Acceptor Chromophores Research Articles

Theory of exciton transfer and diffusion in conjugated polymers.

We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ~10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.

Effect of a pi-bridging unit in triphenylamine-benzothiadiazole based donor acceptor chromophores for dye-sensitized solar cells

ABSTRACT Two new metal free sensitizers based on triphenylamine-benzothiadiazole were synthesized and used in dye sensitized solar cells (DSSCs). Electrochemical and optical properties of the dyes were studied by cyclic voltammetry, UV-vis absorption and fluorescence spectroscopy. A red shift was observed by the attachment of a thiophene (MZ-259) instead of a phenylene (MZ-255) bridge. The reverse effect was observed in the electrochemical properties because of the more electron-donating thiophene incorporated into the cyanoacrylic acid as an anchoring-acceptor unit in this system resulted in a higher LUMO level. The charge distribution of the dyes was also proved by DFT calculations using the Spartan10 program with the parameters of B3LYP and 6-31G** basis set. Finally the photovoltaic performances were investigated under standard AM 1.5 illumination with a power of 100 mW/cm2 using an ionic liquid based electrolyte. The power conversion efficiencies, η, were increased from 3.16% to 3.81% by using thiophene instead of phenylene as a π-bridge unit.

Control of Luminescence Properties in Naphthalene Diimide-Based Gel with Azodibenzoic Acid by Charge Transfer Interaction

The self-sorting between aromaticcompounds that can serve as electron donors and electronacceptors allows the formation of distinct p- and n-typebundles of fibers that further intermix and generate multiplejunction points for efficient electron transfer and transport ofthe charge carriers. However, a difficulty associated withsimple mixing of such compounds is the possibility of adifferent mode of assembly, i.e., charge-transfer interactionmediated by alternate donor–acceptor stacking. Thus, ex-amples related to self-sorting among donor–acceptor chromo-phores have been limited.

Action-FRET: probing the molecular conformation of mass-selected gas-phase peptides with Förster resonance energy transfer detected by acceptor-specific fragmentation.

The use of Förster resonance energy transfer (FRET) as a probe of the structure of biological molecules through fluorescence measurements in solution is well-attested. The transposition of this technique to the gas phase is appealing since it opens the perspective of combining the structural accuracy of FRET with the specificity and selectivity of mass spectrometry (MS). Here, we report FRET results on gas-phase polyalanine ions obtained by measuring FRET efficiency through specific photofragmentation rather than fluorescence. The structural sensitivity of the method was tested using commercially available chromophores (QSY 7 and carboxyrhodamine 575) grafted on a series of small, alanine-based peptides of differing sizes. The photofragmentation of these systems was investigated through action spectroscopy, and their conformations were probed using ion mobility spectrometry (IMS) and Monte Carlo minimization (MCM) simulations. We show that specific excitation of the donor chromophore results in the observation of fragments that are specific to the electronic excitation of the acceptor chromophore. This shows that energy transfer took place between the two chromophores and hence that the action-FRET technique can be used as a new and sensitive probe of the structure of gas-phase biomolecules, which opens perspectives as a new tool in structural biology.

Random copolyesters containing perylene bisimide: flexible films and fluorescent fibers.

Random copolyesters of poly(l-lactic acid) (PLLA) and [poly(1,4-cyclohexylenedimethylene-1,4- cyclohexanedicarboxylate)] (PCCD) incorporating varying mol ratios of perylene bisimide (PBI) were developed via a high-temperature solution-blending approach. PCCD incorporating PBI was developed by melt polycondenzation followed by a polyester-polyester transesterification reaction between PCCD-PBI and PLLA. The polymers exhibited good solubility in common organic solvents and formed free-standing films, which showed bright red emission upon irradiation with ultraviolet radiation. A solid state fluorescence quantum yield of 10% was observed for this PBI based polyester, which was much higher than that reported in literature for PBI based polymers in the solid state (<1%). Strong red fluorescent nanofibers of these polymers were successfully constructed by electrospinning technique. A random copolyester incorporating donor based on oligo(p-pheneylenevinylene) (OPV) and PBI as acceptor chromophore was also synthesized and fluorescence microscopy images of the electrospun fibers of this polymer exhibited blue, green and red emission upon excitation at different wavelengths. The high temperature solution blending approach involving a high molecular weight polymer and a suitably functionalized π conjugated molecule described here is a unique method by which 1D nanostructures of a wide range of π-conjugated chromophores could be fabricated having strong fluorescence, with the scope of application in nanoscale optoelectronics, biological devices, as well as sensing.

