Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

Abstract

-Properties of the triplet states of octaethylporphyrins with the steric hindrance (free bases and Pd complexes) are studied by the methods of stationary and kinetic spectroscopy in the temperature range from 77 to 293 K. The mono-mesophenyl substitution results in a decrease in the quantum yield and shortening of the phosphorescence lifetime of Pd complexes by 250-3500 times in degassed toluene at 293 K. The phosphorescence quenching is caused by nonplanar dynamic conformations of the porphyrin macrocycle in the T 1 state, which also lead to the appearance of new bands at λ ~ 1000 nm in the T -T absorption spectra. As the number of meso-phenyls (Pd-octaetyltetraphenylporphyrin) increases, the quantum yield of phosphorescence further decreases ( <10 -5 ) at 293 K, the lifetime of the T 1 state shortens (<50 ns), and the efficiency of the singlet oxygen generation abruptly decreases (<0.01). The intense bathochromic emission of this compound at 705 nm with a lifetime of 1 ms at 77 K is assigned to the phosphorescence of a nonplanar conformation. Upon meso-orthonitrophenyl substitution, the quenching of phosphorescence of Pd complexes (by more than 10 4 times at 293 K) is caused by direct nonadiabatic photoinduced electron transfer from the T 1 state to the nearest charge-transfer state with the probability = (1.5-4.0) × 10 6 s -1 . The induced absorption of ortho-nitro derivatives in the region between 110 and 1400 nm is caused by mixing of pure ππ * states with charge-transfer states. © 2001 MAIK "Nauka/Interperiodica".

The effect of heavy atom to two photon absorption properties and intersystem crossing mechanism in aza-boron-dipyrromethene compounds

“…This result could be explained by considering our DFT calculation given in the previous section.According to DFT calculations, dihedral angle for 1,7 positions are smaller than that of 3,5 positions. It is known that greater dihedral angle causes smaller the charge transfer due to the non-planarity of the compound[52]. This explains why 1,7 position (compound 5c) is more efficient for charge transfer than 3,5 positions (compound 5b) for the investigated compounds.…”

Quantum-Chemical Investigation of the Conformational Dynamics of Mono-meso-phenyl-Substituted Octaalkylporphyrins in the Triplet Excited State

“…It should be mentioned also that at 295 K in liquid toluene the transient T 1 → T n absorption spectra (in the region of 700−1600 nm) for sterically strained porphyrins ( meso -phenyl-substituted OEP and PdOEP) are distinguished from those measured for usual planar tetrapyrroles. , The appearance of new bands in the red and IR regions of T 1 → T n absorption spectra of sterically strained porphyrins with respect to the spectra of normal planar tetrapyrroles may be connected with the nonplanar distortion of the porphyrin macrocycle in the former molecules.…”

Extended Conjugation Platinum(II) Porphyrins for use in Near-Infrared Emitting Organic Light Emitting Diodes

“…The photophysics of various expanded conjugation porphyrins have been previously investigated, and it has been demonstrated that substituents play a major role in determining porphyrin photophysical properties. − For example, Vinogradov et al demonstrated that Pd-5,15-diphenyltetrabenzoporphyrin has a phosphorescence quantum yield of 0.15, whereas Pd-5,10,15,20-tetraphenyltetrabenzoporphyrin has a lower quantum yield of only 0.08 . This increased quantum yield for the disubstituted derivative originates from the more planar structure, as out-of-plane distorted porphyrins display increased nonradiative decay pathways. ,, Furthermore, Beeby et al have demonstrated an approximate doubling of solution ϕ em from 0.11 to 0.21 for 5,10,15,20-tetrasubstituted free base porphyrins when monophenyl substituents are substituted for more bulky fluorene or terphenyl substituents . Drawing from this previous work, a family of Pt-tetrabenzoporphyrins (Pt-TBPs, Figure ) incorporating 5,15-diaryl and 5,10,15,20-tetraaryl derivatives with varying substituent groups has been synthesized, and the 5,15-diaryl derivatives display a 50% enhancement in solution quantum yield as compared to the 5,10,15,20-tetraaryl derivatives .…”