Singlet fission in pentacene dimers
Edited by David R. Reichman, Columbia University, New York, NY, and accepted by the Editorial Board March 9, 2015 (received for review November 26, 2014)
Significance
In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different regioisomeric pentacene dimers. Pump–probe experiments, which were complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 ± 5%. To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through-bond or through-space interactions, are decisive in tuning the rates of singlet fission.
Abstract
Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.
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Singlet fission (SF) is a spin-allowed process to convert one singlet excited state into two triplet excited states, namely a correlated triplet pair (1). The ability to effectively implement SF processes in solar cells could allow for more efficient harvesting of high-energy photons from the solar spectrum and allow for the design of solar cells to circumvent the Shockley–Queisser performance limit (2). Indeed, several recent studies have demonstrated remarkably efficient solar cell devices based on SF (3–6).
One requirement that needs to be met to achieve SF is that the photoexcited chromophore in its singlet excited state must share its energy with a neighboring ground-state chromophore. As such, the potential of coupled chromophores to afford two triplet excited states via SF has been elucidated in, for example, a tetracene dimer with an SF yield of around 3% (3, 7). Additionally, past experiments in single-crystal, polycrystalline, and amorphous solids of pentacene have documented that the efficiency of SF relates to the electronic coupling between these two chromophores (8, 9). Hence, molecular ordering in terms of crystal packing, that is, proximity, distances, orbital overlap, etc., is decisive with respect to gaining full control over and to fine-tuning interchromophoric interactions in the solid state (10, 11). Of equal importance are the thermodynamic requirements, namely that the energy of the lowest-lying singlet absorbing state must match or exceed the energy of two triplet excited states (S1 ≥ 2T1) (11). In light of both aspects, hydrocarbons such as acenes––tetracene, pentacene, hexacene––and their derivatives are at the forefront of investigations toward a sound understanding and development of molecular building blocks for SF. In tetracenes, the singlet- and triplet-pair energy levels are nearly degenerate (S1 = 2T1), leaving no or little standard enthalpy of reaction for SF (12). In solution, the latter is, however, offset by sizable entropy rendering the process rather slow and, thus, inefficient (13). In addition, the low SF yield relates to the dimer geometry. Its nature hinders electronic coupling through space, leaving only through-bond coupling effective. The latter is, however, insufficient to enhance the SF rate (7, 14). In stark contrast, the relaxed triplet excited state in pentacenes has significantly less than half the energy of the singlet excited state. In turn, the thermodynamic SF requirement, that is (S1 ≥ 2T1), is fulfilled for pentacenes rendering this process exothermic and unidirectional (13). Finally, from SF in hexacenes two triplet excited states plus a certain amount of phonons are derived as the main relaxation products. In other words, the dissipated heat leads to a decrease in SF yields and rates (15).
In terms of exploiting SF for improving device performances, e.g., hybrid solar cells, it is necessary to efficiently dissociate correlated triplet pairs as they are formed, to overcome triplet–triplet annihilation (5, 16). Rapid injection of electrons into fullerenes, perylene diimides, colloidal nanocrystals, semiconductor substrates, etc. suggests a viable strategy. If successful, two charge carriers might be produced per absorbed photon and the photocurrents of the device can reach external quantum efficiencies of more than 100% (5, 17, 18).
A provocative debate has been ignited about the mechanism of SF at the molecular level (19, 20). Controversy exists around the electronic states that are involved in the process, the coupling among them, and the effective nuclear dynamics (14, 21–27). Two contrasting SF mechanisms have been traditionally postulated––the direct and the two-step mechanism. These mechanisms differ in the number and nature of the electronic states that are involved in the SF process. For the direct mechanism, the nonradiative relaxation of the initially populated bright state proceeds via a correlated triplet pair state of singlet character––sometimes called multiexcitonic (ME) state––which then dissociates into two separated triplet excited states (28). For the two-step mechanism, the relaxation of the bright state occurs via an intermediate charge transfer (CT) state to the ME state. As in the direct mechanism, the ME state eventually allows the two triplet excited states involved to separate and undergo separate spin relaxation (11, 21–23). Recent works have, however, challenged these traditional viewpoints on the SF mechanism and several new models have been proposed. In particular, it has been suggested (13, 29, 30) that the initial excitation produces a coherent superposition of the lowest-lying absorbing state and the ME state, with the latter splitting into two separated triplet states after decoherence. On the other hand, recent theoretical works (14, 27) have proposed a model for SF in dimers, in which the ME state is formed from the absorbing state via a superexchange mechanism involving CT states, although the kinetics of the process is one-step–like. All of the above suggests that the traditional classification of SF mechanisms as direct or two-step is presumably too simple to describe the complexity of the process.
