Institut des
NanoSciences de Paris
insp
insp
3.jpg

Nanostructures et systèmes quantiques

Fils quantiques organiques

  • Thierry Barisien
  • Laurent Legrand
  • Sophie Hameau
  • Michel Schott
  • Jérémy Holcman (PhD sept. 2008 - jan. 2012)

Publications

Collaborations :
-  Gerhard Weiser, Philipps-Universität, Marburg, Germany ( - 2010) ;
-  Sylvain Dutremez, Bruno Boury, Jean-Sébastien Filhol, Institut Charles Gerhardt, UMR 5253, University Montpellier II, France (2006 - 2010). Grant : PNANO-ANR 2007-2008, « Nanochemistry and Nanophysics of Red Polydiacetylenes » ;
-  Sylvie Spagnoli, Interdisciplinary Physics Laboratory (LIPhy UMR 5588), University Joseph Fourier, Grenoble, France ;
-  M. Rei Vilar ITODYS, U. Paris-Diderot ;
-  Clémence Allain, Pierre Audebert, Gilles Clavier, Laboratory of Supramolecular and Macromolecular Photophysics and Photochemistry (PPSM UMR 8531, ENS Cachan) (2010 - ) ;

At INSP :
-  Marie-Claude Fauré ;
-  Jean-Louis Fave ;
-  Michel Goldmann.

Scientific context

Polydiacetylenes (PDA) are a class of conjugated polymers with general formula =(RC-CC-CR’)n=, where the side groups R and R’ can have very different chemical formulae. They are prepared by topochemical polymerization of the corresponding monomer diacetylene (DA) crystals, suitable side groups ensuring that the geometry is favorable to the solid-state reaction. These systems can be used as model conjugated polymers as they can be obtained through the topochemical reaction as quasiperfectly ordered linear conjugated chains with a length of about 10 microns (for a review, see [1]). Such a chain is a semiconducting quantum wire. The excited state which dominates all optical properties in the visible and near UV is a strongly bound exciton which carries almost all the oscillator strength and exhibits all theoretical properties expected for a perfect periodic quasi-one-dimensional (1D) system : in one of our PDA chains of interest, poly-3BCMU, we have shown that the exciton is described by a band of states ; the 1/√E density of states dependence expected for 1D topology and the √T dependence of the exciton radiative lifetime, were indeed both evidenced [2,3]. Moreover, the exciton on these chains has been shown to be in a macroscopically spatially coherent state at low temperature [4-6].

Fig. 1 : [click to enlarge] .

 

Our chains are therefore the unique real quasi-1D systems on which the excitonic spatial coherence can be studied at the micrometer scale on a single object in microluminescence experiments where the luminescence detector is coupled to a microscope (Spatial and temporal coherences in PDA quantum wires).

One of our objectives is also the search of new fluorescent model systems in order to learn about the possible generic nature of the electronic properties of PDAs like poly-3BCMU (Fig. 1) and to better understand the relation between chain conformation and electronic structure in systems as close as possible to the ideal theoretical situation (Relation between conformation and electronic structure of PDAs).

We develop also with chemists new systems of coupled dissimilar quantum wires able to support macroscopic coherent excitonic states in order to study the influence of coherence on energy transfer (Coupling between dissimilar quantum wires).

A part of our activities is the study of polymerization of diacetylenes (DA) and color transition in PDA in order to understand the process used in the development of biosensors, and the study of new molecules in Langmuir films (Polymerization of diacetylenes (DA) and color transition in PDA).

Spatial and temporal coherences in PDA quantum wires (CNano CEFQUO)

These works concern physics of conjugated polymers and of excitons in quasi-1D semiconducting quantum systems.

On the individual poly-3BCMU chain, we study dynamics of formation of the extended purely electronic excitonic states we have previously evidenced [4]. The experiments are luminescence experiments spatially resolved at the micrometer scale and temporally at the picosecond time scale, and coherent control experiments on a single object. For these latter two phase-locked picoseconds pulses are used to excite the chain, in a first step, at the same position by the two consecutive pulses and we measure its time-integrated emission. The relative phase between the phase-locked laser pulses is varied by means of a stabilized scanning interferometer.

