Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Jul 22;128(30):6199–6207. doi: 10.1021/acs.jpca.4c02907

Excited State Proton Transfer from Acidic Alcohols to a Quinoline Photobase Can Be Solvated by Non-Acidic Alcohol Solvents

Jonathan Ryan Hunt 1,*, Joseph Hecht 1, Clara Goolsby 1, Jade Hagihara 1, Monique Loza 1, Samantha del Pozo 1
PMCID: PMC11299183  PMID: 39034730

Abstract

graphic file with name jp4c02907_0008.jpg

Photobases are a type of molecule that become more basic upon photoexcitation and can therefore be used to control proton transfer reactions with light. The solvation requirements for excited state proton transfer (ESPT) in photobase systems is poorly understood, which limits their applicability. Here, we investigate the solvation of the ESPT reaction using 5-methoxyquinoline (MeOQ), a well-studied photobase with an excited state pKa (pK*a) of approximately 15.1, as a model system. Previous studies have shown that, in addition to the acidic donor that donates a proton to the photoexcited MeOQ, an additional “auxiliary donor” is necessary to solvate the resulting alkoxide. We investigate whether a nonacidic hydrogen bond donor (an alcohol solvent that MeOQ cannot deprotonate in bulk) can act as the auxiliary donor for the MeOQ ESPT reaction. First, we use steady state spectroscopy, TCSPC, and electronic structure calculations to show that MeOQ can deprotonate the acidic donor 2,2,2-trifluoroethanol (TFE, pKa = 12.5) using ethanol as the auxiliary donor. We show that the degree of ESPT is largely predicted by the degree of ground state hydrogen bonding between the photobase and the acidic donor. Next, we study the deprotonation of the acidic donors TFE and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP, pKa = 9.3) with MeOQ in a variety of nonacidic alcohol solvents of varying chain length and branching. MeOQ ESPT occurs to varying extents in all solvents, suggesting that all studied nonacidic alcohols can function as auxiliary donors. We show that the concentration of the acidic donor is strongly correlated with the degree of ESPT. These results are necessary fundamental steps toward the understanding of the photobase ESPT reaction and its wide application in a variety of chemical systems.

Introduction

Proton transfer is a ubiquitous and fundamental aspect of reactions in all branches of chemistry, including chemical synthesis, catalysis, biochemical processes, and more. It is therefore highly important to find ways to chemically control proton transfer reactions. There are two major classes of molecules that achieve light-initiated control of proton transfer reactions: photoacids, molecules that release protons upon photoexcitation, and photobases, molecules that accept protons upon photoexcitation. Typically, photoacids are aromatic organic molecules with a proton-releasing substituent such as a hydroxyl group.1−6 Excited state pKa values (denoted by pKa*) more than 6 units smaller than their ground state pKa values are commonly reported. Because of their ability to generate acidic protons upon exposure to light, photoacids have found use in a variety of applications such as catalyzing organic reactions,7−9 initiating pH-dependent enzymatic activity,10−12 capturing and releasing CO2,13,14 harvesting photoenergy via the generation of pH gradients,15 creating photoelectric sensors,16 and changing protonic conductivity of electrochemical cells.17 A great deal of work has been dedicated to fundamental understanding of the solvation, thermodynamics, and kinetics of the photoacid reaction.2,18−24

Photobases are the proton-capturing counterparts to photoacids. Photobase molecules are often nitrogen-containing aromatic heterocycles like quinolines25−32 and acridines,33−36 although other photobasic structures have been reported.37−40 Excited state pKa* values more than 10 units larger than ground state pKa values have been observed. There have been several works in the past few years investigating the mechanism of the proton transfer reaction in photobases32,40−42 and developing new applications, such as three-dimensional (3D) laser writing,43 the synthesis of large Stokes shift fluorescent proteins,44 and controlling pH-sensitive chemical reactions.45 Still, there is far less fundamental research and application of the photobase reaction than the photoacid reaction.

Previous studies by Driscoll et al. on quinoline photobases showed that 5-methoxyquinoline (MeOQ) is a promising photobase for fundamental study and applications due to its >10 pKa unit increase after photoexcitation (from pKa = 4.9 to pKa* = 15.1) and its straightforward excited state dynamics.29,30 It has been shown experimentally to readily deprotonate low pKa (<15.5) bulk solvent alcohols.30 For complex and interesting applications, though, one would like to know how such photobases behave in solutions where the acidic proton donor is in dilute concentrations–that is, where the solvation shell predominantly consists of aprotic or weakly acidic molecules. Previous work has demonstrated some of the specific solvation requirements necessary for excited state proton transfer (ESPT) from acidic alcohols to MeOQ. Hunt and Dawlaty showed the necessity for two hydrogen bond donors at the ESPT reaction center: one to donate a proton and the other to stabilize the generated alkoxide.31,32 For the remainder of this work, we refer to the second molecule as the “auxiliary” hydrogen bond donor. These previous studies were carried out in an aprotic background solvent, which was itself unable to solvate the ESPT reaction, necessitating the diffusion of a second acidic hydrogen bond donor to the reaction center. These experiments indicated the necessity of high concentrations of acidic hydrogen bond donor (>0.1 M) for ESPT to occur in aprotic solvents, even though notable hydrogen bonding between the acidic donor and the photobase occurred at much lower concentrations (>5 mM).

