We herein report a theoretical analysis based on a density functional theory/time-dependent density functional theory (DFT/TDDFT) approach to understand the different phosphorescence efficiencies of a family of cyclometalated platinum(II) complexes: [Pt(NCN)Cl] (1; NCN = 1,3-bis(2-pyridyl)phenyl(-)), [Pt(CNN)Cl] (2; CNN = 6-phenyl-2,2'-bipyridyl(-)), [Pt(CNC)(CNPh)] (3; CNC = 2,6-diphenylpyridyl(2-)), [Pt(R-CNN)Cl] (4; R-CNN = 3-(6'-(2''-naphthyl)-2'-pyridyl)isoquinolinyl(-)), and [Pt(R-CNC)(CNPh)] (5; R-CNC = 2,6-bis(2'-naphthyl)pyridyl(2-)). By considering both the spin-orbit coupling (SOC) and the electronic structures of these complexes at their respective optimized singlet ground (S(0)) and first triplet (T(opt)(1)) excited states, we were able to rationalize the experimental findings that 1) 1 is a strong emitter while its isomer 2 is only weakly emissive in CH(2)Cl(2) solution at room temperature; 2) although the cyclometalated ligand of 3 has a higher ligand-field strength than that of 1, 3 is nonemissive in CH(2)Cl(2) solution at 298 K; and 3) extension of pi conjugation at the lateral aryl rings of the cyclometalated ligands of 2 and 3 to give 4 and 5, respectively, leads to increased emission quantum yields under the same conditions. We found that Jahn-Teller and pseudo-Jahn-Teller effects are operative in complexes 2 and 3, respectively, on going from the optimized S(0) ground state to the optimized T(opt)(1) excited state, and thus lead to large excited-state structural distortions and hence fast nonradiative decay. Furthermore, a strong-field ligand may push the two different occupied d orbitals so far apart that the SOC effect is small and the radiative decay rate is slow. This work is an example of electronic-structure-driven tuning of the phosphorescence efficiency, and the DFT/TDDFT approach is demonstrated to be a versatile tool for the design of phosphorescent materials with target characteristics.

All that glitters is gold: highly phosphorescent gold(III) complexes with extended π-conjugated cyclometalating ligands exhibit rich photophysical and photochemical properties. They act as efficient photocatalysts/photosensitizers for oxidative functionalizations of secondary and tertiary benzylic amines and homogeneous hydrogen production from a water/acetonitrile mixture.

Direct functionalization of CÀH bonds is an appealing strategy in organic synthesis [1] but its practical application has so far been difficult to realize. The selective functionalization of primary C À H bonds of alkanes that also contain secondary and/or tertiary C À H bonds is a great challenge, as CÀH bond energy follows an order primary > secondary > tertiary. [1c,d] In seminal works by Bergman, [1b] Jones, [1c] and their respective co-workers, stoichiometric reactions of alkanes with [Cp*(Me 3 P)M] (Cp* = C 5 Me 5 ; M = Rh, Ir) resulted in the formation of C À M bonds by selective activation of primary C À H bonds. Subsequent work by Hartwig and coworkers [1g,i, 2] demonstrated C À B bond formation by stoichiometric and catalytic functionalization of primary CÀH bonds mediated by tungsten, rhodium, or ruthenium complexes. The high selectivity for primary CÀH bond functionalization in these C À M or C À B bond-formation reactions (Scheme S1 in the Supporting Information) is considered to result from kinetic factors or steric interaction between the metal complexes and alkanes. [1i, 3] A well-established process in CÀC bond formation by direct CÀH bond functionalization is the metal-catalyzed intra-and intermolecular carbenoid insertion into C À H bonds, with diazo compounds as the carbene source. [1o, 4] These catalytic C À C bond-formation reactions generally feature lower selectivity for primary CÀH bonds than for secondary and tertiary CÀH bonds. For example, a selectivity order of primary < secondary < tertiary C À H bonds has been observed for the extensively investigated carbene insertion catalyzed by rhodium complexes, [4,5] possibly because of the electron density order of primary < secondary < tertiary CÀH bonds, which renders primary CÀH bonds the least susceptible to attack by electrophilic rhodium-carbene intermediates.[5] By manipulating the steric or electronic properties of the metal catalysts, a selectivity for primary C À H bonds of alkanes comparable to that for secondary or tertiary C À H bonds was observed, [6] with the highest primary/secondary and primary/tertiary ratio per CÀH bond being 1.17:1.0[6b] and 1.0:0.9, [6c] respectively. Herein we report a highly selective primary CÀH bond functionalization by metal-catalyzed carbenoid insertion into the C À H bonds of alkanes (Scheme 1), which features a primary/secondary selectivity (that is, the primary/secondary ratio per C À H bond) of up to 11.4:1. We have also accomplished highly enantioselective functionalization of secondary C À H bonds with ee values of up to 93 % and product turnovers up to 6100 through metal-mediated carbenoid CÀH bond insertion reactions.Our studies in this work were inspired by previous work from the research groups of Callot [6a,b] and Suslick. [7] In the 1980s, Callot and co-workers reported that the primary C À H bond selectivity for the reaction of linear alkanes with ethyl diazoacetate (N 2 CHCO 2 Et, EDA) catalyzed by [Rh(por)I] (H 2 (por) = meso-tetraarylporphyrin) increases with the size o...