Photoexcited Singlet State Research Articles

The photochemical trans → cis and thermal cis → trans isomerization pathways of azobenzo-13-crown ether: A computational study on a strained cyclic azobenzene system.

The isomerization of azobenzo-13-crown ether can be expected to be hindered due to the polyoxyethylene linkage connecting the 2,2'-positions of azobenzene. The mixed reference spin-flip time-dependent density functional theory results reveal that the planar and rotational minima of the first photo-excited singlet state (S1) of the trans-isomer pass through a barrier (2.5-5.0 kcal/mol) as it goes toward the torsional conical intersection (S0/S1) geometry (<CNNC ≈ 98°), which is responsible for the cis isomer formation. The second excited singlet state (S2) of the trans form has a nearly planar minimum along the N-N stretching mode, which approaches a sloped S2/S1 intersection geometry. This excited state has a rotational minimum (<CNNC ≈ 99°) as well. Both these minima have a characteristic S2-S1 energy gap of 9 kcal/mol and may undergo internal conversion. A comparison of this system with an analogous 2,2'-dimethoxy-substituted azobenzene system reveals less strain in the rotational path of the latter on the S1 surface, indicating the possibility of its better trans → cis yield than the azocrown. The completely planar S2 geometry of the dimethoxy system has easy access to the linear concerted inversion path, which seems to be the reason behind its reported slightly lower π-π*(S2) yield than n-π*(S1). The thermal cis → trans isomerization path of the azobenzo-13-crown passes through a transition state (frequency 453i cm-1), which corresponds to Gibbs free energy of activation value of 26 kcal/mol in the gas-phase and isooctane. Our study also confirms that its trans isomer strongly binds Li+ among the alkali metal ions, and this observation may open up possibilities for practical applications of this azobenzo-crown.

Controlling Intramolecular Singlet Fission Dynamics via Torsional Modulation of Through-Bond versus Through-Space Couplings.

Covalent dimers, particularly pentacenes, are the dominant platform for developing a mechanistic understanding of intramolecular singlet fission (iSF). Numerous studies have demonstrated that a photoexcited singlet state in these structures can rapidly and efficiently undergo exciton multiplication to form a correlated pair of triplets within a single molecule, with potential applications from photovoltaics to quantum information science. One of the most significant barriers limiting such dimers is the fast recombination of the triplet pair, which prevents spatial separation and the formation of long-lived triplet states. There is an ever-growing need to develop general synthetic strategies to control the evolution of triplets following iSF and enhance their lifetime. Here, we rationally tune the dihedral angle and interchromophore separation between pairs of pentacenes in a systematic series of bridging units to facilitate triplet separation. Through a combination of transient optical and spin-resonance techniques, we demonstrate that torsion within the linker provides a simple synthetic handle to tune the fine balance between through-bond and through-space interchromophore couplings that steer iSF. We show that the full iSF pathway from femtosecond to microsecond timescales is tuned through the static coupling set by molecular design and structural fluctuations that can be biased through steric control. Our approach highlights a straightforward design principle to generate paramagnetic spin pair states with higher yields.

Toward Simultaneous Coherent Radiation in the Visible and Microwave Bands with Doped Molecular Crystals

AbstractCoherent sources exploiting the stimulated emission of non‐equilibrium quantum systems, i.e., gain media, have proven indispensable for advancing fundamental research and engineering. The operating electromagnetic bands of such coherent sources have been continuously enriched for increasing demands. Nevertheless, simultaneous generation of radiation in multiple widely‐separated bands presents formidable challenges with a single gain medium. Herein, a mechanism of simultaneously realizing the stimulated emission of radiation in the visible and microwave bands, i.e., lasing and masing actions, at ambient conditions by utilizing photoexcited singlet and triplet states of pentacene doped in p‐terphenyl is proposed. The possibility is validated by the observed amplified spontaneous emission (ASE) at 645 nm with a narrow linewidth around 1 nm from the pentacene‐doped p‐terphenyl crystal used for masing at 1.45 GHz and consolidated by a 20‐fold‐lower threshold of ASE compared to the reported masing threshold. The overall threshold of the pentacene‐based multiband coherent source can be optimized by appropriate alignment of the pump‐light polarization with the pentacene's transition dipole moment. This work not only shows a great promise on immediate realization of multiband coherent sources but also establishes an intriguing solid‐state platform for fundamental research of quantum optics in multiple frequency domains.

