Photomechanical Control of the Electronic Properties of Linear p-Conjugated Systems Bruno Jousselme, [a] Philippe Blanchard,* [a] Nuria Gallego-Planas, [b] Eric Levillain, [a] JacquesDelaunay, [b] Magali Allain, [b] PascalRichomme, [c] and Jean Roncali* [a] Abstract: Photodynamic molecular ar- chitectures have been synthesized by covalentfixationofaphotoisomerizable dimethylazobenzene group at two fixed points of conformationally flexible p- conjugated quater- and sexithiophene chains. Theoretical geometry optimiza- tion shows, in excellent agreement with crystallographic structures, that the modeoffixationoftheazogroupplays a determining role in the geometry of the final molecular architecture and on its ability to perform the expected pho- toinduced molecular motion. Thus, co- valentfixationof meta-dimethylazoben- zeneonaquaterthiophenechainresults in a conformationally locked system in which photoisomerization of the azo group is hindered. However, the exper- imentalresultsofoptical, 1 HNMRspec- troscopic, and electrochemical investi- gations show that when an azobenzene groupisconnectedatthe para positions of the phenyl rings, trans-to-cis photo- isomerizationoftheazogroupinducesa conformational transition and dimen- sional changes in the underlying p-con- jugatedoligothiophenechain.Theseex- perimental results unequivocally show that the photochemically induced geo- metrical changes produce in turn an increase in the HOMO level and a narrowingoftheHOMO±LUMOgap. This therefore provides the first evi- denceofphotomechanicalcontrolofthe electronic properties of linear p-conju- gatedsystems. Keywords: azocompounds ¥ molec- ular actuation ¥ nanostructures ¥ oligothiophenes ¥ photochemistry Introduction The synthetic chemistry of nanoscale molecular devices is acquiring an increasing importance in the rapidly expanding field of nanosciences. In addition to the continuing develop- ment of molecular wires, [1] rectifiers, [2] switches, [3] or logic gates, [4] therecentemergenceofdynamicmoleculardevices [5] opensanewfieldofresearchinthechemistryofnano-objects. Thus, in the past few years, molecular architectures such as gears, [6] shuttles, [7] ratchets, [8] motors, [9] andmuscles [10] capable of converting thermal, chemical, or photochemical energy into molecular motion and hence mechanical energy have been described. Such molecular machines and motors are thought to pave the way toward the development of the futuristicfieldofnanorobotics. Initial prototypes of molecular machines capable of pro- ducingarepeatableanddirectionallycontrolledmotionwere essentially powered by means of changes in the oxidation state of a mobile part of the molecular architecture. Thus, Stoddart and co-workers have developed molecular shuttles inwhichmodificationsoftheoxidationstateofaredox-active systemembeddedinarotaxaneinducesitstranslocationalong a linear axis. [7a] Sauvage and co-workers have synthesized molecular machines and muscles powered by chemically or electrochemically induced changes of the redox state and hencecoordinationsphereofametalcomplexedinrotaxanes containingtetra-andpentacoordinationsites. [7b] Among the various possible power sources for molecular dynamic devices, light presents a number of specific advan- tagessuchasrapid,easy,anddirectaccesstothetargetactive siteofthesystemandtheabsenceofforeignchemicals.Some examples of photochemically powered molecular machines have already been described and, in particular, Ferringa and co-workershavesynthesizedmolecularmachinesthatunder- go circular motion, [9] while Hugel etal. have investigated a polymeric system that undergoes a single-molecule optome- chanicalcycle. [11] Thiophene-based monodisperse p-conjugated oligomers have been widely investigated as active materials in field- [a] Dr.P.Blanchard,Dr.J.Roncali,Dr.B.Jousselme,Dr.E.Levillain Groupe Syste ¡mesConjugue ¬sLine ¬aires UMRCNRS6501,Universite ¬ d×Angers 2boulevardLavoisier,49045Angers(France) Fax:( 33)2-41-73-54-05 E-mail:philippe.blanchard@univ-angers.fr jean.roncali@univ-angers.fr [b] N.Gallego-Planas,Dr.J.Delaunay,M.Allain Inge ¬nierieMole ¬culaireetMate ¬riauxOrganiques UMRCNRS6501,Universite ¬ d×Angers 2boulevardLavoisier,49045Angers(France) [c] Prof.P.Richomme ServiceCommund×AnalysesSpectroscopiques,Universite ¬ d×Angers 2boulevardLavoisier,49045Angers(France) FULL PAPER Chem. Eur. J. 2003, 9,5297±5306 DOI: 10.1002/chem.200305010 ¹2003Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim 5297