Exploring structure based charge transport relationships in phenyl diketopyrrolopyrrole single crystals using a 2D π–π dimer model system

This document is the Accepted Manuscript version of the following article: Jesus Calvo-Castro, and Callum J. McHugh, ‘Exploring structure based charge transport relationships in phenyl diketopyrrolopyrrole single crystals using a 2D π–π dimer model system’, Journal of Materials Chemistry C, Issue 16, 2017, first published 28 March 2017. The version of record is available online at DOI: http://dx.doi.org/10.1039/C7TC00434F © Royal Society of Chemistry 2017Crystalline phenyl diketopyrrolopyrroles are often overlooked as charge transfer mediating materials in optoelectronic applications. We report an experimentally ratified two dimensional π–π model dimer system dispelling previous misconceptions regarding the potential of these materials as organic semiconductors and that will enable researchers to screen and predict charge transport potential solely on the basis of their single crystal derived π-stacking architectures. In testing our model system versus the available database of phenyl diketopyrrolopyrrole single crystal structures we reveal that these materials are characterised by intrinsically large thermal integrities and in many cases large charge transfer integrals, not solely restricted to dimeric interactions exhibiting close intermonomer arrangements and bearing low torsion of the core phenyl rings. This study will be of significant interest to the increasingly large community engaged in the quest to engineer π-conjugated organic based semiconducting devices and particularly those employing crystalline diketopyrrolopyrroles.Peer reviewe

Impact of substituent effects on the Raman spectra of structurally related N-substituted diketopyrrolopyrroles

This document is the Accepted Manuscript version of the following article: Jesus Calvo-Castro, et al, ‘Impact of substituent effects on the Raman spectra of structurally related N-substituted diketopyrrolopyrroles’, Vibrational Spectroscopy, Vol. 83, pp. 8-16, March 2016, doi: https://doi.org/10.1016/j.vibspec.2015.12.004. This manuscript version is made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.Control over vibrational frequency modes is important in optimising the performance and behaviour of conjugated organic materials employed as charge transfer mediators and optical components in optoelectronic devices. Raman spectroscopy represents a powerful technique that can be employed to determine the structural implications of molecular substitution on photophysical properties in such conjugated organic environments. Herein, we report for the first time, the optimised geometries for a series of eight systematically varied N-substituted diketopyrrolopyrroles as well as their experimental and computed Raman spectra, with special emphasis placed upon their spectral band assignments. Clear out-of-plane structural re-arrangements, including pyramidalisation of the lactam nitrogens arising from intramolecular H-bonding interactions were observed upon N-substitution in the reported systems, leading to significant vibrational frequency shifts for ν(Nsingle bondC) and ν(Cdouble bond; length as m-dashO) modes. In addition, mode scaling factors were determined and found to be comparable with those reported previously, employed using the same density functional. The following study addresses the implications of structural variation on the progression of those intense Raman modes which play a key role in tuning the photophysical properties of N-substituted diketopyrrolopyrrole systems and as such should be of broad interest to those developing functional materials based upon this molecular motifPeer reviewe

Fluorine Directed Two-Dimensional Cruciform π−π Stacking in Diketopyrrolopyrroles

This is the Accepted Manuscript Version of the following article: Jesus Calvo-Castro, Graeme Morris, Alan R. Kennedy, and Callum J. McHugh, “Fluorine Directed Two-Dimensional Cruciform π–π Stacking in Diketopyrrolopyrroles”, Crystal Growth and Design, Vol. 16 (9): 5385–5393, July 2016. Copyright © 2016 American Chemical Society.Enhanced bulk dimensionality in organic materials employed in optoelectronic devices is desirable and can overcome fabrication issues related to structural defects and grain boundaries. Herein, we report a novel fluorinated diketopyrrolopyrrole single crystal structure, which displays a unique, mutually orthogonal, 2-dimensional cruciform π−π stacking arrangement. The crystal structure is characterized by an unusually large number of nearest neighbor dimer pairs which contribute to a greater thermal integrity than structurally analogous equivalents. Binding energies and charge transfer integrals were computed for all of the crystal extracted dimer pairs by means of the M06-2X density functional at the 6- 311G(d) level. Although weak, a number of intermolecular interactions involving organic fluorine (C−F---H, πF---π, and C−F---πF) were identified to influence the supramolecular assembly of these dimer pairs. Charge transfer integrals for the two π−π stacking crystal dimers were determined using the energy splitting in dimer method. Ambipolar charge transport favoring electron transfer approaching that of rubrene is predicted in both of these π−π stacks, with a greater magnitude of coupling observed from those dimers perpetuating along the crystallographic a-axis. Charge transport behavior in the single crystal is greatly influenced by selective fluorination of the N-benzyl substituents and is consistent with the crystal extracted π−π stacking dimer geometries and their overall influence on wave function overlap. The reported structure is an interesting electron transport material that could be exploited, particularly in thin film based optoelectronic devices, where high bulk dimensionality is required.Peer reviewedFinal Accepted Versio

