Competition of Dexter, Förster, and charge transfer pathways for quantum dot sensitized triplet generation
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Quantum dot (QD) sensitized triplet exciton generation has demonstrated promising applications in various fields such as photon up-conversion through triplet-triplet annihilation. However, how direct triplet energy transfer from the QD to the acceptor through Dexter energy transfer (DET) competes with other processes, including Förster resonance energy transfer (FRET) and charge transfer, remains poorly understood. Herein, the competition of these pathways for QD-sensitized triplet excited state generation in CdSe QD-modified boron dipyrromethene (BODIPY) complexes is studied using transient absorption spectroscopy. After excitation of the CdSe QD with 500 nm pulses, the BODIPY triplet excited state is generated through charge recombination in a charge separated intermediate state (QD-·-BODIPY+·). This intermediate state is populated either through FRET from the excited QD to BODIPY followed by electron transfer from the singlet excited state of BODIPY to the QD or through hole transfer from the excited QD to BODIPY. The triplet excited state generation efficiencies from the FRET and hole transfer pathways are estimated to be (6.18 ± 1.39)% and (13.5 ± 3.1)%, respectively. Compared to these indirect pathways, direct DET from the QD to the BODIPY triplet state is kinetically not competitive. These results demonstrate that sequential charge transfer can be an efficient pathway for triplet excited state generation in QD-acceptor complexes.Keywords:
BODIPY
Singlet fission
Acceptor
Singlet fission
Pentacene
Annihilation
Acene
Exothermic reaction
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Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Singlet fission
Tetracene
Picosecond
Annihilation
Biexciton
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Abstract New boron‐dipyrromethene (BODIPY) dyes linked to viologen are prepared and their photophysical and electrochemical properties are investigated. Both synthesized molecules have similar electronic absorption spectra with the absorption maximum localized at 517 and 501 nm for dye 1 and dye 2 , respectively. They exhibit well‐defined redox behavior, highlighting the presence of BODIPY and viologen subunits, with little perturbation of the redox potential of both subunits with respect to the parent compounds. Both dyes are heavily quenched by photoinduced electron transfer from the BODIPY to the viologen subunit. The transient absorption technique demonstrates that dye 2 forms the viologen radical within a timeframe of 7.1 ps, and that the charge‐separated species has a lifetime of 59 ps. Sustained irradiation of dye 2 in the presence of a tertiary amine allows for the accumulation of BODIPY–methyl‐4,4′‐bipyridinium (BODIPY–MV + ), as observed by its characteristic absorption at 396 and 603 nm. However, dye 2 does not generate catalytic amounts of hydrogen under standard conditions.
BODIPY
Viologen
Photoinduced electron transfer
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We report the first theoretical calculations of excited state absorptions (ESAs) from the singlet and triplet excitons, as well as the key intermediate in the singlet fission (SF) process, the spin singlet multiexciton triplet-triplet state, for solid pentacene with herringbone crystal structure. Our goal is to compare theoretical results against ultrafast transient photoinduced absorption (PA) measurements and their interpretations, which have remained controversial. We show that the elusive triplet-triplet state absorbs both in the visible and near infrared (NIR), at or close to the PA energies assigned to the free triplet exciton. In contrast, the triplet PA has nearly vanishing oscillator strength in the NIR within the rigid herringbone structure. Observable oscillator strength for NIR triplet PA requires photoinduced enhancement of coupling between a pair of neighboring pentacene molecules that confers significant charge-transfer (CT) character to the triplet exciton. We discuss the implication of our results for efficient SF in pentacene and related materials.
Singlet fission
Pentacene
Oscillator strength
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BODIPY
Quantum yield
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Singlet fission (SF) is a process by which one excited singlet state yields two triplet states upon close interaction with a ground-state chromophore of the same kind. This photoreaction was first observed in solid state and has important implications in organic photovoltaics. Singlet fission was also reported in concentrated solutions, where the need for diffusion of the reaction partners slows the dynamics. This helps to single out reaction stages and to identify the involved species. In this work, ultrafast transient absorption spectroscopy and time-correlated single photon counting are applied to the concentration-dependent (from 10–1 to 102 mM) photodynamics of a tetrachlorinated phenazinothiadiazole in toluene. Time-resolved emission shows a monoexponential decay, which is constant across the emission band. The corresponding decay rate depends linearly on the concentration of the phenazinothiadiazole. Femtosecond transient absorption demonstrates that a concentration-dependent singlet-to-triplet conversion hides behind the emission decay which is diffusion controlled. Contrary to previous reports on SF in pentacenes and tetracenes, no indication of intermediate states has been found. Efficient, direct and barrierless SF is concluded. The strong enhancement of the triplet yield at increasingly higher concentrations of the thiadiazole indicates very efficient singlet fission with a triplet yield up to 189 ± 5%.
Singlet fission
Chromophore
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Using ultrafast transient absorption and emission spectroscopy, we probe the spatial dynamics of excitons during intramolecular singlet fission in asymmetric oligoacene heterodimers. Exciton-exciton correlations are crucial for promoting both triplet pair formation and recombination.
