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Letter

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand Sergei V. Lymar, Mehmed Z. Ertem, Anna Lewandowska-Andralojc, and Dmitry E. Polyansky J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01614 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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The Journal of Physical Chemistry Letters

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand Sergei V. Lymar,* Mehmed Z. Ertem, Anna Lewandowska-Andralojc,† and Dmitry E. Polyansky* Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, U.S.A. [email protected], [email protected] †

Present address: Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61614, Poznan, Poland. Abstract Electron-proton transfer, EPT, from phenols to a triplet MLCT-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter

plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to an inconsistent data analysis. Control experiments replacing either 1(T) by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between 1(T) and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT.

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Reactions involving simultaneous movement of an electron and a proton (concerted electronproton transfer, EPT) play an important role in natural and artificial photosynthetic transformations by providing low energy pathways in charge separation, charge transport and redox catalysis.1-7 Rapid photoinduced EPT reactions occurring during the excited state lifetime are of particular interest due to their relevance to charge transfer in photosynthetic reaction centers2-4 and artificial light harvesting systems.8,9 The inherent complexity of EPT and the short-range nature of proton transfer require the formation of a precursor complex with the reactants in close proximity and proper alignment for the EPT step, and hydrogen bonding is the principal interaction that can accomplish such a task. The participation of H-bonding in EPT reactions has been postulated in several studies of: (i) intramolecular EPT in covalently linked molecular assemblies with the hydrogen bond (HB) donor and acceptor moieties within the H-bonding range;10,11 (ii) “multiple-site” or “bidirectional” EPT where a donor transfers its proton to an H-bonded acceptor (basic solute or solvent), and an electron is transferred from the same donor to an oxidizing solute or electrode;1215

(iii) photoinduced EPT from a donor to an electronically excited acceptor.16-24 Here, we

suggest and apply an improved mechanistic model for describing the latter systems; it explicitly accounts for the H-bonding interactions of both reactants with solvent and can be readily extended to include systems listed in (i) and (ii). In this work we investigate the reactivity toward phenols of a Ru polypyridine complex with a peripheral Brønsted basic site (ligand’s uncoordinated N atom, complex 1 in Scheme 1a). Photoexcitation of the metal-to-ligand charge transfer (MLCT) transition promotes an electron from the metal d orbital to a ligand π* orbital producing the triplet excited state 1(T) and rendering the metal center an electron acceptor, while the ligand becomes a base, which creates a driving force 2 ACS Paragon Plus Environment

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for electron transfer, proton transfer, or EPT from a proton-electron donor. Phenols, whose redox properties and acidity can be conveniently tuned by ring substitutions, are particularly useful donors for investigating such reactions.

Scheme 1. (A) General mechanism of photoinduced EPT from a phenol to complex 1 ([Ru(bpy)2(pbn)]2+; pbn = 2-(2-pyridyl)benzo[b]-1,5-naphthyridine, bpy = 2,2′-bipyridine). The complex triplet excited (MLCT) and doublet (radical product) states are denoted 1(T) and 1(D), respectively, and PhOH stands for a generalized reactive phenolic functionality. This mechanism includes the Phenol-Solvent, 1(T)-Phenol, and 1(T)-Solvent H-bonding interactions with equilibrium constants P-S , 1-P , and 1-S, respectively. Although shown as EPT, the irreversible chemical step with the rate constant q can also be an electron or proton transfer or their

combination. (B) Complex 1i ([Ru(bpy)2(i-pbn)]2+; i-pbn = 3-(pyrid-2′-yl)-4-azaacridine), a structural isomer of 1 exhibiting no photoinduced reactivity toward phenol.

