Excited State Quenching of Porphyrins by Hydrogen-Bonded Phenol

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C: Energy Conversion and Storage; Energy and Charge Transport

Excited State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton Coupled Electron Transfer Munisamy Venkatesan, Haraprasad Mandal, Madhu Chakali, and Prakriti Ranjan Bangal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05273 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on September 3, 2019

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The Journal of Physical Chemistry

Excited State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton Coupled Electron Transfer† Munisamy Venkatesan ab, Haraprasad Mandalab, Madhu Chakali a

ab,

Prakriti Ranjan Bangalab*

Analytical Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad. 500007, India bAcademy

of Scientific and Innovative Research, 2-Rafi Marg, New Delhi, 110001, India Corresponding Author *Email: [email protected]

†This article is dedicated to the memory of Prof. Henry Linschitz, pioneer of PCET reaction.

ACS Paragon Plus Environment

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ABSTRACT A series of porphyrins containing methoxy substituted phenols were treated with different pyridine bases. Besides hydrogen bonding (H-bonding), the pyridine bases have imparted oxidation to the phenol rings resulting coupled electron and proton movement. It has been shown that reduction of an excited substrate/porphyrin macrocycle by phenols with adjacent methoxy groups is facilitated by the movement or transfer of the phenolic proton towards H-bonded bases. Rates of electron transfer are accomplished by associated proton displacements within the redox reaction complex. Demonstrated fluorescence quenching of meso-(4-hydroxyphenyl derivatives)substituted porphyrins in aprotic solvents is attributed to electron transfer from phenol moiety by added bases(different pyridine derivatives) and rates of quenching are found to be correlated with Brönsted base strength rather than H-bonding equilibria. The rate of quenching is observed to be the function of the extent of hydroxy and methoxy substitutions to the phenyls and the solvent polarities. Replacement of 4-hydroxy by 4-methoxy completely eliminated the quenching indicating the disappearance of reduction in the porphyrins macrocycle. The dependence of the extent of fluorescence quenching of studied porphyrins on pyridine concentration led to phenolpyridine H-bonding equilibrium constants and these values closely resemble to the values obtained directly from the corresponding absorption spectra. The quenching agent thus revealed to be H-bonded phenol. Further, positive deuterium isotope effects on quenching upon deuteration of the hydroxyl confirms the electron transfer is coupled to the proton movement.


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INTRODUCTION The elementary chemical reactions involving transfer of electrons and protons either in a single reaction step or in stepwise pathways with distinct proton transfer (PT) and electron transfer(ET) steps, where PT either precedes or follows ET, are commonly called as protoncoupled electron transfer (PCET). When the PCET occurs in single reaction step without forming any intermediate is generally termed as concerted- PCET. Such reactions play a pivotal role in a wide range of chemical and biological processes; such as oxygen production and reduction in photosynthesis,1,2 catalytic nitrogen fixation,3 the catalytic oxidation and production of molecular hydrogen,4 in mitochondria and in fuel cells5,6. Since past two decades the PCET reactions have gained immense importance, for their critical role in a variety of chemical and biological processes7-21 and for understanding the phenomena from the fundamental point of view.22-25 Although photophysical behaviors of organo-metallic systems (metal complexes) involved in PCET reaction were explored extensively by several groups

10,26-34

photophysical behaviors of

pure organic systems involved in PCET reaction have not received much attention and literature regarding the fluorescence emission quenching studies in electron transfer (ET) process accompanied by proton movement are remarkably sparse35-43. PCET reactions can be classified broadly in two distinct categories, according to whether the electron and proton move to the same acceptors, 'single-site PCET'14,

or different acceptors 'multiple-site PCET'14 . As in

multiple site PCET the movement of electron and proton occur in two opposite direction it is also classified as bi-directional PCET as well.33 Depending on the extent and phasing of e–/H+ displacement and energetic of the reaction, the first case may correspond to overall H-atom transfer (HAT),44,45 albeit it is difficult to make distinction from PCET, while second may lead to an anion radical, a deprotonated reductant and a protonated base. Recently, such 'single-site


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PCET' process was exemplified for the occurrence of concerted charge movement to the same acceptor by reductive fluorescence quenching of a free base porphyrin, purely organic systems, by an aryl phenol, where pyridine is covalently linked with the porphyrin macrocycle.46 The reaction involved in this case, was ET from the phenol to the singlet porphyrin, coupled with proton transfer of the bonding proton to the associated pyridine. An appreciable kinetic deuterium isotope effect was found in quenching of porphyrin fluorescence by phenol-pyridine adducts. It was therefore concluded that, in these interactions, electron transfer from the phenol moiety to the excited porphyrin molecule is a concerted process with proton movement from the phenol-OH to the pyridine. Here, we report the finest evidences of PCET in 'multiple-site PCET' , where aryl phenols are covalently linked to the porphyrin macrocycle, as in the following meso-substituted porphyrin (Chart-1) and pyridines are added to the solutions. As shown in the Chart-1, substitutions in meso position of the porphyrin macrocycle are systematically designed and synthesized tuning both the electron donating ability as well as H-bonding ability of the substitution by introducing a hydroxy group in 4-position and methoxy groups in 3- and (3,5)-positions of the phenyl, respectively. Finally, to eliminate the H-bonding, methoxy groups are introduced in (3,4,5)positions of the phenyl ring. For brevity, porphyrin derivatives used in this study are annotated as (n,m)TPP, where 'n' and 'm' indicate the number of phenolic and methoxyl groups respectively and TPP stands for tetraphenylporphyrin. Therefore, (4,8)TPP stands for meso-tetrakis(4hydroxy-3,5-dimethoxyphenyl)porphyrin,

