Excited State Charge Separation Research Articles

Formation of Highly Efficient, Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor-Acceptor-Donor Conjugates.

The significance of multiple number of donor-acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor-acceptor-donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), (4-6) by the Pd-catalyzed Sonogashira cross-coupling reactions have been newly synthesized and characterized. Donor-acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, (1-3) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4-6 was found to be quantitatively quenched while for 1-3, this property varied between 73-65 % suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives (4-6) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (ΔGCS ) in these donor-acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc+ -DPP.- -TPA charge separated states (CSS) in the case of 4-6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4-6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc+ -DPP.- -TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4-6 as compared to Fc-DPP-control and simple DPP derived donor-acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in ΔGCS and ΔGCR and the associated electron coupling between different entities.

Fe Single-Atom Catalyst for Visible-Light-Driven Photofixation of Nitrogen Sensitized by Triphenylphosphine and Sodium Iodide

Photosensitizers with charge-separated excited states are commonly introduced into photocatalytic systems to accomplish photon-to-electron transformation. Unfortunately, the photosensitizers in cur...

Electropolymerizable meso-Tetrakis Biphenyl-Bis(bithiophene) Zinc Porphyrin: Ground and Excited State Properties in Solution and in Films with Axially Coordinated C60

An electropolymerizable zinc porphyrin carrying eight entities of peripheral bithiophene, 4 was newly designed and synthesized. In this design, the bithiophene entities were separated by a biphenyl spacer to minimize ground state interactions perturbing porphyrin π-electronic structure. By multi-cyclic voltammetry, thin-films of 4 were formed on transparent FTO electrode and were characterized by optical, electrochemical and STM measurements. Further, the ability of zinc porphyrin in 4 to axially coordinate phenyl imidazole functionalized fullerene, C60Im both in solution and on the film interface was performed and characterized. Fluorescence quenching of zinc porphyrin both in solution and in the film was observed upon binding of C60Im. Femtosecond transient absorption studies revealed excited state charge separation for the dyad in solution wherein the measured rate of charge separation, kCS and charge recombination, kCR were found to be 2 × 1010 s−1 and 1.2 × 109 s−1, respectively. In contrast, transient absorption studies performed on the dyad in the film were suggestive of energy transfer with minimal contributions from electron transfer. The present study brings out the importance of modulating photochemical reactivity of donor-acceptor dyad in film as compared to that in solution.

Sc3N@I h -C80 based donor-acceptor conjugate: role of thiophene spacer in promoting ultrafast excited state charge separation.

Light induced charge separation in a newly synthesized triphenylamine–thiophene-Sc3N@Ih-C80 donor–acceptor conjugate and its C60 analog, triphenylamine–thiophene-C60 conjugate is reported, and the significance of the thiophene spacer in promoting electron transfer events is unraveled.

Triplet photosensitizer-nanotube conjugates: synthesis, characterization and photochemistry of charge stabilizing, palladium porphyrin/carbon nanotube conjugates.

The ability of a triplet photosensitizer to generate long-lived charge separated states, in contrast to traditionally used singlet photosensitizers, in covalently functionalized single-walled carbon nanotube hybrids has been investigated. Enriched single-walled carbon nanotubes with two diameters, namely (6,5) and (7,6), were covalently modified to carry a charge-stabilizing triplet photosensitizer derived from a palladium porphyrin. The nanohybrids were fully characterized and the presence of intramolecular interactions between the porphyrin and nanotubes was established from various spectroscopic, imaging, electrochemical and thermochemical studies. Photoluminescence of palladium porphyrin was found to be quantitatively quenched in the presence of covalently appended SWCNTs and this quenching is due to excited state charge separation and has been established by femtosecond transient absorption studies. Owing to the presence of the triplet photosensitizer, the charge separated states lasted over 3 ns, i.e., much longer than those reported earlier for singlet photosensitizer-derived nanotube hybrids. The nanohybrids also exhibited efficient photocatalytic behavior in experiments involving electron pooling of one-electron reduced methyl viologen in the presence of a sacrificial electron donor. Higher yields of photoproducts were achieved from the present donor-acceptor nanohybrids when compared with those of singlet photosensitizer-derived nanohybrids, more so for (6,5) nanotube derived hybrids compared to (7,6) nanotube derived hybrids. The present findings highlight the importance of triplet photosensitizer derived nanohybrids in artificial photosynthesis of charge separation and photocatalytic applicatons.