Until recently, most studies regarding SF have been carried out in the solid state (11). However, in a groundbreaking report, Friend and coworkers showed that SF could be observed in solution at room temperature for a pentacene derivative (31). Key to this discovery was the formation of an intermediate state via the collision of a singlet excited-state pentacene and a second pentacene that was in the ground state. This study broke for the first time, to our knowledge, the dogma of molecular order as a mandate for SF and demonstrated that the order and packing might not be as crucial as believed, leaving SF as an intrinsic property even in a state of disorder.
In this present contribution, we report on intramolecular SF within a set of three different pentacene dimers (Fig. 1), reaching triplet quantum yields as high as 156 ± 5%, as established by means of pump–probe experiments in solution. Experimental results are complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods. In the series of pentacene dimers, the two pentacenes are linked via a phenylene spacer in an ortho-, meta-, and para-arrangement to impose geometrical control, which influences through-space and through-bond couplings in the ground- and excited state. The electronic coupling element, which governs SF and triplet–triplet annihilation, is mediated through-bond in the linearly and cross-conjugated para- and meta-isomers, respectively. In the ortho-isomer, through-space coupling should dominate due to the unique spatial proximity of the pentacenes (32, 33). Through experiments, we establish a sound picture of the SF mechanism in covalently linked pentacene dimers, showing the involvement of CT states.
Fig. 1.
Results and Discussion
Synthesis.
Synthesis of the pentacene dimers was based on desymmetrization of 6,13-pentacenequinone to give building block 1 (Fig. 1) (34). Dimers o-2, m-2, and p-2 were then assembled to probe the effects of the geometry about the phenylene π-spacer. Sonogashira coupling of 1 with ortho-, meta-, and para-diiodobenzene afforded intermediates 3–5, respectively, in ∼70% yield. SnCl2-mediated reductive aromatization in the presence of aq. H2SO4 yielded finally dimers o-2, m-2, and p-2. The ortho-phenylene–linked pentacene dimer o-2 has reasonable solubility in solvents such as CH2Cl2, CHCl3, and THF, whereas the corresponding meta- and para-derivatives, m-2 and p-2 respectively, have poor solubility in these solvents.
Excited-State Energetics.
Steady-state spectroscopies were used to examine the singlet- (ESinglet) and triplet-state energetics (ETriplet) of the pentacene dimers (SI Appendix, Figs. S14 and S15). First, long wavelength absorption maxima in the 660–678-nm range in toluene and 666–691-nm range in benzonitrile were determined, together with short wavelength fluorescence maxima at 664 nm (m-2), 676 nm (o-2), and 670 nm (p-2) and fluorescence quantum yields ranging from 0.5% to 1.5% in toluene (Table 1).
Table 1.