In all previous studies—both in bulk alcohol solvents and in dilutions where the background solvent was aprotic—the acidic donor has also served as the auxiliary hydrogen bond donor. The central question of this paper is can a nonacidic hydrogen bond donor act as the auxiliary donor for the ESPT process between MeOQ and an acidic donor? In this work, we investigate the ability of MeOQ to deprotonate two acidic alcohols −2,2,2-trifluoroethanol (TFE), pKa = 12.5; and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), pKa = 9.3–in a variety of high pKa alcohol solvents. We first investigate MeOQ in a series of ethanol-TFE solutions to show that ethanol, a molecule unable to be deprotonated by MeOQ, can successfully solvate the ESPT process between MeOQ and TFE. We then study the ESPT process between MeOQ and both TFE and HFIP in a variety of nonacidic alcohol solvents. Interestingly, the degree of ESPT seems relatively independent of the identity of the nonacidic background solvent, including both chain length and branching. We show that the degree of ESPT is well-correlated with the concentration of the acidic proton donor. These results demonstrate new fundamental understandings about the solvation conditions for the photobase ESPT reaction–that while a second hydrogen donor is necessary to solvate the ESPT process in quinoline photobases, the identity of that second hydrogen bond donor is almost entirely irrelevant.

Methods

Materials

5-methoxyquinoline (98%) was purchased from Combi-Blocks. 2,2,2-trifluoroethanol (TFE, ≥99%), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP, ≥99%), ethanol (EtOH, ≥99.5%), 1-pentanol (≥99%), 1-octanol (anhydrous, ≥99%), 2-propanol (99.9%), 2-butanol (≥99%), 2-pentanol (98%), t-butanol (≥99%), t-amyl alcohol (99%), and 12 M HCl were purchased from Sigma-Aldrich. 1-propanol (99.9%), 1-butanol (99.5%), and 1-pentanol (99%) were purchased from Oakwood Chemical. All compounds were used without further purification.

Steady-State Spectroscopy

Absorption spectra were acquired using the Thermoscientific GENESYS 180 UV–vis Spectrophotometer and corrected by subtracting the absorption spectra of the same solution in the absence of MeOQ. Fluorescence spectra were acquired using the Horiba Scientific FluoroMax Plus spectrofluorometer and were corrected for the intensity of the excitation wavelength. All steady state data was collected using 5 × 10–5 M solutions of MeOQ in a 1 cm fused quartz fluorescence cuvette.

Time-Correlated Single-Photon Counting (TCSPC)

TCSPC spectra were acquired using the Horiba Scientific FluoroMax Plus spectrofluorometer with a built-in TCSPC module. The Horiba NanoLED-285 nm pulsed LED (peak wavelength = 281 nm, pulse duration <1.2 ns, repetition rate = 1 MHz) was used as the excitation source. Thorlabs UVFS reflective ND filters were used to attenuate the excitation source to achieve an appropriate collection efficiency with a 10 nm collection monochromator slit width. The instrument response function (IRF) was collected using LUDOX AS-40 colloidal silica for light scattering. The full width at half-maximum of the IRF was consistently around 1.5 ns, indicating a time resolution greater than 150 ps following reconvolution. All TCSPC data was collected using 5 × 10–5 M solutions of MeOQ in a 1 cm fused quartz fluorescence cuvette.

Electronic Structure Calculations

Electronic structure calculations were carried out using Gaussian 16.46 Optimized geometries were visualized for publication using IQmol.47 Ground state thermodynamics calculations for the hydrogen bonding interactions between MeOQ and TFE/ethanol were carried out at the ωB97X-D/aug-cc-PVDZ level of theory with an ethanol IEFPCM model. Excited state geometry optimizations were carried out at the TD-DFT/B3LYP/6-31G* level of theory. Optimized ground state structures at the B3LYP/6-31G* level of theory were used as guess geometries for excited state optimization calculations.

Results and Discussion

Ethanol-TFE Dilution Experiments

First, we set out to determine whether a nonacidic alcohol solvent, which we define here as a bulk solvent that MeOQ is unable to deprotonate in the electronic excited state, could act as the auxiliary hydrogen bond donor for the solvation of the ESPT process between MeOQ and an acidic hydrogen bond donor. Previous work by Hunt and Dawlaty has shown that MeOQ can deprotonate bulk alcohol solvents with pKa values of 15 and lower.30 For these preliminary experiments, we chose 2,2,2-trifluoroethanol (TFE, pKa = 12.5) as the acidic proton donor and ethanol (EtOH, pKa = 15.9) as the nonacidic solvent. As shown by the absorption spectra in Figure 1A, in both TFE and EtOH MeOQ is in the unprotonated form in the ground state. However, we can see that MeOQ’s fluorescence is greatly red-shifted in TFE relative to its emission in EtOH, indicating that MeOQ emits from the protonated form in TFE and from the unprotonated form in EtOH. This shows that MeOQ can capture a proton from TFE during its excited state lifetime, while the same is not true of EtOH. We attribute this to differences in their pKa values. In Figure 1B, we show the hypothesized mechanism of solvation of the ESPT reaction, as discussed in previous works31,32

Figure 1.

Figure 1

Open in a new tab

(A) Normalized absorption and emission of MeOQ in ethanol (EtOH, pKa = 15.9) and 2,2,2-trifluoroethanol (TFE, pKa = 12.5). MeOQ absorbs from the unprotonated form in both EtOH and TFE, indicating a lack of protonation in the ground state. MeOQ emits from the unprotonated form in EtOH and the protonated form in TFE, indicating that MeOQ can participate in excited state proton transfer (ESPT) in the more acidic TFE bulk solvent but not in the less acidic EtOH bulk solvent. (B) The hypothesized solvation mechanism for MeOQ ESPT investigated in previous publications, in which an auxiliary TFE molecule stabilizes the alkoxide generated via ESPT.31,32 (C) The central question of the first section of the paper: can a nonacidic hydrogen bond donor like EtOH act as the auxiliary hydrogen bond donor to stabilize the ESPT process between an acidic donor like TFE and MeOQ? Reproduced from ref (31). Copyright 2019 American Chemical Society.

