Photoinduced intramolecular charge transfer in push-pull polyenes: effects of solvation, electron-donor group, and polyenic chain length

Rational design and investigation of nonlinear optical response properties of pyrrolopyrrole aza-BODIPY-based novel push-pull chromophores

Intramolecular charge transfer (ICT)-based chromophores are highly sought after for designing near-infrared (NIR) absorbing and emitting dyes as well as for designing materials for nonlinear optical (NLO) applications. The properties of these 'push-pull' molecules can easily be modified by varying the electronic donor (D) and acceptor (A) groups as well as the π-conjugation linker. This study presents a methodical approach and employs quantum chemical analysis to explore the relationship between the structural features, electro-optical properties, and the NLO characteristics of molecules with D-π-A framework. The one- and two-photon absorption (2PA), linear polarizability (α), and first hyperpolarizability (β) of some novel chromophores, consisting of a dimeric aza-Boron Dipyrromethene (aza-BODIPY) analogue, called, pyrrolopyrrole aza-BODIPY (PPAB), serving as the acceptor, have been investigated. The electronic donors used in this study are triphenylamine (TPA) and diphenylamine (DPA), and they are conjugated to the acceptor via thienyl or phenylene π-linkers. Additionally, the Hyper-Rayleigh Scattering (βHRS), which enables direct estimation of the second-order NLO properties, is calculated for the studied chromophores with 1064 nm excitation in acetonitrile. The β value shows a significant increase with increasing solvent polarity, indicating that the ICT plays a crucial role in shaping the NLO response of the studied molecules. The enhancement of the 2PA cross-section of the investigated molecules can also be achieved by modulating the combinations of donors and linkers. The results of our study indicate that the novel D-π-A molecules designed in this work demonstrate considerably higher hyperpolarizability values than the standard p-nitroaniline, making them promising candidates for future NLO applications.

Exploring a new class of singlet fission fluorene derivatives with high-energy triplets

We found that a stronger push–pull character favours SF, as long as the ICT does not act as a trap. The unique property of generating high-energy triplets (ca. 2 eV) via SF makes these materials outstanding candidates for photovoltaic applications.

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Li+-Induced fluorescent metallogel: a case of ESIPT-CHEF and ICT phenomenon

A fluorescent metallogel has been synthesized from non-fluorescent ingredients viz. the smallest possible LMW aromatic symmetrical gelator 1 and LiOH, whereas KOH produces a non-fluorescent solution, and regioisomer 2 with LiOH shows an ICT assisted fluorescent precipitate rather than a metallogel.

Microwave Assisted Synthesis, Optical Properties and Physicochemical Investigations on the Powerful Fluorophore: Donor (D) -π-Acceptor (A) Chalcone

(2E)-3-[4-(dimethylamino)phenyl]-1-(2-hydroxyphenyl)prop-2-en-1-one (DPHP) was synthesized by the reaction 4(dimethylamino) benzaldehyde with 1-(2-hydroxyphenyl) ethanone under microwave irradiation. Structure of DPHP was conformed by ¹H and ¹³C NMR, FT-IR, EI-MS spectral studies and elemental analysis. The electronic absorption and fluorescence spectra of DPHP have been studied in solvents of different polarities, and the data were used to study the solvatochromic properties such as extinction coefficient, stokes shift, oscillator strength, transition dipole moment, fluorescence quantum yield and photochemical quantum yield. The absorption maximum and fluorescence emission maximum was observed red shift when increase solvent polarity n-Hexane to DMF. DPHP undergoes solubilization in different micelles and may be used as a probe and quencher to determine the critical micelle concentration (CMC) of CTAB and SDS.

Photoinduced intramolecular charge transfer in trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine: effect of introducing a C[double bond, length as m-dash]C double bond

The spectral characteristics of trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine (t-DMASIP-b) have been investigated using absorption and fluorescence techniques, and compared with 2-(4'-N,N-dimethylamino)imidazo[4,5-b]pyridine (DMAPIP-b). The study reveals that introduction of a C[double bond, length as m-dash]C double bond strongly perturbs the photophysics of the system. Unlike DMAPIP-b, t-DMASIP-b emits a single emission in aprotic and protic solvents. The emission occurs from the locally excited state in nonpolar solvents and from a planar intramolecular charge transfer (PICT) state in polar solvents. Multiple linear regression analysis suggests that among the different solvent parameters, the dipolar interaction contributes more to the stabilization of the system in both the ground and excited states. Theoretical calculations suggest that, unlike in DMAPIP-b, proton coupled twisted intramolecular charge transfer (TICT) emission does not occur in t-DMASIP-b. The higher quantum yield obtained in the viscous solvent glycerol is attributed to the restriction of the twisting of the olefinic bond. The photoirradiation of t-DMASIP-b shows that isomerization takes place in all solvents, including viscous glycerol. The theoretically simulated potential energy surface shows that isomerization occurs via a phantom state, which is a nonradiative process. The rise in temperature favors the photoisomerization, thus, the fluorescence quantum yield decreases. The prototropic study indicates that, unlike in DMAPIP-b, the protonation takes place at different places to form the monocations.

