A Diprotonated Porphyrin as an Electron Mediator in Photoinduced

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C: Energy Conversion and Storage; Energy and Charge Transport

A Diprotonated Porphyrin as an Electron Mediator in Photoinduced Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies Wataru Suzuki, Hiroaki Kotani, Tomoya Ishizuka, Masaki Kawano, Hayato Sakai, Taku Hasobe, Kei Ohkubo, Shunichi Fukuzumi, and Takahiko Kojima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02449 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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The Journal of Physical Chemistry

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conformation due to the steric repulsion of peripheral phenyl groups. Therefore, H2DPP can be easily protonated by stoichiometric amount of weak acids such as carboxylic acids to form diprotonated one (H4DPP2+) derived from high basicity of inner imine N atoms due to the out-of-plane lone pairs, resulting the formation of hydrogen-bonded supramolecular assemblies without introducing hydrogen-bonding sites at the periphery.41-47 So far, hydrogen-bonded supramolecular assemblies based on H2DPP with carboxylic acids such as redox-active ferrocene carboxylic acid (FcCOOH) to form a photofunctional homo-triad (H4DPP2+(FcCOO–)2) have been reported.41 Due to the high reduction potential of diprotonated H2DPP (H4DPP2+), H4DPP2+ can act as an electron acceptor in photoinduced ET to form one-electron reduced H4DPP2+ (H4DPP•+) with the lifetime of pico-seconds. (Figure 1b).41,42,44 While porphyrins act as both electron donors and acceptors to form radical species in hydrogen-bonded supramolecular assemblies, no example of utilization of porphyrins as an electron mediator from an electron donor to an electron acceptor in hydrogen-bonded supramolecular assemblies: In other words, no report on utilization of these photogenerated radical species into subsequent ET reactions (Figure 1c). Given that porphyrins act as an electron mediator in supramolecular assemblies, generation of long-lived charge-separated or ET states as well as application to photocatalysis are expected through stepwise ET in supramolecular hetero-triads. Herein, we report the construction of hydrogen-bonded supramolecular assemblies between H4DPP2+ and an electron accepting molecular unit to utilize one electron reduced H4DPP2+ (H4DPP•+), which can be generated in the course of photoinduced ET reactions, as an electron donor. As an electron donor, a RuII-polypyridyl complex bearing a carboxyl group at the periphery (RuIICOOH) has been selected to form a hydrogen-bonded supramolecular assembly with H4DPP2+. In addition to the electron-donating ability of RuIICOOH, the complex would also be the precursor of an oxidation catalyst in supramolecular hetero-triads enabling photocatalytic oxidation reactions.48,49 On the other hand, benzyl viologen (BV2+) derivatives have been employed as an electron acceptor because of the high reduction potentials.50 Note that the one-electron reduced species (BV•+) is relatively stable,51 enabling to form a long-lived ET state in supramolecules. In addition, viologen derivatives have been often used as electron mediators in photocatalytic reduction reactions.52-55 Thus, we have employed a BV2+ derivative having a carboxyl group (BV2+COOH) as an electron acceptor in a hydrogen-bonded supramolecular assembly with H2DPP, H4DPP2+(BV2+COO–)2. We have examined the photodynamics of H4DPP2+(BV2+COO–)2 to confirm that H4DPP•+ acts as an electron mediator in the hydrogen-bonded supramolecule. RESULTS AND DISCUSSION Synthesis and Characterization of a Ru(II)-polypyridyl Complex and a Benzyl Viologen Derivative Bearing a Carboxylic Group. A RuII-polypyridyl complex, [RuIICl(TPA)(pyCOOH)](ClO4–) (RuIICOOH(ClO4–), TPA = tris(2-pyridylmethyl)amine, pyCOOH = 4-pyridinecarboxylic acid) was synthesized from the reaction of [RuII(µ-Cl)(TPA)]2(ClO4–)256

Scheme 1. Synthesis of RuIICOOH(ClO4–) (ClO4Ð)

2+ (ClO4Ð)2 N N N

RuII

N

RuII N

Cl N

N

N Cl

N

pyCOOH MeOH, reflux

N 2

N

RuII N

N Cl

COOH [RuII(µ-Cl)(TPA)]2(ClO4)2

RuIICOOH(ClO4)

