Divergent excited state proton transfer reactions of bifunctional photoacids 1-ammonium-2-naphthol and 3-ammonium-2-naphthol in water and methanol

Kacie J. Nelson , Paige J. Brown , Holly E. Rudel and Kana Takematsu *
Department of Chemistry, Bowdoin College, Brunswick, ME 04011, USA. E-mail: ktakemat@bowdoin.edu

Received 24th September 2019 , Accepted 24th October 2019

First published on 24th October 2019


This paper highlights the challenge of predicting the excited state proton transfer (ESPT) reactions of small organic compounds with multiple proton transfer sites. Aminonaphthols, naphthalene compounds with both hydroxyl and amino substituents, can be viewed as a combination of two monoprotic photoacids, naphthol and naphthylammonium. Here, the ESPT reactions of 3-ammonium-2-naphthol (3N2OH) and 1-ammonium-2-naphthol (1N2OH) were studied in water and methanol using a combination of steady-state and time-correlated single-photon counting emission spectroscopy. For 3N2OH, ESPT was observed at the OH site in water but at neither of the sites in methanol; for 1N2OH, ESPT was observed at both the OH and NH3+ sites in water but only at the NH3+ site in methanol. Evidence of ESPT at the NH3+ site is limited for aminonaphthols. The divergent dynamics of 3N2OH and 1N2OH in water and methanol are discussed; dependent on the substitution and solvent, the ESPT reactions were analysed within the frameworks of reference photoacids 2-naphthol and 1-naphthylammonium. The application of crown ether and salt to control the release of select protons in non-aqueous media is also discussed.


Introduction

Photoacids, molecules that become more acidic upon excitation, are useful probes for fundamental proton transfer (PT) and proton-coupled electron transfer1–15 and have been utilized in a multitude of applications that require spatial and temporal control of PT.16–21 Naphthol (xOH) and naphthylammonium (xNH) are two prototypical organic photoacids based on the naphthalene structure that have an oxygen and nitrogen-centred PT site, respectively (Scheme 1). Due to their small size and broad commercial availability, there have been numerous studies on the effects of OH and NH3+ on the excited state landscape of naphthalene and the resultant excited state proton transfer (ESPT) reactions of the monoprotic photoacids.2,13,14,22–33 For both systems, substitution at Cα (C1) lowers pKa* compared to addition at Cβ (C2); α-naphthol is a stronger photoacid than β-naphthol (pKa*(1OH) = −0.214vs. pKa*(2OH) = 2.8)23,34 while α-naphthylammonium is only slightly a stronger photoacid than β-naphthylammonium (pKa*(1NH) = −1.0 vs. pKa*(2NH) = −0.8).29 The significant difference in pKa* in naphthols has been attributed to greater mixing of the lowest excited singlet states, La and Lb, (named after the short and long axes of symmetry in naphthalene)35 and subsequent inversion to the more polar La state upon excitation of 1OH; the La and Lb bands remain well-separated in 2OH with emission only observed from the charge-diffusive Lb state.9
image file: c9cp05269k-s1.tif
Scheme 1 The reference photoacids, 2-naphthol (2OH) and 1- and 2-naphthylammonium (1NH and 2NH), and aminonaphthols, 3-amino-2-naphthol (3N2OH) and 1-amino-2-naphthol (1N2OH). The nomenclature for the different protonation states of aminonaphthols is shown for 1N2OH: the cation (NH3+/OH), neutral (NH2/OH), zwitterion (NH3+/O), and anion (NH2/O).

A smaller extent of work has been done on aminonaphthols, which are a combination of the naphthol and naphthylammonium frameworks.36–42 The addition of both functional groups on naphthalene promotes mixing of the La and Lb states, and the dual photoacidic nature of the compound has prompted unique questions on the ESPT mechanism. Aminonaphthols can exist in four different protonation states, the cation, neutral, zwitterion, and anion, with the zwitterion only accessible in the excited state (Scheme 1). Given the greater photoacidity of 2NH versus 2OH (pKa*(2NH) = −0.8 vs. pKa*(2OH) = 2.8),23,29 early works by Ellis and Rogers assumed that excitation of the aminonaphthol cation would lead to ESPT at the NH3+ site, while excitation of the neutral would lead to ESPT at the OH site, followed by proton addition at basic NH2 to form the zwitterion in the excited state.36,37 Decades later, in their study of carboxy naphthol-derivatives, Pines et al. used the Hammett model based on a naphthol framework to predict the photochemistry of aminonaphthols;43 the electron-withdrawing and electron-donating groups (EWG/EDG), NH3+ and NH2, should lower and increase the pKa* of the OH group, respectively.

Our group previously investigated the ESPT mechanism of distally substituted aminonaphthols, 5-amino-2-naphthol (5N2OH), 8-amino-2-naphthol (8N2OH), and 7-amino-2-naphthol (7N2OH), in water, using a combination of steady-state (SS) and time-correlated single-photon counting (TCSPC) emission spectroscopy.39,40 The isomers were chosen such that NH3+ substitution was at both the Cα and Cβ of the distal naphthalene ring with respect to OH. Despite differences in the emission trends between the α- and β-NH3+-substituted aminonaphthols, the studies showed the excited cation to be the zwitterion precursor and the neutral to have dramatically reduced photoacidity, supporting Pines et al.'s prediction described above. The linearity of the Hammett model, however, was broken upon extension of the fit from the EWG to EDG region, hinting that despite the ability to model the ESPT reactions using 2OH, the 1NH/2NH framework may still be needed to fully explain the photochemistry of aminonaphthols.