| Pentacene dimer | Solvent | ESinglet [eV] | ETriplet [eV] | ΦF [%] | kCT [s−1] | kSF [s−1] | kTTA [s−1] | ΦTriplet [%] |
|---|---|---|---|---|---|---|---|---|
| m-2 | Toluene | 1.88 | 0.77* | 0.6 ± 0.1 | 3.97 ± 0.80 × 1010 | 1.11 ± 0.10 × 1010 | 3.85 ± 0.15 × 108 | 125 ± 5† |
| Benzonitrile | 1.86 | 0.4 ± 0.1 | 6.10 ± 0.70 × 1010 | 1.59 ± 0.15 × 1010 | 4.55 ± 0.15 × 108 | 156 ± 5† | ||
| o-2 | Toluene | 1.87 | — | 1.6 ± 0.2 | — | — | — | — |
| Benzonitrile | 1.85 | — | — | 2.00 ± 0.80 × 1012‡ | 8.33 ± 0.20 × 1010 | ‡ | ||
| p-2 | Toluene | 1.83 | — | 0.3 ± 0.1 | — | — | — | — |
| Benzonitrile | 1.79 | — | — | 3.70 ± 1.50 × 1011‡ | 5.78 ± 0.50 × 1010 | 130 ± 10 | ||
Singlet excited-state energy (ESinglet), triplet excited-state energy (ETriplet), fluorescence quantum yields (ΦF), rate constants for charge transfer (kCT), rate constants for triplet growth (kSF), rate constants for triplet decay (kTTA), triplet excited-state quantum yields (ΦTriplet) for m-2, o-2, and p-2 in toluene and benzonitrile.
*
Determined in frozen Me-THF at 77 K.
†
Determined by using the power-dependent method.
‡
The short-lived nature of the triplet excited states in o-2 hampers a meaningful triplet excited-state quantum yield determination.
From the corresponding energetic differences we determined lowest-lying singlet absorbing-state energies of 1.84 ± 0.05 eV for the pentacene dimers o-2, m-2, and p-2. At first glance, all three dimers give rise to absorption patterns that resemble those of 6,13-bis(triisopropylsilylethynyl)pentacene (TPc; see SI Appendix for structure), including vibrational fine structure in toluene and in benzonitrile in the range from 558 to 691 nm, although the absorptions of the dimers are red-shifted relative to TPc. A closer look at the ortho-dimer o-2 suggests, however, strong electronic couplings by means of through-space interactions in the ground state. This stems from the close spatial arrangement of the face-to-face aligned pentacenes (Fig. 1) as seen in the optimized geometries (see SI Appendix for details). Finally, triplet excited-state energies of 0.77 eV were derived for o-2, m-2, and p-2 from maxima at 1,610 nm observed in phosphorescence measurements at 77 K (SI Appendix, Fig. S16). Thus, the singlet and triplet energies of pentacene dimers 2 fulfill the thermodynamic requirement for SF, that is, twice the triplet excitation energy should not exceed the lowest singlet excitation energy. Secondly, to elucidate a possible CT state, we probed m-2 by means of square wave voltammetry. In benzonitrile (SI Appendix, Fig. S17), where solvent stabilization is effective, we derived a first one-electron reduction of −1.43 V and a first one-electron oxidation of 0.44 V (versus Fc/Fc+). Neglecting any Coulombic attractive terms, the energy level of the intramolecular CT state for m-2 would have a lower limit of 1.87 eV and is, thus, quite achievable during excitation. Please note that this intramolecular CT state places the one-electron reduced form on one pentacene and the one-electron oxidized form on the other pentacene, lowering attractive interactions and facilitating its formation.
Transient Absorption Spectroscopy and SF Kinetics.
As a reference, TPc in Me-THF (3.0 × 10−4 M) was examined in pump–probe experiments with an excitation wavelength of 656 nm to stimulate population of only vibrational states of the first singlet excited state (SI Appendix, Fig. S18). Additionally, the photon flux was adjusted to 6.6 × 109 photons per pulse to excite, on average, less than 10% of the ground-state TPc to rule out multiple excitations of a single molecule. The major deactivation pathway for photoexcited TPc includes a 12.3 ± 0.1-ns intersystem crossing of the singlet excited state to afford the corresponding triplet manifold. The corresponding triplet excited state deactivates with a rather long lifetime of 24.0 ± 0.5 µs to reinstate the singlet ground state. In terms of the singlet excited-state characteristics, maxima at 447, 508, 538, 572, 845, and 1,400 nm were noted, as well as minima at 595, 641, and 710 nm. Notably, the feature at 710 nm corresponds to stimulated emission. The triplet excited state has maxima at 465 and 498 nm and minima at 546, 587, and 641 nm in the differential absorption spectra. All of these observations are in excellent agreement with previously reported singlet and triplet excited-state absorptions of TPc in solution (31). When using singlet molar extinction coefficients of 8.3 × 104 M–1⋅cm–1 at the 447-nm maximum, in combination with triplet molar extinction coefficients of 1.45 × 105 M–1⋅cm–1 at the 498-nm maximum, the computed TPc triplet quantum yield is 16%. For this analysis, ground-state extinction coefficients of 3.3 × 104 M–1⋅cm–1 at the 641-nm minimum and 1.6 × 104 M–1⋅cm–1 at the 587-nm minimum were used.
