Tailoring Panchromatic Absorption and Excited-State Dynamics of Tetrapyrrole–Chromophore (Bodipy, Rylene) Arrays—Interplay of Orbital Mixing and Configuration Interaction

Geometry-Independent Ultrafast Energy Transfer in Bioinspired Arrays Containing Electronically Coupled BODIPY Dimers as Energy Donors

In photosynthetic light-harvesting complexes, strong interaction between chromophores enables efficient absorption of solar radiation and has been suggested to enable ultrafast energy funneling to the reaction center. To examine whether similar effects can be realized in synthetic systems, and to determine the mechanisms of energy transfer, we synthesized and characterized a series of bioinspired arrays containing strongly-coupled BODIPY dimers as energy donors and chlorin derivatives as energy acceptors. The BODIPY dimers feature broad absorption in the range of 500-600 nm, complementing the chlorin absorption to provide absorption across the entire visible spectrum. Ultrafast (~10 ps) energy transfer was observed from photoexcited BODIPY dyads to chlorin subunits. Surprisingly, the energy-transfer rate is nearly independent of the position where the BODIPY dimer is attached to the chlorin and of the type of connecting linker. In addition, the energy-transfer rate from BODIPY dimers to chlorin is slower than the corresponding rate in arrays containing BODIPY monomers. The lower rate, corresponding to less efficient through-bond transfer, is most likely due to weaker electronic coupling between the ground state of the chlorin acceptor and the delocalized electronic state of the BODIPY dimer, compared to the localized state of a BODIPY monomer.

Conjugated photothermal materials and structure design for solar steam generation

With the development of solar steam generation (SSG) for clean water production, conjugated photothermal materials (PTMs) have attracted significant interest because of their advantages over metallic and inorganic PTMs in terms of high light absorption, designable molecular structures, flexible morphology, and solution processability. We review here the recent progress in solar steam generation devices based on conjugated organic materials. Conjugated organic materials are processed into fibers, membranes, and porous structures. Therefore, nanostructure design based on the concept of nanoarchitectonics is crucial to achieve high SSG efficiency. We discuss the considerations for designing SSG absorbers and describe commonly used conjugated organic materials and structural designs.

Balancing Panchromatic Absorption and Multistep Charge Separation in a Compact Molecular Architecture

A panchromatic triad and a charge-separation unit are joined in a crossbar architecture to capture solar energy. The panchromatic-absorber triad (T) is comprised of a central free-base porphyrin that is strongly coupled via direct ethyne linkages to two perylene-monoimide (PMI) groups. The charge-separation unit incorporates a free-base or zinc chlorin (C or ZnC) as a hole acceptor (or electron donor) and a perylene-diimide (PDI) as an electron acceptor, both attached to the porphyrin via diphenylethyne linkers. The free-base porphyrin is common to both light-harvesting and charge-separation motifs. The chlorin and PDI also function as ancillary light absorbers, complementing direct excitation of the panchromatic triad to produce the discrete lowest excited state of the array (T*). Attainment of full charge separation across the pentad entails two steps: (1) an initial excited-state hole/electron-transfer process to oxidize the chlorin (and reduce the panchromatic triad) or reduce the PDI (and oxidize the panchromatic triad); and (2) subsequent ground-state electron/hole migration to produce oxidized chlorin and reduced PDI. Full charge separation for pentad ZnC-T-PDI to generate ZnC⁺-T-PDI^- occurs with a quantum yield of ∼30% and mean lifetime ∼1 μs in dimethyl sulfoxide. For C-T-PDI, initial charge separation is followed by rapid charge recombination. The molecular designs and studies reported here reveal the challenges of balancing the demands for charge separation (linker length and composition, excited-state energies, redox potentials, and medium polarity) with the constraints for panchromatic absorption (strong electronic coupling of the porphyrin and two PMI units) for integrated function in solar-energy conversion.

Fast Charge Separation in Distant Donor-Acceptor Dyads Driven by Relaxation of a Hot Excited State

A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (β) while discussing anomalous CS trends.

Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment

The syntheses of two triads are reported. Each triad is composed of two perylene-monoimides linked to a porphyrin via an ethyne unit, which bridges the perylene 9-position and a porphyrin 5- or 15-position. Each triad also contains a single tether composed of an alkynoic acid or an isophthalate unit. Each triad provides panchromatic absorption (350–700 nm) with fluorescence emission in the near-infrared region (733 or 743 nm; fluorescence quantum yield ~0.2). The syntheses rely on the preparation of trans-AB-porphyrins bearing one site for tether attachment (A), an aryl group (B), and two open meso-positions. The AB-porphyrins were prepared by the condensation of a 1,9-diformyldipyrromethane and a dipyrromethane. The installation of the two perylene-monoimide groups was achieved upon the 5,15-dibromination of the porphyrin and the subsequent copper-free Sonogashira coupling, which was accomplished before or after the attachment of the tether. The syntheses provide relatively straightforward access to a panchromatic absorber for use in bioconjugation or surface-attachment processes.

Photophysical Properties and Electronic Structure of Hydroporphyrin Dyads Exhibiting Strong Through-Space and Through-Bond Electronic Interactions

Electronic interactions between tetrapyrroles are utilized in natural photosynthetic systems to tune the light-harvesting and energy-/charge-transfer processes in these assemblies. Such interactions also can be employed to tailor the electronic properties of tetrapyrrolic dyads and larger arrays for use in materials science and biomedical research. Here, we have utilized static and time-resolved optical spectroscopy to characterize the optical absorption and emission properties of a set of chlorin and bacteriochlorin dyads with varying degrees of through-bond (TB) and through-space (TS) interactions between the constituent macrocycles. The dyads consist of two chlorins or two bacteriochlorins joined by a linker that utilizes a triple-double-triple-bond (enediyne) motif in which the double-bond portion is an ester-substituted ethylene or o-phenylene unit. The photophysical studies are coupled with density functional theory (DFT) calculations to probe the ground-state molecular orbital (MO) characteristics of the dyads and time-dependent DFT calculations (TDDFT) to elucidate excited-state properties. The latter include electronic characteristics of the singlet excited-state manifold and the absorption transitions to these states from the electronic ground state. A comparison of the MO and calculated spectral properties of each dyad with the linker present versus disrupted (by eliminating the double-bond portion) gives insight into the relative contributions of TB versus TS interactions to the electronic properties of the dyads. The results show that the TB and TS contributions are additive (constructively interfere), which is not always the case for molecular dyads. Most of the dyads have shorter lifetimes of the lowest singlet excited state compared to the parent monomer, which derives from increased S₁ → S₀ internal conversion. The enhancement is greater for the dyads in benzonitrile than in toluene. The studies provide insights into the nature of the electronic interactions between the constituents in the tetrapyrrole arrays and how these interactions dictate the spectral properties and excited-state decay characteristics.

Electronic Structure and Excited-State Dynamics of Rylene-Tetrapyrrole Panchromatic Absorbers

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (τ_S) and increased fluorescence quantum yield (Φ_f) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a π-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Φ_f and shortened τ_S values. The PMI-P-TMI triad in toluene exhibits Φ_f = 0.038 and τ_S = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φ_f = 0.26 and τ_S = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.

Panchromatic BODIPY dyes: Synthesis and optoelectronic properties

Panchromatic dyes covering the visible and near-infrared (Vis/NIR) region are of considerable interest in the fields of DSSCs and OPVs as well as infrared sensing due to their efficient solar energy harvesting. Herein, we report two BODIPY dyes displaying a panchromatic absorption with an extension to the near-infrared range, by the introduction of bis(4-methoxyphenyl)aminophenylvinyl unit at 3,5-position of BODIPY core through Knoevenagel condensation reactions. The optoelectronic properties were fully investigated by spectroscopy, cyclic voltammetry (CV) measurements and DFT calculations. Our research provides a simple strategy to broaden the absorption spectrum of BODIPY dyes.

Tailoring Photophysical Properties of Diketopyrrolopyrrole Small Molecules with Electron-Withdrawing Moieties for Efficient Solar Steam Generation

The development of photothermal materials (PTMs) for solar steam generation (SSG) has gained tremendous attention in response to the global clean water scarcity issue. However, the investigation in employing organic small-molecule PTMs for SSG applications is rarely found due to their narrow optical absorption range to harvest solar energy and insufficient photostability for long-term use. Herein, we employ a diketopyrrolopyrrole (DPP) core unit together with electron-withdrawing (EW) endcaps and siloxane side chains to introduce stronger intramolecular charge transfer (ICT) characteristics as well as the hydrophobic character. The enhanced ICT characteristics of DPP derivatives render a broad optical absorption range, less emission, and a high nonradiative decay rate for efficient solar energy harvesting and photothermal effects. Meanwhile, the hydrophobic nature of these DPP derivatives allows the facile fabrication of novel Janus photothermal membranes for effective water vaporization and solar-to-vapor conversion efficiency. By embedding DPP derivatives to the SSG device, we showed that the solar-to-vapor efficiency can reach up to 71.8% under relatively low visible light power (∼700 W m^-2), which is, on average, 2.66 times higher than that of bulk water of similar dimension. Moreover, this report demonstrates the great potential of conjugated small molecules for photothermal applications, owing to their versatility and flexibility in structural engineering and its diminishing radiative decay properties. This may inspire more innovation and advancement in SSG applications.

