Co-reporter:Sarah F. Tyler, Eileen C. Judkins, Dmitry Morozov, Carlos H. Borca, Lyudmila V. Slipchenko, and David. R. McMillin
Journal of Chemical Education September 12, 2017 Volume 94(Issue 9) pp:1232-1232
Publication Date(Web):July 14, 2017
DOI:10.1021/acs.jchemed.7b00289
Electronic spectra often exhibit vibronic structure when vibrational and electronic transitions occur in concert. Theory reveals (1) that orbital symmetry considerations determine specific roles played by the nuclear degrees of freedom and (2) that the vibrational excitation is often highly regiospecific, that is, attributable to an identifiable subset of atoms within the molecule. Spectra obtained from a chromium(III) complex involving a macrocyclic ligand and two axially disposed butadiynide groups nicely illustrate many of the concepts involved.Keywords: Coordination Compounds; Inorganic Chemistry; Spectroscopy; Upper-Division Undergraduate; UV−Vis Spectroscopy;
Co-reporter:Lakshmi Nilakantan, David R. McMillin, and Paul R. Sharp
Organometallics 2016 Volume 35(Issue 14) pp:2339-2347
Publication Date(Web):July 6, 2016
DOI:10.1021/acs.organomet.6b00275
We report here a series of emissive biphenyl cyclometalated gold(III) diethyl dithiocarbamate complexes having H, CF3, OMe, and tBu substitutions on the biphenyl moiety. Synthesis of these complexes was accomplished by a single-step reaction of the appropriate dilithio-biphenyl reagent with Au(dtc)Cl2 (dtc = diethyl dithiocarbamate). All four complexes exhibit weak room-temperature phosphorescence in solution and much more intense phosphorescence in the solid state and in low-temperature glasses with lifetimes in the microseconds. From experimental data and computational modeling, the emission originates mainly from a metal-perturbed 3(π–π*) state of the biphenyl moiety with a minor contribution from ligand-to-ligand charge transfer. Weak solution emission is attributed to deactivation via a distorted charge-transfer state that is less accessible in the solid state or in a low-temperature glass.
Co-reporter:Abby J. Gaier and David R. McMillin
Inorganic Chemistry 2015 Volume 54(Issue 9) pp:4504-4511
Publication Date(Web):April 17, 2015
DOI:10.1021/acs.inorgchem.5b00340
This investigation explores binding interactions involving G-quadruplex DNA and two copper(II)-containing porphyrins (5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrinato)copper(II) and the sterically friendlier analogue (trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato)copper(II), or Cu(T4) and Cu(tD4), respectively. The study incorporates five different DNA sequences that support the formation of unimolecular and bimolecular G-quadruplex hosts capable of exhibiting at least nine different structures in toto. Absorbance and emission results establish that G-quadruplex DNA is more adept at sequestering Cu(tD4) compared with the bulkier Cu(T4) ligand, even though the predominant mode of uptake is by end-capping, irrespective of the porphyrin or DNA sequence employed. One of the more impressive observations is that the emission intensities exhibited by Cu(tD4) bound to G-quadruplex DNA are many-fold higher than corresponding signals obtained with single- or double-stranded DNA hosts. With human sequence DNA the Cu(tD4) system is also unusual in that it preferentially binds to structures containing antiparallel strands. Refining the binding properties of porphyrin ligands is of interest because work from many laboratories has established that stabilizing G-quadruplex structure is an effective way to inhibit telomerase, a key enzyme involved in the immortalization of most types of cancer cells.
Co-reporter:Yana A. Lyon; Adrienne A. Roberts
Journal of Chemical Education 2015 Volume 92(Issue 12) pp:2130-2133
Publication Date(Web):August 27, 2015
DOI:10.1021/acs.jchemed.5b00189
The laboratory experiment described provides insight into the energetics of hydrogen evolution at an electrode as well as the intrinsic barrier that typically impedes reaction. In the course of the exercise, students find that Sn(s) is thermodynamically capable of combining with protons to form hydrogen, but that the direct reaction occurs at a negligible rate in the absence of an applied overpotential or a suitable catalyst. Exploring the latter option, students fabricate mini-cells with tin as the anode and either copper or platinum as the cathode. Upon immersing the assemblies in HCl(aq), they observe vigorous formation of H2(g), but only when tin and platinum electrodes are present. They also detect a change in mass that is attributable to the loss of tin, and they attempt to reconcile the stoichiometry of reaction. The exercise requires minimal equipment and provides useful insight into processes that are potentially important to the future energy economy.