To investigate whether it was possible to observe dual or competing ESPT reactions at the NH3+ and OH sites in aminonaphthols, the ESPT reactions of 3-amino-2-naphthol (3N2OH) and 1-amino-2-naphthol (1N2OH) in water and methanol were studied using a combination TCSPC emission spectroscopy and time-dependent density functional theory (TD-DFT). Dynamic studies of 3N2OH and 1N2OH in water and methanol were of particular interest as: (1) addition of NH3+ at adjacent Cα and Cβ with respect to OH could sufficiently alter the intramolecular electronic redistribution to promote competition between the OH and NH3+ ESPT reactions upon excitation; (2) earlier SS emission studies of 3N2OH had conflicting interpretations of the ESPT mechanism;36,42 little work has been done on 1N2OH due to its reported instability in water;37 and (3) the different interactions between the NH3+ and OH protic sites with methanol and water could alter the favoured ESPT reaction. The divergent ESPT reaction schemes of 3N2OH and 1N2OH in water and methanol are discussed in this paper.

Experimental and computational details

A. Sample preparation

1N2OH hydrochloride (Alfa Aesar, 97%), 3N2OH (Sigma Aldrich, 97%), 2OH (Fluka, 99%), and 1NH (TCI, 99%) were purchased and used without further purification. Stock sample solutions were prepared in either deionized water (Barnstead EASYpure RF) or methanol (OmniSolv) and diluted to 40–200 μM for spectral characterization. To minimize photodecomposition, all samples were prepared and handled in the dark. For aqueous measurements, the pH was adjusted to pH = 1–12 with either concentrated HCl or NaOH using a Mettler Toledo SevenEasy pH probe. For highly acidic samples (pH < 1), the pH was directly calculated from titration by 8 M HCl. Of the studied samples, 1N2OH was found to be unstable at pH > 3, most likely forming quinone-like derivatives in water.44,45 Addition of NaOH led to instantaneous decomposition, and even at pH < 3, the sample decomposed slowly during the experiment. Thus, all efforts were made to use 1N2OH immediately upon preparation, and adjustments to pH were made only by addition of HCl. For titration experiments covering a large pH range (pH = −0.24 to 2.8), additional spectra of 1N2OH were later collected at random pH to confirm the observed spectral trends.

For experiments in methanol, all the compounds dissolved more readily in methanol than in water, and 1N2OH was much more chemically stable. An aliquot of 2–4 mL stock solution was added to a 1 cm quartz cuvette (Starna Cell) with a micromagnetic stir bar. Small (μL) amounts of 1–5 mM HCl or NaOH stock solution prepared in methanol (<1% water) were added to adjust the protonation state of the amino group in 3N2OH, 1N2OH, and 1NH. Experiments were done in both air and argon; the latter involved bubbling ultrahigh purity argon (Purity Plus) through septa-capped sample cuvettes for 20–25 minutes before data collection.

B. Crown ether binding experiments

For the crown ether binding experiments, a 1 M stock solution of 18-crown-6-ether (18C6, Strem Chemicals, 99%) in methanol was prepared. To bind crown ether to the NH3+ site, increments of 2–5 μL of 18C6 were added to 2–4 mL of 1N2OH and 1NH cation in methanol (100–200 μM). To reverse or free NH3+ from 18C6, excess >0.01 g KCl (Sigma, 99%) was added directly to the cuvette. Most of the crown ether binding experiments were done in air; however, time-resolved emission measurements were also collected under argon.

C. Spectral characterization

All experiments were done at room temperature. UV/Vis absorption spectra (Agilent 8453) of 1N2OH, 3N2OH, 2OH, and 1NH were collected in both water and methanol to assign protonation states of the samples upon addition of HCl or NaOH. The spectra were also used to check for sample degradation; this was critical for aqueous 1N2OH. SS emission spectra (Hitachi F-2500 FL) were collected at 280 nm excitation over a broad range of wavelengths 300–600 nm. For titration measurements, spectra were normalized at a chosen peak to offset any fluctuating lamp intensity or detector sensitivity. Complementary TCSPC emission were collected at 280 nm excitation (DeltaDiode DD-280, pulse width ∼ 800 ps) using a Horiba DeltaFlex Modular Fluorescence Lifetime System. The validity of the instrument operation was assessed prior to data collection using 4-bis(5-phenyloxazol-2-yl)benzene (τ = 1.32 ns);46 the instrument response (prompt) was collected from light scatter of a colloidal silica suspension (LUDOX TMA, Sigma Aldrich). Sample decays were collected at <2% signal rep rate at select wavelengths in the 325–600 nm region using either single-wavelength mode at 10[thin space (1/6-em)]000 peak count or scan mode at fixed time intervals. Decays collected at λ < 350 nm frequently had too much scatter from the 280 nm excitation and were excluded from analysis. The collected decays were globally fit to multiexponential functions in DAS6 analysis software (Horiba Scientific), and the corresponding lifetimes and relative emission intensities were used in Matlab for kinetic modeling.