(4,4)TPP

is

for

meso-tetrakis

(4-hydroxy-3-

dimethoxyphenyl) porphyrin, (1,2)TPP indicates to 5-(4-Hydroxy-3,5-dimethoxyphenyl)10,15,20-triphenylporphyrin, (4,0)TPP denotes for meso-tetrakis (4-hydroxyphenyl)porphyrin and (0,12)TPP points to meso-tetrakis (3,4,5 -trimethoxyphenyl)

porphyrin throughout the


5


manuscript. Now we show substituted pyridines/ bases which initiate hydrogen bonding with the hydroxy group of phenyls attached to porphyrin, induces static and dynamic fluorescence quenching of the excited porphyrin macrocycle. Here we describe the effects of the extent of hydroxy and methoxy substitution of phenyls, H-bonding power of the base, type of solvent and deuteration of the hydroxyl groups on the fluorescence quenching. These results on reductive quenching of porphyrins by phenols in the presence of a pyridyl base mimics the related event of the appearance of neutral tyrosine radicals in the electron transfer chain of Photosystem II in green plants, algae and cyanobacteria photosynthesis47 and it may have implications for photochemical energy conversion. Porphyrins: R2

OH

OH NH

H3CO

N

OCH3

H3CO

OH OCH3

H3CO

OCH3 OCH3

H3CO

OH

TPP

1 (1,2)TPP

2 (4,8)TPP

3 (4,4)TPP

R1=R2= R3=R4=

R1=R2= R3=R4=

R1=R2= R3=R4=

R4 R1=R2=R3=R4=

R2=R3= R4=

HN

N

R1=R2= R3=R4=

R1

R3

R1=

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

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4 5 (0,12)TPP (4,0)TPP

Bases: NC

N

N

N 4-Cynanopyridine Pyridine (4-CNPy) (Py)

N 2,4-dimethylpyridine (DMPy)

N 2,4,6-trimethylpyridine (TMPy)

N

4-N,N-dimethylaminopyridine (DMAPy)

Chart 1. Structure of synthesized meso-substituted porphyrins and different pyridines used. For The studied porphyrin systems are denoted as (n,m) TPP, where 'n' and ' m' indicate the number of phenolic and methoxy groups respectively and TPP stands tetraphenylporphyrin. Experimental Section Aldehydes, pyridine derivatives and all spectroscopic grade solvents used for spectroscopic studies were used as received from Aldrich. Pyrrole was distilled at atmospheric pressure from CaH2. 1H NMR spectra (300 MHz) and absorption spectra were collected routinely. Porphyrins were analyzed in neat form by laser desorption mass spectrometry (LDMS) without use of a matrix48 CH2Cl2 and CHCl3 (Fisher Certified A.C.S) were distilled from


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K2CO3. Meso-tetrakis(4-hydroxy-3-methoxyphenyl)porphyrin, (4,4)TPP, was prepared as described in the literature.49 Meso-tetrakis(4-hydroxyyphenyl)porphyrin, (4,0)TPP, was obtain from

Sigma-Aldrich,

USA.

UV/vis

spectra

were

recorded

using

Hitachi

U-2910

spectrophotometer. All steady state fluorescence spectra were recorded at room temperature as well as low temperature by Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon, USA. Cyclic voltammograms were recorded on a PC controlled CH Instrument (CHI620C) electro chemical analyzer, USA. For Time Correlated Single Photon Counting, ~100 femtosecond pulses with 80 MHz repetition rate of Ti:Sapphire laser (MaiTai, Spectra Physics) was passed though femtosecond pulse selector, single shot to 8 MHz (3980-5S, spectra physics), and laser pulse of 8 MHz were obtained. The excitation pulses at desired wavelength were generated by frequency doubling of this laser pulse with 0.5 mm BBO crystal. This excitation pulses are focused to the sample using our Fluorescence up-conversion set up describe elsewhere50 The time distribution data of fluorescence intensity were recorded on a SPC-130 TCSPC module (Becker & Hickl). The IRF of the TCSPC system was nearly 200 ps. Synthesis The porphyrins prepared in this study are shown in Chart 1. Porphyrins bearing sterically hindered phenolic groups at the meso-positions were first prepared via the Rothemund method or Adler method.51-53 However, more recently, the porphyrins have been synthesized at room temperature in an aqueous–organic medium containing a surfactant.49

The latter method

provides better solubility for the polar aldehyde, reaction intermediates, and the amphipathic porphyrin, compared to the former methods. The surfactant method typically entails a rather tedious workup procedure owing to the large amount of surfactant present in the reaction mixture, however provides the mildest method for preparing phenolic–porphyrins.

The


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surfactant method was employed to prepare each of the porphyrins described herein, including (1) meso-tetrakis(4-hydroxy-3-methoxyphenyl)porphyrin, first prepared by Triebs and Häberle51 and also synthesized in the first report of the surfactant method;49 (2) meso-tetrakis(4-hydroxy3,5-dimethoxyphenyl)porphyrin, first described by Milgrom;53 and (3) the hybrid porphyrin 5-(4hydroxy-3,5-dimethoxyphenyl)-10,15,20-triphenylporphyrin, which was synthesized via a mixed-aldehyde condensation followed by chromatography to separate the resulting mixture of porphyrins. We note that the syntheses reported herein were initiated in the late 1980s, an era prior to the advent of rational approaches to construction of the macrocycle,54 and continued in later years. The porphyrins with non-identical meso-substituents were prepared in statistical fashion with use of two aldehydes in an appropriate ratio.55 meso-Tetrakis(4-hydroxy-3,5-dimethoxyphenyl)porphyrin(4,8)TPP

53:

Following a standard

procedure,49 samples of syringaldehyde (0.910 g, 5.00 mmol) and pyrrole (0.335 g, 5.00 mmol) were added to an aqueous solution of sodium dodecyl sulfate (500 mL, 0.5 M). The mixture was stirred at room temperature under argon for 15 min. A solution of conc. aqueous HCl (10 mL, 10 M) was added to initiate the reaction. The white, soapy solution turned orange upon the addition of the acid, then slowly changed to brownish-orange. After 1 h, a solution of p-chloranil (1.10 g, 4.47 mmol) in slightly warm THF (30 mL) was added. The reaction mixture was stirred overnight in the open air. The dark brown reaction mixture was transferred to a separatory funnel containing ethyl acetate (500 mL), aqueous KOH (25 mL, 2 M), potassium phosphate (50 mL, 1 M) and aqueous KCl (50 mL, 3 M). The organic layer was separated and washed with water (3 x 100 mL).