Design and Synthesis of Bipyridine Platinum(II) Bisalkynyl Fullerene Donor–Chromophore–Acceptor Triads with Ultrafast Charge Separation

Donor-chromophore-acceptor triads, (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet-triplet energy transfer from the (3)MLCT/LLCT excited state within 4 ps to give the (3)C60* state, while in (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the (3)MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of (3)C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.

Sequence‐Regulated Linear Polymers with Ion‐Sensing Charge‐Transfer Chromophores by the Multiple Click Approach

The multiple click approach is applied to the synthesis of sequence‐regulated linear polymers. The combination and reaction order of the click chemistry reactions are comprehensively investigated with particular focus on the emerging click chemistry reaction of alkyne–acceptor additions. When the copper‐catalyzed azide–alkyne cycloaddition (CuAAC) is combined, no noticeable side reactions occur in both click chemistry reactions. In contrast, alkyne–acceptor addition does not selectively occur in the presence of anthracene units, which are the functional groups for the Diels–Alder cycloaddition. Accordingly, the reaction orders are carefully considered, and double click protocols for achieving high molecular weight and sequence‐regulated linear polymers are successfully established. Since the resulting polymers possess donor–acceptor chromophores formed through the alkyne–acceptor addition, they show a colorimetric chemosensor behavior towards the Ag+ ion. image

A novel near-infrared-emitting cyclometalated platinum (II) complex with donor–acceptor–acceptor chromophores

A novel near-infrared-emitting cyclometalated platinum (II) complex of (TPA-BT-Q)Ptpic with a donor–acceptor–acceptor (D–A–A) chromophores was synthesized and characterized, in which the TPA-BT-Q unit is a cyclometalated ligand of N,N-di(4-octyloxyphenyl)-4-(7-(quinolin-2-yl)-benzo[c][1,2,5]thiadiazol-4-yl)-phenylamine and pic is picolinate anion. Its optophysical, electrochemical and electroluminescent characteristics were primarily studied. An intense UV–vis absorption peak at 540 nm and a strong near-infrared emission peak at 759 nm were observed for (TPA-BT-Q)Ptpic in dichloromethane. Using (TPA-BT-Q)Ptpic as a single dopant and a blend of poly(vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole as a host matrix, the single-emissive-layer polymer light-emitting devices exhibited a near-infrared emission peaked at 760 nm with the maximum external quantum efficiency of 0.12% at 16.6 mA cm−2 and a radiance intensity of 112 μW cm−2 at 11.7 mA cm−2 at the doping concentrations of 2.0 wt%. This work provides an efficient approach to realize near-infrared electrophosphorescent emission with high radiance intensity by employing platinum (II) complex with the D–A–A structure.

Self‐Sorting of Cyclic Peptide Homodimers into a Heterodimeric Assembly Featuring an Efficient Photoinduced Intramolecular Electron‐Transfer Process