D. Computational details

Despite the limitations of TD-DFT,47–49 previous efforts to calculate the excited states of aminonaphthols using this methodology were effective for assignment of the spectral data.39,40 The absorption and emission transitions of water-solvated 1N2OH (cation, neutral, and zwitterion) and 3N2OH (cation, neutral, zwitterion, anion) were calculated using Gaussian 1650 at the B3LYP/6-31++G(d,p) level of theory and a polarizable continuum model (PCM) for water.51 Computational details are available in prior published works.39,40

Results and discussion

A. Spectral Overview of 1N2OH and 3N2OH

The UV/Vis absorption spectra of 1N2OH and 3N2OH in water and methanol are shown in Fig. 1. As neither the absorption of 1N2OH or 3N2OH exhibited any significant differences between solvation by methanol or water, only the spectra collected in the solvent favouring a higher number of protonation states are shown. For 1N2OH, due to instability of the compound in water at higher pH (estimated pKa(NH3+) > 3), absorption spectra of the cation and neutral were obtained in methanol. For 3N2OH, the aqueous sample was chemically stable across a broad pH range, such that the cation, neutral, and anion could be isolated. The ground state equilibria between the cation–neutral and neutral–anion were determined via UV/Vis titration to be pKa(NH3+) = 3.9 ± 0.2 and pKa(OH) = 9.1 ± 0.2, respectively; these values are slightly more acidic than those reported for parent compounds, 2NH (pKa = 4.1)29 and 2OH (pKa = 9.5),2 indicating that the close proximity of the two functional groups leads to some electronic interactions in the ground state.
image file: c9cp05269k-f1.tif
Fig. 1 UV/Vis absorption spectra of 80 μM 1N2OH in methanol (left) and 200 μM 3N2OH in water (right). The different protonation states are shown as: cation (red solid line), neutral (blue dotted line), and anion (green dashed line).

A comparison of the 1N2OH and 3N2OH absorption spectra shows a striking similarity and difference between the cation and neutral spectral features. The cation absorption of both compounds resembles the absorption spectrum of 2OH rather than 1NH, while the neutral absorption is comparable to those of distally-substituted aminonaphthols, 8N2OH and 7N2OH, with NH3+ substitution at Cα and Cβ respectively (see ESI). It would not be unreasonable to predict that subsequent ESPT reactions of 1N2OH and 3N2OH follow a 2OH framework; however, the SS emission spectra attest to the underlying complexity of functionalization of the naphthalene ring(s).

The SS emission spectra of 1N2OH and 3N2OH in water and methanol are shown in Fig. 2. (When possible, the colour and line style have been chosen to indicate the pre-excitation state of the species, as displayed in Fig. 1). The left and right columns represent 1N2OH (left) and 3N2OH (right), respectively, while the top and bottom rows correspond to solvation by water (top) and methanol (bottom). Comparison of the subplots shows that the spectra differ significantly across and down the figure; thus, 1N2OH and 3N2OH have distinct ESPT reaction pathways that are solvent-dependent. In the following sections, the ESPT reactions of each compound are discussed in detail. For reference, the SS emission peaks of the compounds have been summarized in Table 1.


image file: c9cp05269k-f2.tif
Fig. 2 SS emission spectra of (a) 1N2OH in water collected from pH = −0.24 to 2.8 (red to pink line) through HCl titration. The spectra have been normalized to the cation peak at 352 nm for comparison. The solid lines indicate that the pre-excitation state was the cation. The symbol * refers to the three observed peaks at 352 nm, 410 nm, and 456 nm. (b) 3N2OH in water collected at pH = 0.1 (cation, red solid line), 2.4 (cation–zwitterion, orange solid line), 6.5 (neutral, blue dotted line), and 11.9 (anion, green dashed line). (c) 1N2OH in methanol collected from excitation of the cation (red solid line) and neutral (blue dotted line). (d) 3N2OH in methanol collected from excitation of the cation (red solid line) and neutral (blue dotted line).
Table 1 The measured emission peaks (nm) of the various protonation states of 1N2OH and 3N2OH in water (w) and methanol (m). The emission peaks of 2OH and 1N are provided for comparison. The calculated emission peaks at the B3LYP/6-31++G(d,p)-PCM level of theory are provided in parentheses. The x,y,z-coordinates of the optimized S0 and S1 structures of 1N2OH and 3N2OH are available in the ESI
Cation Zwitterion Neutral Anion
a For simplification, for 2OH, the protonated (OH) and deprotonated (O) forms are shown in the cation and anion columns, respectively. b Ref. 29 and 32.
1N2OH (w) 352 (332) 410 (417) 456 (505)
1N2OH (m) 348 450
3N2OH (w) 351 (338) 413 (408) 383 (387) 392 (391)
3N2OH (m) 350 372
2OH (w)a 351 (346) 414 (421)
2OH (m)a 348
1NH (w)b 330 (332) 460 (487)
1NH (m) 331