The combined organic extracts were dried (Na2SO2), filtered, and

concentrated in vacuo. The crude product was purified by column chromatography [silica (6.8 x 10 cm), CH2Cl2/THF (9:1)]. Recrystallization [THF/cyclohexane (1:2)] afforded a purple solid


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(310 mg, 27%): 1H NMR (CDCl3)  –2.8 (s, 2H), 3.96 (s, 24H), 5.84 (s, 4H), 7.43 (s, 8H), 8.89 (s, 8H); calcd mass (C52H46O12N4) 918.3112, obsd m/z = 919.2 (LD-MS); obsd m/z = 918.3126 (FAB-MS); max (CH2Cl2) 426, 518, 556, 592, 648 nm. 5-(4-Hydroxy-3,5-dimethoxyphenyl)-10,15,20-triphenylporphyrin, (1,2)TPP: Following a standard procedure,49 samples of benzaldehyde (0.398 g, 3.75 mmol), syringaldehyde (0.227 g, 1.25 mmol) and pyrrole (0.335 g, 5.00 mmol) were added to an aqueous solution of sodium dodecyl sulfate (500 mL, 0.5 M). The mixture was stirred under argon for 15 min. A solution of conc. aqueous HCl (5 mL, 10 M) was added and the mixture was stirred under argon for 1 h. A solution of p-chloranil (1.13 g, 4.60 mmol) in slightly warm THF (5 mL) was added. The dark reaction mixture was stirred overnight in the open air. The reaction mixture was worked up as described above. Purification by column chromatography (silica, CH2Cl2) gave the desired porphyrin as the second band. Recrystallization [CH2Cl2/hexanes (1:4)] afforded a purple solid (95 mg, 11%): 1H NMR (CDCl3)  –2.77 (s, 2H), 4.01 (s, 6H), 5.88 (s, 1H), 7.49 (s, 2H), 7.77 (m, 9H), 8.23 (m, 6H), 8.89 (m, 8H); calcd. mass (C46H34O3N4) 690.2631, obsd m/z = 691.8 (LD-MS), obsd m/z = 690.2656 (FAB-MS); max (CH2Cl2) 420, 516, 552, 590, 646 nm. meso-Tetrakis(3,4,5-trimethoxyphenyl)porphyrin(0,12)TPP.53

A

solution

of

3,4,5-

trimethoxybenzaldehyde (500 mg, 2.548 mmol) and pyrrole (176 L, 2.548 mmol) in 200 mL CHCl3 under argon was treated with BF3.O(Et)2 (264 L of 2.5 M stock solution in CHCl3). The mixture was stirred at room temperature for 45 min. DDQ (434 mg, 1.91 mmol) was then added and the mixture was stirred for an additional 30 min. The crude reaction mixture was purified by chromatography [silica, CH2Cl2/THF (9:1)] followed by recrystallization [CH2Cl2/hexanes (1:4)] to yield a purple solid (234 mg, 37.7%): 1H NMR (CDCl3)  –2.78 (s, 2H, NH), 3.96 (s, 24H,


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m,m’-OCH3), 4.17 (s, 12H, p-OCH3), 7.46 (s, 8H, ArH), 8.88 (s, 8H, -Py); max (nm) (CH2Cl2) 423, 517, 553, 591, 647. Results and Discussion The photophysical properties of the porphyrins are generally manipulated by peripheral or central substitutions, which lead to charge movement in or out of the macrocycle. Substitutions on the phenyl ring of the tetraphenylporphyrins (TPP) resulted in the change of the shape of absorption spectra depending upon the extent of resonative interactions between porphyrin macrocycle and peripheral substitutions.56-59 In a basic solution, quinonoid resonance structures comprising charge transfer was observed in(4,0)TPP.60 Notably, (4,0)TPP is a class of TPP based molecule, whose sensitive and specific response to peripheral H-bonding to the H atom of -OH group of phenol with bases occurs prior to the H atom of inner core pyrrole and this makes(4,0)TPP and other derivatives useful candidates in exploring the role of H-bonding in electron transfer events. The basic photophysical properties of studied porphyrins (Chart-1) such as spectral band shape, size and peak position of absorption and emission spectra are quite similar to that of TPP in DCM as well as in other aprotic solvents. The absorption spectra of all studied porphyrins (Chart-1)in DCM consist of a Soret band or a B band, conventionally identified as S0 → S2 transition, at around 418 ± 2 nm, with a shoulder at 400 ± 2 nm, and two component Q bands (S0→S1 transition), Qx and Qy, with four distinct peaks at 515±3, 550±4, 588 ±2, and 650±4 nm (Figure 1). Within the framework of Gouterman’s well-described “four orbital” model,61-64 the Soret band at ~418 nm band is attributed to B (0←0) and remaining all peaks in Q-bands are attributed as y-polarized Qy(1←0), Qy(0←0), and x-polarized Qx(1←0), Qx(0←0), respectively, where numbers in parentheses corresponds to number of quanta in the


10

(1-d0/d

-1.9

operative vibrational modes in the excited and ground electronic states of the transitions in -2.1

Franck−Condon principle.

1.02

1.05

1.08

1.11

1.14

d0/d 20

(A)

[TMPy]

[TMPy]

0

 (m M-1 cm-1)

0

300

0.126 M

0.126 M

0.252 M

0.189 M

0.515 M

0.252 M

10

1.101 M

0.315 M

200

15

0.063 M

0.063 M

5.0 M

Q- bands

(mM-1 cm-1)

400

5

100 Soret Band

0

400

425

450 500 550 600 650 700 Wavelength(nm)

-0.22

(B)

-0.26

(1-d0/d)/[Q], M-1

OCH3

NH N

-0.30

OH N HN

-0.34

KHB N

OCH3

PY DMPy TMPy DMAPy

-1.5 -1.7 -1.9 -2.1

1.02

1.05

1.08

1.11

1.14

d0/d 20

(A)