As the HB donor and acceptor, respectively, phenols and 1(T) can also make H-bonds with solvent. These H-bonding interactions can modulate the 1(T) quenching rates and are included in Scheme 1a. Among those solvents that offer sufficient solubility of 1 and phenols, we have



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selected acetonitrile (MeCN), a moderately strong HB acceptor, and dichloromethane (CH2Cl2), a weak HB acceptor, in order to examine the kinetic effects of H-bonding on EPT. Implicit in our kinetic model is the well-corroborated idea that the phenol’s OH group can be involved in H-bonding with only a single HB acceptor at any given time.25-30 The same is presumed here for the H-bonding interactions of a solvent molecule and 1(T). A comprehensive kinetic analysis of Scheme 1a (SI Section S1) shows that the Stern-Volmer expressions for the observed steady-state triplet emission intensity (obs ) and decay lifetime (obs ) are, under ordinary circumstances, identical; namely, o

obs

o

=

obs

=1+

1-P

app P 1-P o

1 o q ⁄o ⁄o app Po 1-P

= 1+

app app P 1-P o app Po 1-P

o q

(1)

where o and o are the corresponding intensity and lifetime in the absence of phenol, Po is the 1-P

analytical concentration of added phenol, o is the natural lifetime of the 1(T)-Phenol exciplex (in the absence of reactive quenching, when q = 0, and the 1(T)-P exciplex decays only by a 1-P

1-P

non-radiative transition and emission with the combined rate constant o = 1⁄o ), q 1-P

app

=

app

q + 1⁄o − 1⁄o is the apparent unimolecular quenching rate constant, and 1-P is the apparent equilibrium constant of the 1(T)-P exciplex formation; that is, app

1-P = (

1-P

(2)

P-S )( 1-S )

Here, the component equilibrium constants for the H-bonded phenol-solvent (P-S) species and 1(T)-Solvent and 1(T)-P exiplexes are defined through the concentrations of unbound 1(T) complex and phenol. Taking into account that solvent activity is unity, we write P-S = app

P-S !P, 1-S = "(#)--S !"(#), and 1-P = "(#)--P !("(#)P). Thus, 1-P is solvent1-P

specific, and the other parameters (o , o , q , and q ) in eq. 1 are generally also solventapp


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dependent. Thermodynamically, our treatment and the resulting eq. 2 are equivalent to assigning activity coefficients of unity to 1(T)-P, 1!(1 + P-S ) to phenol, and 1!(1 + 1-S ) to 1(T)-S in a given solvent. The solvent-phenol H-bonding constant can be evaluated using a well substantiated, empirical, thermodynamic scale for the 1:1 hydrogen bonding between uncharged species in CCl4 developed by Abraham and co-workers,31-33 in which the H-bonding constant is proportional to the so-called HB acidity of the donor ($2H ) and HB basicity of the acceptor (%2H ). Applying the Abraham scale, taking into account solvent molar concentration, [S], and adopting the suggestion of Ingold and co-workers that for a given HB donor-acceptor pair the H-bonding constant is essentially independent of surrounding medium,25 we obtain for P-S in MeCN or CH2Cl2 &'(P-S = 7.354$2H (P)%2H (S) + &'(S − 1.094

(3)

where $2H (P) is phenol’s HB acidity and %2H (S) is solvent’s HB basicity. The estimates obtained through eq. 3 and presented in Table 1 show that for all phenols in MeCN P-S ≫ 1 and vary widely with p-substitution. On the other hand, the P-S values in CH2Cl2 are nearly constant at 22.5. Unfortunately, the HB basicity of 1(T) is unknown and it is not even obvious that a %2H parameter suitable for evaluating 1-S (or 1-P ) can be assigned to this charged HB acceptor. However, both MeCN and CH2Cl2 solvents are very weak HB donors compared to phenols, and we should expect 1-S to be relatively small. This assertion is supported by our computations that give an enthalpy of only −0.5 kcal/mol for H-bonding between MeCN and 1(T). This enthalpy should be more than offset by the negative H-bonding entropy, yielding 1-S < 1. However, until



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more information about 1-S can be obtained, it is difficult to be certain that 1-S in eq. 2 can be neglected. It is evident from eq. 1 that the determination of both q

app

and 1-P from either the steadyapp

state (obs vs Po ) or kinetic (obs vs Po ) quenching measurements requires data in the 1-P Po app

