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Low-Dielectric-Constant Polyimide Aerogel University of Birmingham Identification of a new electron-transfer relaxation pathway in photoexcited pyrrole dimers Neville, Simon P.; Kirkby, Oliver M.; Kaltsoyannis, Nikolas; Worth, Graham A.; Fielding, Helen H. DOI: 10.1038/ncomms11357 License: Creative Commons: Attribution (CC BY) Document Version Publisher's PDF, also known as Version of record Citation for published version (Harvard): Neville, SP, Kirkby, OM, Kaltsoyannis, N, Worth, GA & Fielding, HH 2016, 'Identification of a new electron- transfer relaxation pathway in photoexcited pyrrole dimers', Nature Communications, vol. 7, 11357. https://doi.org/10.1038/ncomms11357 Link to publication on Research at Birmingham portal Publisher Rights Statement: Neville, S., Kirkby, O., Kaltsoyannis, N. et al. Identification of a new electron-transfer relaxation pathway in photoexcited pyrrole dimers. Nat Commun 7, 11357 (2016). https://doi.org/10.1038/ncomms11357 General rights Unless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or the copyright holders. The express permission of the copyright holder must be obtained for any use of this material other than for purposes permitted by law. •Users may freely distribute the URL that is used to identify this publication. •Users may download and/or print one copy of the publication from the University of Birmingham research portal for the purpose of private study or non-commercial research. •User may use extracts from the document in line with the concept of ‘fair dealing’ under the Copyright, Designs and Patents Act 1988 (?) •Users may not further distribute the material nor use it for the purposes of commercial gain. Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document. When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive. If you believe that this is the case for this document, please contact [email protected] providing details and we will remove access to the work immediately and investigate. Download date: 29. Sep. 2021 ARTICLE Received 10 Sep 2015 | Accepted 17 Mar 2016 | Published 21 Apr 2016 DOI: 10.1038/ncomms11357 OPEN Identification of a new electron-transfer relaxation pathway in photoexcited pyrrole dimers Simon P. Neville1,w, Oliver M. Kirkby2, Nikolas Kaltsoyannis2,w, Graham A. Worth1 & Helen H. Fielding2 Photoinduced electron transfer is central to many biological processes and technological applications, such as the harvesting of solar energy and molecular electronics. The electron donor and acceptor units involved in electron transfer are often held in place by covalent bonds, p–p interactions or hydrogen bonds. Here, using time-resolved photoelectron spectroscopy and ab initio calculations, we reveal the existence of a new, low-energy, photoinduced electron-transfer mechanism in molecules held together by an NH?p bond. Specifically, we capture the electron-transfer process in a pyrrole dimer, from the excited p-system of the donor pyrrole to a Rydberg orbital localized on the N-atom of the acceptor pyrrole, mediated by an N–H stretch on the acceptor molecule. The resulting charge-transfer state is surprisingly long lived and leads to efficient electronic relaxation. We propose that this relaxation pathway plays an important role in biological and technological systems containing the pyrrole building block. 1 School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. 2 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK. w Present addresses: Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5 (S.P.N.); School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK (N.K.). Correspondence and requests for materials should be addressed to G.A.W. (email: [email protected]) or to H.H.F. (email: h.h.fi[email protected]). NATURE COMMUNICATIONS | 7:11357 | DOI: 10.1038/ncomms11357 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11357 lectron-transfer (ET) reactions in which an electron moves new ET mechanism in the pyrrole dimer is significant and may from a donor atom or molecule to an acceptor atom or provide a basis for practical applications, such as the rational Emolecule are ubiquitous in nature and technology1,2; design of new photoactive materials. important examples include the ET chains that drive photosystems I and II (ref. 3) and nanoscale electronic devices4. Results ET processes are often mediated by covalently bound molecular Photoelectron spectra of pyrrole and pyrrole clusters.He bridges that hold the donor and acceptor in place; for example, in carrier gas was passed through a reservoir of pyrrole held at 25 °C bridged zinc porphyrin-quinone complexes, which are and the mixture was expanded through a 50 mm diameter, structurally related to the photosynthetic reaction centre, 