Highly Emissive H-Aggregates or Aggregation ... - ACS Publications

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Highly Emissive H‑Aggregates or Aggregation-Induced Emission Quenching? The Photophysics of All-Trans para-Distyrylbenzene Johannes Gierschner,*,†,‡ Larry Lüer,†,‡ Begoña Milián-Medina,† Dieter Oelkrug,‡ and Hans-Joachim Egelhaaf‡,∥ †

Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Campus Cantoblanco, 28049 Madrid, Spain Institute for Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

‡

S Supporting Information *

ABSTRACT: The present Perspective critically re-examines the photophysics of paradistyrylbenzene (DSB) as a prototype of herringbone-arranged H-aggregates to resolve the apparent contradiction of the frequently reported “aggregation-induced emission quenching” in H-aggregates on one side and highly emissive DSB crystals on the other and discusses the signatures and fate of excitons in single- and polycrystalline samples, including size and polarization effects.

C

trap states and their role in excited-state deactivation. We will give some answers to these questions for a common structural motif, that is, H-aggregates of oligomers arranged in a herringbone fashion where the molecules are packed side-byside with their long axes but with an inclination of their short axes; see Figure 1. This is a common motif for rod-shaped unsubstituted or terminally substituted oligomers such as oligophenylenevinylenes,5−8 -phenylenes,9 -thiophenes,10−14 or -acenes,15 which in this way tend to minimize the overlap of the π-systems while maximizing the enthalpic gain by dense packing. Thin films of such materials are known to be low emissive, which is commonly ascribed to H-aggregation, thus referring to as “aggregation-induced emission quenching”;4,16−20 on the other side, there are several reports on highly luminescent H-aggregates.6,21−25 In order to resolve the apparent contradiction and to elucidate the signatures and fate of molecular excitons in these crystalline systems, we critically re-examine the photophysics of para-distyrylbenzene (DSB; see Figure 1), which became a kind of archetypical example of herringbone H-aggregates due to the extensive computational work of F. C. Spano,19,20 based on our early experimental studies of DSB nanoparticle (NP) suspensions.16,26,29 We will start with the photophysics in solution to disentangle later intra- and intermolecular contributions. We will then turn to the steady-state spectra in the solid state, where we will especially pay attention to size

onjugated organic materials have been intensively studied during the last decades for application in (opto)electronic devices, such as organic light-emitting diodes (OLEDs), solar cells (OSCs), sensors, field effect transistors, and optically pumped lasers.1 While the majority of the materials are based on polymers due to their easy processability, there is a clear tendency in recent years toward small molecules; in fact, latest record values for efficiencies of OSCs are based on smallmolecule devices.2 Reasons for this success story are the development of facilitated synthetic routes and improved solubility. Moreover, the possibility to better control the intermolecular arrangement compared to polymeric materials was demonstrated to be beneficial, for example, in multiresponsive materials for sensing applications.3,4 In fact, polymeric materials suffer from energetic and positional disorder and tend to form mixtures of amorphous and crystalline domains, which make morphology control in polymers a demanding issue. In contrast, small, well-defined oligomeric molecules allow for crystalline materials, where clever design of the molecular backbone and substituent pattern can drive the molecules into specific intermolecular arrangements controlled through secondary forces such as local dipoles and hydrogen bonds.3 This might be especially true for the nanoalignment at interfaces needed for optoelectronic functionality. Despite the intense research during the last years, basic optical properties and photophysical processes in crystalline conjugated materials are still not properly understood. This concerns, for example, the correct implications of J- and H-aggregation, size effects on absorption and emission properties, intermolecular contributions to vibronic coupling, mechanisms and efficiency of exciton dynamics, and the precise nature of © XXXX American Chemical Society

Received: May 12, 2013 Accepted: July 25, 2013

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dx.doi.org/10.1021/jz400985t | J. Phys. Chem. Lett. 2013, 4, 2686−2697

The Journal of Physical Chemistry Letters

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Figure 1. (Left) DFT-optimized rotamers of all-trans DSB. For the anti rotamer, the directions of the lowest 11Ag → n1Bu transition dipole moments μn are indicated (for n = 2, 3 enlarged by a factor of 10). For syn, torsions around the vinyl−phenyl bonds are shown. (Right) Herringbone arrangement for Z = 4 (plate-like crystals) and 2 (needle-like crystals), with inclination against the herringbone layer (δL) and herringbone angle (δH).

Figure 2. Excited states term diagram of DSB (C2h) with adiabatic energy levels according to experiment (numbers in italics indicate calculated values). Triplet and singlet states in solution (left) and singlets in the solid state (right). Indicated splitting in the solid state refers to a simple twostate model. Colored arrows indicate experimental one-photon allowed vertical transitions; correlation to the absorption spectra is given on the right. Gray arrows indicate vertical photoinduced absorptions.

agreement with second-order Møller−Plesset perturbation theory (MP2, 19 cm −1) and coupled-cluster singles and doubles (CCSD, 23 cm−1) calculations, with a rotational barrier of 1700 cm−1 between the two forms. From synthesis, none of them is favored, and thus, at ambient conditions, an equilibrium constant of K = exp(−ΔE/kT) = 0.91 is expected, corresponding to 52% of the syn rotamer. Evidence for the coexistence of rotamers in solution arises from low-temperature (LT) fluorescence excitation spectra. In amorphous solid matrixes, the excitation spectrum shifts with the detection frequency by a distinct amount of ∼400 cm−1, ascribed to the coexistence of both rotamers.34 In fact, TD-DFT calculations predict a blue shift of 220 cm−1 for the syn rotamer compared to the anti; on the contrary, in crystalline Shpolskii matrixes (e.g., tetradecane under LT conditions), anti rotamer formation is favored.34 The absorption spectrum of DSB in solution is dominated by the intense S0(11Ag) → S1(11Bu) electronic transition (Figure 2),34 essentially described by a transition between the highest occupied

effects in the absorption spectrum, to the mechanisms for how the lowest excited singlet state gains oscillator strength despite H-aggregation, and to refractive index effects on the fluorescence spectrum. Finally, we will discuss the photophysical deactivation pathways in the crystals. The reader will note that we were integrating the extensive work on DSB from the past, including our own studies5,16,26−40 and, in particular, the work in the groups of A. Spaletti,41−43 F. C. Spano,18−20 V. Vardeny,6,44,45 C. Bardeen,46 and R. H. Friend,47 but putting them into a new, common context, enriched with some additional experimental studies that complete the understanding of the photophysics of DSB. Photophysics in Solution. The DSB equilibrium geometry in the electronic ground state S0 is planar but with substantial torsional flexibility around the vinyl−phenyl bonds.34 Two possible rotamers exist for DSB, that is, anti (C2h symmetry), and syn (C2v); see Figure 1. Density functional theory (DFT; for details, see the Supporting Information (SI)) calculations favor slightly the syn rotamer to anti by ΔE = 20 cm−1, in 2687