Effects of fluorine substitution on the intermolecular interactions, energetics and packing behaviour of N-benzyl substituted diketopyrrolopyrroles

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth and Design, after peer review and technical editing by the publisher. To access the final edited and published work see doi: 10.1021/acs.cgd.6b00157.Rationalizing the effects of molecular substitution in π-conjugated organic materials arising from well-defined intermolecular interactions, which can influence the formation of predefined packing motifs and control the emergence of π–π stacking represents a current challenge in supramolecular design. Significant effort is potentially required to manage the impact on solid state packing behavior in materials that have been molecularly tuned to carry out specific photophysical and electrochemical functions. In this regard, fluorine substitution in π-conjugated systems has seen a recent surge of interest, primarily aimed toward the development of materials with enhanced optical and optoelectronic behavior. In light of this interest, in the following study, we report the synthesis and single crystal structures from a series of four novel and structurally related, symmetric, fluorinated N-benzyl substituted diketopyrrolopyrroles (DPPs). Two of the investigated series exhibit slipped cofacial π–π dimer pairs, which are consistent with those reported by us previously in halogenated DPPs. Significantly, this characteristic stacking motif of N-benzyl substituted DPPs can be carefully modified via the replacement of hydrogen atoms with trifluoromethyl and isosteric fluorine–hydrogen substituents. In the case of trifluoromethyl substitution, we identify a previously unobserved packing motif exhibiting a framework of well-defined channels propagating along the length of the crystallographic c-axis. In each of the reported systems, all of the nearest neighbor dimer pairs have been identified and their intermolecular interaction energies computed by means of the M06-2X density functional at the 6-311G(d) level. Through a detailed theoretical analysis involving the determination of cropped dimer energetics, organic fluorine is shown to play an active role in the stabilization of the crystal extracted dimer pairs through a number of additive and weak C–F---H, C–F---πF, and C–F---π intermolecular contacts. Contrary to recent reports, we demonstrate that substitution of hydrogen by fluorine can also lead to dramatic changes in solid state packing behavior as a consequence of these weak interactions. Given the importance of organic fluorine substitution in the construction of π-conjugated materials for optoelectronic materials, we feel that this work should be of interest to the wider community involved in supramolecular design of organic conjugated systems, and in particular to those investigating organic fluorine as well as diketopyrrolopyrrole containing architecturesPeer reviewedFinal Accepted Versio

Impact of systematic structural variation on the energetics of π−π stacking interactions and associated computed charge transfer integrals of crystalline diketopyrrolopyrroles

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth & Design, copyright © 2014 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see DOI: https://doi.org/10.1021/cg5010165, or ACS Articles on Request http://pubs.acs.org/page/policy/articlesonrequest/index.html/Control over solid state structure is critical for effective performance in optoelectronic devices bearing π-conjugated charge mediating organic materials. A series of five structurally related N-benzyl-substituted diketopyrrolopyrroles (DPPs) differing ... Novel crystal structures demonstrating long molecular axis, slipped, π−π cofacial stacking motifs and associated semiconductor bands in a series of N-benzylated diketopyrrolopyrroles are reported. Through variation of just 2 atoms from 60, clear crystal structure/(computed) charge transport activity interdependency is observed, with two structures exhibiting hole transport integrals comparable to Rubrene, a highly effective positive charge carrying, organic, crystalline materialPeer reviewedFinal Accepted Versio

Intermolecular interactions and energetics in the crystalline π–π stacks and associated model dimer systems of asymmetric halogenated diketopyrrolopyrroles

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth & Design, © 2016 American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see DOI: 10.1021/acs.cgd.5b01656/Four novel structurally analogous asymmetric, halogenated N-benzyl substituted diketopyrrolopyrroles (DPP) have been synthesized, and their crystal structures obtained. All four crystal structures exhibit π–π stacks with very small displacements along their short molecular axes, which based upon our previous studies involving symmetrical DPPs is a characteristic of N-benzyl substitution. Intermolecular interaction energies were computed for extracted crystal π–π dimer pairs by means of M06-2X density functional at the 6-311G(d) level to investigate the most energetically favored position of the halogen atoms in FBDPP and ClBDPP structures. In addition, effective stabilization energies arising from both benzyl and halogen substitution in these derivatives and in BrBDPP and IBDPP π–π dimer pairs were determined in order to probe the impact of these groups on the resulting dimer stability. Effects of the intermonomer displacements along the long molecular axis, which have been shown by us previously to significantly influence wavefunction overlap and effective electronic coupling, were investigated in detail using aligned and anti-aligned model systems of ClDPP and BrDPP. The predictions of these model systems are remarkably consistent with the observed displacements in their crystal derived π–π dimer pair equivalents, offering insight into the effective role of intermolecular contacts in crystal structures involving this molecular motif, particularly with a view toward crystal engineering in these systems. As a result, we believe that this study should be of significant interest to the growing DPP based materials community and in general to those investigating the detailed manner by which substituents can be employed in the supramolecular design of crystalline molecular architectures.Peer reviewedFinal Accepted Versio

