shorter during the peak of the solar irradiation.

Determination of Quantum Yields
Photochemical quantum yields were calculated from the absorption-based rate constant and averaged over a 100 nm window, the approximate width of the main absorption bands. The absorption of each nitrophenol was monitored via UV/Vis for 3-5 h, depending on the reactivity of the nitrophenol. To account for the effect of light-absorbing products, rate constants were determined at the minimum of the normalized absorbance, which would be at 235 nm in the 24DNP example shown in Figure 3. This rate, k (s -1 ), can then be used in Equation S1 to determine the average quantum yield, 〈 〉.
In this equation, F(λ) is the irradiation source shown in Figure S1, and σ(λ) is the absorption cross-section of the molecule. The bounds of the integration, λ2 and λ1, correspond to the wavelengths +50 nm and -50 nm from the point of greatest decay. For example, for 4NP the most decay in normalized absorbance occurred at 319 nm, so the integration window was λ1 = 269 nm and λ2 = 369 nm. The only exception to this was 24DNP, which showed the most change at 235 nm, and the integration center-point was set to 290 nm, the approximate location of the main absorption peak.
To determine estimated atmospheric lifetimes, this equation was flipped to solve for the rate constant resulting from using the 24-h average Los Angeles flux as F(λ), and then taking the inverse of the rate constant to be the lifetime. Figure S2. (a) The absorption spectrum of 2NP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 2NP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 355 nm, indicating loss of 2NP, fit to an exponential decay. For this experiment, the sample was irradiated without use of the light guide, exposing the sample to ~5× larger irradiance than the other samples. Figure S3. (a) The absorption spectrum of 4NP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 4NP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 319 nm, indicating loss of 4NP, fit to an exponential decay. Figure S4. (a) The absorption spectrum of 246TNP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 246TNP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 352 nm, indicating loss of 246TNP, fit to an exponential decay. Figure S5. (a) The absorption spectrum of 2NP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 2NP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 412 nm, indicating loss of 2NP, fit to an exponential decay. Panel (d) illustrates the approximate "product" spectrum at the end of 120 minutes of photolysis, generated using the value of C from the original fit (0.89) to subtract out the contribution of the spectrum of the starting material: Aproducts = A120min -[C × A0min]. This plot is shown to illustrate that this period of small change may be caused by a small portion of the sample returning to the neutral form. Figure S6. (a) The absorption spectrum of 4NP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 4NP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 391 nm, indicating loss of 4NP, fit to an exponential decay. Figure S7. (a) The absorption spectrum of 24DNP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 24DNP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 356 nm, indicating loss of 24DNP, fit to an exponential decay. Figure S8. (a) The absorption spectrum of 246TNP collected over 3 h of exposure to photochemical radiation, (b) the absorption spectrum of 246TNP normalized to the absorption spectrum obtained before photolysis began, and (c) the decrease in the normalized absorbance at 343 nm, indicating loss of 246TNP, fit to an exponential decay.  solvation models for 2-propanol (red) and aqueous (blue) solutions. The 24DNP spectrum required explicit 5 solvation by three solvent molecules, as described in the main text. Subtle (i.e., less than 1 nm) changes in 6 peak position were observed for only some absorption bands. Variations in intensity for the 24DNP spectra 7

Referenced Photochemical Yields in Aqueous Solutions
are the most notable result. respectively. Explicit solvation affects the relative intensities of individual excitations within each 14 absorption band. The notation in the figure legends represent the number of explicit solvent molecules used, 15 i.e. "1x" for 1 explicit 2-propanol solvent molecule. 16 17 Figure S11. The experimental and theoretical absorption spectra of 246TNP as a neutral (a) and anionic 19 (b) species in isopropanol. The insets of these plots show the second derivatives of the absorption spectra. 20 The optimized structures of the neutral form of 246TNP are shown in (c) and (d), with the latter having 21 been rotated to show the rotation of the non-planar NO2 group. 22