Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors and acceptors on the push–pull porphyrins to produce high-potential charge separated states was investigated. The porphyrins were functionalized at the opposite β,β′-pyrrole positions of porphyrin ring bearing triphenylamine push groups and naphthalimide pull groups. Systematic studies involving optical absorption, steady-state and time-resolved emission revealed existence of intramolecular type interactions both in the ground and excited states. The push–pull nature of the molecular systems was supported by frontier orbitals generated on optimized structures, wherein delocalization of HOMO over the push group and LUMO over the pull group connecting the porphyrin π-system was witnessed. Electrochemical studies were performed to visualize the effect of push and pull groups on the overall redox potentials of the porphyrins. Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in polar benzonitrile, the ability of push–pull porphyrins to produce charge-separated states upon photoexcitation was confirmed and the measured rates were in the range of 10⁹ s⁻¹. The lifetime of the final charge separated state was around 5 ns. This study ascertains the importance of push–pull porphyrins in solar energy conversion and diverse optoelectronic applications, for which high-potential charge-separated states are warranted.     

Tetraaryl ZnII Porphyrinates Substituted at β‐Pyrrolic Positions as Sensitizers in Dye‐Sensitized Solar Cells: A Comparison with meso‐Disubstituted Push–Pull ZnII Porphyrinates

A facile and fast approach, based on microwave‐enhanced Sonogashira coupling, has been employed to obtain in good yields both mono‐ and, for the first time, disubstituted push–pull Zn^II porphyrinates bearing a variety of ethynylphenyl moieties at the β‐pyrrolic position(s). Furthermore, a comparative experimental, electrochemical, and theoretical investigation has been carried out on these β‐mono‐ or disubstituted Zn^II porphyrinates and meso‐disubstituted push–pull Zn^II porphyrinates. We have obtained evidence that, although the HOMO–LUMO energy gap of the meso‐substituted push–pull dyes is lower, so that charge transfer along the push–pull system therein is easier, the β‐mono‐ or disubstituted push–pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs. This behavior is apparently not attributable to more intense B and Q bands, but rather to more facile charge injection. This is suggested by the DFT electron distribution in a model of a β‐monosubstituted porphyrinic dye interacting with a TiO₂ surface and by the positive effect of the β substitution on the incident photon‐to‐current conversion efficiency (IPCE) spectra, which show a significant intensity over a broad wavelength range (350–650 nm). In contrast, meso‐substitution produces IPCE spectra with two less intense and well‐separated peaks. The positive effect exerted by a cyanoacrylic acid group attached to the ethynylphenyl substituent has been analyzed by a photophysical and theoretical approach. This provided supporting evidence of a contribution from charge‐transfer transitions to both the B and Q bands, thus producing, through conjugation, excited electrons close to the carboxylic anchoring group. Finally, the straightforward and effective synthetic procedures developed, as well as the efficiencies observed by photoelectrochemical measurements, make the described β‐monosubstituted Zn^II porphyrinates extremely promising sensitizers for use in DSSCs.     

The Photoisomerization Pathway(s) of Push–Pull Phenylazoheteroarenes**

Azoheteroarenes are the most recent derivatives targeted to further improve the properties of azo-based photoswitches. Their light-induced mechanism for trans–cis isomerization is assumed to be very similar to that of the parent azobenzene. As such, they inherited the controversy about the dominant isomerization pathway (rotation vs. inversion) depending on the excited state (nπ* vs. ππ*). Although the controversy seems settled in azobenzene, the extent to which the same conclusions apply to the more structurally diverse family of azoheteroarenes is unclear. Here, by means of non-adiabatic molecular dynamics, the photoisomerization mechanism of three prototypical phenyl-azoheteroarenes with increasing push–pull character is unraveled. The evolution of the rotational and inversion conical intersection energies, the preferred pathway, and the associated kinetics upon both nπ* and ππ* excitations can be linked directly with the push–pull substitution effects. Overall, the working conditions of this family of azo-dyes is clarified and a possibility to exploit push–pull substituents to tune their photoisomerization mechanism is identified, with potential impact on their quantum yield.     