B. The OH-ESPT reaction of 3N2OH: a 2OH framework

The ESPT reactions of 3N2OH are discussed first, as the compound was photochemically similar to previously studied aminonaphthols. In Fig. 2b, select SS emission spectra of 3N2OH collected at various pH in water are shown. Four distinct peaks were observed: the cation at 351 nm (red solid line, pH = 0.1), zwitterion at 413 nm (orange solid line, pH = 2.4), neutral at 383 nm (blue dotted line, pH = 6.5), and anion at 392 nm (green dashed line, pH = 11.9), respectively. These assignments were based on the dominant species in the ground state and are in good agreement with the TD-DFT calculations (Table 1). They also follow a similar emission trend to that observed for 7N2OH (cation 357 nm, zwitterion 425 nm, neutral 398 nm, and anion 407 nm).40

To confirm the assignment, a series of time-resolved emission spectra were collected at low pH (pH = 0.3–3.5) to examine the dynamics of the excited cation. Fig. 3 shows select emission decays collected at 350 nm and 425 nm, near the respective cation and zwitterion peaks, at pH = 1.9. The 350 nm signal decays biexponentially at τ1 = 370 ps (major) and τ2 = 10.4 ns (minor), while the 425 nm signal concomitantly rises at 370 ps and decays at 10.4 ns. The amplitudes of the latter rise and decay are equal in magnitude, a classic feature of a two-state ESPT model.3,23 Application of the two-state model to aminonaphthols has been previously described.39,40 Here, analysis of the emission data was done at low pH to model ESPT at the OH site using Scheme 2; the kinetic parameters have been summarized in Section E (see ESI for details). The photoacidity of the 3N2OH cation was determined to be pKa*(OH) = 0.6 ± 0.2, slightly lower than that reported for the 7N2OH cation, pKa*(OH) = 1.1 ± 0.2.40 There is a slight enhancement from the proximity of NH3+ to OH, although in the Förster cycle,3 the difference in pKa* could also be partially attributed to the difference in ground state acidity. At higher pH, the dynamics of neutral 3N2OH were examined (see ESI); deprotonation of NH3+ in the ground state de-activated the OH photoacidity, such that pKa*(OH) of neutral 3N2OH is slightly more basic or approximately the same as ground state pKa(OH) = 9.1 ± 0.2. The dynamics were similar to that observed for 7N2OH such that even with the proximity of the two functional groups, the kinetic barrier to zwitterion formation remains insurmountable.


image file: c9cp05269k-f3.tif
Fig. 3 Select TCSPC emission decays at 350 and 425 nm of 3N2OH in water at pH = 1.9 with gray markers = prompt (i.e. instrument response function), black markers = emission, and solid line = fit = convolution of exponential function and instrument prompt. Emission at 400–500 nm were globally fit to a biexponential function (τ1 = 370 ± 10 ps, τ2 = 10.4 ± 0.1 ns, χ2 = 1.09), with the 350 nm signal (red, cation) biexponentially decaying at τ1 = 370 ps (major) and τ2 = 10.4 ns and the 425 nm signal (orange, zwitterion) rising at 370 ps and decaying slowly at 10.4 ns.

image file: c9cp05269k-s2.tif
Scheme 2 ESPT reactions of 3N2OH upon excitation of the cation in water and in methanol. The dashed box represents pathways accessible in methanol. No proton quenching pathways are shown, as they did not play a significant role in the observed dynamics of 3N2OH.

In comparison to water, the photochemistry of 3N2OH in methanol is simple. Excitation of the cation (red) and neutral (dotted blue) lead to single emission peaks at 350 nm and 372 nm, respectively (Fig. 2d). Both emission peaks were slightly blue-shifted from those reported in water, consistent with methanol's inability to hydrogen-bond as efficiently as water to the functional groups to stabilize the excited state. The monoexponential decays of the cation and neutral at 350 nm (τ = 8.7 ± 0.1 ns, χ2 = 1.22) and 375 nm (τ = 6.3 ± 0.1 ns, χ2 = 1.23) further support the absence of ESPT pathways for 3N2OH in methanol (see ESI). The difference between the photochemistry of 3N2OH cation in water vs. methanol has been summarized in Scheme 2.

C. The OH and NH3+-ESPT reactions of 1N2OH in water: 2OH and 1NH frameworks

Fig. 2a shows the SS emission spectra collected from excitation of the 1N2OH cation at various acidic pH in water; a quick comparison of the 1N2OH and 3N2OH emission collected at pH = 2.8 (highest-intensity pink line in Fig. 2a) vs. pH = 2.4 (orange line, Fig. 2b) supports a more complex excited state landscape for 1N2OH. At pH = 2.8, upon excitation, a small band at 352 nm and a large broad band at 456 nm with a shoulder at 410 nm appear in the 1N2OH spectrum. As more HCl is added, the small band at 352 nm remains, but the peak at 456 nm gradually disappears such that the 410 nm band becomes prominent (mid-intensity gray-blue line, pH = 1.2). As pH < 0, the 410 nm band decreases until only the band at 352 nm remains (red line, pH = −0.24). The three observed peaks at 352, 456, and 410 nm were assigned to the cation, neutral, and zwitterion, respectively, with the proposition that the excited cation is the precursor to the latter species. Support for the assignment is as follows:

(1) Recall the strong similarity between the absorption spectra of 1N2OH and 8N2OH. Parallel trends in the emission would be reasonable. For 8N2OH, upon excitation in water, the cation forms (only) the zwitterion, with the cation and zwitterion emitting at 358 and 422 nm, respectively; the excited neutral emits at 445 nm.39 For 1N2OH, the observed emission peaks at 352, 410, and 456 nm correspond very well to the 8N2OH cation, zwitterion, and neutral emission. The neutral emission is red-shifted from the zwitterion, contrary to what was observed for 3N2OH; this inversion was also observed in the study of 7N2OH vs. 8N2OH.39,40 A key difference is that ESPT is only observed at OH for these aminonaphthols, while two separate ESPT reactions at OH and NH3+ are proposed for 1N2OH.

