Perylenediimide (PDI) derivatives are essential organic semiconductor materials in a variety of photofunctional devices. By virtue of the large energy gap between the singlet and triplet excited ...states (ΔE ST = 1.1 eV), augmentation of the triplet state population in monomeric PDI is a challenging task. We report the metal atom-free approach in engendering a near-quantitative triplet yield in perbromoperylenediimide/octabromoperylenediimide (OBPDI), absorbing in the visible region of the electromagnetic spectrum. Perbromination of PDI causes significant out-of-plane distortion (θ = 39°) in the aromatic core of OBPDI as compared to the planar PDI (θ = 0°). A substantial decrease (ΔE 0 red = 0.377 V) in the reduction potential of OBPDI, E 1/2(OBPDI/OBPDI· –) = −0.170 V, when compared to the reduction potential, E 1/2 (PDI/PDI· –) = −0.547 V, of bare PDI makes OBPDI a promising electron acceptor. As a consequence of incorporating eight bromine atoms, the fluorescence quantum yield of a bare PDI chromophore (ϕf = 97 ± 1%; τf = 4.54 ns) decreases to a very low value in OBPDI (ϕf = 3 ± 1%; τf = 13.78 ps). Femtosecond transient absorption measurements of OBPDI reveal intersystem crossing (ISC) occurring at an ultrafast time scale (τISC = 14.20 ps), leading to a near-quantitative triplet population (ϕT = 97 ± 1%). Theoretical investigations performed to decode the excited state dynamics in OBPDI propose that (i) cumulative addition of eight bromine atoms enhances the magnitude of spin–orbit coupling (SOC) and (ii) twist on the perylene core moderately reduces the energy gap between the singlet–triplet states. Understanding the structural alterations that control the electronic parameters in accessing the triplet excited states of organic chromophores, like PDI, can lead to the design and fabrication of efficient optoelectronic devices and energy storage materials.
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Over the last several decades, exploring the pathways to access the triplet excited states of organic chromophores has been a stimulating area of research. Among the numerous photoinduced processes ...in organic chromophores, analysis of intersystem crossing (ISC) dynamics has received immense attention. The ISC process involves a spin-forbidden horizontal transition from an excited singlet state to a higher vibrational level of the isoenergetic triplet state. Generally, ISC necessitates a strong driving force from efficient spin-orbit coupling (SOC) between the singlet and triplet wavefunctions. The magnitude of SOC can be tuned by the substituent groups (
e.g.
heavy atoms, carbonyl moieties) or by the out-of-plane vibrational modes in the chromophores. Besides the SOC induced ISC pathway, triplet excited states are also realised in organic chromophores through singlet fission or
via
charge recombination. Accessing the triplet manifold in π-conjugated systems would also include a possible evolution to more aromatically stable configurations in the excited states, an emerging area that needs attention. In the aforesaid mechanisms, the molecular architecture and/or packing arrangement of the chromophores are vital for the effective population of triplet states. We, herein, present a collection of synthetic, spectroscopic and theoretical investigations that provide insights into the diverse pathways to access triplet excited states in organic chromophores. We believe this tutorial review would prove beneficial for researchers to achieve triplet excited states of organic chromophores for numerous biochemical and optoelectronic applications.
Triplet excited states, ubiquitous in organic chromophores, can be accessed through various pathways. The feasibility of each pathway is determined by the molecular and electronic structures of the organic chromophores.
We describe the design, bottom-up synthesis and X-ray single crystal structure of systematically twisted aromatics
and
for efficient intersystem crossing. Steric congestion at the cove region creates ...a nonplanar geometry that induces a significant yield of triplet excited states in the electron-poor core-twisted aromatics
and
. A systematic increase in the number of twisted regions in
and
results in a concomitant enhancement in the rate and yield of intersystem crossing, monitored using femtosecond and nanosecond transient absorption spectroscopy. Time-resolved absorption spectroscopic measurements display enhanced triplet quantum yields (
= 10 ± 1% for
and
= 30 ± 2% for
) in the twisted aromatics when compared to a negligible
(<1%) in the planar analog
. Twist-induced spin-orbit coupling
activated out-of-plane C-H/Cdouble bond, length as m-dashC vibrations can facilitate the formation of triplet excited states in twisted aromatics
and
, in contrast to the negligible intersystem crossing in the planar analog
. The ease of synthesis, high solubility, access to triplet excited states and strong electron affinity make such imide functionalized core-twisted aromatics desirable materials for organic electronics such as solar cells.
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Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behaviour of π‐conjugated materials. Though vital for optoelectronic applications, ideal Greek cross‐dipole ...(α=90°) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7‐dibromoperylene‐3,4,9,10‐tetracarboxylic tetrabutylester (PTE‐Br2) which exhibits null exciton coupling mediated monomer‐like optical characteristics in the crystalline state. In contrast, nonzero exciton coupling in X‐type (α=70.2°, PTE‐Br0) and J‐type (α=0°, θ=48.4°, PTE‐Br4) assemblies have perturbed optical properties. Additionally, the semi‐classical Marcus theory of charge‐transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE‐Br2. Precise rotation angle dependent optoelectronic properties in crystalline PTE‐Br2 can have consequences in the rational design of novel π‐conjugated materials for photonic and molecular electronic applications.