To investigate whether EtOH can function as the auxiliary hydrogen bond donor in the ESPT reaction between MeOQ and the acidic donor TFE, as pictured in Figure 1C, we performed an experimental dilution study of MeOQ in EtOH with gradually increasing amounts of TFE (Figure 2A). The addition of TFE has little impact on the absorbance spectrum of MeOQ (Figure 2A, solid lines), but does result in less emission (Figure 2A, dashed lines) from the unprotonated form of MeOQ (around 400 nm) and more emission from the protonated form (around 500 nm), suggesting more ESPT as more TFE is added. Noticeable amounts of ESPT occur even at [TFE] of 0.5 M and lower (mole ratios of EtOH/TFE around 30 and larger) where it is statistically unlikely that a second TFE molecule is in or near the solvation shell upon photoexcitation.

Figure 2.

Figure 2

Open in a new tab

(A) Normalized absorption (solid lines) and relative fluorescence (dashed lines) spectra of MeOQ in EtOH with various [TFE]. The decrease in emission from the unprotonated form (around 400 nm) and increase in emission from the protonated form (around 500 nm) at higher [TFE] suggests that ESPT between MeOQ and TFE becomes more likely at higher concentrations. (B) The percent of MeOQ that undergoes ESPT as a function of [TFE], acquired from analysis of the experimental data, is compared to a theoretical estimate for the fraction of MeOQ molecules that are hydrogen bonded to TFE in the ground state. The equilibrium constant for the competitive hydrogen bonding reaction between MeOQ and TFE/EtOH was estimated using DFT calculations with thermodynamic corrections at the ωB97X-D/aug-cc-PVDZ level of theory with an ethanol IEFPCM model.

To quantify the degree of ESPT in each TFE-EtOH solution, the following procedure was used: the unprotonated emission spectrum of MeOQ in pure EtOH and the protonated emission spectrum from the acidification of MeOQ with HCl in each dilution were fit using a sum of Gaussian functions. The unacidified emission spectrum from each dilution, where there is some emission from both protonated and unprotonated forms, was fit as a linear combination of those two basis spectra. Those fits, along with the relative quantum yields of the protonated and unprotonated forms, were used to estimate the percentage of MeOQ that undergo ESPT. More details about this analysis may be found in the Supporting Information.

Figure 2B shows the percent of MeOQ that undergoes ESPT as a function of [TFE]. In the same figure, we show a theoretical estimate for the percentage of MeOQ that are hydrogen-bonded to TFE in the ground state at each dilution. The equilibrium constant for the competitive hydrogen bonding reaction between MeOQ and TFE/EtOH was estimated using density functional theory (DFT) calculations with thermodynamic corrections at the ωB97X-D/aug-cc-PVDZ level of theory with an ethanol IEFPCM model. This method and basis set were chosen because they have been shown to provide the best results for describing hydrogen bonding interactions among similar DFT methods.48 More details about these calculations can be found in the SI. Figure 2B suggests that the extent of the ESPT reaction is dictated by the equilibrium concentration of MeOQ-TFE hydrogen-bonded complexes in the ground state. Because MeOQ is such a strong base in the excited state (pKb = −1.1),28 the ability of MeOQ to exchange primary hydrogen bond donors in the excited state is likely severely restricted. These results therefore suggest that the number of prehydrogen bonded complexes with an acidic donor that undergo ESPT is near unity.

Although suggestive that ground state hydrogen bonding is the only requirement for ESPT in these systems, the results shown in Figure 2 do not exclude the possibility of diffusion of a second TFE molecule to the reaction center during the excited state lifetime to function as an auxiliary hydrogen bond donor. Previous work by Hunt and Dawlaty showed that ESPT between MeOQ and TFE in an aprotic background solvent can occur via this mechanism, with proton transfer time scales consistent with diffusion of the auxiliary donor.31 To test the reasonability of this mechanism for the current system, we estimate the time scale of diffusion of TFE to the reaction center as a function of concentration and compare these values to the excited state dynamics observed using time-correlated single photon counting (TCSPC) spectroscopy. The average diffusion length (L) of a molecule is given by

graphic file with name jp4c02907_m001.jpg 1

where D is the diffusion coefficient (cm2/s) and τ is the diffusion time (s). We can estimate the average distance d between adjacent TFE molecules in solution using eq 2

graphic file with name jp4c02907_m002.jpg 2

where NA is Avogadro’s number and c is the concentration (mol/cm3). By assuming that the average distance between adjacent TFE molecules is similar to the average distance between TFE and a MeOQ-TFE hydrogen bonded complex, we can set L = d and combine these equations to estimate the time scale for diffusion of TFE to the MeOQ-TFE reaction center as a function of c

graphic file with name jp4c02907_m003.jpg 3

where D is the mutual diffusion coefficient (cm2/s) between the hydrogen-bonded photobase reaction center and the auxiliary TFE donor. To estimate the mutual diffusion coefficient, we multiply the self-diffusion coefficient of EtOH (D = 1.2 × 10–5cm2/s)49 by 2.