(2) TD-DFT-PCM calculations in Table 1 qualitatively support the peak assignments. While the largest discrepancy between the experiments and calculations is observed for the neutral (Δ = 49 nm), the calculated transition is still red-shifted from the zwitterion. Previous computational work on aminonaphthols showed that addition of explicit water molecules near the amino site drastically affect the calculated emission.40 (The calculated emission of 1N neutral is also red-shifted 27 nm from the observed transition in Table 1). Thus, the addition of explicit water molecules, or a more sophisticated solvent model, would improve the agreement; yet, this approach would still be insufficient to describe the multireference nature of the excited state. Previous calculation of the 7N2OH neutral using EOM-CCSD/aug-cc-PVDZ-PCM showed that the energy gap between La and Lb decreases compared to that of 2OH (ΔE = 0.60 eV vs. 0.75 eV).40,47 As substitution at Cα further reduces the gap (e.g. ΔE = 0.39 eV for 1OH),47 the lowest singlet states of 1N2OH neutral are presumably more closely spaced than 7N2OH. Therefore, it is more surprising how well TD-DFT still captures the respective energies of the different protonation states of excited 1N2OH.

Dynamic measurements were done to further elucidate the mechanism of the ESPT reactions. Select emission decays at 350 nm (cation, red), 400 nm (zwitterion, orange), and 450 nm (neutral, blue) of 1N2OH in water at pH = 2.1 are shown in Fig. 4 (see ESI for the corresponding SS emission spectrum). TCSPC emission were globally fit to a biexponential function, with the 400 nm and 500 nm signals decaying predominantly at τ1 = 790 ps and τ2 = 4.7 ns, respectively. The rise of the signals at 400 and 450 nm (i.e. excited state formation of the zwitterion and neutral) and concomitant cation decay at 350 nm were too fast to be resolved by the instrument and was estimated to be τ3 < 200 ps. Degassing the sample did not alter any of the kinetics. Increasing [H+] further shortened the lifetimes; at pH = 1.3, upon prompt formation, the 420 nm and 450 nm signals decayed at τ1 = 610 ps and τ2 = 1.3 ns, respectively (see ESI for fit: τ1 = 610 ± 10 ps, τ2 = 1.3 ns ± 0.1 ns, χ2 = 1.21). The relative contribution of the 1.3 ns-component at 450 nm grew smaller, consistent with the reduction of the 456 nm peak in the SS emission spectrum.


image file: c9cp05269k-f4.tif
Fig. 4 Select TCSPC emission decays at 350, 400, and 450 nm of 1N2OH in water at pH = 2.1 with gray markers = prompt, black markers = emission, and solid line = fit = convolution of exponential function and instrument prompt. Emission at 400–500 nm were globally fit to a biexponential function (τ1 = 790 ± 10 ps, τ2 = 4.7 ns ± 0.1 ns, χ2 = 1.22), with the 400 nm signal (zwitterion, orange) dominated by the 790 ps decay, and the 450 nm signal (neutral, blue) dominated by the 4.7 ns decay. The 350 nm decay (cation, red) was too fast to be resolved (τ3 < 200 ps).

The drastic decline in the neutral lifetime at lower pH was attributed to quenching by H+. Early work by Förster had noted the susceptibility of neutral 1N fluorescence to H+ quenching.31 Tsutsumi and Shizuka used a combination of SS and time-resolved emission to attribute the H+-quenching to electrophilic protonation of one of the aromatic C atoms in the excited 1N ring.13,29 For 1N2OH, a Stern–Volmer plot of τ2−1vs. [H+] revealed a linear relationship (see ESI), with a slope corresponding to kq(1N2OH)= 1.2 × 1010 M−1 s−1; the value was similar to the quenching rate reported by Tsutsumi and Shizuka for neutral 1N, kq(1N) = 8.9 ± 0.3 × 109 M−1 s−1.29 Thus, the disappearance of the 456 nm peak in Fig. 2a can be assigned to proton-induced fluorescence quenching and not the absence of neutral formation in the excited state. As the 1N2OH cation–neutral excited state dynamics is consistent with 1N (pKa* = −1),29 the photoacidity of 1N2OH with respect to NH3+ is estimated to be pKa*(NH3+) < 0.

