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. 2008 Aug 21;112(33):7723-33.
doi: 10.1021/jp8043626. Epub 2008 Jul 30.

Effects of structural deformations on optical properties of tetrabenzoporphyrins: free-bases and Pd complexes

Artem Y Lebedev  1 , Mikhail A Filatov, Andrei V Cheprakov, Sergei A Vinogradov
Affiliations

Effects of structural deformations on optical properties of tetrabenzoporphyrins: free-bases and Pd complexes

Artem Y Lebedev et al. J Phys Chem A. .

Abstract

A recently developed method of synthesis of pi-extended porphyrins made it possible to prepare a series of tetrabenzoporphyrins (TBP) with different numbers of meso-aryl substituents. The photophysical parameters of free-bases and Pd complexes of meso-unsubstituted TBP's, 5,15-diaryl-TBP's (Ar2TBP's) and 5,10,15,20-tetraaryl-TBP's (Ar4TBP's) were measured. For comparison, similarly meso-arylsubstituted porphyrins fused with nonaromatic cyclohexeno-rings, i.e. Ar(n)-tetracyclohexenoporphyrins (Ar(n)TCHP's, n = 0, 2, 4), were also synthesized and studied. Structural information was obtained by ab initio (DFT) calculations and X-ray crystallography. It was found that: 1) Free-base Ar4TBP's are strongly distorted out-of-plane (saddled), possess broadened, red-shifted spectra, short excited-state lifetimes and low fluorescence quantum yields (tau(fl) = 2-3 ns, phi(fl) = 0.02-0.03). These features are characteristic of other nonplanar free-base porphyrins, including Ar4TCHP's. 2) Ar2TBP free-bases possess completely planar geometries, although with significant in-plane deformations. These deformations have practically no effect on the singlet excited-state properties of Ar2TBP's as compared to planar meso-unsubstituted TBP's. Both types of porphyrins retain strong fluorescence (tau(fl) = 10-12 ns, phi(fl) = 0.3-0.4), and their radiative rate constants (k(r)) are 3-4 times higher than those of planar H2TCHP's. 3) Nonplanar deformations dramatically enhance nonradiative decay of triplet states of regular Pd porphyrins. For example, planar PdTCHP phosphoresces with high quantum yield (phi(phos) = 0.45, tau(phos) = 1118 micros), while saddled PdPh4TCHP is practically nonemissive. In contrast, both ruffled and saddled PdAr(n)TBP's retain strong phosphorescence at ambient temperatures (PdPh2TBP: tau(phos) = 496 micros, phi(phos) = 0.15; PdPh4TBP: tau(phos) = 258 micros, phi(phos) = 0.08). It appears that pi-extension is capable of counterbalancing deleterious effects of nonplanar deformations on triplet emissivity of Pd porphyrins.

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Figures

Figure 1
Figure 1
Absorption and fluorescence (insets) spectra of H2TBP (1, A) (H2TBP was first dissolved in pyridine, and a drop of this solution was added to toluene.), H2Ph2TBP (2, B), H2Ph4TBP (3, C) and H2TCHP(CO2Bu)8 (4, D), H2Ph2TCHP (5, E) and H2Ph4TCHP (6, F) (Small amount of TMEDA was added to prevent dication formation.) in toluene at 295 K.
Figure 2
Figure 2
Fluorescence decays for H2TBP (1), H2Ph4TBP (3) (A) and H2TBP (4) and H2Ph4TBP (6) (B) in toluene at 296 K and the underlying lifetime distributions, recovered by the Maximum Entropy Method.
Figure 3
Figure 3
Computed structures (DFT B3LYP/6–31G(d)) of H2PhnTBP’s (left column) and PhnTCHP’s (right column) (n = 0, 2, 4). Distances are in Å. Main distortion modes, recovered by the NSD analysis, are shown by the bar graphs. In-plane distortions: A1g, red; B2g, green. Out-of-plane distortions: B1u (ruffling), cyan; B2u (saddling), magenta. (For complete list of modes see Supporting Information, V). Doop and Dip designate total mean-square out-of-plane and in-plane displacements, respectively.
Figure 4
Figure 4
X-ray crystal structure of H2Ar2TBP (Ar = 3,5-tBu2C6H3) and its NSD analysis. Distances are in Å. Hydrogen atoms are omitted for clarity.
Figure 5
Figure 5
X-ray crystal structures of PdPh2TBP (Pd-2) (A) and PdAr4TBP (Ar = 3,5-(BuO2C)2C6H3) (B) and their NSD analyses. Hydrogen atoms and substituents in the meso-aryl rings in PdAr4TBP (B) are not shown for clarity (see Supporting Information, IV for complete structure).
Figure 6
Figure 6
Absorption and phosphorescence (insets) spectra of PdTBP (Pd-1, A), PdPh2TBP (Pd-2, B), PdPh4TBP (Pd-3, C) and PdTCHP(CO2Bu)8 (Pd-4, D), PdPh2TCHP (Pd-5a, E) and PdPh4TCHP (6, F) in DMA at 295 K.
CHART 1
CHART 1
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