NIR-Absorbing Donor-Acceptor Based 1,1,4,4-Tetracyanobuta-1,3-Diene (TCBD)- and Cyclohexa-2,5-Diene-1,4-Ylidene-Expanded TCBD-Substituted Ferrocenyl Phenothiazines

NIR absorbing ferrocenyl perylenediimide-based donor-acceptor chromophores

A set of ferrocenyl-functionalized perylenediimide (PDI) compounds and their 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives 1-5 were designed and synthesized using palladium-catalyzed Sonogashira cross-coupling, followed by a thermally activated [2 + 2] cycloaddition-retroelectrocyclization [CA-RE] reaction with a 1,1,2,2-tetracyanoethylene (TCNE) acceptor in good yields. The TCBD group works as an acceptor, whereas the ferrocenyl group acts as a donor at the central PDI core. The effects of varying the number of ferrocenyl and TCNE groups on the photophysical, thermal, electrochemical, and spectroelectrochemical properties were studied. The di-substituted PDI derivatives 3, 4, and 5 exhibit bathochromic shifts in the absorption spectra compared to 1 and 2, attributed to the extended π-conjugation. The electrochemical analysis of derivatives 2, 4, and 5 shows multiple reduction waves in the low potential region due to the presence of TCBD and perylenediimide acceptor units. Spectroelectrochemical studies were performed, showing that upon applying redox potentials, the absorption spectra shifted from the visible to the near-infrared (NIR) region. Computational calculations indicate that in the HOMO, the electron density is localized on the ferrocene unit, while in the LUMO, it is distributed over the PDI-TCBD unit, indicating a strong D-A interaction.

Near-IR Capturing N-Methylbenzene Sulfonamide-Phenothiazine Incorporating Strong Electron Acceptor Push-Pull Systems: Photochemical Ultrafast Carrier Dynamics

Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi-modular donor-acceptor-based push-pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light-emitting devices. Herein, a series of symmetrical and unsymmetrical donor-acceptor chromophores 1-6, are designed and synthesized by the Corey-Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring-opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60-85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low-lying broad absorption band extended towards the near-infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of 3 and 6 are lower than those of 2 and 5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry-optimized structures further support accessing donor-acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.

Near-IR absorbing 1,1,4,4-tetracyanobutadiene-functionalized phenothiazine sulfones

Triphenylamine (TPA) substituted π-conjugated chromophores TPA1-TPA5 were designed and synthesized via Pd-catalysed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions. The effects of acceptor 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (DCNQ) units in the photophysical studies and the HOMO-LUMO energy levels of the phenothiazine sulfones TPA1-TPA5 were evaluated. The absorption spectra of chromophores TPA4 and TPA5 show a significant change due to the incorporation of DCNQ units, resulting in bathochromically shifted broad absorption in the NIR region. The photophysical studies revealed that DCNQ-based chromophores TPA4 and TPA5 have a better D-A interaction than the TCBD functionalized TPA2 and TPA3. Density functional theory calculations and electrochemical studies were performed to examine the molecular geometry and frontier energy levels of the sulfone-based chromophores. Systematic structural modification of the chromophore TPA1 modulated the electrochemical properties and successively tuned the energy gaps for TPA2-TPA5. The theoretically estimated HOMO-LUMO gaps for TPA1-TPA5 exhibit good agreement with the experimental data calculated from the electrochemical studies. The chromophore TPA1 exhibits solvatochromism and aggregation-induced emission (AIE) behavior owing to the emission in the solid state.

Tetracyanobutadiene Bridged Push-Pull Chromophores: Development of New Generation Optoelectronic Materials

This review describes the design strategies used for the synthesis of various tetracyanobutadiene bridged donor-acceptor molecular architectures by a click type [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction sequence. The photophysical and electrochemical properties of the tetracyanobutadiene bridged molecular architectures based on various moieties including diketopyrrolopyrrole, isoindigo, benzothiadiazole, pyrene, pyrazabole, truxene, boron dipyrromethene (BODIPY), phenothiazine, triphenylamine, thiazole and bisthiazole are summarized. Further, we discuss some important applications of the tetracyanobutadiene bridged derivatives in dye sensitized solar cells, bulk heterojunction solar cells and photothermal cancer therapy.

Phenothiazine and phenothiazine-5,5-dioxide based push-pull Derivatives: Synthesis, photophysical, electrochemical and computational Studies

A set of phenothiazine (PTZ) and phenothiazine-5,5-dioxide based π-conjugated push–pull chromophores PTZ 1–6 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition retroelectrocyclic ring opening reaction in...

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization

A series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.

Charge stabilization via electron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl-tetracyanobutadiene donor-acceptor conjugates

Photoinduced charge separation in donor-acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor-acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, the occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors have been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange.

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.

Emission and Color Tuning of Cyanostilbenes and White Light Emission

White-light-emitting diodes are energy efficiency replacement of conventional lighting sources. Herein, we report the luminescent behavior of three simple cyanostilbenes with triphenylamine-donating groups bearing different electron-withdrawing groups (phenyl, pyridyl, and p-nitrophenyl) in a common donor (D)-π-acceptor (A) α-cyanostilbene construct along with their thermal and electrochemical properties. The density functional theory (DFT) studies reveal that aggregation-induced emission characteristic feature of the D-π-A dyes is inversely proportional to the intramolecular charge transfer (ICT) effect, that is, phenyl-and pyridyl-substituted compounds show characteristic aggregation-induced emission in water, whereas the ICT effect is dominant for the nitro derivative. The extent of ICT and the solvatochromic emission shifts, from blue to red, depend on the strength of the electron-withdrawing group. White luminescence and tunable emission colors are obtained by careful admixtures of these cyanostilbenes bearing triphenylamines. The results rationalized through DFT and time-dependent DFT calculations follow a consistent trend with the energy levels measured from the electrochemical and optical studies. Thermogravimetric analysis and differential scanning calorimetry studies showed excellent thermal stability of the compounds. The scanning electron microscopy and dynamic light scattering measurements were performed to reveal the formation of aggregates. This strategy involving synthetically simple and structurally similar molecules with different emission properties has potential applications in the fabrication of multicolor and white-light-emitting materials.

Covalent layer-by-layer thin films with charge-transfer chromophores: side chain engineering for efficient Ag+ ion recognition in aqueous solutions

Covalent layer-by-layer (LbL) thin films are fabricated by a highly efficient [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction of aniline-substituted alkyne and 7,7,8,8-tetracyanoquinodimethane (TCNQ) moieties. Polystyrene bearing aniline-substituted alkyne side chains and TCNQ polyester were prepared as precursor polymers, then sequentially deposited and fixed by covalent bonds on an indium-tin-oxide (ITO) substrate. The successful alternate growth of the films was demonstrated by many techniques including absorption spectroscopy, surface roughness, and redox activities. Interestingly, the water contact angles of the film surface were dependent on the side chains of the polystyrene derivatives. When hydrophilic triethylene glycol was employed, the surface hydrophilicity was similar to that of the TCNQ polyester. In contrast, the use of hydrophobic n-hexyl groups resulted in a significant difference in the water contact angles between the polystyrene and TCNQ polyester. A similar difference occurred when the Ag+ ion recognition was studied by soaking the LbL films in aqueous solutions. The LbL films constructed from the polystyrene with triethylene glycol chains displayed a more rapid and significant recognition of Ag+ ions than those constructed from the polystyrene with n-hexyl chains.