Divergent Hammett Plots of the Ground- and Excited-State Proton

Subscriber access provided by UNIV OF LOUISIANA

B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Divergent Hammett Plots of the Ground and Excited State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds Laura F. Cotter, Paige J. Brown, Ryan C. Nelson, and Kana Takematsu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01295 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Divergent Hammett Plots of the Ground and Excited State Proton Transfer Reactions of 7-Substituted-2Naphthol Compounds Laura F. Cotter, Paige J. Brown, Ryan C. Nelson, and Kana Takematsu* Department of Chemistry, Bowdoin College, Brunswick, ME 04011 *corresponding author: [email protected]

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 38

ABSTRACT

The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here the effect of substituents on the photoacidity and excited state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electrondonating group (EWG vs. EDG, NH3+ vs. NH2) led to a drastic decline in photoacidity: pKa* = 1.1 ± 0.2 vs. 9.6 ± 0.2. TD-DFT calculations with explicit water molecules confirmed that the excited neutral state (x = NH2) is greatly stabilized by water, with EOM-CCSD calculations supporting potential mixing between the La and Lb states. A similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH3, pKa* = 2.7 ± 0.1. Hammett plots of the ground and excited state PT reactions of substituted 7-x-2OH compounds (x = CN, NH3+, H, CH3, OCH3, OH, and NH2) vs. Hammett parameters σp showed breaks in the linearity between the EDG and EWG regions: ρ ~ 0 vs. 1.14 and ρ* ~ 0 vs. 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest Lb state with the La and possible Bb states upon substitution of naphthalene in water.


2

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Photoacidic molecules increase in acidity upon photoexcitation, allowing spatial and temporal control of proton transfer (PT) in various condensed phase environments. Long utilized as tools to study fundamental PT,1-9 applications of photoacids have exploded in the last decade. For example, Das et al. have demonstrated that photoacids 2-naphthol (2OH) and its derivative 7-bromo-2naphthol (7Br2OH) can be used to photocatalytically protonate silylenol ether.10 Liao et al. have developed long-lived photoacids that can photocatalyze esterification reactions and modify pH sensitive polymer gels.11,12 Finally, Li et al. have fabricated silicon nanoreactors embedded with photoacid 8-hydroxypyene-1,3,6-trisulfonate (HPTS) and ATPase-liposomes that can mimic the photophosphorylation process of chloroplasts.13 Thus, the further development and rational design of photoacids is of interest to many scientific and commercial fields. Hydroxyl-substituted aromatic compounds, including naphthol and hydroxypyrene derivatives, constitute many of the prototypical organic photoacids. The abilities of both the solvent and functional groups to tune the photoacidity of these prototypes are well-known.1-3,5,14-20 Of the latter, much of the focus has been on the presence of electron-withdrawing groups (EWG). For example, 1-hydroxypyrene exhibits increased photoacidity in water upon substitution by sulfonate versus sulfonamide EWG: pKa* of 1.3 for HPTS vs. 0.7 for 8-hydroxy-hexamethylpyrene-1,3,6trisulfonamide (HPTA);3 mono- and di-substituted 5-cyano-2-naphthol (5CN2OH) and 5,8dicyano-2-naphthol (DCN2OH) have boosted pKa* of -0.75 and -4.5 compared to 2OH, pKa* = 2.8,14,15,21 such that proton donation can be extended beyond water to select organic solvents. Reversible protonation of functional groups provides additional flexibility in tuning the photoacidity, as in 6-carboxyl-2-naphthol (6COOH2OH), the protonated and deprotonated carboxyl species have pKa*(OH) of 1.4 vs. 2.5, respectively,16,22 while in 8-amino-2-naphthol


3


Page 4 of 38

(8N2OH), the corresponding pKa* vary more greatly, pKa*(OH) = 1.2 vs. 9.5.23 In contrast, there have been few studies exploring the use of electron-donating groups (EDG) to control photoacidity. The assumption is that the addition of EDG will reduce photoacidity; hence simultaneous functionalization of photoacids with EDG and EWG may afford opportunities to develop photoacids for use in narrow chemical windows of interest. Predictive models of the effect of different functional groups on pKa* would aid in the further design of photoacids. The development of “cheap” predictive models of photoacidity that could be applied to both EWG and EDG are of particularly strong interest. Both computational and experimental models have been proposed. Computationally, direct calculation of pKa* remains a challenge24-27 such that many theoretical strategies have shifted to identifying molecular properties (e.g. charge density, bond length, etc.) that correlate to photoacidity.26,28 These studies have typically utilized time-dependent density functional theory (TD-DFT); however, the shortcomings of TD-DFT to calculate the excited state properties of organic aromatic compounds have been well-reviewed.25,29,30 In naphthalene, the first and second excited singlet states, La and Lb,, named with respect to polarization along the short (a) and long (b) axes of symmetry, are nearly degenerate.31 In photoacids, 1-naphthol (1OH) and 2OH, substitution by OH removes the degeneracy; however, the electronic states maintain mixtures of both symmetries.25,32,33 While there have been valiant efforts to use correlated single reference methods to account for mixing,29 higher levels of theory are generally necessary to accurately reproduce the electronic landscape.25 Thus, as a cheap “first-pass” tool, TD-DFT remains the most accessible computational predictor of photoacidity. Experimentally, the Hammett model has been shown to be a promising predictor of photoacidity in substituted naphthol compounds.16,18 The Hammett equation was first formulated to describe


4

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the effect of functional groups on the ground state reactivity of benzoic acids using two empirical terms, the substituent and reaction constants, σ and ρ:34,35 𝐾𝑎

or alternatively,

log 𝐾0 = 𝜎𝜌.

(eqn. 1a)

𝑝𝐾𝑎 = ― 𝜎𝜌 + 𝑝𝐾0

(eqn. 1b)

where Ka and K0 correspond to the reactivity of the substituted and reference benzoic acid, respectively. Pines et al. extended the model with great success to describe the ground and excited state reactivity of photoacids, 5-substituted-1-naphthol (5-x-1OH) and 6-substituted-2-naphthol (6-x-2OH) compounds, using Hammett parameters σp.16,18 The ρ values increased by 2~2.5-fold in the excited state, indicating that the acidity of naphthol was much more sensitive to substituent effects upon excitation. Given the goal of these studies was to increase photoacidity using EWG, it is unclear whether the linear relation holds for EDG. Interestingly, Dawlaty et al. have recently shown that the Hammett model can be extended to 5-substituted-quinoline photobases, using a wide range of EWG and EDG.36 Given the high ease of the approach, the application of the Hammett model to a wider range of functional groups should be tested for photoacids. Aminonaphthols provide a unique opportunity to examine the effect of both a strong EWG (NH3+) and EDG (NH2) on photoacidity within the same chemical framework (Fig. 1). The small size and broad electronic range of aminonaphthols also make them ideal compounds to test the application of both computational and experimental predictors of photoacidity. Our group has recently reported on the photochemistry of isomers 5-amino-2-naphthol (5N2OH) and 8-amino-2naphthol (8N2OH); substitution at the distal α-C sites (C5/C8) enhanced the EWG/EDG-effect such that the protonation state of the amino group acted as an on-off switch for photoacidity.23 7amino-2-naphthol (7N2OH) differs in that the functional groups are placed symmetrically on the


