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Acceleration and Stabilization of Electron Transfer Products with Improved Quantum Yields upon Cation Binding to a Fused Bis-Zinc Porphyrin-Quinone Donor-Acceptor Conjugate Michael B. Thomas, Yi Hu, Wenqian Shan, Karl M. Kadish, Hong Wang, and Francis D'Souza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06764 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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The Journal of Physical Chemistry

Acceleration and Stabilization of Electron Transfer Products with Improved Quantum Yields upon Cation Binding to a Fused BisZinc Porphyrin-Quinone Donor-Acceptor Conjugate Michael B. Thomas,a Yi Hu,a Wenqian Shan,b Karl M. Kadish,b* Hong Wang,a* Francis D’Souzaa* aDepartment

of Chemistry, University of North Texas, 1155 Union Circle #305070, Denton, TX

76203-5017, USA bDepartment

of Chemistry, University of Houston, Houston, TX 77204, USA

1 ACS Paragon Plus Environment

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

ABSTRACT: The role of metal ions in promoting electron transfer processes in biological coenzymes is of great significance not only to understand their mechanistic details but also due to their direct utilization in a number of chemical and biochemical processes. Here, we demonstrate an increased rate of forward electron transfer with improved reaction quantum yield, and an enhanced lifetime of electron transfer products upon cation binding to the acceptor entity in a donor-acceptor conjugate. Selective binding of cations such as Mg2+ or Sc3+ to the electron acceptor, quinone, in a fused, bis-zinc porphyrin-quinone donor-acceptor conjugate leads to facile reduction of the quinone making the electron transfer process thermodynamically highly feasible. Furthermore, the binding of metal ions to the electron transfer product, quinone anion, decelerates the reverse electron transfer process due to ion-pairing interactions, resulting in improving both the lifetime and quantum yields of the electron transfer products. Systematic studies including spectral, electrochemical, computational, spectroelectrochemical, and transient absorption techniques, coupled with data analysis by Global Target Analysis are performed to demonstrate these novel findings that are relevant to further our understanding of electron transfer processes occurring in biological coenzymes and pertinent energy harvesting schemes.


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INTRODUCTION The significance of bound metal ion governed electron transfer processes in biological coenzymes such as NADH, flavins, quinones, and biological redox processes of photosynthesis and respiration are well known.1-10 Metal ion binding to electron acceptors modulates redox potentials making them easier to reduce, thus improving the overall electron acceptor capability. In a number of electron transfer systems, it has been shown that the driving force of electron transfer is tuned by complexation of radical anions– produced in electron transfer reactions – with metal ions that act as Lewis acids.11-12 In the area of light energy harvesting, the generation of long-lived electron transfer products during the process of photoinduced electron transfer (PET) is indispensable for converting light energy into other forms as observed in photosynthesis,13-14 solar cells,15-16 and photocatalysis for fuel production.17-19 In plant photosynthesis, photoexcitation of ‘special pair’ chlorophylls in a well-organized donor-acceptor (DA) sequence in photosystem II triggers a series of electron transfer (ET) reactions in which the rates and direction of ET are governed by metal ions bound to quinones.20-23 This sequential electron transfer affords the long-lived ion-pairs which are involved in the oxidation of water. A large number of DA systems have been studied to attain long-lived ion-pairs through multistep ET.24-33 However, to avoid the energy loss encountered in multi-step processes, directly linked high-potential DA systems are desired.34

O

N N Zn N N

N N Zn N

O

N

1 Figure 1. Structure of the quinone-fused, bis zinc porphyrin donor-acceptor system, 1.



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Positive identification of the ET products is one of the most important issues encountered in PET reactions. Temporal analysis of such signals provides information on kinetics of forward and reverse ET.24-34 Spectra of spectroelectrochemically generated cations and anions are often used in such analysis. Although results secured from this technique work best for DA systems revealing minimal intramolecular interactions, complex spectra are often obtained for strongly interacting DA systems making spectral analysis tedious. Having a better approach to analyze spectral data related to ET products in strongly interacting systems would definitively help in understanding the kinetic and mechanistic aspects of ET in DA systems. Mn+

O

O

O Mn+

Mn+

(1)

O

O

Mn+

eO

-

O

-

O

Mn+

Mn+

O

Mn+

(2) O

Scheme 1. Binding modes of metal ion to neutral and one-electron reduced quinone. In reaction 2, for the sake of simplicity, the negative charge is assumed to be predominantly on one of the oxygen atoms although electron delocalization could occur over the entire molecule while still showing affinity for metal cations. In the present study, we tackle the aforementioned issues with the help of a quinonefused, bis-zinc porphyrin system, 135 (Figure 1). Unlike covalently linked porphyrinquinone systems in the literature that are not fused (mostly through a single covalent bond),13 the fused-quinone in 1 is a relatively poor electron acceptor that could be made into a better electron acceptor through metal ion binding.