Room-Temperature Phosphorescence from Pd(II) and Pt(II) Complexes as Supramolecular Luminophores: The Role of Self-Assembly, Metal-Metal Interactions, Spin-Orbit Coupling, and Ligand-Field Splitting.

The synthesis as well as the structural and photophysical characterization of two isoleptic bis-cyclometalated Pt(II) and Pd(II) complexes, namely [PtL] and [PdL], bearing a tailored dianionic tetradentate ligand (L2-) are reported. The isostructural character and intermolecular interactions of [PtL] and [PdL] were assessed by NMR spectroscopy and X-ray diffraction analysis. Both complexes show fully ligand-controlled aggregation, demonstrating that a judicious molecular design can tune the photophysical properties. In fact, by introduction of fluorine atoms on defined positions and methoxy groups on complementary sites, metal-metal interactions can be forced by a head-to-tail stacking. Hence, [PtL] shows luminescence from metal-perturbed ligand-centered or from metal-metal-to-ligand charge-transfer triplet states in diluted solutions, in frozen glasses and in crystals, with high photoluminescence quantum yields and long lifetimes in the microsecond range. At room temperature (RT) in concentrated fluid solutions, the palladium analogue [PdL] surprisingly emits luminescence from aggregated species involving supramolecular interactions. Time-resolved photoluminescence and transient absorption spectroscopies demonstrated that ultrafast intersystem crossing occurs for both metals, which outruns any competitive relaxation pathway from the photoexcited singlet state. Furthermore, we demonstrate that the radiationless deactivation can be suppressed in frozen glassy matrices at 77 K and by intermolecular interactions in fluid solutions at RT. In both cases and as indicated by density functional theory calculations, the lowest emissive state acts as an energy trap from which the thermal population of dissociative states with formal occupation of an antibonding Pd-centered 4dx2-y2 orbital is suppressed. This occurs as the energy gap between the emissive and the dark states surpasses kT.

Singlet Fission in a Flexible Bichromophore with Structural and Dynamic Control.

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.

Visual Understanding of Vibronic Coupling and Quantitative Rate Expression for Singlet Fission in Molecular Aggregates

Abstract Singlet fission (SF) is an exciton dissociation process that generates two triplet excitons from one singlet exciton. Because the exciton dissociation process involves internal conversion between multiexcitonic states, it is necessary to calculate multiexcitonic vibronic couplings (VCs) and identify what kinds of vibrational modes activate internal conversion to fully understand the SF mechanism. In this study, we developed a method of visually understanding “interstate” vibronic couplings and “quantitatively” evaluating SF rates for dimers in molecular aggregates. We applied the method to tetracene dimers in crystals by computing and visually analyzing interstate VCs between the multiexcitonic states within a configuration interaction scheme. From the calculated VCs, a low-frequency mode (62 cm−1) was found to strongly promote the generation of correlated triplet pairs from a photoexcited singlet state. The spatial overlap between the wave functions of the correlated triplet pairs and singlet state has a large distribution, leading to the large VC of the low-frequency mode. From the calculated VCs, we estimated SF rates in the temperature range from 50 to 300 K. The calculated SF rate was quantitatively in good agreement with an experimental result, which validated our method of calculating VCs and SF rates for dimers in molecular aggregates.

Long Persistent Luminescence Enabled by Dissociation of Triplet Intermediate States in an Organic Guest/Host System.

Organic guest/host systems with long persistent luminescence benefiting from the formation of a long-lived charge-separated state have recently been demonstrated. However, the photogeneration mechanism of such key charge-separated states remains elusive. Here, we report the identification of intermediate triplet states with mixed local excitation and charge-transfer character that connect the initial photoexcited singlet states and the long-lived charge-separated states. Using time-resolved optical spectroscopy, we observe the intersystem crossing from photoexcited singlet charge-transfer states to triplet intermediate states on a time scale of ∼52 ns. Temperature-dependent measurements reveal that the long-lived triplet intermediate states ensure a relatively high efficiency of diffusion-driven charge separation to form the charge-separated state responsible for LPL emission. The findings in this work provide a rationale for the development of new LPL materials that may also improve our understanding of the mechanism of photon-to-charge conversion in many organic optoelectronic devices.