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Table 1. Measured Adiabatic Transition Energies E00 (Reorganization Energies ΔEro in Absorption), Fluorescence Lifetimes τF, Quantum Yields ΦF of DSB, and Rate Constants for Radiative kF and Nonradiative Deactivation knr, Extracted via ΦF = kF·τF = kF/(kF + knr) system solution

temp./K tetradecane DCMc

NPs

f

single crystal

plate-like needle-like

293 10 293 293 10 293 293

ΦF

E00 /eV 3.25 (0.43) 3.15 (0.26) 3.22 3.07 2.98 2.98g 2.98g

a

0.92 0.97a,b 0.90a,d, 0.89e 0.08a ± 0.02 0.43a ± 0.1 0.65e,h,i 0.78d,e

τF/ ns

kF/ ns−1

knr/ ns−1

1.45 0.93 1.26d 2.2 ± 0.3 9.0 ± 0.5 3.1i 3.6d

0.63 1.04 0.71 0.04 0.05 0.21 0.22

0.06 0.03 0.08 0.42 0.06 0.11 0.06

a

From relative measurements against quinine sulfate. bExact data are not yet available due to light scattering. cDCM = dichloromethane. dReference 5. From absolute measurements in an integrating sphere. fMeasured values differ considerably depending on preparation conditions. gAt T = 1.4 K. h From ref 44. iFrom ref 21. e

and lowest unoccupied molecular orbitals (HOMO → LUMO), which is oriented fairly along the long molecular axis (defined by the connection of the terminal C-atoms, see Figure 1, that is, enclosing an angle of 2° in C2h) as determined by TD-DFTbased calculations (see Table S1 in the SI).38 High-resolution spectra at LT conditions34,44 (recently done also under siteselective conditions48) in combination with quantum chemical analysis34 allowed for the determination of the prominent totally symmetrical vibrational modes, which couple to the electronic transition.49 This gives rise to a reorganization energy ΔEro (i.e., the difference of the vertical and adiabatic transition energies50) of 0.26 eV; see Table 1. The LT absorption and emission spectra are approximately mirror-symmetrical (Figure 3) due to planar equilibrium structures in S0 and S1 and

the potentials and thus lead to vibronic structuring of the absorption spectrum. Higher excited 1Bu states as found in experiment are summarized in Figure 2. It should be noted that from the computational side, while 11Bu (due to its high oscillator strength) is reasonably well reproduced by various methodologies, the 21Bu energy is calculated too high, for example, by standard DFT functionals and semiempirical methods, placing 21Bu about 1 eV above 11Bu, while the experimental difference is only 0.57 eV (see Figure 2), as determined from fluorescence excitation anisotropy measurements.38 Higher 1Ag states were investigated by pump−probe experiments (revealing 41Ag; see Figure 2) by Ginocchietti et al.,43 while 21Ag (S2) was assigned by two-photon absorption in ortho-(2,2′)-dimethyl-DSB,52 showing that S2 is energetically well separated from S1 by around 0.4 eV.53 Part of the triplet manifold was investigated by triplet−triplet absorption,43 while the absolute position of T1(13Bu) was estimated experimentally for meta-(t-butyl)4DSB to 1.78 eV,54 close to the calculated (vertical) value for DSB at the DFT level (1.88 eV); see Figure 2. In general, the spectral positions are solvent-dependent; however, strong shifts are only observed for the main absorption band due to its high oscillator strength as suggested by the Onsager relation. In fact, the latter describes the experimental shifts well if an effective cavity radius of 5.6 Å is assumed, reasonably well reproduced by DFT (5.2 Å); see SI.34 The temperature-dependent shift in emission (Figure 3; for experimental details, see the SI) can be equally traced back to the solvent polarizability effect, keeping in mind that the refractive index increases with the density of the environment as induced by cooling.34 It should be noted that contrary to frequent claims, planarization is a minor source for the temperature-induced spectral shifts for emission (while for absorption, it is important, vide supra) because the emission originates from a mainly planar S1 state. The fluorescence quantum yield in solution (tetradecane) at room temperature is as high as ΦF = 0.92, and the fluorescence lifetime is τF = 1.45 ns (see Table 1), so that the radiative rate, which can be calculated via

Figure 3. Fluorescence spectral positions (red circles) of DSB in tetradecane as a function of temperature (the strong shift at 278 K marks the melting point of the solvent); radiative rates (blue dots) as obtained from fluorescence quantum yields and lifetimes. (Inset) Fluorescence (FL) and absorption spectra (ABS) at 10 K (top) at 293 K (bottom).

the minor population of torsional modes.34 The torsional potentials are significantly steeper in S1 compared to S0 because the vinyl−phenyl bonds are considerably shortened in S1, as confirmed by (TD-)DFT calculations.34,51 Hence, at ambient temperatures, thermal population of low-frequency torsional modes leads to strong broadening of the absorption spectrum compared to emission (see Figure 3);34 concomitantly, the reorganization energy increases to ΔEro = 0.43 (Table 1). Restricted environments, for example, in channel-forming host−guest compounds39 or adsorption on alumina,40 steepen

ΦF = kF·τF =

kF (kF + k nr)

(1)

is kF = 0.63 ns−1, and the nonradiative rate is calculated to knr = 0.06 ns−1.55 The radiative rate shows a distinctive temperature dependence (Figure 3), which cannot be solely explained by a polarizability effect. Instead, it should be associated with the excited-state conformational dynamics in 11Bu, allowing for 2688



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effective population of the 21Ag state (being 0.4 eV above 11Bu and thus just within the Franck−Condon region of the latter, with ΔEro = 0.43 eV; Table 1), which will modify the deactivation rates. In rigid solvents under LT conditions, kF increases to 1.0 ns−1. The nonradiative rate is mainly due to internal conversion kIC and shows Arrhenius-like temperature dependence (see the SI), from which an activation barrier of 0.12 eV for IC can be extracted. On the other hand, intersystem crossing (kISC) to the triplet manifold and trans−cis photoisomerization are negligible (ΦISC < 1%; Φiso < 0.1%).41,42 The low ISC rate might be associated with the relatively large energy gap between the S1 state and the accepting state of the triplet manifold, calculated to ΔE(11Bu−13Ag) ≈ 0.6 eV; see Figure 2. The low efficiency of photoisomerization of the all-trans isomer was ascribed to its adiabatic character, owing to substantial energy barriers in the region of 90° double bond twists,54,56 different from the efficient diabatic process observed in stilbene.57,58 At shorter time scales, a component of about 10 ps is found in the fluorescence kinetics as a rise time prior to the nanosecond decay,59 as well as in the transient absorption of the singlet kinetics.43,60,61 While some authors suggest the population of an emitting state somewhat below S1 as the source for the short component,60−62 there is little evidence for such a state from the computational side. Moreover, LT fluorescence emission and excitation spectra give a vanishing Stokes shift and mirror-symmetry relationship,34 both suggesting that the emission indeed originates from the strongly absorbing S1 state. Even more, similar short times were observed both for related compounds with substantial torsional flexibility like alkoxy-substituted DSB63,64 and oligothiophenes65,66 and successfully ascribed to S1 conformational relaxation (CR) associated with the excess energy upon excitation. Correlation of the observed transients with the dynamic Stokes shift during the first 100 ps gave further evidence for this interpretation.64 We thus follow previous authors in their assignment of the 10 ps feature of DSB to the kCR rate constant.43,59 Intermolecular Arrangement in the Solid State. Two different polymorphs were observed for DSB. While preparation by physical vapor transport gives plate-like crystals with four molecules in the orthorhombic unit cell,6 crystallization from solvent mixtures gives needle-like crystals with six molecules in the triclinic unit cell.5 Both structures share the nearestneighbor herringbone arrangement,5,6 where the long molecular axes are arranged in parallel, but the short axes are inclined by a (herringbone) angle of about 60° (Figure 1), similar to longer PPV oligomers;7 this ensures minimized π−π overlap of adjacent molecules and, at the same time, dense packing.67 In both crystals, the long molecular axes are inclined by about 78° against the herringbone layers (see δL in Figure 1). However, while in the needle-like crystals the molecules in the neighboring layer are oriented in the same direction (so that Z = 2),5 the molecules in the plate-like crystals point in the opposite direction (Z = 4).6,38 Solid-State Absorption Spectra. The herringbone motif in the solid state results in a side-by-side arrangement of the S0 → S1 transition dipole moments, which gives rise to pronounced H-type aggregation with a strongly blue-shifted asymmetric absorption band, which is in fact found as the H band at 4.2 eV in the measured extinction spectrum of DSB NP suspensions; see Figure 4. This can already be qualitatively predicted from Kasha’s molecular exciton model based on Coulomb interactions of the transition dipoles68 and was convincingly modeled by Spano in a (semi)quantitative nearest-neighbor approach