Co-reporter:Ryan T. Golkowski, Nicholas S. Settineri, Xikang Zhao, and David R. McMillin
The Journal of Physical Chemistry A 2015 Volume 119(Issue 48) pp:11650-11658
Publication Date(Web):November 16, 2015
DOI:10.1021/acs.jpca.5b08310
The f–f emissions of lanthanide-ion complexes have predictable emission energies and many practical applications, but the emitting states are generally impervious to the surroundings. This investigation explores ligand- and metal-centered emission processes for a series of mixed-ligand complexes of composition M(X-T)(NO3)3, where the metal ion is europium, gadolinium, terbium, or lutetium, and X-T denotes the tridentate ligand 2,2′:6′,2″-terpyridine (H-T), 4′-phenyl-2,2′:6′,2″-terpyridine (Ph-T), or 4′-pyrrolidin-N-yl-2,2′:6′,2″-terpyridine (pyrr-T). The presence of the pyrrolidinyl substituent imparts intraligand charge-transfer (ILCT) character to the ligand-based excited states and reduces the energy gap between the singlet and the triplet excited states. An enhanced rate of intersystem crossing results in a lutetium complex with a relatively small fluorescence quantum yield (0.15%) and a gadolinium complex with an impressive phosphorescence yield of 9.6% in deaerated solution. The Tb(pyrr-T)(NO3)3 system is unique because the relatively low-energy triplet ILCT state equilibrates with the emissive f–f state. The result is a truly remarkable f–f emission signal that is sensitive to the polarity of the local environment as well as the presence of dioxygen.
Co-reporter:Srijana Ghimire, Phillip E. Fanwick, and David R. McMillin
Inorganic Chemistry 2014 Volume 53(Issue 20) pp:11108-11118
Publication Date(Web):October 1, 2014
DOI:10.1021/ic501683t
This investigation explores DNA-binding interactions of various forms of an alkyl-substituted cationic porphyrin, H2TC3 (5,10,15,20-tetra[3-(3′-methylimidazolium-1′-yl)]porphyrin). The motivating idea is that incorporating alkyl rather than aryl substituents in the meso positions will enhance the prospects for intercalative as well as external binding to DNA hosts. The ligands may also be applicable for photodynamic and/or anticancer therapy. Methods employed include absorbance, circular dichroism, and emission spectroscopies, as well as viscometry and X-ray crystallography. By comparison with the classical H2T4 system, H2TC3 exhibits a higher molar extinction coefficient but is more prone to self-association. Findings of note include that the copper(II)-containing form Cu(TC3) is adept at internalizing into single-stranded as well as B-form DNA, regardless of the base composition. Surprisingly, however, external binding of H2TC3 occurs within domains that are rich in adenine–thymine base pairs. The difference in the deformability of H2TC3 versus Cu(TC3) probably accounts for the reactivity difference. Finally, Zn(TC3) binds externally, as the metal center remains five-coordinate.
Co-reporter:Abby J. Gaier, Srijana Ghimire, Sarah E. Fix, and David R. McMillin
Inorganic Chemistry 2014 Volume 53(Issue 11) pp:5467-5473
Publication Date(Web):May 14, 2014
DOI:10.1021/ic403105q
Absorbance, induced circular dichroism, and emission studies establish that the tetrasubstituted cationic porphyrin Cu(T4) preferentially binds externally to single-stranded (ss) DNA sequences, except in a purine-rich system like 5′-(dA)10-3′ where a degree of internalization occurs. On the other hand, the sterically friendly, disubstituted Cu(tD4) system exclusively binds to ss DNA by internalization, that is, pseudointercalation. By and large the results show that double-stranded DNA hosts decisively outcompete more flexible ss hosts for the uptake of a porphyrin, regardless of the binding motif. The findings are relevant because ss domains of DNA appear during replication, in different types of DNA-secondary structure, and as products of the disassembly of multistranded forms.
Co-reporter:Norman Lu, Lauren M. Hight, David R. McMillin, Jyun-Wei Jhuo, Wei-Cheng Chung, Kwan-Yu Lin, Yuh-Sheng Wen and Ling-Kang Liu
Dalton Transactions 2014 vol. 43(Issue 5) pp:2112-2119
Publication Date(Web):11 Nov 2013
DOI:10.1039/C3DT52713A
The yellow (1y) and orange (1o) crystalline polymorphs of [PtBr2(5,5′-bis(CF3CH2OCH2)-2,2′-bipyridine)] exhibit surprisingly short nearest neighbour Pt⋯Pt separations of 3.526 Å and 3.590 Å, respectively, at 295 K. Both distances are much shorter than those found in structures of the unsubstituted [PtBr2(2,2′-bipyridine)] analogue. Consistent with a linear chain structure in 1o and dimer formation in 1y, both solids exhibit emission spectra shifted to much longer wavelengths than that exhibited by the monomer in a low-temperature glass. Furthermore, the emission spectra of 1o and 1y shift to even longer wavelengths as the temperature decreases and the Pt⋯Pt separations contract. Till now delocalized emission of this type has been considered to be restricted to [PtCl2(diimine)] systems and implausible in PtBr2-containing analogues for steric reasons. Ironically, in the system at hand the bulky 5,5′-substituents apparently promote delocalization of the emission by forming a network of hydrogen-bonding-like C–H⋯F–C interactions that help shape the packing.