5


Page 6 of 38

Figure 1. 7-substituted-2-naphthol compounds (7-x-2OH) of interest in this study. Bottom box: the four possible protonation states of 7-amino-2-naphthol (7N2OH) are shown: the cation (NH3+/OH), zwitterion (NH3+/O-), neutral (NH2/OH), and anion (NH2/O-). Top right box: the protonated and deprotonated states of 7-methoxy-2-naphthol (7OMe2OH) are shown. naphthalene framework with respect to the short-axis. It is unclear whether the electronic effects of the functional groups will thus be equally weighted. Previous spectral studies on the

naphthalene framework with respect to the short-axis. It is unclear whether the electronic effects of the functional groups will thus be equally weighted. Previous spectral studies on the photochemistry of aminonaphthol isomers differed on whether the protonated or deprotonated species (i.e. cation or neutral in Fig. 1) leads to zwitterion formation in the excited state.37-40 Our


6

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


time-correlated single-photon counting (TCSPC) emission measurements on 5N2OH and 8N2OH confirmed that the excited cation is the precursor to the zwitterion.23 7N2OH is still assumed to undergo zwitterion formation from the neutral state i.e. undergo dual excited state proton transfer (ESPT) at both functional sites, but there have been no dynamical studies to support this assignment. Here the effect of both EWG and EDG on the photoacidity of 2OH at the symmetric C7 position was investigated through TCSPC emission spectroscopy and TD-DFT and EOM-CCSD calculations of 7N2OH and 7-methoxy-2-naphthol (7OMe2OH) (Fig. 1). 7OMe2OH was an additional control for probing the effect of EDG on photoacidity. The ground and excited state proton transfer reactions of 7N2OH and 7OMe2OH were compared to those of substituted 7-x2OH compounds (x = CN, H, CH3, OH) in the literature6,14,41,42 using the Hammett equation. The possible chemical basis for the nonlinearity observed in the plots between the EWG and EDG regions was explored, with specific emphasis on possible mixing between the La and Lb states. METHODS Sample preparation and characterization 7N2OH hydrochloride (Matrix Scientific) and 7OMe2OH (ACROS Organics, 97%) were purchased and used without further purification. Stock solutions were prepared at pH < 7 in the dark and diluted to 100-140 µM in deionized water (Barnstead EASYpure RF). The pH was adjusted to pH = 1.0-12.5 using concentrated HCl or NaOH and a pH probe (Mettler Toledo SevenEasy), while highly acidic samples (pH < 1.0) were prepared using volumetric addition of concentrated HCl. For spectral measurements, 2-3 mL of the diluted samples were pipetted into 1cm quartz cuvettes (Starna Cell) with micromagnetic stir bars and equilibrated with air before being capped with Teflon seals. For time-resolved emission experiments of 7N2OH, ultrahigh


7


Page 8 of 38

purity argon (Purity Plus) was bubbled through the sample for 20 minutes prior to measurement. No major difference was observed between the air-equilibrated and argon samples. Thus, 7OMe2OH was prepared in air for the time-resolved emission measurements. UV/Vis absorption spectra (Agilent 8453) were recorded at room temperature over pH = 1.012.5 to determine ground state pKa and check for photodecomposition during the emission experiments. (See SI for more information on the spectrophotometric titration). Steady-state emission spectra (Hitachi F-2500 FL) were collected at 280 nm excitation over 300-550 nm and used without further spectral correction. TCSPC emission data were collected in single and multiwavelength mode on the Horiba DeltaFlex Modular Fluorescence Lifetime System using a pulsed 280-nm LED source (DeltaDiode DD-280, pulse width ~ 800 ps, 1 MHZ rep rate). Time-resolved emission spectra (TRES) were recorded in 25-nm intervals over 325-550 nm at either 10,000 peak count or fixed time interval scan mode. The instrument response (prompt) was collected using light scatter by a colloidal silica solution in water (LUDOX TMA, Sigma Aldrich), and an instrument diagnostic was run with 4-bis(5-phenyloxazol-2-yl)benzene (τ = 1.32 ns)43 before each experimental run. The measured TCSPC decays were globally fit to multiexponential functions in DAS6 analysis software (Horiba); the lifetimes and relative emission intensities were analyzed using kinetic models written in Matlab code based on analytical solutions to the two-state ESPT model44 presented in detail in the results and analysis section. Computational details All calculations were done using Gaussian 16.45 As a first pass to understanding photoacidity, TD-DFT calculations of the absorption and emission transitions of the different protonation states of 7N2OH and 7OMe2OH in water were calculated at the B3LYP/6-31++G(d, p) level of theory. B3LYP was chosen over CAM-B3LYP and WB97X-D as reducing the long-range functionals was


8

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown to improve the accuracy of the Lb state, albeit at the price of the La state for 2OH.25 (The Lb state was prioritized, as it should play a more prominent role in the emission pathway). The ground S0 and excited S1 geometries were optimized using DFT and TD-DFT, respectively, under relaxed solvent conditions, with the solvent water being modeled using the polarizable continuum model (PCM).46 In separate calculations, explicit water molecules were added near the OH and functional group moieties and optimized using B3LYP/6-31++G(d,p) and PCM to better model the solvent water environment. Finally, given the limitations of TD-DFT, to compare the energy spacing between the La and Lb states of 2OH and 7N2OH neutral, the multireference-state computational strategy used by Acharya et al. in their study of 1OH and 2OH was employed.25 Acharya et al. showed that equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory accurately predicted the ordering and spacing of the La and Lb states.25 Thus, here the ground state of 7N2OH neutral was optimized at the MP2/aug-cc-PVTZ level of theory in water (PCM), and the excited singlet state energies were determined at the EOM-CCSD/aug-cc-PVDZ level of theory in water (PCM). RESULTS AND ANALYSIS The experimental and calculated spectral peaks (nm) of 7N2OH and 7OMe2OH are summarized in Table 1. The compounds are listed from left to right in the order of the most EWG to EDG, according to Hammett para parameters σp,35 while the absorption and emission data are organized vertically as acid-base pairs in the excited state with respect to OH. The kinetic parameters of ESPT, including pKa*, of 7N2OH and 7OMe2OH are provided in Table 2. Additional tables comparing the experimental and calculated absorption and emission peaks and ESPT kinetic parameters to previously studied 7-substituted-2OH compounds are provided in the SI. Details of


9


Page 10 of 38

the assignment, kinetic modeling, and computational results are provided below in their respective subsections.

Table 1. S1-S0 absorption and emission transitions (nm) of the protonated and deprotonated acidbase pairs: 7N2OH cation-zwitterion (C-Z), 2OH, 7OMe2OH, and 7N2OH neutral-anion (N-A). The TD-DFT PCM values are shown in parentheses. The experimental emission peak for neutral 7N2OH is shown in bold. 7N2OH C-Z

2OH

7OMe2OH

7N2OH N-A

Abs. prot.

328 (296)

328 (303)

325 (295)

332 (323)

Abs. deprot.

n/a (332)

346 (344)

337 (328)

342 (328)

Em. prot.