≫ 1 range, when the Stern-Volmer plot approaches a plateau. If 1-P Po ≪ 1 in the accessible app

phenol concentration range, eq. 1 reduces to the familiar linear Stern-Volmer form for dynamic quenching o

obs

= o ≈ 1 + o q 1-P Po = 1 + o qobs Po app

app

(4)

obs

In this situation, only the q 1-P product can be determined from the Stern-Volmer slope, and app

app

no evidence for the H-bonding between 1(T) and phenols can be obtained by measuring either obs or obs . Contrary to this assessment, Meyer and co-workers suggested a procedure for determining app 1-P from the obs vs Po data despite their observation of a linear Stern-Volmer plot.16 As

detailed in the SI Section S2, this procedure is incorrect and yields erroneous values for both 1-P and q . The same opinion has recently been expressed by Hammarström and coapp

app

workers.24 Unfortunately, the erroneous procedure due to Meyer and co-workers has been uncritically adopted in several subsequent investigations of reactive quenching (including EPT from phenolic donors) that invoked a precursor complex formation through H-bonding or ion pairing.17-23 As a result, none of the reported equilibrium constants for H-bonding between excited metallocomplexes and EPT donors as well as rate constants for the EPT step within these H-bound structures can be considered reliable; only the products of these constants can be determined from the published data (see SI Section S2 for details). In all these studies, with two 6 ACS Paragon Plus Environment

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possible exceptions21,24 (see SI Section S2), no deviations of the Stern-Volmer plots (for obs , obs , or both) from linearity were reported, so it would appear that 1-P ≪ 1 M−1 and the Hbonding between the donor and acceptor in EPT reactions is very weak or absent altogether; in the latter case, 1-P would simply describe the formation of an encounter complex between noninteracting 1(T) and phenol. This is not necessarily so, however, and 1-P can be quite large even if the Stern-Volmer dependence is linear. To recognize this fact, it is necessary to include in the picture H-bonding of both donor and acceptor to solvent as we have done in eq. 2, which shows that the observed smallness of 1-P may be due to the large (1+P-S )(1+1-S ) product; indeed, app

this product is often much greater than unity, especially in strongly HB donating or accepting solvents such as MeCN or water. We also note that eq. 2 predicts a leveling effect of such app

solvents on 1-P in the sense that both 1-P and 1 + P-S increase with the phenol’s HB acidity, which should somewhat dampen the modulation of their ratio by $2H (P). A

B log(kqobs, M-1 s-1)

9

τo/τobs -1

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6 3

0.2

MeO9

PhCl-

8

MeOC(O)NC-

O2N-

7

0.1

6 0 0

0.2

0.4

0.6

0.8

1.0

0.6

TFE (●, ●) or p-MeOPhOH (, ), M

0.7

αH2

0.8

Figure 1. (A) Stern-Volmer plots for quenching of 1(T) by p-MeOPhOH (squares) and 2,2,2trifluoroethanol (TFE, circles) in MeCN (solid lines, o = 190 ns) and CH2Cl2 (dashed lines, o = 350 ns). Although the vertical axis break at 0.27 alters the appearance of the phenol’s plots, these plots are actually linear in the entire range (Figures S7 and S12). The saturating dependence for TFE in CH2Cl2 is fitted to eq. 1, which gave q

app

app

app

= 8.9×105 s−1, 1-TFE = 5.3 M−1, and q 1-TFE


app


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app

= 4.7×106 M−1 s−1. The other 3 lines represent linear fits to eq. 4 with qobs = q 1-TFE = 9.3×105 app

app

M−1 s−1 (lower) and qobs = q 1-P = 3.3×107 (middle) and 1.3×109 M−1 s−1 (upper). For TFE, app

app

1-P and P-S in eq. 1-5 are replaced by 1-TFE and TFE-S, respectively. (B) Correlations of qobs app

with phenol’s HB acidity, $2H . Squares: in CH2Cl2; the solid line shows a linear fit with a slope of −6.4 ± 0.9. Circles: in MeCN; the dashed line gives a linear fit with a slope of −6.5 ± 1.3 to the leftmost 4 data points.