Monomer‐like aggregates: Null exciton coupling in an ideal Greek cross‐dipole (+) assembly affords monomer‐like optical properties and an exceptional charge‐filtering (selective hole transport) effect in orthogonal chromophore stacks.
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Solvent-free crystal structure of N,N-bis(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis(dicarboximide), PDI-Br4, obtained by X-ray diffraction reveals the core-twisted perylene motif ...having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br4 arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br4 in toluene reveal ultrafast intersystem crossing (τISC < 110 fs) when excited at 400 nm. Triplet quantum yield (ΦT) of 19 ± 1% and 105 ± 5% for PDI-Br4 in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br4 can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br4 thin film.
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We report a nonparallel stacked arrangement of donor–acceptor (D–A) pairs for prolonging the lifetime of photoinduced charge-separated states. Hydrogen–hydrogen steric repulsion in ...naphthalimide-naphthalene (NIN) dyad destabilizes the planar geometry between the constituent units in solution/ground state. Sterically imposed nonplanar geometry of the dyad allows the access of nonparallel arrangement of the donor and acceptor stacks having triclinic space group in the crystalline state. Antiparallel trajectory of excitons in nonparallel D–A stacks can result in lower probability of geminate charge recombination, upon photoexcitation, thereby resulting in a long-lived charge-separated state. Upon photoexcitation of the NIN dyad, electron transfer from naphthalene to the singlet excited state of naphthalimide moiety results in radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (τcr a > 1.2 ns) as compared to that of monomeric dyad (τcr m < 110 fs), monitored using femtosecond transient absorption spectroscopy.
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7.
Through-space aromatic character in excimers Vijay, Vishnu; Madhu, Meera; Ramakrishnan, Remya ...
Chemical communications (Cambridge, England),
01/2020, Volume:
56, Issue:
2
Journal Article
Peer reviewed
Aromaticity, though widely used to delineate diverse photochemical phenomena, remains to be examined in excimers, a fundamental and extensively studied entity in the excited states. Herein, the first ...theoretical evidence for the excited state through-space aromatic character in triplet state (T
1
) excimers of benzene, naphthalene and anthracene is reported using multiple aromaticity descriptors based on magnetic, electronic and geometric criteria. The calculated chemical shifts and induced current densities manifest the presence of transannular π-electronic currents in the excimers. The results open up enormous research potential from exploring the possibility of through-space aromatic character in singlet excimers to its possible implications in photoexcited state processes of aromatic supramolecular systems.
Excimers exhibit through-space aromatic character.
Conspectus Spatial noncovalent helical organization of nucleobases in DNA and radial organization of chromophores in natural light-harvesting systems are fascinating yet enigmatic. Understanding the ...numerous weak interactions that drive the formation of elegant supramolecular architectures in native natural systems and developing bioinspired design strategies have seen a surge of interest in recent decades. Self-assembly of functional chromophores in the crystalline phase is a definitive strategy to identify novel molecule–molecule interactions, in particular, atom–atom interactions, and to understand the synergistic nature of noncovalent interactions that stabilizes the supramolecular organization. This Account narrates our recent efforts in developing desirable supramolecular motifs employing weak interaction-based strategies and our observation of deviations from the common motifs chartered in aromatic systems. Modulation of long-range aromatic interactions through chemical modifications (acylation, benzoylation, haloacylation, and alkylation of chromophores) to attain a preferred stacking (herringbone, lamellar, or columnar) is presented. Particular attention has been given to attaining lamellar or columnar packing possessing potential interchromophoric electronic coupling mediated high charge mobility. Supramolecular arrangements of noncovalently or covalently associated donor–acceptor systems that open up additional possibilities of packing modes (segregated, mixed etc.) are explored. Our persistent efforts yielded distinct twisted-segregated and alternate distichous stacks for the nonparallel covalently linked donor–acceptor systems that favor a long-lived photoinduced charge-separated state. We further move on to discuss the unconventional packing motifs that were identified recently. The highly sought-after Greek cross (+) stacking of chromophores in crystalline phase and an elegant crystalline radial arrangement of chromophores are examined. The Greek cross (+) stacked architecture exhibits monomer-like emission characteristics owing to the absence of exciton coupling across the orthogonally stacked chromophores. Crystalline helical chromophore assembly is yet another emerging motif with far-reaching applications in domains ranging from asymmetric catalysis to chiral smart materials and has been accounted here by citing certain phenomenal examples from literature. Thus, this Account demonstrates that identifying and classifying new structural motifs based on topological aspects, such as interchromophoric orientation (cross) and extended chromophore arrangement in the crystal lattice (radial, helical, etc.), are crucial since such fundamental characteristics dictate the properties emerging out of the corresponding motifs. Encouraged from ours and others’ works, we propose the addition of new aromatic supramolecular structural motifs, namely, cross-stacked, helical, and radial arrangements, in order to expand the classification. We believe that identifying new emergent property-based supramolecular motifs and investigating the methods to achieve the desired motif will eventually have implications in fundamental crystal engineering, supramolecular chemistry, and biomimetic design of functional materials.
