One-dimensional exciton diffusion in perylene bisimide aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.     

Generation and decay dynamics of triplet excitons in Alq3 thin films under high-density excitation conditions

We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.     

Picosecond relaxation dynamics in polydiacetylene-pTS

Time-resolved absorption spectroscopy as well as transient grating measurements were performed on polydiacetylene-pTS. Singlet exciton lifetimes of 0.8 ± 0.2 ps at 300 K and 1.1 ± 0.2 ps at 5 K were measured. We propose that the observed non-radiative relaxation proceeds through a conformationally relaxed singlet state which lives for roughly 1.0 and 3.4 ps at 300 and 5 K, respectively. Data pertinent to the formation of triplet excitons are also presented and discussed.     

Time-resolved photoelectron spectroscopy of low-energy excitations of 4×4 C60/Cu(111)

Time-resolved two-photon photoemission is applied to investigate electron dynamics in multiple monolayers (MLs) of ordered fullerite on a copper substrate. The experimental data are analyzed assuming coupled excited state dynamics. Rate equations fitted to these dynamics yield lifetimes of about 80 ps for the lowest unoccupied molecular orbital (LUMO), about 1.2 ns for the singlet exciton and 22 μs for the triplet exciton at a surface temperature of 140 K. For trapped triplet excitons lifetimes up to 200 μs are observed. An increased excitation fluence reduces the lifetime of the excitons due to annihilation. An increased sample temperature slightly reduces the lifetime of the triplet exciton. There is no evident dependence of the exciton lifetimes on the pump photon energy in the range of hν = 2.9 to 3.3 eV. A dependence on the layer thickness (10-20 ML) is not observed as long as more than 9 ML are prepared.     

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.     

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.     

Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2?μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S(1) state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S(0)→S(1) transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T(1)→T(N) absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene's photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.     

Femtosecond time-resolved photophysics of 1,4,5,8-naphthalene diimides

The photophysical properties of a tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds were investigated. The absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500 and 650 nm. This band is sensitive to the polarity of the solvent and is attributed to a CT state. The absorption spectra and lifetime (10+/-2 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5-1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran glass.     

Spin-dependent polaron recombination in conjugated polymers

We simulate the interchain polaron recombination process in conjugated polymer systems using a nonadiabatic molecular dynamics method, which allows for the coupled evolution of the nuclear degrees of freedom and multiconfigurational electronic wavefunctions. Within the method, the appropriate spin symmetry of the electronic wavefunction is taken into account, thus allowing us to distinguish between singlet and triplet excited states. It is found that the incident polarons can form an exciton, form a bound interchain polaron pair, or pass each other, depending on the interchain interaction strength and the strength of an external electric field. Most importantly, we found that the formation of singlet excitons is considerably easier than triplet excitons. This shows that in real organic light emitting devices, the electroluminescence quantum efficiency can exceed the statistical limitation value of 25%, in agreement with experiments.     

Sub‐Picosecond Singlet Exciton Fission in Cyano‐Substituted Diaryltetracenes

Thin films of 5,11‐dicyano‐6,12‐diphenyltetracene ( TcCN ) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T₁)?E(S₁)=?0.17 eV), where S₁ and T₁ are singlet and triplet excitons, respectively. As a result of tuning the triplet‐state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.     

Charge-transfer excitons in DNA

There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data.     

On the excitonic nature of the first UV absorption peak in polyene

The first absorption peak in the UV spectrum of polyene is interpreted in terms of charge transfer excitons. The exciton spectrum has been calculated from first principles using the Green's function formalism of charge transfer exciton theory. Electron correlation effects on the polyene band structure have been included with the help of second order Møller-Plesset perturbation theory and of the electron polaron model of Toyozawa. The spectrum of bound singlet excitons starts at E_K=0 = 1.86 eV above the top of the valence band. A deeper lying triplet level is observed at 0.72 eV. Further correlation effects on the band gap and dielectric screening of the electron-hole ineraction are discussed.     

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4

The ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)₂] [PdCl₂(en)₂] (ClO₄)₄ (en = ethylenediamine) has been investigated by the use of a femtosecond absorption spectrum, calculated from a pump—probe reflection spectrum at room temperature by the Kramers—Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 to 2.5 eV were calculated for three decay components. They are free excitons with a lifetime of about 800 fs, self-trapped excitons with a lifetime of about 3 ps, and polaron pairs which relax within a 100 ps time period.     

Enhanced multiple exciton dissociation from CdSe quantum rods: the effect of nanocrystal shape

A unique ability of semiconductor nanocrystals (NCs) is the generation and accommodation of multiple excitons through either optical or electric current pumping. The development and improvement of NC-based optoelectronic devices that utilize multiple excitons requires the understanding of multiple exciton dynamics and their efficient conversion to emitted photons or external charges prior to exciton-exciton annihilation. Here, we demonstrate that significantly enhanced multiexciton dissociation efficiency can be achieved in CdSe quantum rods (QRs) compared to CdSe quantum dots (QDs). Using transient absorption spectroscopy, we reveal the formation of bound one-dimensional exciton states in CdSe QRs and that multiple exciton Auger recombination occurs primarily via exciton-exciton collision. Furthermore, quantum confinement in the QR radial direction facilitates ultrafast exciton dissociation by interfacial electron transfer to adsorbed acceptors. Under high excitation intensity, more than 21 electrons can be transferred from one CdSe QR to adsorbed methylviologen molecules, greatly exceeding the multiexciton dissociation efficiency of CdSe QDs.     