BODIPY‐Bridged Push–Pull Chromophores for Nonlinear Optical Applications

A set of linear and dissymmetric BODIPY‐bridged push–pull dyes are synthesized. The electron‐donating substituents are anisole and dialkylanilino groups. The strongly electron‐accepting moiety, a 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) group, is obtained by insertion of an electron‐rich ethyne into tetracyanoethylene. A nonlinear push–pull system is developed with a donor at the 5‐position of the BODIPY core and the acceptor at the 2‐position. All dyes are fully characterized and their electrochemical, linear and nonlinear optical properties are discussed. The linear optical properties of dialkylamino compounds show strong solvatochromic behavior and undergo drastic changes upon protonation. The strong push–pull systems are non‐fluorescent and the TCBD‐BODIPY dyes show diverse photochemistry and electrochemistry, with several reversible reduction waves for the tetracyanobutadiene moiety. The hyperpolarizability μβ of selected compounds is evaluated using the electric‐field‐induced second‐harmonic generation technique. Two of the TCBD‐BODIPY dyes show particularly high μβ (1.907 μm) values of 2050×10^?48 and 5900×10^?48 esu. In addition, one of these dyes shows a high NLO contrast upon protonation–deprotonation of the donor residue.     

Highly Asymmetrical Porphyrins with Enhanced Push–Pull Character for Dye‐Sensitized Solar Cells

A porphyrin π‐system has been modulated by enhancing the push–pull character with highly asymmetrical substitution for dye‐sensitized solar cells for the first time. Namely, both two diarylamino moieties as a strong electron‐donating group and one carboxyphenylethynyl moiety as a strong electron‐withdrawing, anchoring group were introduced into the meso‐positions of the porphyrin core in a lower symmetrical manner. As a result of the improved light‐harvesting property as well as high electron distribution in the anchoring group of LUMO, a push–pull‐enhanced, porphyrin‐sensitized solar cell exhibited more than 10 % power conversion efficiency, which exceeded that of a representative highly efficient porphyrin (i.e., YD2)‐sensitized solar cell under optimized conditions. The rational molecular design concept based on highly asymmetric, push–pull substitution will open the possibilities of further improving cell performance in organic solar cells.     

Push–pull effect and synergistic discrimination of β-cyclodextrin and 18-crown-6

The present work was devoted to the study of the effect of one host (18-crown-6, 18C6) on the binding behaviour of the other host (β-cyclodextrin, β-CD) to amphiphilic guests such as sodium dodecylbenzene sulphonate (SDBS) and d- and l-tryptophan (d- and l-Trp). Our results indicated that different combinations of the two hosts exhibited different push–pull effects in their binding process to SDBS, and the extent of the push–pull effect was dramatically dependent on the initial stoichiometric ratios of the two hosts. That is to say, the effect of 18C6 on the binding behaviour of β-CD to SDBS was not linear with its mole fraction, but first decreased and then increased with the increase in its mole fraction. On the other hand, there was a concentration dependence on synergistic effect of 18C6 and β-CD on the binding behaviour to d- or l-Trp. And there were rather remarkable differences in the molecular recognition abilities (K _L/K _D) of β-CD to d- and l-Trp in the presence of 18C6, such as free β-CD (0.48), 18C6–β-CD-a (0.27), 18C6–β-CD-b (0.86), 18C6–β-CD-c (1.17), 18C6–β-CD-d (1.72) and 18C6–β-CD-e (2.31). These results clearly revealed the important role of 18C6 in mediating the intermolecular interaction between the amphiphilic guests and β-CD, providing a new insight into the mutual effect between two hosts in multicomponent systems.     