Effect of Vibronic Coupling on Correlated Triplet Pair Formation in the Singlet Fission Process of Linked Tetracene Dimers.

Tetracene-based singlet fission (SF) materials show application prospects as triplet sensitizers in organic optoelectronics. SF involves internal conversion from photoexcited singlet states 1(S1S0) to correlated triplet pair states 1(T1T1). We derive an expression for the internal conversion rate on the basis of the Fermi golden rule with an artificial Lorentzian broadening. The internal conversion rate depends on the interstate vibronic couplings (VCs) and energy difference (ΔESF) between 1(S1S0) and 1(T1T1). Therefore, understanding the interplay between interstate VCs and ΔESF is necessary to reveal how the structure-property relationship affects the SF efficiency. Here, we propose a method to quantitatively analyze interstate VCs between 1(S1S0) and 1(T1T1). We apply this method to SF in ortho-, meta-, and para-bis(ethynyltetracenyl)benzene and identify an effect of interstate VCs on the 1(T1T1) formation rate. The interstate VCs of the meta dimer are remarkably weak, which reasonably explains the experimentally obtained slow 1(T1T1) formation rate. The weak VCs result from a very small overlap density between 1(S1S0) and 1(T1T1) of the meta dimer. Furthermore, we investigate structure-dependence of the 1(T1T1) formation rate of the para dimer and find that the para dimer shows large VCs and small ΔESF when the rotational angle between the two tetracene units is large, which leads to the faster 1(T1T1) formation rate than those of the ortho and meta dimers. The rotation of the tetracene units is the origin of the experimentally observed fast 1(T1T1) formation rate of the para dimer.

Synthesis of Unsymmetrical Derivatives of Pentacene for Materials Applications.

Pentacene shows unique electronic properties that have long been appreciated and exploited. Over the past 20 years, new synthetic schemes have been developed to address some of the problems encountered with pristine pentacene (e.g., stability and solubility), and pentacene derivatives have become a mainstay in the realm of organic semiconductors in applications such as organic light-emitting diodes, organic field-effect transistors (OFETs), and organic photovoltaics. At the onset of our work, the vast majority of known pentacene derivatives featured a symmetrical structure, often as the result of synthetic protocols that rely on nucleophilic additions to 6,13-pentacenequinone (PQ). The assembly of pentacenes featuring an unsymmetrical framework held great appeal, but the stepwise formation of derivatives, in which a specific function might be incorporated through each individual addition step, did not exist. This Account presents contributions from our lab and others to the synthesis and study of unsymmetrical pentacene derivatives. PQ offers an ideal platform for desymmetrization through the sequential addition of nucleophiles to each of the two ketone groups. Addition can be completed in a one-pot protocol, or through individual steps in which the product of the first addition is isolated and used as a precursor in the divergent synthesis of a series of structurally related molecules. This general approach has been used to assemble pentacene derivatives appended with alkynyl/aryl/alkyl groups, polarized frameworks via substitution with donor and/or acceptor groups, and conjugated oligomers linked by butadiynyl moieties. Stepwise substitution also provides derivatives with remarkable functionality, including pentacene-porphyrin dyads, pendent TEMPO free radicals, cyanoacrylic acid anchor groups (for incorporation into dye-sensitized solar cells), and derivatives with ambipolar behavior for OFET devices. The study of intramolecular singlet fission (iSF) has emerged as one of the most fruitful applications of unsymmetrical pentacene derivatives. SF involves the spontaneous splitting of a photoexcited singlet state (S1) in one chromophore into a pair of triplets (T1) shared with a neighboring chromophore. Pentacene derivatives are particularly well suited for this since E(S1) ≥ 2E(T1) satisfies the thermodynamic requirements for SF, and they have the additional feature that two chromophores can be tethered together by a "spacer" that allows spectroscopic studies of iSF to be done in dilute solution. From a synthetic perspective, the major advantage of the dimeric structure is the ability to modify the spacer, which allows for control over the distance, geometric relationship, and electronic coupling between the two pentacene groups. Dimeric pentacenes are central to providing an in-depth understanding of the molecular mechanism of SF, often providing advances not possible from measurements in the solid state.

