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An Unusual Non-Emissive Behavior of Rubrene J-Aggregates: a Rare Violation Nikhil Aggarwal, and Archita Patnaik J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b02072 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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The Journal of Physical Chemistry

An Unusual Non-Emissive Behavior of Rubrene JAggregates: a Rare Violation Nikhil Aggarwal and Archita Patnaik* Colloid and Interface Chemistry Laboratory, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Abstract: Structure - property correlations in Rubrene (RB) colloidal J-aggregates were unravelled by steady state and time resolved spectroscopy in conjunction with excited state density functional calculations. The RB J-aggregate with a slippage angle θ = 30.4o, estimated from the monomeric transition dipole moment directions, exhibited a broad FWHM of 1073 cm-1 and a 5 λ = 1.80 D) almost nm red shifted absorption band carrying a transition dipole moment ( λ equivalent to the monomeric dye ( = 1.89 D). A significantly low magnitude of exciton coupling energy, ∆Eexc = -358 cm-1 for the rhombic-RB colloidal J-aggregates resulted owing to the weaker electronic communication between the largely separated RB subunits (r = 7.2 Å) and a restricted exciton delocalization over the RB J-dimer (N = 2). The RB J-dimer exhibited a perfect balance between the computed singlet (2.53 eV) and the triplet (1.29 eV) exciton energies for singlet fission (SF). Supporting this, the PL decay profile of the J-aggregates revealed a delayed fluorescence, substantiating triplet pair formation via SF. The experimental evidence for the long-lived triplet formation was furthermore confirmed by its transient absorption (T1 TN) at 530 nm. Consequently, a high probability for SF and a low probability for triplet-triplet recombination, leading to a dramatic lowering in photoluminescence quantum yield from 0.172 1 ACS Paragon Plus Environment


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down to 0.035 was noted. The electronic structure calculations for the RB J-dimer followed TDDFT-M062X/6-31G+(d,p) level of theory following integral equation formalism polarizable continuum model (IEFPCM) in water. S1 excited state for RB J-dimer was carefully analysed using integral overlap of electron and hole density distribution (φ) and the defined t-indexes along all three spatial directions, and was found to be of Locally Excited in character. Introduction: The self-assembly of organic molecules have led to formation of highly ordered functional aggregates with various applications in photovoltaics and field-effect transistors.1-2 Their various morphologies are classified into J- and H-types on the basis of characteristic slippage angle (θ), defined as the angle subtended by the line joining centers of two monomeric units and the polarization axis (cf. Figure 1(a)). Their characteristic photophysical behavior has been primarily governed by the mutual orientation of transition dipole moments (TDM) corresponding to monomeric electronic excitation in the supramolecular arrangements.3 Although θ ≤ 54.7° in J-dimers and θ ≥ 54.7° in H-dimers are validated, the TDM’s direction corresponding to monomeric units has shown a parallel orientation of monomers in both types of molecular dimers, depicted in Figure 1(a). It is well documented that the strong coupling of several similar monomers in J-aggregates results in a coherent excitation at a pronounced redshifted wavelength with reference to the monomer.2-3 In addition, the spectrum gets narrower4-6 along with the enhanced oscillator strength and the vibrational coupling to the molecular modes is seen to be largely absent.2 One of the intriguing photophysical behavior of J-aggregates following the molecular exciton coupling model3 is the aggregation induced enhanced emission.7-9 Numerous J-aggregates exhibit fluorescence with their quantum yield dramatically surpassing that of the monomeric dyes. However, it is well documented that the rapid internal

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conversion of the allowed higher energy exciton state to the forbidden lower energy exciton state quenches the fluorescence in H-aggregates (cf. Figure 1(a)).3 Since the discovery of J-aggregates, scientific interest in the optical, photophysical, and structural properties are limited to certain class of dyes namely cyanine, squaraine, perylene bisimide and boron-dipyrromethene2 with a plenty of space available for exploration of new molecular dyes.

Figure 1. (a) Schematic representation of J- and H- dimeric structures with characteristic slippage angle (θ), defined as angle subtended by the monomeric transition dipole moment (represented with arrows,

and the molecular centers. The exciton coupling energy (∆Eexc) is represented as the energy difference between the in-phase (E’) and the out-of-phase (E’’) arranged TDMs. The allowed transitions and the corresponding aggregation induced enhanced emission (AIEE) in J-dimer, in contrast to emission quenching in H-dimer are also depicted. b) Chemical structure of Rubrene (RB) along with salient features of its J-aggregates.

Polyacenes consisting of linearly fused benzene rings have been used in applications such as light-emitting diodes,10-12 liquid crystal displays,13 and organic field-effect transistors14-15 to name a few. Moreover, the solid polyacenes are known to exhibit a spin allowed singlet fission (SF) process, and have recently been incorporated into solar cells.16-20 A SF process involves conversion of one high energy singlet exciton to two nascent electron-hole pairs via ultrafast triplet state formation.21 Moreover, SF has been observed in polycrystalline and amorphous solids and in single crystals occurring at an ultrafast time scale (80 fs - 25 ps),20-21 and leading to the crystal’s emission from its triplet state relative to the isolated acene chromophore. Therefore, 3 ACS Paragon Plus Environment


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as SF takes place, the singlet excited state population within an acene crystal depletes resulting in a reduction of fluorescence quantum yield. Indeed, Schneider et al. in 1965 introduced SF to explain the fluorescence quenching in crystalline Anthracene22 which was further invoked to explain the temperature-dependent fluorescence quenching in crystalline tetracene.23 Therefore, SF can dramatically alter the photophysical behavior, particularly, the photoluminescence (PL) of acene molecular aggregates. The most recent reports investigating the SF largely centered on molecular solids of tetracenes and their derivatives.21,

24-25

This is because in the solid state,

tetracenes exhibit degenerate singlet excited state (S1) and a pair of correlated triplet excited states following E(S1) ≈ 2E(T1), that renders the energetics of SF negligible “(∆E1 = E(S1)- 2 x E(T1) ≥ 0)”. Rubrene (5, 6, 11, 12-tetraphenyltetracene) in Figure 1(b)) belongs to the group of polyacenes, consisting of a tetracene core with four phenyl group substituents attached to the two internal rings. Rubrene (RB) has intrigued Material Chemists for decades because of its classic field-effect transistor behavior; especially, 20 cm2 V-1 s-1 hole mobility at room-temperature measured in single-crystal organic field-effect transistors.26 Although extensive investigation on high-quality RB crystals towards transport behavior have been well documented in literature, their optical behavior, in particular PL, was explored by considerably fewer studies. In RB single crystals, it has been evident that large optical anisotropy and the experimental conditions had profound influence on its fluorescence behavior.27-28 Recently, a yellow-green and orange light emission both in solution and in the aggregated state was evident for two thienyl peripherally substituted RB analogues.29-30 Irkhin et al. proposed that the most commonly observed absorption and emission in bare RB single crystals were caused by the vibronic-induced depolarization of the electronic transition, predominantly c-polarized between the highest


