Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor-acceptor (D-A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor-acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S0 and S1 states. Mixing a certain degree of local excitation (LE) with CT in the S1 state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

D-A-D Compounds Combining Dithienopyrrole Donors and Acceptors of Increasing Electron-Withdrawing Capability: Synthesis, Spectroscopy, Electropolymerization, and Electrochromism

Five D-π-A-π-D compounds consisting of the same donor unit (dithieno[3,2-b:2',3'-d]pyrrole, DTP), the same π-linker (2,5-thienylene), and different acceptors of increasing electron-withdrawing ability (1,3,4-thiadiazole (TD), benzo[c][1,2,5]thiadiazole (BTD), 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 1,2,4,5-tetrazine (TZ), and benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)) were synthesized. DTP-TD, DTP-BTD, and DTP-DPP turned out to be interesting luminophores emitting either yellow (DTP-TD) or near-infrared (DTP-BTD and DTP-DPP) radiation in dichloromethane solutions. The emission bands were increasingly bathochromically shifted with increasing solvent polarity. Electrochemically determined electron affinities (|EA|s) were found to be strongly dependent on the nature of the acceptor changing from 2.86 to 3.84 eV for DTP-TD and DTP-NDI, respectively, while the ionization potential (IP) values varied only weakly. Experimental findings were strongly supported by theoretical calculations, which correctly predicted the observed solvent dependence of the emission spectra. Similarly, the calculated IP and EA values were in excellent agreement with the experiment. DTP-TD, DTP-BTD, DTP-TZ, and DTP-NDI could be electropolymerized to yield polymers of very narrow electrochemical band gap and characterized by redox states differing in color coordinates and lightness. Poly(DTP-NDI) and poly(DTP-TD) showed promising electrochromic behavior, not only providing a rich color palette in the visible but also exhibiting near-infrared (NIR) electrochromism.

Benzotriazole-EDOT electrochromic conjugated polymers perform sub-second response time and 774 cm2C-1 coloration efficiency

To investigate the effect of double fluorine substitution on the optical, electrochemical, thermodynamic, morphological and electrochromic properties of electrochromic polymers, two benzotriazole-EDOT electrochromic conjugated polymers of PBTz-E and P2F-BTz-E were...

2,1,3-Benzothiadiazole Small Donor Molecules: A DFT Study, Synthesis, and Optoelectronic Properties

We herein report the design and synthesis of small-donor molecules, 2,1,3-benzothiadiazole derivatives (2a-d), by Stille or Suzuki reaction. The synthesized compounds were characterized by spectroscopic and electrochemical methods. The compounds 2a-d absorb the light in a wide range (the UV-green/yellow light (2c)) and emit from green to red/near IR light (2c). Furthermore, these compounds show a narrow energy gap (1.75-2.38 eV), and high Ea values increasing for polymers, which prove their electron-donating nature and semiconductor properties. The measurements were enhanced by theoretical modeling.

Between Aromatic and Quinoid Structure: A Symmetrical UV to Vis/NIR Benzothiadiazole Redox Switch

Reversibly switching the light absorption of organic molecules by redox processes is of interest for applications in sensors, light harvesting, smart materials, and medical diagnostics. This work presents a symmetrical benzothiadiazole (BTD) derivative with a high fluorescence quantum yield in solution and in the crystalline state and shows by spectroelectrochemical analysis that reversible switching of UV absorption in the neutral state, to broadband Vis/NIR absorption in the 1st oxidized state, to sharp band Vis absorption in the 2nd oxidized state, is possible. For the one-electron oxidized species, formation of a delocalized radical is confirmed by electron paramagnetic resonance spectroelectrochemistry. Furthermore, our results reveal an increasing quinoidal distortion upon the 1st and 2nd oxidation, which can be used as the leitmotif for the development of BTD based redox switches.