Uranium Metalla‐Allenes with Carbene Imido R2C=UIV=NR′ Units (R=Ph2PNSiMe3; R′=CPh3): Alkali‐Metal‐Mediated Push–Pull Effects with an Amido Auxiliary

We report uranium(IV)‐carbene‐imido‐amide metalla‐allene complexes [U(BIPM^TMS)(NCPh₃)(NHCPh₃)(M)] (BIPM^TMS=C(PPh₂NSiMe₃)₂; M=Li or K) that can be described as R₂C=U=NR′ push–pull metalla‐allene units, as organometallic counterparts of the well‐known push–pull organic allenes. The solid‐state structures reveal that the R₂C=U=NR′ units adopt highly unusual cis‐arrangements, which are also reproduced by gas‐phase theoretical studies conducted without the alkali metals to remove their potential structure‐directing roles. Computational studies confirm the double‐bond nature of the U=NR′ and U=CR₂ interactions, the latter increasingly attenuated by potassium then lithium when compared to the hypothetical alkali‐metal‐free anion. Combined experimental and theoretical data show that the push–pull effect induced by the alkali metal cations and amide auxiliary gives a fundamental and tunable structural influence over the C=U^IV=N units.     

Dipolar Photosystems: Engineering Oriented Push–Pull Components into Double‐ and Triple‐Channel Surface Architectures

Push–pull aromatics are not popular as optoelectronic materials because their supramolecular organization is difficult to control. However, recent progress with synthetic methods has suggested that the directional integration of push–pull components into multicomponent photosystems should become possible. In this study, we report the design, synthesis, and evaluation of double‐ or triple‐channel architectures that contain π stacks with push–pull components in parallel or mixed orientation. Moreover, the parallel push–pull stacks were uniformly oriented with regard to co‐axial stacks, either with inward or outward oriented push–pull dipoles. Hole‐transporting (p) aminoperylenemonoimides (APIs) and aminonaphthalimides (ANIs) are explored for ordered push–pull stacks. For the co‐axial electron‐transporting (n) stacks, naphthalenediimides (NDIs) are used. In double‐channel photosystems, mixed push–pull stacks are overall less active than parallel push–pull stacks. The orientation of the parallel push–pull stacks with regard to the co‐axial NDI stacks has little influence on activity. In triple‐channel photosystems, outward‐directed dipoles in bridging stacks between peripheral p and central n channels show higher activity than inward‐directed dipolar stacks. Higher activities in response to direct irradiation of outward‐directed parallel stacks reveal the occurrence of quite remarkable optical gating.     

Two Push–Pull Channels Enhance the Dinitrogen Activation by Borylene Compounds

Recently, Braunschweig et al. found that borylene (CAAC)DurB, in which CAAC is a cyclic alkyl(amino) carbene and Dur refers to 2,3,5,6-tetramethylphenyl, can bind and activate N₂, and the resulting [(CAAC)DurB]₂N₂ is of a bent BNNB core. The N₂ ligand in transition metal complexes is generally linear, so herein, the bonding nature of both terminal end-on and end-on bridging borylene-N₂ complexes is investigated with valence bond (VB) theory. In the terminal end-on (CAAC)HBN₂ the bonding follows the mechanism in transition metals with a σ donation and a π back-donation, but in the end-on bridging borylene-N₂ complex, the σ donation comes from the π orbitals of N₂, and thus, there are two opposite and perpendicular push–pull channels. It is the push–pull interaction that governs the enhanced activation of N₂ and the BNNB bent geometry. It is expected that the substituents bonded to B can modulate the bent angle and the strength of the push–pull interaction. Indeed, (CAAC)FB exhibits enhanced catalytic capacity for the activation of N₂.     

Azavinylidenephosphoranes: A Class of Cyclic Push–Pull Carbenes

The synthesis of a novel family of cyclic push–pull carbenes, namely, azavinylidene phosphoranes, is described. The methodology is based on a formal [3+2] cycloaddition between terminal alkynes and phosphine–imines followed by an oxidation/deprotonation step. Carbenes 6 , obtained by simple deprotonation, exhibit typical transient carbene reactivity like the intramolecular C?H insertion reaction and a pronounced ambiphilic character exemplified by [2+1] cycloaddition with electron‐poor methyl acrylate. Owing to the cyclic structure, carbenes 6 also exhibit an excellent coordination ability toward transition metals. Rh^I complex 10 was obtained in excellent yield and was fully characterized by multinuclear NMR spectroscopy and X‐ray crystallography. The corresponding Rh^I–carbonyl complex was also prepared; this indicates that carbenes 6 belong to the strongest σ‐donating ligands to date. DFT calculations confirmed the high σ‐donation ability of 6 and their classification as push–pull carbenes with a relatively small singlet–triplet energy gap of 23.2–24.3 kcal mol^?1.     