Figure 1. Typical UV/vis absorption spectra of (1,2)TPP and titration by 2,4,6-trimethyl Pyridine [TMPy] in DCM 400 [TMPy] solution. Mataga-Tsuno plot based on change of[TMPy] absorption observed at the peak of Soret band of (1,2)TPP upon 0 added different pyridine bases concentration in 0DCM (A), Intercept of the plot determine 15 H-bonding equilibrium 0.063 M 0.063 M constant KHB for respective pyridines (B). 0.126 M 300 0.126 M 0.189 M 0.252 M 0.315 M

200

0.252 M 0.515 M

5.0 M

Q- bands

5

100

0

10

1.101 M

(mM-1 cm-1)

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

-1.7

 (m M-1 cm-1)

Page 11 of 38

PY DMPy The Journal of Physical Chemistry TMPy DMAPy

-1.5

Soret ACS BandParagon Plus Environment

400

425

450 500 550 600 650 700

11


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Hydrogen Bonding in the Ground State The UV/Vis spectroscopic titration of micromolar (4,8)TPP, (4,4)TPP or (1,2)TPP solution in DCM by pyridine or substituted pyridines causes almost no change in the Q bands in the studied concentration range of pyridine base (~0.5 M) but a sizeable decrease in peak height at the Soret band (B-band). A typical change of absorption spectra of (1,2)TPP is shown in Figure 1. Absence of any splitting of the Soret band and almost no change in the FWHM of the Soret band of the respective porphyrins ruled out the possibilities of any -staking systems. Further, lack of appearance of hyperporphyrin spectra or change in shape of Q band absorption spectra in presence of pyridines confirms that no deprotonation of the phenolic-H is occurred.60 Hence, this small change in porphyrin Soret band is assigned to be due to the H-bonding of pyridine base with the meso-substituted phenols. Hence in comparison to earlier observation by Guo et al.,60 two stages deprotonation (first peripheral phenolic-H then inner core pyrrolic-H) of (4,0)TPP, we affirm that in lower additive concentration interaction (H-bonding) occurs at peripheral cites, in the studied systems it is OH group of phenol, while it occurs at inner core cite at very higher concentration of respective additives. However, interaction at core cites of porphyrin macrocycle brings a split in Soret band with a peak at around 450 nm.58,59 As shown in figure, addition of 5 M TMPy in the solution bring a little change in the Soret band with a small red shift of the peak. This suggests that even such high concentration of TMPy does not interact or very weakly interact with N-H group of pyrrole. Hence, possibilities H-bonding or deprotonation in pyrrol N-H is ruled out in these studies, especially when concentration of pyridine bases are kept very low for all these studies. However, H-bonding equilibrium constants (KHB) between phenol-pyridine were determined spectroscopically from the accompanying


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decrease of absorbance at the Soret band upon addition of different studied pyridines. Mataga and Tsuno relation65 was used to estimate KHB, for formation of 1:1 H-bonded complexes (eq 1).

𝑑0 𝑑

(1 ― ) [𝑄]0

= ― 𝐾𝐻𝐵 + 𝐾𝐻𝐵

𝜀𝑐

𝑑0

( )( 𝜀

𝑑)

(1)

Here, d0 and d are the absorbance values at any given wavelength of absorption spectra (here it is peak of the Soret band) in absence and presence of a pyridine base (Q). Excellent linear Mataga– Tsuno plots (Figure 1) at the Soret band of (1,2)TPP (see Figure S1 for (4,8)TPP) measure the H-bonding equilibrium constants KHB for 1:1 complex formation and they are listed in the Table 1 for all porphyrins studied. These values are in agreement with the KHB values obtained from spectroscopic titration of phenol by different pyridines at ~1.4m, an overtone of infrared (IR) absorption peak of OH group of the free phenol where there is no interference of absorption for other species, when H-bonding to OH group reduces the absorbance of IR absorption (Figure S2, Table S1). Note, due to poor solubility of studied porphyrin derivatives similar experiments are not possible to measure KHB for phenol linked porphyrins. However, for free simple phenol observed KHB values reasonably correlate with pKa values of studied pyridines, while this trend is not so prominent for o-methoxyphenol (2-methoxyphenol and 2,6-dimethoxy phenol). This may suggest that competition between intra- and intermolecular H-bonding and steric hindrance are involved in H-bonding to these phenol derivatives. Note, o-methoxyphenols phenols posses nonlinear, five-center intramolecular HBs and form bifurcated intra/inter HBs with HBA molecules,66 which could also play a role in determining KHB values for different pyridines.


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300

200

104

[TMPy] 0 0.4 M

[TMPy] 0 Fl. Counts

250 Fl. Intensity (arb.u)

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 14 of 38

103

0.1 M 102

150 101

100

20

30

40

50

60

70

80

Time (ns)

50 0

640

660 680 700 Wavelength(nm)

720

Figure 2.Typical fluorescence spectra of (1,2)TPP in DCM solution and fluorescence quenching as function of TMPy. Inset shows typical quenching of singlet lifetime of (1,2)TPP by TMPY. λex=590 nm for collection of fluorescence spectra and λex=420 nm for collection of lifetime data.

Fluorescence Quenching All these five porphyrins exhibit fluorescence properties similar to TPP in terms of fluorescence quantum yield, lifetime, and fluorescence band shape67,68 The singlet state lifetime of (4,8)TPP, (4,4)TPP and (1,2)TPP were found to be 7.0, 7.14 and 7.8 ns, respectively in DCM. Interestingly, singlet state lifetime values for those porphyrins were found to be very close to the corresponding values of simple TPP. Hence, it can be concluded that there is no prominent signature of intramolecular electron transfer from the phenol to the respective porphyrin macrocycle in the porphyrin systems studied. However, fluorescence intensities of all three porphyrins, (4,8)TPP, (4,4)TPP and (1,2)TPP were quenched upon addition of pyridine bases to the DCM solution of the corresponding porphyrins respectively (Figure 2). Similarly, singlet state lifetimes of these porphyrins were quenched as function of the concentration of the