Additionally, we show

manipulation of spectroelectrochemical data to deduce the species associated spectrum (SAS, generated from target analysis of transient spectral data, vide infra) of the ET


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products and the overall effect of metal ion binding in stabilizing the electron tranfer products. The two modes of redox inactive metal ion binding to quinones are shown in Scheme 1.10,36 Upto two metal ions can bind to the neutral quinone via Lewis acid-base interactions (route 1) or electrostatically to the reduced product of ET, a quinone anion radical (route 2). While the former type of interaction would increase the electron affinity of the quinone, the latter type would stabilize the anion radical due to an ion-pairing effect. In a DA conjugate undergoing PET, occurrence of the interactions shown in route 1 would facilitate the ET process while the interactions shown in route 2 would stabilize the ET product. If both processes occur, accelerated forward ET followed by slower reverse ET could be envisioned. As demonstrated here, both processes 1 and 2 seem to be operative in the investigated porphyrin-quinone dyad 1 in the presence of metal cations.

EXPERIMENTAL SECTION Chemicals. All the reagents were from Aldrich Chemicals while the bulk solvents utilized in the syntheses were from Fischer Chemicals.

The tetra-n-butylammonium

hexafluorophosphate (TBA)PF6 used in electrochemical studies was from Fluka Chemicals while magnesium perchlorate, Mg(ClO4)2 and scandium triflate used as cation sources were from Aldrich chemicals. Instruments: The UV-visible spectral measurements were carried out with a Shimadzu Model 2550 double monochromator UV-visible spectrophotometer. The fluorescence emission was monitored by using a Horiba Yvon Nanolog coupled with timecorrelated single photon counting with nanoLED excitation sources. A right angle detection method was used. Differential pulse and cyclic voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer using a three electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode

and

an

Ag/AgCl

electrode

was

used


as

the

reference

electrode.


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Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements using argon gas. The computational calculations were performed with the use of GAUSSIAN 09 software package.37 The 1H NMR spectra were recorded using a 400 MHz spectrometer. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane using residual protonated solvent as an internal standard CDCl3, 7.26 ppm. The 1H NMR splitting patterns have been described as “s, singlet; d, doublet; t, triplet and m, multiplet”. HRMS was recorded on TOF-Q mass spectrometer. Spectroelectrochemical study was performed by using a cell assembly (SEC-C) supplied by ALS Co., Ltd. (Tokyo, Japan). This assembly comprised of a Pt counter electrode, a 6 mm Pt Gauze working electrode, and an Ag/AgCl reference electrode in a 1.0 mm path length quartz cell. The optical transmission was limited to 6 mm covering the Pt Gauze working electrode. Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.5 W. For optical detection, a Helios transient absorption spectrometer provided by Ultrafast Systems LLC coupled with an optical parametric amplifier (OPA) provided by Light Conversion was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.5 W, pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental output of the laser was introduced into the OPA, while the rest of the output was used for generation of the white light continuum. In the present study, the maximum absorption wavelength for each compound was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems for initial analysis and later by advanced software Glotaran38 for target analysis. measurements were conducted in degassed solutions at 298 K. 6 ACS Paragon Plus Environment

All

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Synthesis of 1: Compound 1 was synthesized according to previous published method according to Scheme 1 with few modifications.35 The Aldo condensation condition was optimized as following:

N N

Zn N

CHO N

O

N

i) N

CHO

Zn

N

N

N

N

O

Zn

N

N

1

Figure 1. Synthesis of compound 1. Reaction condition: KOH/MeOH, 1,4cyclohexanedione, THF, R.T., 12 h, 75%. Diformylbenzoporphyrin (130 mg, 0.14 mmol) and 1,4-cyclohexanedione (7.6 mg, 0.07 mmol) were dissolved in dry THF (6 mL). The reaction flask was purged with argon, then KOH/MeOH (0.3mL, 0.15 mol/L) was added to the flask. The mixture was stirred at room temperature for 12 h. Then the reaction mixture was washed with water and extracted with CHCl3. The organic layer was separated and the solvent was removed under reduced pressure. The resulting residue was recrystallized using CH2Cl2/MeOH twice to afford pure porphyrin compound 1. The compound was freshly purified over silica gel column prior to performing the experiments (see supporting information, SI for characterization details). The purified compound was stored in dark to avoid any unnecessary photodecomposition. (Note: In previous publication,35 the quality of the 1H NMR spectrum was not good due to low solubility. The problem was solved in this work and a better 1H NMR was obtained using CDCl3 with 1 drop of d5-Pyridine). RESULTS AND DISCUSSION The absorption spectrum of 1 in benzonitrile revealed a split Soret band (436 and 522 nm) and visible bands (600 and 630 nm) accompanied by significant spectral broadening because of strongly interacting -rings (Figure 2a). This is in contrast to 7 ACS Paragon Plus Environment


monomeric porphyrins, such as zinc tetratolylporphyrin and -(benzo-bis-aldehyde) fused zinc porphyrin (see Figure S1a and 1b for structures and absorption spectra) where spectral features were comprised of an intense Soret and two visible bands. In agreement with the literature where strong fluorescence quenching in porphyrin-quinone dyads are reported,13, 39-40

compound 1 was also found to be weakly fluorescent exhibiting broad emission in the

590-800 nm range. The broad emission was a consequence of fusion of benzene ring at one of the pyrrole rings (Figure S1c). The fluorescence lifetime determined using the TCSPC technique was found to be 0.42 ns, significantly smaller than that of the control compounds, zinc tetratolylporphyrin and -(benzo-bis-aldehyde) fused zinc porphyrin, being 1.68 and 1.48 ns, respectively (see Figure S1d for fluorescence decay profiles),34 suggesting the occurrence of excited state events in 1.

Figure 2. (a) Absorption spectrum, (b) fluorescence spectrum (ex = 434 nm), and (c) DPVs of 1 in the absence (i) and presence (ii) of 2 equivalents of Mg2+ in benzonitrile. MgClO4 was used as Mg2+ source and 0.1 M (TBA)ClO4 was used as supporting electrolyte in DPV measurements. 8 ACS Paragon Plus Environment

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Addition of commonly used metal ions for this purpose,11 Mg2+ or Sc3+, to the solution of 1 revealed noticeable changes (see Figure 2a for Mg2+ and S2a for Sc3+ binding). A variation in the intensity ratio of the split Soret and shift in visible bands, especially for Sc3+ binding (596, 632 and 697 nm), were also observed. The 340 nm peak gained in intensity upon metal ion binding suggesting contributions of fused quinone at this wavelength, and enhanced molar absorptivity upon metal ion binding.

These results

indicate interaction of added metal ions with 1 in the ground state (equation 1 in Scheme 1). Interestingly, the added metal ions caused additional fluorescence quenching of 1 indicating promotion of excited state interactions in 1 upon metal ion binding (see Figure 2b for Mg2+ and S2b for Sc3+ binding). Next, electrochemical studies were performed to evaluate redox potentials of the donor-acceptor entities of 1 in order to help establish energy levels (see Figure 2c for Mg2+ and S2c for Sc3+ binding). As expected for zinc porphyrins,41 two one-electron oxidations and reductions of 1 were observed both in benzonitrile and CH2Cl2 (see Figure S3 for CVs in CH2Cl2). The first oxidation and first reduction of ZnP in 1 were located at 0.78 and 1.15 V vs. Ag/AgCl in benzonitrile. In addition, a third process corresponding to quinone reduction at -0.91 V was observed.