Co-reporter:Matthew A. Bork, Christopher G. Gianopoulos, Hanyu Zhang, Phillip E. Fanwick, Jong Hyun Choi, and David R. McMillin
Biochemistry 2014 Volume 53(Issue 4) pp:
Publication Date(Web):January 15, 2014
DOI:10.1021/bi401610t
Studies reveal that it is possible to design a palladium(II)-containing porphyrin to bind exclusively by intercalation to double-stranded DNA while simultaneously enhancing the ability to sensitize the formation of singlet oxygen. The comparisons revolve around the cations [5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(T4), and [5,15-di(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(tD4), in conjunction with A═T and G≡C rich DNA binding sequences. Methods employed include X-ray crystallography of the ligands as well as absorbance, circular dichroism, and emission spectroscopies of the adducts and the emission from singlet oxygen in solution. In the case of the bulky Pd(T4) system, external binding is almost as effective as intercalation in slowing the rate of oxygen-induced quenching of the porphyrin’s triplet excited state. The fractional efficiency of quenching by oxygen nevertheless approaches 1 for intercalated forms of Pd(tD4), because of intrinsically long triplet lifetimes. The intensity of the sensitized, steady-state emission signal varies with the system and depends on many factors, but the Pd(tD4) system is impressive. Intercalated forms of Pd(tD4) produce higher sensitized emission yields than Pd(T4) is capable of in the absence of DNA.
Co-reporter:Lauren M. Hight, Meaghan C. McGuire, Yu Zhang, Matthew A. Bork, Phillip E. Fanwick, Adam Wasserman, and David R. McMillin
Inorganic Chemistry 2013 Volume 52(Issue 15) pp:8476-8482
Publication Date(Web):July 24, 2013
DOI:10.1021/ic4004643
Introducing electron-donating groups extends the excited-state lifetimes of platinum(II)–terpyridine complexes in fluid solution. Such systems are of interest for a variety of applications, viz., as DNA-binding agents or as components in luminescence-based devices, especially sensors. The complexes investigated here are of the form [Pt(4′-X-T)Y]+, where 4′-X-T denotes a 4′-substituted 2,2′:6′,2″-terpyridine ligand and Y denotes the coligand. The π-donating abilities of −X and −Y increase systematically in the orders −NHMe < −NMe2 < −(pyrrolidin-1-yl) and −CN < −Cl < −CCPh, respectively. The results presented include crystal structures of two new 4′-NHMe-T complexes of platinum, as well as absorption, emission, and excited-state lifetime data for nine complexes. Excited-state lifetimes obtained in deoxygenated dichloromethane vary by a factor of 100, ranging from 24 μs for [Pt(4′-pyrr-T)CN]+ to 0.24 μs for [Pt(4′-ma-T)Cl]+, where ma-T denotes 4′-(methylamino)-2,2′:6′,2″-terpyridine and pyrr-T denotes 4′-(pyrrolidin-1-yl)-2,2′:6′,2″-terpyridine. Analysis of experimental and computational results shows that introducing a simple amine group on the terpyridine and/or a π-donating coligand engenders the emitting state with intraligand charge-transfer (ILCT) and/or ligand–ligand charge-transfer (LLCT) character. The excited-state lifetime increases when the change in orbital parentage lowers the emission energy, suppresses quenching via d–d states, and encourages delocalization of the excitation onto the ligand(s). At some point, however, the energy is low enough that direct vibronic coupling to the ground-state surface becomes important, and the lifetime begins to decrease again.
Co-reporter:Matthew A. Bork ; Hunter B. Vibbert ; David J. Stewart ; Phillip E. Fanwick
Inorganic Chemistry 2013 Volume 52(Issue 21) pp:12553-12560
Publication Date(Web):October 15, 2013
DOI:10.1021/ic4016367
Ruthenium(II) in combination with monodentate, bidentate, and tridentate ligands has proven to be a useful design for a variety of applications, but the majority of systems are virtually nonluminescent in solution. The goal of this work has been to design luminescent forms with practicable emission quantum yields, and the focus has been on [Ru(X-T)(dmeb)CN]+ systems, where X-T denotes 2,2′:6′,2″-terpyridine bearing substituent X at the 4′-position and dmeb denotes [2,2′-bipyridine]-4,4′-dicarboxylic acid, dimethyl ester. Results show that varying the π-electron-donating ability of the 4′-X substituent is an effective way to tune the energy and lifetime of the charge-transfer (CT) emission. The lifetime achieved in a room-temperature, fluid solution is as high as 175 ns, depending on the 4′-substituent and the solvent employed because the excited state is very polar. That represents a 20-fold improvement in lifetime relative to that of the prototype, [Ru(trpy)(bpy)CN]+, one of the earliest examples found to be luminescent in a fluid solution. A simple theoretical model proves to be capable of rationalizing all the experimental lifetimes. It suggests that, with the dmeb ligand available to accept the electron, enhancing the donor ability of the 4′-X substituent lowers the energy of the 3CT state and reduces the likelihood of thermally activated decay via a higher-energy d–d state. However, direct nonradiative decay to the ground state begins to reduce the excited-state lifetime whenever the emission maximum shifts beyond 750 nm. Within those limits, there is inevitably a maximal attainable lifetime, regardless of the method of tuning.
