So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. We show here that thanks to fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Occurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes.

Singlet fission (SF), i.e., the splitting of a high-energy exciton into two lower-energy triplet excitons, has the potential to increase the efficiency for harvesting spectrally broad light. The path from the photopopulated singlet state to free triplets is complicated by competing processes that decrease the overall SF efficiency. A detailed understanding of the whole cascade and the nature of the photoexcited singlet state is still a major challenge. Here, we introduce a pentacene dimer with a flexible crown ether spacer enabling a control of the interchromophore coupling upon solvent-induced self-aggregation as well as cation binding. The systematic change of solvent polarity and viscosity and excitation wavelength, as well as the available conformational phase space, allows us to draw a coherent picture of the whole SF cascade from the femtosecond to microsecond time scales. High coupling leads to ultrafast SF (<2 ps), independent of the solvent polarity, and to highly coupled correlated triplet pairs. The absence of a polarity effect indicates that the solvent coordinate does not play a significant role and that SF is driven by intramolecular modes. Low coupling results in much slower SF (∼500 ps), which depends on viscosity, and leads to weakly coupled correlated triplet pairs. These two triplet pairs could be spectrally distinguished and their contribution to the overall SF efficiency, i.e., to the population of free triplets, could be determined. Our results reveal how the overall SF efficiency can be increased by conformational restrictions and control of the structural fluctuation dynamics.

Understanding structure–property relationships in multichromophoric molecular architectures is a crucial step in establishing new design principles in organic electronics as well as to fully understand how nature exploits solar energy. Here, we study the excited state dynamics of three bichromophores consisting of two perylene chromophores linked to three different crown-ether backbones, using stationary and ultrafast electronic spectroscopy combined with molecular dynamics simulations. The conformational space available to the bichromophores depends on the structure and geometry of the crown-ether and can be significantly changed upon cation binding, strongly affecting the excited-state dynamics. We show that, depending on the conformational restrictions and the local environment, the nature of the excited state varies greatly, going from an excimer to a symmetry-broken charge separated state. These results can be rationalised in terms of a structure–property relationship that includes the effect of the local environment.

A series of bis(pyreneamide) macrocycles, synthesized in two steps from THF, THP, oxepane and 1,4-dioxane, are tested as chemosensors for a large range of mono-, di- and trivalent cations. In their native states, these macrocycles exhibit a strong excimer fluorescence that is quenched upon the addition of the metal ions (alkaline, alkaline earth, p-, d-, and f-block metals). UV-Vis spectrophotometric titrations, cyclic voltammetry, excimer fluorescence quenching and transient absorption spectroscopy experiments helped characterize the On-Off changes occurring upon binding and demonstrate that the highest stability constants are obtained with divalent cations Ca²⁺ and Ba²⁺ specifically.

1,4-Dioxepines result from the decomposition of α-diazo-β-keto esters in the presence of oxetanes using the catalytic combination of the (cyclopentadienyl)ruthenium complex [CpRu(CH₃CN)₃][BAr_F] and 1,10-phenanthroline. The regioselective [4+3] insertions follow an S_N1-like mechanism and occur yet enantiospecifically (es 74%). The retention of configuration was ascertained by vibrational circular dichroism (VCD) and solid state analyses. Furans, products of [4+1] insertions, are only observed as traces in the above protocol. To promote their formation under CpRu catalysis, it is necessary to use a two-step process with γ-halogenated alcohols as substrates.

A strategy for late-stage electrophilic functionalizations of cationic helicenes is exposed. Thanks to strongly acidic conditions that permit reversible electrophilic substitutions, regioselective acylations, sulfonylations or alkylations occur at the extremity(ies) of the helical cores. Extended [5] or [6]helicenes can then be generated from cationic [4]helicenes in successive one-pot elongation processes. Retention of configuration and excellent enantiospecificity (up to 99%) are observed for the helicene growth in the enantiopure series.