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occupied and the lowest unoccupied molecular orbitals.31 The PL spectrum of RB exhibited prominent emission near 560 nm and 608 nm.27, 32 Moreover, the intensities of additional meagre emission bands around 645 nm and beyond were found to be more sample dependent and have been partially assigned due to oxidation.27,

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In contrast, Chen et al. presented a systematic

investigation on PL behavior of RB single crystal and concluded that rather than having a chemical impurity related origin, the 650 nm photoluminescence was exhibited by the amorphous RB molecules, energized by the triplets.33 Moreover, Podzorov et al. reported that in highly ordered RB crystals, energy can be transported over 2−8 µm primarily by the triplet excitons.34 In addition, the excited state dynamics of RB single crystals were explored using ultrafast spectroscopic methods under different excitation conditions, and SF was validated to play a predominant role in creating triplet excitons from the excited state (S1) relaxation.24, 34-35 In line with this, Ma et al. found an ultrafast S1 state relaxation with PL decay times of 5.2 and 51 ps in single crystalline RB, which was assigned to SF.24 Even if, the photo-oxidation and SF in RB are largely investigated, their consequential PL origin in RB is still debatable31

36-37

keeping fundamental questions unanswered. The motivation of this study was to explore whether the SF documented in RB crystals can occur in aqueous phase colloidal aggregates, significantly affecting their PL behaviour in line with the exciton coupling model or with finite deviation. While SF has been largely investigated in crystalline forms, solution phase colloidal aggregates enable to investigate SF using suspensions, permitting relevance of molecular arrangement and the associated charge transfer extents. In contrast to solid state SF, SF yields reaching as large as 200% in solution phase has recently been discovered for Pentacenes.38-40 Solution-state SF has also been demonstrated in tetracene dimers with an yield of 3%.25 Additionally, transient absorption and



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time-resolved emission experiments indicated that nanoparticles of photoexcited 5,12diphenyltetracene could undergo SF.41 Therefore, we have synthesized colloidal RB aggregates as aqueous suspensions via reprecipitation. In this report, we present a detailed spectroscopic characterization of aqueous RB colloidal aggregates to unravel their photophysical behaviour, in particular the unusual PL. The monomeric nuclear arrangements in molecular aggregates determine the excitonic behaviour, especially the delocalization extents and the associated charge-transfer contributions in accounting exciton wavefunction.1,

42

In a pioneering work by Scholes et al., the optical

absorption in case of J-aggregate geometry occurred to the lowest bound exciton state with predominant charge transfer (CT) character.1 Interestingly, SF in RB was also found to be highly dependent on the geometric arrangement of RB units. In a challenge to tune the SF and triplettriplet recombination in RB-based OLEDs, the molecular spacing (d) was varied and using magneto-electroluminescence, it was observed that triplet-triplet fusion increased, while the increase in 'd' from 1.8 to 5.0 nm led to a reduction in SF.43 Based on the many-body perturbation calculations, Wang et al. suggested higher experimental SF efficiency in metastable triclinic and monoclinic forms by meeting energy conservation and exhibiting a more CT like character of the singlet exciton wavefunction, as compared to the orthorhombic RB.44 Therefore, in the present investigation, the accurate determination of singlet excited state character in RB colloidal aggregate was established via theoretical calculations. These colloidal J-aggregates displayed excited state characteristics similar to RB films and single crystals along with an unusual PL behaviour, attributed to SF. (i) Experimental Methods: (a) UV-Vis Absorption and Steady State Emission Characteristics:


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RB was purchased from the Sigma–Aldrich and stored in dark at room temperature before and after spectral acquisition. Its colloidal aggregates were synthesized by injecting a 0.1 mL of 1.0 mM RB solution [in Tetrahydrofuran (THF)] into a 2.9 mL of vigorously stirred ultrapure water of 18.2 MΩ.cm-1 resistivity. Coalescence followed by spontaneous formation of RB colloidal aggregates yielded a pale pink suspension, which was preserved in the dark to avoid photo-oxidation. N2 degassed RB aggregate solution for steady-state absorption and emission experiments were taken in a 1 cm path length quartz cells and were carried out using standard methods and instruments. The oscillator strength for an electronic transition was calculated using: f = 4.32 x 10-9 ε. ν ,45 where ε. ν , represents area under the curve in ε(M-1cm-1) vs.

ν (cm-1) plot. The as-obtained value of oscillator strength as a theoretical quantity was converted

, using (D = ( to a more meaningful experimental quantity as the TDM ( π

.

/ =

.ν

0.461 x %& x λ (nm).45 The magnitude of excitonic coupling energy was calculated as ∆Eexc

2.h.c (λ

− λ

=

. The PL quantum yields (PLQY) for RB monomer in THF (nTHF = 1.407) and

RB-aggregate in water (nw = 1.330) were determined using Fluorescein (0.1 N NaOH, nNaOH = 1.335) as a standard.46 (b) HR-Scanning Electron Microscopy Characterization: Morphology of the aggregates was collected using a field emission scanning electron microscope. Sample preparation was done by drop-casting aqueous suspensions of RB aggregates onto a glass microscope slide and allowed to dry overnight in dark. HRSEM experiments were carried out using the instrument Inspect F50 - FEI, USA. (c) Time Correlated Single Photon Counting (TCSPC) Experiments: Time-resolved PL decay curves were measured using the time-correlated single-photon counting technique; here, nanosecond pulsed excitations from light-emitting diodes of 296 nm 7 ACS Paragon Plus Environment


