Yi Lin Wu

1,2,3-Triazoles as Conjugative π-Linkers in Push−Pull Chromophores: Importance of Substituent Positioning on Intramolecular Charge-Transfer

Isomeric charge-transfer chromophores using 1,2,3-triazol-diyl as linker have been studied experimentally and computationally. The instability of the polarized reactants precluded the use of the Huisgen reaction and alternative synthetic methodologies were employed. Charge-transfer absorptions between an N,N-dimethylanilino and a dicyanovinyl group are modest to strong, with maxima from lambda(max) = 400 to 453 nm depending on substituent positioning. TD-B3LYP/6-31G(d) calculations are within 0.6 eV of experiment and assign these bands as HOMO-LUMO transitions.

New donor–acceptor chromophores by formal [2+2] cycloaddition of donor-substituted alkynes to dicyanovinyl derivatives

The efficient methodology of the cycloaddition between electron-rich alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), followed by retro-electrocyclisation, is extended to dicyanovinyl derivatives to produce new donor-acceptor push-pull 1,1-dicyanobutadienyl chromophores in excellent to quantitative yield (63-98%) that express strong charge-transfer (CT) absorptions from 300 to 600 nm. The scope of this reaction is established by both varying the nucleophilic and electrophilic components. Electrochemical studies show that the CT properties of these systems are readily tunable by substitution on the electrophile, which has the largest effect on the lowest unoccupied molecular orbital (LUMO). Non-reversible reduction potentials range from ca. -1.2 to -1.9 V in CH(2)Cl(2), against the ferricinium/ferrocene couple (Fc(+)/Fc) according to cyclovoltammetry (CV) and rotating disk voltammetry (RDV). The chromophores show a significant non-planarity between the N,N-dimethylanilino donor and the 1,1-dicyanovinyl acceptor moieties, with torsional angles around 40 degrees from X-ray analysis, but retain strong quinoidal character. The mechanism of this reaction has been studied computational using density functional methods in the gas-phase and using the polarizable continuum model (PCM) for addressing solvent effects. The complete reaction free-energy profile has been determined for the reaction of 1,1-dicyanoethene and 4-ethynyl-N,N-dimethylaniline. The process proceeds through formal [2+2] cycloaddition followed by retro-electrocyclisation. The formation of a zwitterionic intermediate in the cycloaddition step is shown.

Complexation of Polyoxometalates with Cyclodextrins

Although complexation of hydrophilic guests inside the cavities of hydrophobic hosts is considered to be unlikely, we demonstrate herein the complexation between γ- and β-cyclodextrins (γ- and β-CDs) with an archetypal polyoxometalate (POM)--namely, the [PMo12O40](3-) trianion--which has led to the formation of two organic-inorganic hybrid 2:1 complexes, namely [La(H2O)9]{[PMo12O40]⊂[γ-CD]2} (CD-POM-1) and [La(H2O)9] {[PMo12O40]⊂[β-CD]2} (CD-POM-2), in the solid state. The extent to which these complexes assemble in solution has been investigated by (i) (1)H, (13)C, and (31)P NMR spectroscopies and (ii) small- and wide-angle X-ray scattering, as well as (iii) mass spectrometry. Single-crystal X-ray diffraction reveals that both complexes have a sandwich-like structure, wherein one [PMo12O40](3-) trianion is encapsulated by the primary faces of two CD tori through intermolecular [C-H···O═Mo] interactions. X-ray crystal superstructures of CD-POM-1 and CD-POM-2 show also that both of these 2:1 complexes are lined up longitudinally in a one-dimensional columnar fashion by means of [O-H···O] interactions. A beneficial nanoconfinement-induced stabilizing effect is supported by the observation of slow color changes for these supermolecules in aqueous solution phase. Electrochemical studies show that the redox properties of [PMo12O40](3-) trianions encapsulated by CDs in the complexes are largely preserved in solution. The supramolecular complementarity between the CDs and the [PMo12O40](3-) trianion provides yet another opportunity for the functionalization of POMs under mild conditions by using host-guest chemistry.

Donor-substituted octacyano[4]dendralenes: a new class of cyano-rich non-planar organic acceptors

Double [2+2] cycloaddition/retro-electrocyclisation reactions between tetracyanoethene (TCNE) and various anilino-capped buta-1,3-diynes furnished a series of octacyano[4]dendralene derivatives featuring intense, low-energy intramolecular charge-transfer absorptions. These novel chromophores are strong electron acceptors and undergo facile one-electron reductions at potentials (–0.09 to –0.17 eV vs.Fc+/Fc, in CH2Cl2–0.1 M nBu4NPF6) lower than those reported for the benchmark organic acceptors, such as TCNE (–0.32 eV) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) (–0.25 eV). The electron-accepting power of one octacyano[4]dendralene, as expressed by the computed adiabatic electron affinity (EA), compares to that of the reference acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) used as a p-type dopant in organic light-emitting diodes (OLEDs) and solar cells. Gas-phase density functional theory (DFT) calculations predict a stretched-out conformation as the global energy minimum for octacyano[4]dendralenes. In the solid state however, folded conformations were observed for two structures by X-ray analysis. Taking the solid state environment approximately into account calculations predict a energetical degeneracy between the stretched-out and folded conformation. Therefore conformational preference probably is a result of supramolecular dimer formation, mediated by two pairs of intermolecular, antiparallel dipolar CN⋯CN interactions.