The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pK_R+=20.4, E^red_1/2 =−0.72 V) compared to its azaoxa 2 (pK_R+=15.2, E^red_1/2=−0.45 V) and dioxa 1 (pK_R+=8.8, E^red_1/2=−0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4–15). The electronic absorption is modulated from the orange to the far-red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.

Doubly bridged biphenyl azocines can be prepared in a single step through copper-catalyzed reactions of a doubly bridged biphenyl azepine and diazodiester reagents. Double [1,2]-Stevens rearrangements occur at 100 °C to afford doubly tethered eight-membered rings (49 to 61%) as trans and cis regioisomers (1:1 ratio). These products present an axial chirality. ECD and VCD analyses of the separated enantiomers (CSP-HPLC) were used to assign the absolute configuration. High configurational stability is observed for both regioisomers as racemization does not occur after 1 week of heating at 208 °C in dodecane (ΔG ^‡ > 41 kcal·mol^–1). Interestingly, reactions performed at 40 °C retain a certain level of enantiospecificity (82–86%), avoiding, for the most part, thermal racemization of the starting material.

Unprecendented regioselective post-functionalization of racemic and enantiopure cationic diaza [4]helicenes is afforded. The peripheral auxochrome substituents allow a general tuning of the electrochemical, photophysical and chiroptical properties of the helical dyes (26 examples). For instance, electronic absorption and circular dichroism are modulated from the orange to near-infrared spectral range (575-750 nm), fluorescence quantum efficiency is enhanced up to 0.55 (631 nm) and circularly polarized luminescence is recorded in the red (│g_lum│ ~ 10^-3)

Late endosomes are a major trafficking hub in the cell at the crossroads between endocytosis, autophagy, and degradation in lysosomes. Herein is disclosed the first small molecule allowing their selective imaging and monitoring in the form of a diazaoxatriangulene fluorophore, 1a (hexadecyl side chain). The compound is prepared in three steps from a simple carbenium precursor. In nanospheres, this pH-sensitive (pK_a = 7.3), photochemically stable dye fluoresces in the red part of visible light (601 and 578 nm, acid and basic forms, respectively) with a quantum yield between 14 and 16% and an excited-state lifetime of 7.7–7.8 ns. Importantly, the protonated form 1a·H⁺ provokes a specific staining of late endosome compartments (pH 5.0–5.5) after 5 h of incubation with HeLa cells. Not surprisingly, this late endosome marking depends on the intra-organelle pH, and changing the nature of the lipophilic chain provokes a loss of selectivity. Interestingly, fixation of the fluorophore is readily achieved with paraformaldehyde, giving the possibility to image both live and fixed cells.

Oxidation of ethano-Troeger bases (I) and (III) with DDQ in wet nitromethane results in a highly enantiospecific methylene extrusion to afford ring contracted products (II) and (IV) which are significantly more configurationally stable than regular Troeger bases.

The thermodynamically disfavored isomerization of α,β-unsaturated esters to deconjugated β,γ-unsaturated analogues occurs readily when coupled to an amidation. Within the framework of macrocyclic derivatives, it is shown that 15, 16, and 18 membered macrocycles react with tBuOK and anilines to generate in one-pot the β,γ-unsaturated amides (yields up to 88%). Importantly, single (chiral) diastereomers are isolated (d.r. > 49:1, 1H NMR) irrespective of the size and nature of the rings showing an effective transmission of remote stereochemistry during the isomerization process. CSP-chromatographic resolution and absolute configuration determination by VCD are achieved.

A novel CH₂-extrusion reaction leading to the transformation of ethano-Tröger bases into disubstituted methano derivatives is reported (yields up to 93 %). Under mild and metal-free oxidative conditions, a loss of CH₂ and a ring contraction are provoked. Despite two bond cleavages at stereogenic nitrogen and carbon centers and a temporary rupture of the bicyclic structure, a very high enantiospecificity (es≥98 %) is observed for this unusual reaction.