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and 496 nm with an instrument response function = 260 ps were made use of with a FluoroCube, Horiba Jobin Yvon Inc. fluorescence lifetime system. The resulting fluorescence emission from N2 degassed samples, housed in a 10 mm path length quartz cell, was collected at right angles to the sample. (d) Nanosecond Laser Flash Photolysis Experiments: Transient absorption spectra (TAS) were recorded for RB aggregates using a nanosecond laser flash photolysis instrument (Applied Photophysics, U.K.). The transient signals were detected using a 150 W pulsed xenon lamp, a Czerny−Turner monochromator, and Hamamatsu R-928 photomultiplier tube as a detector. The Agilent infiniium digital storage oscilloscope captured the transient signals, followed by data transferring data to the computer for further analysis. Colloidal aggregates suspended in a long-necked cuvette (10 mm path length) were deoxygenated by constant bubbling with argon gas for 45 min prior to the laser irradiation. Decay profiles were collected typically as the average of 4-8 shots (126 maxia) at 1-2 Hz. (ii) Computational Methods: (a). Electronic Structure Calculation of RB Monomer and the Respective J- dimer: The ground state optimization and electronic structure computation of RB monomer was undertaken at DFT/6-31G+(d,p)/B3LYP and TD-DFT/6-31G+(d,p)/B3LYP levels respectively following an integral equation formalism polarizable continuum model (IEFPCM) in THF using Gaussian 09 set of programs.47 The potential energy curve for the ideal J-dimer (θ = 0o) was computed using a rigid monomer geometry, fully optimized at DFT/6-31G+(d,p)/M062X level, with M062X as a hybrid meta-exchange correlation functional,48 previously bench-marked48-51 for dimers. The equilibrium dimeric conformation of RB was unraveled by computing the globally optimized structure using the single crystal coordinates52 at DFT/6-31G+(d,p)/M062X 8 ACS Paragon Plus Environment

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level in water. All optimizations were performed with tight convergence criteria without any symmetry constraints, followed by frequency calculations to assure the global minimum. The RB dimeric stabilization energy was computed as the absolute energy difference between the RB dimer and double that of the RB monomer, optimized at the same theory level i.e. DFT/631G+(d,p)/M062X with IEFPCM in water. In this contribution, the accurate singlet and triplet state energies of the RB dimer were estimated following TD-DFT/ M062X/6-31G+(d,p) method with IEFPCM in water. The dark or bright character of the singlet excited states was ascertained from the computed magnitudes of the oscillator strength (f). (b). Characteristics of Low-lying Singlet Excited States: Locally excited (LE) and CT states can be unraveled by well-established electronic structure methods, such as, the configuration interaction singles, TD-DFT, or equation-of-motion coupled-cluster singles and doubles (EOM-CCSD). Therefore, with a primary focus towards characterization of low-lying singlet excited states, in this contribution, LE or CT characteristics of the RB dimeric singlet excited states were explored following TD-DFT-M062X/6-31G+(d,p) method with IEFPCM in water. The LE and CT singlet states were visually inspected using the charge density difference (CDD) plots, procured from the computed electron (EDD) and hole (HDD) density distribution maps, following Multiwfn 3.3.8.53 Electron (ρ+ (,) and hole (ρ- (,) density distribution maps are characteristic regions belonging to photoexcited states. Tian Lu and Cheng Zhong described the EDD and HDD in terms of molecular orbital wavefunctions (Φ) and configuration coefficients (w) corresponding to transition of an electron from an occupied MO (i) to a virtual MO (l) upon electronic excitation,53 vide equations 1 and 2. ρ+ (, = /(012 Φ2 (, Φ2 (, + / / 012 01 Φ2 (,Φ (, (1 1→2

1→2 1→52



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ρ- (, = /(012 Φ1 (, Φ1 (, + / / 012 062 Φ1 (,Φ6 (, (2 1→2

1→2 651→2

The above formalism was used in the present investigation for computing EDD and HDD maps for the RB monomer and the dimer at TD-DFT/6-31G+(d,p) level of theory with IEFPCM in THF and water respectively. Following Multiwfn 3.3.8, the spatial extent of LE or CT were computed from the centroids of electron (C-) and hole (C+) density distribution, that are qualitative and easy-to-visualize. The two centroids of charges associated with the positive (C+(r)) and negative (C-(r)) density regions were defined as: 8- (, = 9- : ;−

(< − 2=-@ 2=-B

8+ (, = 9+ : ;−

(< − 2=+@ 2=+B

The direction of RB monomeric TDM was thus defined as the line passing through the centroids of EDD and HDD maps for the associated monomeric electronic transition. This approach in deducing an accurate TDM direction was previously tested by us on a p-nitroaniline monomer.51 Moreover, in the optimized RB dimeric structure, the intermolecular distance 'r' along with the slippage (θ) and the polarization (α) angles were obtained from two defined positions placed at the centers of the RB subunits and the as-computed monomeric TDM. Obtaining quantitative extents of CT is far from trivial both from theoretical and experimental perspectives. With the aim in defining the CT spatial extent associated with the electronic transitions, the indexes EF GF H and t, previously developed for the evaluation of excited state characteristics,54-55 were estimated. Briefly, these indexes were based on the


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computed electron (ρ-(r)) and hole (ρ+(r)) density distribution of the excited states. The overlap integral (EF GF H ) of C- and C+ for the associated electronic transitions was used as a quantitative parameter for differentiating the excited states into either LE or CT state:

EF G F H = I

%8- (, %8+ (, , (5 99+

Finally, the “t” indexes representing the extent of intermolecular degree of separation between hole and the electron were calculated as: K LÅN = OP LÅN − QLÅN (6 The quantitative expressions for root mean square deviation (σ) of C- and C+ along the three axes in equation 3 and 4, normalization factors (A- and A+) in equation 5, Dct and H indexes in equation 6 were originally defined by Bahers et al.54 and in the present work were evaluated following Multiwfn 3.3.8. For t < 0, an overlap between hole and electron was expected and thus the associated electronic excited state was ascribed as LE. However, a clear separation of hole and electron on individual monomeric units leads to a larger positive value of t-index, making the associated electronic excited state to be of predominant CT type. In this regard, using the total EDD and HDD computed for the singlet excited states for RB dimer, the evaluation of the above mentioned indexes on a grid of points with isovalue 0.001 around the RB dimeric structure was established. Numerical integration procedure was adopted to evaluate the above explained density derived quantities using Multiwfn 3.3.8. A combined use of EF GF H and t-indexes in all three spatial directions was used to ascertain the CT or LE character of the lowlying singlet excited states of RB dimer.



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Results and Discussion: (i) Steady State Absorption Characteristics: Validating the J-aggregation of RB Optical spectroscopy is the primary tool to identify aggregation of dye molecules and to unravel the mechanistic details of electronic processes associated with the formed aggregates. Solvent polarity dependent UV-vis spectroscopic signatures of RB were investigated to unravel the aggregation pattern from the monomeric chromophore. THF was a good solvent for RB and no ordered molecular aggregate was observed in this solvent. On the other hand, RB was sparingly soluble in water which promoted the aggregation of this non-polar molecule. So, the absorption characteristics of RB was investigated by first dissolving it in THF and then water was added gradually keeping the RB concentration fixed at 1.67 x 10-5 M. Figure 2(a) shows the solvent polarity-dependent absorption spectra for RB in THF-water binary mixture. At 0 ≤ fw (water volume fraction) ≤ 0.80, no perceptible change with solvent polarity was observed. At 0.80 ≤ fw ≤ 0.97, a new UV-vis absorption band was found at a considerably longer wavelength (λagg = 531 nm) than the monomeric absorption maximum at λmon = 526 nm, indicating the formation of self-assembled RB aggregates. The experimental UV-vis absorption band of RB monomer depicted in Figure 2(b) was deconvoluted into four bands showing maxima at 526 nm, 491 nm, 461 nm and 433 nm, consistent with a previous report.24 The absorption band corresponding to the lowest electronic transition at Rmon = 526 nm possessed a significant

ST = 1.89 D), cf. Table S1, SI. The detailed nature of this oscillator strength of 0.032 ( electronic transition, specifically the involved molecular orbital contributions and the thus calculated TDM direction of the RB monomer are presented under the computational results section.