Co-reporter:Daniel P. Lazzaro, Phillip E. Fanwick, and David R. McMillin
Inorganic Chemistry 2012 Volume 51(Issue 20) pp:10474-10476
Publication Date(Web):October 4, 2012
DOI:10.1021/ic301593b
Many platinum(II) polypyridine complexes are good luminophores, an enigmatic exception being Pt(trpy)Ph+, where trpy denotes 2,2′:6′,2″-terpyridine. A new analysis suggests the complex is nonemissive due to 3SBLCT (sigma-bond-to-ligand charge transfer) character in the lowest energy excited state. Bases for two distinct strategies for inducing emission from aryl derivatives become clear. The standard approach of incorporating a phenyl group into a (N/\N/\C) cyclometalating ligand relies in part on the rigidity of the ligand framework. An alternative strategy, which involves expanding the chromophore and altering the orbital parentage of the emitting state, is capable of suppressing radiationless decay even further. Indeed, the Pt(4′-pyren-1-yl-trpy)Ph+ system emits from a low-lying 3π–π*(pyrene) excited state that has a lifetime of 45 μs in fluid solution.
Co-reporter:Breeze N. Briggs, Abby J. Gaier, Phillip E. Fanwick, Dilek K. Dogutan, and David R. McMillin
Biochemistry 2012 Volume 51(Issue 38) pp:
Publication Date(Web):September 4, 2012
DOI:10.1021/bi300828z
A cationic, copper(II)-containing ligand, derived from bulky 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, Cu(T4), and two sterically friendlier forms, [trans-5,15-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(tD4), and [cis-5,10-di(N-methylpyridinium-4-yl)porphyrinato]copper(II), Cu(cD4), bind to DNA and RNA hosts. Six hairpin-forming RNA 18-mer sequences and two previously studied DNA analogues serve as convenient binding platforms of programmable base composition. A crystal structure shows that the copper center of Cu(tD4) is four-coordinate, establishing compatibility with intercalative binding as well as susceptibility to solvent-induced emission quenching. From the hypochromic responses and the induced emission intensities obtained with all three porphyrins, it is clear that internalization into the RNA host occurs, irrespective of the base pair composition. Further analysis reveals that the porphyrins intercalate into the double-stranded stem domains. Subtle geometric and/or electronic aspects of the binding account for the signs of induced circular dichroic signals and splitting of the Soret band of Cu(tD4).
Co-reporter:Daniel P. Lazzaro ; Robert McGuire ; Jr.
Inorganic Chemistry 2011 Volume 50(Issue 10) pp:4437-4444
Publication Date(Web):April 1, 2011
DOI:10.1021/ic2000359
The carbometalated complex Pt(dppzϕ*)Cl, where dppzϕ* denotes the 6-(4-tert-butylphenyl)-dipyrido[3,2-a:2′,3′-c]phenazine ligand, exhibits emission in a dichloromethane solution at room temperature with a concentration-dependent excited-state lifetime. Extrapolation to zero Pt(dppzϕ*)Cl concentration yields a limiting lifetime of 11.0 μs in the absence of dioxygen along with an impressive emission quantum yield of 0.17. The visible absorption of Pt(dppzϕ*)Cl has intraligand charge-transfer as well as metal-to-ligand charge-transfer character, but the oscillator strength may derive, in part, from π−π* excitation within the phenazine moiety. An intriguing aspect of the Pt(dppzϕ*)Cl system is that its reactive excited state is subject to regiospecific quenching by Lewis bases and hydrogen-bonding Lewis acids. Base-induced quenching involves an attack at the platinum center. The rate constant increases with the donor strength of the quencher and reaches the order of 108 M−1 s−1 with a relatively strong base like dimethyl sulfoxide. The orbital parentage of the excited state probably influences the quenching rates by affecting the charge density at platinum, as well as at the phenazine nitrogen atoms, where attack by Lewis acids occurs. With mildly acidic alcohols like 1,1,1,3,3,3-hexafluoropropan-2-ol and 2,2,2-trifluoroethanol, high concentrations of the quencher are necessary to suppress the emission. Carboxylic acids are stronger quenchers, and the quenching constant increases with the acid strength according to tabulated pKa values. Cyanoacetic acid exhibits the highest measured quenching rate constant (2.6 × 109 M−1 s−1), which only decreases 30% when the acid is in the (NC)CH2CO2D form. A weaker acid, CH3CO2H, exhibits an even smaller kinetic isotope effect. Literature comparisons suggest that acid-induced quenching probably involves hydrogen-bond formation as opposed to net proton transfer.