Next, the three pentacene regioisomers o-2, m-2, and p-2 were examined in solvents of varied polarity (i.e., toluene, THF, and benzonitrile) with photoexcitation at either 610 or 656 nm as shown in Fig. 2 as well as SI Appendix, Figs. S19, and S20, respectively, and the same low photon flux described for TPc––vide supra. The meta-isomer (m-2) was examined in benzonitrile and at time delays as short as 0.5 ps singlet excited-state features are discernible in the form of maxima at 456, 510, 582, and 670 nm. In contrast with the slow intersystem crossing seen for TPc, these singlet excited-state features are rapidly replaced by strong absorptions, which are concentration independent in the range from 1.0 × 10−5 to 1.0 × 10−4 M and, thus, strictly unimolecular. These strong absorptions, which appear at time delays of around 200 ps, bear great resemblance with that of the triplet excited state of m-2. In particular, maxima at 477, 510, 855, and 972 are complemented by minima at 610 and 664 nm and are a perfect match to the photosensitized triplet excited-state spectra of the dimer m-2 (vide infra).
Fig. 2.
A particularly striking effect observed for m-2 during the accelerated triplet excited-state formation is the long wavelength blue shift, from 670 to 664 nm, that is accompanied by a 155 ± 10% intensification of the transient bleaching. This effect is notable upon 610-nm excitation rather than 656-nm excitation. If the ground-state absorption depletion in this spectral region were quantitative for the singlet excited state, this infers more than one triplet excited state is formed per dimer. Nevertheless, the triplet excited state is surprisingly short-lived and decays with concentration-independent kinetics. The only plausible modus operandi is based on intramolecular triplet–triplet annihilation governed by the unique scenario that more than one triplet excited state is localized on each dimer. Multiwavelength analyses attest that the triplet excited-state formation is biexponential in benzonitrile, and it features two distinct steps with CT (τCT) and SF (τSF) lifetimes of 16.4 ± 1.9 and 63.0 ± 6.3 ps, respectively. From the study of solvent dependence vide infra, we postulate the involvement of an additional state that possesses an intramolecular CT character and mediates the transition to the ME state (either directly or virtually). A closer look at the transient absorption spectra taken at delay times of 80–300 ps reveals a 445-nm shoulder, which is likely to stem from the CT state (SI Appendix, Fig. S21). Once formed, the triplet excited-state decay is monoexponential for m-2, with a rather short triplet–triplet annihilation lifetime (τTTA) of 2.2 ± 0.1 ns.
Turning our attention to the para- and ortho-isomers (p-2 and o-2), their singlet excited-state features are discernible immediately after photoexcitation in benzonitrile. For p-2, these are maxima at 462 and 1,117 nm and a minimum at 686 nm (SI Appendix, Fig. S19). The features of the singlet state of p-2, however, transform even faster than in m-2 into the triplet excited-state maxima at 475 and 508 nm as well as its minimum at 670 nm. From multiwavelength analyses we derive lifetimes for the growth and decay of the triplet excited state as 2.7 ± 1.0 and 17.3 ± 1.3 ps, respectively. It is likely that the dramatic difference in behavior in p-2 versus m-2 kinetics derives from enhanced through-bond electronic coupling in the ground state of the para-isomer as a means to impact both SF and triplet–triplet annihilation (32, 33). As such, the electronic coupling matrix elements as the decisive factor to govern CT are larger in p-2 than in m-2 (32). For o-2, the 472- and 507-nm fingerprints for the singlet and triplet excited states, respectively, evolve in pump–probe experiments. Here, in contrast with p-2 and m-2, strong through-space couplings between the two pentacene moieties affect the triplet excited-state kinetics––both in terms of formation and decay––and deconvolution results in upper values of 0.5 ± 0.2 ps and 12.0 ± 0.3 ps, for formation and decay, respectively. The short-lived nature of the triplet excited states in p-2 and o-2 hampers a meaningful triplet excited-state quantum yield determination. The latter is corroborated in the geometry-optimized structures (SI Appendix, Fig. S27), which reveal, on one hand, the coplanarity between the two pentacenes in p-2 and m-2, and, on the other hand, only sizable through-space couplings in o-2.
