Tightly Bound Double‐Caged [60]Fullerene Derivatives with Enhanced Solubility: Structural Features and Application in Solar Cells

Abstract

A series of novel highly soluble double-caged [60]fullerene derivatives were prepared by means of lithium-salt-assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four-membered cycle and a pyrrolizidine bridge with an ester function CO R (R=n-decyl, n-octadecyl, benzyl, and n-butyl; compounds 1 a-d, respectively), as demonstrated by NMR spectroscopy and X-ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one-electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to abstraction of an electron from the nitrogen lone-electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a-d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron-transfer reorganization energy relative to pristine C . Neat thin films of the n-decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10 cm V s , which was comparable to phenyl-C -butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as-cast poly(3-hexylthiophene-2,5-diyl) (P3HT)/1 devices relative to the as-cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge-transport properties - both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double-caged molecules. Test P3HT/1 OSCs demonstrated power-conversion efficiencies up to 2.6 % (1 a). Surprisingly low optimal content of double-caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity.

Electronic Communication between S=1/2 Spins in Negatively‐charged Double‐caged Fullerene C60 Derivative Bonded by Two Single Bonds and Pyrrolizidine Bridge

“…[22] Normally, neutrald oubly bonded( C 60 ) 2 or (C 70 ) 2 dimers are formedo nly under pressure, [23,24] or mechanochemicala ctivation, [6] and the present effect can be regarded as "chemical pressure". The bridging bonds in the cyclobutane ring in the dianionic bispheroid are comparable with those of the neutralm olecule: 1.54(3) and 1.57(3) (Figure1b) vs.1 .553(4) and 1.601(4) , [20] respectively,d espite the repulsion of negative charges. Tight bridging brings fullerene C 60 cages very close togethert hat provides the formationo fs hort contacts between the sp 2 carbon atoms of the two cages:2 .85a nd 2.93 in the neutral bispheroid and 2.89 and 2.91 [20] in the (bispheroid) 2À dianions in 1.C omparison of the geometries of the (bispheroid) 2À dianion and negatively charged (C 60 À ) 2 dimer is given in the Supporting information ( Figure S4).…”

“…The bridging bonds in the cyclobutane ring in the dianionic bispheroid are comparable with those of the neutralm olecule: 1.54(3) and 1.57(3) (Figure1b) vs.1 .553(4) and 1.601(4) , [20] respectively,d espite the repulsion of negative charges. Tight bridging brings fullerene C 60 cages very close togethert hat provides the formationo fs hort contacts between the sp 2 carbon atoms of the two cages:2 .85a nd 2.93 in the neutral bispheroid and 2.89 and 2.91 [20] in the (bispheroid) 2À dianions in 1.C omparison of the geometries of the (bispheroid) 2À dianion and negatively charged (C 60 À ) 2 dimer is given in the Supporting information ( Figure S4). From this comparison it is seen that fullerene spheres approach closely in the (bispheroid) 2À dianion suggestings tronger stacking-like p-p interaction between fullerenec ages.…”

“…Synthesis Pristine double-caged fullerene C 60 derivative( Scheme 1) was obtainedf rom fullerene C 60 ,p araformaldehyde and benzyl glycine ester via as ingle-stage lithium salt-assisted 1,3-dipolar cycloaddition approach. [20] Its molecular structure was determined by means of MALDI mass-spectrometry,U V-vis-NIR and 13 C, 1 Ha nd 1 H- 13 CNMR spectroscopy( see SI for details). The isolated bispheroid compound was subjected to reduction with an excess of potassium graphite in the presence of exact stoichiometric amountso fc ryptand[2.2.2].T he above approach was found to provide selective reduction of the bispheroid compound.…”

“…In contrast to this, pristine bispheroid forms very dense packing in which each bispheroid forms multiple short van der Waals C···C contacts with eight neighbouring bispheroids. [20] Optical properties IR spectra of salt 1 and pristine bispheroid are shown in Figure S3, and the positions of the peaks in these spectra are listed in Table S1. Removal of most of symmetries of the parentC 60 cage causes splitting of the strong F 1u (1) and F 1u (2) modes into three or even four bands in both neutral bispheroid and the (bispheroid) 2À dianion.H owever,t he principal reduction-induced changes are similar to those in pristine C 60 .…”

“…One example that is considered in the presentw ork is shown in Scheme 1. [20] The two C 60 cages are tightly linked via both ac ommonc yclobutaneb ridge and an additional pyrrolizidinem oiety,a ltogether via four sites on each cage. In this work we presentt he reductiono ft he compound showni nS cheme 1w ith potassium graphite in the presence of cryptand[2.2.2] in different stoichiometry.…”