Heterometal incorporation in NH2-MIL-125(Ti) and its participation in the photoinduced charge-separated excited state

Optical and X-ray spectroscopy studies reveal the location and role of Fe3+ sites incorporated through direct synthesis in NH2-MIL-125(Ti). Fe K-edge XAS analysis confirms its metal-oxo cluster node coordination while time-resolved optical and X-ray transient absorption studies disclose its role as an electron trap site, promoting long-lived photo-induced charge separation in the framework. Notably, XTA measurements show sustained electron reduction of the Fe sites into the microsecond time range. Comparison with an Fe-doped MOF generated through post-synthetic modification indicates that only the direct synthesis approach affords efficient Fe participation in the charge separated excited state.

(Invited) Tracking Structures in Solar Fuels Catalysis: In-Situ X-Ray Structure Characterization of Interfacial Water-Splitting Molecular and Thin-Film Catalysts

A key challenge for the design of efficient photoelectrochemical devices that employ nanoscaled interfacial designs for solar energy-to-fuels conversion lies in resolving atomic structures and dynamics at the active interfaces, and relating these to the complex cascade of events which includes excited-state charge separation, charge-accumulation, and multi-step energy conversion and catalysis. Our research programs have been developing in-situ, time-resolved X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function for both interfacial thin-films and molecular photosensitizers and catalysts bound to semiconductor surfaces. This presentation will discuss our development of X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function. We have developed high surface area TiO2 semiconductor and ITO and IZO conducting oxide porous assemblies that duplicate oxide-supported dye and catalyst architectures widely used in dye sensitized solar cells (DSSC) and photoelectrochemial (PEC) electrodes, but have been tailored to allow for interfacial X-ray structure characterization. Thin-Film Metal-Oxide Water Oxidation Catalysts. Non-noble-metal, thin film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L-edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, while CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4p-3d hybridization through Co-O-Co bonding was detected by RXES and the inter-site dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure while CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance OEC performance.The combined electronic and structural analyses demonstrate that hole transfer to catalytic sites in amorphous cobalt oxide thin films is the rate-limiting step for electrochemical water-splitting rather than the multi-step catalytic events themselves. The macroscopic catalytic properties of the thin films were found to be correlated to the electronic structures measured at the atomic scale for the metal-oxo cluster domains. These results show the interplay between intrinsic catalytic activity and charge transport properties of semiconductor thin-film catalysts. In-situ Structure-Function Analysis of Molecular Water Oxidation Catalysts. Building from the microporous electrodes for in-situ X-ray analyses of amorphous oxide thin-film OECs, we have extended this approach by the development of nano-porous electrode architectures which enable the use of combined PDF and X-ray spectroscopy analyses of coordination structures for molecular photosensitizers and catalysts in solution and when bound to semiconductor oxide surfaces. PDF measurements require PEC oxide supports with sufficiently low background scattering to permit detection of the surface bound complexes. We have found that atomic layer deposition of semiconductor oxides on anodic aluminium oxide templates, AAO, provides a suitable layered, high-surface area architecture. PDF patterns measured for the N3 dye, cis-bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II), bound to an amorphous TiO2 surface reveal details on outer sphere ligand structures. Each of the PDF peaks can be assigned to the atom pair distances calculated from crystal and DFT structures. Measurements on ruthenium photosensitizer complexes bound to the TiO2 surface resolve distortions in the surface-coordinating ligand structures. On-going work is investigating the correlation between ligand structural distortion and the metal-to-ligand charge-transfer (MLCT) states with interfacial charge injection function.

Generation of Microsecond Charge-Separated Excited States in Rhenium(I) Diimine Complexes: Driving Force Is the Dominant Factor in Controlling Lifetime.