Triplet Quantum Yield and SF Yield Determination.
Using the differential absorption changes at 610 and 664 nm for m-2, for which ground-state extinction coefficients of 2.8 × 104 and 5.3 × 104 M–1⋅cm–1 have been determined, the singlet and triplet molar extinction coefficients are calculated as 7.9 × 104 and 1.4 × 105 M–1⋅cm–1, respectively. In recent studies, it was hypothesized that the bleaching at 610 nm is elusive for the triplet excited state, whereas that the bleaching at 664 nm serves as an internal standard to calibrate the singlet excited state (31). Because the meta-dimer m-2 has only very weak ground-state absorption at 456 nm, the observed amplitude of differential absorption at 456 nm and 0.5 ps relates entirely to the singlet excited-state concentration formed after excitation. In doing so, differential absorption changes of 0.028 a.u. at 456 nm and 0.0726 a.u. at 510 nm were converted into a triplet quantum yield (ΦTriplet) of 145 ± 10%. This high triplet quantum yield along with its ultrafast formation provides strong evidence that the triplet excited state for m-2 forms by intramolecular SF between the two pentacenes. The triplet quantum yield for p-2 was determined by extrapolation to time zero as 130%.
Taking the ratio from 510/456 nm as an indicator for the respective quantum yield, we note that it increases from toluene (126 ± 3%) to THF (132 ± 2%) and to benzonitrile (145 ± 10%). We surmise from this trend that the mediating step, which dictates the triplet excited-state formation, features CT-type character and this operates through-bond for dimer m-2. Such a postulate fits well with the fact that the triplet excited-state kinetic growth is biexponential and that the kinetics are solvent dependent. Biexponential fits of these data, for example, yields lifetimes of 25.2 ± 4.8/90.2 ± 9.0 ps in toluene, 18.6 ± 2.9/70.3 ± 7.0 ps in THF, and 16.4 ± 1.9/63.0 ± 6.3 ps in benzonitrile. In contrast, the intramolecular triplet–triplet annihilation lacks any appreciable solvent polarity dependence as evidenced by nearly constant lifetimes of 2.6 ± 0.1 ns in toluene, 2.5 ± 1.0 ns in THF, and 2.2 ± 0.1 ns in benzonitrile.
Alternatively, the power-dependent method using the differential absorption changes at 670 and 664 nm was used to calculate the transient extinction coefficient of the bleaching as 5.7 × 104 and 8.3 × 104 M–1⋅cm–1 for the singlet and the triplet excited state, respectively. Consequently, we obtained a triplet quantum yield for the meta-dimer (m-2) of 156 ± 5%. The main advantage of this method is that it allows the determination of transient extinction coefficients correlated by the amount of excited molecules in the exact range of excitation without using an external reference (31).