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Figure 2. (a) Solvent polarity dependent UV-vis absorption spectra at a fixed Rubrene (RB) concentration (1.67 x 10-5 M) with varied volume fraction of water (fw). The formation of self-assembled molecular aggregates of RB is indicated with a red shifted absorption band (λagg, blue curve). (b) The deconvoluted UV-vis absorption spectrum of RB monomer depicting the associated electronic transitions along with λ ) for the lowest FWHM and experimentally calculated monomer transition dipole moment (

electronic transition located at λmon = 526 nm.

The HR-SEM image in Figure 3(a) for RB obtained at fw = 0.97 solvent composition validated the formation of colloidal aggregates with spherical morphology (ca. 50 nm by HRSEM, see Figure 3(a)). Moreover, the abrupt transition from a RB monomer to an aggregated phase was also evident with a concomitant visible color change from orange to pink (cf. Figure 3(b) inset). The deconvoluted UV-vis absorption spectrum of RB colloidal aggregates in Figure 3(b) depicted the transition to fit to Gaussians centered at 531 nm, 494 nm, 464 nm and 437 nm, in agreement with the reported values for RB single crystals and thin films.24 Accordingly, the 531 nm red-shifted feature confirmed the prevalent J-type aggregation of RB. Although this RB J-aggregate exhibited a perturbation of only 5 nm red shift, but a significant modification of the vibronic progression suggested that the phenyl substituents steric bulk was insufficient to limit the coupling between the RB monomers. It was also interesting to note that in contrast to the bare tetracene where the oblique arrangement of monomers in a single crystal resulted into an almost equal distribution of monomer oscillator strength between two absorption bands,56 the RB J13 ACS Paragon Plus Environment


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aggregate predominantly exhibited the lower energy electronic transition (Ragg = 531 nm) with UVV = 1.80 D (f = 0.029). This suggested that the aggregation of tetracene based derivative (RB) in solution strongly depended not only on the π-stacking forces, but on the bay substituents as well.

Figure 3. (a) The HR-SEM image of Rubrene at THF: water volume fraction of 0.1:2.9 depicting the selfassembled colloidal RB-aggregate. (b) Deconvoluted experimental UV−vis absorption spectrum of RB aggregate (1.7 × 10−6 M) in THF-water binary mixture depicting predominant J-absorption feature at λagg= 531 nm with a transition dipole moment of 1.8 D and exciton coupling energy of -358 cm-1. Inset depicting the colour change on aggregation.

The prominent absorption band centred at Ragg = 531 nm for RB molecular J-aggregate showed certain salient photophysical features which were largely atypical to the characteristics of well-studied classical J-aggregates of Cyanines, Perylene Bisimide (PBI) and others. The calculated ∆Eexc = - 358 cm-1 corresponding to 5 nm bathochromic shift for RB J-aggregate (cf. Figure 3(b)) could be considered as a meagre change in comparison to large exciton coupling energies reported for other J-aggregate dyes.2 It was worth noting that the experimental TDM for RB J-aggregate was almost equivalent to the RB monomer, providing an initial indication of weaker electronic communication between the two RB subunits. Moreover, the lower magnitude of ∆Eexc for RB J-aggregate could be rationalized with the molecular exciton coupling theory 14 ACS Paragon Plus Environment

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proposed by Kasha et al.3 Accordingly, the ∆Eexc between the individual monomers and the chromophoric aggregate could be estimated as: ΔEYZ[ =

(cos(α – 3cos (θ (N − 1 2M (7 4πɛ r N

), the monomeric Here, ∆Eexc of molecular aggregates is dependent on the monomeric TDM ( units center-to-center distance (r), the angle subtended by polarization axes for the absorbing units (α), slippage angle (θ), and number of coherently coupled monomeric units (N). Therefore, a significantly low TDM for the RB monomer could be a reason for the observed low ∆Eexc. Moreover, the steric hindrance provided by the phenyl substitutions could be another factor for ∆Eexc lowering as it could minimize the π–π interaction between RB subunits by increasing the intermolecular distance. This meager π–π interaction was substantiated experimentally from the weak diffraction peaks [(002) and (010)] observed in the RB colloidal J-aggregates (cf. XRD pattern in Figure S1, SI). These diffraction peaks at 6.8o and 12.5o indicated a significant crystalline feature resembling Rhombic-RB nanoparticles.57 In contrast, the literature reported strong peak at 6.7o for the (002) plane of RB crystal was attributed to the Hexagonal-RB NPs. It is widely documented that, the exchange narrowing in J-aggregates resulted in a reduction of line-widths compared to the monomers by a factor N-1/2, where N is the number of molecules over which the excitons are delocalized.4-5 A PBI derivative containing hydrogens instead of trialkyloxyphenyl substituents in the imide positions exhibited J-aggregation with strong narrowing of the red-shifted absorption band from a FWHM of 2393 cm-1 down to 885 cm-1.58-59 In contrast, in the present investigation, the almost identical values of the monomeric FWHM = 1042 cm-1 and J-aggregate FWHM = 1073 cm-1 indicated the exciton delocalization to be strictly confined to only a dimeric unit (i.e. N = 2). However, it should be mentioned in this regard that



