Excimer evolution hampers symmetry-broken charge-separated states

Symmetry-Breaking Charge Separation Mediated Triplet Population in a Perylenediimide Trimer at the Single-Molecule Level

Herein, we demonstrate triplet excited-state population in a conformationally rigid perylenediimide trimer (PDI-T) via intramolecular symmetry-breaking charge separation (SB-CS) at the single-molecule level. The single-molecule fluorescence intensity trajectories of PDI-T in nonpolar polystyrene matrix (ε = 2.60) exhibit prolonged fluorescence with infrequent dark states, representing the triplet and/or the charge transfer states. In contrast, in a poly(vinyl alcohol) matrix (ε = 7.80), erratic blinking dynamics resulting in low photon counts were observed, corroborating the feasibility of charge separation in a polar environment. In agreement with the single-molecule measurements, transient absorption spectroscopy of PDI-T reveals ultrafast SB-CS (τCS < 5 ps) in polar tetrahydrofuran (ε = 7.58) and acetone (ε = 20.70), with the population of the triplet excited-state through charge recombination. The current investigation shows the utility of rigid and weakly coupled molecular constructs in controlling triplet generation and SB-CS for potential applications in optoelectronic devices.

Sculpting photoproducts with DNA origami

Natural light-harvesting systems spatially organize densely packed dyes in different configurations to either transport excitons or convert them into charge photoproducts, with high efficiency. In contrast, artificial photosystems like organic solar cells and light-emitting diodes lack this fine structural control, limiting their efficiency. Thus, biomimetic multi-dye systems are needed to organize dyes with the sub-nanometer spatial control required to sculpt resulting photoproducts. Here, we synthesize 11 distinct perylene diimide (PDI) dimers integrated into DNA origami nanostructures and identify dimer architectures that offer discrete control over exciton transport versus charge separation. The large structural-space and site-tunability of origami uniquely provides controlled PDI dimer packing to form distinct excimer photoproducts, which are sensitive to interdye configurations. In the future, this platform enables large-scale programmed assembly of dyes mimicking natural systems to sculpt distinct photophysical products needed for a broad range of optoelectronic devices, including solar energy converters and quantum information processors.

Ultrafast symmetry-breaking charge separation in Perylenemonoimide-embedded multichromophores: impact of regioisomerism

Symmetry-breaking charge separation (SB-CS) has recently evolved as an emerging concept offering its potential to the latest generation of organic photovoltaics. However there are several concerns that need to be addressed to reach the state-of-the-art in SB-CS chemistry, for instance, the desirable molecular geometry, interchromophoric distance and extent of electronic coupling. To shed light on those features, it is reported herein, that ortho-functionalized perylene monoimide (PMI) constituted regioisomeric dimer and trimer derivatives with varied molecular twisting and electronic conjugation have been synthesized. In steady-state photophysical studies, all the dimers and trimer derivatives exhibit a larger bathochromic shift in the emission spectra and a significant reduction of fluorescence quantum yield in polar DMF. Among the series of multichromophores, ortho- and self-coupled dimers display the strikingly different optical feature of SB-CS with a very fast charge separation rate (τCS = 80.2 ps) upon photoexcitation in DMF, which is unveiled by femtosecond transient absorption (fs-TA) studies. The SB-CS for two dimers is well-supported by the formation of PMI˙+ and PMI˙- bands in the fs-TA spectra. Further analysis of fs-TA data revealed that, among the other multichromophores the trimer also exhibits a clear charge separation, whereas SB-CS signatures are less prominent, but can not be completely disregarded, for the meta- and para-dimers. Additionally, the charge separation dynamics of those above-mentioned PMI derivatives are devoid of a kinetically favorable excimer or triplet formation. The evidence of a profound charge transfer phenomenon in the ortho-dimer is characterized by density functional theory (DFT) calculations on excited state electronic structures. The excitonic communications in the excited state electronic arrangements unravel the key role of dihedral twisting in SB-CS. The thermodynamic feasibility of CS (ΔGCS) and activation barrier (ΔG≠) of the derivatives in DMF are established from the Rehm-Weller equation and Marcus's theory, respectively. This work is an in-depth study of the effect of mutual orientation of PMIs and regioisomerism in determining sustainable guidelines for using SB-CS.

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

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 of 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.

