Copper 5,10,15,20-Tetra(N-ethyl-3-carbazolyl) Porphyrin

Abstract

The copper complex of 5,10,15,20-Tetra(N-ethyl-3-carbazolyl) porphyrin was synthesized and characterized by electronic absorption spectrophotometry and cyclic voltammetry. The spectral data were in agreement with the proposed structure. The copper complex exhibited a shift in the Soret band in comparison to the non-metallated porphyrin and the extinction coefficient for the Soret band was on the order of 10⁵ cm⁻¹M⁻¹. Trends observed in the oxidation and reduction potentials were consistent with the nature of the porphyrin. That is, the electron donating group in 5,10,15,20-tetra(N-ethyl-3-carbazolyl) porphyrin enhances oxidation and inhibits reduction.

Introduction

Porphyrins are of interest in many areas of study including photodynamic cancer therapy, optical data storage, sensors, and solar energy [1,2,3,4,5]. The focus of this paper is to report the synthesis, electronic absorption spectroscopy, and cyclic voltammetry of the copper complex of 5,10,15,20-tetra(N-ethyl-3-carbazolyl) porphyrin, TECP.

Experimental Section

Chemicals and Reagents

Reagents and solvents used in the synthesis and metallation of TECP, NMR measurements, and UV-vis were purchased from Aldrich Chemical Company and used as received. Methylene chloride used in cyclic voltammetry experiments was distilled over calcium hydride before use. Tetrabutylammonium tetrafluoroborate was purchased from Fisher Scientific Company and used as received.

Instrumentation

Ultraviolet-visible (UV-vis) absorption spectra were recorded on a Hewlett Packard 8451A spectrophotometer. Cyclic voltammograms (CV) were recorded using a computer linked EG&G Princeton Applied Research Potentiostat/Galvanostat Model 273 equipped with an IR compensator and model 270 Electrochemical software. The CVs were obtained in a four-necked cell with a platinum working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. A capacitor (0.1 μF), was connected between the counter electrode and the reference electrode to reduce background noise. The porphyrin solutions were 1 × 10⁻³ M in 0.1 M methylene chloride solutions of tetrabutylammonium tetrafluoroborate. Potentials were reported versus a S.C.E. and not corrected for liquid junction potentials. Deaeration of the solution was achieved by passing a stream of nitrogen through the solution. The platinum working electrode was cleaned with solvent, using a lint free wipe, between each run and polished periodically.

Synthesis

5,10,15,20-Tetra(N-ethyl-3-carbazolyl) Porphyrin: TECP was synthesized as reported previously [6]. The structure was confirmed using ¹H NMR, ¹³C NMR, UV-Vis and high resolution mass spectrometry. UV (CHCl₃) [λ,nm (ε, cm⁻¹M⁻¹)]: 433(2.8 × 10⁵), 523(1.3 × 10⁴), 562.5(1.1 × 10⁴), 654.5(5.6 × 10³). ¹H NMR (360 MHz, CDCl₃), δ (ppm): 8.97 (s, 4H), 8.89 (s, 8H), 8.37(d, 4H, J = 8.1 Hz), 8.19 (d, 4H, J = 7.8 Hz), 7.75 (d, 4H, J = 8.2 Hz), 7.58 (m, 8H), 7.28 (t, J = 7.8 Hz)), 4.63 (q, J = 7.1), 1.68 (t, J = 7.0), –2.4 (s). ¹H NMR (360 MHz, Acetone-d6), δ (ppm): 9.03 (s), 8.93 (s), 8.40 (d, J = 8.4 Hz), 8.32(d, J = 7.6 Hz), 7.99 (d, J = 8.4 Hz), 7.76 (d, J = 8.3 Hz), 7.58 (t, J = 7.7 Hz), 7.27 (t, J = 7.5 Hz), 4.76 (q, J = 7.1), 1.65 (t, J = 7.0), –2.4 (s). HRMS-FAB (m/z): calcd for C₇₆H₅₉N₈: 1083.486 a.m.u.; observed: 1083.487 a.m.u.

Copper 5,10,15,20-Tetra(N-ethyl-3-carbazolyl) Porphyrin. The copper 5,10,15,20-Tetra(N-ethyl-3-carbazolyl) porphyrin was prepared using chromatographed TECP (0.2002 g, 1.848 × 10⁻⁴ mol), in approximately 50 mL of boiling chloroform. To this solution a saturated solution of copper (II) acetate hydrate in 3 mL of methanol was added. After 1 minute of refluxing the reaction mixture was found to have a λmax of 429 nm. This shift in the electronic absorption spectrum indicated the completion of the reaction. The reaction mixture was rotavaped to dryness, rinsed with methanol, and filtered giving 0.2189 g (1.912 × 10⁻⁴ moles) of crude Cu(II)TECP. The Cu(II)TECP was purified using a silica gel column with CHCl₃ as the eluent coming off as a brilliant red band. For 0.1093 g crude compound chromatographed, 0.0253 g of pure compound was obtained. Thus, for the overall reaction 0.0507 g, 4.43 × 10⁻⁵ moles, of pure CuTECP was obtained, giving a yield of 23.9%. UV-vis Data [λ,nm (ε, cm⁻¹M⁻¹)], CHCl₃ 429(2.4 × 10⁵), 545(1.2 × 10⁴), 583(3.4 × 10³).

Results and Discussion

In the electronic absorption spectra of the metallated porphyrin a shift in the Soret band and the disappearance of at least one of the Q bands is expected and observed. This disappearance is due to the increased symmetry acquired by the porphyrin with introduction of a metal ion to the ring. The extinction coefficient for the Soret band are on the order of 10⁵ cm⁻¹M⁻¹.

CuTECP undergoes two successive one-electron oxidations at 0.875 V and 1.248 V vs. S.C.E. A single electron reduction of CuTECP is observed at −1.29 V vs. S.C.E. These values for the electro-oxidation and reduction of CuTECP correspond extremely well with what is reported in the literature for CuTPP. It is reported that CuTPP in benzonitrile with 0.1 M nBu₄NClO₄ undergoes two, one electron oxidations at 0.99 V and 1.33 V vs. S.C.E. [7]. A reduction of CuTECP was observed at −1.20 V vs. S.C.E. This corresponds to a reduction of CuTPP in THF-DMF solvent reported to occur in at −1.20 V vs. S.C.E. [8,9].

Conclusions

All data observed for the copper complex of TECP follow trends expected when compared to the TPP complexes.