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Photoinduced Charge Shifts and Electron Transfer in ViologenTetraphenylborate Complexes: Push-Pull Character of the Exciplex Willy Glen Santos, Darya S. Budkina, Victor Marcelo Deflon, Alexander N. Tarnovsky, Daniel Rodrigues Cardoso, and Malcolm D. E. Forbes J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Journal of the American Chemical Society

Photoinduced Charge Shifts and Electron Transfer in ViologenTetraphenylborate Complexes: Push-Pull Character of the Exciplex Willy G. Santos,† Darya S. Budkina,† Victor M. Deflon,‡ Alexander N. Tarnovsky† Daniel R. Cardoso,*,‡ Malcolm D. E. Forbes*,† † Center for Photochemical Sciences, Department of Chemistry, Bowling Green State University, Bowling Green, OH 43403 ‡ Instituto de Química de São Carlos, Universidade de São Paulo, Caixa Postal 780, 13560-970 São Carlos, SP, Brazil Supporting Information Placeholder ABSTRACT: Viologen-tetraarylborate ion-pair complexes were prepared and investigated by steady-state and time-resolved spectroscopic techniques such as fluorescence and femtosecond transient absorption. The results highlight a charge transfer transition that leads to changes in the viologen structure in the excited singlet state. Femtosecond transient absorption reveals the formation of excited-state absorption and stimulated emission bands assigned to the planar (kobs < 1012 s-1) and twisted (kobs ~ 1010 s-1) structures between two pyridinium groups in the viologen ion. An efficient photoinduced electron transfer from the tetraarylborate anionic moiety to the viologen dication was observed less than 1 µs after excitation. This is a consequence of the push-pull character of the electron donor twisted viologen structure, which helps formation of the borate triplet-state. The borate triplet state is deactivated further via a second electron transfer process, generating viologen cation radical (V•+).

The viologen (V) molecular framework finds utility in many important redox and photochemically based applications,1–5 including photovoltaics,6–8 water splitting for solar fuels,9 photocleavage of DNA,10,11 electron transfer in supramolecular assemblies,12,13 and as an herbicide (“paraquat”). The canonical structure methylviologen (MV2+, 1a, Scheme 1) is an excellent electron acceptor with a very stable radical cation (MV+•). While its most common electron transfer reactions involve the ground state of MV2+, its first photoexcited singlet state is also an excellent electron acceptor whose photoacidity in water14 and reactivity with alcohol solvents15 have previously been investigated using ultrafast spectroscopy under dilute (non-ion paired) conditions. In this paper we report ultrafast spectroscopic results for viologen ion-pair complexes16–20 with a tetraarylborate anion, where the charge transfer transition leads to significant structural changes in the MV framework.

The bipyridinium dication readily forms outer-sphere charge transfer (CT) complexes20,21 with Lewis base– type species with low ionization potentials. A better understanding of these structural changes may help in the development of new solar energy technologies, and to exploit the photochromic and electrochromic properties of organic molecules or organometallic complexes. The charge transfer process may involve a torsional coordinate in the excited state, and can be related to the concept of twisted intramolecular charge transfer (TICT).22 This type of conformational change can facilitate the transfer of charge or electron from the donor to the acceptor species.22 Tetraarylborates only display electrons in the sigma and π-bonding orbitals and are typically difficult to oxidize (0.5 – 1.5 V, vs. AgCl).23 Consequently, oxidation of the borate molecule will result in either α-cleavage of one of the Boron-Aryl bonds, or two concomitant α-cleavages, as observed for the oxidation of the tetrahedral arylboron compounds yielding aryl and boranyl radicals.24,25 Indeed, the charge transfer character of an ion-pair complex containing the V moiety is usually attributed to the fact that the donor–V bonding interaction can occur when the latter occupies either the singlet excited state or the ground state. Herein, we describe an interesting effect of exchanging the viologen ion in the photophysics of the ion-pair complexes formed between three Scheme 1. Chemical structure for the ions studied. Viologen (V) Borate (B)