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Figure 2. (a) An ORTEP drawing of RuIICOOH(ClO4–). Hydrogen atoms except for that attached to the carboxyl oxygen atom (O2) were omitted for clarity. (b) Intermolecular hydrogen bonding (blue dotted lines) between O2-H and Cl of RuIICOOH(ClO4–) to form a dimeric structure.

with pyCOOH in MeOH with 38% yield (Scheme 1).57 RuIICOOH(ClO4–) was characterized by 1H NMR and UV-Vis spectroscopy, ESI-TOF-MS, and elemental analysis. In the 1H NMR spectrum of RuIICOOH in acetone-d6 (Figure S1 in Supporting Information), two doublets derived from the pyCOOH ligand were observed at 7.69 and 8.47 ppm. The coordination geometry of RuIICOOH was confirmed by measurements of a nuclear Overhauser effect (NOE); the NOE was clearly observed between the 1H NMR signals assigned to the protons at the 6-positions of the equatorial pyridine moieties of TPA (9.17 ppm) and those at the 2-position of pyCOOH (8.47 ppm, Figure S2). This result suggests that pyCOOH coordinates to the RuII center at the trans position of the axial pyridine ring of the TPA ligand as shown in Scheme 1. ESI-TOF-MS of RuIICOOH in MeOH showed a peak cluster at m/z = 572.04, whose isotropic pattern was consistent with the computer-simulated one for [RuIICl(TPA)(pyCOONa)]+ (calcd: m/z = 572.04, Figure S3a). In the UV-Vis spectrum of RuIICOOH in acetone, an absorption band was observed at 436 nm, assignable to a ligand-to-metal charge-transfer (LMCT) band due to CT from the chloro ligand to the RuII center on the basis of literature (Figure S3b).58 X-ray crystallographic analysis of RuIICOOH(ClO4–) was conducted using a single crystal of RuIICOOH(ClO4–), which was obtained from a vapor diffusion of diethyl ether into an acetone solution of RuIICOOH(ClO4–). As shown in Figure 2a, the crystal structure of RuIICOOH(ClO4–) revealed that pyCOOH coordinated to the RuII center at the cis position of the tertiary amino nitrogen atom of the TPA ligand, that is, the trans position of the axial pyridine moiety. The coordination geometry in the crystal structure is consistent with the argument on the 1H NMR spectrum of the complex. The position of hydrogen atom attached to the carboxyl oxygen atom was determined in light of the packing structure of RuIICOOH(ClO4–): As shown in Figure 2b, the complex forms a hydrogen-bonding dimer in the crystal with the distance of 3.031(7) Å between O2 atom of one RuIICOOH(ClO4–) molecule and the chloro ligand of another RuIICOOH(ClO4–). To determine the pKa value of the appended carboxyl group of RuIICOOH(ClO4–), UV-vis spectroscopic titration of the complex was performed with an NaOH aqueous solution (NaOHaq) in Briton-Robinson buffer at 298 K (Figure S4). Upon addition of NaOHaq, the LMCT band of RuIICOOH(ClO4–) (414 nm at pH 1.52) slightly decreased together with a blue shift to 409 nm at pH 4.21 (Figure S4a). Then, the pKa value of the carboxyl group of RuIICOOH(ClO4–) was determined to be 2.18 ± 0.03 from the titration curve based on the absorbance change at 414 nm vs pH values (Figure S4b). The pKa value is low enough to diprotonate H2DPP to form H4DPP2+ in acetone quantitatively, judging from the result of quantitative diprotonation of H2DPP by meta-nitrobenzoic acid (pKa = 3.4 in H2O).47,59 In electrochemical measurements on RuIICOOH(ClO4–) in an acetone solution containing 0.1 M TBAPF6


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(2)

Ð!GBET = e(Eox Ð Ered)

(3)