Co-reporter:Robert McGuire Jr., Meaghan Clark McGuire, David R. McMillin
Coordination Chemistry Reviews 2010 Volume 254(21–22) pp:2574-2583
Publication Date(Web):November 2010
DOI:10.1016/j.ccr.2010.04.013
There are many possible applications for luminescent platinum terpyridine (trpy) complexes, but the emission quantum yield and lifetime vary greatly depending upon the design. One reason is that potentially emissive metal-to-ligand charge-transfer (MLCT) states occur at relatively high energies because a planar coordination geometry is not the best supporting environment for a Pt(III) center. At the same time, strain in the Pt–N sigma bond framework often results in low-lying d–d excited states that effectively quench the emission. One way of differentially lowering the energy of the emitting state, and thereby reducing the effect of d–d states, involves delocalizing the π*(trpy) acceptor orbital onto a 4′-aryl substituent. Delocalizing the ‘hole’ orbital is an alternative approach capable of producing dramatic results. Thus, with the addition of an electron-rich group like –NMe2 or 1-naphthyl to the 4′-position of trpy ligand, the emitting state takes on intraligand charge-transfer (ILCT) character and the excited-state lifetime extends to tens of microseconds in dichloromethane solution. In some systems introduction of a π-donating co-ligand enhances the emission yield, and when the co-ligand is a very electron-rich group like an ethynylarene, the emitting state takes on an admixture of ligand-to-ligand charge-transfer (LLCT) character. Finally, it is possible to destabilize deactivating states by incorporating an ethynylalkane as a strong-field co-ligand, or by utilizing a carbometalating derivative of the trpy ligand. Complexes of the latter support another type of ILCT excitation because of the presence of the formally anionic phenyl moiety, and the emission energy vary greatly depending upon which ligand axis contains the Pt–C bond.
Co-reporter:David J. Stewart ; Phillip E. Fanwick
Inorganic Chemistry 2010 Volume 49(Issue 15) pp:6814-6816
Publication Date(Web):July 1, 2010
DOI:10.1021/ic1010117
Delocalization of the charge-transfer excitation in the series [Ru(NNN)(bpy)CN]+ [bpy = 2,2′-bipyridine; 1, NNN = 2,2′:6′,2′′-terpyridine; 2, NNN = 2-(2′-pyridyl)-1,10-phenanthroline or php; 3, NNN = 6′-(2′′-pyridyl)dipyrido[3,2-a:2′,3′-c]phenazine or dppzp] proves to be an effective way of tuning photophysical properties. Red-emitting 3 functions as a DNA “light switch”, shows emission from a state with charge-transfer-to-phenazine character, and exhibits a significantly enhanced emission signal relative to 1 and other dppzp-containing ruthenium(II) complexes.
Co-reporter:Breeze N. Briggs, David R. McMillin, Tanya K. Todorova, Laura Gagliardi, Frederic Poineau, Kenneth R. Czerwinski and Alfred P. Sattelberger
Dalton Transactions 2010 vol. 39(Issue 47) pp:11322-11324
Publication Date(Web):01 Nov 2010
DOI:10.1039/C0DT00751J
The emission spectra of the solids [n-Bu4N]2Tc2X8 (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the 1δ–δ* excited state, as with the rhenium homologues, but has a shorter lifetime.
Co-reporter:Robert McGuire Jr. and David R. McMillin
Chemical Communications 2009 (Issue 47) pp:7393-7395
Publication Date(Web):03 Nov 2009
DOI:10.1039/B919273E
The binding motifs of copper(II) porphyrins with G quadruplex DNA structures vary markedly depending on the steric demands of the ligand and the host structure.
Co-reporter:Robert McGuire ; Jr.; Michael H. Wilson ; John J. Nash ; Phillip E. Fanwick
Inorganic Chemistry 2008 Volume 47(Issue 8) pp:2946-2948
Publication Date(Web):March 20, 2008
DOI:10.1021/ic800220r
This report describes platinum(II) complexes of 6-(2-pyridyl)-dipyrido[3,2-a:2′,3′-c]phenazine (dppzp) and 6-phenyl-dipyrido[3,2-a:2′,3′-c]phenazine (dppzϕ). The [Pt(dppzp)Cl]+ (1) system exhibits an excited-state lifetime of 5.0 µs in deoxygenated dichloromethane. Lewis bases quench the emission with rate constants on the order of 107 M−1 s−1; however, acetic acid is definitely not a quencher. The carbometalated [Pt(dppzϕ)Cl] (2) complex is novel in that it is subject to quenching by acid as well. In deoxygenated 2-chloronaphthalene, the excited-state lifetime of 2 is 270 ns, and acetic acid quenches the emission with a rate constant of 2 × 108 M−1 s−1. In addition, Lewis bases like dimethyl sulfoxide and dimethylformamide quench the emission of 1 and 2 with similar efficiencies. The coordinatively unsaturated platinum center provides a logical place for attack by Lewis bases, whereas the phenazine extension of dppzϕ introduces potentially acid-sensitive nitrogen centers. The emissive states of 1 and 2 exhibit mainly intraligand character, but enhanced charge-transfer character in 2 accounts for the differences in reactivity.