1-mm-long nozzle. The distribution of pyrrole cluster ions excitation of the porphyrin results in rapid ET to the quinone formed by 1 þ 1 ionization of the clusters was 100% monomer 5,6 accompanied by large-scale vibrations of the bridges . when using 0.4 bar He and B89% monomer, 10% dimer and 1% Photoinduced ET can also take place between non-covalently trimer when using 1.8 bar He. bonded donors and acceptors; for example, between p-stacked The spectral signatures of the pyrrole dimer were identifed by 7 nucleobases . Recently, there has been a great deal of interest in comparing photoelectron spectra obtained with different backing ET reactions occurring between hydrogen-bonded donors and pressures of He gas. Figure 1 shows the 240 nm (5.17 eV) 1 þ 1 acceptors accompanied by proton transfer. This class of ET multiphoton photoelectron spectra, recorded with helium carrier reaction has been shown to provide an efficient mechanism gas pressures of 0.4 and 1.8 bar. At 0.4 bar, the spectrum is for excited state deactivation following absorption of UV photons. identical to the one-photon, vacuum UV photoelectron spectrum Such proton-coupled ET reactions are important in the of pyrrole22, indicating that it is predominantly a 1 þ 1 photostability of proteins, DNA and other essential bio- multiphoton photoelectron spectrum of the monomer. The 8 macromolecules and in the photorepair cycles of UV-damaged maximum electron kinetic energy (eKE) possible following 9 DNA . Proton-coupled ET has also been found to play an 1 þ 1 photoionization of pyrrole, calculated using the central important role in the photostability of hydrogen-bonded wavelength of the laser pulse and the experimental adiabatic 10,11 heterocycles, such as the pyrrole dimer . ionization energy (8.207 eV) (refs 23,24), is 2.13 eV and is marked The pyrrole molecule is a common motif in biology; for on Fig. 1. The photoelectron spectrum at 1.8 bar has an additional example, in tryptophan and porphyrins. It is the key to the feature B2.45 eV, which lies above the maximum eKE possible 12 photoactivation of the phytochrome enzyme . Pyrrole is also a following 1 þ 1 photoionization of the monomer. The intensity of basic building block in many biologically and technologically this feature is independent of laser power but increases with important systems, such as polyamide DNA-binding agents in chemical biology13,14, dye-sensitized solar cells15,16, 17 the polypyrrole conducting polymer and self-assembled 1.0 0.4 bar 18 architectures containing donor–acceptor units . Moreover, it is 0.8 a prototypical heteroaromatic molecule that undergoes highly efficient non-radiative relaxation on a ps* potential surface19–21. 0.6 Dimerization of pyrrole has been found to open up additional 0.4 relaxation pathways, such as a high-energy channel involving 0.2 proton-coupled ET10,11. In this article, we report the observation of a new, low-energy 0.0 1.0 relaxation channel in pyrrole dimers in the gas-phase, involving 1.8 bar electron transfer between the two pyrrole molecules. Using 0.8 time-resolved photoelectron spectroscopy, we find that at high 0.6 gas pressures, when dimers are present, a spectroscopic feature appears on a sub-picosecond timescale that is not observed at low Intensity 0.4 pressures when only monomers are present. High level ab initio 0.2 calculations show that the lowest excited state of the dimer 0.0 corresponds to a charge-transfer (CT) state and that the 0.3 Difference ionization energy of this state is consistent with the spectral feature associated with the dimers. Analysis of the orbitals and 0.2 (dimer) 0 molecular structures shows that ET takes place from a p-orbital D (monomer) on the donor pyrrole molecule to a Rydberg orbital localized on 0.1 0 the N-atom of the acceptor pyrrole molecule. Time-resolved D photoelectron spectra recorded following excitation at different 0.0 wavelengths, together with calculations of potential energy 0.0 0.5 1.0 1.5 2.0 2.5 3.0 surfaces, suggest that the N–H bond of the acceptor pyrrole eKE (eV) must stretch for ET to take place; in the framework of Marcus theory, the N–H bond of the acceptor pyrrole stretches to bring Figure 1 | Photoelectron spectra of pyrrole at different backing the dimer to a geometry in which there is a strong vibronic pressures. 240 nm (5.17 eV) 1 þ 1 photoelectron spectra of pyrrole with He coupling between neutral and CT states. As with the high-energy, backing pressures of 0.4 bar (top) and 1.8 bar (middle) together with the proton-coupled ET process identified by Slavı´cˇek and difference spectrum (bottom) obtained by subtracting the 0.4 bar spectrum co-workers10,11, this new channel competes with dynamics on from the 1.8 bar spectrum.
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