Intrasupramolecular photoinduced ET occurs from the ground state of RuII centers to 1[H4DPP2+]* in H4DPP2+(RuIICOO–)2 to form the ET state, H4DPP•+(RuIIICOO–)(RuIICOO–), and BET proceeds to afford the ground state of H4DPP2+ rather than the triplet excited state (3[H4DPP2+]*); because the energy level of the ET state (1.13 eV) is lower than that of 3[H4DPP2+]* (1.46 eV). The lifetime ($) of the ET state (150 ps) is comparable with that of H4DPP2+(FcPhCOO–)2 ($ = 150 ps in PhCN, FcPhCOO– = 4-ferrocenebenzoate), which has almost the same donor-acceptor distance (10.1 Å from DFT calculations)41 with that of H4DPP2+(RuIICOO–)2 (9.501 Å from crystal structure of [H4DPP2+(Cl–)(RuIICOO–)](ClO4–) in Figure 5). Therefore, photoinduced ET properties of H4DPP2+(RuIICOO–)2 is concluded to be similar to that of hydrogen-bonded donor-acceptor supramolecular assemblies based on H4DPP2+ with ferrocene derivatives.41 Photophysical and ET properties of H4DPP2+(BV2+COO–)2. To examine the redox properties of H4DPP2+(BV2+COO–)2, CV and DPV measurements on H2DPP were performed in the presence of 2 equivalents of BV2+COOH in deaerated acetone in the presence of 0.1 M TBAPF6 as an electrolyte. As shown in Figure S14, a reversible redox wave derived from one-electron reduction of the BV2+COO– moiety in H4DPP2+(BV2+COO–)2 was observed at –0.75 V vs Fc/Fc+, which was the same with that of free BV2+COOH (Ered = –0.75 V vs Fc/Fc+ in Figure 3). On the other hand, an irreversible redox wave derived from one-electron reduction of H4DPP2+ was observed at –1.01 V vs Fc/Fc+ in contrast to the case of H4DPP2+(RuIICOO–)2, in which a quasi-reversible two-electron redox wave of H4DPP2+ was observed at –0.99 V vs Fc/Fc+ (Figure 7). This irreversible one-electron reduction of H4DPP2+ in H4DPP2+(BV2+COO–)2 suggests that one-electron reduced H4DPP2+ (H4DPP•+) would reduce free BV2+COO– through intermolecular ET in the CV time scale. Thus, judging from the higher reduction potential of BV2+COO– moieties than that of H4DPP2+, intrasupramolecular ET from H4DPP•+ to BV2+COO– should occur in H4DPP2+(BV2+COO–)2. Fluorescence spectrum of H4DPP2+(BV2+COO–)2 showed emission maxima at 779 nm (Figure S15a) in acetone, which was slightly red-shifted compared with H4DPP2+(CF3COO–)2 (Figure S12). From the absorption and emission maxima of H4DPP2+(BV2+COO–)2 (733 and 779 nm, respectively), 1[H4DPP2+]* in H4DPP2+(BV2+COO–)2 was determined to be 1.64 eV, comparable with that in H4DPP2+(RuIICOO–)2 (1.65 eV). Phosphorescence spectrum of H4DPP2+(BV2+COO–)2 in 2-MeTHF showed a shoulder around 850 nm (Figure S15b). Thus, the energy level of 3[H4DPP2+]* in H4DPP2+(BV2+COO–)2 was determined to be 1.46 eV, which was identical to that of H4DPP2+(RuIICOO–)2.

(a)