Synthesis of Novel Anellated Pyranosides as Precursors of C‐Nucleoside Analogues Using Isopropyl 6‐O‐Acetyl‐3‐deoxy‐4‐S‐ethyl‐4‐thio‐α‐D‐threo‐hexopyranosid‐2‐ulose

Abstract

Isopropyl 6‐O‐acetyl‐3‐deoxy‐4‐S‐ethyl‐4‐thio‐α‐D‐threo‐hexopyranosid‐2‐ulose (3) was converted to the corresponding 3‐[bis(methylthio)methylene] derivative 4 with a push–pull activated C–C double bond. Treatment of 4 with hydrazine and methylhydrazine afforded the pyrano[3,4‐c]pyrazol‐5‐ylmethyl acetates 5a and 5b, respectively. Desulfurization of compound 4 with sodium boron hydride yielded the 3‐[(methylthio)methylene]hexopyranosid‐2‐ulose 7. Compound 7 was reacted with amines to furnish 3‐aminomethylene‐hexopyranosid‐2‐uloses 8, 9. Reaction of 7 with hydrazine hydrate, hydrazines, hydroxylamine, and benzamidine afforded the pyrazolo, isoxazalo, and pyrimido anellated pyranosides (10–13).     

N‐Méthyl succinimide

The molecular structure of 1‐methylpyrrolidine‐2,5‐dione, C₅H₇NO₂, corresponds to the dicarbonyl tautomer with an envelope ring conformation. The packing is stabilized by weak intermolecular hydrogen bonds and presents push–pull nucleophile–electrophile interactions of the carbonyl groups.     

Functionalization of a Ruthenium–Diacetylide Organometallic Complex as a Next‐Generation Push–Pull Chromophore

The design and preparation of an asymmetric ruthenium–diacetylide organometallic complex was successfully achieved to provide an original donor–π–[M]–π–acceptor architecture, in which [M] corresponds to the [Ru(dppe)₂] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge‐transfer processes occurring upon photoexcitation of the push–pull metal–dialkynyl σ complex were investigated by combining experimental and theoretical data. The novel push–pull complex, appropriately end capped with an anchoring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye‐sensitized solar cell devices, showed a good spectral response with a broad incident photon‐to‐current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.     

Donor–acceptor‐integrated conjugated polymers based on carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole with tight π–π stacking for photovoltaics

An original strategy to construct a new donor–acceptor (D–A)‐integrated structure by directly imposing “pull” unit on the “push” moiety to form fused ring architecture has been developed, and poly{N‐alkyl‐carbazole[3,4‐c:5,6‐c]bis[1,2,5]thiadiazole‐alt‐thiophene} (PCBTT) with D–A‐integrated structure, in which two 1,2,5‐thiadiazole rings are fixed on carbazole in 3‐, 4‐ and 5‐, 6‐position symmetrically and thiophene is used as bridge, has been synthesized. The interaction between pull and push units has fine tuned the HOMO/LUMO energy levels, and the resulting copolymer covers the solar flux from 300 to 750 nm. The interaction between pull and push units is worth noting that due to the fused five rings inducing strong intermolecular interaction, an extremely short π–π stacking distance of 0.32 nm has been achieved for PCBTT both in powder and solid states. This is the shortest π–π stacking distance reported for conjugated polymers. Additionally, an obvious intramolecular charge transfer and energy transfer from donor units to acceptor units have been detected in this D–A integration. A moderate‐to‐high open‐circuit voltage of ～0.7 V in PCBTT:[6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) (w/w = 1/2) solar cells is achieved due to the low‐lying HOMO energy level of PCBTT. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

Low‐lying excited electronic states of an important class of molecules known as push–pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high‐sensitivity nanosecond time‐resolved dispersive IR spectroscopy and DFT calculations on p‐nitroaniline (PNA), a prototypical push–pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO₂ group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge‐transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited‐state structure of highly polarizable molecular systems.     