Mechanistic Investigation of the Dipolar [2+2] Cycloaddition–Cycloreversion Reaction between 4‐(N,N‐Dimethylamino)phenylacetylene and Arylated 1,1‐Dicyanovinyl Derivatives To Form Intramolecular Charge‐Transfer Chromophores

The kinetics and mechanism of the formal [2+2] cycloaddition-cycloreversion reaction between 4-(N,N-dimethylamino)phenylacetylene (1) and para-substituted benzylidenemalononitriles 2 b-2 l to form 2-donor-substituted 1,1-dicyanobuta-1,3-dienes 3 b-3 l via the postulated dicyanocyclobutene intermediates 4 b-4 l have been studied experimentally by the method of initial rates and computationally at the unrestricted B3LYP/6-31G(d) level. The transformations were found to follow bimolecular, second-order kinetics, with DeltaH(exp)(not equal)=13-18 kcal mol-1, DeltaS(exp)(not equal) approximately -30 cal K-1 mol-1, and DeltaG(exp)(not equal)=22-27 kcal mol-1. These experimental activation parameters for the rate-determining cycloaddition step are close to the computational values. The rate constants show a good linear free energy relationship (rho=2.0) with the electronic character of the para-substituents on the benzylidene moiety in dimethylformamide (DMF), which is indicative of a dipolar mechanism. Analysis of the computed structures and their corresponding solvation energies in acetonitrile suggests that the rate-determining attack of the nucleophilic, terminal alkyne carbon onto the dicyanovinyl electrophile generates a transient zwitterion intermediate with the negative charge developing as a stabilized malononitrile carbanion. The computational analysis predicted that the cycloreversion of the postulated dicyanocyclobutene intermediate would become rate-determining for 1,1-dicyanoethene (2 m) as the electrophile. The dicyanocyclobutene 4 m could indeed be isolated as the key intermediate from the reaction between alkyne 1 and 2 m and characterized by X-ray analysis. Facile first-order cycloreversion occurred upon further heating, yielding as the sole product the 1,1-dicyanobuta-1,3-diene 3 m.

Proaromaticity: Organic Charge-Transfer Chromophores with Small HOMO-LUMO Gaps

Novel donor- and/or acceptor-substituted cross-conjugated carbocycles based on quinoids or expanded quinoids, with radiaannulene perimeters, were prepared and investigated to validate proaromaticity as a concept for reducing HOMO-LUMO gaps in push-pull chromophores. Analyses of IR, (1)H NMR, and UV/Vis/NIR spectra in conjunction with molecular structures determined by X-ray diffraction show that these push-pull quinoids have significant charge-separated ground states. This feature results in small optical gaps (near IR region) and diatropic magnetic environments inside the carbocycles, as suggested by nucleus-independent chemical shift (NICS) calculations. The NICS results, together with the bond-length analysis of the quinoid spacers, provide strong support that proaromaticity, that is, aromatized zwitterionic mesomeric contributions in the ground state, is effective. A push-pull tetrakis(ethynediyl)-expanded quinoid chromophore represents the first proaromatic radiaannulene.

Structural, optical, and electrochemical properties of three-dimensional push-pull corannulenes.

Electrochemically active corannulene derivatives with various numbers of electron-donating 4-(N,N-dimethylamino)phenylethynyl (1-4) or electron-withdrawing cyanobutadienyl peripheral substitutents (5-8) were prepared. The latter derivatives resulted from formal [2 + 2] cycloaddition of cyanoolefins to 1-4 followed by retro-electrocyclization. Conformational properties were examined by variable-temperature NMR and X-ray diffraction and opto-electronic properties by electronic absorption/emission spectra and electrochemical measurements; these analyses were corroborated by dispersion-corrected density functional calculations at the level of B97-D/def2-TZVPP. In CH(2)Cl(2), 1-4 exhibit intramolecular charge-transfer (ICT) absorptions at 350-550 nm and green (λ(em) ~ 540 nm) or orange (600 nm) fluorescence with high quantum yields (56-98%) and are more readily reduced than corannulene by up to 490 mV. The variation of optical gap and redox potentials of 1-4 does not correlate with the number of substituents. Cyanobutadienyl corannulenes 5-8 show red-shifted ICT absorptions with end-absorptions approaching 800 nm. Intersubstituent interactions lead to distortions of the corannulene core and lower the molecular symmetry. NMR, X-ray, and computational studies on 5 and 8 with one cyanobutadienyl substituent suggested the formation of intermolecular corannulene dimers. Bowl-inversion barriers around ΔG(‡) = 10-11 kcal/mol were determined for these two molecules.

Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films

Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.

Extending photoinduced charge separation lifetimes by using supramolecular design: guanine-perylenediimide G-quadruplex.

We report here a potassium-induced guanine quadruplex as a supramolecular platform for controlled assembly of electron donor-acceptor systems. A monodisperse, C4-symmetric octamer of a guanine-perylene-3,4,9,10-bis(dicarboximide) conjugate (GPDI) was prepared in tetrahydrofuran. The two layers of cyclic guanine tetramers have the same direction of rotation, and the PDI moiety between the layers adopts a nearly eclipsed relationship (H-aggregation), as revealed by small- and wide-angle X-ray scattering, NMR spectroscopy, and steady-state UV/vis absorption. Following photoexcitation of the PDI moiety in the quadruplex, charge separation occurs in τCS = 98 ± 12 ps to give G(+•)-PDI(-•) that recombines in τCR = 1.2 ± 0.2 ns, which is >100 times longer than that in the monomeric GPDI dyad. The transient absorption spectrum of G(+•)-PDI(-•) within the GPDI quadruplex suggests the formation of a radical anion delocalized over the neighboring PDI units, and this result is consistent with the more favorable electrochemical reduction potential for PDIs in the quadruplex relative to the monomer.

Sub-Picosecond Singlet Exciton Fission in Cyano-Substituted Diaryltetracenes.

Thin films of 5,11-dicyano-6,12-diphenyltetracene (TcCN) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T1)-E(S1)=-0.17 eV), where S1 and T1 are singlet and triplet excitons, respectively. As a result of tuning the triplet-state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.