(b) 0.05

0.05 0.04

ÐÐ 0.8 ns ÐÐ 6.4 ns

560

!Abs at 560 nm

Ð!GET = Ðe(Eox Ð Ered) + 1E*

Next, we examined intrasupramolecular ET from H4DPP•+ that was formed by intermolecular ET from an external electron donor to BV2+COO– in H4DPP2+(BV2+COO–)2 in acetone at 298 K. In the absence of any external electron donors, the formation and the decay of 3 [H4DPP2+]* in H4DPP2+(BV2+COO–)2 were only observed by time-resolved ns-transient absorption spectra of H4DPP2+(BV2+COO–)2 in deaerated acetone, indicating that no photoinduced electron transfer from 3[H4DPP2+]* to BV2+COO– occurred and vice vasa (Figure S16). From the decay at 560 nm derived from 3[H4DPP2+]*, the decay rate constant (kT) of 3[H4DPP2+]* in H4DPP2+(BV2+COO–)2 was determined to be (1.73 ± 0.01) # 104 s–1 (Figure S16b), which was comparable with that of 3[H4DPP2+]* in H4DPP2+(CF3COO–)2 (kT = (1.60 ± 0.05) # 104 s–1) in MeCN (Figure S17). On the other hand, in the presence of decamethylferrocene (Me10Fc) as an external electron donor (Eox = –0.49 V vs Fc/Fc+ in acetone), the decay at 560 nm derived from 3[H4DPP2+]* in H4DPP2+(BV2+COO–)2 was accelerated with increasing the concentration of Me10Fc as observed in time-resolved ps-transient absorption spectra (Figure 9a,b). By changing the concentration of Me10Fc, the rate constant (ket) of ET from Me10Fc to 3[H4DPP2+]* in H4DPP2+(BV2+COO–)2 was determined to be (5.60 ± 0.01) # 109 M–1 s–1 (Figure 9c, blue circle), which was comparable to that ((6.86 ± 0.01) # 109 M–1 s–1) from Me10Fc to 3[H4DPP2+]* in H4DPP2+(CF3COO–)2 (Figure 9c red circle). Finally, the transient absorption spectrum showing the absorption maximum around 610 nm was observed at 6.4 ns after laser excitation at 532 nm (Figure 9a), clearly different from those in the absence of Me10Fc (Figure S16). This spectrum showed a good agreement with that of one-electron reduced BV2+COO–(BV•+COO–), which was generated by one-electron reduction of BV2+COOH with 1 equivalent of Na2S2O4 aqueous solution in acetone (Figure S18). Therefore, these spectral changes suggested intermolecular ET from Me10Fc to 3[H4DPP2+]* to form H4DPP•+, following by faster intrasupramolecular ET from H4DPP•+ to BV2+COO– to afford H4DPP2+ and BV•+COO–. In addition, the lifetime of BV•+COO– was determined to be 1.1 ms from the decay of 610 nm in transition absorption spectra of H4DPP2+(BV2+COO–)2 in the presence of Me10Fc (Figure S19). The energy diagram of photoinduced ET system of hydrogen-bonded

0.03 0.02 0.01

610

0 Ð0.01 500

600

0.03 0.02 0.01 0

700

¥ 0.2 mM ¥ 0.4 mM ¥ 0.6 mM ¥ 0.8 mM ¥ 1.0 mM

0.04

0

1

Wavelength, nm

2 3 Time, µs

4

5

(c) 2.0

kobs, 107 sÐ1

and the lifetime of electron-transfer state ($) was determined to be (1.60 ± 0.06) # 1011 s–1, (6.7 ± 0.9) # 109 s–1, and 150 ps, respectively, based on the time profile of the absorbance at 1040 nm (Figure 8b). The energy diagram of intrasupramolecular photoinduced ET and BET in hydrogen-bonded H4DPP2+(RuIICOO–)2 in acetone is depicted in Scheme 3. The driving forces of photoinduced ET (–!GET, eV) and BET (–!GBET, eV) were determined from the energy level of the ET state of H4DPP2+(RuIICOO–)2 (1.13 eV) and the energy level of 1[H4DPP2+]* using eq 2 and eq 3, where e is the elementary charge, Eox is the one-electron oxidation potential of an electron donor that is RuIICOO– in this case, Ered is the reduction potential of an electron acceptor that is H4DPP2+ in this case, and 1E * is the excitation energy of the singlet excited state of H4DPP2+.

!Abs

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¥ CF3COOH ¥ BV2+COOH

1.5 1.0 0.5 0

0

0.5

1.0

1.5

2.0

[Me10Fc], mM

Figure 9. (a) Transient absorption spectra (%&x = 532 nm) of H4DPP2+(BV2+COO–)2 (0.20 mM) with Me10Fc (0.60 mM) in deaerated acetone at 0.8 ns (red), and 6.4 ns (blue) after picosecond laser excitation at 532 nm. (b) Decays time profiles of 3[H4DPP2+]* monitored at 560 nm with single exponential decay curve fitting: [H2DPP] = 2.0 # 10–4 M, [BV2+COOH] = 8.0 # 10–4 M. (c) Plots of kobs vs [Me10Fc] for H4DPP2+(BV2+COO–)2 (blue) and H4DPP2+(CF3COO–)2 (red).