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the aggregation number “N” does not indicate the molecular aggregate dimensions, but relates to the coherently coupled monomeric units that undergo mutual spectral perturbation. (ii) Unusual PL Behavior of RB J-aggregate: Steady State Emission Characteristics To obtain insight into the aggregation-induced PL characteristics, solvent polarity controlled emission signatures of RB were collected (Figure 4(a)) using steady state emission spectroscopy. The inset micro-imaging in Figure 4(a) depicted the RB monomer at 1.67 x 10-5 M in pure THF exhibiting a bright yellow fluorescence under UV lamp, illuminated at 365 nm. The emission spectrum in Figure 4(a) for the RB monomer in pure THF (fw = 0) was acquired at a monomer absorption maximum of 525 nm. The two peaks at 548 and 590 nm could be identified as the 0–0 transition and its 0–1 vibronic band respectively. The calculated PLQY for RB monomer using Fluorescein as a reference (see Figure S2, SI) was found to be ΦF = 0.172. In literature, the J-type aggregation is primarily associated with a dramatic improvement in fluorescence quantum efficiency.7-9 Therefore, RB J-aggregate could be thought of supporting 'aggregation induced enhanced emission' or superradiance phenomenon. Much to our surprise, at fw = 0.97, the emission of the RB J-aggregate was drastically reduced (cf. micrograph in Figure 4(a) inset) in reference to the pure monomeric THF solution, indicating a virtually non-emissive behavior of the former. As a consequence of enhanced intermolecular interaction in the RB Jaggregate, spectroscopically the monomeric emission intensity at 548 nm abruptly decreased with the appearance of a red shifted broad emission band at ca. 552 nm (Figure 4(a)). Subsequently, the experimental PLQY for the RB J-aggregate in water (fw = 0.97) was estimated to be low (ΦF = 0.035, see Figure S3, SI for calculation). Although, this reduction in PLQY upon aggregation provided a clue for the possibility of a faster non-radiative deactivation channel for


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S1 state, but the reason for such emission quenching, especially in J-aggregates, was a huge deviation.

Figure 4. (a) Emission spectra of Rubrene (RB) monomer in THF (excitation wavelength = 526 nm) and J-aggregate in water (excitation wavelength = 531 nm) depicting the dramatic fluorescence quenching of RB J-aggregate with Photoluminescence quantum yield (PLQY) of 0.172 down to 0.035. Inset depicting the visual color change on J-aggregation under UV lamp exposure at 365 nm. (b) Time dependent UV-vis absorption and corresponding emission feature of RB J-aggregate in air depicting the stability towards photo-oxidation. In RB single crystal, previous investigations have found evidence of band gap states

introduced by the oxidation of RB. Mitrofanov et al. found that an oxygen-related state in RB crystals provided an additional radiative recombination path for molecular excitons resulting in emission at 1.92 eV, positioned about 0.25 eV below the lowest 0-0 transition. Here, the impurity state quenched the radiative recombination (S0S1= 2.17 eV) observed in high quality oxygenfree RB single crystals.27 While the nature of the state is still unclear, rubrene peroxide was thought to be the dominant oxygen-related impurity. Indeed, Kafer et al. in their recent report found rubrene peroxide (C42H45O2) on the surface of RB crystals.60 Another study on RB photooxidation indicated a bleaching effect due to rubrene peroxide formation.32 Thus, in order to establish the RB J-aggregates to be unperturbed by oxidation, their absorption spectra as a function of irradiation time were recorded in air (cf. Figure 4(b)) which showed meager changes in the absorption intensity (inset Figure 4(b)). Moreover, upon prolonged illumination of RB J17 ACS Paragon Plus Environment


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aggregate in an atmosphere of pure oxygen, no qualitative change in their PL spectra was noted. In both cases, other than a meagre reduction of absorption and PL intensities, the absence of any new spectral features discarded the formation of rubrene peroxide. Another process which could complicate the excited state dynamics in RB J-aggregates is SF, supported from unprecedented experimental and theoretical reports in single crystals, polycrystalline and amorphous solids of RB.

24, 34-35

In a most recent report, Ma et al. found an

ultrafast decay channel for singlet excited states with decay times of 5.2 and 51 ps in RB single crystal, attributing it to SF.24 In the present investigation, in order to assess the possibility of SF in RB J-aggregate and thus to explain the corresponding dramatic drop in PLQY, time-correlated single photon counting experiments were undertaken. (iii) Delayed Fluorescence and Transient Triplet Absorption in RB J-aggregate and Correlation with its Unusual PL Behavior: SF was detected in acene crystals via classic signatures of delayed emission in timecorrelated single photon counting measurements.22, 61-62 SF involves a singlet excited molecule sharing its energy with a ground-state neighbour. Therefore, it is unlikely that SF can occur in any isolated chromophore.21 The lifetime measurements in the present experiments were performed on the very dilute 1×10−7 M RB solution in THF with well separated RB monomers and their negligible molecular interaction. Concomitantly, the decay profile in Figure 5(a) depicted a single exponential decay with a singlet excited state lifetime, τ = 9.8 ns.


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Figure 5. (a) Time correlated photoluminescence decay profiles (with their fits) for Rubrene (RB) monomer (blue dotted) and J-aggregate (red dotted) acquired at 496 nm excitation, depicting a short lifetime component along with an onset of long lifetime component in J-aggregate. (b) The emission decay profile corresponding to the RB J-aggregate plotted on a log-scale to further indicate the long-lived emission tail observed in (a). (c) Transient absorption spectra for the rubrene colloidal J-aggregates at different delay times depicting triplet absorption. Inset depicts transient kinetics at 530 nm (d) TCSPC emission decay profiles at varied emission wavelengths for RB J-aggregate acquired at 295 nm excitation, depicting an absence of delayed fluorescence at lower emission wavelength.

The time-resolved emission spectral profile for RB J-aggregates excited at 496 nm and monitored at 550 nm emission is depicted in Figure 5(a). Three exponential decays with the fastest component's lifetime, closer (τ1 = 0.85 ns, relative amplitude = 22.3%) to the IRF is noted. A slower time constant of τ2 = 2.62 ns (relative amplitude = 50.8%) was attributed to the decay of S1 state via radiative fluorescence emission. In comparison with the monomeric RB, the



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lifetime of the RB J-aggregates PL decay showed obvious shortening. Notably, in the highly polar solvent (water), the red-edge emission in PL decay profile did not converge to a zero baseline, indicating the presence of a long-lived emitting species. In Figure 5(b), the emission decay profile corresponding to the RB J-aggregate was plotted on a log-scale to provide a clear depiction of the long-lived emission tail. In fact, this type of delayed fluorescence following an essential energetic criterion E(S1) ~ 2xE(T1) can be understood as : the initially created triplet excitons63-64 from the singlet excitons subsequently underwent triplet-triplet fusion following radiative decay from the singlet state.63, 65 Indeed, the delayed fluorescence on the microsecond time scale in RB single crystal was first observed by Liu et al., and was attributed to triplet−triplet annihilation.66 Since then, the transfer from singlet to triplet via SF followed by delayed fluorescence has been confirmed in RB by many studies35,