As demonstrated by eq 3, if ESPT occurred according to a diffusive mechanism, the ESPT time scale should be highly dependent on [TFE]. However, TCSPC data for MeOQ and TFE in EtOH shows similar excited state decay kinetics for the protonated form at 575 nm for all [TFE] (Figure 3), suggesting that τESPT is independent of the concentration of TFE and is therefore inconsistent with a diffusive mechanism. Furthermore, if diffusion were necessary, we would expect exponential rise times in the protonated decay curve corresponding to diffusion time scales, as observed in our previous publication using an aprotic background solvent.31 Applying eq 3, we would expect rise times of τ ≈ 2.7 ns at [TFE] = 0.1 M and τ ≈ 580 ps at [TFE] = 1.0 M. Instead, we see protonated decay curves following ESPT that are nearly identical to the decay curves obtained via acidifying the solutions in the ground state (Figures S5–S9), implying that τESPT must be below the time resolution of our TCSPC instrument, which we estimate to be ≈150 ps. This analysis strongly suggests that MeOQ that are hydrogen bonded to TFE in the ground state and then excited undergo rapid ESPT (τESPT < 150 ps) where an EtOH molecule acts as the auxiliary donor. Note that this is consistent with the proton transfer time scale between MeOQ and TFE in pure TFE (τESPT = 2.3 ps) observed via femtosecond transient absorption spectroscopy in our previous publication.30

Figure 3.

Figure 3

Open in a new tab

Time-correlated single photon counting (TCSPC) decay curves of the protonated form of MeOQ (575 nm) following ESPT as a function of [TFE]. Except for some vertical displacement due to differences in the intensity of emission at 575 nm, the decay is almost identical in all solutions, suggesting that τESPT is shorter than the time resolution of our TCSPC apparatus (≈150 ps). Note that both this time scale and the lack of an exponential rise time suggest that the mechanism of ESPT is nondiffusive. Exponential fits, decay time scales, and relevant statistics for this data may be found in the SI.

We then set out to verify these experimental observations using DFT calculations. We studied MeOQ in the presence of only one hydrogen bond donor and in the presence of a primary and auxiliary hydrogen bond donor, both in vacuum and in high dielectric. First, hydrogen-bonded MeOQ geometries were optimized in the ground state using the B3LYP/6-31G* level of theory. These geometries are provided in the SI. The ground state geometries were then used as the initial guess geometries for optimization in the first singlet excited state using the TDDFT/B3LYP/6-31G* level of theory. The resulting excited state geometries are visualized in Figure 4 and provided in the SI. In the presence of one TFE molecule, in vacuum or in high dielectric (ϵ = 150, produced via CPCM model), the proton remains on the TFE molecule, indicating a lack of ESPT reaction (Figure 4A,B). However, when EtOH is added as an auxiliary hydrogen bond donor, the proton migrates from TFE to MeOQ during the excited state geometry optimization, indicating that the ESPT reaction occurs, even in vacuum (Figure 4C,D). These observations seem to confirm both the need for an auxiliary hydrogen bond donor and the ability of EtOH to act as the auxiliary hydrogen bond donor. A similar calculation (shown in Figure S10) for MeOQ hydrogen bonded to EtOH with a TFE auxiliary donor showed no evidence of ESPT. Note that, following proton transfer in the MeOQ-TFE-EtOH complexes, the calculations repeatedly failed to converge. The authors were therefore unable to produce fully optimized excited state geometries for the complexes following ESPT, so the provided geometries are presented here qualitatively.

Figure 4.

Figure 4

Open in a new tab

Excited state geometries for MeOQ hydrogen bonded to TFE without (A, B) and with (C, D) an auxiliary EtOH hydrogen bond donor, either in vacuum (A, C) or high dielectric (B, D). Excited state geometry optimization was carried out at the TDDFT/B3LYP/6-31G* level of theory in Gaussian46 and visualized using IQmol.47 Ground state optimized geometries, obtained using the B3LYP/6-31G* level of theory, were used as the initial guess geometries. (A, B) In the absence of an auxiliary hydrogen bond donor, no ESPT is observed, regardless of the dielectric environment. (C, D) In the presence of an auxiliary EtOH, the acidic proton from TFE moves to MeOQ’s nitrogen heteroatom and the TFE molecule rotates to hydrogen bond with EtOH, indicating ESPT. Note that the geometry optimizations for the structures shown in (C, D) did not converge following proton transfer, so these are nonoptimized excited state geometries.

MeOQ ESPT as a Function of Non-Acidic Alcohol Solvent

Now that we have shown that ethanol, a nonacidic alcohol solvent, can act as an auxiliary hydrogen bond donor and stabilize the ESPT process between MeOQ and the acidic donor TFE, we wish to investigate a variety of other nonacidic alcohol solvents for similar purposes. We investigate several primary (ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-octanol), secondary (2-propanol, 2-butanol, 2-pentanol), and tertiary (t-butanol, t-amyl alcohol) nonacidic alcohol solvents. For the acidic proton donor, we investigate both TFE and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, pKa = 9.3). In our initial investigations, a fixed mole ratio between the acidic proton donor and the solvent of approximately 1:7.3 was used.

In Figure 5, some representative corrected emission spectra are shown for MeOQ in three pure primary alcohol solvents of varying chain length (1-propanol, 1-pentanol, 1-octanol), MeOQ in the same solvents in the presence of TFE and HFIP, and MeOQ in the same solution but acidified with HCl. In all three solvents, noticeable ESPT is observed in the presence of the acidic donor, as indicated by the decrease in emission intensity from the unprotonated form of MeOQ (400 nm) and the increase in emission intensity from the protonated form of MeOQ (520 nm). A similar phenomenon can be observed via emission spectra for all the alcohol solvents used in this study, including the secondary and tertiary solvents, all of which can be found in the SI (Figures S10–S29). This suggests that all nonacidic solvent molecules can solvate the ESPT process. A cursory glance shows that emission from the protonated form, and thus the degree of ESPT, decreases when the acidic donor is switched from HFIP to TFE (A → D, B → E, C → F) and when the chain length increases (1-propanol →1-pentanol →1-octanol). We suspect that more ESPT is observed in the presence of HFIP than in the presence of TFE because HFIP is a significantly stronger hydrogen bond donor (αHFIP = 1.96)50 than TFE (αTFE = 1.51)50 and there will therefore be a higher equilibrium concentration of MeOQ-HFIP complexes than MeOQ-TFE complexes at the same concentration. Note that, because of the fixed mole ratio used in these experiments, the concentration of acidic donor decreases as the chain length of the solvent alcohol increases.