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A combined theoretical and experimental investigation into the role of concerted long‐ (dipole coupling) and short‐range (orbital overlap mediated excimer) electronic interactions in modulating the ...emission of six crystalline acetylanthracenes (1–3) is reported. Friedel–Crafts acylation of anthracene rendered crystalline acetylanthracenes with discrete close packing, varied orbital overlap, and resultant distinct emission (blue–green–yellow) from cooperative excimer and dipole coupling. Time‐resolved emission spectroscopy (TRES) studies and the Kasha's exciton theory based quantitative estimation of dipole coupling (mean‐field approximation) substantiates the exciton dynamics in crystalline 1–3. Extension of the Kasha's exciton model beyond the traditional nearest‐neighbor approach, and consistent agreement among the computed spectral shifts and TRES temporal components, corroborate a holistic approach to decipher the exciton relaxation dynamics in the molecular assembly of novel photonic materials.
Balancing the force: A combined theoretical and experimental investigation into the role of concerted long‐ (dipole coupling) and short‐range (orbital overlap mediated excimer) electronic interactions in modulating the emission of six crystalline acetylanthracenes is reported. A subtle interplay between these interactions dictate the optical signatures of the six crystalline anthracene assemblies.
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Among the various donor–acceptor (D‐A) charge‐transfer co‐crystals investigated in the past few decades, tetrathiafulvalene‐tetracyanoquinodimethane (F⋅Q, popularly known as TTF⋅TCNQ)‐based ...co‐crystals have fascinated materials chemists owing to their exceptional conducting and magnetic properties that arise from the packing in crystal structures. Here, crystallographic information files of eighteen F⋅Q‐based co‐crystals are extracted from the Cambridge Structural Database (CSD) and classified into Class 1 (D‐on‐D and A‐on‐A segregated stacks; F⋅Q, F1⋅Q‐F6⋅Q, and F⋅Q1), Class 2 (‐A‐D‐A‐D‐A‐D‐ mixed stacks; F6a⋅Q‐F11⋅Q and F⋅Q2), and Class 3 ‐A‐D‐A‐A‐D‐A‐; Class 3a (F12⋅Q and F13⋅Q) and ‐D‐D‐A‐A‐; Class 3b (F14⋅Q) systems according to their packing modes. Hirshfeld surface analysis, PIXEL energy calculations, and quantum theory of atoms in molecules (QTAIM) analysis are performed on the selected multicomponent charge‐transfer crystals for the first time, in an attempt to explore the driving forces that give rise to different classes of 3 D crystal packing, which in turn mandates the expedient electronic properties exhibited by the investigated co‐crystals. PIXEL calculations reveal that the dispersion energy component makes the maximum contribution to the total lattice energy for most of the F⋅Q‐based co‐crystals under study. Although the Q‐on‐Q dimer is the energetically most favored dimer in F⋅Q, the substituents on F capable of forming hydrogen‐bonding, C⋅⋅⋅S, and other weak intermolecular interactions result in the greater stability of the F‐on‐F dimer for F1⋅Q‐F6⋅Q (except F2⋅Q). The C⋅⋅⋅S, Csp⋅⋅⋅S, S⋅⋅⋅N, and π⋅⋅⋅π interaction‐driven D‐on‐A dimer is found to be the most stable dimer of all the Class 2 co‐crystals. Band structure and density‐of‐state calculations of the representative co‐crystals in each class indicate different electronic structures according to the packing arrangement. F⋅Q and F6⋅Q with a high interaction of electronic orbitals between D‐on‐D and A‐on‐A in segregated stacks are found to be metal‐like (bandgap, Eg=0.003 eV) and metallic (overlapping bands in the Fermi level), respectively, whereas the polymorph of F6⋅Q belonging to Class 2 (F6a⋅Q) displays a semiconductor‐type band structure (Eg=0.053 eV). F12⋅Q of Class 3a exhibits a metal‐like band structure (Eg=0.001 eV). The fine tuning of chromophores with diverse functional substituents capable of triggering weak intermolecular interactions that give rise to the desired packing and charge‐transfer properties has the potential to open floodgates of opportunity for research in the chemistry of materials and fabrication of efficient electronic devices.
Crystal math: Eighteen tetrathiafulvalene‐tetracyanoquinodimethane (TTF⋅TCNQ)‐based co‐crystals extracted from the Cambridge Structural Database are classified according to their packing modes. PIXEL calculations yield lattice energies and stable dimer energies with new insights into energy contributions from Coulombic, polarization, dispersion, and exchange‐repulsion components (see figure). Band‐structure and density‐of‐states calculations of representative co‐crystals in each Class reveal different electronic structures and thereby 1 D/2 D conductivity characteristics according to their packing arrangement.
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