We describe the thermodynamic characterisation of the self-sorting process experienced by two homodimers assembled by hydrogen-bonding interactions through their cyclopeptide scaffolds and decorated with Zn-porphyrin and fullerene units into a heterodimeric assembly that contains one electron-donor (Zn–porphyrin) and one electron-acceptor group (fullerene). The fluorescence of the Zn-porphyrin unit is strongly quenched upon heterodimer formation. This phenomenon is demonstrated to be the result of an efficient photoinduced electron-transfer (PET) process occurring between the Zn-porphyrin and the fullerene units of the heterodimeric system. The recombination lifetime of the charge-separated state of the heterodimer complex is in the order of 180 ns. In solution, both homo- and heterodimers are present as a mixture of three regioisomers: two staggered and one eclipsed. At the concentration used for this study, the high stability constant determined for the heterodimer suggests that the eclipsed conformer is the main component in solution. The application of the bound-state scenario allowed us to calculate that the heterodimer exists mainly as the eclipsed regioisomer (75-90 %). The attractive interaction that exists between the donor and acceptor chromophores in the heterodimeric assembly favours their arrangement in close contact. This is confirmed by the presence of charge-transfer bands centred at 720 nm in the absorption spectrum of the heterodimer. PET occurs in approximately 75% of the chromophores after excitation of both Zn-porphyrin and fullerene chromophores. Conversely, analogous systems, reported previously, decorated with extended tetrathiafulvalene and fullerene units showed a PET process in a significantly reduced extent (33%). We conclude that the strength (stability constant (K) x effective molarity (EM)) of the intramolecular interaction established between the two chromophores in the Zn-porphyrin/fullerene cyclopeptide-based heterodimers controls the regioisomeric distribution and regulates the high extent to which the PET process takes place in this system.

Cross‐Conjugation vs. Linear Conjugation in Donor–Bridge–Acceptor Nitrophenol Chromophores

AbstractThe influence of cross‐conjugation vs. linear conjugation on the electronic communication between donor and acceptor groups in phenol(ate)–bridge–nitrobenzene chromophores was investigated by solution and gas‐phase absorption spectroscopy, fluorescence spectroscopy, and quantum chemical calculations. The compounds studied include, among others, geminally and trans‐substituted diethynylethenes prepared by stepwise Sonogashira cross‐coupling reactions, and the cross‐conjugated analogue of stilbene. A butadiyne‐bridged donor–acceptor chromophore was prepared by an unsymmetrical Pd‐catalysed coupling between a chloroalkyne and a terminal alkyne. While the linearly conjugated chromophores showed a strong and redshifted charge‐transfer (CT) absorption maximum upon deprotonation in solution, a new redshifted absorption was either absent or present as a weak shoulder for the cross‐conjugated derivatives. Calculations on the nonplanar 1,1‐diaryl‐substituted ethylene derivative revealed that the S0–S1 absorption has a very low oscillator strength due to insignificant coupling between the donor and acceptor ends. The energy of this CT absorption for a nonsolvated molecule is smaller than that of the linearly conjugated stilbene analogue by 0.4 eV. This result cannot directly be inferred from the solution studies. A gas‐phase absorption spectrum of this chromophore was measured by action spectroscopy, which only revealed a higher‐energy absorption band. Based on calculations, this band was assigned to a higher‐lying π–π* transition. For the planar diethynylethene chromophores, cross‐conjugation leads to a decrease of 0.3 eV in the excitation energy, according to the calculations. Finally, in contrast to the linearly conjugated chromophores, the cross‐conjugated ones did not show any fluorescence, which may be due to deexcitation via the low‐lying CT “dark” state.

Highly Efficient Near‐Infrared Organic Light‐Emitting Diode Based on a Butterfly‐Shaped Donor–Acceptor Chromophore with Strong Solid‐State Fluorescence and a Large Proportion of Radiative Excitons

The development of near-infrared (NIR) organic light-emitting diodes (OLEDs) is of growing interest. Donor-acceptor (D-A) chromophores have served as an important class of NIR materials for NIR OLED applications. However, the external quantum efficiencies (EQEs) of NIR OLEDs based on conventional D-A chromophores are typically below 1 %. Reported herein is a butterfly-shaped D-A compound, PTZ-BZP. A PTZ-BZP film displayed strong NIR fluorescence with an emission peak at 700 nm, and the corresponding quantum efficiency reached 16 %. Remarkably, the EQE of the NIR OLED based on PTZ-BZP was 1.54 %, and a low efficiency roll-off was observed, as well as a high radiative exciton ratio of 48 %, which breaks through the limit of 25 % in conventional fluorescent OLEDs. Experimental and theoretical investigations were carried out to understand the excited-state properties of PTZ-BZP.

