Singlet-singlet Exciton Annihilation Research Articles

A high fluorescence rate is key for stable blue organic light-emitting diodes

Singlet–singlet exciton annihilation (SSA) is found to be a critical factor for the decomposition of blue thermally activated delayed fluorescence (TADF) emitters in organic light-emitting diodes (OLEDs).

(Invited) Transient Absorption Study of Singlet Excitons in Crystalline Peropyrene Derivatives: Examination of Singlet Fission Process

Transient absorption study of singlet excitons in crystalline peropyrene derivatives: examination of singlet fission process Akihiro Furube*1, Daiki Yamanaka1, Shinichiro Yanagiya1, Hiroyuki Matsuzaki2, Kazuyuki Uchida3, Takashi Kubo3 1Tokushima Univ,. 2-1, Minamijosanjima-cho, Tokushima, 770-8506, Japan 2AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 3Osaka Univ., 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan Furube.akihiro@tokushima-u.ac.jp Singlet fission process in organic crystal is attracting considerable interest due to its potential application of organic photovoltaic (OPV) with the internal quantum efficiency of 200%, since one absorbed photon can produce two triplet excitons that can generate two electron-hole pairs at the interface between the organic crystal and a suitable electron donor material such as fullerene derivatives and carbon nanotubes. Peropyrene, which is one of polycyclic aromatic hydrocarbon compounds, is expected to exhibit singlet fission in the crystal form. The SF is a reaction in which the excited singlet state (S1) spilt into two triplet excited states (T1 + T1). Energy matching (S1 ~ T1 + T1) reported [1] for peropyrene molecule interests researchers for development of derivatives of this molecule to control the energy levels and crystal structure for efficient singlet fission. In the present study, two kinds of 2,9-disubstituted peropyrene derivatives, one with two phenyl groups and the other with two n-butyl groups, were newly synthesized and tested. Time-resolved transient absorption spectroscopy in the femtosecond and nanosecond timescales, time-resolved fluorescence spectroscopy, and steady-state absorption/fluorescence measurements were carried out in order to reveal the relaxation processes of the singlet excitons of these peropyrene derivatives in the crystalline and solution phases. Transient absorption spectra and kinetics of phenyl peropyrene in the crystalline state are shown in Figure 1a. Here, diffuse reflectance spectroscopy was applied [2, 3]. At 10 ps after excitation, a broad absorption band appeared around 700 nm, which showed gradual decay up to 500 ps. It should be noted that the excitation laser intensity (at 400 nm wavelength) was adjusted to be weak enough so as not to undergo any non-linear decay process such as exciton–exciton annihilation. Since the absorption peak wavelength (700 nm) is close to the one observed in solution (775 nm), the absorption band can be ascribed to the singlet exciton in crystalline state. The decay component in the picosecond timescale showed a time constant of 150 ps. The involvement of singlet fission in the decay of a singlet exciton would result in the appearance of the T1absorption band being similar to the band observed in solution around 525 nm. However, we did not observe such an absorption band in the picosecond time scale. This experimental finding implies that the energy level of the S1 state is lower than twice the energy of the T1 state in the crystalline state. In this case, singlet fission is expected to occur from highly excited singlet states (Sn). Under high-density excitation conditions, two singlet excitons collide each other to produce Sn and S0 through singlet-singlet exciton annihilation (S1 + S1 → Sn + S0). Thereafter the resulting Sn state can undergo singlet fission. Figure 1b shows the transient absorption. Indeed, a broad absorption band centered at 570 nm was observed in the transition absorption spectrum measured at 1 ns, after the disappearance of the absorption band of the S1 state around 700 nm. This spectral feature is similar to that of the T1 state observed in solution, suggesting that the T1 state is generated from the highly excited singlet state Snthrough singlet fission. Interestingly, n-butyl peropyrene crystal did not show singlet fission either in weak or strong excitation condition. Based on these experiments, it was revealed that S1exciton in phenyl peropyrene is energetically low for undergoing singlet fission, therefore requiring additional energy. Further detailed discussion is given with other experimental results. Figure.1Transient absorption spectra of phenyl peropyrene crystal power samples excited by 400 nm laser with weak (a) and strong (b) intensity, respectively. Acknowledgement: A part of this work is supported by KAKENHI (16K13669) Japan.