Co-reporter:Meaghan L. Clark ; Robert L. Green ; Olivia E. Johnson ; Phillip E. Fanwick
Inorganic Chemistry 2008 Volume 47(Issue 20) pp:9410-9418
Publication Date(Web):September 24, 2008
DOI:10.1021/ic8009354
This paper focuses on DNA-binding interactions exhibited by Pt(dma-T)CN+, where dma-T denotes 4′-dimethylamino-2,2′:6′,2′′-terpyridine, and includes complementary studies of the corresponding pyrr-T complex, where pyrr-T denotes 4′-(N-pyrrolidinyl)-2,2′:6′,2′′-terpyridine. The chromophores are useful for understanding the interesting and rather intricate DNA-binding interactions exhibited by these and related systems. One reason is that the terpyridine ligands employed provide intense visible absorption and enhanced photoluminescence signals. Incorporating cyanide as a coligand further aids analysis by suppressing covalent binding. Physical methods utilized include X-ray crystallography for structures of the individual inorganic complexes. Viscometry as well as spectral studies of the absorbance, emission, and circular dichroism (CD) yield information about interactions with a variety of DNA hosts. Although there is no sign of covalent binding under the conditions used, most hosts exhibit two phases of uptake. Under conditions of high loading (low base-pair-to-platinum ratios), the dma-T complex preferentially binds externally and aggregates on the surface of the host, except for the comparatively rigid host [poly(dG-dC)]2. Characteristic signs of the aggregated form include a bisignate CD signal in the charge-transfer region of the spectrum and strongly bathochromically shifted emission. When excess DNA is present, however, the complex shifts to intercalative binding, preferentially next to G≡C base pairs if available. Once the complex internalizes into DNA it becomes virtually immune to quenching by O2 or solvent, and the emission lifetime extends to 11 μs when [poly(dI-dC)]2 is the host. On the other hand, the host itself becomes a potent quenching agent when G≡C base pairs are present because of the reducing strength of guanine residues.
Co-reporter:MeaghanL. Clark;Stéphane Diring;Pascal Retailleau Dr.;DavidR. McMillin Dr.;Raymond Ziessel Dr.
Chemistry - A European Journal 2008 Volume 14( Issue 24) pp:7168-7179
Publication Date(Web):
DOI:10.1002/chem.200701975
Abstract
Neutral orthometalated platinum(II) complexes of the deprotonated 6-phenyl-2,2′-bipyridine ligand (bearing a trialkoxygallate, tolyl, ethynyltrialkoxygallate, or ethynyltolyl substituent) and a σ-bonded Cl, ethynyltolyl, or ethynyltrialkoxygallate coligand have been prepared by a stepwise procedure based on copper-promoted cross-coupling reactions. The X-ray structure of the [2-(p-tolyl)ethynyl][4-{2-(p-tolyl)ethynyl}-6-phenyl-2,2′-bipyridyl)]platinum(II) complex revealed a coplanar arrangement of all residues bound to platinum, although the tolylethynyl groups exhibit position-dependent bending in the solid state. The complexes exhibit charge-transfer absorption in the visible region. All except two of the complexes also exhibit charge-transfer emission, typically from an excited state that has a submicrosecond lifetime at room temperature in deoxygenated dichloromethane solution. In accordance with the presence of a carbometalated polypyridine ligand, the emitting state is assumed to have a mixture of metal-to-ligand charge-transfer (MLCT) and intra-ligand charge-transfer (ILCT) character. However, spectral comparisons and electrochemical data suggest that the emissive state also exhibits interligand charge-transfer (LLCT) character when an electron-rich ethynylaryl group is bound to platinum. In keeping with altered orbital parentage in the latter systems, the emission occurs at longer wavelength. The excited-state lifetime is also shorter, evidently due to vibronic interactions. The decay is so efficient when an ethynyltrialkoxygallate group binds to platinum that there is no detectable emission in fluid solution, although the complexes do emit in a frozen glass. The excited states are subject to associative (exciplex) quenching by Lewis bases, but the admixture of ILCT and/or LLCT character diminishes efficiency, except for relatively strong bases like dimethyl sulfoxide and dimethylformamide.