Detection of nitroaromatic and peroxide-based explosives with amine- and phosphine-functionalized diketopyrrolopyrroles

Effective strategies for the detection and identification of explosives are highly desirable. Herein, we illustrate the efficient optoelectronic detection of nitroaromatic and peroxide-based explosives using amine- and phosphine-substituted diketopyrrolopyrroles. Selective quenching and an unprecedented enhancement of thin-film emission in the presence of nitroaromatic vapors are demonstrated via the judicious choice of amine substituents. The modulation of fluorescence emission in each case is shown to be dominated by electronic and thermodynamic effects, the vapor pressure of explosives, and the thin-film morphology. For peroxide detection, we describe an approach exploiting redox-mediated functional group transformation. The rapid oxidation of triphenylphosphine to phosphine oxide with hydrogen peroxide affords a significant increase in fluorescence emission, facilitating the sensitive turn-on detection of an important class of explosives at ppb concentrations

True absolute determination of photoluminescence quantum yields by coupling multiwavelength thermal lens and photoluminescence spectroscopies

© Royal Society of Chemistry 2020. This is the accepted manuscript version of an article which has been published in final form at https://dx.doi.org/10.1039/D0CP03794J.Photoluminescence quantum yields denote a critical variable to characterise a fluorophore and its potential performance. Their determination, by means of methodologies employing reference standard materials, innevitably leads to large uncertainties. In response to this, herein we report for the first time an innovative and elegant methodology, whereby the use of neat solvent/reference material required by thermal lens approaches is eliminated by coupling it to photoluminescence spectroscopy, allowing for the discrimination between materials with similar photoluminescence quantum yields. To achieve that, both radiative and non-radiative transitions are simultaneously measured by means of a photoluminescence spectrometer coupled to a multiwavelength thermal lens spectroscopy setup in a mode-mismatched dual-beam configuration, respectively. The absorption factor independent ratio of the thermal lens and photoluminescence signals can then be used to determine the fluorescence quantum yield both accurately and precisely. We validated our reported method by means of rhodamine 6G and further applied in three novel structurally related diketopyrrolopyrrole based materials to, contrary to results obtained by other methods, unveil significant differences in their photoluminescence quantum yields.Peer reviewe

Detection of nitroaromatic vapours with diketopyrrolopyrrole thin films exploring the role of structural order and morphology on thin film properties and fluorescence quenching efficiency

This document is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. The Version of Record is available at: https://doi.org/10.1039/C4CC08468C.Sensitive optical detection of nitroaromatic vapours with diketopyrrolopyrrole thin films is reported for the first time and the impact of thin film crystal structure and morphology on fluorescence quenching behaviour demonstratedPeer reviewedFinal Accepted Versio

Torsional angle dependence and switching of inner sphere reorganisation energies for electron and hole charge transfer processes involving phenyl substituted diketopyrrolopyrroles; a density functional study

This document is the Accepted Manuscript version of the following article: Jesus Calvo-Castro, Callum J. McHugh, Andrew J. McLean, ‘Torsional angle dependence and switching of inner sphere reorganisation energies for electron and hole transfer processes involving phenyl substituted diketopyrrolopyrroles; a density functional study’, Dyes and Pigments, Vol. 113, pp. 609-617, February 2015. The Version of Record is available online at doi: https://doi.org/10.1016/j.dyepig.2014.09.031. Published by Elsevier.Determination of inner sphere reorganisation energies is important in the development of organic charge mediating materials and electron transfer reactions. In this study, hole and electron inner sphere reorganisation energies, lambda(h) and lambda(e) respectively, have been computed for the first time for a series of structurally related diketopyrrolopyrrole (DPP) molecular motifs. Inner sphere reorganisation energies for self-exchange electron transfer reactions are calculated as being lower than those associated hole transfer processes in model planar phenyl and thiophenyl substituted DPP systems. It is found that lambda(e) lambda(h).Peer reviewedFinal Accepted Versio