Our recent results concerning the purely electronic excitonic state are (1) the measurement of an upper time limit for the formation of the extended state on the wire : after a picosecond laser pulse, in a few picoseconds, the exciton centre of mass is delocalized over the wire with a uniform probability of presence on micrometers [5, 6] (Figure 2) ;

Fig. 2 : [click to enlarge] On the left : the image of a chain is formed on the entry slit of a streak camera. The 2D figure shows the spatial extension of the emitting zone and its temporal evolution. Here we show the spatio-temporal evolution of the emission at 7K of a single chain with a length of about 15 microns, the temporal resolution of the experiment being estimated to 5 ps. On the right : emission profiles as cross sections of the image at different delays (the time origin is arbitrary). After normalization the profiles are almost identical ; this indicates that the non resolved delocalization dynamics of the emission is faster than 5 ps [6].

 

(2) the low temperature measurement of the decoherence times of the excitons on single chains. Our results in the temporal domain are in good agreement with those deduced from the spectral analysis of homogeneous luminescent lines [7] ;

(3) the evidence of the control of the whole emission of a chain from a local excitation [7] which is fully consistent with the macroscopic coherence of the single exciton states we had demonstrated in [4] (Figure 3) ;

Fig. 3 : [click to enlarge] Upper panel : spatially resolved emission of a single chain with varying phase between excitation pulses (T = 6 K) ; there is nearly no temporal overlap between the pulses at the considered delay, tau= 2.5 ps. Spatial resolution is about 1 um well below the chain length estimated here around 18 um. Correction for the background (due to dark counts) was applied to the data. Lower panel : visibility associated with the emission pattern [7].

 

(4) the development of a model allowing to better understand the influence on decoherence of exciton thermalization in its band due to phonons scattering [7] ;

(5) the development of an original experiment where the single chain is excited in two spatially distant points. We have in this manner evidenced the sensitivity of the emission with respect with the relative phase between pulses. This result has to be analyzed in the framework of a more sophisticated model taking into account the non local interaction between the incident field and the exciton centre of mass delocalized over distances higher than the incident wave length. We evidence here the possibility of an optical control of the energy transfer on a quantum wire.

We have also used sub-picosecond pulses pairs to couple coherently the fundamental state (absence of exciton) and a vibronic state (exciton-phonon state) of the chain. The results lead to a direct measure of the coherence time and lifetime of such a composite state [6].

Relation between conformation and electronic structure of PDAs (PNANO NANOPDAR)

PDA may have at least two different electronic structures, corresponding to the same chemical formula but presumably to different chain geometries. The two most frequent structures are conventionally named “blue” and “red” these colors corresponding to excitonic absorption near 630 nm and 550 nm, respectively. The most spectacular difference is that, while blue PDA chains are almost not fluorescent (yields less than 10−4), red ones show an intense resonance emission with yields 0.1 or higher at low temperature. The absence of fluorescence in blue chains corresponds to the presence in the optical gap of “dark” exciton states which provide a very efficient non radiative decay channel for the exciton states responsible for light absorption and emission. By implication, the dark states in the red chains are no longer in the optical gap, so their energies have increased much more than that of the radiating exciton on passing from blue to red chains. Crystal-structure determinations have shown that blue chains are planar and that successive repeat units are translationally equivalent : the 1D unit cell contains a single repeat unit. It has also been shown that bond lengths in blue and red PDA do not differ significantly.

- Thanks to a crystalline engineering approach we have indeed succeeded in obtaining new PDA phases [9, 10] : these are the only ones allowing a quantitative experimental study of the influence of the chain conformation and electronic properties in systems where chromophores are tightly bounded via pi conjugation.

- The structures of the excitons (binding energy, Bohr radius) on new luminescent wires and the emission properties of the chains have been studied and discussed : the nature of quasi-1D perfect systems has been evidenced. An intense Franz Keldysh effect and a large coherence length of the photogenerated carriers have been measured [9, 11]. The characteristic temperature dependence of the radiative lifetime and quantum yield have been measured [12] (Figure 4).

Fig. 4 : [click to enlarge] (a) Inverse of the lifetime, 1/tau_eff, as a function of T for the emission of PDA chains ensembles in a new luminescent developed system. The red line is a fit 1/tau_eff = 1/tau_nrad + 1/b√T. The radiative lifetime varies therefore as √T ; (b) Quantum yield deduced from the fit of the radiative lifetime tau_rad [12].