In contrast, the excited state dynamics of 1N2OH cation–zwitterion is consistent with that of 2OH. While the zwitterion peak can never be isolated, its disappearance can be attributed to the shifted excited state acid equilibrium, as proton quenching for 2OH is much slower than that reported for 1N.13,34 Comparisons of the cation and zwitterion emission peaks were made in pH regions that the neutral emission was quenched. From Fig. 2a and repeated titration measurements, the photoacidity of 1N2OH with respect to OH is estimated to be pKa*(OH) = 0.6 ± 0.3, slightly more acidic than that reported for the 8N2OH cation, pKa*(OH) = 1.1 ± 0.2.39 Thus, the proximity of NH3+ to OH enhances photoacidity. The competing OH and NH3+-ESPT reactions and rates have been summarized in Scheme 3 and section E, respectively.


image file: c9cp05269k-s3.tif
Scheme 3 ESPT reactions upon excitation of the 1N2OH cation in water and in methanol. The dashed box represents ESPT reaction pathways accessible in methanol.

D. The NH3+-ESPT reaction of 1N2OH in methanol: a 1NH framework

Fig. 2c shows the SS emission spectra collected for 1N2OH in methanol. Upon excitation of the cation, two emission peaks appear at 346 and 450 nm (red solid line). As 1N2OH is stable in methanol, the neutral can also be isolated in the ground state and excited to obtain an emission peak at 450 nm (blue dotted line). The neutral peak is slightly blue-shifted from that reported for aqueous 1N2OH, but they overlap relatively well (Fig. 2avs.2c). This is at odds with the significant blue-shifts (Δ = 27–40 nm) observed for 8N2OH and 1N neutral emission upon solvation in methanol: 445 nm vs. 418 nm for 8N2OH and 460 nm29,32vs. 420 nm for 1N. Despite the spectral similarity between 8N2OH and 1N, excitation of the 8N2OH cation leads only to a single peak in methanol (see ESI). Thus, dynamically, the 1N2OH cation is proposed to be more similar to 1N, undergoing ESPT at the NH3+ site.

To confirm the ESPT reaction pathway, binding studies with crown ether 18C6 were done in methanol. Crown ethers are known to bind strongly to ammonium cations.52,53 Shizuka et al. previously studied the ESPT reactions of 1N-18C6 complexes in 9[thin space (1/6-em)]:[thin space (1/6-em)]1 methanol[thin space (1/6-em)]:[thin space (1/6-em)]water mixtures;54 the hydrogen atoms of the ammonium group form hydrogen bonds with the oxygen atoms in the crown ether to significantly inhibit the proton dissociation rate. Here, the 18C6 binding studies with 1N2OH were done in methanol, with minimal water content ≪1%. While 1N2OH has two functional groups, the crown ether should preferentially bind to NH3+ (Fig. 5, top inset). Indeed, when 18C6 was added to 1N2OH cation in methanol, the neutral peak decreased in the emission spectra, implying that the formation of the neutral in the excited state was blocked (Fig. 5, top). (The emission and absorption spectra of the cation and cation–18C6 complex were nearly identical, suggesting that addition of 18C6 did not greatly perturb the electronic redistribution in the excited state but only acted as a kinetic inhibitor.) To reverse or free the hydrogens of the ammonium group from crown ether, potassium chloride was added to the solution. K+ should preferentially bind to 18C6, and indeed upon KCl addition, the emission spectrum of the 1N2OH-18C6 mixture resembled that of the free 1N2OH cation (Fig. 5, top). For comparison, the emission spectra of the titration of 1N cation in methanol with 18C6 and KCl were also collected (Fig. 5, bottom). Similar trends were observed, as the neutral peak decreased upon addition of crown ether and increased upon addition of KCl.


image file: c9cp05269k-f5.tif
Fig. 5 SS emission spectra of the titration of 1N2OH cation (top) and 1NH cation (bottom) with 18C6 (↓) and KCl (↑). All measurements were done in air-saturated solutions. The thick black line represents the cation while the thin lines represent addition of 18C6. To reverse the binding, KCl was added to the solution (dashed line). The emission has been normalized to the cation peak to highlight the relative changes in the neutral peak intensity. Inset of top plot: top view of the 1N2OH cation bound to 18-crown-6-ether. The cation (red) is most likely located above the crown ether plane to form hydrogen bonds with the oxygen atoms of the ether. (R = 2OH). As shown, the presence of the OH group should not obstruct complex formation.

If the quantum yields of 1N and 1N2OH in methanol are similar, the proportion of 1N cation undergoing ESPT is larger than that of the 1N2OH cation (Fig. 5). To shed light on this mechanism, time-resolved emission decays were collected near the maximum cation and neutral emission peaks and globally fit for both compounds. For the 1N2OH cation (Fig. 6), the 350 nm signal was fit to a monoexponential decay at τ1 = 1.5 ns, while the corresponding 450 nm signal was fit to a concomitant 1.5 ns rise and 15.7 ns decay that had opposite but identical amplitudes. (A minor 3.3 ns component was also observed at 450 nm; it is unclear whether this component was due to impurities or was a possible decomposition pathway of 1N2OH.) The 15.7 ns component is approximately the same lifetime observed for the isolated neutral (τN = 16.5 ± 0.1 ns, χ2 = 1.18). The kinetics are consistent with a two-state ESPT model of the 1N2OH cation in which the proton recombination pathway is negligible (kr ∼ 0). Thus, analogous to the reported bulky hydronium ions in water in the 1N-18C6 complex study,54 the proton may be behaving as a protonated methanol ion rather than a diffusing proton surrounded by methanol and thus, may not be conducive to the back PT reaction.


