David R. McMillin

Luminescence that lasts from Pt(trpy)Cl+ derivatives (trpy=2,2′;6′,2″-terpyridine)

Abstract This report outlines progress to date in preparing Pt(trpy)Cl + derivatives that exhibit long-lived photoluminescence in solution (trpy=2,2′;6′,2″-terpyridine). The trpy complex itself is a versatile binding agent/reporter probe for biological macromolecules but is non-emissive, except in solids or low-temperature glasses. Derivatives investigated are of the type Pt(4′-R-T)Y + , where 4′-R-T denotes a 4′-substituted form of the trpy ligand and Y is a pseudohalide co-ligand. Simple halide replacement yields the Pt(trpy)OH + derivative which has facilitated luminescence-based DNA-binding studies. Other complexes with long-lived excited states in fluid solution include Pt(4′-NMe 2 -T)Cl + and Pt(4′-Pyre1-T)Cl + , where Pyre1 designates a 1-pyrenyl substituent. Due to intra-ligand charge-transfer character, Pt(4′-NMe 2 -T)Cl + and Pt(4′-Pyre1-T)Cl + each exhibit relatively low energy absorption maxima. Open coordination sites support quenching by Lewis bases, however, the effect is less evident with increased intra-ligand orbital parentage in the excited state.

Exciplex quenching of photo-excitd copper complexes

Abstract Exciplexes are less common in transition metal photochemistry than in organic photochemistry. However, Lewis bases and coordinating anions quench metal-to-ligand charge-transfer excited states of Cu(I) systems, and the quenching is ascribed to exciplex formation. A variety of structural arguments and steric effects lend strong support to the model. These results are related to the broader context of transition metal systems, and future areas of potential interest are briefly discussed.

On the luminescence of bis (triphenylphosphine) phenanthroline copper (I)

The luminescence spectra and decay tunes of the emission of the [Cu(PPh3)2(phen)]+ ion have been measured down to liquid helium temperature. The long decay time at low temperatures is interpreted in terms of the triplet level of the charge- transfer state.

Remarkable substituent effects on the photophysics of Pt(4′-X-trpy)Cl+ systems (trpy = 2,2′; 6′,2″-terpyridine)

Abstract In view of the interest in probing the binding interactions that occur between platinum complexes and biological macromolecules, the aim of this work has been to develop systems that exhibit enhanced excited-state lifetimes and emission yields in fluid solution. The investigation focuses on a series of complexes of the type Pt(4′-X-T)Cl + where 4′-X-T denotes a 4′-substituted derivative of 2,2′; 6′,2″-terpyridine. In all cases the counterion is the non-coordinating ion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. The substituents employed include electron-withdrawing groups like CN and SO 2 Me as well as electron-donating groups like SMe and NMe 2 . Within the series of complexes, the first reduction wave ranges over about 0.7 V in DMF. Although the process probably entails ligand reduction, the acceptor orbital appears to have some platinum 6p z character. Even though electron-donating substituents destabilize the reduced form of the ligand, all substituents induce a red-shift in the charge-transfer (CT) absorption band system that occurs around 400 nm. Furthermore, there is generally an increase in the CT absorption intensity, the emission lifetime and the emission quantum yield in methylene chloride. Thus, at room temperature, the complex with the terpyridine ligand itself is a very poor emitter with an emission lifetime of 10 ns or less, while the Pt(4′-SMe-T)Cl + and Pt(4′-NMe 2 -T)Cl + systems exhibit lifetimes of 140 ns and 1.9 μs, respectively. With the electron-donating substituents in particular, the lifetime enhancement reflects a configuration interaction between the original CT state and an intraligand charge-transfer excited state. Substituents also influence a thermally activated pathway to radiationless decay.