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pyridines added to the solution. A typical change in fluorescence intensity and singlet lifetime of (1,2)TPP are shown in figure 2. In contrast (0,12)TPP, with no hydroxy group but three methoxy groups at 3,4,5 position of phenyl ring, and (4,0)TPP, with no methoxy group, did not exhibit such quenching. Similarly,4-CNPy, which is a weak base and a weak H-bond acceptor, did not quench the fluorescence of any porphyrins tested. Figure 3 shows Stern-Volmer (S-V) plots based on fluorescence intensity (I0/I) and singlet state lifetime (0/)quenching of (4,8)TPP, (4,4)TPP and (1,2)TPP for the different pyridines studied. The observed Stern-Volmer constant (KSV) values are found to be increasing on going from weaker to stronger bases, i.e. simple pyridine (Py) to 4-N,N-dimethylaminopyridine (DMAPy), for the respective porphyrin systems. The S-V plots based on (I0/I) as function of base concentrations are mostly non-linear and they shift from convex (decreasing slope) to concave (increasing slope) on increasing base strength from Py to DMAPy. Fluorescence quenching of (4,4)TPP was observed to be relatively weak and S-V plots were found mostly convex in nature for Py and its other two methyl derivatives, namely, 2,4-dimethylpyridine (DMPy) and 2,4,6-trimethylpyridine (TMPy), while it is very strong and concave in nature for DMAPy (Figure 3S). Such non-linear S-V plots generally indicate the occurrence of both dynamic and static quenching of fluorescence by added reagents. However, most interestingly it was found that S-V plots based on singlet state lifetime change (0/) of (4,8)TPP and (1,2)TPP were exactly similar to the convex S-V plots observed based on fluorescence intensity change (I0/I) upon addition of Py (Figure 3). In contrast, neither singlet state lifetime quenching nor fluorescence intensity quenching was observed for (4,4)TPP when titrated with weak base, Py and DMPy, while quenching of both fluorescence and singlet state lifetime were observed for (4,4)TPP upon addition of TMPy and S-V plot based on lifetime change replicated to that of fluorescence intensity change (Figure S4) which were convex in


15


Page 16 of 38

nature. Finally, fluorescence intensity quenching of (4,4)TPP by DMAPy showed concave S-V plot while lifetime quenching followed linear S-V plot (Figure S4). Occurrence of concave S-V plot is well established, consisting of both static and dynamic quenching of the substrates when ground state complexes show no fluorescence. Irrespective of the cases studied here, linear change of S-V plots based on singlet state lifetime change (0/) represent the dynamic part of the quenching and there can only be two possibilities for this dynamic quenching; (i) the electron transfer from H-bonded phenol to excited porphyrin macrocycle (ii) energy transfer from excited porphyrin macrocycle to H-bonded phenol. The energetic position of H-bonded phenol/Hbonded is much higher (absorption of phenol/H-bonded phenol below 320 nm) than porphyrin macrocycle and the porphyrin has absorption above 390 nm (Figure 1). Hence, neither Förster resonance energy transfer (FRET) nor excited state energy transfer (EET) mediated by the Dexter-type electron exchange between the excited state porphyrin macrocycle to ground state phenol/H-bonded phenol are energetically feasible process to show dynamic quench of porphyrin fluorescence. Therefore, the dynamic part of the quenching is attributed to the electron transfer from H-bonded phenol to excited porphyrin macrocycle. The thermodynamic driving force, -G, for electron transfer from phenol to excited porphyrin macrocycle are found to be endergonic considering first reduction potential of TPP (-1.4 V),69 and E1/2OX of (2,6)-dimethoxyphenol (~1.3 V) in DCM (Figure S5) when E00 (peak of porphyrin fluorescence ~650 nm) is 1.9 eV. However, H-bonding of the phenol to the added pyridine led to have quasi-reversible voltammograms with a shift of an electrochemical wave peak (Epox ) into cathodic direction, which is correlated with basicity of the pyridines and a new proton-coupled oxidation pathways was accessed at lower driving force (or less positive potential) (see Figure S5 and Table S2) 36,37 Mayer and his group have also shown one electron oxidation of a H-bonded phenol occurs by


16

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concerted proton-coupled electron transfer

70,71

and they interpreted most of the previous

electrochemical studies of phenol-base system by considering the E1/2, average of anodic and cathodic peaks, as potential for PCET. Note, E1/2 values from cyclic voltammetry can be taken as the reduction potential of the proton-coupled reduction potential E(PhO˙/PhOH) just for electrochemically reversible systems

70,71

and since that is not the case for the studied phenol

systems here (Figure S5 and Table S2), those E1/2 values are only used in qualitative manner to show the effect of pyridine pKa under similar conditions on the oxidation peak potentials of (2,6)-dimethoxyphenol. Likewise, addition of pyridines to the solution of studied methoxy substituted phenol linked porphyrins ((4,8)TPP, (4,4)TPP and (1,2)TPP) indeed open up a new reaction pathways of PCET at less positive potential (at lower driving force) which causes electron transfer from H-bonded phenol-pyridine adduct to the adjacent porphyrin macrocycle. Therefore the association of the proton of the attached phenol with the porphyrin is essential for reductive quenching of excited porphyrin. In addition, the decrease in Epox of the phenols on increasing the basicity of the pyridines implicated the increase of rate of quenching. Hence, we propose H-bond induced electron transfer as the reason for dynamic quenching and the quenching of porphyrin fluorescence is due to proton coupled electron transfer (PCET). Therefore, calculated rates of dynamic quenching kq (shown in Table 1) correspond to the second order PCET rate constant for respective porphyrins. Notably, the second order PCET rate constants kq grossly increases with the increase of base strength for respective porphyrins (Table 1). Further, 4-CNPy, is a weak base, which undergoes through very feeble H-bonding interaction with phenol and apparently produces no shift of an electrochemical wave into cathodic direction opening any new proton-coupled oxidation pathways at lower driving force36,37 Thus, reduction of porphyrin macrocycle by phenol remains endergonic process and fluorescence of porphyrin


