“Two‐Point” Self‐Assembly and Photoinduced Electron Transfer in meso ‐Donor‐Carrying Bis(styryl crown ether)‐BODIPY–Bis(alkylammonium)fullerene Donor–Acceptor Conjugates

Photoinduced Energy and Electron Transfer in a 'Two-Point' Bound Panchromatic, Near-Infrared-Absorbing Bis-styrylBODIPY(Zinc Porphyrin)2 - Fullerene Self-Assembled Supramolecular Conjugate

Structurally well-defined self-assembled supramolecular multi-modular donor-acceptor conjugates play a significant role in furthering our understanding of photoinduced energy and electron transfer events occurring in nature, e. g., in the antenna-reaction centers of photosynthesis and their applications in light energy harvesting. However, building such multi-modular systems capable of mimicking the early events of photosynthesis has been synthetically challenging, causing a major hurdle for its growth. Often, multi-modularity is brought in by combining both covalent and noncovalent approaches. In the present study, we have developed such an approach wherein a π-extended conjugated molecular cleft, two zinc(II)porphyrin bearing bisstyrylBODIPY (dyad, 1), has been synthesized. The binding of 1 via a 'two-point' metal-ligand coordination of a bis-pyridyl fulleropyrrolidine (2), forming a stable self-assembled supramolecular complex (1 : 2), has been established. The self-assembled supramolecular complex has been fully characterized by a suite of physico-chemical methods, including TD-DFT studies. From the established energy diagram, both energy and electron transfer events were envisioned. In dyad 1, selective excitation of zinc(II)porphyrin leads to efficient singlet-singlet excitation transfer to (bisstyrly)BODIPY with an energy transfer rate constant, kEnT of 2.56×1012 s-1. In complex 1 : 2, photoexcitation of zinc(II)porphyrin results in ultrafast photoinduced electron transfer with a charge separation rate constant, kCS of 2.83×1011 s-1, and a charge recombination rate constant, kCR of 2.51×109 s-1. For excitation at 730 nm corresponding to bisstyrylBODIPY, similar results are obtained, where a biexponential decay yielded estimated values of kCS 3.44×1011 s-1 and 2.97×1010 s-1, and a kCR value of 2.10×1010 s-1. The newly built self-assembled supramolecular complex has been shown to successfully mimic the early events of the photosynthetic antenna-reaction center events.

Multimodular Wide-Band Capturing Nanohybrids: Role of Carbon Nanotubes in Slowing Charge Recombination in Supramolecular C60-BisstyrylBODIPY-(Zinc Porphyrin)2 Donor-Acceptor Molecular Cleft

The importance of diameter-sorted single-wall carbon nanotubes (SWCNTs) noncovalently bound to a donor-acceptor molecular cleft, 1, in prolonging the lifetime of charge-separated states is successfully demonstrated. For this, using a multistep synthetic procedure, a wide-band capturing, multimodular, C60-bisstyrylBODIPY-(zinc porphyrin)2, molecular cleft 1, was newly synthesized and shown to bind diameter-sorted SWCNTs. The molecular cleft and its supramolecular assemblies were characterized by a suite of physicochemical techniques. Free-energy calculations suggested that both the (6,5) and (7,6) SWCNTs bound to 1 act as hole acceptors during the photoinduced sequential electron transfer events. Consequently, selective excitation of 1 in 1:SWCNT hybrids revealed a two-step electron transfer, leading to the formation of charge-separated states. Due to the distant separation of the cation and anion radical species within the supramolecules, improved lifetimes of the charge-separated states could be achieved. The present supramolecular strategy of improving charge separation involving SWCNTs and donor-acceptor molecular clefts highlights the potential application of these hybrid materials for various light energy harvesting and optoelectronic applications.

Panchromatic Light-Capturing Bis-styryl BODIPY-Perylenediimide Donor-Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer

Two wide-band-capturing donor-acceptor conjugates featuring bis-styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the 1 PDI* to BODIPY, and a subsequent electron transfer from the 1 BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground-state interactions between the donor and acceptor entities. Steady-state fluorescence and excitation spectral recordings provided evidence of singlet-singlet energy transfer in these dyads, and quenched fluorescence of bis-styrylBODIPY emission in the dyads suggested additional photo-events. The facile oxidation of bis-styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S1 and S2 states, derived from time-dependent DFT calculations, supported excited charge transfer in these dyads. Spectro-electrochemical studies on one-electron-oxidized and one-electron-reduced dyads and the monomeric precursor compounds were also performed in a thin-layer optical cell under corresponding applied potentials. From this study, both bis-styrylBODIPY⋅+ and PDI⋅- could be spectrally characterizes and were subsequently used in characterizing the electron-transfer products. Finally, pump-probe spectral studies were performed in dichlorobenzene under selective PDI and bis-styrylBODIPY excitation to secure energy and electron-transfer evidence. The measured rate constants for energy transfer, kENT , were in the range of 1011  s-1 , while the electron transfer rate constants, kET , were in the range of 1010  s-1 , thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

Direct synthetic routes to functionalised crown ethers

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

Excitation-dependent electron exchange energy and electron transfer dynamics in a series of covalently tethered N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline - [C60] fullerene dyads via varying π-conjugated spacers

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C60)-based donor-π conjugated bridge-acceptor dyads (D-B-A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (kEET ≥ 2 × 1012 s-1) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from 1BBA* to C60 followed by electron transfer from BBA to 1C60*. Cohesive rise and decay dynamics of conjugated BBA˙+-C60˙- anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (kCS ≥ 2.2 × 1011 s-1) with near unity quantum efficiency (φCS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [kCR ∼ (1010-108) s-1]. In contrast, excitation of the C60 moiety (λex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to 1C60*, involving non-adiabatic (diabatic) and distance-dependent CS (kCS in the range of 0.59-1.78 × 1011 s-1) with 98.86-99.6% (Dyad-3-Dyad-1) quantum efficiency and a CR process with kCR values [kCR ∼ (1010-108) s-1] up to three orders greater than kCS of the respective dyads. Both the processes, CS and CR, upon C60 excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor β ≤ 0.5 Å-1, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CShot) and thermally relaxed (CSeq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C60. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.

Sequential energy transfer followed by electron transfer in a BODIPY-bisstyrylBODIPY bound to C60 triad via a 'two-point' binding strategy

Excitation transfer from ¹BODIPY* to bisstyrylBODIPY followed by electron transfer to C₆₀ leading to a charge separated state of appreciable lifetime in a supramolecularly assembled triad is demonstrated, as a mimic of the photosynthetic 'antenna-reaction centre'.

Photophysical and optical limiting properties of a novel distyryl-BODIPY with fused crown ether moieties

The synthesis and characterization of a crown-ether-substituted 3,5-distyrylBODIPY dye with a 4-dimethylaminophenyl group at the meso-position is reported. The optical limiting properties were investigated at 532 nm, and the dye was found to have enhanced reverse saturable absorption responses during z-scan measurements. Theoretical calculations suggest that this may be due to the large dipole moment that is introduced by the benzo-fused crown ether and 4-dimethylaminophenyl substituents.