Tetrathiafulvalene (TTF)-based donor-acceptor (D–A) ensembles have attracted a lot of attention due to their unique (opto)electronic properties and potential applications in organic semiconductors, photovoltaics, sensors, switches and molecular electronics.1-3 To develop high-performance electronic devices, control over multiple charge-transfer (CT) pathways in D-A ensembles is of prime importance. Recently, we have demonstrated chemical and ultrafast optical regulation of distinct photo-induced charge flows within such D-A systems.4,5 As a continuation of our ongoing work, we herein describe redox and optical properties of new D–A ensembles (Chart 1) which were prepared by covalent linkage of two TTF donor units to a central azapyrene acceptor either with or without two tert-butyl groups. A detailed experimental and theoretical study of electronic interactions between D and A units and ICT processes in these triads is presented

Aschauer, Ulrich

Feurer, Thomas

Häner, Robert

Liu, Shi-Xia

Zhou, Ping

English

Bern Open Repository and Information System (BORIS)

Organic Chemistry, Virtual Poster (3min video) OC-154Photo-induced Charge Transfer in Azapyrene-Tetrathiafulvalene TriadsP. Zhou1, U. Aschauer1, T. Feurer2, R. Häner1, S. Liu1*1Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern,Freiestrasse 3, 3012 Bern, Switzerland, 2Institute of Applied Physics, University of Bern,Sidlerstrasse 5, 3012 Bern, SwitzerlandTetrathiafulvalene (TTF)-based donor-acceptor (D–A) ensembles have attracted a lot of attentiondue to their unique (opto)electronic properties and potential applications in organicsemiconductors, photovoltaics, sensors, switches and molecular electronics.1-3 To develop high-performance electronic devices, control over multiple charge-transfer (CT) pathways in D-Aensembles is of prime importance. Recently, we have demonstrated chemical and ultrafast opticalregulation of distinct photo-induced charge flows within such D-A systems.4,5 As a continuation ofour ongoing work, we herein describe redox and optical properties of new D–A ensembles (Chart 1)which were prepared by covalent linkage of two TTF donor units to a central azapyreneacceptor either with or without two tert-butyl groups. A detailed experimental and theoreticalstudy of electronic interactions between D and A units and ICT processes in these triads ispresented.Chart 1. Chemical structures of triads 2TTF-pyrene and 2TTF-t-butyl-pyrene.[1] Bergkamp, J. J.; Decurtins, S.; Liu, S. X., Current advances in fused tetrathiafulvalene donor-acceptor systems. Chem. Soc. Rev. 2015, 44, 863-74.[2] Jana, A.; Ishida, M.; Park, J. S.; Bahring, S.; Jeppesen, J. O.; Sessler, J. L., Tetrathiafulvalene-(TTF-) Derived Oligopyrrolic Macrocycles. Chem. Rev. 2017, 117, 2641-2710.[3] Pfattner, R.; Pavlica, E.; Jaggi, M.; Liu, S.-X.; Decurtins, S.; Bratina, G.; Veciana, J.; Mas-Torrent, M.; Rovira, C., Photo-induced intramolecular charge transfer in an ambipolar field-effecttransistor based on a π-conjugated donor–acceptor dyad. J. Mater. Chem. C 2013, 1, 3985-3988.[4] Zhou, P.; Aschauer, U.; Decurtins, S.; Feurer, T.; Häner, R.; Liu, S. X. Chemical control ofphotoinduced charge-transfer direction in a tetrathiafulvalene-fused dipyrrolylquinoxalinedifluoroborate dyad. Chem. Commun. 2020, 56, 13421-13424.[5] Rohwer, E. J.; Geng, Y.; Akbarimoosavi, M.; Daku, L. M. L.; Aleveque, O.; Levillain, E.; Hauser, J.;Cannizzo, A.; Haner, R.; Decurtins, S.; Stanley, R. J.; Feurer, T.; Liu, S. X. Optically ControlledElectron Transfer in a Re(I) Complex. Chem. Eur. J. 2021, 27, 5399-5403.Powered by TCPDF (www.tcpdf.org)