Figure 5.

Figure 5

Open in a new tab

Comparison of emission of 5 × 10–5 M MeOQ solutions in various nonacidic alcohol solvents (blue) with MeOQ emission in the presence of acidic alcohol (HFIP or TFE, green) and with emission of MeOQ acidified with HCl in the presence of the acidic alcohol (red). The mole ratio of acidic alcohol to nonacidic alcohol solvent is kept at 1:7.3 in all cases. (A, D) are in 1-propanol; (B, E) are in 1-pentanol; (C, F) are in 1-octanol. (A–C) contain HFIP; (D–F) contain TFE. The degree of ESPT is higher in the presence of HFIP than in the presence of TFE, as indicated by the larger amount of emission from the protonated form (around 500 nm). Note that, because of the fixed mole ratio used in these experiments, the concentration of acidic donor decreases as the chain length of the solvent alcohol increases.

To evaluate the origin of these differences, we quantified the percentage of MeOQ that undergo ESPT for each MeOQ-solvent-acidic donor system. In brief, unprotonated emission spectra of MeOQ in each pure solvent and protonated emission spectra from the acidification of MeOQ with HCl in each solvent were fit as a sum of Gaussian functions. The spectrum of MeOQ in the presence of acidic donor, where there is some population of both protonated and unprotonated forms, was fit as a linear combination of those two basis spectra. Those fits, along with the relative quantum yields of the protonated and unprotonated forms, were used to estimate the percentage of MeOQ that undergo ESPT. More details about the analysis may be found in the SI.

As shown in Figure 6, the degree of ESPT is well-correlated with the concentration of the acidic donor. The correlation is more robust for only primary alcohols (Figure 6A) than when all solvents are included (Figure 6B). Figure S31 shows the same data as Figure 6B, but with each data point explicitly labeled as a primary, secondary, or tertiary solvent system. Figure S31 seems to suggest that the less robust correlation when all solvents are included is due to slightly different solvation behaviors for each type of branched solvent. The correlation between degree of ESPT and the concentration of acidic donor is consistent with that observed by Lahiri et al. when studying the deprotonation of bulk primary alcohol solvents with the photobase FR0-SB.40

Figure 6.

Figure 6

Open in a new tab

(A) Fraction of MeOQ molecules that undergo excited state proton transfer (ESPT) as a function of the concentration of the acidic donor in primary nonacidic background alcohol solvents. (B) Fraction of MeOQ molecules that undergo excited state proton transfer (ESPT) as a function of the concentration of the acidic donor in all nonacidic background alcohol solvents, including primary, secondary, and tertiary alcohol solvents. Lines of best fit and their R2 values are provided as insets.

Next, we studied the same systems but with a fixed concentration (1 M) of HFIP or TFE rather than a fixed mole ratio. Small but systematic differences in the degree of ESPT were observed even with a fixed concentration of acidic donor. We therefore investigated the correlation of ESPT % with excited state lifetime, viscosity, dielectric constant, and Kamlet–Taft parameters for solvent hydrogen-bond donation strength (α), hydrogen bond accepting strength (β), and polarity (π*). Correlation analyses were performed in two ways: for only the five primary alcohol solvents and for all 10 alcohol solvents, including primary, secondary, and tertiary alcohols. These analyses are predominantly included in the SI (Figures S34–S39). In brief: convincing correlations between ESPT % and excited state lifetimes (obtained via TCSPC), viscosity, dielectric constant, and the π* parameter were observed for both HFIP and TFE in primary alcohol solvents, but these correlations did not extend to the inclusion of all types of alcohol solvents for both acidic donors.

The only solvent parameter that showed a correlation for all solvents and for both HFIP and TFE as the acidic proton donor is a ratio of the excited state lifetime of MeOQ in the solvent of interest and the solvent’s viscosity (τ0/η). This correlation is shown in Figure 7A for only primary solvents and in Figure 7B for all solvents. As with the fixed mole ratio experiments, the correlation is more robust for only primary alcohols (Figure 7A) than when all solvents are included (Figure 7B). Figure S33 shows the same data as Figure 7B, but with each data point explicitly labeled as a primary, secondary, or tertiary solvent system. Once again, Figure S32 seems to suggest that the less robust correlation when all solvents are included is due to slightly different solvation behaviors for each type of branched solvent.

Figure 7.

Figure 7

Open in a new tab

(A) Fraction of MeOQ molecules that undergo excited state proton transfer (ESPT) in the presence of 1 M acidic donor (HFIP or TFE) in primary nonacidic alcohol solvents as a function of excited state lifetime (τ0) in the pure solvent divided by solvent viscosity (η).50 (B) Fraction of MeOQ molecules that undergo ESPT in the presence of 1 M acidic donor (HFIP or TFE) in all nonacidic background alcohol solvents, including primary, secondary, and tertiary alcohol solvents, as a function of excited state lifetime (τ0) in the pure solvent divided by solvent viscosity (η).50 Lines of best fit and their R2 values are provided as insets.

Conclusions

We have investigated the ability of high pKa alcohol solvents to stabilize the ESPT reaction between the photobase MeOQ and acidic hydrogen bond donors by acting as the auxiliary hydrogen bond donor. We show that the identity of the auxiliary hydrogen bond donor is almost entirely irrelevant: only small changes in ESPT % are observed when the background solvent is changed and the clearest predictor for degree of ESPT is the concentration of the acidic donor. The small differences that are observed seem to be related to the time scale for excited state decay and the viscosity of the solvent.