Control experiments performed using 6,13-

pentacenequinone confirmed assignment of this redox process (Figure S3). The second expected reduction of the quinone in 1 was overlapped with the second reduction of ZnP. Addition of metal ions to the solution of 1 caused an anodic shift of the first quinone reduction by nearly 400 mV. Under these conditions, the redox process of ZnP revealed a potential shift of less than 40 mV, suggesting that the added metal ion binds to the Q center and not to the ZnP entities. It may be mentioned here that of the two metal ions used, Mg2+ was redox inactive upto the negative potential window of -2.0 V while Sc3+ revealed a peak at -0.09 V. The redox data is summarized in Table 1.



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Table 1. Redox potential of 1 in benzonitrile containing 0.1 M (TBA)ClO4, and energy of the charge separated states and free-energy change of electron transfer in the absence and presence of added metal ions. 𝐸𝑜𝑥 1 ,V

Compound 1 1+Mg2+ 1+Sc3+ 6,13pentacenequinone

𝐸𝑟𝑒𝑑 1 ,V

2

0.78, 1.01 0.76, 0.99 0.74, 0.99 --

ECS, eV

2

-0.91, -1.15 -0.56, -1.00 -0.61, -1.10 -0.90

-ΔGET, eV

1.69 1.31 1.34 --

0.17 0.54 0.51 --

Using the redox potentials and spectral data, the energy of the charge-separated states (ECS) and the free-energy changes of electron transfer (ΔGET) of 1 in the absence and presence of metal ions was evaluated using Rehm-Weller’s approach, according to equations 1-3.42 (1)

𝑟𝑒𝑑 𝐸𝐶𝑆 = 𝐸𝑜𝑥 1 (𝐷𝑜𝑛𝑜𝑟) ― 𝐸1 (𝐴𝑐𝑐𝑒𝑝𝑡𝑜𝑟) + 𝐺𝑆 2

2

ΔGET = ECS – E0-0

(2)

where E0–0 is the singlet-state energy of 1, calculated from the mid-point energy of 0,0 1 (𝐷𝑜𝑛𝑜𝑟) is the first oxidation potential transitions of absorption and fluorescence peaks. 𝐸𝑜𝑥 2

of zinc porphyrin entity in 1 and

𝐸𝑟𝑒𝑑 1 (𝐴𝑐𝑐𝑒𝑝𝑡𝑜𝑟) 2

is the first reduction potential corresponding to

fused quinone in 1. GS is the ion-pair stabilization and incorporates both the solvent-

dependent Coulomb energy change upon ion-pair formation or recombination and the free energy of solvation of the ions. Due to fused system introducing sufficient rigidity, the terms GS was ignored in the present study. These calculations yielded ΔGET values of 0.17 eV in the absence and -0.51 to -0.54 eV in the presence of metal ions (Table 1). Figure 3 shows the energy level diagram for the occurrence of electron transfer from 1ZnP* to the fused quinone with and without added Mg2+ ions (see Figure S4 for energy level diagram with Sc3+ binding). From this, metal ions binding induced facile reduction of quinone in 1 and promotion of the weak exergonic process to a more favorable ET process was borne out. The energy of the triplet state was established by recording phosphorescence of the control of -(benzo-bis-aldehyde) fused zinc porphyrin at liquid nitrogen (Figure S5 in SI). 10 ACS Paragon Plus Environment

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Mg2+ 1 1

ZnP*-Q-ZnP ZnP*-Q-ZnP ~ 1.85 eV

ET ET ZnP.+-Q.--ZnP

ISC

~1.69 eV ZnP*-Q-ZnP ~1.65 eV

3

Mg2+ ZnP.+-Q.--ZnP ~1.31 eV

E

hf

ZnP-Q-ZnP or ZnP-Q-ZnP Mg2+

Figure 3. Energy level diagram showing intramolecular PET from the 1ZnP* to quinone in 1 in the absence (blue) and presence of Mg2+ (red). The presence of Mg2+ makes the electron transfer process exergonic by an additional 380 mV. As pointed out earlier, one of the challenging issues often encountered in PET is to characterize the products, especially in strongly interacting DA conjugates. In such systems, either oxidation or reduction would perturb the spectral features of the entire molecule. Figure S6 shows the molecular electrostatic potential map (MEP) and frontier HOMO and LUMO of 1 on the B3LYP/6-31G* optimized structure. The MEP shows the electron rich (red) and poor (blue) regions of 1 in which the two ZnP rings are found to be sufficiently nonplanar. Interestingly, the majority of the HOMO was on the porphyrin ring although some contributions extend all the way to quinone skeletal carbons because of the fused ring structure. Similarly, the majority of the LUMO was on the quinone with significant contributions on the porphyrin π-system. These results suggest that either oxidation of ZnP or reduction quinone in 1 would cause drastic spectral perturbations 11 ACS Paragon Plus Environment