Slow Singlet Fission Observed in a Polycrystalline Perylenediimide Thin Film.

Singlet exciton fission (SF) is a process wherein an exciton in an organic semiconductor divides its energy to form two excitations. This process can offset thermalization losses in light harvesting technologies, but requires photostable materials with high SF efficiency. We report ultrafast kinetics of polycrystalline films of N-N'-dioctyl-3,4,9,10-perylenedicarboximide (C8-PDI), a chromophore predicted to undergo SF on picosecond time scales. While transient absorption measurements display picosecond dynamics, such kinetics are absent from low-fluence time-resolved emission experiments, indicating they result from singlet-singlet exciton annihilation. A model that accounts for annihilation can reproduce both measurements and highlights that care must be taken when extracting SF rates from time-resolved data. Our model also reveals SF proceeds in C8-PDI over 3.8 ns. Despite this slow rate, SF occurs in high yield (51%) due to a lack of competing singlet deactivation pathways. Our results show perylenediimides are a promising class of SF materials that merit further study.

Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductors

Excitation energy transfer from host to guest is studied in spin-cast blends of 4,4′-bis(N-carbazolyl)biphenyl (CBP) and a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)-cored dendrimer using time resolved fluorescence. The kinetics of energy transfer are consistent with homogeneous dispersion of the dendrimers in the CBP host. Diffusion-controlled singlet-singlet exciton annihilation is observed in the CBP host at moderate excitation densities, similar to those encountered in high brightness light-emitting devices and organic lasers. The results are important for organic lighting and the understanding of exciton diffusion in guest-host systems for electrophosphorescence.

Influence of the dendron chemical structure on the photophysical properties of bisfluorene-cored dendrimers

A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications.

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.

Time-dependent fluorescence depolarization arising from exciton annihilation in confined molecular domains

A theory of fluorescence depolarization arising from singlet-singlet exciton annihilation is presented for an ensemble of randomly oriented molecular domains. This paper extends our previous work including the effects of the orientational distribution of chromophores forming a domain. The orientational order of the domain is described by the distribution function ƒ(θ) depending on one angle θ, the angle between the transition dipoles of chromophores (or their aggregates) and the symmetry axis of the domain. By using a kinetic equation of a continuous model for exciton annihilation, the time evolution of the fluorescence anisotropy is investigated. For large t<τ, the anisotropy was shown to decrease as t − 1 2 , t −1 (ln t+const.) and t −1 for S = 1, S = − 1 2 and − 1 2 < S<1 ( S ≠ 0), respectively, τ being the excited - state lifetime and S = P 2 = (3 cos 2θ -1)/2 being the order parameter of the domain. The residual anisotropy at long times in analysed using both continuous and discrete models of exciton annihilation. It is shown that from the residual anisotropy against intesity curves one can estimate both the domain size and the order parameter S. The theory accounts for the intensity-dependant depolarization observed recently in the J-aggregated pseudoisocyanine dye (PIC) solution. Applications to photosynthetic systems are also discussed.

Fluorescence depolarization due to exciton annihilation in molecular domains

A theory of time-resolved fluorescence depolarization due to singlet-singlet exciton annihilation is presented for the ensemble of randomly oriented molecular domains formed of chromophores (or their associates) with parallel transition moments. For large t<τ, the fluorescence anistropy induced by the delta pulse excitation is shown to decrease as t -1/2, while the anisotropy after the long-pulse excitation decreases as t -1 (const+ln t), τ being the excited-state lifetime. The residual fluorescence anisotropy at long times is estimated. The results obtained are applied to J-aggregated pseudoisocyanine dye (PIC) solutions to account for the intensity-dependent depolarization recently observed. From the comparison of the theory with the experiment, the number of PIC molecules forming a domain was evaluated to be more than 10 4.

High intensity effects in biological and medical samples

Intense excitation sources are known to cause nonlinear optical effects in biological samples. Similarly, high power lasers, used as medical and surgical tools, can induce nonlinear processes in tissues. In this paper we describe and research the theory of one such high intensity effect: bimolecular singlet-singlet exciton annihilation.