Eq. 1 suggests that the rates of reactive and physical quenching cannot be separated, only their combination q

app

1-P

= q + 1⁄o − 1⁄o is experimentally determinable, and q

app

can be equated to

q only under the assumption that H-bonding does not alter the natural lifetime of 1(T); that is, 1-P

o

= o . Although never mentioned or discussed, this assumption is implicit in all previous

studies that postulated H-bonding between an excited metallocomplex and a quencher.16-24 To investigate the validity of this assumption, we have compared the quenching of 1(T) by pmethoxyphenol, MeOPhOH, and 2,2,2-trifluoroethanol, TFE. With their $2H of 0.573 and 0.567, respectively, these compounds are HB donors of essentially the same strength, and their Hbonding to 1(T) are expected to be quantitatively similar; this conjecture is supported by the computational data (Table S4). However, the chemical reactivity of MeOPhOH and TFE is different, as revealed by transient absorption experiments (Figures S4-S6). Whereas MeOPhOH predominantly reacts through EPT producing the phenoxyl radical and 1(D)-H as shown in Scheme 1a, the only effect of TFE is the acceleration of the 1(T) decay with no net chemical change, which suggests that q = 0 for TFE and it can serve as a surrogate for evaluating the relative contribution of physical and chemical quenching by MeOPhOH. Undoubtedly, the


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observed difference in EPT reactivity between TFE and MeOPhOH is due to the much higher OH bond dissociation energy in the former (∆BDE ≈ 23 kcal/mol).34 The Stern-Volmer plots in Figure 1a reveal several salient features. First, the decrease of 1(T) lifetime upon TFE addition is consistent with the H-bonding and attendant physical quenching in both solvents. Second, the data for TFE in CH2Cl2 conform to the functional form of eq. 1 with 1-TFE

app

1-TFE = 5.3 M−1 and o

= 270 ns; the latter value corresponds to a 23% decrease of the 1(T)

lifetime brought about by its H-bonding with TFE. This relatively small effect is not unreasonable considering the weakness of H-bonding interactions. Using eq. 2 and 3 we evaluate 1-TFE ⁄(1+1-S ) = 16 M−1 in CH2Cl2; this value represents the lower limit of “true” 1-TFE in an HB-inert solvent, where 1-S = 0. Third, the linear plot obtained for TFE in MeCN suggests that app

1-TFE ≪ 1 M−1 in this solvent and eq. 4 applies, which prevents separating the contributions from q

app

and 1-TFE into qobs = 9.3×105 M−1 s−1. For the lifetime of the H-bonded 1(T)-TFE app

1-TFE

species in MeCN we can only evaluate the upper limit, o

≪ 160 ns. Considering eq. 2 and app

that the estimated factor 1+TFE-S is ∼100 for MeCN, the smallness of 1-TFE in MeCN is not surprising. Fourth, for MeOPhOH with the chemical quenching via EPT present (q > 0), the values of qobs are 1-2 orders of magnitude greater than for TFE in both solvents suggesting that 1-P q ≫ 1⁄o − 1⁄o for phenols in eq. 1 and the observed bimolecular rate constants can be

approximated by qobs ≈ q 1-P = q ( app

1-P

P-S )( 1-S )

(5)

For all phenols examined in this work, the Stern-Volmer plots remained linear even at phenol concentrations producing over 80% quenching of 1(T) (Figures S7-S12). Thus, in all cases 9 ACS Paragon Plus Environment


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app 1-P Po ≪ 1 in the accessible phenol concentration range (up to 0.8 M in MeCN and 0.1 M in

CH2Cl2 limited by phenol solubility and/or the extent of quenching), and eq. 4 is applicable for obtaining qobs . These rate constants are summarized in Table 1 along with their corrections for the phenol-solvent H-bonding, qobs (1+P-S ) = q 1-P!(1+1-S ). Since we would definitely detect greater than 10% downward deviations from linearity in the Stern-Volmer plots in Figures app

S7-S12, we can estimate that even for the strongest HB donor, p-nitrophenol, 1-P < 0.3 M−1 in app

MeCN and 1-P < 3 M−1 in CH2Cl2. The upper limits for the corresponding H-bonding constants proper, 1-P, are by a factor of (1+P-S )(1+1-S ) greater (eq. 2).

