emissions between 77 and 295 K, in contrast to the small red shifts observed for the -ir-ir* emissions of analogous Rh(III) complexes.This type of nuclear barrier may be sufficiently important in ortho-metalated Ir(III) complexes to prohibit thermal equilibration of their close-lying MLCT excited states even though the electronic coupling of these states is promoted by their Ir-C bonds. In fact, these same Ir-C bonds may, through trans effects, be responsible for distortions which give rise to barriers. The nature of the nuclear distortions in which these barriers originate is presently under investigation.

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We have obtained 27 Al, 69 Ga, 71 Ga, 113 In, and 115 In static and ‘‘magic-angle’’ sample-spinning (MASS) solid-state nuclear magnetic resonance (NMR) spectra of a series of polycrystalline III–V semiconductors (AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb) at magnetic field strengths of 8.45 and 11.7 T. Line-broadening mechanisms have been identified by comparing static and MASS NMR results. By applying the MASS technique, dipolar, first-order quadrupolar, and pseudodipolar interactions are removed. Thus, the MASS spectral linewidth (of the central, 1/2↔−1/2, transition) is dominated by exchange and/or second-order quadrupolar interactions. For compounds having the cubic zinc blende structure, the exchange interaction dominates, and exchange interaction constants can be determined. For AlP, AlAs, AlSb, GaP, GaAs, InP, and InAs, first-order quadrupole effects are evident as spinning sidebands (SSBs), due to the satellite transitions. These effects are due to a small distribution of electric field gradients caused by lattice defects, and result in an overall Lorentzian profile for the SSB envelope. For compounds having the hexagonal wurtzite structure (GaN and AlN), the second-order quadrupolar interaction is the main line-broadening mechanism, and we find isotropic chemical shifts δi of 333 ppm [from 1 M Ga(NO3 )3 ] for GaN, and of 115 ppm [from 1M Al(H2 O)6 Cl3 ] for AlN, and nuclear quadrupole coupling constant e2 qQ/h of 2.8 MHz for 69 GaN, 1.7 MHz for 71 GaN, and of 2.2 MHz for AlN.