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different types of viologen ions with the same tetraphenylborate counter ion (Scheme 1). We also describe the influence of the twisted viologen structure on the nature of the relaxation pathways that result in transient radicals. A single crystal with 1:2 (dication/anion) stoichiometry has been observed for the 1a:2 and 1b:2 complexes, and their structures were determined by single crystal X-ray diffraction. The crystal structure refinement reveal a triclinic system with space group P-1 for the 1a:2 complex and orthorhombic system with space group Pccn for the 1b:2 complex. Crystallographic details are shown in Table S1 (supplementary information). Figure S1a shows that the 1a dication has a planar structure. However, a twisted structure (ϕ = 48ο) between the two pyridinium groups is observed in the 1b dication. The difference in the dihedral angle is attributed to the presence of π-stacking interaction between the pyridinium groups and a phenyl group in the 1a:2 complex. Indeed, this interaction is not observed in the aniondication-anion arrangement of the 1b:2 complex. The 1a:2 fragment (Figure S1a) shows a viologen dication with two tetraphenylborates where one of these anions is closer to the pyridinium viologen group than the other. The shortest distance between the aromatic rings of donor (one borate anion) and acceptor (pyridinium) is found to be 3.232 Å, which is a distance short enough to allow charge transfer.26 We could not obtain a crystal structure for the 1c:2 complex. However, when 1c and 2 are in 9:1 v/v THF:ACN solution, a charge-transfer (CT) transition is observed as a shoulder around 370 nm. This band disappears in acetonitrile (See Figure S2a and S2b), consistent with the fact that ion-pair species dissociate in polar environments. Figure S2b shows the absorption spectrum of the 1c:2 complex, deconvoluted into two major contributions, one from a higher energetic transition of 1c itself (2 absorbs outside the spectral region shown) and another from the CT transition. A Job’s plot (Figure S2c) reveals a 1:1 stoichiometry for 1c:2 complex. For 1a:2 complex, despite a 1:2 stoichiometry (viologen:borate) obtained from the X-ray crystallographic analysis, the Job’s plot (Figure S3c) further supports that the CTtransition is formed by interaction of one donor (borate anion) with the acceptor (viologen).

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Figure S1b shows the fragment crystal structure of complex 1b:2. Both borate anions are close enough to, and at about the same distance from, each pyridinium group on the benzylviologen structure (3.578 Å). The general description of the wave function (ψ) for the excited-state ion-pair complex between a tetraphenylborate and viologen is shown in eq 1. ψ = αψ1(D-A+) + βψ2(D+A-) + γψ3(D*A) + δψ4(DA*) (1) where the coefficients α, β, γ and δ have varying magnitudes depending on the extent of mixing of the charge resonance (αψ1 and βψ2) and exciton resonance (γψ3 and δψ4) states. In the special case of ion-pair CT complexes between viologen compounds and tetraphenylborate, the excitation in the CT-band involves the additional requirement of |α| ≠ |β| and |γ| = |δ|. The equality in the second term is due to equal contributions of both ions to the CT-excited state. In pure THF, ion 1a is complexed with 2 in contrast to pure acetonitrile as solvent. In comparison with the absence of visible bands of precursor salts in their UV-vis absorption spectra, the ion-pair complexes (1a:2, 1b:2, and 1c:2) show low-energy absorption bands. This confirms the presence of a strong interaction between both ions in the ground state. The linear relationship between absorbance and concentration of an absorbing 1a:2 complex at 390 nm is shown in Figure S3a. Figure 1a shows absorption and fluorescence spectra of the 1b:2 complex in THF solution. The difference between the fluorescence excitation and absorption spectra suggests that the excited states of the complex relax via a mechanism that bypasses this emissive state. The Stokes-shift between the fluorescence excitation and fluorescence is large (3100 cm-1 or ~ 9 kcal mol-1), indicating a significant structural change taking place between the initial Franck-Condon state and the emissive state. The contribution of the different conformers can be observed by a red-shift in the emission spectra recorded in solvents of different polarity (Figure S3e and S3f). Analysis of the fluorescence decay reveals three time components with % populations and lifetimes strongly dependent on the ion-pair structure.

