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Organic conjugated compounds are envisaged as functional materials for fabricating devices able to drive low-cost, lowperformance consumer electronics. [1][2][3][4] To reach this goal, however, a better understanding of their electrical behavior is needed. In this view, organic single crystals offer the interesting and unique opportunity to investigate the intrinsic electrical behavior of organic materials, excluding hopping phenomena due to grain boundaries and structural imperfections. Their structural asymmetry also allows the investigation of the correlation between their 3D order and their charge-transport characteristics. [5,6] One useful investigation tool in this sense may be found in organic field-effect transistors (OFETs), which can provide precious information on the nature of the chargetransport phenomena in organic materials. Indeed, single-crystal organic transistors, where the active channel is a single crystal, exhibited up to now the best performances in terms of charge-carriers mobility, reaching time-of-flight (TOF)-measured values as high as 400 cm 2 V À1 s À1 , [7] and FET-measured mobilities of several units, up to tens of cm 2 V À1 s À1. [5,8,9] In this light, macroscopic (millimeter-sized) self-standing crystals suitable for being manipulated and selectively deposited on any surface and in any position with respect to existing electrodes are very useful, and recent studies showed that macroscopic crystals of rubrene present 2D electrical anisotropy. [8,10,11] For obtaining crystals suited for these investigations, vacuumbased methods are up to now the most exploited strategy. [5] However, macroscopic organic crystals may be easily grown also from solution, permitting a considerable degree of control over the final crystal characteristics in terms of dimensions [12] and of the developed crystallographic phase. [13] The suitability of solution grown (SG) organic crystals for electronic studies has been recently confirmed by a report on dicyclohexyl-a-quaterthiophene single crystals. The crystals were grown from solution and used as active materials in FETs, demonstrating 2D electrical anisotropy [14] even though in this case the studied crystals had dimensions in the micrometers domain, and an investigation of their electrical behavior in the third dimension was not presented.Here, we report on millimeter-sized SG organic single crystals based on 4-hydroxycyanobenzene (4HCB, Fig. 1), which exhibited 3D anisotropic electrical properties along the three crystallographic axes a, b (constituting the main crystal flat face), and c (the crystal thickness), measured over several different samples. FET devices were used to estimate the directional carrier mobilities in the dark at room temperature and atmosphere along the two main axes a and b, reaching top values up to 8 Â 10 À2 and 9 Â 10 À3 cm 2 V À1 s À1 for m a and m b , respectively.[15]Along the crystal thickness, axis c, the mobility was determined by means of space-charge-limited current (SCLC) measurements, which delivered a maximum value...

Four Cu(I) complexes with general formulas [Cu(N^N)(2)][BF(4)] and [(P^P)Cu(N^N)][BF(4)] were prepared, where N^N stands for 2-(2-tert-butyl-2H-tetrazol-5-yl)pyridine and P^P is a chelating diphosphine, namely bis-(diphenylphosphino)methane (dppm), bis-(diphenylphosphino)ethane (dppe) or bis[2-(diphenylphosphino)phenyl]ether (POP). In an acetonitrile medium, the Electro-Spray Ionization Mass Spectrometry (ESI-MS) determination provided the preliminary evidence for the occurrence of the dppm-containing complex as a mixture of a cationic mononuclear [Cu(N^N)(dppm)](+) species and a bis-cationic dinuclear [Cu(2)(N^N)(2)(dppm)(2)](2+)-type compound. Definitive evidence of peculiar structural features came from X-ray crystallography, which showed both the dppm- and, unexpectedly, the dppe-based heteroleptic compounds to crystallize as diphosphine-bridged Cu(I) dimers, unlike [Cu(N^N)(2)](+) and [(POP)Cu(N^N)](+) which are mononuclear species. In solutions of non-coordinating solvents, (31)P NMR studies at variable temperatures and dilution titrations confirmed that the dppm-based complex undergoes a monomer-dimer dynamic equilibrium, while the dppe-containing complex occurs as the bis-cationic dinuclear species, [Cu(2)(N^N)(2)(dppe)(2)](2+), within a concentration range comprised between 10(-2) and 10(-4) M. Differences among heteroleptic complexes might be related to the smaller natural bite angle displayed by dppm and dppe phosphine ligands (72° and 85°, respectively), with respect to that reported for POP (102°). The electrochemical features of the new species have been investigated by cyclic voltammetry. Despite the irreversible and complicated redox behaviour, which is typical for copper complexes, the reductions have been attributed to the tetrazole ligand whereas the oxidations are characterized as Cu(I/II) processes with a substantial contribution from the P^P-based ligands in the case of the heteroleptic species. All the four complexes are weakly or not luminescent in CH(2)Cl(2) solution, but heteroleptic complexes are bright green luminophores in a solid matrix, with quantum yields as high as 45% (dppm complex) even at room temperature. This makes them potential candidates as cheap emitting materials for electroluminescent devices.

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