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Scheme 4. Energy Diagram of H4DPP2+(BV2+COO–)2 in Acetone. 1[H DPP2+(BV2+COOÐ) ]* 4 2

+ Me10Fc

1.65 eV ISC

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details and analytical data, including Figures S1-S20 and Table S1 (PDF), X-ray crystallographic data for RuIICOOH(ClO4–) and [H4DPP2+(Cl–)(RuIICOO–)](ClO4–) (CIF).

3[H DPP2+(BV2+COOÐ) ]* 4 2

+ Me10Fc

Energy

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


1.46 eV

AUTHOR INFORMATION

Intermolecular ET

H4DPP¥+(BV2+COOÐ)2 + Me10Fc+ h!

0.52 eV Intrasupramolecular ET (fast)

H4DPP2+(BV2+COOÐ)(BV¥+COOÐ) Intrasupramolecular BET " = 1.1 ms

+

[email protected]

Me10Fc+ 0.34 eV

H4DPP2+(BV2+COOÐ)2 + Me10Fc

H4DPP2+(BV2+COO–)2 with Me10Fc in acetone is depicted in Scheme 4. In Scheme 4, the energy levels of each state were calculated on the basis of the emission and UV-Vis absorption spectral data and redox potentials of the components by using eq 2 and eq 3. To confirm the requirement of association of H4DPP2+ with BV2+COO– for generation of the benzyl viologen radical cation (BV•+) through photoinduced electron transfer from H4DPP•+ to the benzyl violgen derivative, ns-transient spectra of H4DPP2+ with Me10Fc were measured in the presence of CF3COOH as a proton source and BV2+COOMe as a terminal electron acceptor, respectively. As shown in Figure S20, no absorption band around 610 nm due to BV•+COOMe was observed, while the decay of 3[H4DPP2+]* at 560 nm was accelerated upon addition of Me10Fc. This spectral change clearly indicates that no intermolecular ET from H4DPP•+ to BV2+COOMe occurred under the reaction conditions. Therefore, the hydrogen bonding between H4DPP2+ and BV2+COO– to form H4DPP2+(BV2+COO–)2 should be essential to generate H4DPP2+(BV2+COO–)(BV•+COO–) in the presence of external electron donors. CONCLUSION In conclusion, we have described successful construction of hydrogen-bonded supramolecular assemblies based on H4DPP2+ with redox-active molecules such as a RuII-polypyridyl complex (RuIICOOH) as an electron donor and a benzyl viologen derivative (BV2+COOH) as an electron acceptor. Formation of supramolecular homo-triads, H4DPP2+(RuIICOO–)2, H4DPP2+(BV2+COO–)2, were confirmed by spectroscopic measurements in acetone and the crystallographic analysis. Photoinduced ET reactions in the supramolecular assemblies have been also scrutinized by using laser flash photolysis. In H4DPP2+(RuIICOO–)2, intrasupramolecular ET from the RuII center of RuIICOO– to the singlet excited state of H4DPP2+ to afford the ET state involving one-electron reduced H4DPP2+ (H4DPP•+) with lifetime of 150 ps. On the other hand, in the presence of an external electron donor, intermolecular ET from the electron donor to the triplet excited state of H4DPP2+ in H4DPP2+(BV2+COO–)2 to form H4DPP•+(BV2+COO–)2. In addition, H4DPP•+ was able to reduce the hydrogen-bonded BV2+COO– through intrasupramolecular ET to afford one-electron reduced BV2+COO– (BV•+COO–), which exhibited relatively long lifetime. This is the first example of utilization of porphyrins as an electron mediator in hydrogen-bonded supramolecular assemblies. The role of an electron mediator of H4DPP2+ in supramolecules are expected to be applied to charge-separation systems to achieve the formation of long-lived electron-transfer states in supramolecular hetero-triads and to perform photocatalytic reductions such as photocatalytic hydrogen evolution. SUPPORTING IMFORMATION

Corresponding Author

ACKNOWLEDGMENTS This work was supported by Grant-in-Aid (17H03027 and 18J12184) from the Japan Society of Promotion of Science (JSPS, MEXT) of Japan. Financial supports through CREST (JST) are also appreciated (JPMJCR16P1).

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