Co-reporter:Yi-Zhen Hu, Michael H. Wilson, Ruifa Zong, Celine Bonnefous, David R. McMillin and Randolph P. Thummel
Dalton Transactions 2005 (Issue 2) pp:354-358
Publication Date(Web):07 Dec 2004
DOI:10.1039/B415021J
The ligand 2-(8′-quinolinyl)-1,10-phenanthroline (1) was prepared in 79% yield by the Friedländer condensation of 8-amino-7-quinolinecarbaldehyde and 8-acetylquinoline. The complex [Pt(1)Cl]+ was prepared and compared with the isomeric 2-(2′-quinolinyl)-1,10-phenanthroline (2) complex. An X-ray analysis indicated that the six-membered chelate ring in the tridentate complex resulted in a relief of angle strain as well as some non-planarity in the bound ligand 1. The control system for photophysical studies is [Pt(3)Cl]+ where 3 denotes 2-(2′-pyridyl)-1,10-phenanthroline. Relative to the complex of 3, in dichloromethane solution [Pt(1)Cl]+ exhibits noticeably higher energy charge-transfer absorption but slightly lower energy emission. The gap between the onset of absorption and emission is larger because the emission from [Pt(1)Cl]+ originates from a triplet excited state with substantial intra-ligand character. At room temperature in deoxygenated dichloromethane, [Pt(1)Cl]+ has an excited-state lifetime of 310 ns vs. 230 ns for [Pt(3)Cl]+. Within the series, [Pt(1)Cl]+ also exhibits the largest activation barrier for thermally induced quenching at 2730 cm−1 in fluid dichloromethane solution. However, the barrier is only about 50% larger than that found for [Pt(3)Cl]+. There is reduced ring strain in [Pt(1)Cl]+, but inter-ligand steric interactions weaken the ligand field.
Co-reporter:Michael H. Wilson, L. Peter Ledwaba, John S. Field and David R. McMillin
Dalton Transactions 2005 (Issue 16) pp:2754-2759
Publication Date(Web):11 Jul 2005
DOI:10.1039/B508127K
As part of an effort to develop new lumaphors involving late transition metal ions, this report describes the synthesis and characterization of the first platinum(II) derivatives containing 2,2′:6′,2″-terpyridine (trpy) and cyanide as co-ligands. According to existing models, including cyanide in the coordination sphere should raise the energies and minimize the influence of short-lived d–d excited states that otherwise compromise the excited-state lifetime. Both [Pt(trpy)(CN)]+ and the 4′-cyano-2,2′:6′,2″-terpyridine analogue [Pt(CN–T)(CN)]+ are emissive in dichloromethane solution, but the signals are weak. Part of the problem is that the d–π* charge-transfer excited states also rise in energy, so that the emission actually originates from a 3π−π* state with a relatively low radiative rate constant. However, another member of the series, the 4′-dimethylamino-2,2′:6′,2″-terpyridine (dma–T) derivative [Pt(dma–T)(CN)]+, proves to be a very promising platform with an emission quantum yield of ϕ
= 0.26 and an excited-state lifetime of τ
= 22 µs in room-temperature, deoxygenated dichloromethane solution. In the dma–T complex the electron-rich dimethylamino substituent provides the basis for an emissive, but largely ligand-based, charge-transfer excited state. The orbital parentage is such that the photoluminescence persists in donating solvents like dimethylformamide, which ordinarily quenches d–π* excited states in complexes of this type.
Co-reporter:Stephanie A. Bejune and David R. McMillin
Chemical Communications 2004 (Issue 11) pp:1320-1321
Publication Date(Web):10 May 2004
DOI:10.1039/B402330G
DNA hairpins are extremely versatile hosts for investigating DNA-binding interactions, but studies with a dicationic zinc(II) porphyrin reveal that the choice of loop sequence is critical when the aim is to understand adduct formation with very much longer, naturally occurring sequences.
Co-reporter:Breeze N. Briggs, David R. McMillin, Tanya K. Todorova, Laura Gagliardi, Frederic Poineau, Kenneth R. Czerwinski and Alfred P. Sattelberger
Dalton Transactions 2010 - vol. 39(Issue 47) pp:NaN11324-11324
Publication Date(Web):2010/11/01
DOI:10.1039/C0DT00751J
The emission spectra of the solids [n-Bu4N]2Tc2X8 (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the 1δ–δ* excited state, as with the rhenium homologues, but has a shorter lifetime.
Co-reporter:Srijana Ghimire, Matthew A. Bork, Hanyu Zhang, Phillip E. Fanwick, Matthias Zeller, Jong Hyun Choi and David R. McMillin
Dalton Transactions 2016 - vol. 45(Issue 36) pp:NaN14284-14284
Publication Date(Web):2016/08/15
DOI:10.1039/C6DT01918H
The goal of this work has been to synthesize and investigate Pd(TC3), an intercalating porphyrin that has conformable substituents capable of groove binding to B-form DNA. (TC3 denotes the doubly deprotonated form of 5,10,15,20-tetra[3-(3′-methylimidazolium-1′-yl)prop-1-yl]porphyrin.) Palladium(II) is an apt choice for the central metal ion because it remains strictly four-coordinate and provides for a luminescent triplet excited state with a long lifetime. The DNA hosts are hairpin-forming sequences programmed to differ in base composition. Luminescence, absorbance, and circular dichroism results are consistent with the idea that congruent structural reorganization takes place at the host and ligand during uptake. Photoexcitation of DNA-bound Pd(TC3) generates a comparatively modest steady state concentration of singlet oxygen, due to a relatively slow reaction with molecular oxygen in solution. The sheer size of the substituent groups disfavors quenching, but groove-binding interactions compound the problem by inhibiting mobility. The results show how ligand design affects adduct structure as well as function.
