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The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX2(X = Cl–, Br–, I–) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX2perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X2] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpbto a mixture of ISC and fluorescence in [Zn(dpb)Cl2] and [Zn(dpb)Br2] and to nonradiative decay in [Zn(dpb)I2]. Coordination of ZnX2and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet–triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

In an effort to probe the influence of structural isomerism on the excited-state properties of a naphthalene-based donor–acceptor (D–A) system, four 1-naphthalenecarbonitrile compounds with amine substituents in the 2-, 3-, and 4-positions were synthesized and their photophysical properties were examined. Specifically, the molecules 2-dimethylamino-1-naphthalenecarbonitrile (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P), 3-dimethylamino-1-naphthalenecarbonitrile (3DA), and 4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were studied. The substitution position of the amine donor has a significant impact on both the ground-state absorption and excited-state properties of the complexes in toluene solution. The energy, band shape, and extinction coefficient of the ground-state absorption spectra are highly dependent on the substitution position of the amine donor. All of the derivatives exhibit fluorescence at room temperature. The fluorescence observed from 2DA, 2P, and 3DA demonstrates a vibronic structure with all three molecules possessing Stokes shifts on the order of 40 nm, whereas the fluorescence observed from 4P is broad and has a Stokes shift 2 times greater than the other derivatives. The fluorescence lifetimes, fluorescence quantum yields, and intersystem crossing quantum yields vary greatly with the substitution position of the amine donor. 2DA and 2P display intermediate fluorescence lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing the greatest intersystem quantum yield (0.80). 3DA has a much greater fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82) at the expense of the intersystem crossing quantum yield (0.12). 4P has the shortest lifetime (0.53 ns), with the lowest fluorescence and intersystem crossing quantum yields (<0.05). The singlet–triplet energy gaps are nearly identical for 2DA, 2P, and 3DA with values on the order of 0.70 eV. This singlet–triplet gap is larger in 4P, with a calculated value of 0.94 eV. The triplet–triplet absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in the UV and visible regions with maxima around 330 and 500 nm characterize all three spectra. The triplet excited-state extinction coefficient values for 3DA were found to be 1.5 times larger than those in 2DA and 2P. The triplet–triplet absorption spectrum of 4P is markedly different from the triplet–triplet absorption spectra of the other derivatives. The spectrum is broad, with the four local maxima observed at 374, 445, 624, and 774 nm. All four molecules display delayed fluorescence and laser-power-dependent triplet excited-state decay kinetics, indicating the involvement of triplet–triplet annihilation in the deactivation of the triplet excited states. Both the intrinsic triplet lifetimes and triplet–triplet annihilation rate constants were determined. These values are similar for all of the derivatives with triplet lifetimes on the order of 100 μs and diffusion-controlled rates of triplet–triplet annihilation.

Four new laterally asymmetric dinuclear gold(I) complexes made from a new chromophoric ligand (Au-DiBTF0–3) have been characterized. Changing the organic ancillary ligand from a phosphine ((Au-DiBTF0(PMe3) and Au-DiBTF1(PCy3)) to an N-heterocyclic carbene (Au-DiBTF2) with an alkynyl linking moiety or modifying the alkynyl linkage to a triazolyl moiety with the ancillary ligand being an N-heterocyclic carbene (Au-DiBTF3) impacts the ground- and excited-state properties of these systems. All four complexes exhibit structured absorption and emission spectra with weak phosphorescence. A bathochromic shift is observed in both the ground-state absorption and luminescence spectra as the series varies from Au-DiBTF0to Au-DiBTF3. The dinuclear alkynyl complexes (Au-DiBTF0–2) display decreased fluorescence lifetimes and fluorescence quantum yields along with more efficient intersystem crossing when the capping ligand is changed from a trialkylphosphine to an N-heterocyclic carbene. This change results in comparable rates of radiative decay and intersystem crossing and negligible rates of nonradiative decay. Changing the π-bridging moiety (Au-DiBTF3) results in a diminished fluorescence quantum yield, shorter fluorescence lifetime, and increased intersystem quantum yield, resulting in faster intersystem crossing accompanied by slower radiative decay and more efficient nonradiative decay relative to the alkynyl-bridged complexes. Density functional theory calculations are in accord with the observed photophysics, with nearly identical S1-to-T2energy gaps for the dinuclear alkynyl complexes (Au-DiBTF0and -2) and a smaller energy gap for Au-DiBTF3. Experimentally, Au-DiBTF3has the highest rate constant and quantum yield of intersystem crossing of the new gold(I) organometallics.

The dipyrrolonaphthyridinedione (DPND) core can be readily converted into a series of acid-responsive quinazolinoindolizinoindolizinoquinazolines through a two-step route involving direct arylation followed by acid-catalyzed condensation. Unlike the majority of previously obtained DPNDs, these nonplanar dyes bearing eight fused rings are almost nonfluorescent, which is attributed to fast internal conversion relative to radiative decay and intersystem crossing.

The syntheses and photophysical characterization of five new gold(I) complexes bearing diphenylamine-substituted fluorenyl moieties are reported; four are characterized by X-ray diffraction crystallography. Ancillary ligation on gold(I) is provided by organophosphine and N-heterocyclic carbene ligands. Two complexes, Au-DPA0and Au-DPA1, are σ-aryls, two, Au-ADPA0and Au-ADPA1, are σ-alkynyls, and one, Au-TDPA1,is a σ-triazolyl bound through carbon. All complexes show vibronically structured absorption and luminescence bands that are assignable to π–π* transitions localized on the diphenylamine-substituted fluorenyl π system. The excited-state dynamics of all five chromophores are governed by selection of the ancillary ligand and σ attachment of the diphenylamine-substituted fluorenyl moiety. All of these chromophores are dual luminescent in a toluene solution at 298 K. The luminescence from the aryl derivatives, Au-ADPA0and Au-DPA1, appears green. The alkynyl derivative containing a phosphine ancillary ligand, Au-ADPA0, is a white-light emitter, while the alkynyl derivative containing an N-heterocyclic carbene ancillary ligand, Au-ADPA1, is a yellow-light emitter. The luminescence from the triazolyl-linked chromophore, Au-TDPA1, appears as yellow-green. Spin-restricted density functional theory calculations support the assignments of ligand-centric optical transitions but with contributions of ligand-to-metal charge transfer involving the vacant Au 6p orbital.