instead of a particular segment of the molecule being electrolyzed. Indeed, this prediction is proven true as shown in the spectroelectrochemical results represented in Figure 4. During the first oxidation process, the original peaks of 1 diminished in intensity with new peaks growing at 775 and 862 nm. An isosbestic point was also observed at 668 nm. During the first reduction, new peaks at 522 and 600 nm were observed with isosbestic points at 535, 585, and 687 nm.

Figure 4. Absorption spectra obtained during (a) first oxidation and (b) first reduction of 1 in benzonitrile containing 0.2 M (TBA)PF6. Figure (c) shows the differential absorption spectrum generated for CS species, ZnP.+-Q.-, by averaging the differential spectrum of the radical cation and radical anion shown in (a) and (b), and subtracting that with the initial spectrum of neutral 1. According to the energy level diagrams (Figures 3 and S4), photoexcitation of the donor ZnP entity in 1 produces ZnP.+-Q.- electron transfer product with the positive charge mainly located on the easily oxidizable ZnP and the negative charge mainly on the easily reducible Q entities. Under these conditions, the spectrum of the electron transfer product 12 ACS Paragon Plus Environment

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would be close to the average of the one-electron oxidized and one-electron reduced products. In the present study, we have made an effort to generate such a spectrum using the above obtained spectroelectrochemical results. For this, we averaged the differential absorption spectrum of the oxidized and reduced products of 1, and subtracted this from the spectrum of initial neutral compound. Such analysis resulted in the differential spectrum for the ET product as shown in Figure 4c, and a spectrum is expected to closely match with the SAS generated from target analysis of the transient data corresponding to the ET product.43 In order to verify our hypothesis and to secure kinetic information of the photoprocesses, we performed femtosecond transient absorption spectral studies. Figure 5a shows transient absorption spectra at the specified delay times of 1 in benzonitrile at the excitation wavelength of 511 nm. Immediately after excitation (see spectrum at 1 ps), the 1ZnP*

formed in 1 revealed positive peaks at 570, 668, 740, 834, and 1100 nm and negative

peaks at 525, 602, 630, and 702 nm. By comparison with the absorption and fluorescence spectrum of 1, the first three negative peaks were assigned to ground state bleaching while the 702 nm peak was assigned to stimulated emission. The decay of the positive peaks and recovery of the negative peaks was accompanied by new peaks at 738 and 860 nm (see spectrum at 20 ps); by comparison with earlier discussed spectroelectrochemical data, these peaks were assigned to the formation of ZnP.+. The expected peak in the 600 nm range corresponding to the reduced species of 1 was buried in the strong ground state bleaching signal. These results provide evidence of excited state ET in 1. Figure 5b and c show the transient spectra upon addition of 1 and 2 equivalents of Mg2+. For the most part, spectral features were similar to that observed in the absence of added metal ions – although relaxation of the transient peaks of 1ZnP* were relatively faster. Nonetheless, excited state ET in the presence of Mg2+ or Sc3+ were also observed.



Figure 5. Femtosecond transient spectra at the indicated delay times of 1 in the presence of 0, 1, and 2 equivalents of Mg2+ in benzonitrile (λex = 511 nm). Figures 5d-f show the SAS while Figures 5g-i show the population time profiles from GloTarAn analysis of transient data in the presence of 0, 1, and 2 equivalents of Mg2+. Next, the transient spectral data of 1 was subjected to GloTarAn target analysis38 to generate SAS and the corresponding population time profiles. Figures 5d-f show the SAS while Figures 5g-i show the population time profiles in the presence of 0, 1, and 2 equivalents of Mg2+. The transient data could be satisfactorily fitted to four components following the energy level diagrams shown in Figures 3 and S4. The SAS spectral features of the first component was close to that of the S2 state (black traces; ~ 1 ps) while the second component (red traces) was for 1ZnP*. Importantly, the spectrum for the third component (blue traces) was close to that generated for the CS state in Figure 5c by manipulating the 14 ACS Paragon Plus Environment