66-68

and the triplet-state

lifetime in single crystals was observed as high as 100 µs.35 Additionally, in RB polycrystalline thin films, Piland et al. observed geminate triplets formed via thermally activated SF process, that remained correlated with each other for >100 ns and concomitant initial fast decay of the primary fluorescence followed by a long-lived delayed PL signal were witnessed.69 During such a time span, the triplets diffused throughout the crystals and either a) experienced triplet–triplet recombination that produced emissive singlet excitons or b) non-radiatively decayed with emission of phonons. In view of this, the slowest third PL decay component (τ3 = 24.9 ns, relative amplitude = 26.8%) in RB J-aggregate (cf. Figure 5 (a) and (b)) was ascribed to the delayed fluorescence occurring via SF followed by triplet−triplet recombination. Subsequently, a triplet formation via SF in RB J-aggregates was assessed via Flash photolysis experiments. In Figure 5(c), the transient absorption spectra in RB J-aggregates measured under 500 nm excitation led to a positive absorption band at 530 nm, in agreement


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with the reported triplet-triplet absorption (T1 TN) in RB solution at 510 nm70-71 where the 20 nm red-shift could be attributed to the solvation effect. Furthermore, in view of the reported very low intersystem crossing efficiency for RB solution (φisc < 0.05),70 triplet formation via intersystem crossing in the RB J-aggregates was ruled out. Therefore, the triplet absorption signal (T1 TN) in the RB colloidal J-aggregates in figure 5 (c) provided concrete evidence of the triplet state (T1) formation via singlet fission. For the emission characteristics of the RB colloidal J-aggregates in water, a lower excitation wavelength of 296 nm was also used and the corresponding emission decay profiles were monitored at varied emission wavelengths. As shown in Figure 5(d), the PL decay at different emission wavelengths in the range 540 nm - 560 nm exhibited signatures of delayed fluorescence, similar to that observed at 496 nm excitation. In sharp contrast, the decay profile collected for 320 nm emission was greatly accelerated with 99% of the total decay occurring within the instrument’s time resolution (260 ps) along with complete diminishing of delayed fluorescence. Accordingly, we assigned RB J-aggregate to undergo SF from both the lower as well as the higher energy singlet excited states with fluorescence quenching, a rare case for RB J-aggregates in solution. Recently, an ultrafast (200 fs) direct SF from a higher energy singlet excited state was observed in the RB crystal.24 Furthermore, two-photon-induced direct SF in RB single crystal was observed from low-lying singlet excited states, competing with the internal conversion rate.72 (iv). Corroborating Singlet Fission via Computationally Estimated RB J-dimer Energetics: The computed ground state optimized structure of RB monomer in Figure 6(a) depicted the non-planar tetracene core (dihedral angle, C1-C2---C3-C4 = 13.5o) with the four fused phenyl rings lying almost perpendicular to the inner plane, consistent with the previous report.52 The 21 ACS Paragon Plus Environment


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computed zero dipole moment for the RB monomer reflected its centrosymmetric nature. The calculated excited state energies for the RB monomer at TD-DFT–B3LYP/6-31G+(d,p) level with IEFPCM in THF were computed as E(S1) = 2.22 eV , and E(T1) = 1.22 eV, leading to a large negative ∆E1 = E(S1) – 2x(T1) value (-0.22 eV), and thus evidenced a thermodynamically inactive SF in RB monomer (cf. Figure 6(b)). The computed 2.22 eV electronic transition for the RB monomer involved 97% HOMO LUMO weighing contribution for which the computed HDD and EDD maps (Figure S4, SI) showed a symmetrically distributed isosurface on tetracene core unit. Further, the similarity of EDD and HDD maps with the ground state LUMO and HOMO isosurfaces of the RB monomer was attributed to a single MO pair excitation process. The computed configuration coefficients for allowed electronic excitations along with the centroid coordinates of EDD and HDD for RB monomer are tabulated in Table S2, SI, which were further used for estimation of the TDM direction. Accordingly, the computed TDM direction in Figure 6(c) was depicted as a vector passing through the longer molecular axis of RB monomer with a magnitude of 4.68 D.

Figure 6. (a) The ground state optimized structure of Rubrene (RB) monomer obtained at DFT/B3LYP/631G+(d,p) level with IEFPCM in THF depicting non-planar tetracene backbone with a dihedral angle of 13.51o. Hydrogen atoms are removed for clarity. (b) The electronic structure of RB monomer computed at TD-DFT/B3LYP/6-31G+(d,p) level with IEFPCM in THF depicting energetically unfavorable SF. (c) The computed transition dipole moment of RB monomer procured from the computed electron (yellow) and hole (red) density distribution centroids.


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Since excitonic delocalization for RB colloidal J-aggregate was established to be restricted to only a dimeric unit, to obtain a deeper insight of PL behavior in RB J-aggregate, the computed energetics and characteristics of low-lying singlet excited states in its dimeric form were detailed in Figures 7 and 8 respectively. The equilibrium structure of RB monomer could be thought of supporting an ideal J-dimer (θ = 0o) formed via minimal steric repulsion while ruling out the ideal H-dimer (θ = 90o) formation due to steric hindrance. In Figure S5, SI, the computed potential energy curve as a function of intermolecular separation depicted positive interaction energy at an equilibrium conformation, implying a thermodynamically non-existent head-to-tail packed RB ideal J-dimer. Subsequently, the ground state optimized structure of RB dimer, using its single crystal coordinates52 in Figure 7(a) converged to a J-dimeric conformation with slippage and polarization angles of 30.4o (< 54.7o) and 0o respectively, substantiated with directions of monomeric TDMs from monomeric EDD and HDD maps (cf. Figure S4, SI). Moreover, the intermolecular distance of r = 7.2 Å (albeit at a larger than typical intermolecular distance due to the phenyl side groups)73 and an interplanar separation of d = 3.64 Å, typical of π-π stacked dimers,74 were also evident for this J-dimeric structure. The computed stabilization energy for dimerization was evaluated to be -6.28 kcal.mol-1 (∆Estabilization = (-3233.55 - 2.(1616.77) a.u.) x 627.51)) and was attributed to the π-π stacking interactions.

Figure 7. (a) The computed ground state optimized structure of Rubrene (RB) dimer converging into a J23 ACS Paragon Plus Environment


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dimeric form with the slippage angle θ = 30.4o, intermolecular distance (r) and interplanar separation (d). The two red circles depict the molecular centers of RB monomeric subunits. Hydrogen atoms are removed for clarity. (b) The electronic structure of optimized RB J-dimer computed at TDDFT/M062X/6-31G+(d,p) level with IEFPCM in water depicting energetically feasible singlet fission.