Models to study photoinduced multiple proton coupled electron transfer processes

In water-oxidizing photosynthetic organisms, excitation of the reaction-center chlorophylls (P680) triggers a cascade of electron and proton transfer reactions that establish charge separation across the membrane and proton-motive force. An early oxidation step in this process involves proton-coupled electron transfer (PCET) via a tyrosine-histidine redox relay (Yz-H190). Herein, we report the synthesis and structural characterization of two isomeric dyads designed to model this PCET process. Both are based on the same high potential fluorinated porphyrin (model for P680), linked to isomeric pyridylbenzimidazole-phenols (models for Yz-H190). The two isomeric dyads have different hydrogen bond frameworks, which is expected to change the PCET photooxidation mechanism. In these dyads, 1H NMR evidence indicates that in one dyad the hydrogen bond network would support a Grotthuss-type proton transfer process, whereas in the other the hydrogen bond network is interrupted. Photoinduced one-electron, two-proton transfer is expected to occur in the fully hydrogen-bonded dyad upon oxidation of the phenol by the excited state of the porphyrin. In contrast for the isomer with the interrupted hydrogen bond network, an ultrafast photoinduced one-electron one-proton transfer process is anticipated, followed by a much slower proton transfer to the terminal proton acceptor. Understanding the nature of photoinduced PCET mechanisms in these biomimetic models will provide insights into the design of future generations of artificial constructs involved in energy conversion schemes.

Conjugated-linker dependence of the photophysical properties and electronic structure of chlorin dyads

The synthesis, photophysical properties and electronic structure of seven new chlorin dyads and associated benchmark monomers are described. Each dyad contains two identical chlorins linked at the macrocycle [Formula: see text]-pyrrole 13-position. The extent of electronic communication between chlorin constituents depends on the nature of the conjugated linker. The communication is assessed by modification of prominent ground-state absorption and redox properties, rate constants and yields of excited-state decay processes, and molecular-orbital characteristics. Relative to the benchmark monomers, the chlorin dyads in toluene exhibit a substantial bathochromic shift of the long-wavelength absorption band (30 nm average), two-fold increased radiative rate constant [average (10 ns)[Formula: see text] vs. (22 ns)[Formula: see text]], reduced singlet excited-state lifetimes (average 5.0 ns vs. 8.2 ns), and increased fluorescence quantum yields (average 0.56 vs. 0.42). The excited-state lifetime and fluorescence yield for the chlorin dyad with a benzothiadiazole linker are reduced substantially in benzonitrile vs. toluene due largely to [Formula: see text]25-fold accelerated internal conversion. The results aid design strategies for molecular architectures that may find utility in solar-energy conversion and photomedicine.

Excited-state photophysical processes in a molecular system containing perylene bisimide and zinc porphyrin chromophores

Multichromophoric systems with efficient photoinduced excited-state processes are important for the conversion of solar energy in artificial photosynthesis. However, a low molecular absorption coefficient of these multichromophoric systems in the near-infrared region limits their power conversion efficiency in organic solar cells. It is critical to design molecules with a broad absorption range in the whole spectral region, to better harvest solar energy, and to reveal their important multiple-step photophysical processes for the design of organic solar cells. Here, we investigate a novel compound having three chromophores, namely two near-by N,N'-bis(1-pentyl)hexyl-3,4,9,10-perylenebiscarboximide (PDI) units linked to a zinc porphyrin core side by side (in the form of PDI-ZnPor-PDI), which absorbs solar energy ranging from the ultraviolet (UV) to near-infrared regions. The photophysical behavior of PDI-ZnPor-PDI in both film and solution forms, has been investigated using steady-state and transient spectroscopy measurements. Charge-transfer species and triplet excited-state species are observed, the excited-state evolutions of which are monitored using molecular vibrations as probes. These observations support the idea that PDI-ZnPor-PDI on photoexcitation generates the radical anion and triplet species of the PDI unit (PDI˙- and 3PDI*). Our results demonstrate the effect of solid film state on the photophysical properties in such multichromophoric system, and are valuable for guiding the design and utilization of novel near-infrared electron donors or acceptors for use in organic solar cells.