These results suggest future directions for research, such as investigating the role of solvation in similar photobases to determine whether the observed phenomena are general, performing transient absorption spectroscopy to quantify the time scale of ESPT in these systems and thus the typical auxiliary hydrogen bond reorganization time scale, investigating the impact of acidic donor pKa to determine whether the identity of the auxiliary donor is more important when the thermodynamic drive for the ESPT reaction is smaller, and investigating ESPT reactions with very large thermodynamics drives to determine whether ESPT can occur in the absence of an auxiliary donor. We hope that the results in this work encourage additional fundamental work on photobase molecules and provide understanding necessary for their continued application in interesting chemical systems, especially where the acidic proton donor of interest is in low concentration.

Acknowledgments

The authors gratefully acknowledge support from Loyola Marymount University and Seaver College in the form of startup funds and research stipends via RAINS, SURP, SOAR, and the Continuing Faculty Grant. We thank Dr. Emily Jarvis of Loyola Marymount University for graciously allowing use of her Gaussian license.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.4c02907.

  • Details of the method used for quantitative analysis of emission data; details of electronic structure calculations, including methods and optimized molecular geometries; TCSPC data, fits, and time scales; steady state absorption and emission data in all alcohol solvents; and correlations between degree of ESPT and solvent characteristics (PDF)

The authors declare no competing financial interest.

Supplementary Material

jp4c02907_si_001.pdf (4.2MB, pdf)