Singlet exciton annihilation in the picosecond fluorescence decay of 1, 1'-diethyl-2,2'-cyanine chloride dye j-aggregate

Time-resolved fluorescence decay of the J-aggregate of the title dye has been measured with a jitter-free picosecond streak camera and by time-correlated single photon counting. The observed decay is sensitive to the excitation intensity, and the dependence can be simulated by a model involving singlet-singlet exciton annihilation at high exciton densities. The estimate of exciton mobility from this exciton annihilation experiment is similar to results from fluorescence quenching experiments.

Probing fluorescence induction in chloroplasts on a nanosecond time scale utilizing picosecond laser pulse pairs

The fluorescence induction and other fluorescence properties of spinach chloroplasts at room temperature were probed utilizing two 30-ps wide laser pulses (530 nm) spaced Δt (s) apart in time ( Δt = 5–110 ns). The energy of the first pulse (P 1) was varied (10 12–10 16 photons · cm −2), while the energy of the second (probe) pulse (P 2) was held constant (5 · 10 13 photons · cm −2). A gated (10 ns) optical multichannel analyzer-spectrograph system allowed for the detection of the fluorescence generated either by P 1 alone, or by P 2 alone (preceded by P 1). The dominant effect observed for the fluorescence yield generated by P 1 alone is the usual singlet-singlet exciton annihilation which gives rise to a decrease in the yield at high energies. However, when the fluorescence yield of dark-adapted chloroplasts is measured utilizing P 2 (preceded by pulse P 1) an increase in this yield is observed. The magnitude of this increase depends on Δt, and is characterized by a time constant of 28 ± 4 ns. This rise in the fluorescence yield is attributed to a reduction of the oxidized (by P 1) reaction center P-680 + by a primary donor. At high pulse energies (P 1 = 4 · 10 14 photons · cm −2) the magnitude of this fluorescence induction is diminished by another quenching effect which is attributed to triplet excited states generated by intense P 1 pulses. Assuming that the P 1 pulse energy dependence of the fluorescence yield rise reflects the closing of the reaction centers, it is estimated that about 3–4 photon hits per reaction center are required to close completely the reaction centers, and that there are 185–210 chlorophyll molecules per Photosystem II reaction center.

EXCITON ANNIHILATION IN THE ISOLATED PHYCOBILIPROTEINS FROM THE BLUE-GREEN ALGA NOSTOC SP. USING PICOSECOND ABSORPTION SPECTROSCOPY

— Phycobilisomes from the blue-green alga Nostoc sp. contain the phycobiliproteins: c-phycoerythrin (c-PE), c-phycocyanin (c-PC) and allophycocyanin (APC). The depletion and the recovery of the ground states for the individual phycobiliproteins were measured using picosecond (ps) absorption spectroscopy. In all cases the depletion time was Glops. The recovery was found to be non-exponenzial which could be fitted to a single exponential (‘fast’ component) and a second component with a relaxation time of > 300ps. The recovery times of the fast component were found to be intensity dependent and for c-PE, c-PC and APC were measured to be 19, 27 and 35ps, respectively, at intensity (I) ∼ 7 × 1020 photons/m2 and increased to 54, 55 and 67 ps, respectively, at I∼ 8 × 1019 photons/m2. The ps absorption data support the assignment of the‘fast’ component to singlet-singlet exciton annihilation.

Exciton fission and annihilation in crystalline tetracene

The time-resolved fluorescence of tetracene crystals excited by a single 7-ps pulse of 530-nm light has been measured with a streak camera/OMA system. The singlet exciton lifetime was found to be 300�30 ps at 293 K. Singlet-singlet exciton annihilation was observed, and the rate constant found to be 5 x 10-9 cm3 s-1. In order to fit the observed decay curves, an upper limit of c. 1 x 10-9 cm3 s-1 must be set on the singlet-triplet exciton annihilation rate. The measured singlet-singlet exciton annihilation rate is consistent with a hopping model for singlet exciton motion at room temperature.

Bimolecular quenching of excitons and fluorescence in the photosynthetic unit

The recent results of Campillo et al. and Mauzerall on the quenching of the fluorescence of chlorophyll a in Chlorella pyrenoidosa as a function of the intensity of the laser excitation pulses are rationalized by applying a model invoking singlet-singlet exciton annihilation.