357 (350)

351 (346)

343 (345)

398 (413)

Em. deprot.

425 (413)

414 (421)

413 (418)

407 (409)

Table 2. Kinetic parameters for ESPT obtained from fitting the TCSPC emission data of 7N2OH cation-zwitterion (C-Z), 7N2OH neutral-anion (N-A), and 7OMe2OH. Literature values of the kinetic parameters for 2OH have been provided for comparison. Rate constant

7N2OH C-Z

2OHa

7OMe2OH

7N2OH N-A

kHA (s-1)

1.5(0.1) × 108

1.38 × 108

1.2(0.1) × 108

1.3 × 108

kA- (s-1)

7.4(0.1) × 107

1.06 × 108

2.5(0.2) × 108

4.4 × 108

kr (M-1 s-1)

1.9(0.6) × 1010

kd (s-1)

1.4(0.1) × 109

pKa*

1.1 ± 0.2

4.70 × 1010

4.3(0.2) × 1010

n/a

7.0 × 107

8.2(0.8) × 107

n/a

2.8

2.7 ± 0.1

~9.6 ± 0.2

*aRef

[41]


10

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spectral and kinetic measurements of 7N2OH The UV/Vis absorption spectra of the cation, neutral, and anion of 7N2OH are shown in Fig. 2 (top). In comparison to the isomer 8N2OH,23 the spectral features are similar; however, at higher pH, the S1 peak of the 7N2OH anion is blue-shifted 10 nm from the corresponding 8N2OH species. The ground state equilibria between the cation-neutral (NH3+/NH2) and neutral-anion (OH/O-) were determined spectrophotometrically to be pKa1 = 4.4 ± 0.2 and pKa2 = 9.6 ± 0.2, respectively (see SI), which are consistent to those reported for 8N2OH (pKa1 = 4.2 ± 0.2; pKa2 = 9.5 ± 0.2).23 The similarity of these values to the pKa of the parent compounds, 2-naphthylammonium (pKa = 3.9) and 2OH (pKa = 9.5),41,47 suggests that the ground state acidities of the two distal functional groups are independent of each other. The steady-state emission spectra of 7N2OH at select pH are shown in Fig. 2 (bottom). In spectra collected at low pH (pH < pKa1 = 4.4), with only the cation present in the ground state, an isoemissive point was observed between emission bands at 357 nm and 425 nm. Increasing the pH above 4.4 resulted in diminishing of the 357-nm emission band and blue-shifting of the 425-nm band to a new peak at 398 nm. At higher pH (pH > pKa2 = 9.6), the 398-nm band began to decrease and red-shift to 407 nm. The 357-nm and 407-nm emission bands were assigned to the excited cation and anion, respectively. Assignment of the 425-nm and 398-nm bands was more complex. For 8N2OH, the peak emission of the zwitterion and neutral species were observed at 422 nm and 445 nm respectively;23 for 7N2OH, it was not clear whether the neutral emission would red-shift from the zwitterion peak or whether the zwitterion would similarly emit at 425 nm. Kinetic experiments, as described below, were conducted to ultimately assign the neutral and zwitterion excited states to the 398-nm and 425-nm bands, respectively.


11


Page 12 of 38

Figure 2. Top: UV/Vis absorption spectra of the different protonation states of 7N2OH at 100 µM in water: cation (pH = 2.2, red), neutral (pH = 7.1, green), and anion (pH = 11.2, black). pKa1(NH3+/NH2) and pKa2(OH/O-) were determined to 4.4 ± 0.2 and 9.6 ± 0.2, respectively. Bottom: Steady-state emission spectra of the different protonation states of 7N2OH: predominant cation and zwitterion (pH = 0.5, red), cation and predominant zwitterion (pH = 3.0, blue), neutral (pH = 7.4, green), and anion (pH = 12.1, black).


12

Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The simplest photochemical scheme for monoprotic photoacids is the two-state ESPT (Scheme 1).7,8 The proton quenching pathways in 2OH are reported to be minor, in contrast to systems such as 1OH.41 Thus excluding the proton quenching pathways, the concentrations of the protonated and deprotonated species at different pH can be described analytically as:44 𝑡

[𝐻𝐴 ] = 𝛼1𝑒

―𝜏

[𝐴 ― ∗ ] = 𝛽1𝑒

―𝜏

∗

1

𝑡

+ 𝛼2𝑒

𝑡 1

―𝜏

𝑡

+ 𝛽2𝑒

―𝜏

(eqn. 3)

2

1

1

(eqn. 2)

2

1

𝜏1 ―1,𝜏2 ―1 = [2{(𝑀 + 𝑁)2 ± [(𝑁 ― 𝑀)2 + 4𝑘𝑑𝑘𝑟[𝐻 + ]]2}]

(eqn. 4)

with M = 𝑘𝐻𝐴 + 𝑘𝑑, 𝑁 = 𝑘𝐴 ― + 𝑘𝑟[𝐻 + ] where τ1, τ2 are the lifetimes; α, β are the relative abundances of the protonated and deprotonated species; kHA, kA- correspond to the rates of nonradiative and radiative relaxation of the protonated (HA) and deprotonated (A-) species, respectively; and kd, kr are the rates of excited state proton dissociation and recombination, respectively. Note that this model does not include photo-induced electron transfer (PET) pathways. While there have been reports of radical formation in naphthol

HA

hv

kd

*

kr

A-* + H+

kHA kQ[H+] hv

kA- kQ[H+]

A - + H+

HA protonated

deprotonated

Scheme 1. Schematic of the two-state ESPT model between protonated (HA*) and deprotonated (A-*) acid. Proton quenching of HA* and A-* (dashed arrows) can also play a role in the kinetics.


13


Page 14 of 38

water-cluster studies,48 it is unclear whether PET is competitive with ESPT in bulk water. Within the limitations of the measurements, there was no sufficient evidence to include PET pathways in the kinetic model. For 7N2OH, the protonated species HA in Scheme 1 can be viewed as either the cation, with NH3+ as an EWG on 2OH, or the neutral, with NH2 as an EDG on 2OH. pH will dictate which 7N2OH species are involved in the photochemical scheme. Select single wavelength emission decays of 7N2OH from TRES collected at various pH are shown in Fig 3. At pH = 6.8, with only the neutral species present in the ground state, a monoexponential decay (τ =7.6 ns) was observed at 400-nm. At higher pH (pH = 6.8-12.1), with both the neutral and anion species present in the ground state, the 400-nm signal became biexponential (Fig. 3a). The lifetimes, however, remained constant at τ ~ 7.6 ns and τ ~ 2.3 ns, with the relative contributions corresponding to the neutralanion population in the ground state. At pH = 12.6, with only the anion present, the signal became monoexponential again, with lifetime τ = 2.3 ns. Thus, the two lifetime decays were assigned to the neutral and anion, respectively, with the neutral emission being attributed to the 398-nm band. The relatively constant lifetimes suggested that there was no enhanced acidity between the neutral and anion in the excited state, i.e. pKa* ~ pKa = 9.6. There was no evidence of a third species in the excited state in the pH region. At low pH = 1.4 (Fig. 3b), with only the cation present in the ground state, a biexponential decay was observed at both 350 nm and 425 nm. The fast and slow decay at 350 nm (τ1 = 350 ps and τ2 = 9.3 ns) was assigned to the cation, while the concomitant fast rise and slow decay at 425 nm was assigned to the zwitterion. The amplitudes of the zwitterion rise (i.e. the formation) and decay were