Cationic azaoxa[4]helicenes can be prepared in a single step from a common xanthenium precursor by addition of nucleophilic amines under monitored conditions (160 °C, 2 min, MW). The (−)-(M) and (+)-(P) enantiomers can be separated by chiral stationary-phase chromatography. Determination of the absolute configuration and racemization barrier (ΔG^⧧ (433 K) 33.3 ± 1.3 kcal·mol^–1) was achieved by VCD and ECD spectroscopy, respectively.

Configurationally stable diaza[4]helicenes have been prepared in two steps by using a particularly facile N–N bond-cleavage reaction. The synthetic procedure uses hydrazine (NH₂NH₂) for the introduction of a single nitrogen atom. The strategy is general, modular and highly tolerant to functional groups. A mechanistic rationale is proposed for the spontaneous N–N bond-cleavage reaction. The resulting helical quinacridines are dyes that present absorption and emission properties that can be modulated as a function of pH; the pink quinacridine and green protonated forms (pK_a ≈ 9.0) display distinct optical features in the near-IR region. Single enantiomers were obtained by chiral stationary phase HPLC resolution. The absolute configurations were assigned by comparison of the ECD spectra of the conjugated acids with those of known dialkylquinacridinium derivatives. A rather high racemization barrier was measured by means of variable-temperature ECD experiments (ΔG^‡ = 30.7 ± 4.0 kcal mol^–1 at 140 °C).

A C-C bond forming reaction occurs spontaneously between tris-(2,6-dimethoxyphenyl)carbenium ion and indoles / anilines. The carbocation acts both as electrophile and oxidant. Effective cationic π-conjugated dyes are formed resulting in a strong hyper- and bathochromism

Rather than lead to the usual deoxygenation pathway, metal carbenes derived from α-diazo-β-ketoesters undergo three-atom insertions into epoxides using a combination of 1,10-phenanthroline and [CpRu(CH₃CN)₃][BAr_F] as the catalyst. Original 1,4-dioxene motifs are obtained as single regio- and stereoisomers. A perfect syn stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an S_N1-like transformation.

The interaction of a series of chiral cationic [4]helicene derivatives, which differ by their substituents, with double-stranded DNA has been investigated by using a combination of spectroscopic techniques, including time-resolved fluorescence, fluorescence anisotropy, and linear dichroism. Addition of DNA to helicene solutions results to a hypochromic shift of the visible absorption bands, an increase of fluorescence quantum yield and lifetime, a slowing down of fluorescence anisotropy decay, and a linear dichroism in flow-oriented DNA, which unambiguously points to the binding of these dyes to DNA. Both helicene monomers and dimeric aggregates, which form at higher concentration, bind to DNA, the former most probably upon intercalation and the latter upon groove binding. The binding constant depends substantially on the dye substituents and is, in all cases, larger with the M than the P enantiomer, by factors ranging from 1.2 to 2.3, depending on the dye.

Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions (see scheme). Reductions, cross-coupling, or condensation reactions introduce additional diversity and allow tuning of the absorption properties up to the near-infrared region. The diaza salts can be resolved into single enantiomers.

The photophysical properties of a series of helicene cations in various solvents have been investigated using stationary and time-resolved spectroscopy. These compounds fluoresce in the near infrared region with a quantum yield ranging between 2 and 20% and a lifetime between 1 and 12 ns, depending of the solvent. No clear solvent dependence could be recognized except for a decrease of fluorescence quantum yield and lifetime with increasing hydrogen-bond donating ability of the solvent. In water, the helicene cations undergo aggregation. This effect manifests itself by the presence of a slow fluorescence decay component, whose amplitude increases with dye concentration, and by a much slower decay of the polarization anisotropy in water compared to an organic solvent of similar viscosity. However, aggregation has essentially no effect on the stationary fluorescence spectrum, whereas relatively small changes can be seen in the absorption spectrum. Analysis of the dependence of aggregation on the dye concentration reveals that the aggregates are mostly dimers and that the aggregation constant is substantially larger for hetero- than homochiral dimers.