References

  1. Gutman M.; Huppert D.; Pines E. The pH Jump: A Rapid Modulation of pH of Aqueous Solutions by a Laser Pulse. J. Am. Chem. Soc. 1981, 103, 3709–3713. 10.1021/ja00403a016. [DOI] [Google Scholar]
  2. Agmon N. Elementary Steps in Excited-State Proton Transfer. J. Phys. Chem. A 2005, 109, 13–35. 10.1021/jp047465m. [DOI] [PubMed] [Google Scholar]
  3. Prémont-Schwarz M.; Barak T.; Pines D.; Nibbering E. T. J.; Pines E. Ultrafast Excited-State Proton-Transfer Reaction of 1-Naphthol-3,6-Disulfonate and Several 5-Substituted 1-Naphthol Derivatives. J. Phys. Chem. B 2013, 117, 4594–4603. 10.1021/jp308746x. [DOI] [PubMed] [Google Scholar]
  4. Pines E.; Huppert D. pH Jump: A Relaxational Approach. J. Phys. Chem. A 1983, 87, 4471–4478. 10.1021/j100245a029. [DOI] [Google Scholar]
  5. Spry D. B.; Fayer M. D. Charge Redistribution and Photoacidity: Neutral versus Cationic Photoacids. J. Chem. Phys. 2008, 128, 084508 10.1063/1.2825297. [DOI] [PubMed] [Google Scholar]
  6. Spry D. B.; Goun A.; Fayer M. Deprotonation Dynamics and Stokes Shift of Pyranine (HPTS). J. Phys. Chem. A 2007, 111, 230–237. 10.1021/jp066041k. [DOI] [PubMed] [Google Scholar]
  7. Badillo J. J.; Saway J.; Salem Z. M. Recent Advances in Photoacid Catalysis for Organic Synthesis. Synthesis 2020, 53 (03), 489–497. 10.1055/s-0040-1705952. [DOI] [Google Scholar]
  8. Dempsey J. L.; Winkler J. R.; Gray H. B. Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime. J. Am. Chem. Soc. 2010, 132, 16774–16776. 10.1021/ja109351h. [DOI] [PubMed] [Google Scholar]
  9. Das A.; Ayad S.; Hanson K. Enantioselective Protonation of Silyl Enol Ether Using Excited State Proton Transfer Dyes. Org. Lett. 2016, 18, 5416–5419. 10.1021/acs.orglett.6b02820. [DOI] [PubMed] [Google Scholar]
  10. Kohse S.; Neubauer A.; Pazidis A.; Lochbrunner S.; Kragl U. Photoswitching of Enzyme Activity by Laser-Induced pH-Jump. J. Am. Chem. Soc. 2013, 135, 9407–9411. 10.1021/ja400700x. [DOI] [PubMed] [Google Scholar]
  11. Peretz-Soroka H.; Pevzner A.; Davidi G.; Naddaka V.; Kwiat M.; Huppert D.; Patolsky F. Manipulating and Monitoring On-Surface Biological Reactions by Light-Triggered Local pH Alterations. Nano Lett. 2015, 15, 4758–4768. 10.1021/acs.nanolett.5b01578. [DOI] [PubMed] [Google Scholar]
  12. Moreno S.; Sharan P.; Engelke J.; Gumz H.; Boye S.; Oertel U.; Wang P.; Banerjee S.; Klajn R.; Voit B.; Lederer A.; Appelhans D. Light-Driven Proton Transfer for Cyclic and Temporal Switching of Enzymatic Nanoreactors. Small 2020, 16 (37), 2002135 10.1002/smll.202002135. [DOI] [PubMed] [Google Scholar]
  13. de Vries A.; Goloviznina K.; Reiter M.; Salanne M.; Lukatskaya M. R. Solvation-Tuned Photoacid as a Stable Light-Driven pH Switch for CO2 Capture and Release. Chem. Mater. 2024, 36 (3), 1308–1317. 10.1021/acs.chemmater.3c02435. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Alghazwat O.; Elgattar A.; Liao Y. Photoacid for Releasing Carbon Dioxide from Sorbent. Photochem. Photobiol. Sci. 2023, 22 (11), 2573–2578. 10.1007/s43630-023-00472-8. [DOI] [PubMed] [Google Scholar]
  15. Bae J.; Lim H.; Ahn J.; Kim Y. H.; Kim M. S.; Kim I.-D. Photoenergy Harvesting by Photoacid Solution. Adv. Mater. 2022, 34 (24), 2201734 10.1002/adma.202201734. [DOI] [PubMed] [Google Scholar]
  16. Wu L.; Qi J.; Zhang L.; Yu L.; Gao H.; Gao J.; Ju J.; Yao X. Self-Powered Photoelectric Sensors Based on Hydrogel Diodes Doped with Photoacid. Chem. Eng. J. 2024, 489, 151215 10.1016/j.cej.2024.151215. [DOI] [Google Scholar]
  17. Haghighat S.; Ostresh S.; Dawlaty J. M. Controlling Proton Conductivity with Light: A Scheme Based on Photoacid Doping of Materials. J. Phys. Chem. B 2016, 120, 1002–1007. 10.1021/acs.jpcb.6b00370. [DOI] [PubMed] [Google Scholar]
  18. Simkovitch R.; Shomer S.; Gepshtein R.; Huppert D. How Fast Can a Proton-Transfer Reaction Be beyond the Solvent-Control Limit?. J. Phys. Chem. B 2015, 119, 2253–2262. 10.1021/jp506011e. [DOI] [PubMed] [Google Scholar]
  19. Förster T. Elektrolytische Dissoziation Angeregter Moleküle. Z. Elektrochem. Angew. Phys. Chem. 1950, 54, 42–46. 10.1002/bbpc.19500540111. [DOI] [Google Scholar]
  20. Weller A. Fast Reactions of Excited Molecules. Prog. React. Kinet. Mech. 1961, 1, 187–214. [Google Scholar]
  21. Tolbert L. M.; Solntsev K. M. Excited-State Proton Transfer: From Constrained Systems to Super Photoacids to Superfast Proton Transfer. Acc. Chem. Res. 2002, 35, 19–27. 10.1021/ar990109f. [DOI] [PubMed] [Google Scholar]
  22. Simkovitch R.; Akulov K.; Shomer S.; Roth M. E.; Shabat D.; Schwartz T.; Huppert D. Comprehensive Study of Ultrafast Excited-State Proton Transfer in Water and D2O Providing the Missing RO(−)H(+) Ion-Pair Fingerprint. J. Phys. Chem. A 2014, 118, 4425–4443. 10.1021/jp5002435. [DOI] [PubMed] [Google Scholar]
  23. Pines D.; Nibbering E. T. J.; Pines E. Monitoring the Microscopic Molecular Mechanisms of Proton Transfer in Acid-Base Reactions in Aqueous Solutions. Isr. J. Chem. 2015, 55, 1240–1251. 10.1002/ijch.201500057. [DOI] [Google Scholar]
  24. Cotter L. F.; Brown P. J.; Nelson R. C.; Takematsu K. Divergent Hammett Plots of the Ground- and Excited-State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds. J. Phys. Chem. B 2019, 123, 4301–4310. 10.1021/acs.jpcb.9b01295. [DOI] [PubMed] [Google Scholar]
  25. Mataga N.; Kaifu Y.; Koizumi M. On the Base Strength of Some Nitrogen Heterocycles in the Excited State. Bull. Chem. Soc. Jpn. 1956, 29, 373–379. 10.1246/bcsj.29.373. [DOI] [Google Scholar]
  26. Pines E.; Huppert D.; Gutman M.; Nachliel N.; Fishman M. The pOH Jump: Determination of Deprotonation Rates of Water by 6-Methoxyquinoline and Acridine. J. Phys. Chem. A 1986, 90, 6366–6370. 10.1021/j100281a061. [DOI] [Google Scholar]