Electronic structure calculation on this optimized RB J-dimer showed the lowest-lying singlet excited state (S1) as a bright state of Au symmetry located 2.53 eV above the ground state (cf. Table S3 for oscillator strength, SI). The second (S2) and the third (S3) singlet excited states were dark in absorption, having Bg symmetry, and were found 2.55 and 3.08 eV above the ground state respectively. Finally, the S4 and S6 states were bright in absorption with Au and Bu symmetry respectively, and were located at 3.14 and 3.56 eV above the ground state. The computed excited state energy for the RB J-dimer complied with the experimental results for RB crystal,24, 33, 37 thus validating the level of theory adopted for computation. With the presently computed lowest triplet state (T1) at 1.29 eV, in agreement with the literature reports,24, 33, 37 the ∆E1 for SF from S1 state was evaluated to be negligibly small (-0.05 eV), implying the occurrence of SF in RB colloidal J-aggregate via thermal activation. It was also interesting to note that RB J-dimer satisfied the primary energy criterion, E(S1) ~ 2x(T1) for delayed fluorescence as well and was experimentally observed in its colloidal J-aggregate. Furthermore, in RB J-dimer, a clear exoergic energy balance (∆E1 = 3.14 – 2 x 1.29 = 0.56 eV) for an SF process from the S4 bright state was anticipated. In conclusion, our theoretical results corroborated the occurrence of energetically feasible SF in RB colloidal J-aggregate, leading to delayed fluorescence and thus dramatically reducing the PLQY from 0.172 down to 0.035. A similar observation of SF in 2-(4-pentylphenylvinyl)tetracene resulted in low PLQY (7%).75 In contrast, a high PLQY (70%) achieved for 2-(4-hexylphenylvinyl)anthracene crystals, attributed to 2E (T1) > E (S1), turned off the emission quenching through SF.76


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Figure 8. (a) and (b) depict the locally excited characteristics of Rubrene J-dimer bright S1 and S4 excited states, visualized as localized electron (EDD), hole (HDD) and resulting charge (CDD) density distribution maps. The quantitative indexes: EDD and HDD centroid overlap (φ) ~1 and negative t-values along all three spatial directions, signifying the LE nature of both S1 and S4 excited states are depicted. Hydrogen atoms are removed for clarity.

In the next step, we established the LE or CT character to the bright S1 and S4 states. Figure 8(a) and (b) depict the computed vertical excitation energies, EF G F H , t-index and the assigned LE or CT character to the bright singlet states for RB J-dimer. The S1 and S4 states could be predominantly of LE nature with certain amount of CT character, visualized in the uniformly distributed CDD plots in Figures 8(a) and (b) respectively. Furthermore, the calculated overlap integrals (EF GF H ) of hole and electron density distribution centroids for S1 and S4 states were found close to unity, signifying the meager delocalization of electron density in a closer proximity of hole density, and thus providing an additional evidence for S1 and S4 LE character. Nonetheless, it is important to note that EF GF H is the initial indicative of CT character. Thus, the final assessment of excited state character in the present context warranted a more detailed



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investigation. A tangible proof was obtained by evaluating the t-index in all three spatial directions; a positive t-index defined the quantitative extent of intermolecular CT through space.54-55 The calculated values of t-index using the computed EDD and HDD for S1 and S4 bright states are depicted in Figures 8(a) and (b) respectively. The obtained negative values of tx, ty and tz for both S1 and S4 states in RB J-dimer provided a concrete evidence for the absence of intermolecular CT character. Consequently, these low-lying S1 and S4 bright states in the RB Jdimer were assigned LE type, analogous to the results obtained in solid RB crystals.77 Further, the S2, S3, and S5 dark states were also carefully analyzed using the above defined indexes and were found to be of LE character as well (cf. Table S3, SI). The above computed LE character of S1 state in RB J-dimer was similar with the result obtained for orthorhombic RB single crystal.44, 77

However, Wang et al. indicated the metastable monoclinic and triclinic forms of RB exhibiting

a more CT like character of the singlet exciton wavefunction.44 In conclusion, the RB J-dimeric unit being a highest limit of exciton spectral perturbation in RB colloidal J-aggregate, exhibited S1 and S4 bright locally excited singlet states, which could result in a low singlet fission yield. Conclusions: Rubrene as a prototypical molecular material of tetracene formed colloidal J-aggregates via reprecipitation method in controlled THF-water binary mixtures. The J-aggregates with a slippage angle θ = 30.4o (< 54.7o) exhibited a broad and a red shifted absorption band, λ = 1.80 D), almost equivalent to the monomeric dye possessing a transition dipole moment ( λ = 1.89 D). The significantly low magnitude of exciton coupling energy (∆Eexc = - 358 ( cm-1) for the rhombic-RB colloidal J-aggregate was due to the weak electronic communication between the largely separated RB subunits (r = 7.2 Å) and a restricted exciton delocalization on


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the RB J-dimer (N = 2). The unusual photoluminescence quenching in the RB J-aggregate nuclear conformation was quantified with a dramatic drop in the PL quantum yield and was ultimately attributed to the availability of an alternate ultrafast decay channel for S1 state to quench via formation of a correlated triplet pair. The PL decay profile of RB colloidal J-aggregate depicted a long-lived delayed fluorescence as a result of triplet pair formation via singlet fission. The positive transient absorption band (T1 TN) at 530 nm further confirmed the long-lived triplet formation. The SF and delayed fluorescence in RB J-dimer was further supported by TD-DFT-M062X/6-31G+(d,p) calculations following IEFPCM in water. The theoretical calculations revealed an energy balance between the bright singlet and the triplet state for a thermally activated SF, followed by a triplettriplet recombination in RB J-dimer. In addition, the S1 excited state was proven to be of locally excited in nature by carefully analysing the integral overlap of electron and hole density distributions, and the defined t-indexes along all three spatial directions. The present investigation has thus paved way for using the functionalized tetracene J-aggregate exhibiting singlet fission in organic photovoltaics. Supporting Information: The optical signatures of RB monomer and RB J-aggregate, photoluminescence quantum yield calculations for RB monomer and J-aggregates, computed RB monomer HDD and EDD coordinates for TDM calculation, RB ideal J-dimer potential energy curve, computed indexes towards characterization of low lying dark and bright singlet states in Rubrene J-dimer are provided in SI. Corresponding Author. *(A.P.). *Fax: (+91) 2257 4202 E-mail: [email protected] 27 ACS Paragon Plus Environment


