π-Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

Synthesis and Characterization of Novel Thienothiadiazole-Based D-π-A-π-D Fluorophores as Potential NIR Imaging Agents

As fluorescence bioimaging has increased in popularity, there have been numerous reports on designing organic fluorophores with desirable properties amenable to perform this task, specifically fluorophores with emission in the near-infrared II (NIR-II) region. One such strategy is to utilize the donor-π-acceptor-π-donor approach (D-π-A-π-D), as this allows for control of the photophysical properties of the resulting fluorophores through modulation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Herein, we illustrate the properties of thienothiadiazole (TTD) as an effective acceptor moiety in the design of NIR emissive fluorophores. TTD is a well-known electron-deficient species, but its use as an acceptor in D-π-A-π-D systems has not been extensively studied. We employed TTD as an acceptor unit in a series of two fluorophores and characterized the photophysical properties through experimental and computational studies. Both fluorophores exhibited emission maxima in the NIR-I that extends into the NIR-II. We also utilized electron paramagnetic resonance (EPR) spectroscopy to rationalize differences in the measured quantum yield values and demonstrated, to our knowledge, the first experimental evidence of radical species on a TTD-based small-molecule fluorophore. Encapsulation of the fluorophores using a surfactant formed polymeric nanoparticles, which were studied by photophysical and morphological techniques. The results of this work illustrate the potential of TTD as an acceptor in the design of NIR-II emissive fluorophores for fluorescence bioimaging applications.

From gas separation to ion transport in the cavity of hyperbranched polyamides based on triptycene aimed for electrochromic and memory devices

Introducing 3D triptycene as core with methoxy-diphenylamine into hyper- branching polyamides will greatly improve robust electrochemical cycling stability crucial for the application of ECDs.

Functional Materials Based on Cyclometalated Platinum(II) β-Diketonate Complexes: A Review of Structure-Property Relationships and Applications

Square planar organoplatinum(II) complexes have garnered immense interest in the area of materials research. The combination of the Pt(II) fragment with mono-, bi- tri- and tetradentate organic ligands gives rise to a large variety of complexes with intriguing properties, especially cyclometalated Pt(II) complexes in which ligands are connected through covalent bonds demonstrate higher stability, excellent photoluminescence properties, and diverse applications. The properties and applications of the Pt(II)-based materials can be smartly fine-tuned via a judicious selection of the cyclometalating as well as ancillary ligands. In this review, attempts have been made to provide a brief review of the recent developments of neutral Pt(II) organometallic complexes bearing bidentate cyclometalating ligands and β-diketonate ancillary ligands, i.e., (C^N)Pt(O^O) and (C^C)Pt(O^O) derivatives. Both small (monomeric, dimeric) and large (polymeric) materials have been considered. We critically assessed the role of functionalities (ligands) on photophysical properties and their impact on applications.

Evaluating Donor Effects in Isoindigo-Based Small Molecular Fluorophores

Small molecular organic fluorophores have garnered significant interest because of their indispensable use in fluorescence imaging (FI) and optoelectronic devices. Herein, we designed triphenylamine (TPA)-capped donor-acceptor-donor (D-A-D)-based fluorophores having a variation at the heterocyclic donor (D) units, 3,4-ethylenedioxythiophene (EDOT), furan (FURAN), thiophene (THIO), and 1-methyl-1H-pyrrole (MePyr), with isoindigo as the core electron acceptor (A) unit. Synthesis of these fluorophores (II-X-TPA) resulted in four symmetrical dye molecules: II-EDOT-TPA, II-FURAN-TPA, II-THIO-TPA, and II-MePyr-TPA, where TPA functioned as a terminal unit and a secondary electron donor group. Photophysical, electrochemical, and computational analyses were conducted to investigate the effect of heterocyclic donor units on the II-X-TPA derivatives. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations provided insightful features of structural and electronic properties of each fluorophore and correlated well with experimental observations. Electron density distribution maps, overlapping frontier molecular orbital diagrams, and highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) electron transfer indicated intramolecular charge transfer (ICT). Theoretical studies confirmed the experimental HOMO energy trend and demonstrated its crucial importance in understanding each heterocycle's donor ability. Stokes shifts of up to ∼178 nm were observed, whereas absorptions and emissions were shifted deeper into the NIR region, resulting from ICT. Results suggest that this isoindigo fluorophore series has potential as a molecular scaffold for the development of efficient FI agents. The studied fluorophores can be further tuned with different donor fragments to enhance the ICT and facilitate in shifting the optical properties further into the NIR region.

Linear and Star-Shaped Extended Di- and Tristyrylbenzenes: Synthesis, Characterization and Optical Response to Acid and Metal Ions

Two linear 1,4-distyrylbenzenes and five star-shaped 1,3,5-tristyrylbenzene derivatives (L2a and L2b , Y0 -Y3 and YNBu ) were synthesized and spectroscopically characterized. The photophysical properties, optical response to acid and metal ions were investigated. Upon backbone extension of linear distyrylbenzenes or the introduction of dibutylanilines, the electronic spectra are redshifted. Incorporation of electron-deficient pyridyl units does not significantly affect the optical properties. Variation of the number of pyridine rings and substitution pattern tune the fluorescence response to acids and metal ions. The novel arenes discriminate Al3+ , Mn2+ , Fe3+ , Fe2+ , Cd2+ , Ag+ and Hg2+ .