Figure 4. (a) Extinction spectra (in units of the molar extinction coefficient, εm) of NP suspensions with two different average diameters (d1 = 90 nm, black solid line; d2 = 800 nm, red dashed line). (b) Fluorescence emission (left, λex = 295 nm) and excitation (right, λem = 440 nm) spectra with steady-state anisotropies (rF) for d1 (black open and closed circles), d2 (red open squares), and μm size crystals (size ∼1−100 μm; blue dashed− dotted line). (c) Simulation of the absorbance for d = 90, 200, 400, and 1000 nm (see text).

(NNA) based on quantum chemically calculated exciton couplings and an effective intramolecular vibronic mode;19,20 nevertheless, for a quantitative understanding, it will be certainly necessary to go beyond the NNA.69 In a classical description, the asymmetric band shape of the measured extinction results from the strong anisotropy of the optical constants (ñj = nj + i·kj, with j = x, y, z), which arise from the quasi-uniaxial (z) arrangement of the S0 → S1 transition dipole moments within the herringbone layer. While the imaginary part of ñz reveals a rather symmetrical (Gaussian) band shape similar to the absorption in solution, it is the real part, nz, that is responsible for the asymmetry of the absorbance by reaching high values of about 3 in the red part of the band, whereas it goes down to about 1 in the blue part; for details, see the discussion on a related H-aggregate herringbone-forming system (quinquethiophene; 5T) by Egelhaaf et al.70 The real part nz remains high also below the absorption region to the red. This is revealed by doping experiments of DSB with suitable energy acceptors like 5T, which has the same molecular length as DSB and is thus incorporated into the regular herringbone structure of the DSB crystal.35 From the spectral position of the 5T high-energy fluorescence sub-band, E(F1) = 2.5 eV (500 nm), a refractive index of nz = 1.84 eV is extracted, whereas in systems with missing long-range ordering (based, e.g., on t-butyl-substituted DSB), an isotropic refractive index of niso = 1.57 is obtained upon doping with 5T.35 Following Spano’s theoretical work,18−20 the low-energy absorption features are attributed to vibrationally dressed excitons. The A1 band (at 3.14 eV; see Figure 4a) was identified in this context as a false origin, which borrows intensity in an 2689



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“aggregate-type” version of Herzberg−Teller (AHT) coupling19,72 from the H band via a nontotally symmetrical vibrational mode νAHT that separates A1 from the low-lying J band (at 2.98 eV).19 The latter represents the lower Davydov component of the exciton band and gains only little intensity. This arises mainly from the small inclination of the transition dipole moment against the molecular axis in the “pinwheel” arrangement of DSB within the herringbone layer; however, other factors might contribute such as the inclination of the molecules against the herringbone layer (vide supra), disorder,20 and the coexistence of anti and syn rotamers in the crystal.5,71 Spano’s assignment agrees with the remarkably uniform fluorescence anisotropy rF over the spectral range of the main absorption band for small particle sizes (Figure 4b). In nanocrystal suspensions with a spatially isotropic particle distribution, the reduction of the steady-state value from the maximum possible value of rF = 0.4 to 0.16−0.22 reflects the inclination of the molecules against the plane of the herringbone layer through energy transfer between translationally nonequivalent molecules in the nanocrystals, that is, mainly between the herringbone layers.38 Concomitantly, the anisotropy decay (43 ps) corresponds to the time constant of this transfer step (vide infra).30 For larger particle sizes, the steadystate anisotropy drops fast at higher energies and rises higher toward the red edge of the spectrum (Figure 4), which is due to the optical artifact induced by the particle size, as discussed in the following.

of the absorbance by Mie theory with increasing d indeed reproduced the changes of the spectral band shape in experiment; see Figure 4. The simulations demonstrate the masking of the intrinsic properties by the particle size, showing a different impact of d in the blue and red regions of the spectrum. In the red part, the absorbance is low, and the extinction increases with d mainly because of light scattering. In the blue part, the absorbance is high and saturates with d, so that Cabs (and thus εm) decreases. For large particles, the interplay of scattering and absorbance becomes complex, and an accurate description requires a proper treatment of the optical and geometrical anisotropy of the particles. Solid-State Fluorescence Spectra. The solid-state fluorescence spectrum is in many aspects similar to the solution spectra, displaying the same energy spacings between the apparent subbands but with a smaller halfwidth at ambient conditions due to the (homogeneous) rigid crystalline environment. The intensity of the high-energy sub-band F1 is significantly lowered against the dilute state, compare Figures 3 and 4.16,31,38,44 Although for optically thick samples (d ≫ 1 μm) reabsorption plays a significant role (see, e.g., the microcrystallite spectrum in Figure 431 and the reported single crystal21,44), the reduction of F1 intensity is considerable also in optically thin samples (Figure 4), in particular, also at LTs.31,38 It was conjectured earlier that F1 could possibly represent the first sub-band of the fluorescence from bulk DSB, whereas the Fi sub-bands with i ≥ 2 could be due to trap states, for example, structural dislocations.31 Such deep traps are indeed present as minority species as observed on longer time scales (vide infra). However, for the prompt fluorescence of the Fi (i ≥ 2) features, neither thermal activation nor rise components in the time traces were found,31 which would be signatures of diffusive exciton transport toward energy acceptors acting as trap states in crystalline DSB, vide infra.26,31,35 Such trap state emission was observed in polycrystalline samples by us26 and others.46 Thus, careful cleaning of the material and inspection of the emission properties of the samples is required to ensure that the intrinsic crystal properties are observed. In fact, all polycrystalline samples (NPs and films) reported in the present work coincide in their spectral positions (and relative heights of the apparent sub-bands) with the reported single crystals.5,21,44 More insight into the origin of the emission spectrum is gained by LT single-crystal spectra as examined by Wu et al.,44 exhibiting a red shift of 0.06 eV against the spectrum at ambient conditions. While spectral narrowing is essentially due to cooling of the lattice phonons, the red shift is attributed to the increase in density, which increases the effective polarizability just like that in solution. Also here, planarization is a minor issue for the temperature-induced shift because the crystal structure reveals a planar DSB conformation5 and close packing does not permit substantial torsional freedom. The low intensity of F1 in the solid state was ascribed by Spano19 and by Meskers et al.74 to emitting excitons18,20 in H-aggregates, where the aggregate F1 band should coincide with the F1 band in solution. While the original model was based on just one coupling mode, it was later extended to a multimode picture,75 which can in fact explain the suppression of the F1 band in total. The latter contains progressions and intercombinations of low-frequency in-plane vibrational modes. This can be seen from the LT spectra in solid solution (Figure 3), which in the high-energy region are dominated by the in-plane stretch at 150 cm−1 (with a Huang−Rhys factor of 1.05).34 According to the model, a different polarization is predicted for F1 and Fi (i ≥ 2), in agreement with experiment.44 For polycrystalline

