Electrochemistry of platinum(II) porphyrins: effect of substituents and π-extension on redox potentials and site of electron transfer.

Fourteen platinum(II) porphyrins with different π-conjugated macrocycles and different electron-donating or electron-withdrawing substituents were investigated as to their electrochemical and spectroscopic properties in nonaqueous media. Eight compounds have the formula (Ar(4)P)Pt(II), where Ar(4)P = the dianion of a tetraarylporphyrin, while six have π-extented macrocycles with four ß,ß'-fused benzo or naphtho groups and are represented as (TBP)Pt(II) and (TNP)Pt(II) where TBP and TNP are the dianions of tetrabenzoporphyrin and tetranaphthoporphyrin, respectively. Each Pt(II) porphyrin undergoes two reversible one-electron reductions and one to three reversible one-electron oxidations in nonaqueous media. These reactions were characterized by cyclic voltammetry, UV-visible thin-layer spectroelectrochemistry and in some cases by ESR spectroscopy. The two reductions invariably occur at the conjugated π-ring system to yield relatively stable Pt(II) π-anion radicals and dianions. The first oxidation leads to a stable π-cation radical for each investigated porphyrin; but in the case of tetraarylporphyrins containing electron-withdrawing substituents, the product of the second oxidation may undergo an internal electron transfer to give a Pt(IV) porphyrin with an unoxidized macrocycle. The effects of macrocycle structure on UV-visible spectra, oxidation/reduction potentials, and site of electron transfer are discussed.

Measuring oxygen in living tissue: intravascular, interstitial, and "tissue" oxygen measurements.

Oxygen dependent quenching of phosphorescence has been used to measure the oxygen pressure in both the vasculature of the microcirculation and the interstitial spaces of resting muscle tissue. Oxygen sensitive molecules were either dissolved in the blood (intravascular space) or micro-injected into the interstitial space and the distributions, histograms, of the oxygen pressure were measured. The mean oxygen pressures are higher in the blood than in the interstitial space but the oxygen pressures in the lowest 10% of the two spaces were not significantly different, indicating there is minimal (< 1 mm Hg) oxygen gradient between the two spaces in the capillary bed.

Dynamic Quenching of Porphyrin Triplet States by Two-Photon Absorbing Dyes: Towards Two-Photon-Enhanced Oxygen Nanosensors.

Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [1]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern-Volmer quenching constants were correlated with the corresponding ET driving forces (DeltaG(ET)), calculated using the Rehm-Weller equation. FRET-pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.

Oxygen microscopy by two-photon-excited phosphorescence.

High-resolution images of oxygen distributions in microheterogeneous samples are obtained by two-photon laser scanning microscopy (2P LSM), using a newly developed dendritic nanoprobe with internally enhanced two-photon absorption (2PA) cross-section. In this probe, energy is harvested by a 2PA antenna, which passes excitation onto a phosphorescent metalloporphyrin via intramolecular energy transfer. The 2P LSM allows sectioning of oxygen gradients with near diffraction-limited resolution, and lifetime-based acquisition eliminates dependence on the local probe concentration. The technique is validated on objects with a priori known oxygen distributions and applied to imaging of pO(2) in cells.

Energy and electron transfer in enhanced two-photon-absorbing systems with triplet cores.

Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for several biomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. The performance of so far developed molecules, however, is substantially below expected. In this study we take a detailed look at the processes occurring in these systems and propose ways to improve their performance. We focus on the interchromophore distance tuning as a means for optimization of two-photon sensors for oxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective 2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecular electron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensor efficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescence emission using Rhodamine B-Pt tetrabenzoporphyrin (RhB-PtTBP) adducts as model compounds. 2PA cross sections (sigma2) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800 nm), making TBPs superior 2PA chromophores compared to regular porphyrins (sigma2 values typically 1-2 GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800-850 nm range) and their high photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (lambda(fl) = 590 nm, phi(fl) = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (lambda(abs) = 615 nm, epsilon = 98 000 M(-1) cm(-1), phi(p) approximately 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence via fluorescence resonance energy transfer (FRET) might occur. However, most of the excitation energy in RhB-PtTBP assemblies is consumed in several intramolecular ET processes. By installing rigid nonconducting decaproline spacers (Pro10) between RhB and PtTBP, the intramolecular ETs were suppressed, while the chromophores were kept within the Förster r0 distance in order to maintain high FRET efficiency. The resulting assemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the number of 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possess unexpectedly strong forbidden S0 --> T1 bands (lambda(max) = 762 nm, epsilon = 120 M-1 cm-1). The latter may overlap with the laser spectrum and lead to unwanted linear excitation.

Synthesis and luminescence of soluble meso-unsubstituted tetrabenzo- and tetranaphtho[2,3]porphyrins.