14

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nearly equal but opposite in magnitude, indicative of a two-state ESPT model. The lifetimes and amplitudes indeed changed over pH = 0.4-4.1. Lifetimes, τ1 and τ2, were collected and fit using

Figure 3. TCSPC emission data for 7N2OH. The gray markers and line correspond to the instrument prompt, while the colored markers and line correspond to the following pH sample data and fit. (a) Normalized emission decays at 400 nm corresponding to the neutral and anion: green: pH 6.8, τ = 7.60 ns; purple: pH 9.3, τ = 7.6 ns (77%) and τ = 2.3 ns (23%); brown: pH 10.5, τ = 7.6 ns (17%) and τ = 2.3 ns (83%); and black: pH 12.6, τ = 2.3 ns. (b) Normalized emission decays


15


Page 16 of 38

at 350 nm (red) and 425 nm (blue) corresponding to the cation and zwitterion, respectively, at pH = 1.4. Both decays were fit to the biexponential functions with τ = 350 ps and τ = 9.3 ns. equation 4 (see SI), and the resultant kinetic parameters {kHA, kA-, kd, kr} are summarized in Table 2. Uncertainties in the values primarily arose from limitation of the instrument time resolution. Using kd and kr, pKa* between the excited cation and zwitterion state of 7N2OH was determined to be 1.1 ± 0.2. This is similar to the value reported for the 8N2OH cation (pKa* = 1.2).23 Thus, despite their spectral differences, the cation is the precursor to the zwitterion in the excited state for both isomers. The photochemistry of the different protonation states of 7N2OH has been summarized in Scheme 2.

Scheme 2. Photochemical scheme of the different protonation states of 7N2OH.

Spectral and kinetic measurements of 7OMe2OH The UV/Vis absorption and steady-state emission spectra of the protonated and deprotonated 7OMe2OH are shown in Fig. 4. The absorption spectra were similar to those observed for the neutral and anion of 7N2OH, and the pKa was determined spectrophotochemically to be 9.55 ±


16

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10. The peak emission of deprotonated 7OMe2OH at 413-nm matched the anion of 7N2OH; however, the peak emission of protonated 7OMe2OH at 343-nm resembled that of the cation, and not the neutral state, of 7N2OH. Thus, the photoacidity of 7OMe2OH diverges from the 7N2OH neutral-anion picture. TRES were collected from pH = 1.5-10.0 and analyzed using the two-state ESPT model to determine pKa*. Here, the protonated HA species can be viewed as protonated 7OMe2OH, with OCH3 as an EDG on 2OH. Select biexponential decays at 350 nm and 425 nm, corresponding to the protonated and deprotonated species, at pH = 2.7 are shown in Fig. 5. The 350-nm signal decays at τ = 2.8 ns and τ = 6.1 ns, while the 425-nm signal rises (τ = 2.8 ns) and decays (τ = 6.1 ns) concomitantly. Given the proximity of the observed lifetimes, global fitting of the TRES was critical, as the rise of the deprotonated species was distinctly fit from the decay. At very high pH (pH = 11), where only the deprotonated species is present in the ground state, the TRES data were fit to a monoexponential decay (τ = 4.2 ns). Full analysis of the lifetimes at different pH can be found in the SI, and a summary of the kinetic parameters is shown in Table 2. A comparison of the rates of proton dissociation (kd) of 7OMe2OH and analogous photoacids (see SI) showed that the dissociation rate was similar to that of 2OH; likewise, the pKa* of 7OMe2OH at 2.7 ± 0.1 is identical to the photoacidity of 2OH rather than 7N2OH neutral. The surprising photoacidity of 7OMe2OH will be addressed in the discussion.


17


Page 18 of 38

Figure 4. (a) UV/Vis absorption of the protonated (green) and deprotonated (black) states of 7OMe2OH in water: neutral (pH = 6.1) and anion (pH = 11.7). (b) Steady-state emission spectra of predominantly neutral (pH = 1.5) and anion (pH 11.6) states of 7OMe2OH. Note that the neutral


18

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


emission spectrum had to be collected at much lower pH than the UV/Vis absorption spectrum, suggesting photoacidity of the species.

Figure 5. Select single wavelength emission decays at 350 nm (protonated) and 425 nm (deprotonated) from TRES collected at pH = 2.7 for 7OMe2OH. The gray markers and line correspond to the instrument prompt, while the colored markers and black lines correspond to the emission data and biexponential fit (τ1 = 2.8 ns and τ2 = 6.1 ns), respectively. Computational results DFT and TD-DFT calculations were performed on the different protonation states of 7N2OH and analogous 7-substituted-2OH compounds. While the limitations of TD-DFT to describe the exact energy and ordering of excited aromatics are known;25,29,30 here the method was examined as a cheap “first-pass” approach to predicting the effect of EDG and EWG on the absorption and emission properties of the stated compounds. The calculated S0- S1 absorption and emission transitions of the relaxed species in water are summarized in Table 1, along with the observed


19


Page 20 of 38

values (see SI for additional reference compounds). In general, there was good agreement between the experimental and calculated values, especially for the emission. TD-DFT-PCM, however, was insufficient for assignment of the zwitterion and neutral states of 7N2OH, as emissions of both species were computed to be at 413 nm, the halfway point between the experimentally observed transitions.

Figure 6. Addition of explicit water molecules to B3LYP/6-31++G(d,p)-PCM models of neutral 7N2OH. The oxygen of the first water molecule is hydrogen-bonded to the hydrogen of the hydroxyl group. In (a), the hydrogen of the second water molecule is hydrogen-bonded to the nitrogen of the amino group, while in (b) the oxygen of the second water molecule is hydrogenbonded to the hydrogen of the amine group.

To probe possible direct interactions between the solvent water and functional groups, one or more explicit water molecules were added to select TD-DFT-PCM calculations. As a benchmark, a single water molecule was added to 2OH such that the oxygen atom of water was hydrogen-