Identifying (BN)2-pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution.

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)2-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)2-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)2-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry.

Role of Spin-Coupled Polaron Pairs in the Recombination of Charges in Electroluminescent Conjugated Polymers

We study the delayed fluorescence following geminate recombination of photogenerated polaron pairs (PPs) in films of a model conjugated polymer F8BT. Doping with tiny gold nanoparticles also enables us to observe phosphorescence from triplet states that arise via intersystem crossing from the directly photoexcited singlet state. Small magnetic fields ∼5 mT have large effects on the delayed fluorescence by causing demixing of the hyperfine coupled singlet and triplet PP configurations, implying PP spin equilibration times much shorter than 1 μs. Nevertheless, the magnetic field effect on the fluorescence persists to tens of microseconds, and we argue that this apparent inconsistency implies that photogenerated polarons reside primarily as uncoupled spin half particles with long spin memory times. We consider the implications for whether recombination of charges in organic light-emitting diodes is likely to be governed by quantum spin statistics.

Pentacene Dimers as a Critical Tool for the Investigation of Intramolecular Singlet Fission.

Singlet fission (SF) involves the spontaneous splitting of a photoexcited singlet state into a pair of triplets, and it holds great promise toward the realization of more efficient solar cells. Although the process of SF has been known since the 1960s, debate regarding the underlying mechanism continues to this day, especially for molecular materials. A number of different chromophores have been synthesized and studied in order to better understand the process of SF. These previous reports have established that pentacene and its derivatives are especially well-suited for the study of SF, since the energetic requirement E(S1 )≥2E(T1 ) is fulfilled rendering the process exothermic and unidirectional. Dimeric pentacene derivatives, in which individual pentacene chromophores are tethered by a "spacer", have emerged as the system of choice toward exploring the mechanism of intramolecular singlet fission (iSF). The dimeric structure, and in particular the spacer, allows for controlling and tuning the distance, geometric relationship, and electronic coupling between the two pentacene moieties. This Minireview describes recent advances using pentacene dimers for the investigation of iSF.

Exploring the isomerization paths of push–pull hexatrienes

The first photo-excited singlet state of all-trans donor (amine), acceptor (cyano)-substituted hexatriene forms donor side-twisted minima and its passage towards the cis–trans–trans isomer is characterized by an S0/S1 conical intersection.

Excited‐State Dynamics of Pentacene Derivatives with Stable Radical Substituents

AbstractThe excited‐state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited‐state characteristics of the derivatives.

Excited-state dynamics of pentacene derivatives with stable radical substituents.

The excited-state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited-state characteristics of the derivatives.

Ultrafast Hole/Electron Transfer Dynamics in a CdSe Quantum Dot Sensitized by Pyrogallol Red: A Super-Sensitization System

To find a suitable hole-transporting adsorbate for CdSe quantum dots (QDs), a pyrogallol red (PGR) molecule was chosen where PGR also can sensitize CdSe QDs. Energy level diagrams suggest that the photoexcited hole can be transferred to PGR and photoexcited PGR can inject an electron into CdSe QDs. Steady-state and time-resolved emission studies suggest that the photoexcited hole is transferred to PGR; however, the process is too fast to monitor with the subnanosecond time-resolution spectroscopic technique. Femtosecond transient absorption spectroscopy has been employed to monitor the charge-transfer behavior of the above system in an early time scale. Photoexcitation of pure PGR and CdSe QDs at 400 nm laser light gives the transient absorption due to the photoexcited singlet state of PGR and charge carriers (electron/hole) in CdSe QDs, respectively, in the visible/near-IR region of the absorption spectra. However, on photoexcitation of the CdSe/PGR composite at 400 nm, the PGR cation radical and electro...

Singlet exciton fission in solution

Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.