Exciplex quenching of photoexcited platinum(II) terpyridines: influence of the orbital parentage

Abstract Quenching studies involving a range of Lewis bases establish that exciplex quenching can affect the lifetime of the emissive charge-transfer state of a platinum(II) terpyridine. The evidence comes from studies of Pt(trpy)SCN + , where trpy denotes 2,2′:6′,2″-terpyridine, and a Pt(4′-XT)Cl + series where 4′-XT denotes a 4′-substituted trpy derivative and X is a CN, SMe or NMe 2 substituent. Thus, in dichloromethane the quenching rate constant increases with the donor number as the quencher varies from a relatively weak base like acetonitrile or acetone to a stronger donor like DMSO or pyridine. For the thiocyanate complex in particular, the quenching rate increases by almost three orders of magnitude. Within the Pt(4′-XT)Cl + series, the rates show a marked variation with the electron-donating ability of the substituent X. Thus, with pyridine as the quencher, the rate constant varies from 3.5×10 8 to 1.0×10 10 M −1 s −1 as X changes from NMe 2 to CN. Variations in the orbital parentage of the excited state account for the trend because the Lewis acidity of the metal center decreases with the delocalization of the hole onto the ligand. When the rate of exciplex formation is slow, an outer-sphere complex accumulates in solution and the kinetic plots show saturation behavior at high quencher concentrations.

DNA-binding studies of Cu(T4), a bulky cationic porphyrin

Abstract The potential for therapeutic applications has provided much of the impetus for investigating the DNA-binding interactions of cationic metalloporphyrins, but there are also signs of intriguing base and sequence dependences. More recent work has established that the stem regions of DNA hairpins are extremely useful as model duplexes. Mainly the focus has been on 16-mers with generic sequence 5′-GANNAC-NNNN-GTNNTC-3′, where the letters A, C, G, and T denote adenine, cytosine, guanine, and thymine nucleotides, respectively. N is the symbol for a variable nucleotide in the string; and the inner dashes enclose the loop sequence: TTTT, GTTA or GAAA. Because of its spectroscopic properties, the porphyrin of interest has been Cu(T4), the Cu(II) derivative of H 2 T4 (H 2 T4, meso -tetrakis (4-( N -methylpyridiniumyl))porphyrin). Spectral data have revealed that the hairpins retain their stem structures in the adducts with Cu(T4) and have provided important new insights into the interactions that occur. One is that the local rigidity of the DNA is the most important factor that shapes the binding. A run of DNA that includes at least 50% G C base pairs can support intercalative binding with or without contiguous G C base pairs in the sequence. As demonstrated by inosine-for-guanine base replacement experiments, the real pre-requisite for intercalative binding is a robust hydrogen bonding framework that overcomes the steric strain generated within the minor groove by pyridyl substituents pressing against the sugar–phosphate backbones. Cu(T4) binds externally to DNA that is richer in the more flexible A T base pairs. The intercalated form of Cu(T4) is unique in exhibiting an appreciable photoluminescence signal, and the intensity appears to increase with the rigidity of the overall DNA structure. Changes in the stability of the loop structure can perturb external binding interactions as indicated by the shape of the porphyrins induced circular dichroism (CD) signal. However, the stem composition determines whether intercalation or external binding occurs, though these two adducts may actually be the limiting structures of a continuum of binding motifs. One technique rarely suffices to characterize an adduct but the combination of absorbance, emission and CD spectroscopies is very powerful.

The pH-dependent structure of calf thymus DNA studied by Raman spectroscopy

The pH-dependent structure of calf thymus DNA is analyzed using Raman spectroscopy. The Raman spectra in the acidic region demonstrate that denaturation occurs in several steps. The binding of H+ to adenine and cytosine residues is accompanied by a decrease in the percentage of DNA in the B-conformation and a concurrent increase in a conformation most probably related to the C-form. The denaturation of DNA is observed at pH 3.3 and parallels the protonation of guanine bases. The Raman spectra of calf thymus DNA in the basic region (above pH 10) show that guanine residues are deprotonated at lower pH value than are thymine residues. In addition, Raman spectra in the basic region detect conformational changes of the phosphate backbone different from those found in the acidic region.