Polymerization-induced photothermy: A non-donor-acceptor approach to highly effective near-infrared photothermal conversion nanoparticles

Photothermal conversion nanoagents based on conjugated polymers (CPs) are attracting increasing attention for in vivo disease theranostics and high-performing ones are in urgent pursuit. Herein, we report a new and non-donor-acceptor approach to photothermal conversion CPs that combine several merits including low bandgaps, strong near-infrared absorption, low intersystem crossing rate and non-emissive nature. Three CPs based on 6,7; 6',7'-fused isoindigos (nIIDs), i.e., P2IIDV, P3IIDV and P4IIDV that have optical bandgaps of 1.30, 1.22 and 1.17 eV, respectively, are synthesized. The nanoparticles (NPs) of the CPs in water are prepared via nanocoprecipitation, which are non-fluorescent due to the rapid intramolecular twisting in the CP backbone within NPs, enabling most of the excitation energy flow to generate heat. The photothermal conversion efficiencies of the NPs as measured under irradiation at 808, 880 and 980 nm are 62.4%, 40.5% and 15.8% for P2IIDV, 65.1%, 41.0% and 38.9% for P3IIDV and 71.5%, 48.9% and 41.7% for P4IIDV, which are significantly higher than indocyanine green and many popularly reported photothermal conversion materials. In vivo studies using xenograft 4T1 tumor-bearing mouse model demonstrate that the P4IIDV NPs can serve as a rather effective photothermal conversion nanoagent for enhanced photoacoustic imaging and photothermal therapy of tumors.

Synthesis and Characterization of Isoindigo-Based Push-Pull Chromophores

Symmetrical and unsymmetrical chromophores of isoindigo 3-7 were designed and synthesized, in which isoindigo was used as the central unit (electron acceptor unit A), triphenylamine as the end capping unit (electron donor group D), 1,1,4,4-tetracyanobutadiene (TCBD, A') and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (A″) as the acceptor unit. The effects of multiacceptor units on photophysical, electrochemical, and computational studies were investigated. The photophysical properties of isoindigo 6 and 7 exhibit a strong intramolecular charge transfer (ICT) absorption band in the near IR region. The isoindigo 4-7 shows multi-redox waves with a low electrochemical band gap, which signifies the tuning of highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels and enhance the π-conjugation. The computational studies demonstrate that there is a good agreement with experimental data. The molecular design and synthesis of isoindigo 4-7 gives a new avenue for the development of building blocks in organic electronics.

Population dynamics of multiple triplet excitons revealed from time-dependent fluorescence quenching of single conjugated polymer chains

The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV-vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.

Pyridine-terminated low gap π-conjugated oligomers: design, synthesis, and photophysical response to protonation and metalation

Pyridine terminated donor–acceptor π-conjugated oligomers have been prepared and their absorption and emission properties have been evaluated with respect to protonation and metalation.

Lewis Acid-Base Chemistry of 7-Azaisoindigo-Based Organic Semiconductors

Low-band-gap organic semiconductors are important in a variety of organic electronics applications, such as organic photovoltaic devices, photodetectors, and field effect transistors. Building on our previous work, which introduced 7-azaisoindigo as an electron-deficient building block for the synthesis of donor-acceptor organic semiconductors, we demonstrate how Lewis acids can be used to further tune the energies of the frontier molecular orbitals. Coordination of a Lewis acid to the pyridinic nitrogen of 7-azaisoindigo greatly diminishes the electron density in the azaisoindigo π-system, resulting in a substantial reduction in the lowest unoccupied molecular orbital (LUMO) energy. This results in a smaller highest occupied molecular orbital-LUMO gap and shifts the lowest-energy electronic transition well into the near-infrared region. Both H⁺ and BF₃ are shown to coordinate to azaisoindigo and affect the energy of the S₀ → S₁ transition. A combination of time-dependent density functional theory and UV/vis and ¹H NMR spectroscopic titrations reveal that when two azaisoindigo groups are present and high concentrations of acid are used, both pyridinic nitrogens bind Lewis acids. Importantly, we demonstrate that this acid-base chemistry can be carried out at the solid-vapor interface by exposing thin films of aza-substituted organic semiconductors to vapor-phase BF₃·Et₂O. This suggests the possibility of using the BF₃-bound 7-azaisoindigo-based semiconductors as n-type materials in various organic electronic applications.

Direct Observation of an Efficient Triplet Exciton Diffusion Process in a Platinum-Containing Conjugated Polymer

We report the synthesis and characterization of a conjugated polymer incorporated with cyclometalated platinum complexes on the main chain. The polymer may serve as an efficient triplet sensitizer in light-harvesting systems. The photophysical properties of the polymer were studied by nanosecond and femtosecond time-resolved transient absorption spectroscopies. After excitation, an energy-transfer process from the thiophene units on the conjugated main chain to the singlet excited state of the Pt complex moieties occurred in less than 150 fs. The subsequent intersystem crossing process resulted in the formation of a triplet excited state at the Pt complex moieties in ∼3.2 ps, which was then followed by an efficient triplet diffusion process that led to the formation of triplet excitons on the polymer main chain in ∼283 ps. This proposed efficient triplet sensitized polymer system not only enhances the exciton diffusion length but also reduces energy loss in the process, which displays remarkable implications in the design of novel materials for triplet sensitized solar cells.