20

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bonded to the hydroxyl group of 2OH (Table 3). The calculated emission (354 nm) was slightly red-shifted from the PCM-calculated value (346 nm) and remained in excellent agreement with the experimental data (351 nm). Addition of water molecules near both the hydroxyl and ammonium groups of the 7N2OH cation also only slightly red-shifted the calculated emission (357 nm vs. 350 nm), closer to the experimental value (357 nm). For the 7N2OH neutral and zwitterion, TD-DFT-PCM computations had resulted in identical emission transitions at 413 nm, in contrast to the expt. values at 398 nm and 425 nm, respectively. When a single water molecule was added near the ammonium group of the 7N2OH zwitterion, no change was observed in the calculated emission. Similarly, when a single water molecule was added near the hydroxyl group of 7N2OH neutral, no change was observed. If two water molecules, however, were added near the hydroxyl and amino groups of the 7N2OH neutral, respectively, the calculated emission changed. Two stable configurations of the water molecule near the amino group were found (Fig. 6); the water molecule was either (a) hydrogen-bonded to the nitrogen atom of NH2 or (b) hydrogen-bonded to the hydrogen atom of NH2. In the first configuration, the calculated emission is blue-shifted to 377 nm; in the latter, the calculated emission is red-shifted to 421 nm. Thus, the energy landscape of 7N2OH is very sensitive to the presence of the second water molecule near the amino group. Given that the neutral is observed to emit at 398 nm, the blue-shift observed in configuration (a) is in qualitative agreement with the data and aligns with the physical picture of a basic amino group. Application of solvent fields other than PCM could improve the calculations and therein shed light on the nature of the solvation. This was, however, beyond the scope of this current study. Given the peculiar nature of the 7N2OH neutral, the excited singlet states were also examined using EOM-CCSD-PCM. The first and second excited states were calculated to be 4.11 eV (302


21


Page 22 of 38

nm) and 4.70 eV (264 nm), respectively (Table 4). For 2OH, EOM-CCSD-PCM was reported to overestimate the excitation energy by ~0.5 eV due to the lack of both triple excitations in the theory and absence of explicit solute-solvent interactions.25 The difference between the experimental and calculated Lb state of 7N2OH neutral (3.73 eV vs. 4.11 eV) is within this error. More importantly, EOM-CCSD has been shown to accurately predict the relative spacing between the two low-lying electronic states for 1OH and 2OH: 0.39 eV and 0.75 eV, respectively.25 These calculations are in agreement with spectral analysis of the two naphthol isomers, which show stronger mixing between the La and Lb states in both the absorption and emission spectra of 1OH.1 Here, the difference between the Lb and La states of 7N2OH neutral is almost at the halfway point, 0.60 eV. Thus, there is potential for substantial mixing between the La and Lb states of 7N2OH neutral compared to 2OH.

Table 3. Comparison of experimental and calculated B3LYP/6-31++G(d,p)-PCM emission transitions (nm) with and without explicit water molecules for 2OH and 7N2OH species. The two emission values for neutral 7N2OH correspond to the different orientation of the explicit water molecules as described in the text. Gas phase values were also calculated using B3LYP/631++G(d,p). 2OH

7N2OH cation

7N2OH zwitterion

7N2OH neutral

Expt. water

351

357

425

398

PCM

346

350

413

413

PCM + 1 H2O

354

411

412

PCM + 2 H2O Calc. gas

357 335

377, 421 357


22

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



23


Page 24 of 38

Table 4. EOM-CCSD/aug-cc-pVDZ-PCM excitation energies of the Lb and La states of 1OH, 2OH, and 7N2OH neutral. 1OHa

7N2OH neutral

2OHa

Lb

4.30 (288 nm)

4.11 (302 nm)

4.28 (290 nm)

La

4.69 (264 nm)

4.71 (263 nm)

5.03 (246 nm)

La-Lb

0.39 eV

0.60 eV

0.75 eV

*aRef[25]

DISCUSSION 7N2OH photochemistry and the peculiar neutral state Given the spectral reassignment of 7N2OH in the excited state, this work supports formation of the zwitterion from the excited cation, and not neutral state, of 7N2OH. This is parallel to the mechanism reported for 8N2OH despite the two isomers’ divergent emission behavior: while the zwitterion for both compounds emits at 425 nm, the neutral emissions are blue- and red-shifted from the zwitterion for 7N2OH and 8N2OH, respectively. A thermodynamic square of the zwitterion formation from the excited neutral state is shown in Scheme 3. Energetically, the concerted path, i.e. the exchange of protons between the hydroxyl and amino sites via a water network, should be favored for both compounds (more favorable for 7N2OH, see SI for discussion). In practice, however, given the lack of zwitterion formation under neutral conditions, the reaction must be kinetically hindered for both compounds, with either the electronic interaction between the two sites slowing the breaking and forming of the OH and NH3+ bonds in the excited state and/or the organization of the water network, i.e. the purported water wire,2 being too costly for the reaction to occur between the two sites.


24

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 3. Diagram of the thermodynamic square of the zwitterion formation from the excited neutral state of 7N2OH. The formation can occur in a stepwise mechanism such that the proton is transferred independently at each protonation site (black and gray) or in a concerted mechanism such that the proton exchange is coupled between the two sites (dashed). The change in free energy of the reaction is represented by the pKa* of the reaction.

The extent of water reorganization or entropic change upon excitation of 7N2OH and 8N2OH can be inferred from the Förster cycle:8 1

𝑝𝐾𝑎∗ = 𝑝𝐾𝑎 + 2.303𝑘𝐵𝑇(ℎ𝑣𝐴 ― ― ℎ𝑣𝐻𝐴)

(eqn 5)

where energies of the protonated and deprotonated states, hvHA and hvA-, are calculated as the average of the measured peak absorption and emission values. This approach has been used to determine pKa* of many photoacids and photobases. For example, applying eqn. 5 to 2OH, the pKa* was calculated to be 2.9, comparable to the kinetically-modelled value, pKa* = 2.8.41 While the Förster analysis cannot be applied to the zwitterion formation directly, it can be used to study the stepwise neutral-cation (or cation-neutral) equilibrium. While the water network necessary for the stepwise and concerted PT mechanism may differ, there should be similarities in the organization of water about the neutral species. Using equation 5, the calculated pKa*(cation-


25


Page 26 of 38

neutral) for 7N2OH and 8N2OH were 0.6 and -3.0, respectively. These low values are in direct conflict with the experimental results as the excited cation would have preferentially dissociated to the neutral and not zwitterion species. The derivation of the Förster equation assumes that the change in entropy in the ground and excited state are the same (∆S = ∆S*). To increase pKa* or to compensate for the experimental and calculated discrepancy, ∆S* must decrease considerably, i.e. water molecules must become more ordered around the excited neutral state (and concomitant hydronium ion) upon proton dissociation of the excited cation. This physical picture is supported by the computational models, which showed that the addition of explicit water, specifically to the excited neutral state calculations, is critical for reproducing experimental data (Fig. 6 and Table 3). pKa*(cation-neutral) describes the photoacidity at the amino site. To reproduce the neutral emission spectra, an explicit water molecule had to be oriented in the calculations such that it could potentially donate a proton to the nitrogen atom of the amino group. Both the neutral species of 7N2OH and 8N2OH have abnormally long emission lifetimes compared to the other protonation states: ~7.6 ns and ~20.1 ns, respectively. These extended lifetimes could be further indications of the high degree of water organization required to stabilize the charge distribution in the excited state. Ultrafast measurements of the solvent reorganization in these systems, accompanied by molecular dynamic modeling, could shed more light on these events. The longer lifetimes could also be due to the peculiar electronic properties of the neutral state, which will be discussed in the next section. The enhanced stabilization of the neutral state by water contributes to the peculiar photoacidity of 7N2OH. In the literature, there has been much discussion on whether the pKa* strength is due to charge transfer of the photoacid or conjugate base.2,3,5,20 Here the neutral state plays the dual role of potential conjugate base to the cation with respect to the NH3+ group and potential photoacid


