Abstract
Chiral thiophene-diketopyrrolopyrrole derivatives have been synthesised to investigate the potential of stereochemistry and symmetry as a means of modulating properties by influencing self-assembly of these purely organic materials. In particular, derivatives of diketopyrrolopyrrole were employed because of their proven interest as dyes, especially for organic solar cells. The natural product myrtenal was used as the source of stereochemistry, introduced through a Kröhnke reaction of a thiophene-bearing pyridinium salt and diketopyrrolopyrroles were prepared through Suzuki coupling with this chiral moiety at one end only as well as at both ends. Absorption spectroscopy and electrochemistry confirmed the potential suitability of the compounds for photovoltaic devices. The nanostructures formed by the compounds have been probed with circular dichroism spectroscopy in solution and in films. It is shown that a chiral C2 symmetric molecule assembles in solution giving a strong circular dichroic signal while as a film this optical activity is nulled, whereas an asymmetric homologue is most optically active as a thin film. The X-ray crystal structure of the asymmetric compound shows a polar order of the molecules that might explain this observation. The lack of optical activity in solution is very likely a result of the high solubility of the compound. The results reaffirm the sensitivity of circular dichroism spectroscopy to inter-chromophore organisation, whereas absorption spectroscopy in the visible region reveals only slight changes to the bands. The differing order in the compounds also affects their performance in bulk heterojunction photovoltaic devices. Atomic force microscopy of the blended thin films with the fullerene derivative usually employed (PC61 BM) showed that smooth and well mixed films were achieved, with the conditions required during spin coating depending greatly on the derivative, because of their differing solubility. The apparently better performance of the symmetrical compound (although with very low efficiency) is probably a result of the alignment of the molecules inferred by the circular dichroism experiments, whereas the asymmetric compound presumably adopts a twisted supramolecular organisation.
Highlights
₸ Pr es en taddr es s : La b or a t oir e MOLTE CH -An jou, UMR 6 2 0 0 CNRS UFR S cien ces, Bâ t
Th eappar en t ly b et t er p er for m a n ce of t h e s ym m et r ica l com p ou n d is p r ob a b ly a r es u lt of t h e a lign m en t of t h e m olecu les in fer r ed bythe cir cu la r d ich r ois m exp er im en t s, wh er ea stheas ym m et r ic com pou n d pres u m a bly a dopts a twis ted s u pra m olecu la r orga n is a tion
Com pou n d 3 wa s con s tru cted by Kr öh n k e r ea ct ion of p yr id in iu m s a lt 2 (m a d e fr om 1 ) a n d (1 R )-(−)-m yr t en a l.[36,37]
Summary
Wh ile ch ir almat er ia ls s h ow r em arkab le s elf-a s s em b led n a n os t r u ct u r es , [1,2] in t h e con t ext of fu n ct ion almat er ia ls for or ga n ic p h ot ovolt a ic d evices (OPVs ) t h e in flu en ce of s ym m et ryandst er eoch em is t r y p os s es s ed b y m olecu la r or p olym er ic com p on en t s d oes n ot s h ow a n y con s is t en t t r en d s . [3-8] Th e d egr ee andnature of m olecu la r or d er in t h e a ct ive la yer of OPVs is a vit a l fa ct or in flu en cin g p er for m a n ce, [9-16] wh er esupram olecu la rarran gem en t [17] in flu en ces m or p h ology, excit on s p lit t in g a n d ch a r ge ca r r ier m ob ilit y.[18]. Ma t er ia ls b a s ed on 1 ,4 -d ik et op yr r olo[3 ,4 -c]p yr r ole (DPP, Figu r e 1 ) h a ve ga in ed con s id er a b le in t er es t a s a ct ive la yer con s t it u en t s in OPVs a n d or ga n ic field -effect trans is t or s (OFE Ts ) a s a r es u lt of t h eir h igh ch a r ge ca r r ier m ob ilit y, fa vou r a b le p h ot op h ys ica l p r op er t ies , s yn t h et ic a cces s ib ilit yandstab ilit y.[26-29]. Gen e ra l ch em ica l s tru ctu re of th e DPP ch rom op h ore a n d p os s ib le p o s ition s for ch ira lsubs titu e n ts , la tera l in b lu e ("s id e" s u b s titu tion ) andma in ch a in in red ("en d " s u b s titu tion w h e n th e s tereoge n ic ce n tre is a t th e extrem e of th e red s y m b ol)
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