Table 1. Apparent bimolecular rate constants (in 107 M−1 s−1) derived from the Stern-Volmer slopes (qobs, eq. 4) and corrected for the solvent-phenol H-bonding (q 1-P !(1+1-S ), eq. 5) for the 1(T) quenching by p-substituted phenols (R-PhOH) in MeCN and CH2Cl2 obtained through the transient emission measurements. Also shown is the kinetic solvent effect (KSE).

R- ($2H)a

a

In MeCN

In CH2Cl2

($2H = 0.09; %2H = 0.44)a

($2H = 0.13; %2H = 0.05)a

KSEc

P-S b

qobs

q 1-P 1+1-S

P-S b

qobs

q 1-P 1+1-S

MeO- (0.573)

110

3.3

360

2.0

130

390

39

Ph- (0.595)

130

1.4

180

2.1

44

140

31

Cl- (0.670)

230

0.43

97

2.2

12

39

28

MeOC(O)- (0.730)

350

0.28

98

2.3

4.6

15

16

NC- (0.787)

540

0.48

260

2.4

3.8

13

7.9

O2N- (0.824)

710

0.88

620

2.5

2.4

8

2.7

Shown in parentheses are the literature values for HB acidities, $2H, and basicities, %2H, derived by

Abraham and co-workers;31-33 bCalculated from eq. 3; c78 = qobs (CH2 Cl2 )/qobs (MeCN). 10 ACS Paragon Plus Environment

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Only in the case of p-methoxyphenol the evaluation of the quenching rate constant through app

EPT is possible due to the independent measurement of 1-TFE = 5.3 M−1 in CH2Cl2 (vide supra). app

app

Assuming that 1-P ≈ 1-TFE , we obtain q ≈ 2.5×108 s−1 in CH2Cl2. If we additionally assume 1-P to be solvent-independent, ignore the presumably small K 1-S term in eq. 5, and use the K P-S values from Table 1, we obtain q ≈ 2.3×108 s−1 in MeCN. To the extent of their accuracy, these estimates suggest that the kinetic solvent effect on the EPT step described by q is practically absent, and the large KSE on qobs shown in Table 1 is mainly due to the much stronger Hbonding between MeOPhOH and solvent in MeCN. The plots of &'(qobs vs $2H in Figure 1b show a good linear correlation for all phenols in CH2Cl2, but not in MeCN, where a significant positive deviation is apparent for the two most acidic phenols. On the other hand, for the less acidic phenols, the linear correlation holds in MeCN with essentially the same slope as in CH2Cl2. We believe that this pattern signals an alteration of the reactive quenching mechanism. This conjecture, deuterium isotope effects, energetics, and mechanisms of the reactive quenching of 1(T) by phenols will be discussed in a forthcoming publication. Despite the nearly identical excited state and redox energetics of the isomeric complexes 1 and 1i (Scheme 1b and Table S1), complex 1i(T) was found to be completely unreactive toward phenols (Figure S13). This result demonstrates that exposure of the ligand’s uncoordinated N atom to the surrounding medium is a prerequisite to the Ru complex-phenol interaction and attendant quenching reaction. Further evidence for the intermediacy of the H-bonded 1(T)-P precursor comes from the activation parameters compiled in Table 2 and featuring zero or even negative activation 11 ACS Paragon Plus Environment


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enthalpies in MeCN. Due to the high HB basicity and low HB acidity of this solvent (P-S ≫ 1, and expected 1-S ≪ 1), eq. 5 simplifies to qobs ≈ q 1-P⁄P-S , and the observed activation enthalpy is ‡ ≈ ∆q ; ‡ + ∆1-P ; − ∆P-S ; ∆;obs