image file: c9cp05269k-f6.tif
Fig. 6 TCSPC emission decays at 350 and 450 nm from the excitation of 1N2OH cation in methanol with gray markers = prompt, black markers = emission, and solid line = fit = convolution of exponential function and instrument prompt. The signals were fit simultaneously to a triexponential function (τ1 = 1.5 ± 0.1 ns, τ2 = 15.7 ± 0.1 ns, τ3 = 3.3 ± 0.1 ns, χ2 = 0.95), with the 350 nm signal (cation, red) dominated by the 1.5 ns decay, and the 450 nm signal (neutral, blue) fit to a 1.5 ns rise and 15.7 ns decay of equal but opposite amplitude. The 3.3 ns was a minor component that is from either an impurity or minor photodecomposition channel.

The addition of crown ether to 1N2OH cation introduced an additional lifetime-component in the 350 nm decay: τ1 = 1.66 ns and τ2 = 7.1 ns (global fit of 350 nm and 450 nm data: τ1 = 1.66 ± 0.05 ns, τ2 = 7.1 ± 0.1 ns, τN = 16.3 ± 0.1 ns, χ2 = 1.15); the latter τ2 component was assigned to relaxation of the cation–18C6 complex. TCSPC emission collected in parallel to the SS emission titration studies showed that at 350 nm, contributions by the τ1 and τ2 components varied by the fraction of cation and cation–18C6 complex species in solution, while at 450 nm, the relative contributions of the rise (τ1) and decay (τN) remained fixed, implying that the kinetics at 450 nm were most likely due to the free cation remaining in solution (see ESI). Similar measurements and observations were made for 1N in methanol, with the main distinction being differences in the lifetimes: τ1 = 9.0 ns (cation), τ2 = 53 ns (cation–18C6-complex), and τN = 18 ns (neutral) (see ESI). In the absence of proton recombination (kr ∼ 0), if the complex is treated as the “isolated” cation, the rate of excited cation relaxation (kC), neutral relaxation (kN), and proton dissociation (kd) can be determined from the two-state ESPT model shown in Scheme 3 using the following equations:

 
image file: c9cp05269k-t1.tif(1)
 
image file: c9cp05269k-t2.tif(2)
 
image file: c9cp05269k-t3.tif(3)

The rate constants for both 1N2OH and 1N will be discussed in the next section.

E. Comparison of the rates of ESPT for the various photoacids

Table 2 summarizes the kinetic rate constants and pKa* of 1N2OH, 3N2OH, 2OH, and 1NH in water and methanol. For the reference photoacids, 1NH is a significantly stronger photoacid than 2OH (pKa* = −1.0 vs. 2.8)2,29 in water. Given Ka* as the ratio of the rates of proton dissociation and recombination, the rate of proton dissociation is much faster for 1NH than 2OH (kd = 1.3 vs. 0.07 × 109 s−1),23,29 while the rate of proton recombination or back PT to nitrogen is much slower in water (kr = 0.12 vs. 47 × 109 M−1 s−1), indicating a different water cluster network about the different oxygen and nitrogen-centred protic sites. Proton dissociation is also favoured in excited 1NH given its extended lifetime compared to that of 2OH (τ = 48 ns vs. 7.3 ns).23,29 In methanol, the 1NH and 2OH excited acid and base relaxation rates remain relatively the same, but only 1NH undergoes ESPT. The ESPT reaction is significantly slowed down in methanol (kd = 0.092 vs. 1.3 × 109 s−1), with the corresponding back PT too slow to be observed. Indeed, if it were not for the significantly long lifetime of the 1NH cation in methanol, ESPT would not have been observed.
Table 2 Summary of the kinetic rate constants, kacid and kbase (relaxation of the acid and base), kd (proton dissociation), and kr (proton recombination), and pKa* of 3N2OH, 1N2OH, 1NH, and 2OH in water (w) and methanol (m). In water, the respective acid/base pairs refer to the cation/zwitterion of 3N2OH and 1N2OH1, cation/neutral of 1N2OH2 and 1NH, and neutral/anion of 2OH. In methanol, the acid/base pair refers to the cation/neutral of 3N2OH, 1N2OH, and 1NH
k acid × 108 (s−1) k base × 107 (s−1) k d × 109 (s−1) k r × 109 (M−1 s−1) pKa*(NH3+) pKa*(OH)
a Estimated pKa* from the SS emission spectra. b The neutral is quenched by H+: kq = 1.2 × 1010 M−1 s−1. c Ref. 29: the neutral is quenched by H+: kq = 8.9 × 109 M−1 s−1. d Ref. 23.
3N2OH (w) 2.1 ± 0.1 8.9 ± 0.2 2.3 ± 0.3 9.5 ± 0.7 0.62 ± 0.20
1N2OH (w)1a >1 0.6 ± 0.3
1N2OH (w)2b >1 <0
1N (w)c 0.21 3.7 1.3 0.12 −1.0
2OH (w)d 1.38 10.6 0.07 47 2.8
3N2OH (m) 1.1 ± 0.2 16 ± 2
1N2OH (m) 1.4 ± 0.2 6.1 ± 0.2 0. 49 ± 0.02 ∼0
1N (m) 0.19 ± 0.02 5.5 ± 0.2 0.092 ± 0.002 ∼0
2OH (m) 1.2 ± 0.2