Co-reporter:Robert McGuire Jr. and David R. McMillin
Chemical Communications 2009(Issue 47) pp:NaN7395-7395
Publication Date(Web):2009/11/03
DOI:10.1039/B919273E
The binding motifs of copper(II) porphyrins with G quadruplex DNA structures vary markedly depending on the steric demands of the ligand and the host structure.
Co-reporter:Norman Lu, Lauren M. Hight, David R. McMillin, Jyun-Wei Jhuo, Wei-Cheng Chung, Kwan-Yu Lin, Yuh-Sheng Wen and Ling-Kang Liu
Dalton Transactions 2014 - vol. 43(Issue 5) pp:NaN2119-2119
Publication Date(Web):2013/11/11
DOI:10.1039/C3DT52713A
The yellow (1y) and orange (1o) crystalline polymorphs of [PtBr2(5,5′-bis(CF3CH2OCH2)-2,2′-bipyridine)] exhibit surprisingly short nearest neighbour Pt⋯Pt separations of 3.526 Å and 3.590 Å, respectively, at 295 K. Both distances are much shorter than those found in structures of the unsubstituted [PtBr2(2,2′-bipyridine)] analogue. Consistent with a linear chain structure in 1o and dimer formation in 1y, both solids exhibit emission spectra shifted to much longer wavelengths than that exhibited by the monomer in a low-temperature glass. Furthermore, the emission spectra of 1o and 1y shift to even longer wavelengths as the temperature decreases and the Pt⋯Pt separations contract. Till now delocalized emission of this type has been considered to be restricted to [PtCl2(diimine)] systems and implausible in PtBr2-containing analogues for steric reasons. Ironically, in the system at hand the bulky 5,5′-substituents apparently promote delocalization of the emission by forming a network of hydrogen-bonding-like C–H⋯F–C interactions that help shape the packing.
![Ferrate(2-), [7,12-diethenyl-3,8,13,17-tetramethyl-21H,23H-porphine-2,18-dipropanoato(4-)-κN21,κN22,κN23,κN24]-, hydrogen (1:2), (SP-4-2)-](/data/chemimg/522800/14875-96-8.png)
![Ferrate(2-), [7,12-diethenyl-3,8,13,17-tetramethyl-21H,23H-porphine-2,18-dipropanoato(4-)-κN21,κN22,κN23,κN24]-, hydrogen (1:2), (SP-4-2)-](/data/chemimg/522800/14875-96-8_b.png)
![Zinc, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)-](/data/chemimg/478800/14074-80-7.png)
![Zinc, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)-](/data/chemimg/478800/14074-80-7_b.png)
![Chloro[3,3'-(3,7,12,17-tetramethyl-8,13-divinyl-2,18-porphyrindiyl-κ2n21,n23)dipropanoato(2-)]gallium](/data/chemimg/410200/210409-12-4.png)
![Chloro[3,3'-(3,7,12,17-tetramethyl-8,13-divinyl-2,18-porphyrindiyl-κ2n21,n23)dipropanoato(2-)]gallium](/data/chemimg/410200/210409-12-4_b.png)
![[2,2':6',2''-Terpyridin]-4'-amine, N,N-dimethyl-](/data/chemimg/2124600/145533-41-1.png)
![[2,2':6',2''-Terpyridin]-4'-amine, N,N-dimethyl-](/data/chemimg/2124600/145533-41-1_b.png)
![Zinc(4+),[[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-kN21,kN22,kN23,kN24)tetrakis[1-methylpyridiniumato]](2-)]-,(SP-4-1)-](/data/chemimg/59500/40603-58-5.png)
![Zinc(4+),[[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-kN21,kN22,kN23,kN24)tetrakis[1-methylpyridiniumato]](2-)]-,(SP-4-1)-](/data/chemimg/59500/40603-58-5_b.png)
![Dipyrido[3,2-a:2',3'-c]phenazine](/data/chemimg/1778900/19535-47-8.png)
![Dipyrido[3,2-a:2',3'-c]phenazine](/data/chemimg/1778900/19535-47-8_b.png)
![[(sulfonatoperoxy)sulfonyl]oxidanide](http://img.cochemist.com/ccimg/15100/15092-81-6.png)
![[(sulfonatoperoxy)sulfonyl]oxidanide](http://img.cochemist.com/ccimg/15100/15092-81-6_b.png)
![Copper(4 ), [[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-κN21,κN22,κN23,κN24)tetrakis[1-methylpyridiniumato]](2-)]-, (SP-4-1)-](/data/chemimg/2567900/48242-70-2.png)
![Copper(4 ), [[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-κN21,κN22,κN23,κN24)tetrakis[1-methylpyridiniumato]](2-)]-, (SP-4-1)-](/data/chemimg/2567900/48242-70-2_b.png)