17


Page 18 of 38

macrocycle remains unaffected by added 4-CNPy. Similarly, oxidation potential of phenol with no methoxy group is too high34 to trigger proton-coupled oxidation pathways upon addition of pyridines even though strong H-bonding with pyridine bases are occurred and fails to reduce the excited porphyrin macrocycles and to quench the fluorescence of porphyrins in (4,0)TPP. However, the cases, when fluorescence quenching are observed and S-V plots are convex in nature, then S-V plots obtained based on fluorescence intensity change (I0/I) and singlet state lifetime change (0/) replicate to each other. The concurrence of fluorescence intensity quenching to the fluorescence lifetime quenching can be rationalized by assuming the formation of ground state complexes/adducts, phenol linked porphyrin to the respective bases, and these ground state H-bonded complexes are weakly fluorescent compared to free porphyrins. In such cases, singlet state lifetime values of these complexes are very similar to that of free porphyrins. Notably, TMPy, which is a stronger base than DMPy, showed dynamic quenching for (4,4)TPP. However, it showed lower quenching rate constant for (4,8)TPP and (1,2)TPP than that of DMPy. These results clearly indicate that the steric hindrance also plays a significant role in fluorescence quenching process i.e. in PCET reactions. In the case studied here, the competition between the radiative rate of porphyrin and the rate of reductive quenching of porphyrins depends coherently on two factors: firstly, the oxidation potential of the H-bonded phenol which in turn depends on substitution of methoxy group on the phenol and secondly, the strength of the H-bonding base (Table 1). Thus the strong reductant, (2,5)-dimethoxyphenol in (1,2)TPP or (4,8)TPP, effectively reduces porphyrin macrocycle when H-bonded to all pyridine derivatives studied except Py, a relatively weak base (Figure 3) and fluorescence of porphyrin macrocycle is quenched. This produces concave S-V plots. However, with weak reductant, monomethoxyphenol in (4,4)TPP, reduction of porphyrin macrocycle upon addition of H-bonding


18

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Table 1. Ground state H-bonding equilibrium constant and static and dynamic quenching parameters of (4,8), (4,4) and (1,2)TPP by studied bases in DCM solvent.

(4, 8)TPP kqa x109, (M-1s-1 )

0.30.1

KSV(0/), (M-1) b

2.10.2d

16.7  2d

12.1  1

1.90.3

2.5  0.3

2.80.3c

43.5 3c

42  2.0

6.50.7

7.4

0.930.2

1.20.2c

35.0  3c

33  2.0

5.10.6

DMAPy 9.7

16  2.0

19.03c

152  10c

62  2.0

9.51.0

0.3 0.1

b

b

7.01d

4.4 0.3

0.6,  0.1 (3.5)

Base

pKa KHB, (M-1)

4-CNPy

----

b

Py

5.2

1.9  0.2

DMPy

6.9

TMPy

fK

HB (M-1)

KSV(I0/I), (M-1)

b

(1, 2)TPP 4-CNPy

----

b

Py

5.2

1.6  0.2

DMPy

6.9

0.95 0.1 -----

15.01

12  1

1.5,  0.2 (4.3)

TMPy

7.4

1.2  0.1

-----

11.01

9.5  1

1.2, 0.2 (4.2)

DMAPy 9.7

5.4  0.5

6.20.5

30.02 c

17  1

2.1,  0.3 (4.5)

0.30 0.1

b

b

1.50.3

(4, 4)TPP 4-CNPy

----

b

Py

5.2

2.8 0.2

-----

0.400.1d

-----

0.07  0.01

DMPy

6.9

2.8 0.3

-----

4.40.2d

-----

0.64  0.07

TMPy

7.4

4.6 0.5

4.60.5d

18.0 2 d

12

d

3.4  0.40

DMAPy 9.7

20 2.0

253.0d

97.05c

57, 70 e

8.0  0.70

a--Values

based on KSV (0/), b -- No quenching, c-- Concave S-V plot ; KSV based on slope at origin of quadratic fit, d-- Convex S-V plot with plateau ; e--KSV based on slope of quadratic fit to initial points, Values in the parenthesis in the last column are the ratio of kPCET { kPCET for (4,8)TPP)/kPCET for (1,2)TPP)} determined for (4,8)TPP and (1,2)TPP by respective bases. f -- KHB obtained from fitting of eq(2)to concave S-V plot up to 80% of quenching and eq(3) to convex S-V plots. Lifetime of free porphyrins (0): (4,8)TPP, 7 ns; (4,4)TPP, 7.14 ns; (1,2)TPP, 7.8ns. Uncertainty arises mainly from the concentration measurement of quencher and it is about 10-20%


19


bases only partially competes with porphyrin fluorescence. This leads to the convex S-V plots and plateaus (Figure 3S), which lie in the order of decreasing base strength from TMPy to Py. For relatively strong base DMAPy, the S-V plot for (4,4)TPP is found to be again concave in nature (Figure 3S). 0.0

0.3

[Q],M

[Q],M-1

-1

0.9 0.0

0.6

0.1

0.2

0.3

(4,8) TPP

(1,2) TPP

Fl.Quenching

Fl.Quenching

0.4

10

0.5 12

Fit, eq(2)

8

I0/I

4

10

6

8

I0/I

4 2

3

0.00

0.05

[Q], M-1

Py DMPy TMPy DMAPy

2

Py DMPy, TMPy, DMAPy,

6 4 2

(4,8) TPP

Linear Fit Linear Fit Linear Fit

Lifetime Quenching

3

/

5

0.15

Py DMPy TMPy DMAPy

1 6

0.10

I0/I

5

0/

4

2

3 2

Py DMPy, TMPy, DMAPy,

(1,2) TPP Lifetime. Quenching

1 0.00

0.12

0.24

0.36 0.00

0.05

[Q],M-1

Linear Fit Linear Fit Linear Fit

0.10

[Q],M

0.15

1

0.20

-1

Figure 3. Stern–Volmer plot based on change in fluorescence intensity (I0/I) (upper panel) and on change in fluorescence lifetime (0/) (lower panel) for (1,2)TPP and (4,8)TPP by different pyridines in DCM. Inset shows the prominent concave S-V plot for (4,8)TPP and dotted lines are fits to eq(2).

