Electron donor–acceptor compounds: exploiting the triptycene geometry for the synthesis of porphyrin quinone diads, triads, and a tetrad

Excited Charge Separation in a π-Interacting Phenothiazine-Zinc Porphyrin-Fullerene Donor-Acceptor Conjugate

We have designed, synthesized, and characterized a donor-acceptor triad, SPS-PPY-C60, that consists of a π-interacting phenothiazine-linked porphyrin as a donor and sensitizer and fullerene as an acceptor to seek charge separation upon photoexcitation. The optical absorption spectrum revealed red-shifted Soret and Q-bands of porphyrin due to charge transfer-type interactions involving the two ethynyl bridges carrying electron-rich and electron-poor substituents. The redox properties suggested that the phenothiazine-porphyrin part of the molecule is easier to oxidize and the fullerene part is easier to reduce. DFT calculations supported the redox properties wherein the electron density of the highest molecular orbital (HOMO) was distributed over the donor phenothiazine-porphyrin entity while the lowest unoccupied molecular orbital (LUMO) was distributed over the fullerene acceptor. TD-DFT studies suggested the involvement of both the S2 and S1 states in the charge transfer process. The steady-state emission spectrum, when excited either at porphyrin Soret or visible band absorption maxima, revealed quenched emission both in nonpolar and polar solvents, suggesting the occurrence of excited state events. Finally, femtosecond transient absorption spectral studies were performed to witness the charge separation by utilizing solvents of different polarities. The transient data was further analyzed by GloTarAn by fitting the data with appropriate models to describe photochemical events. From this, the average lifetime of the charge-separated state calculated was found to be 169 ps in benzonitrile, 319 ps in dichlorobenzene, 1.7 ns in toluene for Soret band excitation, and ∼320 ps for Q-band excitation in benzonitrile.

Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.

Synthetic methodologies leading to porphyrin-quinone conjugates

This review focuses on synthetic strategies that have been established for the preparation of porphyrin-quinone conjugates of potential biological significance and as donor–acceptor compounds for electron transfer processes.

Electron transfer in electron donor-acceptor ensembles containing porphyrins and metalloporphyrins

Recent advances in electron transfer chemistry of electron donor-acceptor ensembles reported containing porphyrins and metalloporphyrins reported in ICPP-2 are reviewed briefly by focusing on the fundamental aspects of electron transfer reactions.

The photoinduced electron transference of porphyrin-anthraquinone dyads bridged with different lengths of links

The photoinduced electron transference (PET) interaction in porphyrin containing donor-acceptor (D-A) molecules is of great importance in nature and a significant part of the PET research has been devoted to the study of its mechanism ("through-space" or "through-bond") in these decades. Herein we synthesized a series of covalently linked porphyrin-anthraquinone dyads (Por-C(n)-AQ) bridged with flexible alkoxy chains at different lengths (n=1, 4, 10) and investigated their intramolecular PET using a combination of electronic absorption, steady-state fluorescence and decayed luminescence spectra. The experimental results show that the PET efficiency depends on the length of the flexible linkage between the porphyrin and anthraquinone moieties. Meanwhile, theoretical calculation applying the density functional theory (DFT) was also carried out to give the frontier orbital distribution and the optimized structures of these dyads. It is found that the orientation of the dyad with high PET efficiency is disadvantageous to π-π interaction. Thus, the PET of these dyads seemingly is best compatible with a "through-bond" (superexchange) mechanism.

Synthesis and spectral property study of porphyrin-anthraquinone dyads bonded through azo

To determine if the dihedral angle between the phenyl group and the porphyrin ring in the meso-phenyl porphyrin (H2TPP) could be adjusted by selective substitution of the naphthyl group for phenyl groups, novel dyads with a free-base porphyrin donor and anthraquinone acceptor linked by a rigid azo bond were synthesized and their spectral properties were investigated in detail. The Soret absorption band of dyads with meso-naphthyl groups in porphyrin shows 5–7 nm red shift as compared to the corresponding raw materials due to the introduction of the anthraquinone moiety. Fluorescence from the porphyrin moiety was found to be intensely quenched with the addition of anthraquinone moiety in the dyads; the highest quenching rate was observed to be 95% in the dyad prepared by meso-tetranaphthyl porphyrin. This indicates that the efficiency of the intramolecular photoinduced electron transfer in porphyrin-anthraquinone dyads could be changed by substituting the meso-phenyl group in porphyrin for the meso-naphthyl group.

Synthesis and stereochemistry of highly unsymmetric beta,meso-linked porphyrin arrays

Porphyrin arrays with tailor-made photophysical properties and well-defined three-dimensional geometries constitute attractive synthetic targets in porphyrin chemistry. The paper describes a variable, straightforward synthetic procedure for the construction of beta,meso-linked porphyrin multichromophores in good to excellent yields. In a Suzuki-type coupling reaction beta-borylated 5,10,15,20-tetraarylporphyrins (TAPs) served as versatile building blocks for the preparation of a plethora of directly linked, unsymmetrically substituted di- and triporphyrins. Besides their interesting photophysical properties, especially the trimeric porphyrin arrays show exciting stereochemical features. The established protocols thus open a convenient entry into the synthesis of achiral and chiral, unsymmetrically substituted beta,meso-linked oligoporphyrins, e.g., for applications in biomedicine or nonlinear optics.