The Aminopropenal‐ and Aminopentadienal Rearrangement

Scope and mechanism of the aminopropenal rearrangement are reviewed: Various 3‐acyloxy‐3‐dialkylaminopropenals ( 2 , formed by addition of acids to ‘push‐pull’‐acetylenes 1 ) rearranged quantitatively to give 3‐acyloxyacrylic amides ( 3 , Schemes 1 and 22). Since these activated enol esters reacted very selectively with amino groups of polyfunctional amino acids, ‘push‐pull’‐acetylenes are versatile peptide reagents. Similarly, 5‐X‐5‐dialkylaminopentadienals ( 38 , formed by addition of acids to ‘push‐pull’‐enynes 37 ) could be rearranged (aminopentadienal rearrangement). In this case, the rearrangement 38 → 40 → 42 (Schemes 16 and 22) normally stopped at the level of the quite stable 2‐dialkylamino‐pyrylium salts 40 . Ring opening 40 → 42 of these intermediates was quite tricky, but could be realized in several cases.     

Synthesis of Reversed C-Nucleosides

A number of C-nucleoside analogs of the D-furanose series were synthesized by using push–pull activated monosaccharide α-dimethylaminomethyleneulose (1) as a precursor. Enaminoketone 1 was reacted with o-phenylenediamine, cyanamide, and dialkyl-3-oxoglutarates to obtain benzodiazepine nucleoside analog 2, reversed C-nucleoside analog with pyrimidine ring 3, and isophthalic acid derivative 4, respectively.     

Design of Weak‐Donor Alkyl‐Functionalized Push–Pull Pyrene Dyes Exhibiting Enhanced Fluorescence Quantum Yields and Unique On/Off Switching Properties

We designed, synthesized, and evaluated environmentally responsive solvatochromic fluorescent dyes by incorporating weak push–pull moieties. The quantum yields of the push (alkyl)–pull (formyl) pyrene dyes were dramatically enhanced by the introduction of alkyl groups into formylpyrene (1‐formylpyrene: Φ_F=0.10; 3,6,8‐tri‐n‐butyl‐1‐formylpyrene: Φ_F=0.90; in MeOH). The new dyes exhibited unique sensitivity to solvent polarity and hydrogen‐bond donor ability, and specific fluorescence turn‐on/off properties (e.g., 3,6,8‐tri‐n‐butyl‐1‐formylpyrene: Φ_F=0.004, 0.80, 0.37, and 0.90 in hexane, chloroform, DMSO, and MeOH, respectively). Here, the alkyl groups act as weak donors to suppress intersystem crossing by destabilizing the HOMOs of 1‐formylpyrene while maintaining weak intramolecular charge‐transfer properties. By using alkyl groups as weak donors, environmentally responsive, and in particular, pH‐responsive fluorescent materials may be developed in the future.     

Two Novel Push?Pull Series of Benzothiazole‐Based Dyes: Synthesis and Characterization

New dichromophoric cyanine dyes based on benzothiazol‐2‐amines as push? pull systems were synthesized in two series of disperse and cationic forms (see 4 in Scheme 1 and 5 in Scheme 2, resp.). Their thermal stabilities, UV/VIS, fluorescence, and solvatochromic behavior, which are important parameters in push? pull systems, were also studied.     

Mechanosensitive Oligodithienothiophenes: Transmembrane Anion Transport Along Chalcogen‐Bonding Cascades

The design, synthesis, and evaluation of multifunctional dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) trimers is described. Twisted push‐push‐pull or donor‐donor‐acceptor (DDA) trimers composed of one DTT acceptor and two DTT donors show strong mechanochromism in lipid bilayer membranes. Red shifts in excitation rather than emission and fluorescence recovery with increasing membrane order are consistent with planarization of the twisted, extra‐long mechanophores in the ground state. The complementary pull‐pull‐pull or AAA trimers with deep σ holes all along the scaffold are not mechanochromic in membranes but excel with submicromolar anion transport activity. Anion transport along membrane‐spanning strings of chalcogen‐bond donors is unprecedented and completes previous results on transmembrane cascades that operate with equally unorthodox interactions such as halogen bonds and anion‐π interactions.