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A series of paracyclophane derivatives that hold chromophores of varying conjugation lengths has been synthesized using palladium-mediated coupling reactions. These molecules mimic solid-state interactions in main-chain polychromophores and conjugated emissive polymers such as poly(p-phenylenevinylene) (PPV). Their optical properties give insight into the energetics of photoexcitations localized in a discrete chromophore relative to a state containing the through-space delocalized paracyclophane core. Thus, 4-vinyl[2.2]paracyclophane (5) is obtained by reaction of 4-bromo[2.2]paracyclophane (3) and ethylene using Pd(OAc)2 and P(o-tol)3. Similar reactions starting with pseudo-o- or pseudo-p-dibromo[2.2]paracyclophane (4a and 4b, respectively) give the pseudo-o- and pseudo-p-divinyl products (6a and 6b, respectively). Using styrene instead of ethylene provides the styryl-substituted products. Thus, 4-styryl[2.2]paracyclophane (7) is obtained from 3 while pseudo-p- and pseudo-o-distyryl[2.2]paracyclophane (1a and 1b) are obtained from 4a and 4b, respectively. Compounds 1a and 1b can be viewed as stilbene dimers that have a pair of cofacial arene units at a fixed distance. Pseudo-p-bis(4-vinyl-styryl)[2.2]paracyclophane (9) was prepared by reaction of CH2PPh3 with pseudo-p-bis(4-carboxaldehyde-styryl)[2.2]paracyclophane. Reacting 4-(4-tert-butylstyryl)styrene with 3, 4a, or 4b under Heck-type conditions gives 4-[4-(4-tert-butylstyryl)styryl][2.2]paracyclophane (10) and pseudo-p- and pseudo-o-bis[4-(4-tert-butylstyryl)styryl][2.2]paracyclophane (2a and 2b), respectively. The observed trends in absorption, fluorescence and radiative lifetime of these compounds are reported and analyzed using collective electronic oscillators (CEO) representing the changes induced in the reduced single-electronic density matrix upon optical excitation. Comparison of the CEO of the aggregates with the corresponding monomers using two-dimensional plots provides an efficient method for tracing the origin of the various optical transitions by identifying the underlying changes in charge densities and bond orders. For 5, 6a,b, 7, and 1a,b the emission is red-shifted from the “monomeric” compound and featureless, reminiscent of excimer qualities. The emissions of 9, 10, and 2a,b are similar to the “monomer” and display vibronic structure. Thus, for the smaller chromophores, emission occurs from a state containing the through-space delocalized paracyclophane core. In the situation where extended chromophores, with more stable excited states, are held together with the paracyclophane core, the photophysics of the individual chromophores dominates. The present analysis is relevant to the design and synthesis of organic molecules with desired optical properties.

Protonation of TpM(PR 3 )H 2 (M ) Rh, Ir) complexes with HBF 4 ‚Et 2 O or [H(Et 2 O) 2 ][B(Ar) 4 ] (Ar ) 3,5-(CF 3 ) 2 C 6 H 3 ) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the 1 H NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T 1 (min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond lengths of 0.88-1.11 Å in the iridium complexes and 0.73-0.92 Å in the rhodium complexes depending on the relative rate of the dihydrogen rotational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J H-D or J H-T coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the 1 J H-D coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.Since the first report of a stable molecular hydrogen complex by Kubas, 1 the possibility that a fluxional polyhydride complex might also contain a dihydrogen ligand has been actively investigated. 2 In general, transition metal polyhydride complexes are characterized by high coordination numbers (CN 7-9) and high formal oxidation states. 3 Because several structures of nearly equivalent energy are available to seven-, eight-, and nine-coordinate complexes, rapid permutation of the hydride positions is often observed by 1 H NMR spectroscopy. As a result, structural characterization in solution depends upon indirect methods, in which the observed NMR parameters are a population-weighted average of all the hydride environments. For example, Crabtree and co-workers have employed T 1 measurements to detect short H-H contacts in a range of polyhydride complexes, including [Ir(PCy 3 ) 2 H 6 ] + and Fe-(PEtPh 2 ) 3 H 4 . 4 A quantitative treatment of relaxation in polyhydride complexes has been developed which allows useful structural information to be obtained from T 1 (min) data. [4][5][6] We have previously reported the structure and properties of cationic iridium complexes of the form [CpIr(L)H 3 ]BF 4 (L ) various PR 3 ), which have been shown to adopt iridium(V) trihydride structures in the solid state. 7,8 These complexes undergo a rapid hydride rearrangement which leads to a single hydride resonance in the 1 H NMR spectrum above ca. 220 K. However, at very low temperatures, spectra consistent with the solid state structure are obtained.In this paper we investigate the effect of substituting the Cp ligand of [CpIr(L)H 3 ]BF 4 complexes with the hydrotris(1-pyrazolyl)borate (Tp) 9 ligand. The new Tp complexes, [TpIr-(L)(H 2 )H]BF 4 (L ) PMe 3 , PPh 3 ), are formulated as dihydrogen/ hydride complexes, although only a single hydride resonance is observed...