5

(I0/I)or 0/

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 20 of 38

4

3

2

1

0.0

(1,2)TPP-Py, (4,8)TPP-Py, (4,4)TPP-TMPy,

0.2

0.4

0.6

Fit eq(3) Fit eq(3) Fit eq(3)

0.8

1.0

[Base],M

Figure 4. Convex S-V plots both based on fluorescence intensity change (I0/I) and fluorescence lifetime change (0/) as function of Py for (1,2)TPP and (4,8)TPP , as function of TMPy for (4,4) TPP in DCM solvent. Dotted lines are corresponding fit of eq(3).


20

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Linear S-V plots based on singlet state lifetime (0/) change (Figure3) conclusively agree to the interpretations of reductive quenching. A closer look in the case of (1,2)TPP where possible multi bonding equilibria are absent, helps to rationalize the nature of non-linear S-V plots more accurately. For strongly bonded DMAPy, the S-V plots based on singlet state lifetime change (0/) have much lower slopes than that of S-V plots based on fluorescence intensity change (I0/I), while for weakly bonded Py, the two curves agree closely. For latter case, the singlet lifetime at the final plateau value is simply singlet lifetime of the H-bonded fluorescent complex. For concave S-V plots, the quenching of fluorescence is considered via static and dynamic pathways by added bases and they are fitted well in accordance with the modified S-V equation as below I0/I ={1+KHB[Q]}{1+kq0[Q]}

(2)

where KHB and kq0 (KSV(0/)) are the H-bonding equilibrium constant and Stern–Volmer constant based on singlet state lifetime quenching respectively, and Q is the concentration of base. Fitting of eq(2) to the concave S-V plots with kq0 generates the KHB values which closely agree to the values obtained from the change in absorption spectra for the respective porphyrins. Similarly, the convex S-V plots are explained considering H-bonded complexes possess fluorescence at the overlapping region of the free porphyrin. Based on this assumption a modified S-V equation is developed (see supporting info for derivation) as shown below: I0/I={1+KHB[Q]}/{1/(1+kq0[Q])+(cI0/I0) KHB[Q]}

(3)

where cI0 is the fluorescence intensity of H-bonded porphyrins, a ground state complex between phenol linked porphyrin and base, and it is estimated to be the fluorescence intensity at plateau region of the convex S-V plots. Fitting of the eq (3) to the convex S-V plots with estimated cI

0values

extract kq, (Table 1),rate constant for dynamic part of the quenching and KHB values,


21


Page 22 of 38

which closely reciprocate to the values obtained using eq (1) to the change in absorption spectra for the respective porphyrins. A typical comparison of the observed convex S-V plot and fit of the eq(3) shown in the figure4 for the case of [(1,2)TPP····Py], [(4,8)TPP····Py] and [(4,4)TPP····TMPy].The

ratios

of

observed

dynamic

quenching

rate

constants

(kPCET(4,8)TPP/kPCET(1,2)TPP) for (4,8)TPP and (1,2)TPP by all the pyridines, when quenching is observed, were found to be very close to 4. This result suggests that the dynamic quenching for these similar systems statistically depend simply on the number of phenol H-bonding sites and the reaction involved here is a physical process. Furthermore, observed quenching of (4,8)TPP by different bases are faster than that of (4,4)TPP, which reflects the effect of methoxy on the phenol oxidation potential. The more the number of methoxy groups added to the phenol, the more decrease in oxidation potential is obvious36. Moreover, replacement of hydroxy by methoxy in (0,12)TPP completely eliminates the fluorescence quenching of porphyrin macrocycle by any of the base studied which substantiates the involvement of proton in quenching reaction leading to PCET reaction. However, a mechanism of stepwise PCET where PT either precedes or follows ET, suggested by Rhile et al.,70,71 can also explain the observation of concave S-V plots in the case of strong bases studied here. In this mechanism, a fraction of adducts presumably lead to ratelimiting pre-equilibrium proton transfer forming phenolate species which very fast transfers electron to porphyrin macrocycle resulting fluorescence quenching, a the first order PCET of the adducts. The accessibility of this quenching is inhibited in TCSPC measurement in our system, which has IRF around 200 ps. Femtosecond time resolved florescence studies could reveal the process in realization wand we plan this studies soon. The other possibility which could be very fast intramolecular electron transfer from phenol to porphyrin macrocycle yielding the phenol


22

Page 23 of 38

radical cation which would rapidly reorganize, (preferably below our IRF ) by proton transfer. However, this possibility is ruled out as the thermodynamic driving force of ET from phenol to porphryrin macrocycle is endergonic in nature. 8

DCM

6

kq(109 M-1 s-1)

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


CH3CN

Cl-Bz DCE

CH3 NO2

4 Ph-CN

2

Propylene Carbonate

CHCl3

CCl4

0 32

34

36

38

40

42

44

46

48

50

ET(30) (kcal/M)

Figure 5. Variation of second order PCET rate constant of (4,8)TPP by DMPy in different solvents as function of ET(30) indices. Dotted line is the guide to the eyes.

Role of solvents on proposed PCET reactions has also been explored for (4,8)TPP with DMPy, a case where second order quenching is predominant over static quenching or first order quenching if any. It has been observed that over 60% quenching of porphyrin fluorescence occurred mostly in dynamic process below 0.05 M concentration of DMPy (Figure 3). However, figure 5 shows the quenching rate of (4,8) TPP by DMPy as a function of Dimroth‐Reichardt ET(30) polarity indices of aprotic solvents covering wide range of solvent polarities. As shown in figure 5, the quenching rates grossly correlate with ET(30) indices, it increases on increasing the ET(30 ) values. In general, the rate of electron transfer is enhanced on increasing solvent polarity by lowering the thermodynamic driving force, whereas H-bonding equilibrium constant decreases on increasing solvent polarity. Since the quenching process engages with both electron transfer coupled with proton movement, effect of solvents in such two situations is critical composite of different events. These effects arising not only from differences in oxidation


23


potentials but also different reorganization energies associated with various proton-acceptor configurations. In addition, solvent specific solute-solvent interactions for studied porphyrins and added pyridines and nonspecifice solvation of the phenol-pyridine pairs, solvent segregation and local polarity by the excess of pyridines can modulate the second order PCET rate constants. 4.0

(4,8)TPP( with phenol) + 0.1% CH3OH (4,8)TPP (with deuterated phenol) + 0.1% CH3OD

3.5 3.0

0/

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 24 of 38

2.5 2.0 1.5 (1,2)TPP(with phenol) + 0.1% CH3OH

1.0

(1,2)TPP(with deuterated phenol) + 0.1% CH3OD

0.0

0.1

0.2

0.3

0.4

[Py], M

Figure 6. Dynamic isotope effect on fluorescence quenching of (1,2)TPP and (4,8)TPP by Py. Comparative plots of 0/ vs pyridine concentration for (1,2)TPP and (4,8)TPP and their respective deuterated phenol counterpart.