26

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the anion with respect to the OH group. As a potential conjugate base, the neutral “loses” to the zwitterion as the cation preferentially dissociates to the zwitterion. While the two functional groups interact to tune their respective photoacidic properties, the Förster analysis showed that the entropic cost of stabilizing the conjugate base, rather than the enthalpic cost of breaking the respective bonds, was the determining factor in the reaction. As a potential photoacid, the neutral again “loses,” as there is little to no enhanced proton dissociation at the OH site upon excitation. In the spectral analysis of 7OMe2OH, the absorption features of the protonated and deprotonated species were observed to be analogous to the 7N2OH neutral and anion; however, the spectral symmetry was broken in the emission spectra. A comparison of all the 7-substituted-2OH photoacid pairs (see SI) shows that with the exception of 7CN2OH, the compounds are remarkably similar in their spectral features despite the broad range of pKa* values (1.1 ~ 9.6): the protonated species absorb at 325-332 nm while the deprotonated species absorb at 337-346 nm and emit at peak 407-425 nm. The only discrepancy lies in the emission of the 7N2OH neutral state, which is redshifted ~50 nm from the other emissions at 343-357 nm. A closer examination of the absorption and emission peaks of the neutral shows a significant Stokes shift (~66 nm), indicating that much of the charge redistribution occurs after excitation but before proton dissociation. Thus, the “overstabilization” of the neutral by the surrounding water limits its reactivity as a photoacid. The different roles that the neutral (and solvent) play in limiting its photoactivity underlies the difficulty of finding a common mechanism to explain photoacidity. The photochemistry of 7N2OH highlights the remarkable tunability of 2OH by the presence of a single functional group. The contrasting photoacidity between the neutral and cation of 7N2OH is shown in Scheme 2. Compared to 2OH (pKa* = 2.8), with NH3+ at the C7 position (i.e. the cation), the photoacidity is enhanced by nearly two orders of magnitude (pKa* = 1.1); with NH2 at


27


Page 28 of 38

the C7 position (i.e. the neutral), the photoacidity is decreased or shut off (pKa* ~ pKa = 9.6). The difference in protonation state leads to over an eight-order of magnitude change in acidity. These new assignments for 7N2OH led us to examine more closely the electronic effect of substituents on the ground and excited state acidity. Effect of substituents on photoacidity in 7-substituted-2OH naphthol derivatives Motivated by previous analyses of photoacids and photobases using the Hammett model,16,18,36 the ground and excited state pKa of various 7-substituted-2OH naphthol derivatives were plotted against the para-Hammett parameters (Fig. 7; see SI for table and additional information on pKa* determined using the Förster cycle). With the exception of substituent x = NH2, the general shape of the ground and excited state plots can be divided into two regions: (i) a sloped region (ρ >1) for EWG (σp > 0, gray region in Fig. 7); (ii) a flat linear region (ρ ~ 0) for EDG (σp < 0, white region in Fig. 7). Region (i) represents photoacids that have been the most targeted, primarily due to their enhancement in acidity. Using the σp ≥ 0 values, the ground and excited state ρ values were determined from the Hammett plots to be 1.14 and 3.86, respectively. While ideally there would be more data points in this region, the ~3.4-fold increase in ρ is in agreement with the ~2.5 and ~5-fold increase in ρ reported by Pines et al. and Dawlaty et al upon excitation of 6-x-2OH and 5substituted quinoline.16,36 Furthermore, work by Tolbert et al. on 6- and 7-CN2OH showed that substitution at the adjacent C6 and C7 positions result in similar electron densities upon excitation.14,15,21 Thus, the Hammett plots for 6- and 7-substituted-2OH should be similar in the excited state. Our reported values (ρ = 1.14, ρ* = 3.86) are indeed comparable to the reported reaction constants for 6-substituted-2OH (ρ = 1.71, ρ* = 4.37), which focused primarily on EWG.16


28

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Hammett plots of the ground (left) and excited (right) state pKa of 7-substituted-2OH compounds vs. σp. Note the range of pKa values is significantly larger for the excited state plot. Regions (i, gray) and (ii, white) represent compounds with EWG and EDG, respectively. The solid lines correspond to fits pKa = -1.14 σp + 9.50 (x = CN and H) and pKa* = -3.86 σp + 2.85 (x = CN, NH3+, H; R2 = 0.87), while the dashed lines correspond to pKa = 9.55 and pKa* = 2.8, i.e. ρ = ρ* = 0. The blue and red colors denote compounds with substituents, CH3 and NH2, respectively.

Region (ii) is more complex. For OH- and OCH3-substituted 2OH, the Hammett plots of the ground and excited state acidity are similar in shape, indicating that the excited state trend can be interpreted as an amplification of the ground state thermodynamic driving force. While ρ (=0) does not increase by three-fold in the excited state, the excited state pKa* does decrease by three-fold on the logscale compared to the ground state pKa (2.8 vs. 9.6). Generally, a break in the linearity or change in ρ indicates a change in mechanism or establishment of a new rate-determining step for a multi-step mechanism.34,49 It is not immediately clear what the change in mechanism is for these acidic species. In both the ground and excited state Hammett plots, the linearity of region (i)


29


Page 30 of 38

could be argued to extend to x = CH3 (σp = -0.17, shown in blue in Fig. 7). The few EDG included in the earlier Hammett studies of 5-substituted-1OH and 6-substituted-2OH consisted of alkyl groups, x = CH3 and C(CH3)3.16,18 In our plot, the linearity is noticeably broken at x = OCH3 (σp = -0.27), which is similar to the electron-donating capability of OH (σp = -0.37). Thus, it may be enticing to assign the breaking of the linearity to a “tug-of-war” effect between the OH and substituent; if the substituent cannot push or pull electron density away from OH, the acidity will remain unchanged. This explanation, however, underplays the role of the naphthalene scaffold in determining acidity. In benzoic acid, addition of the carboxyl group COOH at the para-position was observed to affect reactivity (σp=0.45), while its conjugate base COO- led to no change (σp=0.00).35 Thus, simple symmetry of the substituent and acidic site COOH does not negate reactivity for benzoic acid. The inherent differences between the naphthalene and benzene scaffold must lead to different electronic effects in the EDG region. Spectral analysis of substituted naphthol compounds provide further insight into the electronic effects leading to the breakdown of the Hammett equation. In the configuration analysis of naphthalenediols with C2v symmetry, Fuji and Suzuki examined the distinct UV/Vis spectra of 2,3diol and 2,7-diol (i.e. 7OH2OH) in isooctane.50 For both diols, the lowest excited state is the Lb state; the spectral features, however, vary due to different mixing mechanism between the Lb and Bb bands. The group found that the electronic states of 2,3-diol could be expanded in terms of the electronic states of 2OH (total weight of the coefficients ~ 90%), but the expansion of 7OH2OH resulted in poor fits (total weight ~ 60-70%).50 Comparison of the absorption spectra of 2OH and 7OMe2OH collected in water shows that indeed the two spectra have very different features (see SI). If 7OMe2OH cannot be described within the 2OH framework, it is not surprising that a linear relation fails to exist between their energy states. (It is instead more curious why the two