Photoinduced symmetry-breaking intramolecular charge transfer in a quadrupolar pyridinium derivative

We report here a joint experimental and theoretical study of a quadrupolar, two-branched pyridinium derivative of interest as a potential non-linear optical material. The spectral and photophysical behaviour of this symmetric system is greatly affected by the polarity of the medium. A very efficient photoinduced intramolecular charge transfer, surprisingly more efficient than in the dipolar asymmetric analogue, is found to occur by femtosecond resolved transient absorption spectroscopy. TD-DFT calculations are in excellent agreement with these experimental findings and predict large charge displacements in the molecular orbitals describing the ground state and the lowest excited singlet state. The theoretical study also revealed that in highly polar media the symmetry of the excited state is broken giving a possible explanation to the fluorescence and transient absorption spectra resembling those of the one-branched analogous compound in the same solvents. The present study may give an important insight into the excited state deactivation mechanism of cationic (donor-π-acceptor-π-donor)(+) quadrupolar compounds characterised by negative solvatochromism, which are expected to show significant two-photon absorption (TPA). Moreover, the water solubility of the investigated quadrupolar system may represent an added value in view of the most promising applications of TPA materials in biology and medicine.

Rational design of a fluorescent NADPH derivative imaging constitutive nitric-oxide synthases upon two-photon excitation

We report the structure-based design and synthesis of a unique NOS inhibitor, called nanoshutter NS1, with two-photon absorption properties. NS1 targets the NADPH site of NOS by a nucleotide moiety mimicking NADPH linked to a conjugated push-pull chromophore with nonlinear absorption properties. Because NS1 could not provide reducing equivalents to the protein and competed with NADPH binding, it efficiently inhibited NOS catalysis. NS1 became fluorescent once bound to NOS with an excellent signal-to-noise ratio because of two-photon excitation avoiding interference from the flavin-autofluorescence and because free NS1 was not fluorescent in aqueous solutions. NS1 fluorescence enhancement was selective for constitutive NOS in vitro, in particular for endothelial NOS (eNOS). Molecular dynamics simulations suggested that two variable residues among NOS isoforms induced differences in binding of NS1 and in local solvation around NS1 nitro group, consistent with changes of NS1 fluorescence yield. NS1 colocalized with eNOS in living human umbilical vein endothelial cells. Thus, NS1 constitutes a unique class of eNOS probe with two-photon excitation in the 800-950-nm range, with great perspectives for eNOS imaging in living tissues.

Synthesis, photophysical, and two-photon absorption properties of elongated phosphane oxide and sulfide derivatives

A series of rod-shaped and related three-branched push-pull derivatives containing phosphane oxide or phosphane sulfide (PO or PS)-as an electron-withdrawing group conjugated to electron-donating groups, such as amino or ether groups, with a conjugated rod consisting of arylene-vinylene or arylene-ethynylene building blocks-were prepared. These compounds were efficiently synthesized by a Grignard reaction followed by Sonogashira coupling. Their photophysical properties including absorption, emission, time-resolved fluorescence, and two-photon absorption (TPA) were investigated with special attention to structure-property relationships. These fluorophores show high fluorescence quantum yields and solvent-dependent experiments reveal that efficient intramolecular charge transfer occurs upon excitation, thereby leading to highly polar excited states, the polarity of which can be significantly enhanced by playing on the end groups and conjugated linker. Rod-shaped and related three-branched systems show similar fluorescence properties in agreement with excitation localization on one of the push-pull branches. By using stronger electron donors or replacing the arylene-ethynylene linkers with an arylene-vinylene one induces significant redshifts of both the low-energy one-photon absorption and TPA bands. Interestingly, a major enhancement in TPA responses is observed, whereas OPA intensities are only weakly affected. Similarly, phosphane oxide derivatives show similar OPA responses than the corresponding sulfides but their TPA responses are significantly larger. Finally, the electronic coupling between dipolar branches promoted by common PO or PS acceptor moieties induces either slight enhancement of the TPA responses or broadening of the TPA band in the near infrared (NIR) region. Such behavior markedly contrasts with triphenylamine-core-mediated coupling, which gives evidence for the different types of interactions between branches.

Synthesis and study of the use of heterocyclic thiosemicarbazones as signaling scaffolding for the recognition of anions

A family of heterocyclic thiosemicarbazone dyes (1-9) linked to different furan, thiazole, (bi)thiophene, and arylthiophene pi-conjugated bridges were synthesized in good yields, and their response toward anions was studied. Acetonitrile solutions of 1-9 show bands in the 326-407 region that are modulated by the electron-donor or -acceptor strength of the heterocyclic systems appended to the phenylthiosemicarbazone moiety. Anions of different shape such as fluoride, chloride, bromide, iodide, dihydrogen phosphate, hydrogen sulfate, nitrate, acetate, cyanide, and thiocyanate were employed for the recognition studies. From these anions, only fluoride, cyanide, acetate, and dihydrogen phosphate displayed sensing features. Two different effects were observed, (i) a low bathochromic shift of the absorption band due to coordination of the anions with the thiourea protons and (ii) the growth of a new red-shifted band with a concomitant change of the solution from yellow or pale yellow to orange-red due to deprotonation. The extent of each process is a balance between the acidity tendency of the thioureido-NH donors modulated by the donor or acceptor groups in the structure of the receptors and the basicity of the anions. Fluorescence studies were also in agreement with the different effects observed on the UV/vis titrations. Stability constants for the two processes (complex formation + deprotonation) for selected receptors and the anions fluoride and acetate were determined spectrophotometrically using the program HYPERQUAD. Semiempirical calculations to evaluate the hydrogen-donating ability of the dyes and (1)H NMR titrations experiments with fluoride were carried out. A prospective electrochemical characterization of compound 3 in the presence of anions was also performed.