Two new platinum(II) porphyrins have been synthesized and their luminescent properties have been studied. The platinum porphyrins exhibited strong phosphorescence in the red with narrow line widths. When they were doped into aluminum(III) tris(8-hydroxyquinolate) (AlQ3) in the electron-transporting and -emitting layer of an organic light-emitting device, energy transfer occurred between the host AlQ3 and the platinum porphyrin. Bright saturated red emission with high efficiency at low to moderate current density has been achieved. In the high current regime, the electroluminescence efficiency decreased and the perceived emission color blue shifted as a result of mixed emission from the platinum porphyrin and AlQ3. This current dependence was due to the saturation of triplet emissive sites, because of the long-lived phosphorescence state of the platinum porphyrin complex.

Efficient, Saturated Red Organic Light Emitting Devices Based on Phosphorescent Platinum(II) Porphyrins

The ultrafast photodynamics of four‐coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand‐field electronic excited state continues to evolve and reaches the steady‐state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.

Ultrafast vibrational dynamics of a photoexcited metalloporphyrin

To examine the influence of mixed substituents on the structural, electrochemical redox behavior of porphyrins, two new classes of β-pyrrole mixed substituted free-base tetraphenylporphyrins H2(TPP(Ph)4X4) (X = CH3, H, Br, Cl, CN) and H2(TPP(CH3)4X4) (X = H, Ph, Br, CN) and their metal (M = Ni(II), Cu(II), and Zn(II)) complexes have been synthesized effectively using the modified Suzuki cross-coupling reactions. Optical absorption spectra of these porphyrins showed significant red-shift with the variation of X in H2(TPPR4X4), and they induce a 20−30 nm shift in the B band and a 25−100 nm shift in the longest wavelength band [Qx(0,0)] relative to the corresponding H2TPPR4 (R = CH3, Ph) derivatives. Crystal structure of a highly sterically crowded Cu(TPP(Ph)4(CH3)4)·2CHCl3 complex shows a combination of ruffling and saddling of the porphyrin core while the Zn(TPP(Ph)4Br4(CH3OH))·CH3OH structure exhibits predominantly saddling of the macrocycle. Further, the six-coordinated Ni(TPP(Ph)4(CN)4(Py)2)·2(Py) struc...

Mixed Substituted Porphyrins: Structural and Electrochemical Redox Properties

The influence of substituents with increasing steric demands on the structure of nickel(II) 5,15-disubstituted porphyrins has been investigated with X-ray crystallography, UV−visible absorption spectroscopy, resonance Raman spectroscopy, molecular energy optimization calculations, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporphyrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrins (where the alkyl group is propyl, isopropyl, and tert-butyl) are calculated to be in a predominantly gabled (gab) conformation resulting from an αα orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear combination of distortions along the lowest-frequency out-of-plane macrocycle normal coordinates of A2u (doming) and B1u (ruffling) symmetry types. A higher energy stable αβ conformer is also predicted for dialkylporphyrins, and its nonplanar structure can be rep...

Representation of Nonplanar Structures of Nickel(II) 5,15-Disubstituted Porphyrins in Terms of Displacements along the Lowest-Frequency Normal Coordinates of the Macrocycle

Relaxation processes of excited copper porphyrins were studied with relevance to the structure of the substates of the lowest excited states. Lifetimes of luminescence at room temperature were determined as 17, 29, 69, and 105 ns for T(EtO)PPCu [T(EtO) PP: 5, 10, 15, 20‐tetra (p‐ethoxyphenyl)porphin], TPPCu(TPP: 5, 10, 15, 20‐tetraphenylporphin), TFPPCu[TFPP: 5, 10, 15, 20‐tetra(pentafluorophenyl)porphin], and OEPCu(OEP: 2, 3, 7, 8, 12, 13, 17, 18‐octaethylporphin) in toluene, respectively. Emission intensities and lifetimes of OEPCu and TFPPCu measured as a function of temperature show a variation ascribed to a Boltzmann distribution between the lowest trip–doublet and –quartet with an energy gap of 300–400 cm−1. The anomalous temperature dependence for TPPCu and T(EtO)PPCu is explained by a larger energy gap and larger vibronic distortions in the excited state. The difference in behavior is attributed to the orbital nature of the triplet: ‖2,4[b31(a1e)]〉 for OEPCu and TFPPCu but ‖2,4[b31(a2e)]〉 for TPPC...

The lowest excited states of copper porphyrins

The effects of substituents at the periphery of the porphyrin ring on its a - a* absorption spectrum are determined in a quantitative fashion by using a new approach based on the four-orbital model. To accomplish this we systematically investigated with UV-visible absorption spectroscopy the Mg (Sn), Zn, Cu, Ni, and Pt series of metals incorporated into porphyrins with different substituents at the periphery of the macrocycle. The metal dependence of the UV-visible spectrum is used to determine relative energy splittings of the frontier orbitals, intensity parameters for a - a* transitions, and configuration interaction energies for metalloporphyrins with a variety of patterns of peripheral substitution (P, OEP, UroP, CoproP, ProtoP, and TPP). Differences in these MO parameters for distinct porphyrins are related to inductive and conjugative effects of the substituents. In particular, a good correlation between the splitting of the Lu-band and the Soret band for a series of copper porphyrins and the Hammett constants for the substituents is explained by a relationship between the electron-withdrawing ability of substituents and the strength of the configuration interaction that mixes one-electron excited states of the porphyrin. Variations among the porphyrins in the configuration interaction energy result from delocalization of ring charge caused by increased mixing of p-orbitals of the substituent a-carbons with ring orbitals as the "electronegativity" of the substituents increases. Delocalization of ring charge onto substituent carbons lowers the electron-electron repulsion, and therefore the configuration interaction. Anomalous features of the spectra of the metalloporphines and metalloprotoporphyrins are also explained. The spectra of the metalloporphines are unusual because suitable substituent p-orbitals are not available on the hydrogens for mixing with ring a-orbitals. The spectra of the metalloprotoporphyrins are of biological interest and are found to be only mildly anomalous. The unusual features can be traced to enhanced mixing with and delocalization of ring charge onto the a-system of the two vinyl substituents. For a complete understanding of substituent effects on the absorption spectra, changes in the configuration interaction, orbital energies, and transition dipoles must all be considered.

V. Ortiz

Papers

Substituent effects on the electronic structure of metalloporphyrins: a quantitative analysis in terms of four-orbital-model parameters