To complement the power-dependent studies, we also generated the triplet excited-state signatures of all three dimers by means of triplet sensitization experiments in toluene. In these experiments, an N-methylfulleropyrrolidine (N-MFP; see SI Appendix for structure) with a triplet quantum yield close to unity and a triplet excited-state energy of 1.5 eV was taken and selectively excited at 480 nm, which coincides with a minimum in the pentacene ground-state absorption. N-MFP (8.0 × 10−5 M) was photoexcited, leading to the sequential formation of its short-lived singlet and its long-lived triplet excited states with their 695- and 900-nm fingerprint absorptions, respectively. In the presence of the three pentacene regioisomers o-2, m-2, and p-2 as well as TPc in concentration regimes from 1.0 × 10−5 to 1.0 × 10−4 M, a set of newly developing transient absorption features evolves with the depletion of the N-MFP triplet excited state (Fig. 3 as well as SI Appendix, Figs. S22 and S23). The meta-dimer m-2, for example, shows a transient maximum at 506 nm and minima at 559, 605, and 661 nm. While for TPc pentacene, a 503-nm maximum as well as 547-, 592-, and 643-nm minima were noted, for o-2 and p-2, 512/507-nm maxima as well as 570-, 614-, 664-nm and 562-, 614-, and 668-nm minima, respectively, were recorded. Importantly, a closer look reveals that the decay and growth are pentacene concentration dependent. Complementary analyses of the N-MFP triplet excited-state decay as well as of the pentacene triplet excited-state growth provide the means to derive the second-order energy transfer rate constants (SI Appendix, Fig. S24). The rate constants in deoxygenated toluene are 1.6 ± 0.1 × 109 M–1⋅s–1 for o-2, 3.4 ± 0.3 × 109 M–1⋅s–1 for m-2, and 8.8 ± 0.5 × 109 M–1⋅s–1 for p-2, which infers that diffusion controls the transduction of triplet excited-state energy. Once formed, the pentacene triplet excited states in o-2, m-2, and p-2 are subject to concentration-dependent deactivations via efficient intermolecular triplet–triplet annihilation with lifetimes in the range of tens of microseconds. Intramolecular contributions under these conditions play no major role. A rate constant of 1.5 ± 0.5 × 108 M–1⋅s–1 was derived for m-2 from pseudo–first-order analyses (SI Appendix, Fig. S25). From Figs. 2 (i.e., direct excitation experiments) and 3 (i.e., triplet sensitization experiments) the singlet molar extinction coefficient at 456 nm, the triplet molar extinction coefficient at 506 nm, and the triplet quantum yield were determined for m-2 as 7.4 × 104 M–1⋅cm–1, 1.5 × 105 M–1⋅cm–1, and 130 ± 10%, respectively (see SI Appendix for details).
Fig. 3.
Theory.
To characterize the electronic states involved in the SF process, we have used the second-order complete active space perturbation theory with a complete active space self-consistent field reference wavefunction (CASPT2/CASSCF) approach (see SI Appendix for details). This ab initio multireference perturbation theory method is known to provide accurate results for electronic states of very different chemical character like those involved in SF (35). The results, depicted in SI Appendix, Table S1, reveal a set of closely lying electronic excited states of ME and locally excited (LE) character. Whereas in m-2 and p-2 the first excited state, S1, is of ME character and S2 and S3 are of LE character, respectively, this order changes in o-2. In all cases, however, the ME state is quasi-degenerate with the initially excited LE states and, therefore, a direct pathway for SF is energetically possible. The results obtained in vacuo predict CT states located at significantly higher energies than the LE and ME states. Additional calculations using the polarization continuum model (see SI Appendix for details) for benzonitrile reveal a pronounced stabilization of the CT states which locates them ∼0.35 eV above the lowest-lying LE and ME states in m-2 (SI Appendix). Taking into account the experimentally observed solvent dependence of the triplet yield and the kinetics, our results indicate that CT states participate in SF either as virtual states via a superexchange mechanism (14) or directly if nuclear relaxation of the lowest-lying absorbing state facilitates population of the CT state via internal conversion. In addition to the characterization of the excited singlet states, we also have characterized the excited triplet states of m-2. The results in SI Appendix, Table S3 reveal a strong T1 → T9 excitation at 2.43 eV, which corresponds to the pronounced triplet feature at 510 nm observed in the transient absorption spectra (Fig. 3).
