Phenol Complexes

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A: Spectroscopy, Photochemistry, and Excited States

Light Driven Electron Transfer in MethylBipyridine Phenol Complexes Is Not Proton Coupled Robin Tyburski, Jens Föhlinger, and Leif Hammarström J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02221 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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The Journal of Physical Chemistry

Light Driven Electron Transfer in Methyl-Bipyridine Phenol Complexes Is Not Proton Coupled Robin Tyburski, Jens Föhlinger, Leif Hammarström* Department of Chemistry - Ångström Laboratories, Uppsala University, Box 523, SE75120 Uppsala, Sweden

ABSTRACT

The pursuit of systems that undergo optical electron proton transfer (photo-EPT) is very attractive, due to the wealth of information contained in the absorption spectra of such complexes. However, separating photo-EPT transitions from conventional charge transfer states remains a major challenge. In this study, we show that optical charge transfer in a complex between 4methoxyphenol and N-methyl-4,4’-bipyridyl, previously assigned to occur through photo-EPT involving a hydrogen bond between the reactants (Gagliardi, C. J.; Wang, L.; Dongare, P.; Brennaman, M. K.; Papanikolas, J. M.; Meyer, T. J.; Thompson, D. W., Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 11106-11109), does not lead to protonation of the acceptor molecule. Additionally, we propose that association of the complex is likely due to donor-acceptor interactions rather than hydrogen bonding.

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Introduction The importance of proton transfer coupled to many biological charge transfer reactions is widely recognized. Proton coupled electron transfer (PCET) reactions can occur sequentially or in one concerted step. Concerted electron proton (CEP) reactions avoid the high-energy intermediates necessarily formed through the sequential pathway and may thus significantly lower the activation barrier. However, as the proton and electron have to transfer simultaneously (from the same transition state), this often comes at a large kinetic penalty due to a significantly lower overall tunneling probability. First theoretical considerations of rate constants for CEP reactions were formulated by Cukier et al in 1994.1 More recent models are mainly based on a theory for concerted transfer of multiple charges first proposed (and since then developed extensively) by HammesSchiffer and Soudackov.2-4 The rate constants take a form similar to those of Marcus’ widely applied theory for electron transfer.5 Electron transfer can be accompanied by characteristic charge transfer absorption bands. Here, vertical excitation in the Franck- Condon region yields the charge-separated product. All thermodynamic and electronic parameters relevant to the Marcus theory rate expression for thermal electron transfer between the same states can in these cases be extracted from the absorption spectrum, as described by Hush.5, 6 Due to the similarity of PCET theory and MarcusHush theory, the idea of optical electron proton transfer (photo-EPT), where both the proton and electron are transferred upon optical excitation, has attracted considerable interest. 7-12 These transitions would not violate the Born-Oppenheimer approximation, as the position of the proton could be fixed during excitation. The transition would then result in the excited PCET product with an elongated acceptor-proton bond, which rapidly relaxes to the thermally equilibrated geometry.


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Very few systems have been proposed to undergo photo-EPT. Westlake et al. have reported a photo-EPT transition in 4-paranitrophenyl-phenol and tert-butylamine,8 and this system was subsequently investigated by theoretical methods.9-12 The optical transition is, however, masked by the intrinsic charge transfer transition of the nitrophenyl-phenol, making it difficult to deconvolute the photo-EPT band from the ground state absorption spectrum. Another study has recently been published by Gagliardi et al.7 Here, the absorption spectra of a series of phenols forming an adduct with N-methyl-4,4’-bipyridyl (MQ+) were measured (Scheme 1). The adduct, showing a distinct absorption band centered around 390 nm, was presumed to form a hydrogen bond between the phenolic proton and the free nitrogen of MQ+, thus preparing the system for intermolecular PCET. An estimation of the complexation constants gave free energies of association ranging from 17 to 32 kcal mol−1, consistent with the formation of hydrogen bonded complexes. Ultrafast transient absorption spectroscopy following excitation at 388 nm was used to show the formation of a product with an absorption band around 560 nm. The transient species decayed with a lifetime of approximately 2 ps, and through exchanging the protons for deuterons (by changing the aqueous solvent from protonated to deuterated water and buffer) a kinetic isotope effect of 1.6 was measured. The product resulting from excitation was interpreted as the protonated reduced HMQ•+ radical, and the adduct absorption was assigned to be a photo-EPT band. In the above mentioned series of complexes, both the electron and proton transfer are proposed to occur intermolecularly. This is in contrast to photo-EPT in the 4-paranitrophenyl-phenol tertbutylamine complex,8 where the electron transfer is intramolecular (from the phenol to the nitro group). It is therefore not trivial to distinguish photo-EPT bands from conventional charge transfer bands in the MQ+/phenol systems, as both would only arise in the associated complex.


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In the present study, we show that the observed charge transfer absorption in a MQ+/methoxyphenol complex reported in ref. 7 is due to a conventional charge transfer band rather than a photo-EPT transition. Furthermore, we propose that association of the complex is not only due to hydrogen bonding, but likely the result of interactions between the electron deficient and electron rich aromatic rings of MQ+ and methoxyphenol, respectively.


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Experimental Section N-methyl-4,4’-bipyridyl chloride (MQCl) was available from a previous study.15 4methoxyphenol, sodium ascorbate and glycine were purchased from Sigma-Aldrich. Tetra(hydroxymethyl)aminomethane (Tris) was obtained from VWR chemicals. All chemicals were used without further purification. The femtosecond to picosecond transient absorption set-up has been described in detail previously.23 Shortly, short pulses (800 nm, ~150 fs) were generated from a Ti: Sapphire regenerative amplifier with a repetition rate of 1 kHz. A fraction of the 800nm fundamental was frequency doubled to 400 nm in a BBO crystal and used to excite the sample. Pump energies were kept at around 300 nJ/pulse. Probe light was obtained from white light generation from another fraction of the 800 nm fundamental in a CaF2 crystal. Different time delays were obtained by changing the relative path length between pump and probe beam. The orientation of the polarization of pump and probe pulse was kept to 54.7° in order avoid anisotropy effects. Samples for ultrafast measurements were saturated in 4-methoxyphenol (soluble to ~100 mM) and contained 20 mM MQCl. Reference transient absorption spectra of MQ• and HMQ•+ were collected on a Spectra Physics Quanta Ray system with excitation from a frequency doubled Q-Switched Nd:YAG laser providing pulses at 532 nm, with a pulse length of approximately 10 ns. The pulse energy was kept at 15 mJ/pulse. The spectra were collected with an Edinburgh instruments spectrometer, with white probe light from a pulsed Xe-arc lamp and a CCD camera detector. The sample contained 50 μM Ru(bpy)32+ , 40 mM ascorbate and 350 μM MQCl. The pH of the solution was varied through titration of concentrated NaOH solution.


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Results and discussion In order to understand the nature of complexation, the degree of association at pH values below and above the pKa of HMQ2+ (pKa=3.5)13 was investigated (figure 1). At pH values below 3.5, the pyridyl group will be protonated (see scheme 1) and a hydrogen bonding interaction with the phenolic proton is not possible. As can be seen from the appearance of a new absorption band around 400 nm in figure 1 b), an adduct is still formed. This is likely due to interactions between the electron rich/deficient aromatic rings of the methoxyphenol and HMQ2+, respectively. The absorption of this complex is necessarily a pure charge transfer reaction and excitation in the charge transfer band will thus yield the protonated HMQ•+ radical. At pH=8.5, the methoxyphenol may be hydrogen-bonded to the methylbipyridine. However, the possibility of complexation due to similar donor-acceptor interactions as at pH

Phenol Complexes

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