The profound changes in the measured extinction for optically thick NPs are due to the effective decrease of the molar absorption coefficient and to strong light scattering. In fact, to monitor the intrinsic H-aggregate absorption properties in the solid state, optically thin samples in the sub-50 nm range have to be used, such as thin vapor-deposited films or NP suspensions (e.g., prepared from solvent mixtures);16,26,27,30,31,35,38 see Figure 4a. Only here, in the Rayleigh limit, does the cross section of absorption Cabs for a single particle (or the absorption coefficient for a particle suspension) scale linearly with the particle volume Vp (while the cross section for scattering Cabs scales quadratic with Vp; see the SI for details). For optically thicker particles, profound changes of the spectral characteristics in the measured extinction (i.e., absorption + scattering) are observed, which are due to the effective decrease of the molar absorption coefficient (and thus Cabs) as well as to strong light scattering, which in all mask the intrinsic H-aggregate features of DSB. In a first approximation, absorbance without light scattering is accessible from fluorescence excitation spectra (Figure 4b), which allows for deeper insight into size effects. In fact, both, decreasing absorbance and increasing scattering with increased thickness, are, for example, described by Mie theory.73 Although the latter was developed for optically isotropic spherical particles (with diameter d), the observed effects for the size dependence of DSB particles can be modeled in a qualitative way. For this, the absorption spectrum of the NP suspension was simulated by a number of Lorentz oscillators, extracting the averaged isotropic complex refractive index in the Rayleigh limit (see the SI); recalculation 2690



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the trap concentration should be low because of the absence of contaminations and structural dislocations, ΦF might reach high values despite H-aggregation. In fact, ΦF = 65% was reported for plate-like DSB single crystals;6,21,44,77 for needle-like crystals, we obtained even ΦF = 78%.5 Taking the (monoexponential) fluorescence lifetime of the needle-like single crystal, τF = 3.6 ns,5 the radiative rate is calculated as kF = 0.22 ns−1; basically, the same rate is found for the plate-like crystals; see Table 1. The rather moderate reduction of kF in the single crystals compared to that for rigid solvents (where kF ≈ 1.0 ns−1, vide supra) reflects the non-negligible oscillator strength of the emitting state as caused by the AHT mechanism, as discussed before. The nonradiative rate in the single crystal (knr = 0.06 ns−1 for the needle-like crystal; see Table 1) is basically the same as that in solution and is thus attributed mainly to IC. The somewhat higher value in the plate-like crystal (knr = 0.11 ns−1; see Table 1) might be due to additional trapping by excimers and/or charge or triplet formation at the crystal surface (vide infra), possibly due to a smaller crystal size.78 It should be stressed that although kF is reduced against solution, it is still high enough to provide highly luminescent single crystals. In fact very low radiative rate constants, which would lead to nonemissive single crystals, have to fulfill two conditions, (i) an optimized parallel alignment of the transition dipoles (i.e., single crystals with one molecule per unit cell) and (ii) strong excitonic coupling, that is, small slips of the transition dipole moments. For smaller excitonic couplings, substantial enhancement of the S1 oscillator strength is expected through AHT coupling because the efficiency of the latter depends on the energetic difference between the forbidden and the allowed state from which intensity is borrowed. Excited-State Deactivation in Polycrystalline Samples. The contradiction of highly luminescent H-type single crystals and the frequent statement of fluorescence quenching in H-aggregates actually originated from the fact that most of the examples of quantum yield measurements in the past were done in vapordeposited or spin-coated thin films and in NP suspensions. In fact, DSB thin films and NP suspensions show drastically reduced fluorescence quantum yields of 5−10%,16,26,27,30,31,35,38 although the self-assembled crystal structure of the emissive species in the monocrystalline domains (i.e., the bulk) within the NPs and films practically coincides with the single-crystal structure, as revealed from polarized fluorescence measurements.38,79 It should be stressed that the actual intrinsic quantum yields can be considerably higher because relative measurements of ΦF rely on the correct estimation of the absorbed amount of light. As shown in earlier reports,27,70 light scattering in NP suspensions can amount to more than 50% of the measured extinction, so that the actual ΦF and thus kF can be much higher than the values reported in Table 1. Nevertheless, the main difference with the single crystals is seen in drastically enhanced nonradiative rates in the films and NP suspensions; see Table 1. This is ascribed to the polycrystallinity of these samples along with a large surface area, so that structural dislocations, for example, π-stacked configurations at the boundaries between monocrystalline domains as well as surface states (inter alia oxidized species) become relevant as trapping sites for the initially generated exciton; for a scheme on deactivation processes in DSB single- and polycrystalline samples, see Figure 5. Furthermore, for DSB NP suspensions, also the particle shapes and size distributions are far from homogeneous, as revealed from AFM images by Lim et al.46 Due to these factors, nonexponential fluorescence decay traces

samples, the polarization varies due to disorder effects as shown by Spano, clearly seen in enhanced intensity of the F1 band compared to the single-crystalline samples.20 Alternatively, an explanation of the complex solid-state fluorescence spectrum can be based on an “aggregate version” of Herzberg−Teller (AHT) coupling to the weakly allowed 0−0 origin at E00 = 2.98 eV (i.e., the J band, vide supra). Inspecting the LT spectrum,44 we assign the false origin at 2.78 eV, so that νAHT = 1597 cm−1 (0.198 eV); in fact, a vibrational mode of bu symmetry is calculated with the same frequency. The vibronics of the single-crystal spectra (Fi+1,SC, with i = 1, 2, ...) follow in a remarkable way those of the molecules in frozen solution (Fi,Mol), that is, Fi+1,SC ≈ Fi,Mol.34 The latter are determined by the intercombinations of the totally symmetrical vibrational modes νi. The vibronics Eij of the LT single-crystal spectrum could be thus described by Eij = E00 + h·νAHT + h· ∑ ni ·νi