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Figure 1. (a) UV-vis absorption, excitation and fluorescence spectra of the 1a:2 complex in 9:1 v/v THF:ACN solution; λexc = 450 nm and λem = 600 nm. (b) Fluorescence decay for 1a:2, 1b:2, and 1c:2 at 600 nm. (c) Photochromic changes and radical formation and decay for 1a:2 complex in solid-phase upon light excitation and d) luminescence in 9:1 v/v THF:ACN solution, λirr = 410 ACS nm. Paragon Plus Environment

Scheme 2. Photochemical and photophysical mechanisms expected for the 1a:2 exciplex after CT-band absorption. Ground state has a predominance of the αconformer in 9:1 v/v THF:ACN solution. 15

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Fluorescence decay data are shown in Figure 1b and the global fit results are shown in Table S2. The fluorescence decay of 1c:2 has a short–lived component with a major contribution (98%) to the fluorescence emission. However, when the 1c ion is replaced by 1a or 1b, the percentage of the short–lived component (τ1) decreases, with a concomitant percentage increase of the other two long–lived components (τ2 and τ3, Table S2). In the time–resolved fluorescence results, the fast component (1) has a lifetime decay of about 470 ps (also see Table S2). This corresponds to the longest lifetime decay component with a similar rate constant (2.8×109 s-1) observed within the stimulated emission band in ultrafast transient absorption experiments (vide infra). The other two long lifetime components (τ2 and τ3) in Table S2 were not observed in our ultrafast transient absorption measurements. Indeed, the lifetime components τ2 and τ3 have a higher percentage contribution for the 1a:2 complex compared to the 1b:2 complex, because of an effective π-stacking interaction between one pyridinium group of the viologen and a phenyl group in the tetraphenylborate. This π-stacking interaction is not observed for the 1b:2 complex, which also shows low fluorescence emission (see Table S2). Transient absorption spectra of 1a:2 measured at different delay times following excitation at 350 nm (away from the CT band) are shown in Figure 2a. The transition at 350 nm has an electron acceptance character, which induces the capture of one electron from the tetraarylborate anion by MV2+ to form MV●+. This radical cation has been observed by Kohler et al.15 after excitation of uncomplexed MV2+ in methanol at 265 nm. Figure 2b shows the transient absorption spectra upon excitation at 480 nm, which represents excitation exclusively into the CT band. Between 100 and 200 fs after excitation, a ground state bleach (negative signal) is observed at 490 nm, with concomitant observation of the stimulated emission band (SE, negative signal) at 520 nm and induced absorption bands (positive signals) at 390 nm and 600 nm. These two absorption bands have a decay time constant of ~650 fs, and the ground state bleach signal partially recovers on the same time scale. Scheme 2 summarizes the photophysical processes. As the transient absorption features decay, a stimulated emission band peaking at 550 nm starts to develop, along with a red shift of the signal (from 550 nm to 625 nm). The complex does not appreciably absorb at wavelengths longer than 520 nm, and this stimulated emission (SE) band stretching from 510 nm to 625 nm shows a different shape from the initial 100-200 fs stimulated emission. Therefore, it is assigned to a new stimulated emission band. This new SE band decays on a longer time scale (time constant = 360 ps). The SE band corresponds to the steady-state emission with a maximum at 550 nm observed in Figure 1a and to the shortest component observed in our fluorescence time-resolved measurements (component 1, 470 ps).