Conclusions
In conclusion, we used pump–probe experiments and complementary theoretical calculations using multireference perturbation theory to corroborate for the first time, to our knowledge, intramolecular SF in pentacene dimers. Three regioisomeric pentacene dimers, in which the two pentacenes are linked via a phenylene spacer in ortho-, meta, or para-position––o-2, m-2, and p-2––feature triplet quantum yields as high as 156 ± 5% mediated either by through-space or through-bond electronic couplings in the ground- and excited state. As recently hypothesized, mixing between the singlet excited state and the CT state might be helpful in terms of overcoming the activation barrier (14). It is in m-2 where the right balance between triplet formation and triplet decay optimizes the SF yield. Such an efficient SF reaches well beyond the 3% mark found in tetracene dimers, for which, incidentally, it is the para-isomer that revealed the highest SF yields (7). The demonstrated depletion of the ground state, the quantitative analysis of forming more than one triplet excited state per singlet, and the very low concentrations used corroborate the intramolecular nature of SF. Although our studies demonstrated that the singlet excited states in all three different pentacene dimers are rapidly and efficiently converted to triplet excited states via SF, this is considered only as a preliminary step toward realizing SF enhanced devices. In this context, a sound platform en route toward charge separation with suitable electron acceptors and charge collection at suitable electrodes is forming.
Materials and Methods
The synthesis of o-2, m-2, and p-2, materials, general methods, and computational details are described in SI Appendix.
For the photophysical characterization (see SI Appendix for details), the samples were placed in fluorometric cuvettes with different pathways and, when necessary, purged with argon. Steady-state UV-vis absorption spectra were acquired at room temperature (RT) using a Perkin Elmer Lambda 2 spectrometer. Steady-state fluorescence spectra were carried out at a FluoroMax3 spectrometer from Horiba in the visible detection range (RT) and at a FluoroLog3 spectrometer from Horiba in the near infrared. Femtosecond (fs) transient absorption experiments were carried out with an amplified Ti:Sapphire CPA-2110 fs laser system (Clark MXR: output 775 nm, 1 kHz, 150-fs pulse width) using transient absorption pump/probe detection systems (Helios and Eos, Ultrafast Systems).
Details of the calculations can be found in the SI Appendix. Briefly, the geometries of o-2, m-2, and p-2 were optimized using density functional theory. In all systems, i-Bu3Si− groups have been modelled using H3Si− to ensure computational tractability. For the different systems investigated, vertical excitation energies were calculated at the CASPT2/CASSCF/ANO-S-VDZP level of theory (35, 36). In all calculations, an active space of 8 electrons in 8 orbitals (comprising the HOMO-1, HOMO, LUMO, and LUMO+1 orbitals per pentacene unit; see SI Appendix, Fig. S26) was used.
Acknowledgments
Generous allocation of computing time at the computing centers Erlangen (Regionales Rechenzentrum Erlangen), Munich (Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften in München), and Jülich (Jülich Supercomputing Centre) is gratefully acknowledged. Funding is gratefully acknowledged from the Emerging Fields initiative “Singlet Fission” supported by Friedrich-Alexander-Universität Erlangen-Nürnberg, as well as the Cluster of Excellence Engineering of Advanced Materials and “Solar Technologies Go Hybrid”––an initiative of the Bavarian State Ministry for Science, Research, and Art.
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Published online: April 9, 2015
Published in issue: April 28, 2015
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Acknowledgments
Generous allocation of computing time at the computing centers Erlangen (Regionales Rechenzentrum Erlangen), Munich (Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften in München), and Jülich (Jülich Supercomputing Centre) is gratefully acknowledged. Funding is gratefully acknowledged from the Emerging Fields initiative “Singlet Fission” supported by Friedrich-Alexander-Universität Erlangen-Nürnberg, as well as the Cluster of Excellence Engineering of Advanced Materials and “Solar Technologies Go Hybrid”––an initiative of the Bavarian State Ministry for Science, Research, and Art.
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This article is a PNAS Direct Submission. D.R.R. is a guest editor invited by the Editorial Board.
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The authors declare no conflict of interest.
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Singlet fission in pentacene dimers, Proc. Natl. Acad. Sci. U.S.A.
112 (17) 5325-5330,
https://doi.org/10.1073/pnas.1422436112
(2015).
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