An approach towards artificial quinone pools by use of photo- and redox-active dendritic molecules

Molecules with one porphyrin and multiple quinone groups are described. The molecules are based on dendritic frameworks, with the quinone groups attached at the "internal" positions and the porphyrin attached at the focal point, leading to a characteristic layer architecture. When irradiated with visible light in the presence of 4-tert-butylthiophenol, the quinones were converted to quinols. Such a behavior mimics the function of quinone pools in photosynthesis.

Photoinduced Energy and Electron Transfer Processes in Hexapyropheophorbide a- Fullerene [C60] Molecular Systems

The photophysical properties of the novel hexapyropheophorbide a (P6), and hexakis (pyropheophorbide a)-C60 (FP6) were studied and compared with those of hexakis (pyropheophorbide a)-fullerene [5:1] hexaadduct (FHP6). It was found that after light absorption the pyropheophorbide a molecules in all three compounds undergo very efficient energy transfer as well as partly excitonic interactions. The last process results in the formation of energy traps, which could be resolved experimentally. For P6, due to shorter distances between neighboring dye molecules, stronger interactions between pyropheophorbide a units than for FHP6 were observed. As a consequence, the excitation energy is delivered rapidly to traps formed by stacked pyropheophorbide a molecules resulting in the reduction of fluorescence, intersystem crossing, and singlet oxygen quantum yields compared to the values of FHP6. For FP6 the reduction of these values is much stronger due to an additional fast and efficient deactivation process, namely photoinduced electron transfer from pyropheophorbide a to the fullerene moiety. Consequently, FP6 can be considered as a combination of a light-harvesting system consisting of several separate pyropheophorbide a molecules and a charge-separating center.

Acid-Catalyzed Cyclization of Anthracenol Derivatives to Homotriptycenes

10-Benzyl-9,10-dihydroanthracen-9-ols, having high electron densities in the benzene ring, exhibit in the presence of acid a transannular ring closure to the corresponding homotriptycenes in almost quantitative yields. Since the starting compounds are easily accessible from 9(10H)-anthracenone, this process represents the most facile route to such pentacyclic systems. An electron-releasing methoxy group enables the intramolecular electrophilic substitution in its para position. In the absence of such an activation, a number of alternative processes can occur, namely the acid-catalyzed dehydration to anthracene derivatives with (R not = H) or without (R = H) rearrangement or a disproportionation reaction of the secondary alcohol to the corresponding ketone and hydrocarbon.

Exercises in molecular gymnastics--bending, stretching and twisting porphyrins

The functional versatility of tetrapyrroles as natural cofactors is related to their conformational flexibility where manipulation of the macrocycle conformation allows a fine-tuning of their physicochemical properties. This feature article gives a personal account of the synthesis and solid state structural characterization of highly substituted, non-planar porphyrins. Their conformational analysis identifies sterically strained tetrapyrroles as a versatile class of biomimetic compounds with tailor-made properties.

Conjugated Polymers Containing Large Soluble Diethynyl Iptycenes

[structures: see text] An efficient synthesis of large iptycenes appended with alkoxy and ethynyl substituents is reported. The rigid shape-persistent iptycene scaffold prevents interactions between the polymer backbones and can be used to solubilize polymers containing less soluble but readily accessible comonomers to prepare functional, solution-processible poly(p-phenyleneethynylene) (PPE)-conjugated polymers. These polymers are highly emissive in thin films without significant excimer/exciplex formation as a result of the effective chain isolation enforced by the iptycene units.

Iptycene quinones: synthesis and structure

A practical and efficient method to synthesize iptycene quinones has been developed. As a result, a series of pentiptycene quinones 8-16 were conveniently synthesized by one-pot reaction of triptycene quinone 4 or 5 with anthracene 1 or its derivatives 2-3 in refluxing acetic acid in the presence of p-chloranil, followed by CAN oxidative demethylation. Similarly, a series of heptiptycene quinones 17-23 with U-shaped cavities were achieved with pentiptycene quinone 10 and triptycene diquinone 6 as precursors. Non-iptycene triquinones 24 with one tweezer-shaped cavity and 25 with two U-shaped cavities were synthesized by one-pot reactions of anthracene with pentiptycene triquinones 16a and 16b, respectively. Non-iptycene triquinone 26 with a dendritic structure was conveniently obtained by the reaction of anthracene with either pentiptycene diquinone 12 or triptycene triquinone 7. The structures of regioisomers 16a and 16b were determined by the single-crystal structure analysis of 16b. The structures of other regioisomers, including heptiptycene tetraquinones 19a/19b/19c and heptiptycene triquinones 23a/23b, were identified by comparative reactions.