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Figure 2. Evolution of the transient absorption spectra for complex 1a:2 after excitation at (a) 350 nm and (b) at 480 nm. Finally, we observed that in comparison to the SE band, the transient absorption at 430 nm decays slower; however, its decay constant couldn’t be accurately resolved within the 1.2 ns probing window of our ultrafast experiments. Excitation at 480 nm leads to the initial Franck-Condon charge transfer state (CT). The emissive band at 550 nm is due to the singlet excited state from structural rearrangement of this initial CT state on the time scale of ~650 fs. This involves a change of the dihedral angle of the viologen moiety leading to significant stabilization.27 The absorption of the twisted viologen radical is also confirmed by our time dependent theoretical results (TD-DFT). As shown in Figure S4, the maximum absorption band of the viologen cation radical red shifts with the increasing torsion dihedral angle. This red-shift effect in the absorption band was also observed by Benniston and coworkers,27 using viologen derivatives with restricted dihedral angle. The contrast in photophysical behavior between excitation at 350 nm and at 480 nm (or 450 nm, see Figure S5) reveals the peculiar exciplex characteristics, where the change of conformation of the viologen molecule is observed only after excitation in the CT band. Arylborates clearly have a significant influence in stabilizing both viologen conformers. Our results are distinct from previously reports, where the planar free viologen cation radical is formed by S1 deactivation with little or no flu-



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orescence emission.1,15,28 The emission of the twisted viologen structure reveals the potential of the ionic complex and/or radical pairs to modulate their photochromic (solid state) and luminescence (solution) properties, as shown for the 1a:2 complex in Figure 1c and 1d, respectively. Figure S6 shows nanosecond transient absorption spectra recorded after 1a:2 complex excitation at 355 nm, with a short-lived component around 450 nm (τdecay = 2.3 µs) and a long-lived component at 650 nm (τdecay > 1ms). The rise time of the long-lived component has a mono-exponential growth with a lifetime around ~2.8 µs, which is very close to the mono-exponential decay obtained for the short-lived component at 450 nm. In other words, the similarities in the kinetic behavior of each component show an inverse dependence between them. The absorption at 450 nm is most likely the triplet-triplet absorption transient for uncomplexed species. This is based on the same spectral location of the maximum absorption, and is in agreement with a decay lifetime previously reported for sodium tetraphenylborate in polar solvents (e.g. acetonitrile).29 This indicates that the triplet exciplex has a strong contribution from the tetraphenylborate structure (following the predicted term γψ3(D*A), see eq 1). The absence of a short-lived triplet species (100 ns < τdecay > 1 ms) of methylviologen in previously ns-transient absorption studies also support this hypothesis.30 Figure S7 shows the EPR-spectrum of viologentetraphenylborate ion-pair complexes in THF upon light excitation at 120 K. The borate-radical is confirmed as a first paramagnetic species formed before the viologen radical builds up, which increases in liquid-phase at high values of temperature (196 K and 303 K). Figure S8a shows EPR spectra of the 1a:2 complex in the presence of the spin-trap 3,3,5,5-Tetramethyl-1-pyrroline N-oxide (TMPO). The first radical adduct detected in the beginning of the irradiation at 410 nm is the phenyl radical, as consequence of its release from the tetraphenylborate structure. Figure S8b and S8c show the simulated EPR spectra of the phenyl-TMPO adduct radical and the viologen radical, respectively. The end product of irradiation in the CT-transition band (450 nm) was detected such as triphenylborane and two viologen derivatives (with one or two additional phenyl groups in the viologen structure), which were detected by high resolution mass spectrometry (see SI). In summary, two interdependent processes were identified by femtosecond transient absorption after excitation of the CT absorption band, and attributed to two different conformers of the viologen. The second, more stable conformer is deactivated by radiative processes and intersystem crossing, forming of the triplet-state of the tetraphenylborate, which further decomposes to photoproducts. ASSOCIATED CONTENT

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Supporting Information. Experimental details, additional spectra, characterization and crystallographic data (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge support FAPESP (2011/5155-7, 2008/54011-8, 2012/19823-4 and 2015/13756-1). MDEF and ANT acknowledges the continued support from NSF (CHE-0923360, CHE-1464817 and DMR-1006761).

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Photoinduced Charge Shifts and Electron Transfer ... - ACS Publications

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