Supramolecular Assemblies by Charge‐Transfer Interactions between Donor and Acceptor Chromophores

We have collated various supramolecular designs that utilize organic donor-acceptor CT complexation to generate noncovalently co-assembled structures including fibrillar gels, micelles, vesicles, nanotubes, foldamers, conformationally restricted macromolecules, and liquid crystalline phases. Possibly inspired by nature, chemists have extensively used hydrogen bonding as a tool for supramolecular assemblies of a diverse range of abiotic building blocks. As a structural motif, CT complexes can be compared to hydrogen-bonded complexes in its directional nature and complementarities. Additional advantages of CT interactions include wider solvent tolerance and easy spectroscopic probing. Nevertheless the major limitation is their low association constant. This article shows different strategies have evolved over the years to overcome this drawback by reinforcing the CT interactions with auxiliary noncovalent forces without hampering the alternate stacking mode. Emerging reports on promising CT complexes in organic electronics are intimately related to various supramolecular designs that one can postulate based on donor-acceptor CT interactions.

Highly Efficient Near‐Infrared Organic Light‐Emitting Diode Based on a Butterfly‐Shaped Donor–Acceptor Chromophore with Strong Solid‐State Fluorescence and a Large Proportion of Radiative Excitons

AbstractThe development of near‐infrared (NIR) organic light‐emitting diodes (OLEDs) is of growing interest. Donor–acceptor (D–A) chromophores have served as an important class of NIR materials for NIR OLED applications. However, the external quantum efficiencies (EQEs) of NIR OLEDs based on conventional D–A chromophores are typically below 1 %. Reported herein is a butterfly‐shaped D–A compound, PTZ‐BZP. A PTZ‐BZP film displayed strong NIR fluorescence with an emission peak at 700 nm, and the corresponding quantum efficiency reached 16 %. Remarkably, the EQE of the NIR OLED based on PTZ‐BZP was 1.54 %, and a low efficiency roll‐off was observed, as well as a high radiative exciton ratio of 48 %, which breaks through the limit of 25 % in conventional fluorescent OLEDs. Experimental and theoretical investigations were carried out to understand the excited‐state properties of PTZ‐BZP.

Constructing (super)alkali–boron-heterofullerene dyads: an effective approach to achieve large first hyperpolarizabilities and high stabilities in M3O–BC59(M = Li, Na and K) and K@n-BC59(n = 5 and 6)

Using DFT methods, the electronic properties and the first hyperpolarizabilities of boron-heterofullerene-(super)alkali dyads: M3O-BC59 (M = Li, Na and K) and K@n-BC59 (n = 5 and 6) were systematically investigated. It is found that both M3O and K can effectively bind to BC59 with high binding energies (2.50-2.69 eV for K and 4.24-5.14 eV for M3O). The interaction between K and BC59 in K@n-BC59 is identified as primarily ionic in nature, whereas that between the superalkali M3O unit and BC59 becomes much stronger owing to the formation of a strong chemical bond (B-O bond). Moreover, compared with the sole parent cluster BC59 (619 au), both K@n-BC59 (n = 5 and 6) and M3O-BC59 (M = Li, Na and K), possess large first hyperpolarizabilities (β0), which are 3352, 2621 and 4921, 5440 and 7800 au, respectively, where the superalkali doped dyads (M3O-BC59) are much superior to the simple alkali exo-hedral species (K@n-BC59), and heavier superalkali can be more powerful in enhancing the β0 values of M3O-BC59. Clearly, these superalkali doped dyads M3O-BC59, formal donor-acceptor (DA) chromophores, exhibit not only excellent stability but also large first hyperpolarizability; therefore, they are expected to be potential candidates for excellent second-order NLO materials.

