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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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