(2)

where ni is the quantum number of mode νi. Also within the AHT framework, disorder will be noted in the intensity of F1, and a different polarization for the F1 spectral region of the single-crystal spectrum is expected, as actually observed experimentally.44 It should be noted that the AHT is compatible with a picture of localized emission from the rapidly relaxed exciton as induced by the strong exciton−phonon (ep) coupling in DSB. The close resemblance of the vibronics in the LT spectra in the single crystal and solid solution furthermore demonstrates that intermolecular vibrational modes, although present in the crystal as lattice phonons, do not couple to the lowest electronic transition in DSB. The purely intramolecular vibronic coupling thus proves that the geometrical change upon electronic excitation in the herringbone arrangement is intramolecular due to missing charge-transfer (CT) contributions in the transition as demonstrated through electron−hole wave function analysis.38 Differently, significant CT character of the S0 ↔ S1 transition is observed in systems with substantial π−π overlap. This can be induced, for example, through structural modification of the DSB backbone by fluorination, which switches the DSB herringbone motif to a π-stack while keeping the side-by-side arrangement (H-aggregation).38 The CT character gives rise to a new intermolecular coordinate in the geometry change upon electronic excitation in such π-stacks and thus leads to efficient coupling of breathing modes, which provokes strongly red-shifted excimer-like emission spectra.76 Such π-stack arrangements exist in polycrystalline DSB as a minority species, vide inf ra. Excited-State Deactivation in Single Crystals. H-aggregates are often referred to as non- or low emissive systems, being the textbook example of what is sometimes called “aggregationinduced emission quenching”. However, it is not always true that H-aggregates are weakly emissive; excitonic coupling in H-aggregates leads primarily to a low oscillator strength of the emitting state and thus to a low rate constant of emission kF. A small kF however does not inevitably imply low fluorescence quantum yields ΦF because the latter depends not only on kF but also on all competing nonradiative processes knr. The total rate for nonradiative processes may include again kIC and kISC in analogy to the situation in solution (and are expected to be as low as those in solution), but it also includes loss processes by trapping; this point will be addressed later. Thus, for singlecrystalline materials with a small surface-to-volume ratio, where 2691



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has to travel to reach the surface or grain boundary. In fact, a strong polaron absorption is observed at around 2.2 eV (calculated at 2.3 eV with f = 1.6, see the SI), which might thus be one of the possible quenching pathways in DSB. The trapping efficiency depends critically on the exciton motion process. The exciton migration between the herringbone layers can be measured through the fluorescence anisotropy decay due to the inclination of the transition dipole moments, which leads to depolarization upon energy migration. Starting from an initial value of r0 = 0.4, the anisotropy decays in 43 ps to the steady state value r∞ = 0.22,30,81 where the latter corresponds well to the expectation value from the 130° inclination between the herringbone layers.38 The high steadystate value thus also implies either that NPs are monodomain objects or that exciton migration over grain boundaries to a neighboring domain is not taking place during the exciton lifetime due to the high trapping probability (by structural dislocations) when the exciton reaches the grain boundary. The picosecond kinetics for the interlayer migration also suggests that exciton motion within the herringbone layer (where fluorescence depolarization is not observed) should be much faster, that is, in the subpicosecond regime because the exciton coupling is much larger as it is promoted by H-type aggregation. Subpicosecond exciton dynamics are also suggested from the aforementioned doping experiments of DSB NPs with appropriate energy acceptors (xA = 10−6−10−2).30,31,35 The doping experiments further allow for the discrimination between “bulk” and surface traps. Upon doping the system with small fractions of acceptors A, which for sterical reasons cannot be incorporated in the bulk, for example, higher un/substituted homologues of DSB, the acceptors must populate the boundary regions; see situation AS in Figure 5. Upon excitation of DSB, the total fluorescence intensity of the system increases by a factor of 6 going from xA = 0 to 10−3.30,31 The increase is dominated by the fluorescence of A, whereas the DSB fluorescence intensity remains almost unchanged with xA. Thus, AS is a luminescent competitor to the other surface acceptor sites, but AS does not change the diffusion rate φED. In a second experiment, we doped with an acceptor AL = 5T that forms mixed crystals with DSB.35 In this case, φED is reduced, and the fluorescence of A increases at the expense of the DSB fluorescence. The latter disappears completely at doping levels of xA > 6 × 10−4. Importantly, exciton migration is a thermally activated process, which becomes obvious from the increase of the fluorescence quantum yield of the undoped system upon a decrease of temperature; see Figure 6. This is in stark contrast to, for example, naphthalene,82 which due to its rigid nature shows very weak ep coupling where exciton diffusion scales like D ≈ T1/2.83 Differently, ep coupling in DSB is strong (vide supra), and thus, exciton hopping is predicted in agreement with our results. Rewriting eq 1, the temperature dependence of the nonradiative rate can be estimated by knr = (1/ΦF − 1)·kF, where knr = kIC + kISC + ∑ kQ,i·xQ,i, with kQ,i as the bimolecular quenching rates and xQ,i as the molar quencher ratios. Because kF, kIC, and kISC should depend little on T, the main contribution of the temperature dependence will be due to quenching (driven by exciton migration), decreasing by 1 order of magnitude when going from 198 to 10 K, so that at 10 K, the quantum yields of polycrystalline samples are similar to the single-crystal value.38,47 The thermal activation is linear in T, giving knr = (0.05 + 1.4 × 10−3T) ns−1; see Figure 6. We ascribe the thermal activation to the pronounced temperature

Figure 5. Simplified scheme of the photophysics in single- and polycrystalline samples of DSB (D = localized DSB state, AL = acceptor in the lattice, AS = acceptor at the phase boundary).

with varying mean decay times and quantum yields are observed. Our fluorescence studies on single NPs36 indicated that indeed imperfections (impurities etc.) are responsible for the diminution of the fluorescence quantum yields. In fact, the time-resolved fluorescence study on single DSB NPs by Lim et al.46 revealed emission properties with varying spectral positions and decay times, which the authors attributed to the coexistence of an energetically higher-lying “free exciton” state (which in our terminology corresponds to the intrinsic luminescence observed in the single crystal) and low-lying “defect” states. The latter are red shifted by up to 0.3 eV and reveal a remarkable similarity with our doping experiments of DSB NPs (done in ensemble measurements) with small concentrations of energy acceptors (A).30,31,35−37 In those studies, it was shown that sensitized fluorescence is observed already at doping ratios xA in the ppm range; the transfer efficiency to the traps reaches >90%,30,35 vide infra. Thus, even at trap concentrations in the sub-ppm range, single NP measurements will show significant sensitized fluorescence for some particles, while for others, they will not, just as seen in the experiments of Lim et al.