Competitive Charge Separation Pathways in a Flexible Molecular Folda-Dimer

We report the photophysical properties of a molecular folda-dimer system PDI-AnEt2-PDI, where the electron-donating N,N-diethylaniline (AnEt2) moiety bridges two electron-accepting perylene diimide (PDI) chromophores. The conformationally flexible PDI-AnEt2-PDI adopts either an open (two PDIs far apart) or folded (two PDIs within π-stacking distance) conformation, depending on the solvent environment. We characterized the photoinduced charge separation dynamics of both open and folded forms in solvents of varying polarity. The open form undergoes charge separation to give PDI•--AnEt2•+-PDI (Bridge electron transfer) independent of solvent polarity. The folded form exhibits two charge separation photoproducts, yielding both PDI•--AnEt2•+-PDI and PDI•--AnEt2-PDI•+, the latter of which is formed via symmetry-breaking charge separation (SBCS) between the two π-stacked PDI chromophores. Our results further indicate that the conformational flexibility of the folda-dimer leads to unexpected excimer formation in some open form conditions. In contrast, no excimer formation is observed in the folded form, indicating that this geometry preferentially yields the SBCS instead. Our results provide insight into how conformationally flexible folda-dimer systems can be designed and built to tune competitive photophysical pathways.

Excimer Formation Driven by Excited-State Structural Relaxation in a Covalent Aminonaphthalimide Dimer

Strongly coupled excimer formation from interchromophoric charge transfer driven by the ultrafast excited-state structural dynamics of a 5,5'-linked 4-amino-1,8-naphthalimide covalent homodimer was investigated by ultrafast transient spectroscopy and chemical calculations. Theoretical calculations indicate that the structural relaxation associated with the dihedral motion leads to significantly enhanced interchromophoric charge transfer (CT) coupling, which favors the formation of an excimer-like symmetry-broken CT state. The formation and relaxation dynamics of the excimer state in the dimer are identified via ultrafast transient absorption and fluorescence spectroscopy. The structural relaxation following the photoexcitation occurs in tens of picoseconds and stabilizes the dimer to the strongly coupled excimer state. The highly polar solvents further stabilize the excimer state and enhance the CT character, which enable efficient electron and excitation energy transport in covalent molecular aggregates.

Unveiling the intersystem crossing dynamics in N-annulated perylene bisimides

The efficient population of the triplet excited states in heavy metal-free organic chromophores has been one of the long-standing research problems to molecular photochemists. The negligible spin-orbit coupling matrix elements in the purely organic chromophores and the large singlet-triplet energy gap (ΔES-T) pose a hurdle for ultrafast intersystem crossing (ISC). Herein we report the unprecedented population of triplet manifold in a series of nitrogen-annulated perylene bisimide chromophores (NPBI and Br-NPBI). NPBI is found to have a moderate fluorescence quantum yield (Φf = 68 ± 5%), whereas Br-NPBI showcased a low fluorescence quantum yield (Φf = 2.0 ± 0.6%) in toluene. The femtosecond transient absorption measurements of Br-NPBI revealed ultrafast ISC (kISC = 1.97 × 1010 s-1) from the initially populated singlet excited state to the long-lived triplet excited states. The triplet quantum yields (ΦT = 95.2 ± 4.6% for Br-NPBI, ΦT = 18.7 ± 2.3% for NPBI) calculated from nanosecond transient absorption spectroscopy measurements showed the enhancement in triplet population upon bromine substitution. The quantum chemical calculations revealed the explicit role of nitrogen annulation in tuning the excited state energy levels to favor the ISC. The near degeneracy between the singlet and triplet excited states observed in NPBI and Br-NPBI (ΔES-T = -0.01 eV for NPBI, ΔES-T = 0.03 eV for Br-NPBI) facilitates the spin flipping in the molecules. Nitrogen annulation emerges as a design strategy to open up the ISC pathway and the rate of which can be further enhanced by the substitution of a heavier element.

Symmetry-Breaking Charge Separation in a Chiral Bis(perylenediimide) Probed at Ensemble and Single-Molecule Levels

Chiral molecular assemblies exhibiting symmetry-breaking charge separation (SB-CS) are potential candidates for the development of chiral organic semiconductors. Herein, we explore the excited-state dynamics of a helically chiral perylenediimide bichromophore (Cy-PDI2) exhibiting SB-CS at the ensemble and single-molecule levels. Solvent polarity-tunable interchromophoric excitonic coupling in chiral Cy-PDI2 facilitates the interplay of SB-CS and excimer formation in the ensemble domain. Analogous to the excited-state dynamics of Cy-PDI2 at the ensemble level, single-molecule fluorescence lifetime traces of Cy-PDI2 depicted long-lived off-states characteristic of the radical ion pair-mediated dark states. The discrete electron transfer and charge separation dynamics in Cy-PDI2 at the single-molecule level are governed by the distinct influence of the local environment. The present study aims at understanding the fundamental excited-state dynamics in chiral organic bichromophores for designing efficient chiral organic semiconductors and applications toward charge transport materials.