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ACKNOWLEDGEMENT: N.A. thanks CSIR, New Delhi, for providing a Research Fellowship. P. G. Senapathy center for computing resources, Sophisticated Analytical Instrumental Facility (SAIF) and Department of Chemical Engineering, IIT-Madras are gratefully acknowledged for providing excellent Computing, TCSPC and HRSEM facilities. The authors thank the Laser Flash Photolysis Laboratory at IIT-Madras for providing nanosecond Flash Photolysis experimental facility. CSIR, New Delhi, India, Grant no. 01(2830)/15/EMR-II is acknowledged for financial support. ABBREVIATIONS RB: rubrene, SF: singlet fission, PLQY: photoluminescence quantum yield, EDD: electron density distribution, HDD: hole density distribution, CDD: charge density distribution, LE: locally excited, CT: charge transfer, TDM: transition dipole moment, THF: Tetrahydrofuran, IEFPCM: integral equation formalism polarization continuation model. References: 1. Scholes, G. D.; Rumbles, G., Excitons in Nanoscale Systems. Nat Mater 2006, 5, 683-696. 2. Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R., J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem., Int. Ed. Engl. 2011, 50, 3376-3410. 3. Kasha, M.; Rawls, H.; Ashraf El-Bayoumi, M., The Exciton Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371-392. 4. Knapp, E. W., Lineshapes of Molecular Aggregates, Exchange Narrowing and Intersite Correlation. Chem. Phys. 1984, 85, 73-82. 5. Knapp, E. W.; Scherer, P. O. J.; Fischer, S. F., On the Lineshapes of Vibronically Resolved Molecular Aggregate Spectra. Application to Pseudoisocyanin (Pic). Chem. Phys. Lett. 1984, 111, 481-486. 6. Choi, S.; Bouffard, J.; Kim, Y., Aggregation-Induced Emission Enhancement of a Meso-Trifluoromethyl Bodipy Via J-Aggregation. Chem. Sci. 2014, 5, 751-755. 7. Hong, Y.; Lam, J. W.; Tang, B. Z., Aggregation-Induced Emission. Chem. Soc. Rev. 2011, 40, 5361-5388. 8. Hong, Y.; Lam, J. W.; Tang, B. Z., Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332-4353. 9. An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y., Enhanced Emission and Its Switching in Fluorescent Organic Nanoparticles. J. Am. Chem. Soc. 2002, 124, 14410-14415.


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10. Picciolo, L. C.; Murata, H.; Kafafi, Z. H., Organic Light-Emitting Devices with Saturated Red Emission Using 6,13-Diphenylpentacene. Appl. Phys. Lett. 2001, 78, 2378-2380. 11. Wolak, M. A.; Jang, B.-B.; Palilis, L. C.; Kafafi, Z. H., Functionalized Pentacene Derivatives for Use as Red Emitters in Organic Light-Emitting Diodes. J. Phys. Chem. B 2004, 108, 5492-5499. 12. Xu, Q.; Duong, H. M.; Wudl, F.; Yang, Y., Efficient Single-Layer “Twistacene”-Doped Polymer White Light-Emitting Diodes. Appl. Phys. Lett. 2004, 85, 3357-3359. 13. Sheraw, C.; Zhou, L.; Huang, J.; Gundlach, D.; Jackson, T.; Kane, M.; Hill, I.; Hammond, M.; Campi, J.; Greening, B., Organic Thin-Film TransistorDriven Polymer-Dispersed Liquid Crystal Displays on Flexible Polymeric Substrates. Appl. Phys. Lett. 2002, 80, 1088. 14. Merlo, J. A.; Newman, C. R.; Gerlach, C. P.; Kelley, T. W.; Muyres, D. V.; Fritz, S. E.; Toney, M. F.; Frisbie, C. D., P-Channel Organic Semiconductors Based on Hybrid Acene−Thiophene Molecules for Thin-Film Transistor Applications. J. Am. Chem. Soc. 2005, 127, 3997-4009. 15. Tang, M. L.; Reichardt, A. D.; Miyaki, N.; Stoltenberg, R. M.; Bao, Z., Ambipolar, High Performance, Acene-Based Organic Thin Film Transistors. J. Am. Chem. Soc. 2008, 130, 6064-6065. 16. Lee, J.; Jadhav, P.; Reusswig, P. D.; Yost, S. R.; Thompson, N. J.; Congreve, D. N.; Hontz, E.; Van Voorhis, T.; Baldo, M. A., Singlet Exciton Fission Photovoltaics. Acc. Chem. Res. 2013, 46, 1300-1311. 17. Jadhav, P. J.; Mohanty, A.; Sussman, J.; Lee, J.; Baldo, M. A., Singlet Exciton Fission in Nanostructured Organic Solar Cells. Nano Lett. 2011, 11, 1495-1498. 18. Ehrler, B.; Wilson, M. W. B.; Rao, A.; Friend, R. H.; Greenham, N. C., Singlet Exciton Fission-Sensitized Infrared Quantum Dot Solar Cells. Nano Lett. 2012, 12, 1053-1057. 19. Rao, A.; Wilson, M. W. B.; Hodgkiss, J. M.; Albert-Seifried, S.; Bässler, H.; Friend, R. H., Exciton Fission and Charge Generation Via Triplet Excitons in Pentacene/C60 Bilayers. J. Am. Chem. Soc. 2010, 132, 12698-12703. 20. Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A., External Quantum Efficiency above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell. Science 2013, 340, 334-337. 21. Smith, M. B.; Michl, J., Recent Advances in Singlet Fission. Ann. Rev. Phys. Chem. 2013, 64, 361-386. 22. Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G., Laser Generation of Excitons and Fluorescence in Anthracene Crystals. J. Chem. Phys. 1965, 42, 330-342. 23. Swenberg, C. E.; Stacy, W. T., Bimolecular Radiationless Transitions in Crystalline Tetracene. Chem. Phys. Lett. 1968, 2, 327-328. 24. Ma, L.; Zhang, K.; Kloc, C.; Sun, H.; Michel-Beyerle, M. E.; Gurzadyan, G. G., Singlet Fission in Rubrene Single Crystal: Direct Observation by Femtosecond Pump-Probe Spectroscopy. Phys. Chem. Chem. Phys. 2012, 14, 83078312. 25. Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J., Exciton Fission and Fusion in Bis(Tetracene) Molecules with Different Covalent Linker Structures. J. Am. Chem. Soc. 2007, 129, 1424014250. 26. Podzorov, V.; Menard, E.; Borissov, A.; Kiryukhin, V.; Rogers, J. A.; Gershenson, M. E., Intrinsic Charge Transport on the Surface of Organic Semiconductors. Phys. Rev. Lett. 2004, 93, 086602. 27. Mitrofanov, O.; Lang, D. V.; Kloc, C.; Wikberg, J. M.; Siegrist, T.; So, W.-Y.; Sergent, M. A.; Ramirez, A. P., Oxygen-Related Band Gap State in Single Crystal Rubrene. Phys. Rev. Lett. 2006, 97, 166601.



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Unusual Nonemissive Behavior of Rubrene J ... - ACS Publications

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