Photo-induced Charge Transfer in Azapyrene-Tetrathiafulvalene Triads

source: https://doi.org/10.48350/159587 | downloaded: 12.11.2021Introduction: Tetraazapyrene (TAP), the prototype of nitrogenatedpolycyclic aromatic hydrocarbons (N-PAHs), exhibits intrinsicoptoelectronic and electrochemical properties as well as high thermaland chemical stability, all important requirements for its diverseapplications in the field of organic (opto)electronics.1,2 However, onlyquite limited reports on synthetic approaches to the TAP scaffold and itsderivatization appear in the literature. In 2012, a series of 2,7-substitutedTAP derivatives was prepared and tested as n-type semiconductors inorganic field-effect transistors (OFETs).1 It has been demonstrated thatthe electronic properties are significantly affected by the nature ofsubstituents at the core positions. Inspired by these appealing results, wehave embarked on the design and synthesis of tetrathiafulvalene (TTF)-functionalized TAPs to create novel triads (Scheme 1) with promisingmaterial properties.Photo-induced Charge Transfer in Azapyrene-Tetrathiafulvalene TriadsREFERENCES[1] S. Geib, S. C. Martens, U. Zschieschang, F. Lombeck, H. Wadepohl, H. Klauk and L. H. Gade, J. Org. Chem., 2012, 77, 6107.[2] S. C. Martens, L. Hahn, F. Lombeck, A. Rybina, H. Wadepohl and L. H. Gade, Eur. J. Org. Chem., 2013, 5295-5302.Figure 1. Differential Cyclic voltammograms of TTF-TAP (blue), TTF-t-Bu-TAP(red), t-Bu-TAP (black) and TTF precursor (pink) were measured indichloromethane solution, containing 0.1 M TBAPF6 as the supporting electrolyte atroom temperature, Pt working electrode, Ag/AgCl electrode as the referenceelectrode and the scan rate at 100 mV s-1.Ping Zhou1, Ulrich Aschauer1, Thomas Feurer2, Robert Häner1 and Shi-Xia Liu11Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland2Institute of Applied Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, SwitzerlandAbstract: Tetraazapyrene (TAP) and its tert-butyl substituted derivative are symmetrically functionalised with two tetrathiafulvalene (TTF) units. Electroniccommunication between TTF units and the central TAP core in resultant triads is evaluated by UV-Vis absorption, cyclic voltammetry and spectroelectrochemicalmeasurements. Notably, the insertion of tert-butyl groups raises the TAP-localised LUMO level by 0.21 eV.Scheme 1 Chemical structures of the TTF-TAP and TTF-t-Bu-TAP.Figure 1 shows that TTF-TAP undergoes three distinct reversibleoxidation processes at 0.63 V, 0.77 V and at 1.06 V (vs Ag/AgCl),indicating that two TTF units are successively oxidized to TTF radicalcation species and simultaneously to TTF dication species. This splittinginto two oxidation processes for the generation of its radical cationpoints to intramolecular through-bond interactions between two TTFunits. In the negative potential window, two reversible reduction wavesat -0.46 V and -0.98 V are observed, corresponding to sequentialaddition of electrons to the TAP unit. Interestingly, the incorporation oftert-butyl group has an appreciable influence on the redox properties ofTTF and TAP units. TTF-t-Bu-TAP shows only two reversible oxidationwaves at 0.70 V and 1.03 V, suggesting that two TTF units aresimultaneously oxidized to their radical cation and dication species,respectively. It also undergoes two reversible reductions at -0.67 V and -1.19 V, which, however, are negatively shifted compared to TTF-TAP.Consequently, the insertion of tert-butyl groups raises the LUMO levelby 0.21 eV, very probably due to the hyperconjugation and electron-donating effect of the tert-butyl groups.Figure 2. UV-Vis absorption spectra of TTF-TAP (blue), TTF-t-Bu-TAP (red), t-Bu-TAP(black) and TTF precursor (pink) in CH2Cl2 at r.t. Insert: Vis-NIR absorption spectra of TTF-TAP, TTF-t-Bu-TAP in CH2Cl2 at r.t.In the UV and blue part of the optical spectra, the observed strongabsorption bands are attributed to π–π* transitions localized on the TTFand TAP cores, only with bathochromic shifts owing to some extendedconjugation. By comparison of TTF-t-Bu-TAP (red) to TTF-TAP (blue), theinsertion of the tert-butyl groups leads to noticeable bathochromic shifts ofthe absorption bands in a range of 360 nm and 450 nm and a slighthypsochromic shift of the absorption band around 320 nm. As thesetransitions correspond mainly to π–π* excitations of the TAP core, theobserved shifts can be accounted for by the fact that the presence of bulkytert-butyl groups at 2,7-positions of the TAP is unfavorable for possible H-and J-aggregation and impedes effective intermolecular interactions. Moreimportantly, both triads exhibit two weak and broad absorption bandscentered at 680 nm and 1100 nm for TTF-TAP, and 630 nm and 1100 nmfor TTF-t-Bu-TAP. Consequently, these absorptions reflect ICT transitionsdominated by excitations from HOMOs localized on the TTF units to theLUMO localized on the TAP core.Figure 3. Variation of UV-Vis-NIR absorption spectra of TTF-TAP (left) and TTF-t-Bu-TAP(right) in CH2Cl2 upon successive addition of aliquots of NOSbF6 at room temperature.As shown in Fig. 3 for TTF-TAP, a progressive reduction of the absorbanceof high energy of π–π* transitions is accompanied with a concomitantappearance of new absorption bands peaked at 452 nm and 835 nm whichreach their maximum values upon addition of 12 equiv. of NOSbF6. Thelowest energy absorption band is continuously blue-shifted during theoxidation process. These new transitions are characteristic of the newlyformed cationic TTF radical TTF·+ within a D-A-D triad. In contrast, the initialoxidation of TTF-t-Bu-TAP leads to a broad absorption band whichemerges at 905 nm, and after addition of 1.2 equiv. suddenly movestowards higher energy (830 nm) with increasing amounts of NOSbF6 .Conclusion: Both TTF-TAP and TTF-t-Bu-TAP absorb in the UV-Vis-NIRspectral region due to ICT excitations from TTF units to the central TAPcore. Upon chemical oxidation, a reverse ICT from the TAP core to theTTF·+ moieties occurs. Interestingly, the two TTF units of TTF-TAP andTTF-t-Bu-TAP are sequentially and simultaneously oxidized to the TTFradical cation species, respectively.

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Photo-induced Charge Transfer in Azapyrene-Tetrathiafulvalene Triads

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