30

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compounds share the same ground and excited state acidity). In sharp contrast, replacing OCH3 with NH3+ results in an absorption spectrum with features very similar to 2OH (see SI). Thus, if an EWG is added to the distal site on the naphthalene scaffold, the electronic states of the substituted 2OH must still be able to be expanded in terms of the 2OH basis functions; if an EDG is added, the 2OH-basis functions must become insufficient. The divergence in the Hammett plot is due to the inability of the energy states of the substituted compounds to be expressed linearly with respect to the 2OH terms. The root of the problem must lie in the different mixing interactions between the Lb band and other electronic transitions. The neutral state of 7N2OH (x = NH2) is an outlier in region (ii). The high entropic cost of the solvation and relaxation of the excited neutral state of 7N2OH by water has already been discussed. Electronically, EOM-CCSD calculations showed that there was significant lowering of the energy difference between the La and Lb states upon substitution of 2OH by an amine group (Table 4). The electronic gap approaches that of 1OH, which has been spectroscopically and theoretically shown to undergo significant mixing between the La and Lb states in the excited state.1,25 The large impact of the amine group on the electronic landscape is also consistent with previous spectral work; in the configurational analysis of 2OH and 2-naphthylamine in isooctane by Suzuki et al., amino-substitution was shown to have a greater effect on the naphthalene electronic structure than 2OH due to larger contributions from charge transfer states in the amine.33 A close inspection of the spectrum of neutral 7N2OH in water shows that while it resembles the spectrum of 7OMe2OH (see SI), there is broadening in the 340-nm region such that the band resembles the convolution of the 2OH and 2-naphthylamine Lb bands. Thus, to further complicate the electronic picture, the 7N2OH neutral can be electronically interpreted in two photoacidic frameworks: 2OH and 2-naphthylamine. Advanced (multireference) calculations of 2-


31


Page 32 of 38

naphthylamine and other 7-substituted-2OH compounds would be of strong interest for comparison. Inclusion of refined solvent models will be key, as water may play a significant role in coupling the various electronic states. CONCLUSIONS The photochemical studies of 7N2OH showed that the photoacidity of the hydroxyl group with respect to 2OH is extremely sensitive to the presence of an EWG (NH3+) vs. EDG (NH2): for the excited cation (NH3+), the photoacidity is enhanced (pKa* = 1.1) such that it forms the zwitterion; for the excited neutral (NH2), the photoacidity is suppressed (pKa* ~ pKa = 9.6), as thermodynamic and computational calculations showed that the excited neutral state is substantially stabilized by water. In contrast, investigations of 7OMe2OH showed that the presence of EDG (OCH3) did not affect the photoacidity (pKa* = 2.7). Hammett plots of the ground and excited pKa of 7-substituted2OH compounds vs. σp revealed nonlinearity between the EDG and EWG regions that was tentatively attributed to mixing between the La and Lb states. EOM-CCSD calculations of the 7N2OH neutral supported narrowing of the energy gap between the two states. In their study of photobasicity in 5-substituted quinoline, Dawlaty et al concluded that the relative interaction of the substituent with the lowest La state was the determining factor for pKa*.36 Here, for 7-substituted-2OH, the driving factor for photoacidity is the interaction of the substituent with the lowest Lb state. Thus, the rational design of photoacid-photobase switches based on the naphthalene-scaffold will require better theoretical and experimental understanding of the mixing of the La and Lb states upon substitution. In practice, the knowledge will also have to be translated to easily accessible predictors of photoacidity/photobasicity. Extension of the compounds to new media, e.g. ionic liquids, supramolecular assemblies, etc., which may further tune the photoacidic


32

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and photobasic properties of these functionalized compounds, will introduce further experimental and theoretical challenges.

ASSOCIATED CONTENT Supporting Information. Additional information on the spectrophotometric titration, kinetic modeling of 7N2OH and 7OMe2OH, and tables comparing compounds in this study to 7-substituted-2OH compounds in the literature are provided in the SI. A thermodynamic explanation of the concerted ESPT path and the thermodynamic-derived pKa*/Hammett plot are also included. (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to acknowledge Bowdoin College for support of this work. P. J. Brown was funded by the Maine Space Grant Consortium (SG-18-17). Bowdoin College is an affiliate of


33


Page 34 of 38

the Maine Space Grant Consortium; any findings and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration or of the Maine Space Grant Consortium.


34

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Tolbert, L. M.; Solntsev, K. M. Excited-State Proton Transfer: From Constrained Systems to “Super” Photoacids to Superfast Proton Transfer. Acct. Chem. Res. 2002, 35, 19-27. (2) Agmon, N. Elementary Steps in Excited-State Proton Transfer. J. Phys. Chem. A 2005, 109, 13-35. (3) Spry, D. B.; Fayer, M. D. Charge redistribution and photoacidity: Neutral versus cationic photoacids. J. Chem. Phys. 2008, 128, 084508. (4) Mohammed, O. F.; Pines, D.; Dreyer, J.; Pines, E.; Nibbering, E. T. J. Sequential Proton Transfer Through Water Bridges in Acid-Base Reactions. Science 2005, 310, 83-86. (5) Tran-Thi, T. H.; Prayer, C.; Millié, P.; Uznanski, P.; Hynes, J. T. Substituent and Solvent Effects on the Nature of the Transitions of Pyrenol and Pyranine. Identification of an Intermediate in the Excited-State Proton-Transfer Reaction. J. Phys. Chem. A 2002, 106, 22442255. (6) Rosenberg, J. L.; Brinn, I. Excited state dissociation rate constants in naphthols. J. Phys. Chem. 1972, 76, 3558-3562. (7) Weller, A. Fast reactions of excited molecules. Prof. React. Kinet. 1961, 1, 187. (8) Forster, T. Z. Elektro. Diss. ang. Molek. Elektrochem. 1950, 54, 42-46. (9) Pines, E.; Huppert, D.; Agmon, N. Geminate recombination in excited‐state proton‐transfer reactions: Numerical solution of the Debye–Smoluchowski equation with backreaction and comparison with experimental results. J. Chem. Phys. 1988, 88, 5620-5630. (10) Das, A.; Banerjee, T.; Hanson, K. Protonation of silylenol ether via excited state proton transfer catalysis. Chem. Comm. 2016, 52, 1350-1353. (11) Liao, Y. Design and Applications of Metastable-State Photoacids. Acct. Chem. Res. 2017, 50, 1956-1964. (12) Shi, Z.; Peng, P.; Strohecker, D.; Liao, Y. Long-Lived Photoacid Based upon a Photochromic Reaction. J. Amer. Chem. Soc. 2011, 133, 14699-14703. (13) Li, Y.; Feng, X.; Wang, A.; Yang, Y.; Fei, J.; Sun, B.; Jia, Y.; Li, J. Supramolecularly Assembled Nanocomposites as Biomimetic Chloroplasts for Enhancement of Photophosphorylation. Angew. Chem. Int. Ed. 2019, 58, 796-800. (14) Huppert, D.; Tolbert, L. M.; Linares-Samaniego, S. Ultrafast Excited-State Proton Transfer from Cyano-Substituted 2-Naphthols. J. Phys. Chem. A 1997, 101, 4602-4605. (15) Tolbert, L. M.; Haubrich, J. E. Enhanced photoacidities of cyanonaphthols. J. Am. Chem. Soc. 1990, 112, 8163-8165. (16) Ditkovich, J.; Mukra, T.; Pines, D.; Huppert, D.; Pines, E. Bifunctional Photoacids: Remote Protonation Affecting Chemical Reactivity. J. Phys. Chem. B 2015, 119, 2690-2701. (17) Solntsev, K. M.; Huppert, D.; Agmon, N. Solvatochromism of β-Naphthol. J. Phys. Chem. A 1998, 102, 9599-9606. (18) 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. (19) Spies, C.; Shomer, S.; Finkler, B.; Pines, D.; Pines, E.; Jung, G.; Huppert, D. Solvent dependence of excited-state proton transfer from pyranine-derived photoacids. Phys. Chem. Chem. Phys. 2014, 16, 9104-9114.


