Supramolecular organization of liquid-crystal dimers-bis-cyanobiphenyl alkanes on HOPG by scanning tunneling microscopy

2D supramolecular organization of a series of six cyanobiphenyl bimesogens deposited on highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM). The adsorbatesare 1,ω-bis(4-cyanobiphenyl-4’-yl)alkanes (CBnCB) with different length of their flexible alkyl spacer (containing 7 to 12 methylene groups). Microscopic investigations at the molecular resolution allow fordetailed analysis of the effect of the alkyl spacer length on the type and the extent of the resulting 2Dorganization. It has been demonstrated that bimesogens with shorter spacers (7 and 8 methylene units)organize in a similar manner characterized by the formation of two types of differently ordered monolayers: dense packed, wherein the molecules are oriented in one direction and ordered into parallel rows(layer structure), or less densely packed wherein they are organized into a chiral windmill-like structure. For derivatives with longer spacers (ranging from 9 to 12 methylene units) additional effect of parity of carbon atoms in the spacer (even versus odd) is observed. In this range of the spacer length even membered bimesogens are still organized in a typical layer structure. However, odd-membered dimers exhibit a much complex 2D supramolecular organization with a larger unit cell and a helical arrangement of the molecules. Careful comparison of this structure with the 3D structural data derived from the X-ray diffraction investigations of single crystals indicates that for these bimesogens a clear correlation exists between the observed complex 2D supramolecular organization in the monolayer and the organization in one of the crystallographic planes of the 3D nematic twist-bent phas

Supramolecular organization of liquid-crystal dimers-bis-cyanobiphenyl alkanes on HOPG by scanning tunneling microscopy

2D supramolecular organization of a series of six cyanobiphenyl bimesogens deposited on highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM). The adsorbatesare 1,ω-bis(4-cyanobiphenyl-4’-yl)alkanes (CBnCB) with different length of their flexible alkyl spacer (containing 7 to 12 methylene groups). Microscopic investigations at the molecular resolution allow fordetailed analysis of the effect of the alkyl spacer length on the type and the extent of the resulting 2Dorganization. It has been demonstrated that bimesogens with shorter spacers (7 and 8 methylene units)organize in a similar manner characterized by the formation of two types of differently ordered monolayers: dense packed, wherein the molecules are oriented in one direction and ordered into parallel rows(layer structure), or less densely packed wherein they are organized into a chiral windmill-like structure. For derivatives with longer spacers (ranging from 9 to 12 methylene units) additional effect of parity of carbon atoms in the spacer (even versus odd) is observed. In this range of the spacer length even membered bimesogens are still organized in a typical layer structure. However, odd-membered dimers exhibit a much complex 2D supramolecular organization with a larger unit cell and a helical arrangement of the molecules. Careful comparison of this structure with the 3D structural data derived from the X-ray diffraction investigations of single crystals indicates that for these bimesogens a clear correlation exists between the observed complex 2D supramolecular organization in the monolayer and the organization in one of the crystallographic planes of the 3D nematic twist-bent phas

Copolymers Containing 1-Methyl-2-phenyl-imidazole Moieties as Permanent Dipole Generating Units: Synthesis, Spectroscopic, Electrochemical, and Photovoltaic Properties

New donor–acceptor conjugated alternating or random copolymers containing 1-methyl-2-phenylbenzimidazole and benzothiadiazole (P1), diketopyrrolopyrrole (P4), or both acceptors (P2) are reported. The specific feature of these copolymers is the presence of a permanent dipole-bearing moiety (1-methyl-2-phenyl imidazole (MPI)) fused with the 1,4-phenylene ring of the polymer main chain. For comparative reasons, polymers of the same main chain but deprived of the MPI group were prepared, namely, P5 with diketopyrrolopyrrole and P3 with both acceptors. The presence of the permanent dipole results in an increase of the optical band gap from 1.51 eV in P3 to 1.57 eV in P2 and from 1.49 eV in P5 to 1.55 eV in P4. It also has a measurable effect on the ionization potential (IP) and electrochemical band gap (EgCV), leading to their decrease from 5.00 and 1.83 eV in P3 to 4.92 and 1.79 eV in P2 as well as from 5.09 and 1.87 eV in P5 to 4.94 and 1.81 eV in P4. Moreover, the presence of permanent dipole lowers the exciton binding energy (Eb) from 0.32 eV in P3 to 0.22 eV in P2 and from 0.38 eV in P5 to 0.26 eV in P4. These dipole-induced changes in the polymer properties should be beneficial for photovoltaic applications. Bulk heterojunction solar cells fabricated from these polymers (with PC71BM acceptor) show low series resistance (rs), indicating good electrical transport properties. The measured power conversion efficiency (PCE) of 0.54% is limited by the unfavorable morphology of the active layer