Trap states are responsible for the diminution of the fluorescence quantum yield in polycrystalline samples. Trapping might involve emitting species like contaminations or structural dislocations like different herringbone arrangements (by changing herringbone angles and/or slips along the long axis) or excimer-like arrangements. While the former should give rise to structured emission and shallow traps due to rather small energy differences to the bulk, the latter should provide somewhat deeper traps and unstructured weak emission due to the optimized H-aggregate arrangement, which will deactivate mainly nonradiatively (vide infra).38 Further nonradiative channels in the (poly)crystalline samples might be due to triplet states and to surface quenching by oxygen giving rise to the formation of singlet oxygen and/or radical cations, that is, polarons. Such quenching is an efficient loss channel80 in polycrystalline samples compared to that in single crystals due to their high surface area and the short distance that the exciton 2692



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Figure 6. Temperature dependence of 1/ΦF − 1 of DSB NPs. (Inset) Fluorescence emission (left) and excitation spectra (right) at 298 and 10 K.

dependence of the DSB spectral overlap JDA between absorption and emission (see the inset of Figure 6), which governs exciton motion in the crystalline material if a resonant hopping process is assumed. For a donor/acceptor (D/A) system, JDD can be calculated from the normalized donor emission spectrum f D(ν) and the acceptor absorption spectrum (in units of the molar extinction coefficient) εA(ν). JD/A =

∫0

∞

fD (ν) ·εA (ν) ·ν−4 dν

Figure 7. Delayed fluorescence (DF) of DSB films at T = 77 K. (a) DF spectra as a function of gate delay against an 8 ns pump pulse at 355 nm (bold curves) and multi-Gaussian fits (thin curves). (b) Same as (a) but normalized to the emission intensity at 2.3 eV, where emission is dominated by excimers. (Inset) Spectral weight (integral spectral width divided by gate length) for the contributions of the excimer and structured emission to the DF spectra (red and black symbols, respectively), as obtained from the multi-Gaussian fits in (a), and fits according to the rate equations given in the SI (curves).

(3)

For exciton migration in the case of A = D (homotransfer), JDD is extracted from the overlap of the DSB emission and absorption spectra, giving a room-temperature value of JDD,298K = 1.9 × 10−15 M−1 cm3, whereas at 10 K, JDD,10K = 0.2 × 10−15 M−1 cm3. Thus, upon cooling, a decrease of one order of magnitude is found, which agrees with the observed temperature dependence of exciton migration and demonstrates that the main factor for its temperature dependence is the spectral overlap of DSB. The thermal activation of the exciton motion process is as well clearly seen in the doping experiments by a drastic drop of the sensitized fluorescence upon decreasing temperature,30 as well as in the recovery of the intrinsic bulk fluorescence in the single NP spectra of Lim et al. upon cooling.46 LT experiments also allow monitoring of the beforementioned low emissive species, which are not seen at room temperature due to the fast depopulation by exciton motion. Figure 7 shows time-resolved emission spectra at delay times of 100−1000 ns at 77 K for a 100 nm thick vapor-deposited film. After 100 ns, the (steady-state) bulk intrinsic fluorescence features of Figure 4 have decayed, and an unstructured emission band appears, with its maximum shifted by 0.56 eV against the electronic origin (2.98 eV). We ascribe these features to a π-stacked (face-to-face) arrangement of adjacent molecules, which might be easily present at the domain boundaries and give rise to this kind of excimer-like emission, as frequently observed in, for example, in PPV-type oligomer crystals,33,38,84,85 covalently bound dimeric model compounds,76,86 and polymer samples.87,88 The excimer characteristics are due to a change in the intermolecular separation,76 as induced by partial CT character of the transition, as observed for fluorinated DSB, where almost perfect π-stacking is found.38 At longer time scales (>200 ns), the vibronically structured (intrinsic) emission of the pristine localized S1 state reappears;

compare Figures 7 and 4. By means of a multi-Gaussian fitting, we obtain the time-resolved spectral weights of the excimer and structured emissions as a function of time (inset of Figure 7). For times t < 1 μs, the excimer emission decays much faster than the delayed structured emission. Then, for t > 1 μs, the ratio excimer/structured emission becomes constant, an indication of an equilibrium reaction. The time-resolved spectral weights of excimer and structured fluorescence, thus obtained, show dynamics on the microsecond time scale that is consistent with excimer generation via (i) migration of directly generated excitons toward predimer sites in a pseudo-first-order process followed by fast decay in the submicrosecond time scale (left part of the red curve in the inset of Figure 7b) and (ii) delayed exciton regeneration on the microsecond time scale by second-order annihilation from precursor states. Continuing excimer decay causes a stationary excimer concentration linked to the precursor lifetime (right part of the red curve in the inset of Figure 7b). From the initial excimer decay, we extract an excimer lifetime of about 83 ns. Candidates for precursor states are long-lived polarons, stabilized at defects, or triplet states. It has been shown that after 400 ps, triplets are the most abundant long-lived states in DSB films.89 Triplets were also found to dominate the microsecond time scale at LTs.47 Therefore, we assign the longlived precursors to triplet states, regenerating singlet excitons by triplet−triplet annihilation (Figure 5), which is expected to occur under the conditions of the experiment (high irradiation densities, LT);90 for details, see the SI. All loss mechanisms described above efficiently compete with the radiative decay channel in the polycrystalline samples and 2693



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thus give rise to small fluorescence quantum yields at ambient conditions, while the loss pathways get effectively suppressed under LT conditions. This clearly demonstrates that the loss channels become operative only through the high efficiency of exciton migration in DSB at ambient conditions, which permits reaching the trap sites. Apparently, this gives a unique chance to achieve high fluorescence quantum yields in polycrystalline samples through sensitization; highly emissive acceptor molecules with good spectral overlap with the DSB emission might efficiently compete with the loss channels already at moderate doping ratios xA. Indeed, we showed in the past that doping of DSB NPs with longer PPV-type oligomers revealed very efficient sensitization already at xA = 10−3 with quantum yields close to unity30,31 and the possibility for facile tuning of the emission color from blue to the red, corresponding to spectral shifts of up to 0.7 eV.35 This might be an additional lesson of the present work, that is, how a detrimental loss mechanism can be positively utilized if the underlying photophysical processes are properly understood. In conclusion, DSB forms pronounced H-aggregates, which are highly luminescent in single crystals but become low emissive in polycrystalline NP suspensions and vapor-deposited or spin-coated films. The reason for this on first glance surprising difference is the low trap concentration in the single crystals due to negligible contaminations and structural defects as well as a low surface-to-volume ratio. In the polycrystalline samples, trap states can be easily reached during the relative long lifetime of the excited state through efficient exciton motion. Both long lifetime and fast exciton migration are promoted by strong H-aggregation in the DSB samples. Among the possible trap states, the main contributions seem to arise from polarons, structural dislocations (here, in particular, excimers at the grain boundaries), triplets, and contaminations, which might be all important even down to the ppm range. Exciton motion is a thermally activated process, essentially driven by the temperature-dependent spectral overlap of the absorption and emission processes; thus, at LTs, also polycrystalline samples can reach high fluorescence quantum yield. Therefore, at ambient conditions, intrinsically highly emissive polycrystalline samples of H-aggregates will be difficult to achieve, while for single crystals, this should rather be the norm. This might be shown more frequently in the future due to the widespread use of absolute quantum yield measurements as well as the increasing interest in single crystals of conjugated materials. Low emissive polycrystalline samples can be however made highly emissive by sensitization at doping ratios already in the ‰ range, making proper use of the per se detrimental exciton migration process.