The ion pair of the stereolabile C₃-symmetric, i⁺o proton complex [1â‹…H]⁺ of diaza-macropentacycle 1 and the configurationally stable Δ-TRISPHAT ([Δ-3]^−) anion exists in the form of two diastereomers, namely, [Δ-(1â‹…H)][Δ-3] and [Λ-(1â‹…H)][Δ-3], the ratio of which, in terms of diastereomeric excess (de) decreases in the order [D₈]THF (28 %)>CD₂Cl₂ (22 %)>CDCl₃ (20 %)>[D₈]toluene (16 %)>C₆D₆ (7 %)>[D₆]acetone (0 %) at thermodynamic equilibrium. Except in the case of [D₆]acetone, the latter is reached after a period of time that increases from 1 h ([D₈]THF) to 24 h (CDCl₃). Moreover, the initial value of the de of [1â‹…H][Δ-3] in CDCl₃, before the thermodynamic equilibrium is reached, depends on the solvent in which the sample has been previously equilibrated (sample “historyâ€�). This property has been used to show that the crystals of [1â‹…H][Δ-3] formed by slow evaporation of CH₂Cl₂/CH₃OH mixtures had 100 % de, which indicates that [1â‹…H][Δ-3] has enjoyed a crystallization-induced asymmetric transformation. Structural studies in solution (NMR spectroscopy) and in the gas phase by calculations at the semiempirical PM6 level of theory suggest that the optically active anion is docked on the i⁺ (endo) external side of the proton complex such that one of the aromatic rings of [Δ-3]^− is inserted into a groove of [1â‹…H]⁺, a second aromatic ring being placed astride the outside i⁺ pocket. Solvent polarity controls the thermodynamics of inversion of the [1â‹…H]⁺ propeller. However, both polarity and basicity control its kinetics. Therefore, the rate-limiting steps correspond to the ion-pair separation/recombination and [1â‹…H]⁺/1deprotonation/protonation processes, rather than the inversion of [1â‹…H]⁺, the latter being likely to take place in the deprotonated form (1).

Using simple organic synthetic transformations, a novel diazaoxatricornan derivative, the 12c-methyl-12-phenyl-8-propyl-12,12c-dihydro-8H-4-oxa-8,12-diazadibenzo[cd,mn]pyrene (6a), was prepared. This novel chiral cup-shaped molecule was isolated in racemic form and in excellent yield after the addition of methyl lithium to the BF₄ salt of a novel unsymmetrical diazaoxatriangulenium cation. Compound 6a was found to be stable under classical laboratory conditions—something not obvious considering the extreme stability of the carbenium ion precursor, the electron-rich nature of the core, and the strain induced by the pyramidalization of the central carbon. The enantiomers were readily separated by chiral stationary phase chromatography, and the absolute configuration of (−)-(S)-6a was determined by a comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. This isolation of (−)-(S)-6a and (+)-(R)-6a constitutes thus the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality is due to the intrinsic dissymmetry of the central core of the structure only.

Herein is reported an experimental and theoretical study of the circular dichroism properties of TRISPHAT (1) anion. ECD analysis of the [tetramethylammonium][Δ-1] salt confirms the absolute configuration assignment obtained through X-ray crystallographic analysis of the parent cinchonidium salt. The structure, infrared, and vibrational circular dichroism (VCD) spectra derived from density functional theory (DFT) calculations are compared with experimental data.

Highly symmetric spirobi[dibenzazepinium] cation 3 reacts with P₄-t-Bu to form exclusively a ring-expanded tertiary amine; this unusual reactivity can be traced back to the geometry of the ylide.

A [4](hetero)helicenium cation was resolved using the hexacoordinated phosphorus-containing binphat anion (see picture: N, blue; O, red; C, gray). Its absolute configuration was determined by vibrational circular dichroism spectroscopy. The barrier of interconversion of its enantiomers is higher than that of [6]helicene.