Green Synthesis, Physicochemical and Polarity Studies of Some Novel Biologically Active Donor Acceptor Chromophores

Green Synthesis, Physicochemical and Polarity Studies of Some Novel Biologically Active Donor Acceptor Chromophores

7-Azaisoindigo as a new electron deficient component of small molecule chromophores for organic solar cells

The use of 7-azaisoindigo as an electron accepting moiety in donor–acceptor chromophores is reported for the first time. Four chromophores were synthesized that contained either 2,2′-bithiophene or 3,3′-bis(dodecyloxy)-2,2′-bithiophene as an electron-donating sub-unit and either isoindigo or 7-azaisoindigo as an electron acceptor. The inclusion of a heteroatom in the isoindigo framework was designed to probe its effect on the LUMO, while 3,3′-bis(dodecyloxy)-2,2′-bithiophene was used as an electron rich alternative to 2,2′-bithiophene in order to similarly evaluate its effect on the HOMO level. The electronic properties of the resulting chromophores were fully characterized by UV/visible spectroscopy, cyclic voltammetry, and density functional theory. All four dyes were evaluated for their performance as electron donors in bulk heterojunction solar cells using PC61BM as the electron acceptor. Devices that used the isoindigo and 3,3′-bis(dodecyloxy)-2,2′-bithiophene-based dye displayed the best performance, reaching a maximum power conversion efficiency of 0.93%.

Structural effect on controllable resistive memory switching in donor–acceptor polymer systems

Controllable bistable electrical conductivity switching behavior and resistive memory effects have been demonstrated in Al/polymer/indium-tin oxide (ITO) sandwich structure devices, constructed from non-conjugated vinyl copolymers of PTPAnOXDm with pendant donor–acceptor chromophores. The observed electrical bistability can be attributed to the field-induced intra- and intermolecular charge transfer interaction between triphenylamine electron donor (D) and oxadiazole electron acceptor (A) entities, and is highly dependent on the chemical structure of the copolymers. The vinyl copolymers showed different memory behaviors, which depended on the loading of D/A ratios. The polymers containing only donor or acceptor moieties showed as insulators, the polymers containing both donor and acceptor moieties showed as WORM, flash and DRAM as D/A ratio increased. The structural effect on the physicochemical and electronic properties of the PTPAnOXDm copolymers, viz surface morphology, thermal stability, optical absorbance and photoluminescence, and molecular orbital energy levels, were investigated systematically to study the factors that influence the memory characteristics of the devices.

Synthesis and Properties of Azulene‐Substituted Donor–Acceptor Chromophores Connected by Arylamine Cores

Abstract1‐Ethynylazulenes connected by several arylamine cores reacted with tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) in a formal [2+2] cycloaddition–cycloreversion reaction to afford the corresponding tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) chromophores, respectively, in excellent yields. The intramolecular charge‐transfer (ICT) characters between the donor (azulene and arylamine cores) and acceptor (TCBD and DCNQ units) moieties were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the new TCBD and DCNQ derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, significant color changes were observed by visible spectroscopy under the electrochemical reduction conditions.

Dipolar Ferrocene and Ruthenocene Second-Order Nonlinear Optical Chromophores: A Time-Dependent Density Functional Theory Investigation of Their Absorption Spectra

The origin of the two prominent solvatochromic near-UV/visible/near-IR absorptions observed for donor–(π-bridge)–acceptor chromophores with ferrocene donors has been investigated using TD-DFT metho...

Tuning Light Absorption in Pyrene: Synthesis and Substitution Effects of Regioisomeric Donor–Acceptor Chromophores

Three isomeric donor-acceptor (DA) chromophores based on pyrene were synthesized to study the effects of substitution pattern on intramolecular charge-transfer absorption through pyrene. These chromophores are nonfluorescent and absorb light in the long-wavelength region approaching 700 nm, making them promising light-harvesters. Their optical properties depend greatly on the substitution pattern of the donor, but their electrochemical properties are relatively unaffected.