channel-forming supramolecular host−guest systems,91 in crystals of quasi-isolated molecules,92 but even more in J-aggregate crystalline materials,31,33 high radiative rates efficiently compete with exciton migration. Several examples of DSB-based systems were reported, which exhibit high fluorescence quantum yields not only in the single- but also in the polycrystalline state.17,22,31,33,91−95 Such structures generally utilize secondary forces such as local dipole interactions (in particular by cyano functionalities in the vinylene units) and/or H-bonding by specific substituents to generate specific intermolecular arrangements.3 For DSB, a large library of molecular crystals with known structure and photophysics is available in the meantime, which allows one to establish profound structure−property relationships, as reviewed in a recent feature paper.97 Facing the rapidly increasing number of publications in this field, our present study might also be understood as a reminder that proper assignments, valid structure−property relationships, and correct interpretations require a deep understanding of optical and photophysical properties of conjugated materials.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental and computational details, calculation of solvent shifts and of absorption spectra, fitting of the microsecond spectra, DFT-calculated Raman and IR spectra, excited states, T-dependent nonradiative decay, and reabsorption effects on fluorescence. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

Belectric OPV GmbH, Landgrabenstr. 94, 90443 Nürnberg, Germany. Notes

The authors declare no competing financial interest. Biographies Johannes Gierschner received his Ph.D. in Physical Chemistry in Tübingen (2000). After stays in Tübingen, in Mons, and at Georgia Tech, Atlanta, he became Ramón y Cajal fellow of the Spanish Science Ministry (2008−2013) and joined IMDEA Nanoscience in 2008 as a senior researcher. His work integrates optical spectroscopy and computational chemistry to elucidate the structure−property relationships in conjugated organic materials. www.nanoscience.imdea.org/

For good reasons, the quest for highly luminescent polycrystalline materials has focused in the last years on systems that avoid strong H-type coupling.

Larry Lüer started his research career in the photoconductivity of short molecules (Ph.D., 2001, Tübingen). As Marie Curie Fellow at Politecnico di Milano (Polimi), femtosecond spectroscopy became his main research tool. He became senior researcher in 2003 at Polimi, focused on elementary photophysics in nanostructured systems. Since 2009, he has been senior researcher at IMDEA Nanoscience, working on the improvement of femtosecond spectroscopy for optoelectronic devices.

Nevertheless, for good reasons, the quest for highly luminescent polycrystalline materials has focused in the last years on systems that avoid strong H-type coupling. Weakly coupled H-aggregation with small spectral overlap effectively slows down exciton motion and thus the trapping probability, offering highly luminescent polycrystalline samples.22 In weakly coupled

Begoña Milián-Medina obtained her Ph.D. in Theoretical Chemistry in 2004 at the University of Valencia. After postdoctoral stays in Mons (2004−2007) and GeorgiaTech, Atlanta (2005), she returned to Valencia with a Juan de la Cierva grant of the Spanish Science Ministry. Since 2011, she has been researcher at IMDEA Nanoscience, working on computational approaches to conjugated (metal−)organic materials. 2694



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(11) Siegrist, T.; Fleming, R. M.; Haddon, R. C.; Laudise, R. A.; Lovinger, A. J.; Katz, H. E.; Bridenbaugh, P.; Davis, D. D. The Crystal Structure of the High-Temperature Polymorph of α-Hexathienyl (α6T/HT). J. Mater. Res. 1995, 10, 2170−2173. (12) Siegrist, T.; Kloc, C.; Laudise, R. A.; Katz, H. E.; Haddon, R. C. Crystal Growth, Structure, and Electronic Band Structure of α-4T Polymorphs. Adv. Mater. 1998, 10, 379−382. (13) Antolini, L.; Horowitz, G.; Kouki, F.; Garnier, F. Polymorphism in Oligothiophenes with an Even Number of Thiophene Subunits. Adv. Mater. 1998, 10, 382−385. (14) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Growth and Characterization of Sexithiophene Single Crystals. Chem. Mater. 1995, 7, 1337−1341. (15) Holmes, D.; Kumaraswamy, S.; Matzger, A. J.; Vollhardt, K. P. C. On the Nature of Nonplanarity in the [N]Phenylenes. Chem.Eur. J. 1999, 5, 3399−3412. (16) Oelkrug, D.; Egelhaaf, H.-J.; Gierschner, J.; Tompert, A. Electronic Deactivation in Single Chains, Nano-Aggregates and Ultrathin Films of Conjugated Oligomers. Synth. Met. 1996, 76, 249−253. (17) Shimizu, M.; Himaya, T. Organic Fluorophores Exhibiting Highly Efficient Photoluminescence in the Solid State. Chem. Asian J. 2010, 5, 1516−1531. (18) Spano, F. C. The Spectral Signatures of Frenkel Polarons in Hand J-Aggregates. Acc. Chem. Res. 2010, 43, 429−439. (19) Spano, F. C. The Fundamental Photophysics of Conjugated Oligomer Herringbone Aggregates. J. Chem. Phys. 2003, 118, 981− 994. (20) Spano, F. C. Excitons in Conjugated Oligomer Aggregates, Films and Crystals. Annu. Rev. Phys. Chem. 2006, 57, 217−243. (21) Wang, H.; Li, F.; Gao, B.; Xie, Z.; Liu, S.; Wang, C.; Hu, D.; Shen, F.; Xu, Y.; Shang, H.; et al. Doped Organic Crystals with High Efficiency, Color-Tunable Emission toward Laser Application. Cryst. Growth Des. 2009, 9, 4945−4950. (22) Yoon, S.-J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.G.; Kim, D.; Park, S. Y. Multi-Stimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets. J. Am. Chem. Soc. 2010, 132, 13675−13683. (23) Stampfl, J.; Tasch, S.; Leising, G.; Scherf, U. Quantum Efficiencies of Electroluminescent Poly(para-phenylenes). Synth. Met. 1995, 71, 2125−2128. (24) Kabe, R.; Nakanotani, H.; Sakanoue, T.; Yahiro, M.; Adachi, C. Effect of Molecular Morphology on Amplified Spontaneous Emission of Bis-styrylbenzene Derivatives. Adv. Mater. 2009, 21, 4034−4038. (25) Yoon, S.-J.; Park, S. Y. Polymorphic and Mechanochromic Luminescence Modulation in the Highly Emissive Dicyanodistyrylbenzene Crystal: Secondary Bonding Interaction in Molecular Stacking Assembly. J. Mater. Chem. 2011, 21, 8338−8346. (26) Egelhaaf, H.-J.; Gierschner, J.; Oelkrug, D. Characterization of Oriented Oligo- (phenylenevinylene) Films and Nano-Aggregates by UV/Vis-Absorption and Fluorescence Spectroscopy. Synth. Met. 1996, 83, 221−226. (27) Gierschner, J.; Egelhaaf, H.-J.; Oelkrug, D. Absorption, Fluorescence, and Light Scattering of Oligothiophene and Oligophenylenevinylene Nanoaggregates. Synth. Met. 1997, 84, 529−530. (28) Egelhaaf, H.-J.; Brun, M.; Reich, S.; Oelkrug, D. Luminescence and Photoconductivity Studies on Bonding, Mobility and Electronic Deactivation in Submonolayers and Thin Films of Distyrylbenzene. J. Mol. Struct. 1992, 267, 297−302. (29) Oelkrug, D.; Haiber, J.; Lege, R.; Stauch, H.; Egelhaaf, H.-J. Temporal Stability of Vapor-Deposited Molecular Films as Studied by Laser Light Scattering. Thin Solid Films 1996, 284−285, 581−584. (30) Gierschner, J.; Egelhaaf, H.-J.; Oelkrug, D.; Müllen, K. Electronic Deactivation and Energy Transfer in Doped Oligophenylenevinylene Nanoparticles. J. Fluoresc. 1998, 8, 37−44. (31) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Hohloch, M.; Steinhuber, E. Tuning of Fluorescence Yields in Films and Nanoparticles of Oligophenylenevinylenes. J. Phys. Chem. B 1998, 102, 1902−1907.

