Joceline
Zeitouny
a,
Abdelhalim
Belbakra
b,
Anna
Llanes-Pallas
a,
Andrea
Barbieri
b,
Nicola
Armaroli
*b and
Davide
Bonifazi
*ac
aDipartimento di Scienze Farmaceutiche, Università di Trieste, Piazzale Europa 1, 34127 Trieste, Italy
bIstituto per la Sintesi Organica e la Fotoreattività del CNR, Via Gobetti 101, 40129 Bologna, Italy. E-mail: nicola.armaroli@isof.cnr.it; Fax: +39 051 6399844
cDepartment of Chemistry, University of Namur (FUNDP), Rue de Bruxelles 61, B-5000, Namur, Belgium. E-mail: davide.bonifazi@fundp.ac.be; Fax: +32 81 725433
First published on 4th October 2010
The synthesis, photoswitchability and NIR emitting properties of a novel π-extended pyrene derivative, peripherally decorated with four azobenzenyl-ethynyl legs, are reported.
Scheme 1 Reagents and conditions: i, TMSA, [Pd(PPh3)4], CuI, iPr2NH, 50 °C, 20 h (82%); ii, 1 M aq. KOH, MeOH/CH2Cl2, 2 h (98%); iii, 4, [Pd(PPh3)4], CuI, Et3N/THF, rt, 20 h; iv, TMSA, [PdCl2(PPh3)2], CuI, iPr2NH/THF, rt, 20 h; v, TBAF/THF, rt, 20 h; vi, 8, [PdCl2(PPh3)2], CuI, PPh3, iPr2NH/THF, rt, 20 h; vii, TMSA, [PdCl2(PPh3)2], CuI, PPh3, iPr2NH/THF, 80 °C, 12 h; viii, 1 M aq. KOH, MeOH/CH2Cl2, 12 h; ix, 2, [PdCl2(PPh3)2], CuI, Et3N, 45 °C, 12 h. Right bottom: electronic absorption spectra of molecules tttt-1, t-8, 11 and tt-12 in toluene solution at 298 K. |
Fig. 1 Schematic representation of the full switching of the molecular arachnoid 1 from the 1-tttt to the 1-cccc configuration. |
UV irradiation of azobenzene derivatives 1, 8 and 12 afforded the all-trans isomers in toluene solutions, their electronic absorption spectra are depicted in Scheme 1 (bottom right) along with that of tetrasubstituted pyrene reference 11. The spectral onset is progressively red-shifted from 8 to 12 to 1 with enhanced π-delocalization. Upon irradiation of tttt-1 at 470 nm (see exp. section in ESI†), spectral changes are observed, which are stopped after about 2 min. An isosbestic point is detected at 372 nm, suggesting the occurrence of a clean photoreaction (Fig. 2a). By keeping the solution in the dark, the initial spectrum is recovered after 2 hours. The same result is obtained upon light excitation at 313 nm but in a much shorter time, i.e. about 90 seconds (Fig. 2b). Taken together, the above observations suggest the occurrence of a reversible trans–cis interconversion. Photoisomerization experiments have been carried out also with the linear derivative t-8. Due to the lack of the strongly-absorbing pyrenyl unit, this molecule showed more pronounced spectral changes upon illumination (Fig. S1, ESI†). The intense absorption features of 1 and 8, which are attributable to the pyrene core and/or the extended π-delocalization, do not allow us to individuate the weak characteristic absorption fingerprints of the azobenzene moieties around 450 nm, thus hampering the determination of the quantum yield of photoisomerisation.13 In order to get further insight on the control of the molecular movements, semi-empirical (PM3) quantum mechanical calculations on the minimised geometrical structures of the nine possible configurational isomers of molecule 1 have been performed and their relative stability has been assessed (Table S2 and Fig. S3, ESI†). Results show that the all-trans (tttt) is the most stable isomer and that each trans → cisisomerization destabilizes the structure by roughly 2.6 kcal mol−1.14 Furthermore, it has been found that the isomers with the same number of cis and transazobenzene moieties show similar enthalpies of formation (Fig. S2 and S3, ESI†), thereby supporting the idea that the four legs behave like independent units in the gas phase, as the central pyrenyl core is large enough to keep them far apart, avoiding steric congestion and repulsion.
Fig. 2 (a) Time evolution of the absorption spectra of a 4.0 × 10−6 M solution of tttt-1 in toluene under 460 nm irradiation, until the photostationary state is reached. (b) Back reaction observed over time under 313 nm irradiation. The initial spectrum of tttt-1 is fully recovered showing complete reversibility. |
Despite a few scattered reports claiming weak emission,15azobenzene systems are known to be virtually non-luminescent. Indeed, a detailed analysis of azobenzene 8 does not reveal any reliable luminescence signals in a variety of experimental conditions. On the contrary, pyrene and phenylacetylene derivatives are typically strong emitters16 and in fact reference molecule 11 exhibits an intense luminescence band (λmax = 578 nm, Φfl = 0.70, τ = 3.1 ns, CH2Cl2), comparable to that of unsubstituted pyrene. Molecule 1 unexpectedly shows a weak emission band centred at 740 nm, only observable with NIR sensitive photodetectors (quantum yield ≤ 0.001, Fig. 3). Very interestingly, also bisazobenzene derivative 12 exhibits an almost identical NIR-centred emission, with perfect matching of absorption and excitation spectra (Fig. 3). This proves that the population of the electronic excited states of molecules 1 and 12 yields sensitization of the NIR emitting state. Structurally, both molecules 1 and 12 are, respectively, linked to four and two fragments of 8 through a π-conjugated spacer (a bisethynyl and pyrenyl, respectively). In both cases, the central spacer allows extensive electronic delocalization between the 8-type branches, generating a low-lying NIR emitting level. This shows that both bisethynyl and pyrene π-spacers have a similar effect on the electronic delocalization between the azobenzene units.
Fig. 3 Emission spectra of tttt-1, t-8, 11 and tt-12 (λexc = 330 nm) in toluene at 298 K. Inset: absorption (colored lines) vs.excitation spectra (black lines) of tttt-1 and tt-12. The emission spectrum of 11 is recorded with a conventional UV-Vis photodetector, the others with a NIR sensitive one. Luminescence spectra are corrected for the detector responses. |
In summary, we have prepared an arachnoid-like molecular system made of a chromophoric pyrene core and azobenzenyl-ethynyl photoswitchable legs, which exhibits NIR emission, a quite uncommon feature for organic switchable chromophores. We are now carrying out more detailed photophysical investigations to exploit the photoswitchable character of this complex system also on surfaces and to rationalize and tune its intriguing luminescence properties.
This work was supported by EU (MC-RTN “PRAIRIES” MRTN-CT-2006-035810 and MC-ITN FINELUMEN PITN-GA-2008-215399), the CNR (PM.P04.010, MACOL), the FRS-FNRS (2.4.625.08.F & F.4.505.10.F), the “Loterie Nationale”, the ‘TINTIN’ ARC project (09/14-023), the University of Namur and the University of Trieste.
Footnotes |
† Electronic supplementary information (ESI) available: Details for the experimental procedures, computational studies and photophysical characterization. See DOI: 10.1039/c0cc03045g |
‡ This article is part of the ‘Emerging Investigators’ themed issue for ChemComm. |
This journal is © The Royal Society of Chemistry 2011 |