(6)

where ∆1-P ; and ∆P-S ; are the H-bonding enthalpies in 1(T)-P and P-S, respectively, and ∆q ; ‡ is the activation enthalpy of the chemical quenching step. The observed activation entropy should obey a similar equation. As expected, the values of ∆P-S ; estimated from an empirical correlation due to Ingold and co-workers28 are negative for all phenols (Table 2). The observed negative activation enthalpy is consistent with and best explained by the precursor exciplex formation, whose ∆1-P ; term in eq. 6 is more negative than the combined positive contributions from the ∆q ; ‡ and ∆P-S ; terms. This suggestion is supported by the computational data that predict more negative ∆1-P ; than ∆P-S ; for all 3 phenols (Tables 2 and S4); that is, the phenols’ H-bonding to 1(T) should be stronger than to MeCN. The values of ‡ ∆1-P ; − ∆P-S ; ≈ −4 kcal/mol and the ∆;obs in the 0 to −3.1 kcal/mol range in Table 2 suggest

that the ∆q ; ‡ terms in eq. 6 lie within 0-3.1 kcal/mol; that is the chemical quenching steps are not substantially activated.

‡ ‡ = 0.3 cal/(mol K) and ∆;obs = 0.1 kcal/mol) for Table 2. Observed activation parameters (∆7obs

reactions between p-substituted phenols (R-PhOH) and 1(T) in MeCN obtained from the temperature dependencies of obs (Figures S14-S16). Also shown are enthalpy of solvent-phenol H-bonding (∆P-S ;) estimated through an empirical correlation (eq. S6.3) and the

computationally-evaluated enthalpy difference (∆1-P ; − ∆P-S ;) between phenol H-bonding to 1(T) and to solvent (SI section S7 and Table S4). 12 ACS Paragon Plus Environment

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‡ −∆;obs

‡ −∆7obs

−∆P-S ;

−(∆1-P ;−∆P-S ;)

MeO-

0.0

25

4.6

3.8

NC-

3.1

39

6.5

4.1

O2N-

3.1

37

6.9

4.4

R-

Interpreting the considerably negative observed activation entropies in Table 2 is more ‡ complicated and consigned to SI Section S6. Qualitatively, negative ∆7obs result from the greater

phenol immobilization upon H-bonding to 1(T) than to MeCN and a possibility of the activation entropies ∆q 7 ‡ for the chemical quenching step being substantially negative. In conclusion, an improved mechanistic model has been proposed for describing reactivity, including EPT, of a photo-excited metallocomplex containing an uncoordinated H-bonding site with an HB acidic solute. In addition to the H-bonding between the reactants and attendant physical and chemical quenching, this model accounts for the H-bonding of both reactants to solvent and suggests that proper corrections for these interactions and physical quenching in the H-bonded reactant exciplex are required for comparing reactivity of different electron-proton donors toward the same acceptor. The model and its implications have been corroborated by the data on the photochemical reactivity of Ru polypyridine complexes (Scheme 1) toward psubstituted phenols and trifluoroethanol. For increasing the likelihood of separating unimolecular EPT rate constants from reactants’ H-bonding constants in photoinduced EPT reactions similar to those investigated in this work, our results suggest the following approaches, each subject to certain limitations: (a) use of as HB-inert solvents as possible; apart from possible solubility issues, CCl4 with its $>? = %>? = 0 appears to be the best choice; (b) use of unreactive H-bonding proxies for the EPT quenchers, as 13 ACS Paragon Plus Environment


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we have done in this work; in addition to yielding the reactants’ H-bonding constant, this approach also provides information on the physical quenching associated with H-bonding; (c) extending the Stern-Volmer plot into its saturation region; however, this seemingly straightforward approach may require the use of sub-nanosecond excitation and detection and is limited by the quencher solubility, variation of activity coefficients at high quencher concentrations, and the requirement of maintaining the emitter-quencher H-bonding equilibrium.

Supporting Information Derivation of Stern-Volmer dependencies and their experimental plots, comparison with previous studies, transient absorption spectra, activation plots and their interpretation, computational methods and results, experimental details.

Acknowledgemens We are grateful to Koji Tanaka for providing the Ru compounds and to Jacob Schneider, Norman Sutin, Gábor Merényi, and David Grills for their assistance and helpful discussions. This work was carried out at Brookhaven National Laboratory and supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, & Biosciences under contract DE-SC0012704.


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