CONCENTRATION-DEPENDENT LIFETIMES OF Cu(NN)+2 SYSTEMS: EXCIPLEX QUENCHING FROM THE ION PAIR STATE

Abstract— The concentration dependence of the lifetimes of the charge transfer excited states of Cu(dmp)+2 and Cu(dpp) +2 has been investigated in CH2C12 solution at 20°C. (dmp denotes 2,9‐dimethyf‐1,10‐phenanthroline, and dpp denotes 2,9‐diphenyl‐l,10‐phenanthroline.) In dilute solution (< 30 μM) the lifetime of Cu(dmp)+2, is 95 ± 5 ns, independent of the anion. At higher concentrations the lifetime decreases, in most cases, to a limiting value that depends upon the counterion. The measured limiting lifetimes range from 38 ± 3 ns for CIO‐4 to 78 ± 5 ns for PF‐6. The anion‐induced quenching is attributed to exciplex quenching which is mediated by an ion pair which exists in the ground state. The results imply that the quenching ability of the anions follows the order BPh‐4 < PF ‐6, < BF‐4 < CIO ‐4 < NO‐3 which is consistent with previous estimates of donor strength. The lifetime of Cu(dpp)+2 is also concentration dependent, but the effect is much smaller because the phenyl substituents impede attack by the anion.

Luminescence of some CuI complexes

Temperature-dependence measurements of the luminescence spectra, the decay times and the luminescence intensities of some CuI complexes are reported. The results are interpreted in terms of a three-level scheme. The lifetimes of these complexes vary considerably due to the differences in quantum efficiencies of the complexes. The charge-transfer character of the transitions involved is confirmed.

Crystal structures and photoluminescent properties of the orange and yellow forms of [Pt{4′-(o-ClC6H4)trpy}(CN)]SbF6: an example of concomitant polymorphism

Vapour diffusion of diethyl ether into a concentrated solution of [Pt{4′-(o-ClC6H4)trpy}(CN)]SbF6 (trpy = 2,2′:6′,2″-terpyridine) in acetonitrile induces the concomitant formation of orange and yellow single crystals that have been shown by means of X-ray crystal structure determinations at 200 K to be different polymorphs of the compound. The [Pt{4′-(o-ClC6H4)trpy}(CN)]+ cations in the orange polymorph are stacked parallel and head-to-tail in an extended chain of tetramers with Pt⋯Pt distances of 3.6 A linking the outer cations of successive tetramers. The [Pt{4′-(o-ClC6H4)trpy}(CN)]+ cations in the yellow polymorph form a “staircase” motif, comprising a continuous sequence of stepped cation pairs where the members of each pair are coplanar, and linked by C—H⋯N hydrogen bonds between the ortho- and meta-H atoms of an outer pyridine ring of one trpy moiety and the cyano ligand N atom of the other cation. Successive cation pairs (or dimers) are parallel to each other, and linked by π(trpy)–σ(trpy) interactions that follow from the offset geometry of the trpy overlap and a short perpendicular separation between the trpy planes of 3.30 A. The emission spectra recorded on crystalline samples of the two polymorphs are very different and reflect the different crystal structures. That of the orange polymorph is typical of emission from a 3MMLCT (MMLCT = metal–metal bond-to-ligand charge transfer) excited state that derives from finite dz2(Pt)–dz2(Pt) orbital overlap within the tetramers: λ(em)max = 650 nm at 200 K. On the other hand, a much broader emission band was recorded for the yellow polymorph that peaks at 583 nm and which has higher energy shoulders at ca. 545 and 505 nm. Deconvolution of this band suggests that it comprises two emission origins, most likely 3IL (IL = intraligand) emission peaking at 543 nm and 3MLCT (MLCT = metal-to-ligand charge transfer) emission with a maximum at 588 nm.