Towards developing efficient metalloporphyrin-based hybrid photocatalysts for CO2 reduction; an ab initio study

A series of thiophene-based donor-acceptor-donor (D-A-D) oligomer substituted metalloporphyrins (MPors) with different 3d central metal-ions (M = Co, Ni, Cu, and Zn) were systematically investigated to screen efficient hybrid photocatalysts for CO2 reduction based on density functional theory (DFT) and time-dependent DFT simulations. Compared with base MPors, the newly designed hybrid photocatalysts have a lower bandgap energy, stronger and broader absorption spectra, and enhanced intermolecular charge transfer, exciton lifetime, and light-harvesting efficiency. Then, the introduction of D-A-D electron donor (ED) groups into the meso-positions of MPors is a promising method for the construction of efficient photocatalysts. According to the calculated adsorption distance, adsorption energy, Hirshfeld charge and electrostatic potential analysis, it was revealed that CO2 physically adsorbed on the designed photocatalyst surface. In addition, among the studied model systems the ZnPor(ED)4 catalyst with four D-A-D electron donors exhibits the best photocatalytic performance due to its broadest absorption spectra with λmax = 500.12 nm and the highest adsorption energy of about 26 kJ mol-1. Finally, the sensing ability of the ZnPor(ED)4-based multi-terminal molecular junction for CO2 gas detection is determined using Green's functions. The transmission plots of this molecular junction are barely changed due to the physical adsorption of CO2 on the molecular surface, leading to the low sensitivity of the device. We believe that such a theoretical design can provide a general approach for further experimental and computational studies of photocatalysts used in the CO2 reduction process.

The Effect of Acceptor Structure on Emission Color Tuning in Organic Semiconductors with D-π-A-π-D Structures

A series of novel donor-acceptor D-π-A-π-D compounds were synthesized and characterized in order to determine the influence of different acceptor units on their properties. The introduction of acceptor moieties had a direct impact on the HOMO and LUMO energy levels. Fluorescence spectra of compounds can be changed by the choice of an appropriate acceptor and were shifted from the green to the near-infrared part of spectra. Due to observed concentration induced emission quenching, the green exciplex type host was used to evaluate the potential of synthesized molecules as emitters in organic light emitting diodes (OLEDs).

The Effect of Aromatic Diimide Side Groups on the π-Conjugated Polymer Properties

The presented study describes the method for the synthesis and characterization of a new class of conjugated copolymers containing a perylenediimide (PDI) and naphthalene diimide (NDI) side groups. The main conjugated backbone is a donor-acceptor polymer poly[3,6-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] containing thiophene and carbazole as donor units and benzothiadiazole as an acceptor unit. The presented compounds were synthesized in a multistep synthesis. The polymerization was carried out by Suzuki or Stille coupling reaction. Redox properties of the studied polymers were tested in different conditions. Electrochemical investigation revealed independent reduction of the main polymer chain and diimide side groups. UV-Vis spectroscopy revealed the overlap of two absorption spectra. The difference between the electron affinity of the polymer main chain and that of the diimides estimated electrochemically is approximately 0.3 eV.

Low Band Gap Donor-Acceptor Conjugated Systems Based on 3-Alkoxy or 3-Pyrrolidino-4-cyanothiophene and Benzothiadiazole Units

3-Hexyloxy-4-cyanothiophene, 3-pyrrolidil-4-cyanothiophene, and 3,4-ethylenedioxythiophene (EDOT) units are used with benzothiadiazole as building blocks for the development of three new conjugated donor-acceptor-donor (DAD) derivatives. The DAD molecules have the central acceptor part, which is formed by combining electron-withdrawing cyano groups and the benzothiadiazole moiety, in common. Theoretical calculations and UV/Vis and electrochemical data reveal the key role of the end-capped donor to tune the electronic properties of the derivatives. A study of the electropolymerization process of the three derivatives shows the strong influence of the donor parts on both the reactivity of the precursors and the electronic properties of the resulting polymers. Derivatives end-capped with pyrrolidinocyano thiophene or EDOT units lead to films of polymers presenting low band gaps of around 0.9-1.4 eV. Upon oxidation, the two polymers present different behavior. In the presence of the pyrrolidinocyano thiophene moieties, oxidation leads to a blueshift of the absorption bands, whereas with EDOT units a classical redshift, giving high absorption in the near-IR region, is observed for the oxidized states.