 

- Our recent density functional theoretical calculation [13] relates stable non planar chain conformations to the spectroscopic properties (visible absorption, vibrational frequencies, and NMR chemical shifts) in a simple model PDA and in the known PDA poly-THD [9]. It was proposed early on the luminescent (red) chains are non planar, successive repeat units being rotated in opposite directions relative to the chain’s average plane so that the 1D unit cell contains two non-translationally equivalent repeat units. That is the result we obtain here : different energetically stable conformations corresponding to different degrees of torsion for the successive units are evidenced, these conformations leading to the luminescent nature of the system.

To sum up, the PDA we have developed during these last years add to the existing set of available structures thus providing an experimental basis for the study of the connection between conformational and electronic properties in model conjugated structures with large electron correlations. The structure of excitons in perfectly ordered luminescent polydiacetylenic wires do not differ significantly for different chain conformations. Comparisons between our systems indicate semi-quantitatively a significant evolution of the different parameters with the angle giving the chain its “twisted” conformation. The outstanding quantum wire properties of isolated red poly-3BCMU chains are shared by other PDA, and we propose that they should be considered as generic of this class of materials. We conjecture that similar properties would be found in other conjugated polymers, if only they could be prepared in the same degree of order.

Coupling between dissimilar quantum wires

This part of our work is more prospective. It is based on a strong synergy between chemists and physicists. It is axed on the investigation of electronic energy transfer processes in truly novel systems involving spatially extended molecular assemblies in quasi-1D geometry (like J-aggregates), pi-conjugated polymers and coupled systems of the previous structures. This project are based on the expertise gained at INSP in the study of PDA chains which were shown to exhibit remarkable properties as ideal quantum wires, and the expertise of the PPSM for a new class of chromophores, the tetrazines. Properties of emission delocalization will be, in a first step, investigated in scaffold of non-covalently bound chromophores forming J-aggregates prepared in a still unequalled state of order. Those aggregates will then be coupled to perfectly ordered PDA chains allowing the study of energy transfer in hetero-structures of closely packed quantum wires with a controlled and adjustable distance between them allowing different regimes of coupling from Förster-type process (large interwire distance) to strong quantum coupling. Signatures of interactions will be searched in the relaxation dynamics of the photoexcitations and the possible formation of composite charge transfer excited states (leading to charge separation) will be studied. Due to its importance in quantum transport a special attention will be paid to the possible emergence of long range spatial coherence (resulting from the utmost state of order expected from the elaboration processes and already observed in PDA chains), how it may be preserved against increasing dynamic disorder or transferred in the explored situations of coupling. Electronic properties will be investigated through optical spectroscopy, at the molecule scale whenever possible and as a function of temperature to modulate the degree of dynamic disorder.

The first molecules containing a DA unit and a tetrazine chromophore and leading to the topochemical polymerization in the crystalline state have been synthesized. The study of the sprectroscopic properties of the coupled system evidences the transfer of energy from the chromophore to the PDA chain.

Polymerization of diacetylenes (DA) and color transition in Polydiacetylenes (PDA)

Collaborations : S. Spagnoli (LIPhy, Grenoble), M. Rei Vilar (ITODYS, U. Paris-Diderot), C.Allain, P. Audebert, G. Clavier (PPSM, ENS Cachan), M.-C. Fauré, M. Goldmann (INSP).

Motivation : The color transition in PDA-containing Langmuir (L) films, vesicles and liposomes is being used for developing biosensors. Usually the color change itself is used for detection, but there is growing interest in using the weak room temperature fluorescence of the red phase (the blue one is not). This is a very active resarch field. However, the color transition process is still not understood. We are working on two problems :

1-Understanding the color transition process :

- We have shown that photopolymerization of a lamellar polycrystalline film (hence similar to a L film) does not go to completion, due to quenching of the reactivity by energy transfer to already formed chains [14]. Hence, contrary to what is generally assumed, usually investigated films are not pure PDA but mixed solids containing ≈ 50 % polymer. Remaining monomer is neglected but may play a role in the observed transition.
- It is always claimed in the literature that the transition is dure to disordering of the PDA chain, itself produced by a disordering of the side-groups. But we have shown (see above) that red chains in crystals are not disordered and may even be the best ordered polymer chains ever produced. We have now directly shown by IR that a color transition may occur without any disordering of the side groups [15].
- The kinetics of thermally produced transition is now under study, and this work is being extended to Langmuir and Langmuir-Schäefer films including structural studies by WAXS.