The photoacidity and kinetics of the 3N2OH cation are most consistent with 2OH. In water, the addition of NH3+ to the naphthol ring boosts the photoacidity compared to 2OH, pKa* = 0.6 vs. 2.8. Kinetically, the excited acid and base of 3N2OH and 2OH relax at similar rates, but the proton dissociation for 3N2OH occurs much faster (kd = 2.3 vs. 0.07 × 109 s−1). Previous work by Pines et al. showed a correlation between increased kd and decreased pKa* for substituted naphthol and pyranol derivatives;43,55 given the 100× increase in photoacidity for 3N2OH, the heightened kd for 3N2OH is not unexpected. In methanol, the proton dissociation must slow drastically such that the ESPT reaction cannot compete with the relaxation, which only decreases by a factor of 2 in methanol.

The dynamics of 1N2OH are more complex. Due to its reactivity and fast ESPT dynamics, the kinetics of 1N2OH could not be resolved in water. Nevertheless, the estimated pKa* of the OH and NH3+ sites are consistent with both 2OH and 1NH. For pKa*(OH), the photoacidity of 1N2OH is approximately the same as that of 3N2OH, and the estimated rate of proton dissociation is magnitudes larger than that of 2OH. For pKa*(NH3+), the negative pKa* and lower limit of the proton dissociation rate are consistent with that of 1NH.

The ability to measure the kinetics of 1N2OH in methanol provided direct quantitative comparisons to 2OH and 1NH. The lifetime of the excited cation interestingly resembles that of 2OH (τ = 7.1 vs. 8.1 ns). Given the similarity between the OH-ESPT reactions of 1N2OH and 3N2OH cation in water, proton dissociation at the OH site must also be too slow in methanol to compete with relaxation. ESPT, however, was still observed at NH3+. Surprisingly, the rate of proton dissociation at NH3+ was higher for 1N2OH than 1NH (kd = 0.49 vs. 0.092 × 109 s−1). From the Hammett model,56 the addition of OH near the acidic site should reduce the PT rate. In our study of 7-substituted-2OH compounds, addition of OH and methoxy group OCH3 had little to no effect on the OH photoacidity.40 In contrast, addition of OCH3 on 5-substituted quinolines decreased the photoacidity of the N-protic site,57 demonstrating that there are substantial differences among the naphthalene-based frameworks.

Despite the increased rate of proton dissociation for 1N2OH, comparison of the SS emission spectra of 1NH and 1N2OH (Fig. 5) showed higher proton donation yield for 1NH. The neutral 1NH and 1N2OH had nearly identical lifetimes in methanol, τ = 17 vs. 18 ns; thus, the apparent discrepancy in the proton yield must be due to the inherent nature of the 1N2OH cation which resembles 2OH. Due to the fast relaxation of 1N2OH in the excited state, the ratio of the competing pathways between the proton dissociation and cation relaxation is higher for the 1NH than 1N2OH cation (4.9 vs. 3.5). While a major focus of photoacid design is stabilization of the conjugate base, stabilization of the acid must also be considered to optimize proton transfer. A thorough computational investigation of the electronic states of 1N2OH in water and methanol and the subsequent coupling of explicit solvent molecules around the ESPT sites would further shed light on this problem.

Conclusions

The contrast in the dynamics of simple aminonaphthols 1N2OH and 3N2OH show that despite the large body of work on naphthalene-based photoacids, the ESPT reactions and proton yields of such systems remain difficult to model. Dependent on the solvent, 1N2OH can undergo ESPT at both protic sites; 3N2OH is limited to ESPT at the OH site in water. The addition of crown ether also selectively hinders ESPT at the NH3+ site but not the OH site. Experiments on these small compounds are ideal as they provide excellent tests in the development of theory that must address not only the coupling of multiple electronic states to describe the electronic landscape but also the solvent and ESPT dynamics at the different oxygen and nitrogen-centred protic sites. Moving forward, experiments examining the effects of a broader range of solvents on the 1N2OH cation dynamics would be of interest. Previous studies have shown that the energies of the lowest singlet states of 1NH and 2OH are greatly influenced by the polarity of the solvent;2,9,27,33,58–60 it is unclear how the energy levels of 1N2OH will be affected and how the changes will impact the OH and NH3+-ESPT reactions. Methods to chemically modify the compounds to provide more control of the multiple ESPT reactions would also be of interest. In these cases, the ability to use an external combination of crown ethers and salts to control the release of select protons may be useful.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge Bowdoin College and Research Corporation for Science Advancement (Cottrell Scholar Award) for support of this work. P. J. B. and H. E. R. were funded by the Maine Space Grant Consortium (SG-18-17). Bowdoin College is an affiliate of the Maine Space Grant Consortium; any findings and conclusions expressed in this material are those of the authors and do not necessarily reflect the view of the National Aeronautics and Space Administration or of the Maine Space Grant Consortium.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp05269k

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