A robust and efficient aqueous electrochemiluminescence emitter constructed by sulfonate porphyrin-based metal-organic frameworks and its application in ascorbic acid detection

The robust and strong electrochemiluminescence (ECL) emission of organic emitters in an aqueous solution is crucial for expanding their applications in early diagnosis. Herein, a Zn porphyrin-based metal-organic framework ((Zn)porphMOF) was facilely obtained by chelating Zn(ii)meso-tetra (4-sulfonatophenyl) porphine (Zn-TSPP) with Zn ions, showing substantially enhanced ECL radiation with K₂S₂O₈ as the coreactant via the "reduction-oxidation" route in aqueous media. In contrast with Zn-TSPP, (Zn)porphMOF displayed 22-fold increase in the ECL intensity because of the agglomeration effect. By virtue of the dramatic confinement towards the energy and electron transfer of ascorbic acid (AA) during the ECL process, an ultrasensitive biosensor was developed with a wide linear range (3.77 to 26.4 μM) and ultra-low detection limit of 0.29 μM at 3 times of the signal-to-noise ratio (3S/N). This work offers a feasible avenue to harvest the steady and boosted ECL responses of organic molecules in aqueous media, also greatly expanding the MOF applications in bioanalysis.

Strongly Coupled Phenazine-Porphyrin Dyads: Light-Harvesting Molecular Assemblies with Broad Absorption Coverage

The development of light-harvesting architectures with broad absorption coverage in the visible region continues to be an important research area in the field of artificial photosynthesis. Here, we introduce a new class of ethynyl-linked panchromatic dyads composed of dibenzophenazines coupled ortho and meta to tetrapyrroles with an anchoring group that can be grafted onto metal oxide surfaces. Quantum chemical calculations and photophysical measurements of the synthesized materials reveal that both of the dibenzophenazine dyads absorb broadly from 300 to 636 nm and exhibit absorption bands different from those of the constituent chromophore units. Moreover, the different points of attachment of dibenzophenazines to tetrapyrroles give different absorption profiles which computations suggest result from differences in the planarity of the two dyads. Applicability of the dyads in artificial photosynthesis systems was assessed by their incorporation and characterization of their performance in dye-sensitized solar cells.

Buchwald-Hartwig Coupling at the Naphthalenediimide Core: Access to Dendritic, Panchromatic NIR Absorbers with Exceptionally Low Band Gap

The first successful Buchwald-Hartwig reaction at the naphthalenediimide core is reported, leading to the coupling of diverse secondary aromatic amines including dendritic donors. The G1-dendrimer-based donor exhibit blackish color, providing access to black absorbing systems. λ_onset values up to 1070 nm was achieved, which is the maximum from a single NDI scaffold. These dyes also manifest multielectron reservoir properties. A total of eight-redox states with a band gap of ∼0.95 eV was accomplished.

Effect of Side Groups on the Photovoltaic Performance Based on Porphyrin-Perylene Bisimide Electron Acceptors

In this work, we developed four porphyrin-based small molecular electron acceptors for non-fullerene organic solar cells, in which different side groups attached to the porphyrin core were selected in order to achieve optimized performance. The molecules contain porphyrin as the core, perylene bisimides as end groups, and the ethynyl unit as the linker. Four side groups, from 2,6-di(dodecyloxy)phenyl to (2-ethylhexyl)thiophen-2-yl, pentadecan-7-yl, and 3,5-di(dodecyloxy)phenyl unit, were applied into the electron acceptors. The new molecules exhibit broad absorption spectra from 300 to 900 nm and high molar extinction coefficients. The molecules as electron acceptors were applied into organic solar cells, showing increased power conversion efficiencies from 1.84 to 5.34%. We employed several techniques, including photoluminescence spectra, electroluminescence spectra, atomic force microscopy, and grazing-incidence wide-angle X-ray to probe the blends to find the effects of the side groups on the photovoltaic properties. We found that the electron acceptors with 2,6-di(dodecyloxy)phenyl units show high-lying frontier energy levels, good crystalline properties, and low nonradiative recombination loss, resulting in possible large phase separation and low energy loss, which is responsible for the low performance. Our results provide a detailed study about the side groups of non-fullerene materials, demonstrating that porphyrin can be used to design electron acceptors toward near-infrared absorption.