2 - To increase the sensitivity of fluorescence detection, a possible new detection is under study.

Publications :

[1] M. Schott, in Photophysics of Molecular Materials, edited by G. Lanzani (Wiley-VCH, Berlin, 2006), p. 49.

[2] Fluorescence yield and lifetime of isolated polydiacetylene chains : Evidence for a one-dimensional exciton band in a conjugated polymer R. Lécuiller, J. Berréhar, J. D. Ganiere, C. Lapersonne-Meyer, P. Lavallard, and M. Schott, Phys. Rev. B 66, 125205 (2002).

[3] Optical evidence of a purely one-dimensional exciton density of states in a single conjugated polymer chain, F. Dubin, J. Berréhar, R. Grousson, T. Guillet, C. Lapersonne-Meyer, M. Schott, and V. Voliotis, Phys. Rev. B 66, 113202 (2002).

[4] Macroscopic coherence of a single exciton state in an organic quantum wire, F. Dubin, R. Melet, T. Barisien, R. Grousson, L. Legrand, M. Schott, V. Voliotis, Nature Physics 2, 32 (2006)

[5] T. Barisien, L. Legrand, V. Voliotis, Un fil quantique idéal, Images de la physique 2006, éd. du département MPPU du CNRS, 92 (2007)

[6] Excitons in a perfect quasi-1D organic quantum wire, an isolated polydiacetylene chain, L. Legrand, A. Al Choueiry, J. Holcman, A. Enderlin, R. Melet, T. Barisien, V. Voliotis, R. Grousson, M. Schott, Phys. Stat. Sol. (b) 245, 2702 (2008)

[7] Coherent Control of the Optical Emission in a Single Organic Quantum Wire, J. Holcman, A. Al Choueiry, A. Enderlin, S. Hameau, T. Barisien, L. Legrand, Nano Lett. 11, 4496 (2011), UPMC PhD Thesis, J. Holcman, Cohérence temporelle d’un exciton sur un fil quantique organique unique, Jan. 2012

[8] Red Ionic Water-soluble Imidazonium-Containing Polydiacetylene, K. Chougrani, J. Deschamps, S. Dutremez, A. Van der Lee, T. Barisien, L. Legrand, M. Schott, J.-S. Filhol, B. Boury, Macromolecular Rapid Communications 29, 580 (2008)

[9] Exciton spectroscopy of red polydiacetylene chains in single crystals, T. Barisien, L. Legrand, G. Weiser, J. Deschamps, M. Balog, B. Boury, S. Dutremez, M. Schott, Chemical Physics Letters 444, 309 (2007)

[10] Tuning Topochemical Polymerization of Diacetylenes : a Joint Synthetic, Structural, Photophysical, and Theoretical Study of a Series of Analogues of a Know Reactive Monomer, 1,6-Bis(diphenylamino)-2,4-hexadiyne (THD), J. Deschamps, M. Balog, B. Boury, M. Ben Yahia, J.-S. Filhol, A. van der Lee, A. Al Choueiry, T. Barisien, L. Legrand, M. Schott, and S. G. Dutremez, Chemistry of Materials 22, 3961 (2010)

[11] Stark effect and Franz-Keldysh effect of a quantum wire realized by conjugated polymer chains of a novel diacetylene 3NPh2, G. Weiser, L. Legrand, T. Barisien, A. Al Choueiry, M. Schott, S. Dutremez, Phys. Rev. B 81, 125209 (2010).

[12] A twisted polydiacetylene quantum wire : Influence of conformation on excitons in polymeric quasi-1D systems, A. Al Choueiry, T. Barisien, J. Holcman, L. Legrand, G. Weiser, M. Schott, J. Deschamps, M. Balog, B. Boury, J.-S. Filhol, S. Dutremez, Phys. Rev. B 81, 125208 (2010).

[13] Polymorphs and colors of polydiacetylenes : a first principles study, J.-S. Filhol, J. Deschamps, S. Dutremez, B. Boury, T. Barisien, L. Legrand, M. Schott, JACS 131, 6976 (2009).

[14] Photopolymerization of Thin Polycrystalline Diacetylene Films and Quenching of the Precursor Excited State, S. Spagnoli, J.-L. Fave, M. Schott, Macromolecules 44, 2613 (2011)

[15] S. Spagnoli, M. Rei Vilar, M. Schott, submitted.