Singlet fission
Biexciton
Transient (computer programming)
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A series of new compounds in which various Bodipy dyes are grafted logically on triptycene rigid structures are synthesized and characterized, and their absorption spectra and photophysical properties are studied, also by pump-probe transient absorption spectroscopy. The studied compounds are: the mono-Bodipy species TA, TB, and TC (where A, B, and C identify different Bodipy subunits absorbing and emitting at different wavelengths), the multichromophore species TA3 , which bears three identical A subunits, and the three multichromophoric species TAB, TBC, and TABC, all of them containing at least two different types of Bodipy subunits. The triptycene moiety plays the role of a rigid scaffold, keeping the various dyes at predetermined distances and allowing for a three-dimensional structural arrangement of the multichromophoric species. The absorption spectra of the multichromophoric Bodipy species are essentially additive, indicating that negligible inter-chromophoric interaction takes place at the ground state. Luminescence properties and transient absorption spectroscopy indicate that a very fast (on the picosecond time scale) and efficient photoinduced energy transfer occurs in all the multi-Bodipy species, with the lower-energy Bodipy subunits of each multi-Bodipy compounds playing the role of an electronic energy collector. In TAB, an energy transfer from the A-type Bodipy subunit to the B-type one takes place with a rate constant of 1.6×10(10) s(-1), whereas in TBC an energy transfer from the B-type Bodipy subunit to the C-type subunit is bi-exponential, exhibiting rate constants of 1.7×10(11) and 1.9×10(10) s(-1); the possible presence of different conformers with different donor-acceptor distances in this bichromophoric species is proposed to cause the bi-exponential energy-transfer process. Interpretation of the intricate energy-transfer pathways occurring in TABC is made with the help of the processes identified in the bichromophoric compounds. In all cases, the measured energy-transfer rate constants agree with a Förster mechanism for the energy-transfer processes.
BODIPY
Triptycene
Chromophore
Picosecond
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A donor–acceptor dyad composed of a BF2-chelated dipyrromethene (BODIPY) and a C60 fullerene has been newly synthesized and characterized. The two moieties are linked by direct addition of an azido substituted BODIPY on the C60, producing an imino–fullerene–BODIPY adduct. The photoinduced charge transfer process in this system was studied by ultrafast transient absorption spectroscopy. Electron transfer toward the fullerene was found to occur selectively exciting both the BODIPY chromophore at 475 nm and the C60 unit at 266 nm on a time scale of a few picoseconds, but the dynamics of charge separation was different in the two cases. Eletrochemical studies provided information on the redox potentials of the involved species and spectroelectrochemical measurements allowed to unambiguously assign the absorption band of the oxidized BODIPY moiety, which helped in the interpretation of the transient absorption spectra. The experimental studies were complemented by a theoretical analysis based on DFT computations of the excited state energies of the two components and their electronic couplings, which allowed identification of the charge transfer mechanism and rationalization of the different kinetic behavior observed by changing the excitation conditions.
BODIPY
Chromophore
Photoinduced charge separation
Photoinduced electron transfer
Picosecond
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Transient absorption study of singlet excitons in crystalline peropyrene derivatives: examination of singlet fission process Akihiro Furube* 1 , Daiki Yamanaka 1 , Shinichiro Yanagiya 1 , Hiroyuki Matsuzaki 2 , Kazuyuki Uchida 3 , Takashi Kubo 3 1 Tokushima Univ,. 2-1, Minamijosanjima-cho, Tokushima, 770-8506, Japan 2 AIST, 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568, Japan 3 Osaka 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 (S 1 ) spilt into two triplet excited states (T 1 + T 1 ). Energy matching (S 1 ~ T 1 + T 1 ) 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 T 1 absorption 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 S 1 state is lower than twice the energy of the T 1 state in the crystalline state. In this case, singlet fission is expected to occur from highly excited singlet states (S n ). Under high-density excitation conditions, two singlet excitons collide each other to produce S n and S 0 through singlet-singlet exciton annihilation (S 1 + S 1 → S n + S 0 ). Thereafter the resulting S n 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 S 1 state around 700 nm. This spectral feature is similar to that of the T 1 state observed in solution, suggesting that the T 1 state is generated from the highly excited singlet state S n through 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 S 1 exciton in phenyl peropyrene is energetically low for undergoing singlet fission, therefore requiring additional energy. Further detailed discussion is given with other experimental results. Figure.1 Transient 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. References [1] Nichols, V. M.; Rodriguez, M. T.; Piland, G. B.; Tham, F.; Nesterov, V. N.; Youngblood, W. J.; Bardeen, C. J. J. Phys. Chem. C 2013 , 117 , 16802–16810. [2] Asahi, T.; Furube, A.; Fukumura, H.; Ichikawa, M.; Masuhara, H. Rev. Sci. Instrum. 1998 , 69 , 361. [3] Singh, R. B.; Matsuzaki, H.; Suzuki, Y.; Seki, K.; Minegishi, T.; Hisatomi, T.; Domen, K.; Furube, A. J. Am. Chem. Soc. 2014 , 136 , 17324–17331. Figure 1
Singlet fission
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