Photophysical and DNA-Binding Properties of Cytochrome c Modified with a Platinum(II) Complex

Cytochrome c (cyt c) derivatives modified with a platinum(II) complex at the lysine residue, cyt c(III)-[Pt(bpy)(dapap)](1) {bpy = 2,2'-bipyridine, and dapap = 3-(2,3-diaminopropionylamino)propionic acid}, have been prepared. The modified residues are Lys8, Lys13, Lys55, Lys60, Lys73, and Lys88. In the case of the cyt c(III)-[Pt(bpy)(dapap)](1) dyad, the photoexcited singlet state of (1)([Pt(bpy)(dapap)](1))* was quenched by the heme Fe(III) moiety through the intramolecular photoinduced energy-transfer reaction via a through-space mechanism. Next, in the presence of calf thymus (CT)-DNA, the DNA-responsive fluorescence properties of cyt c(III)-[Pt(bpy)(dapap)](1) isomers were investigated. The order of the obtained binding constants between the cyt c(III)-[Pt(bpy)(dapap)](1) isomer and CT-DNA in an aqueous solution suggested that the electrostatic interaction is one of the important factors to stabilize the cyt c-DNA complex. Finally, we discussed the rotational motion of the [Pt(bpy)(dapap)](2+) moiety at the surface of cyt c by fluorescence anisotropy measurement. The increase in the anisotropy parameter, r, for each cyt c isomer clearly revealed that the noncovalent recognition of the [Pt(bpy)(dapap)](2+) moiety by CT-DNA is an essential event in the formation of the cyt c-DNA complex and generation of DNA-sensitive fluorescence signals.

Electron Transfer from Hydrophobic Porphyrin to Hydrophilic Viologen via the Photoexcited Singlet State in Water

Abstract Viologen-linked porphyrins are a typical compound for photoinduced electron-transfer models via the photoexcited singlet state. The singlet-state electron transfer, however, does not occur in water, because solvent cage formation, which is motion of solvent molecules during electron transfer, is slow in the case of water. In order to boost the solvent cage formation of water and achieve the electron transfer via the singlet state in water, viologen-linked hydrophobic porphyrin was prepared.

Triplet States of the Nonlinear Optical Chromophore DCM in Single Crystals of Potassium Hydrogen Phthalate and Their Relationship to Single-Molecule Dark States

Single-molecule dark states are often attributed to photoexcited triplets with scant evidence of the participation of paramagnetic molecules. The photodynamics of blinking single molecules of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in crystals of potassium hydrogen phthalate (KAP) were compared with the lifetimes of DCM triplet states, likewise in KAP, whose zero-field splitting (ZFS) tensors were fully characterized by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. Luminescent mixed crystals of KAP were grown from solutions containing 10(-4) -10(-9) M DCM, a model optically nonlinear chromophore. The luminescent dye was localized in the {111} crystalline growth sectors. The photoexcited triplets states of DCM in the heavily dyed (10(-4) M) crystals were analyzed by TR-EPR spectroscopy. The photoexcited singlet states of DCM in lightly dyed crystals (10(-9) M) were analyzed by single-molecule microscopy. Large blue shifts in the absorption and emission spectra of DCM in KAP were interpreted as a consequence of protonation at the dimethylamino nitrogen atom, an assignment supported by calculations of the zero-field splitting (ZFS) tensors of molecules in their triplet states. Experimental ZFS tensors with eigenvalues comparable to those of the computed tensors were determined from the angular dependence of the EPR spectra of DCMH(+) triplets within KAP single crystals with respect to the applied magnetic field. Data from individual growth sectors failed to show magnetically equivalent site occupancies, evidence of the kinetic ordering during growth. The intermittent fluorescence of individual chromophores was analyzed. The distributions of on(off) times were characterized by distributed rates fit to power laws. The lifetime of the triplet states was analyzed from the time decay of the EPR signals between 100 and 165 K. The data were well fit with a single time constant for the signal decay, a result wholly inconsistent with the blinking of single molecules with off times commonly of tens of seconds. Triplet decay was extrapolated to approximately 25 micros at room temperature. Therefore, the assumption that single-molecule dark states originate with triplet excited states is not sustainable for single DCM molecules in KAP.