A transition-metal-based donor-(linker)-acceptor system can produce long-lived charge transfer excited states using visible excitation wavelengths. The ground- and excited-state photophysical properties of a series of [ReCl(CO)3(dppz-(linker)-TPA)] complexes, with varying donor and acceptor energies, have been systematically studied using spectroscopic techniques (both vibrational and electronic) supported by computational chemistry. The long-lived excited state is 3ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR2), and time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Modulation of the donor and acceptor energies results in changes of the 3ILCT lifetime by 1 order of magnitude, ranging from 6.1(±1) μs when a diphenylamine donor is used to 0.6(±0.2) μs when a triazole linker and triphenylamine donor is used. The excited-state lifetime may be rationalized by consideration of the driving force within the framework of Marcus theory and appears insensitive to the nature of the linker.

Photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in ionic liquids: Effect of cation alkyl-chain length

Transient absorption spectroscopy has been applied to study the photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in twelve imidazolium/phosphonium cation-based ionic liquids (ILs) with alkyl chains of different lengths. The transient absorption spectra showed an initial bleach signal around the absorption center (400 nm) and a stimulated emission band around 500 nm. The bleach signal was recovered mostly within a few picoseconds, and a hot band absorption appeared around 450 nm. The time profiles of the band integrals for the bleach and absorption spectral regions were simulated by a sum of four exponential decays. The internal conversion rate corresponding to the back electron transfer (b-ET) rate from the charge-separated excited state, was determined by the time profile of the band integral of the bleach region. The vibrational energy relaxation (VER) rate in the ground state was determined from the time profile of the hot band absorption decay band integral. The b-ET rate decreased with increasing number of alkyl carbons in the cation (#C). The solvent dependence was discussed in terms of the free energy change at the Marcus inverted region, which was indicated by the blue-shift of the ground state absorption band. The change of the free energy is ascribed to the change of the charge concentration of ILs. The VER rate decreased slightly with increasing #C. The reason was discussed in terms of the interaction of DMpNA with the polar region of ILs. The intersystem crossing yield also increased with increasing #C, reflecting an increase in the non-polar region of the ILs.

Mechanism of the charge separation improvement in carbon-nanodot sensitized g-C3N4

Graphitic carbon nitride (g-C3N4), an environment-friendly metal-free photocatalysts represent a promising alternative to conventional metal-based semiconductors. Although sp2-structured carbon-dot modified g-C3N4 has shown its effectiveness to enhance the photocatalytic efficiencies, a precise, comprehensive and molecular level understanding of this enhancement is demanded for advance progress, which is lacking due to the complexity of the samples in experiments. Here, our density functional theory (DFT) and time-dependent DFT calculations reveal that a precise graphene quantum dot (GQD) sensitization of g-C3N4 nanosheet significantly enhance excited-state charge separation, which is crucial for the photocatalytic activity. Nitrogen to carbon ratio is critical for the spatial separation of photogenerated electron-hole pair via GQD/g-C3N4 π-π stacking. Red-shift in the lowest excitation of GQD/g-C3N4 with size increment stretches the absorption to the visible wavelengths, facilitating the visible-light photocatalytic activity of g-C3N4. The mechanism revealed in this study supports the recent experimental reports on the competence of the environmental friendly, low cost and stable GQD sensitized g-C3N4 photocatalyst and provides guidance for optimal design to achieve higher efficiency.

Accelerated Charge Separation and Stabilization in a Fused Bis Zinc Porphyrin-Quinone Conjugate Via Cation-Quinone Interaction

Accelerated singlet excited state charge separation, improved quantum yields, and ultimate charge stabilization upon cation binding to the electron acceptor quinone in a fused bis zinc porphyrin-quinone (zinc porphyrin located on each side of the pentacenequinone) donor-acceptor conjugate has been accomplished. Selective binding of either Mg2+ or Sc3+ to quinone in the donor-acceptor conjugate led to facile reduction of quinone making the charge separation process thermodynamically more feasible. Consequently, increased population of the charge separated state was witnessed in femtosecond transient absorption studies with a near linear trend in population of the charge separated state versus amount of metal-ion added. GloTarAn software was utilized to process the spectral data which enabled determination of the species associated spectrum and population profiles which thereby allowed deconvolution of the femtosecond spectral data and appropriate designations of each state. The present findings may provide further insight into the role of the interaction between quinones and iron metal ions in the electron transport chain of natural photosynthesis, which may have further implications in other biological systems where metal cations are present in electron transport chains.