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spectroelectrochemical results. The SAS features of the fourth component (magenta traces) was close to that of 3ZnP*. The time constants for 1ZnP* evaluated from population time profiles shown in Figures 5g-i (red traces) were found to be 112, 29.2, and 18.4 ps in the presence of 0, 1, and 2 equivalents of Mg2+. The decrease in lifetime indicates promotion of ET in the presence of metal ions, as predicted by route 1 in Scheme 1. The lifetimes for the forward ET process from Figures 5g-i (blue traces) were found to be 120, 151, and 197 ps in the presence of 0, 1, and 2 of Mg2+, and stabilization of ion-pairs upon increased

Figure 6. Femtosecond transient spectra at the indicated delay times of 1 in the presence of 0, 1, and 2 equivalents of Sc3+ in benzonitrile (ex = 511 nm). Figures 5e-g show the species associated spectra while Figures 5h-k show the population time profiles from Glotaran global analysis of transient data in the presence of 0, 1, and 2 equivalents of Sc3+.



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addition of Mg2+ was observed, as predicted by route 2 in Scheme 1. The lifetime of the fourth component was beyond 3 ns, an indication for population of 3ZnP*. Interestingly, through use of the population kinetic profiles, it was also possible to estimate the quantum yields of ET (shown by the relative height of blue traces in Figure 5g-i). Such analysis provided quantum yields of ET products on the order of 21, 40, and 56% for 1 in the presence of 0, 1, and 2 equivalents of Mg2+. Similar results and experimental trends were also obtained for Sc3+ bound 1 as shown in Figure 6. For Sc3+, the lifetimes of the ion-pairs were found to be 112, 167, and 181 ps with quantum yields of 21, 33, and 50%, respectively, in the presence of 0, 1, and 2 equivalents of Sc3+. It is also worth pointing out that with the increase in ET quantum yields there was a corresponding decrease in the 3ZnP* quantum yields (see peak heights of magenta lines in Figure 5g-i and Figure S7g-i) due to more 1ZnP* being involved in the ET process upon metal ion interactions. It apperas that the added metal ions not only helped in the acceleration and stabilization of the ET process, but also helped in achieving significantly improved quantum yields. A summary of key kinetic data, as determined by generating species associated spectra from target analysis, is given in Table 2 below. Table 2. Kinetics of photochemical events occuring in 1 in the absence and presence of metal ions in benzonitrile, evaluated from GloTaRan target analysis. Compound

1ZnP*,

ps

RIP, ps

 

1

112

120

21

1+Mg2+ (1:1)

29.2

151

40

1+Mg2+ (1:2)

18.4

197

56

1+Sc3+ (1:1)

32.4

167

33

1+Sc3+ (1:2)

30.0

181

50

CONCLUSIONS In summary, the present study brings out several unprecedented results related to cation binding in promotion of PET. First, a relatively weak electron acceptor, fused 16 ACS Paragon Plus Environment

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quinone in 1 was successfully demonstrated to become a better electron acceptor upon metal ion binding which resulted in accelerated ET. Second, through manipulation of spectroelectrochemical results, construction of a spectrum corresponding to the electron transfer product in the strongly interacting fused, donor-acceptor conjugate was revealed. Third, the spectrum of the ET product derived from SAS is shown to track that deduced from spectroelectrochemical results, thus providing an alternative path for multifaceted transient spectral analysis. Finally, the much desired stabilization of electron transfer product with improved quantum yields was possible to achieve as a result of binding metal ions to the ET product. Further studies to fully explore the current approach in probing excited state ET in fused DA conjugates–relevant to biological and chemical ET covering a wide range of research interests – is underway in our laboratories.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS publications website at DOI. Additional spectral, electrochemical, and computational data (PDF). AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Francis D’Souza: 0000-0003-3815-8949 Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT 17 ACS Paragon Plus Environment


The U.S. Department of Energy, Office of Science, Basic Energy Sciences (DESC0016766) supported research conducted by YH and HW. The Robert A. Welch Foundation (K.M.K., Grant E-680) supported research conducted at the University of Houston. MBT is thankful to NSF for GRFP. We also acknowledge NSF (CHE-1531468) for the computation facilities at UNT.

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