Ultrafast dynamics of exciton fission in polycrystalline pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ～80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.     

Triplet exciton stimulated current pulses in anthracene crystals

Current pulses of nanoseconds duration accompanied by delayed luminescence are induced by triplet excitons in the presence of a steady state hole current. The current pulses set in at 15.kV/cm and are brought about by the cooperation of two triplet excitons. The current pulse probability depends on the nature of the cathode. Current and luminescence pulses are attributed to triplet exciton induced free electrons in the vicinity of a blocking cathode.     

Dynamics and transient absorption spectral signatures of the single-wall carbon nanotube electronically excited triplet state

We utilize femtosecond-to-microsecond time domain pump-probe transient absorption spectroscopy to interrogate for the first time the electronically excited triplet state of individualized single-wall carbon nanotubes (SWNTs). These studies exploit (6,5) chirality-enriched SWNT samples and poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), which helically wraps the nanotube surface with periodic and constant morphology (pitch length = 10 ± 2 nm), providing a self-assembled superstructure that maintains structural homogeneity in multiple solvents. Spectroscopic interrogation of such PNES-SWNT samples in aqueous and DMSO solvents using E(22) excitation and a white-light continuum probe enables E(11) and E(22) spectral evolution to be monitored concomitantly. Such experiments not only reveal classic SWNT singlet exciton relaxation dynamics and transient absorption signatures but also demonstrate spectral evolution consistent with formation of a triplet exciton state. Transient dynamical studies evince that (6,5) SWNTs exhibit rapid S(1)→T(1) intersystem crossing (ISC) (τ(ISC) ~20 ps), a sharp T(1)→T(n) transient absorption signal (λ(max)(T(1)→T(n)) = 1150 nm; full width at half-maximum ≈ 350 cm(-1)), and a substantial T(1) excited-state lifetime (τ(es) ≈ 15 μs). Consistent with expectations for a triplet exciton state, T(1)-state spectral signatures and T(1)-state formation and decay dynamics for PNES-SWNTs in aqueous and DMSO solvents, as well as those determined for benchmark sodium cholate suspensions of (6,5) SWNTs, are similar; likewise, studies that probe the (3)[(6,5) SWNT]* state in air-saturated solutions demonstrate (3)O(2) quenching dynamics reminiscent of those determined for conjugated aromatic hydrocarbon excited triplet states.     

Singlet-triplet and triplet-triplet interactions in conjugated polymer single molecules

Single molecule fluorescence correlation spectroscopy has been used to investigate the photodynamics of isolated single multichromophoric polymer chains of the conjugated polymers MEH-PPV and F8BT on the microsecond to millisecond time scale. The experimental results (and associated kinetic modeling) demonstrate that (i) triplet exciton pairs undergo efficient triplet-triplet annihilation on the <30 micros time scale, (ii) triplet-triplet annihilation is the dominant mechanism for triplet decay at incident excitation powers > or =50 W/cm(2), and (iii) singlet excitons are quenched by triplet excitons with an efficiency on the order of (1)/(2). The high efficiency of this latter process ensures that single molecule fluorescence spectroscopy can be effectively used to indirectly monitor triplet exciton population dynamics in conjugated polymers. Finally, correlation spectroscopy of MEH-PPV molecules in a multilayer device environment reveals that triplet excitons are efficiently quenched by hole polarons.     

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)_n: n=2, 4, 6]. Then, a comparison of (Tc)_n and Tc-hetero-oligomer [TcF₃-(Tc)₄-TcF₃] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF₃ units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: Φ_TrT=176 %).     

Triplet excited state in platinum-acetylide oligomers: triplet localization and effects of conformation

An experimental and theoretical investigation was carried out on a series of platinum-acetylide oligomers of the general structure Ph-CC-[PtL2-CC-(1,4-Ph)-CC-]n-PtL2-CC-Ph (where n = 1, 2, 3, 4, 6; Ph = phenyl, 1,4-Ph = 1,4-phenylene; L = P(n-Bu)3, and the geometry at Pt = trans). The objective of this work is to understand the geometry and electronic structure of the ground and triplet excited states of Pt-acetylide oligomers. The experiments carried out include temperature-dependent absorption and photoluminescence spectroscopy (80-298 K range) and ambient temperature transient absorption spectroscopy. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were carried out on several of the oligomers using the hybrid Becke's three-parameter functional with the B3LYP correlation functional and the SDD basis set. The combined experimental and theoretical results provide very clear evidence that the triplet excited state is localized on a chromophore consisting approximately of a single -[PtL2-CC-(1,4-Ph)-CC-PtL2]- repeat unit. DFT calculations indicate that in the ground state conformers that differ in the (rotational) orientation of the 1,4-phenylenes with respect to the plane defined by the PtL2(C)2 units (twisted = t and planar = p) are very close in energy (difference of <1 kcal.mol-1). By contrast, in the triplet excited state, the p conformer is 3 kcal.mol-1 lower in energy than the t conformer. The torsional geometry change in the triplet state is reflected in the low-temperature phosphorescence spectra of the short oligomers, where separate emission bands are seen from the t and p conformers.