  27. Munitz N.; Avital Y.; Pines D.; Nibbering E. T. J.; Pines E. Cation-Enhanced Deprotonation of Water by a Strong Photobase. Isr. J. Chem. 2009, 49, 261–272. 10.1560/IJC.49.2.261. [DOI] [Google Scholar]
  28. Driscoll E. W.; Hunt J. R.; Dawlaty J. M. Photobasicity in Quinolines: Origin and Tunability via the Substituents’ Hammett Parameters. J. Phys. Chem. Lett. 2016, 7, 2093–2099. 10.1021/acs.jpclett.6b00790. [DOI] [PubMed] [Google Scholar]
  29. Driscoll E. W.; Hunt J. R.; Dawlaty J. M. Proton Capture Dynamics in Quinoline Photobases: Substituent Effect and Involvement of Triplet States. J. Phys. Chem. A 2017, 121, 7099–7107. 10.1021/acs.jpca.7b04512. [DOI] [PubMed] [Google Scholar]
  30. Hunt J. R.; Dawlaty J. M. Photodriven Deprotonation of Alcohols by a Quinoline Photobase. J. Phys. Chem. A 2018, 122 (40), 7931–7940. 10.1021/acs.jpca.8b06152. [DOI] [PubMed] [Google Scholar]
  31. Hunt J. R.; Dawlaty J. M. Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase. J. Phys. Chem. A 2019, 123 (48), 10372–10380. 10.1021/acs.jpca.9b08970. [DOI] [PubMed] [Google Scholar]
  32. Hunt J. R.; Tseng C.; Dawlaty J. M. Donor–Acceptor Preassociation, Excited State Solvation Threshold, and Optical Energy Cost as Challenges in Chemical Applications of Photobases. Faraday Discuss. 2019, 216, 252–268. 10.1039/C8FD00215K. [DOI] [PubMed] [Google Scholar]
  33. Kellmann A.; Lion Y. Acid-Base Equilibria of the Excited Singlet and Triplet States and the Semi-Reduced Form of Acridine Orange. Photochem. Photobiol. 1979, 29, 217–222. 10.1111/j.1751-1097.1979.tb07042.x. [DOI] [Google Scholar]
  34. Nachliel E.; Ophir Z.; Gutman M. Kinetic Analysis of Fast Alkalinization Transient by Photoexcited Heterocyclic Compounds: pOH Jump. J. Am. Chem. Soc. 1987, 109, 1342–1345. 10.1021/ja00239a009. [DOI] [Google Scholar]
  35. Ryan E. T.; Xiang T.; Johnston K. P.; Fox M. A. Absorption and Fluorescence Studies of Acridine in Subcritical and Supercritical Water. J. Phys. Chem. A 1997, 101, 1827–1835. 10.1021/jp962676f. [DOI] [Google Scholar]
  36. Eisenhart T. T.; Dempsey J. L. Photo-Induced Proton-Coupled Electron Transfer Reactions of Acridine Orange: Comprehensive Spectral and Kinetics Analysis I. Experimental Methods. J. Am. Chem. Soc. 2014, 136, 12221–12224. 10.1021/ja505755k. [DOI] [PubMed] [Google Scholar]
  37. Akulov K.; Simkovitch R.; Erez Y.; Gepshtein R.; Schwartz T.; Huppert D. Acid Effect on Photobase Properties of Curcumin. J. Phys. Chem. A 2014, 118, 2470–2479. 10.1021/jp501061p. [DOI] [PubMed] [Google Scholar]
  38. Vogt B. S.; Schulman S. G. Reversible Proton Transfer in Photoexcited Xanthone. Chem. Phys. Lett. 1983, 97, 450–453. 10.1016/0009-2614(83)80527-2. [DOI] [Google Scholar]
  39. Sheng W.; Nairat M.; Pawlaczyk P. D.; Mroczka E.; Farris B.; Pines E.; Geiger J. H.; Borhan B.; Dantus M. Ultrafast Dynamics of a “Super” Photobase. Angew. Chem., Int. Ed. 2018, 57 (45), 14742–14746. 10.1002/anie.201806787. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Lahiri J.; Moemeni M.; Magoulas I.; Yuwono S. H.; Kline J.; Borhan B.; Piecuch P.; Jackson J. E.; Blanchard G. J.; Dantus M. Steric Effects in Light-Induced Solvent Proton Abstraction. Phys. Chem. Chem. Phys. 2020, 22 (35), 19613–19622. 10.1039/D0CP03037F. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Lahiri J.; Moemeni M.; Kline J.; Borhan B.; Magoulas I.; Yuwono S. H.; Piecuch P.; Jackson J. E.; Dantus M.; Blanchard G. J. Proton Abstraction Mediates Interactions between the Super Photobase FR0-SB and Surrounding Alcohol Solvent. J. Phys. Chem. B 2019, 123 (40), 8448–8456. 10.1021/acs.jpcb.9b06580. [DOI] [PubMed] [Google Scholar]
  42. Alamudun S. F.; Tanovitz K.; Fajardo A.; Johnson K.; Pham A.; Araghi T. J.; Petit A. S. Structure–Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline. J. Phys. Chem. A 2020, 124 (13), 2537–2546. 10.1021/acs.jpca.9b11375. [DOI] [PubMed] [Google Scholar]
  43. Carlotti M.; Tricinci O.; Mattoli V. Novel, High-Resolution, Subtractive Photoresist Formulations for 3D Direct Laser Writing Based on Cyclic Ketene Acetals. Adv. Mater. Technol. 2022, 7 (9), 2101590 10.1002/admt.202101590. [DOI] [Google Scholar]
  44. Santos E. M.; Sheng W.; Salmani R. E.; Tahmasebi Nick S.; Ghanbarpour A.; Gholami H.; Vasileiou C.; Geiger J. H.; Borhan B. Design of Large Stokes Shift Fluorescent Proteins Based on Excited State Proton Transfer of an Engineered Photobase. J. Am. Chem. Soc. 2021, 143 (37), 15091–15102. 10.1021/jacs.1c05039. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Yucknovsky A.; Amdursky N. Controlling pH-Sensitive Chemical Reactions Pathways with Light - a Tale of Two Photobases: An Arrhenius and a Brønsted. Chem. – Eur. J. 2024, 30 (9), e202303767 10.1002/chem.202303767. [DOI] [PubMed] [Google Scholar]
  46. Frisch M. J.; Trucks G. W.; Schlegel H. B.; Scuseria G. E.; Robb M. A.; Cheeseman J. R.; Scalmani G.; Barone V.; Petersson G. A.; Nakatsuji H.. et al. Gaussian 16, Gaussian Inc..
  47. IQmol Molecular Viewer, 2024http://iqmol.org/. (accessed May 2, 2024).
  48. Thanthiriwatte K. S.; Hohenstein E. G.; Burns L. A.; Sherrill C. D. Assessment of the Performance of DFT and DFT-D Methods for Describing Distance Dependence of Hydrogen-Bonded Interactions. J. Chem. Theory Comput. 2011, 7 (1), 88–96. 10.1021/ct100469b. [DOI] [PubMed] [Google Scholar]
  49. Pratt K. C.; Wakeham W. A. Self-Diffusion in Water and Monohydric Alcohols. J. Chem. Soc., Faraday Trans. 2 1977, 73 (7), 997–1002. 10.1039/f29777300997. [DOI] [Google Scholar]
  50. Stenutz Tables for Organic Chemistry, 2024https://www.stenutz.eu/chem/. (accessed May 2, 2024).

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

jp4c02907_si_001.pdf (4.2MB, pdf)

Articles from The Journal of Physical Chemistry. a are provided here courtesy of American Chemical Society

RESOURCES