High-Efficiency Photoinduced Charge Separation in Fe(III)carbene Thin Films

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a 2LMCT excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L2]PF6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate]-). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the 2LMCT excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Anomalous deep-red luminescence of perylene black analogues with strong π-π interactions

Perylene bisimide (PBI) dyes are known as red, maroon and black pigments, whose colors depend on the close π-π stacking arrangement. However, contrary to the luminescent monomers, deep-red and black PBI pigments are commonly non- or only weakly fluorescent due to (multiple) quenching pathways. Here, we introduce N-alkoxybenzyl substituted PBIs that contain close π stacking arrangement (exhibiting d_π-π ≈ 3.5 Å, and longitudinal and transversal displacements of 3.1 Å and 1.3 Å); however, they afford deep-red emitters with solid-state fluorescence quantum yields (Φ_F) of up to 60%. Systematic photophysical and computational studies in solution and in the solid state reveal a sensitive interconversion of the PBI-centred locally excited state and a charge transfer state, which depends on the dihedral angle (θ) between the benzyl and alkoxy groups. This effectively controls the emission process, and enables high Φ_F by circumventing the common quenching pathways commonly observed for perylene black analogues.

Direct Observation of Ultrafast Relaxation Dynamics of a Mixed Excimer State in Perylene Monoimide Dimer by Femtosecond Transient Absorption

A J-type dimer PMI-2, two perylene monoimides linked by butadiynylene bridger was prepared, and its excited-state dynamics was studied using ultrafast femtosecond transient absorption spectroscopy, along with steady-state spectroscopy and quantum chemical calculations. It is evidently demonstrated that the symmetry-breaking charge separation (SB-CS) process in PMI-2 is positively mediated by an excimer, which is mixed by localized Frenkel excitation (LE) and an interunit charge transfer (CT) state. Kinetic studies show that, with the polarity increasing of the solvent, the transformation of excimer from a mixture to the CT state (SB-CS) is accelerated, and the recombination time of the CT state is reduced obviously. Theoretical calculations indicate that these are due to PMI-2 obtaining more negative free energy (ΔG_cs) and lower CT state energy levels in highly polar solvents. Our work suggests that the mixed excimer can be formed in a J-type dimer with suitable structure, in which the charge separation the process is sensitive to the solvent environment.

A Symmetry-Broken Charge-Separated State in the Marcus Inverted Region

We report a long-lived charge-separated state in a chromophoric pair (DC-PDI₂ ) that uniquely integrates the advantages of fundamental processes of photosynthetic reaction centers: i) Symmetry-breaking charge-separation (SB-CS) and ii) Marcus-inverted-region dependence. The near-orthogonal bichromophoric DC-PDI₂ manifests an ultrafast evolution of the SB-CS state with a time constant of τ S B - C S ${{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ =0.35±0.02 ps and a slow charge recombination (CR) kinetics with τ C R ${{\tau }_{{\rm C}{\rm R}}}$ =4.09±0.01 ns in ACN. The rate constant of CR of DC-PDI₂ is 11 686 times slower than SB-CS in ACN, as the CR of the PDI radical ion-pair occurs in the deep inverted region of the Marcus parabola ( - Δ G C R ${{-{\rm \Delta }G}_{{\rm C}{\rm R}}}$ >λ). In contrast, an analogous benzyloxy (BnO)-substituted DC-BPDI₂ showcases a ≈10-fold accelerated CR kinetics with τ C R / τ S B - C S ${{\tau }_{{\rm C}{\rm R}}/{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ lowering to ≈1536 in ACN, by virtue of a decreased CR driving force. The present investigation demonstrates a control of molecular engineering to tune the energetics and kinetics of the SB-CS material, which is essential for next-generation optoelectronic devices.

From symmetry breaking to symmetry swapping: is Kasha's rule violated in multibranched phenyleneethynylenes?

The phenomenon of excited-state symmetry breaking is often observed in multipolar molecular systems, significantly affecting their photophysical and charge separation behavior. As a result of this phenomenon, the electronic excitation is partially localized in one of the molecular branches. However, the intrinsic structural and electronic factors that regulate excited-state symmetry breaking in multibranched systems have hardly been investigated. Herein, we explore these aspects by adopting a joint experimental and theoretical investigation for a class of phenyleneethynylenes, one of the most widely used molecular building blocks for optoelectronic applications. The large Stokes shifts observed for highly symmetric phenyleneethynylenes are explained by the presence of low-lying dark states, as also established by two-photon absorption measurements and TDDFT calculations. In spite of the presence of low-lying dark states, these systems show an intense fluorescence in striking contrast to Kasha's rule. This intriguing behavior is explained in terms of a novel phenomenon, dubbed "symmetry swapping" that describes the inversion of the energy order of excited states, i.e., the swapping of excited states occurring as a consequence of symmetry breaking. Thus, symmetry swapping explains quite naturally the observation of an intense fluorescence emission in molecular systems whose lowest vertical excited state is a dark state. In short, symmetry swapping is observed in highly symmetric molecules having multiple degenerate or quasi-degenerate excited states that are prone to symmetry breaking.