35


Page 36 of 38

(20) Silverman, L. N.; Spry, D. B.; Boxer, S. G.; Fayer, M. D. Charge Transfer in Photoacids Observed by Stark Spectroscopy. J. Phys. Chem. A 2008, 112, 10244-10249. (21) Tolbert, L. M.; Haubrich, J. E. Photoexcited Proton Transfer from Enhanced Photoacids. J. Am. Chem. Soc. 1994, 116, 10593-10600. (22) Ditkovich, J.; Pines, D.; Pines, E. Controlling reactivity by remote protonation of a basic side group in a bifunctional photoacid. Phys. Chem. Chem. Phys. 2016, 18, 16106-16115. (23) Groves, M. S.; Nelson, K. J.; Nelson, R. C.; Takematsu, K. pH switch for OHphotoacidity in 5-amino-2-naphthol and 8-amino-2-naphthol. Phys. Chem. Chem. Phys. 2018, 20, 21325-21333. (24) Houari, Y.; Jacquemin, D.; Laurent, A. D. Methodological keys for accurate simulations. Phys. Chem. Chem. Phys. 2013, 15, 11875-11882. (25) Acharya, A.; Chaudhuri, S.; Batista, V. S. Can TDDFT Describe Excited Electronic States of Naphthol Photoacids? A Closer Look with EOM-CCSD. J. Chem. Theor. and Comp. 2018, 14, 867-876. (26) Wang, Y.-F.; Cheng, Y.-C. Molecular electrostatic potential on the protondonating atom as a theoretical descriptor of excited state acidity. Phys. Chem. Chem. Phys. 2018, 20, 4351-4359. (27) Granucci, G.; Hynes, J. T.; Millié, P.; Tran-Thi, T.-H. A Theoretical Investigation of Excited-State Acidity of Phenol and Cyanophenols. J. Am. Chem. Soc. 2000, 122, 1224312253. (28) Agmon, N.; Rettig, W.; Groth, C. Electronic Determinants of Photoacidity in Cyanonaphthols. J. Am. Chem. Soc. 2002, 124, 1089-1096. (29) Prlj, A.; Sandoval-Salinas, M. E.; Casanova, D.; Jacquemin, D.; Corminboeuf, C. Low-Lying ππ* States of Heteroaromatic Molecules: A Challenge for Excited State Methods. Journal of Chemical Theory and Computation 2016, 12, 2652-2660. (30) Laurent, A. D.; Jacquemin, D. TD-DFT benchmarks: A review. International Journal of Quantum Chemistry 2013, 113, 2019-2039. (31) Klevens, H. B.; Platt, J. R. Spectral Resemblances of Cata‐Condensed Hydrocarbons. J. Chem. Phys. 1949, 17, 470-481. (32) Nishimoto, K. ELECTRONIC SPECTRA AND STRUCTURE OF - AND NAPHTHOL. The Journal of Physical Chemistry 1963, 67, 1443-1446. (33) Suzuki, S.; Fujii, T.; Baba, H. Interpretation of electronic spectra by configuration analysis: Absorption spectra of monosubstituted naphthalenes. J. Molec. Spectro. 1973, 47, 243251. (34) Anslyn, E. V.; Dougherty, D. A.: Modern Physical Organic Chemistry; Univ. Sci. Books: USA, 2006. (35) Hansch, C.; Leo, A.; Taft, R. W. A survey of Hammett substituent constants and resonance and field parameters. Chem. Rev. 1991, 91, 165-195. (36) 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, 20932099. (37) Ellis, D. W.; Rogers, L. B. Zwitterion formation by 2,3-aminonaphthol in its excited state. Spectrochim. Acta 1962, 18, 265-267. (38) Ellis, D. W.; Rogers, L. B. The measurement of ionization constants of electronically excited species from aminonaphthols. Spectrochim. Acta 1964, 20, 1709-1720.


36

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Gahlaut, R.; Joshi, H. C.; Joshi, N. K.; Pandey, N.; Pant, S. Photochemistry and excited state prototropic behaviour of 8-amino 2-naphthol. Spectrochim. Acta A: Mol. Biomol. Spectro. 2013, 109, 164-172. (40) S. Sarpal, R.; K. Dogra, S. Prototropic Equilibria in Few β-HydroxySubstituted 1-Naphthylamines in the Ground and Excited Singlet States. Bullet. Chem. Soc. Japan 1989, 62, 3283-3290. (41) Harris, C. M.; Selinger, B. K. Proton-induced fluorescence quenching of 2naphthol. J. Phys. Chem. 1980, 84, 891-898. (42) Brinn, I. M.; Rosa, M. F. Determination of protonic dissociation constants of 2.7naphthalenediol in the first excited singlet state by experiments with temporal resolution and mathematical modeling. Acta Scient. Technol 2006, 28, 5-12. (43) Lakowicz, J. R.: Principles of fluorescence spectroscopy; Springer: New York, 2006. (44) Laws, W. R.; Brand, L. Analysis of two-state excited-state reactions. The fluorescence decay of 2-naphthol. J. Phys. Chem. 1979, 83, 795-802. (45) 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 Rev. B.01: Wallingford, CT, 2016. (46) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999-3094. (47) Tsutsumi, K.; Shizuka, H. Excited state pK*a values of naphthylamines: protoninduced fluorescence quenching. Chem. Phys. Lett. 1977, 52, 485-488. (48) Novak, J.; Prlj, A.; Basarić, N.; Corminboeuf, C.; Došlić, N. Photochemistry of 1and 2-Naphthols and Their Water Clusters: The Role of 1ππ*(La) Mediated Hydrogen Transfer to Carbon Atoms. Chemistry – A European Journal 2017, 23, 8244-8251. (49) Schreck, J. O. Nonlinear Hammett relationships. J. Chem. Ed. 1971, 48, 103. (50) Tsuneo, F.; Satoshi, S. Absorption Spectra of Four Naphthalenediols with C2V Symmetry. Interpretation by Means of Configuration Analysis. Bullet. Chem. Soc. Japan 1976, 49, 2892-2898.


37


Page 38 of 38

TOC Graphic


38

Divergent Hammett Plots of the Ground- and Excited-State Proton

Recommend Documents