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Low band gap molecular and polymeric organic materials are currently being considered for the active component in a variety of optoelectronic devices, most notably as the emissive layer in light-emitting diodes. 1 It is generally believed that the morphology adopted by these materials plays a fundamental role in defining their bulk performance. For example, completely amorphous and ultrapure thin films appear to provide the longest device lifetimes, combined with the highest efficiency and emissive brightness. 2 While sufficiently pure films of low molecular weight fluorescent compounds can be obtained by sublimation under high vacuum, these tend to recrystallize over time, thus precipitating device failure. 3 Luminescent polymeric materials can be designed to remain amorphous, even at elevated temperatures, but they are more difficult to obtain in high purity. 4,5 Furthermore, the close association of polymer chains in the solid state can lead to crystalline domains. 6 Ordered polymer regions appear to reduce emission efficiency by promoting low-energy non-or weakly emissive excimer or aggregate states. 7 Ultimately, because of facile energy migration, these low-energy sites can dominate the optical properties, even when present in small concentration. 8 In this paper, we describe a strategy to obtain precisely defined and readily purified luminescent materials of intermediate molecular size with amorphous morphology. For this purpose, new molecules containing a tetrahedral array of four stilbenoid units coupled to a central sp 3 -hybridized carbon atom have been prepared. 9 The rigid tetrahedral framework orients the chromophores such that the possibility of intramolecular π-stacking is minimized. These molecules are highly symmetrical and might be expected to favor a crystalline morphology, as observed for most structures which incorporate a tetraphenylmethane core. 10 However, we show that when the stilbenoid units are sufficiently long, crystalline packing becomes unfavorable and a stable amorphous phase is obtained. 11The parent compound in this series, tetrastilbenylmethane (2), is obtained via palladium-catalyzed Heck coupling of tetrakis(4iodophenyl)methane 12 (1) with styrene (see Scheme 1). 13 Reactions carried out under phase-transfer conditions 14 were found to give the highest yields (86%), and this procedure was used exclusively in subsequent coupling reactions. Slightly reduced yields (70%) were obtained using the Herrmann catalyst, 15 and only traces of 2 were observed using the traditional catalyst system 16 composed of a mixture of Pd(OAc) 2 and P(o-tol) 3 . Incomplete reaction was observed, even under optimized conditions, if tetrakis(4-bromophenyl)methane 11 was substituted for 1. Reactions carried out with pentafluorostyrene gave tetrakis-(pentafluorostilbenyl)methane (3), which is only slightly soluble in aromatic or chlorinated solvents. 17 Longer stilbenoid arms were obtained by coupling 4,4′-tertbutylvinylstilbene 18 with 1 to yield the pale yellow and freely soluble tetrakis(4-tert...

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Solutions of TpM(C 2 H 4 ) 2 (M ) Rh (1a) and Ir (1b)) react with 1 equiv of PPh 3 to yield TpM(PPh 3 )(C 2 H 4 ) (2a,b). The new complexes adopt trigonal-bipyramidal structures in solution with triphenylphosphine coordinated in the axial site and ethylene positioned in the equatorial plane. For 2a the axial and equatorial pyrazolyl arms of the Tp ligand exchange positions on the NMR time scale (∆G q ) 14.3 kcal mol -1 , 279 K); however, no exchange is observed in the case of 2b, even at 353 K (∆G q > 18.4 kcal mol -1 ). Complexes 2a,b react with molecular hydrogen to yield TpM(PPh 3 )H 2 (3a,b) and free ethylene. Kinetic studies of the iridium system show that this reaction is first order in both 2b and H 2 and is not inhibited by a 10-fold excess of ethylene or PPh 3 (k H 2 /k D 2 ) 1.26 ( 0.18). These results indicate that the H 2 addition reaction proceeds by rapid reversible dissociation of a pyrazolyl arm, through a square-planar (η 2 -Tp)Ir(PPh 3 )(C 2 H 4 ) intermediate. The hydridotris(1-pyrazolyl)borate (Tp) class of ligands generally forms stable metal complexes containing either a bidentate or tridentate array of nitrogen-ligated pyrazolyl arms: 1The subtle interplay between steric and electronic factors which favors one structure over another is clearly evident in the series of low-valent rhodium and iridium complexes of the form Tp R2 ML 2 (L ) CO, CNR, olefin). 2-9 The solution-phase, ground-state coordination geometry in these species has been shown to be either trigonal bipyramidal (tbp) or square planar (sp) or a mixture of both, depending on the metal center, the substituents of the Tp ligand, and the donor ligands. The tbp structure is related to the sp form by simple dissociation of an equatorial pyrazolyl arm. Since little additional ligand rearrangement is required, this dynamic process is often observed with only small activation barriers. In fact, a dynamic equilibrium between tbp and sp structures has recently been carefully examined by Venanzi and co-workers for an extensive series of rhodium complexes, Tp 3R,4R,5R Rh(LL) (LL ) 2CO, norbornadiene (NBD), cyclooctadiene (COD)). 9,10 Solution-phase IR spectroscopy of the bis-CO complexes showed that, in certain cases, both tbp and sp forms were present in solution. By altering the substituents of the Tp ligand and/or the solvent, it was possible to shift the equilibrium to favor either five-coordinate tbp complexes or four-coordinate sp complexes. Similar isomeric mixtures have been observed by Trofimenko and coworkers in more elaborately substituted Tp 3R,4R,5R Rh(CO) 2 complexes. 11 In most cases the solution-phase structure mirrors that determined in the solid state by single-crystal X-ray diffraction. However, small crystal packing forces can favor selective crystallization of the minor isomer, thereby providing evidence for its existence in solution. 12 The ground-state structure of TpM(C 2 H 4 ) 2 (M ) Rh (1a), 13 Ir (1b) 14,15 ) is not known with certainty. A static sp or tbp structure is expected to show a 2:1 pattern of pyrazolyl ...

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