In order to establish the role of proton movement to the quenching of porphyrin fluorescence, electron transfer, the typical effect of deuteration in dynamic quenching of (4,8)TPP and (1,2)TPP by Py were observed, upon replacing the OH group of phenol by OD, This reaction was performed by carrying out the substitution reaction on phenolic groups using CH3OD. The large molar excess of CH3OD induced high isotopic enrichment at the exchangeable OH position. After that, the solution was dried and fresh solutions of as-prepared deuterated porphyrins were made by adding required amount of DCM. To eliminate the effect of residual proton content in deuterated methanol, 0.1% CH3OH was added to the deuterated porphyrin solution, as well as in the normal porphyrin solutions of the respective porphyrins. Despite the high value of kq, a clear positive isotope effect was observed. Figure 6, shows the comparative S-V plots as function of Py concentration for deuterated phenol linked porphyrin


24

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and phenol linked porphyrin. Deuterated phenol linked porphyrin shows much lower quenching than that of simple phenol linked porphyrin. The ratio of rate constants (kq (H)/kq (D) were found to be 1.7 and 2,2 for (1,2)TPP and 2.2 for (4,8)TPP), respectively. These results establish the fact of involvement of concerted proton movement in the quenching process of PCET 36,72 and may also affirm the reaction avoids high-energy intermediates, although it is debateable

70,71,

of

sequential movement of proton and electron transfer in the quenching process. Much further work specially ultrafast transient spectroscopic studies are needed to identify the reaction products to distinguish more clearly the involved reaction pathways on these prototype systems. Conclusion In conclusion, using simple spectroscopic methods we were able to delineate that Hbonded phenols covalently linked to porphyrins quenches fluorescence of the porphyrin macrocycle by both static and dynamic pathways. The effect increased with the base strength of the H-bonded pyridines and number of methoxy groups attached to the phenol rings. The reaction involved for dynamic part of the quenching was identified as PCET, where electron transfer from H-bonded phenol to the excited singlet porphyrin macrocycle was coupled with the movement of the bound proton to the pyridine. Such movement of the proton lowered phenol oxidation potential was consistent with electron movement. Although, described qualitative results strongly implicate the reaction involved here is concerted-PCET, further studies are needed to establish it firmly. Critical detection of the PCET reaction products and time scale of the reaction for such systems require ultrafast transient absorption studies which will be planned in the near future. However, an important instance of this effect occurs in photosynthesis, where tyrosine, which is H-bonded to histidine, participates in the electron transfer chain of


25


Page 26 of 38

Photosystem-II. Hence, demonstrated results will aid in understanding of PCET reactions in biological processes and other systems as well.

Conflicts of interest “There are no conflicts to declare”. ASSOCIATED CONTENT SUPPORTING INFORMATION Mataga-Tsuno plot for (4,8)TPP, H-bonding equilibrium constants of free phenol derivatives, Fluorescence quenching profiles, S-V Plots for (4,4)TPP as function of different bases concentration, Electrochemical data for phenol in absence and presence of pyridines, Calculated (I0/I) plot for convex and concave S-V plots, Derivation for equation (3).The following files are available free of charge. See DOI: 10.1039/x0xx00000x AUTHOR INFORMATION Corresponding Author: Prakriti Ranjan Bangal, Email: [email protected] ACKNOWLEDGEMENTS The synthetic porphyrins were prepared by Prof. Jonathan S. Lindsey and co-workers for a collaborative project with Prof. Henry Linschitz under support by a grant from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, of the U.S. Department of Energy (DE-FG02-05ER15661).We thank Profs. J. S Lindsey and Henry Linschitz for the gift of samples and for the perception of this work. PRB acknowledges L. Biczók, and N. Gupta towards initiation of PCET reaction. PRB also acknowledges the support from the Department of Science and Technology, Government of India, Grant No.


26

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EMR/2014/000441 and MV thanks Council of Scientific and Industrial Research, Government of India for providing CSIR-SRF fellowship. Manuscript communication no. IICT/pubs./2019/169.

REFERENCES 1. Keough, J.M.; Jenson, D.L.; Zuniga, A. N.; Barry, B.A. Proton Coupled Electron Transfer and Redox-Active Tyrosine Z in the Photosynthetic Oxygen-Evolving Complex. J. Am. Chem. Soc. 2011, 133, 11084–11087. 2. Weinberg, D. R.; Gagliardi, C. J.; Hull, J.F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev.2012,112, 7, 4016-4093. 3. Pappas,I.; Chirik, P. J.; Catalytic Proton Coupled Electron Transfer from Metal Hydrides to Titanocene Amides, Hydrazides and Imides: Determination of Thermodynamic Parameters Relevant to Nitrogen Fixation. J. Am. Chem. Soc. 2016, 138, 13379−13389. 4. Horvath, S.; Fernandez, L. E.; Soudackov, A. V.; Hammes-Schiffer, S.; Insights into Proton-coupled Electron Transfer Mechanisms of Electrocatalytic H2 Oxidation and Production. Proc. Natl. Acad. Sci.2012,39, 15663–15668. 5. Kaila, V. R. I.; Verkhovsky, M. I.; Wikström,M. Proton-Coupled Electron Transfer in Cytochrome Oxidase. Chem. Rev. 2010, 110, 7062–7081.


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Excited State Quenching of Porphyrins by Hydrogen-Bonded Phenol

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