[Structure: see text]. Syntheses of soluble tetrabenzoporphyrins (TBP) and tetranaphtho[2,3]porphyrins (TNP), with multiple substituents in the conjugated aromatic rings but bearing no substituents in the meso-positions, is reported. Both types of porphyrins were obtained by direct aromatization of precursor porphyrins, annealed with either cyclohexene or dihydronaphthalene fragments. TBPs and TNPs possess powerful absorption bands in the near-infrared (lambda = 610-710 nm, epsilon = 100,000-300,000 M(-1) cm(-1)) and exhibit strong luminescence. Free bases and Zn complexes fluoresce with quantum yields of up to 50%, whereas Pd and Pt complexes phosphoresce in solutions at ambient temperatures. Remarkably, the phosphorescence quantum yields of Pd and Pt TBPs reach as high as 20-50%, which places them among the brightest near-infrared phosphors known to date.

Synthesis of symmetrical tetraaryltetranaphtho[2,3]porphyrins.

A new method of synthesis of meso-tetraaryltetranaphtho[2,3]porphyrins (Ar4TNP) has been developed. Ar4TNPs with peripheral functional groups are obtained by oxidative aromatization of meso-tetraarylporphyrins in which pyrrole units are fused with either octahydro- or dihydronaphthalene moieties. These precursor porphyrins are synthesized in four to five steps from readily available starting materials, such as naphthalene or 1,4-benzoquinone. The pathway originating in dihydronaphthalene, i.e., the "dialine" route, was found to be superior to the alternative "octaline"route in that it (1) enables the shortening of the overall reaction sequence, (2) has a broader scope in terms of the peripheral substitution in Ar4TNPs, and (3) affords higher yields of the target porphyrins. Pd complexes of the synthesized Ar4TNPs exhibit remarkably strong absorption bands at 710-720 nm (epsilon approximately 200,000 M(-1) cm(-1)) and phosphoresce at room temperature with moderate quantum yields (phi = 2-3%, lambda(max) = 900-1000 nm). The absorption maxima of naphthoporphyrins substituted with eight methoxy groups (Ar4TNP(OMe)8) were found to be about 15-20 nm red shifted compared to the corresponding maxima of unsubstituted Ar4TNPs. The X-ray crystallographic data suggest that these spectral shifts are caused not by the differences in nonplanar distortions of the macrocycles but by the purely electronic effects of the substituents.

Novel versatile synthesis of substituted tetrabenzoporphyrins.

A novel general synthetic route to tetraaryltetrabenzoporphyrins (Ar(4)TBP) with various peripheral functional groups is developed. The procedure includes (i) Barton-Zard condensation of 1-nitro- or 1-phenylsulfonylcyclohexenes with isocyanoacetic acid esters, (ii) condensation of the resulting 4,5,6,7-tetrahydroisoindoles with aromatic aldehydes to give fused tetraaryltetracylohexenoporphyrins (Ar(4)TCHP), and (iii) aromatization of the metal complexes of Ar(4)TCHP's into the corresponding Ar(4)TBP's. Cu and Zn complexes of Ar(4)TBP's are further demetalated to give the corresponding Ar(4)TBP free bases. The overall yields for the sequence range from 15% to 40%, making the method suitable for the preparation of gram quantities of Ar(4)TBP's in a single run. The scope of the method, the selection of the peripheral substituents, the choice of the metal ions, and their influence on the yields of aromatization are discussed. The basic spectroscopic properties of newly synthesized Ar(4)TBP's and Ar(4)TCHP's are reported together with the first X-ray crystallographic structure of the NiAr(4)TBP complex.

Novel route to functionalized tetraaryltetra[2,3]naphthaloporphyrins via oxidative aromatization.

A novel general route to substituted meso-tetraaryltetra[2,3]naphthaloporphyrins (Ar(4)TNP) and meso-tetraaryloctamethoxytetra[2,3]naphthaloporphyrins (Ar(4)(MeO)(8)TNP) via oxidative aromatization of nonaromatically fused porphyrin precursors is described. Ar(4)(MeO)(8)TNPs exhibit more red-shifted absorption bands than Ar(4)TNPs and differ dramatically in solubility. The first X-ray crystallographic structure of tetranaphthaloporphyrin, i.e., PdAr(4)TNP (Ar = 4-MeO(2)CC(6)H(4)), revealed that the degree of nonplanar distortion of this macrocycle is only slightly higher than that of the homologous tetrabenzoporphyrins (Ar(4)TBP).

Influence of nonplanarity and extended conjugation on porphyrin basicity.

Thermodynamic basicities of several new nonplanar water soluble tetraaryltetracyclohexano- (Ar(4)TCHP) and tetraaryltetrabenzoporphyrins (Ar(4)TBP) have been measured and correlated with their structural parameters. While the degrees of nonplanarity in these porphyrins are similar, Ar(4)TCHPs exhibit significantly higher basicities than Ar(4)TBPs and than planar tetraarylporphyrins. Low basicities of distorted Ar(4)TBPs are believed to be due to extended pi-conjugation, which causes delocalization of the core electron density in these porphyrins and reduces negative charges at the protonation site.