A series of N-heterocyclic carbene (NHC) Ag(I) complexes have been prepared and used to study the dynamics of NHC ligand exchange in these Ag(I) complexes. These studies used solution-state variable-temperature (VT) 13 C NMR spectroscopy and the temperature-dependent changes in 13 C− 107/109 Ag coupling to determine activation energies for the ligand exchange process. The effects of concentration, bridging anions, and additives on the exchange process have been studied. The experimental activation energies for the NHC ligand exchange processes of these silver complexes are also compared with DFT calculations. The results are consistent with an associative mechanism for the Ag(I)−NHC exchange process.

Fluorescence excitation spectra for transition à 1 A u-X 1 A g of trans-glyoxal in a supersonic jet were recorded with laser excitation in a wavelength region near 395 nm. With resolution 0.04 cm Ϫ1 , most lines in these spectra are resolved and are assigned rotationally; of six bands assigned, three have c-type rotational structure, and another three have types a/b hybrid, a and b. Fluorescence decays with quantum beats, resulting from coherent excitation of S 1 and T 1 states, are observed for most rotational levels in this region. In spectra transformed to the frequency domain for these decays, the widths are larger than those obtained at low excitation energy. The widths from nonzero frequency lines, reflecting mostly the triplet character, indicate that the triplet state is dissociating. Fluorescence spectra of fragment HCO confirm that the dissociation channel correlated to the triplet surface of glyoxal involves formation of radical products. From an abrupt decrease of lifetime of the triplet state and onset of disappearance of fluorescence of glyoxal, the threshold for formation of fragment HCO from trans-glyoxal is determined to be 394.4 nm. With data for heat of formation this threshold yields an exit barrier 2.5Ϯ0.5 kcal/mol. Signal of HCO for wavelengths greater than the threshold position up to 400.5 nm is observed and is assigned to arise from the S 0 surface. Rotationally state-resolved appearance rates of HCO for a vibrational state near the dissociation threshold are measured. Relative to lifetimes obtained from eigenstates in the quantum-beat data, the appearance time of product HCO reflecting the overall depletion of glyoxal is on average longer. Some observed gateway states with enhanced yields of HCO are considered to result from strong singlet-triplet interaction.

By measuring the vibrational wavenumbers of their stretching modes in Raman and surface-enhanced Raman scattering (SERS) spectra, we investigated the strength of the CrÀCr bonds in metal-string complexes Cr 5 (tpda) 4 X 2 and Cr 7 (teptra) 4 (NCS) 2 (tpda = tripyridyldiamido; teptra = tetrapyridyltriamido; X = Cl À , NCS À ). The bands in SERS and Raman differ insignificantly in spectral positions, indicating no major structural variation between the solid and solution forms. For SERS measurements, these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify the stable structure. We identified both penta-and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. For pentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for the heptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of the vibrational normal modes, we assign the band at 280 cm À1 to the CrÀCr symmetric stretching mode of the s-form pentachromium complex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign 570 cm À1 to the stretching mode of the CrÀCr quadruple bond in the u-form for the pentachromium complex and 554/571 cm À1 analogously for the heptachromium complex. The bands for metal-related modes in SERS spectra might be enhanced because of interaction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increased absorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metalÀmetal stretching modes, yielding information about the strength of chromiumÀchromium multiple bonding.

Supporting information: Raman spectra at excitation wavelengths 532, 473, 632 and 1064 nm and SERS spectra of Ni 5 (tpda) 4 Cl 2 and Co 5 (tpda) 4 Cl 2 and a list of line positions and assignments for H 2 tpda.

The red and orange emitters (ANA-1-3) consisting of a 4-amino-1,8-naphthalic anhydride group were synthesized. The lowest absorption band of these ANA molecules centered at approximately 450 nm is assigned to be a charge-transfer transition with emission at 514-536 nm in nonpolar solvents such as n-hexane and at approximately 590-640 nm in polar solvents such as THF and CH(2)Cl(2) and in the solid states. Emission lifetimes are measured with time-correlated single photon counting. Shorter lifetimes are observed for the ANA molecules when dissolved in polar solvents compared with those in nonpolar solvents. Strong dipole-dipole interaction of ANA molecules with solvents is indicated. At high concentrations the measured emission lifetimes, generally shortened from self-quenching, are found to remain about the same order of magnitude in ANAs. This implies that the exciton states of aggregates are formed and they exhibit a relatively long lifetime. Crystallographic data of 4-(phenyl antracen-9-yl) (ANA-2) and 4-(phenyl-2-naphthyl) amino-1,8-naphthalic anhydrides (ANA-3) show that the molecules exist as dimeric structures with antiparallel head-to-tail stacking of naphthalic anhydride planes in addition to other pi-pi stacking. The strong dipole-dipole interactions and the pi-pi stacking account for the observed red-shifted emissions of ANAs in the powders. For films prepared from vacuum sublimation, a structure similar to that in the crystal but with less crystalline order is expected based on the emission wavelength. Several electroluminescent devices based on these ANAs are reported here; they emit orange-red light at 602-628 nm with high brightness and steady external quantum efficiency.