Metal Substitution as the Method of Modifying Electronic Structure of Metal-Organic Frameworks.

Targeted modification of electronic structure is an important step in the optimization of metal–organic frameworks (MOFs) for photovoltaic, sensing, and photocatalytic applications. The key parameters to be controlled include the band gap, the absolute energy position of band edges, the excited state charge separation, and degree of hybridization between metal and ligand sites. Partial metal replacement, or metal doping, within secondary building units is a promising, yet relatively unexplored route to modulate these properties in MOFs. Therefore, in the present study, a general method for selecting metal dopant is worked out in theory and validated by experiment, retaining MIL-125 and UiO-66 as the model systems. Metal mixing enables targeted optimization of key electronic structure parameters. This method is applicable to any MOF architecture and can serve as a roadmap for future synthesis of MOFs with predefined properties.

Wave Function Control of Charge-Separated Excited-State Lifetimes.

Control of excited-state processes is crucial to an increasing number of important device technologies that include displays, photocatalysts, solar energy conversion devices, photovoltaics, and photonics. However, the manipulation and control of electronic excited-state lifetimes and properties continue to be a challenge for molecular scientists. Herein, we present the results of ground-state and transient absorption spectroscopies as they relate to magnetic exchange control of excited-state lifetimes. We describe a novel mechanism for controlling these excited-state lifetimes that involves varying the magnetic exchange interaction between a stable organic radical and the unpaired electrons present in the open-shell configuration of a charge-separated excited state. Specifically, we show that the excited-state lifetime can be controlled in a predictable manner based on an a priori knowledge of the pairwise magnetic exchange interactions between excited-state spins. These magnetic exchange couplings affect the excited-state electronic structure in a manner that introduces variable degrees of spin forbiddenness into the nonradiative decay channel between the excited state and the electronic ground state.

Occurrence of excited state charge separation in a N-doped graphene-perylenediimide hybrid formed via 'click' chemistry.

Hetero-atom doped graphene is a two-dimensional material with a band gap, needed to build optoelectronic devices. However, research progress in this area has been sluggish due to synthetic challenges to build energy harvesting materials, especially donor–acceptor type hybrids. In the present study, using click chemistry, we have successfully synthesized a donor–acceptor hybrid comprised of N-doped graphene and perylenediimide (PDI), a well-known electron-accepting photosensitizer. The TGA and XPS results revealed the attachment of the PDI moiety in the hybrid. Ground and excited state interactions were monitored by a variety of spectral and electrochemical techniques. Finally, the ability of the present donor–acceptor hybrid to undergo photoinduced charge separation from singlet excited PDI was systematically probed using femtosecond transient spectral techniques. Evidence of charge separation was possible to achieve from comparison of transient and spectroelectrochemical results. These results suggest the potential use of covalently functionalized, substitutional N-doped graphene as a functional material for building optoelectronic devices.

Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes.

Dichalcogenolene platinum(II) diimine complexes, (LE,E')Pt(bpy), are characterized by charge-separated dichalcogenolene donor (LE,E') → diimine acceptor (bpy) ligand-to-ligand charge transfer (LL'CT) excited states that lead to their interesting photophysics and potential use in solar energy conversion applications. Despite the intense interest in these complexes, the chalcogen dependence on the lifetime of the triplet LL'CT excited state remains unexplained. Three new (LE,E')Pt(bpy) complexes with mixed chalcogen donors exhibit decay rates that are dominated by a spin-orbit mediated nonradiative pathway, the magnitude of which is proportional to the anisotropic covalency provided by the mixed-chalcogen donor ligand environment. This anisotropic covalency is dramatically revealed in the 13C NMR chemical shifts of the donor carbons that bear the chalcogens and is further probed by S K-edge XAS. Remarkably, the NMR chemical shift differences also correlate with the spin-orbit matrix element that connects the triplet excited state with the ground state. Consequently, triplet LL'CT excited state lifetimes are proportional to both functions, demonstrating that specific ground state NMR chemical shifts can be used to evaluate spin-orbit coupling contributions to excited state lifetimes.

Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots.

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

Paddle-Wheel BODIPY-Hexaoxatriphenylene Conjugates: Participation of Redox-Active Hexaoxatriphenylene in Excited-State Charge Separation to Yield High-Energy Charge-Separated States.

Hexaoxatriphenylene, a scaffold linker often utilized in building covalent organic frameworks, is shown to be electroactive and a useful entity to build light energy-harvesting donor-acceptor systems. To demonstrate this, new donor-acceptor conjugates have been synthesized by employing BODIPY as a sensitizer. Excited-state electron transfer leading to high-energy charge-separated states, useful to drive energy-demanding photocatalytic reactions, from the electron-rich hexaoxatriphenylene to 1BODIPY*, in the synthesized tri-BODIPY-hexaoxatriphenylene "paddle-wheel" conjugates, has been successfully demonstrated using femtosecond transient absorption spectroscopy. The measured rate of charge separation was in the range of ∼3-10 × 1011 s-1, revealing ultrafast charge separation.

Long-Lived Photoinduced Charge Separation in a Trinuclear Iron-μ3-oxo-based Metal–Organic Framework

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100(Fe), composed of Fe3-μ3-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ3-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to furthe...

The Cation-π Interaction Enables a Halo-Tag Fluorogenic Probe for Fast No-Wash Live Cell Imaging and Gel-Free Protein Quantification.

The design of fluorogenic probes for a Halo tag is highly desirable but challenging. Previous work achieved this goal by controlling the chemical switch of spirolactones upon the covalent conjugation between the Halo tag and probes or by incorporating a “channel dye” into the substrate binding tunnel of the Halo tag. In this work, we have developed a novel class of Halo-tag fluorogenic probes that are derived from solvatochromic fluorophores. The optimal probe, harboring a benzothiadiazole scaffold, exhibits a 1000-fold fluorescence enhancement upon reaction with the Halo tag. Structural, computational, and biochemical studies reveal that the benzene ring of a tryptophan residue engages in a cation−π interaction with the dimethylamino electron-donating group of the benzothiadiazole fluorophore in its excited state. We further demonstrate using noncanonical fluorinated tryptophan that the cation−π interaction directly contributes to the fluorogenicity of the benzothiadiazole fluorophore. Mechanistically, this interaction could contribute to the fluorogenicity by promoting the excited-state charge separation and inhibiting the twisting motion of the dimethylamino group, both leading to an enhanced fluorogenicity. Finally, we demonstrate the utility of the probe in no-wash direct imaging of Halo-tagged proteins in live cells. In addition, the fluorogenic nature of the probe enables a gel-free quantification of fusion proteins expressed in mammalian cells, an application that was not possible with previously nonfluorogenic Halo-tag probes. The unique mechanism revealed by this work suggests that incorporation of an excited-state cation−π interaction could be a feasible strategy for enhancing the optical performance of fluorophores and fluorogenic sensors.

Long lived charge separated states in vinylbenzonitrile substituted derivatives of pyrene and anthracene

Abstract The present study investigates the photophysical properties of two highly conjugated pyrene and anthracene derivatives viz ( E )-4-(2-(pyren-1-yl) vinyl) benzonitrile (PyCN) and ( E )-4-(2-(anthracen-9-yl) vinyl) benzonitrile (AnCN) in the presence and absence of electron donors such as N , N -diethylaniline (DEA) and N , N -diphenylamine (DPA). The observed rates of back electron transfer (BET) in the case of PyCN-DEA and AnCN-DEA are 1.3 × 10 5 and 4.8 × 10 4 s −1 , respectively, which are 3–4 orders of magnitude lower compared to the well-known pyrene–amine and anthracene–amine systems. Mechanistic investigations indicate that the donor-acceptor systems undergo different pathways in presence of tertiary and the secondary amine. The results indicate that the design strategy is highly efficient in reducing the rate of back electron transfer, which results in long lived charge-separated excited states.