Origin of Panchromaticity in Multichromophore-Tetrapyrrole Arrays

Panchromatic absorbers that have robust photophysical properties enable new designs for molecular-based light-harvesting systems. Herein, we report experimental and theoretical studies of the spectral, redox, and excited-state properties of a series of perylene-monoimide-ethyne-porphyrin arrays wherein the number of perylene-monoimide units is stepped from one to four. In the arrays, a profound shift of absorption intensity from the strong violet-blue (B _y and B _x) bands of typical porphyrins into the green, red, and near-infrared (Q _x and Q _y) regions stems from mixing of chromophore and tetrapyrrole molecular orbitals (MOs), which gives multiplets of MOs having electron density spread over the entire array. This reduces the extensive mixing between porphyrin excited-state configurations and the transition-dipole addition and subtraction that normally leads to intense B and weak Q bands. Reduced configurational mixing derives from moderate effects of the ethyne and perylene on the MO energies and a more substantial effect of electron-density delocalization to reduce the configuration-interaction energy. Quantitative oscillator-strength analysis shows that porphyrin intensity is also shifted into the perylene-like green-region absorption and that the ethyne linkers lend absorption intensity. The reduced porphyrin configurational mixing also endows the S₁ state with bacteriochlorin-like properties, including a 1-5 ns lifetime.

Expanding π-Conjugation in Chlorins Using Ethenyl Linker

A series of chlorin monomers and dyads has been prepared to probe the effect of ethenyl vs ethynyl linkers on the electronic conjugation and optical properties in resulting derivatives. Styryl-substituted chlorins have been prepared either by a Heck reaction or by microwave-assisted olefin metathesis, while β-β ethenyl-linked dyads have been synthesized from the corresponding vinyl-substituted chlorin monomer using microwave-assisted olefin metathesis. It has been found that when an ethenyl linker is connected at the β-position of chlorin it provides stronger electronic conjugation than an ethynyl one, which is manifested by a greater bathochromic shift of the longest wavelength absorption (Q _y) and emission bands. Stronger electronic coupling is particularly evident for dyads, where ethenyl-linked dyad exhibits a strong near-IR absorption band emission (λ_abs = 707 nm, λ_em = 712 nm, Φ_f = 0.45), compared to the deep-red absorption and emission of a corresponding ethynyl-linked dyad (λ_abs = 689 nm, λ_em = 691 nm, Φ_f = 0.48). The reactivity of ethenyl-linked dyads with singlet oxygen is discussed as well. The results reported here provide further guidelines for molecular design of deep-red and near-IR absorbing and intensely emitting chlorin derivatives and chlorins with extended π-electronic conjugation for a variety of photonic applications.

Synthesis of arrays containing porphyrin, chlorin, and perylene-imide constituents for panchromatic light-harvesting and charge separation

Achieving solar light harvesting followed by efficient charge separation and transport is an essential objective of molecular-based artificial photosynthesis. Architectures that afford strong absorption across the near-UV to near-infrared region, namely panchromatic absorptivity, are critically important given the broad spectral distribution of sunlight. A tetrapyrrole–perylene pentad array was synthesized and investigated as a means to integrate panchromatic light harvesting and intramolecular charge separation. The pentad consists of three moieties: (1) a panchromatically absorbing triad, in which a porphyrin is strongly coupled to two perylene-monoimides via ethyne linkages; (2) a perylene-diimide electron acceptor; and (3) a chlorin hole-trapping unit. Integrating the three components with diphenylethyne linkers generates moderate electronic coupling for intramolecular energy and hole/electron transfer. The construction of the array relies on a stepwise strategy for incorporating modular pigment building blocks. The key building blocks include a trans-A₂BC porphyrin, a chlorin, a perylene-monoimide, and a perylene-diimide, each bearing appropriate (halo, ethynyl) synthetic handles for Pd-catalyzed Sonogashira coupling reactions. One target pentad, three tetrads, four triads, and four monomeric benchmark compounds were synthesized from six building blocks (three new, three reported) and 10 new synthetic intermediates. Four of the tetrapyrrole-containing arrays are zinc chelated, and four others are in the free base form. Absorption and fluorescence spectra and fluorescence quantum yields were also measured. Collectively, investigations of the arrays reveal insights into principles for the design of novel reaction centers integrated with a panchromatic antenna for artificial photosynthetic studies.

Twelve arrays containing porphyrin, chlorin, and/or perylene-imide units were synthesized to investigate panchromatic absorption integrated with charge separation.