Dieter Oelkrug, Professor emeritus of Physical Chemistry, is the supervisor of the Ph.D. theses of more than 60 diploma students in chemistry and physics. He has more than 150 original scientific publications in the fields of heterogeneous chemistry, fluorescence and transient absorption spectroscopy, photo processes on surfaces and in thin organic films, the theory of multiple elastic, and inelastic scattering of light in packed turbid media. Hans-Joachim Egelhaaf, after finishing his Ph.D. (1996) and habilitation (2005) in Tübingen, joined the ultrafast spectroscopy group at the Politecnico di Milano. In 2006, he moved to the Johannes-Kepler-University in Linz from where he relocated to Nuremberg in 2009 to become Stability Group Manager at the German R&D site of Konarka Technologies. Since 2012, he has been the Director R&D at Belectric OPV GmbH in Nuremberg.

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ACKNOWLEDGMENTS The work at IMDEA was supported by the Spanish Ministerio de Economı ́a y Competitividad (MINECO; Project CTQ201127317) and by the Comunidad de Madrid (Projects S2009/ MAT-1726, S2009/PPQ-1533). Ramón y Cajal fellowships of the MINECO are acknowledged by J.G. (2008−2013) and L.L. (2010−2015). L.L. also acknowledges funding from the EC via the COFUND program AMAROUT. J.G. and H.-J.E. are regular visiting researchers of the IPTC Tübingen. E. Ortı ́ (Valencia) is acknowledged for access to computing facilities, and R. Kessler (IAF Reutlingen) is acknowleged for access to the NIR reflection spectrometer.

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(32) Oelkrug, D.; Gierschner, J.; Egelhaaf, H.-J.; Lüer, L.; Tompert, A.; Müllen, K.; Stalmach, U.; Meier, H. Evolution of Optical Absorption from Small Oligomers to Ideally Conjugated PPV and MEH-PPV Oligomers. Synth. Met. 2001, 121, 1693−1694. (33) Schweikhart, K.-H.; Hohloch, M.; Steinhuber, E.; Hanack, M.; Lü er, L.; Gierschner, J.; Egelhaaf, H.-J.; Oelkrug, D. Highly Luminescent Oligo(phenylenevinylene) Films: The Stereochemical Approach. Synth. Met. 2001, 121, 1641−1642. (34) Gierschner, J.; Mack, H.-G.; Lüer, L.; Oelkrug, D. Fluorescence and Absorption Spectra of Oligophenylenevinylenes: Vibronic Coupling, Band Shapes and Solvatochromism. J. Chem. Phys. 2002, 116, 8596. (35) Egelhaaf, H.-J.; Gierschner, J.; Oelkrug, D. Polarizability Effects and Energy Transfer in Quinquethiophene Doped Bithiophene and OPV Films. Synth. Met. 2002, 127, 221−227. (36) Gierschner, J.; Egelhaaf, H.-J.; Oelkrug, D.; Botchway, S.; Parker, A. W. Time-Resolved Fluorescence Spectroscopy on Single Oligophenylenevinylene Nanoparticles. Tech. Rep. - Counc. Cent. Lab. Res. Counc. 2004, 131−133. (37) Gierschner, J.; Oelkrug, D. Optical Properties of Oligophenylenevinylenes. Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H., Ed.; American Scientific Publishers: Valencia, CA, 2004; Vol. 8, pp 219−238. (38) Gierschner, J.; Ehni, M.; Egelhaaf, H.-J.; Milián Medina, B.; Beljonne, D.; Benmansour, H.; Bazan, G. C. Solid State Optical Properties of Linear Polyconjugated Molecules: π-Stack Contra Herringbone. J. Chem. Phys. 2005, 123, 144914. (39) Srinivasan, G.; Villanueva-Garibay, J. A.; Müller, K.; Oelkrug, D.; Milián Medina, B.; Beljonne, D.; Cornil, J.; Wykes, M.; Viani, L.; Gierschner, J.; et al. Dynamics of Guest Molecules in PHTP Inclusion Compounds as Probed by Solid-State NMR and Fluorescence Spectroscopy. Phys. Chem. Chem. Phys. 2009, 11, 4996−5009. (40) Oelkrug, D.; Reich, S.; Wilkinson, F.; Leicester, P. A. Photoionization on Insulator Surfaces. Diffuse Reflectance Laser Flash Photolysis of Distyrylbenzenes Adsorbed on Silica and Alumina. J. Phys. Chem. 1991, 95, 269−274. (41) Marri, E.; Pannacci, D.; Galiazzo, G.; Mazzucato, U.; Spalletti, A. Effect of the Nitrogen Heteroatom on the Excited State Properties of 1,4-Distyrylbenzene. J. Phys. Chem. A 2003, 107, 11231−11238. (42) Ginocchietti, G.; Galiazzo, G.; Mazzucato, U.; Spalletti, A. Competitive Radiative and Reactive Relaxation Channels in the Excited State Decay of Some Thio-Analogues of EE-Distyrylbenzene. Photochem. Photobiol. Sci. 2005, 4, 547−553. (43) Ginocchietti, G.; Cecchetto, E.; De Cola, L.; Mazzucato, U.; Spalletti, A. Photobehaviour of Thio-Analogues of Stilbene and 1,4Distyrylbenzene Studied by Time-Resolved Absorption Techniques. Chem. Phys. 2008, 352, 28−34. (44) Wu, C. C.; Korovyanko, O. J.; DeLong, M. C.; Vardeny, Z. V.; Gutierrez, J. J.; Ferraris, J. P. Optical Studies of Distyrylbenzene Single Crystals. Synth. Met. 2003, 139, 735−738. (45) Wu, C. C.; Ehrenfreund, E.; Gutierrez, J. J.; Ferraris, J. P.; Vardeny, Z. V. Apparent Vibrational Side Bands in π-Conjugated Systems: The Case of Distyrylbenzene. Phys. Rev. B 2005, 71, 081201. (46) Lim, S.-H.; Bjorklund, T. G.; Bardeen, C. J. Characterization of Individual Submicron Distyrylbenzene Aggregates Using Temperature-Dependent Picosecond Fluorescence and Atomic Force Microscopy. J. Phys. Chem. B 2004, 108, 4289−4295. (47) Colaneri, N. F.; Bradley, D. D. C.; Friend, R. H.; Burn, P. L.; Holmes, A. B.; Spangler, C. W. Photoexcited States in Poly(pPhenylene Vinylene): Comparison with trans, trans-Distyrylbenzene, a Model Oligomer. Phys. Rev. B 1990, 42, 11670−11681. (48) Vasil’eva, I. A.; Voitova, N. A.; Nurmukhametov, R. N. Effect of the Fluorine Substitution in Ethyl Groups of 1,4-Distyrylbenzene on the Fine Structure Fluorescence and Fluorescence Excitation Spectra. Opt. Spectrosc. 2012, 112, 443−452. (49) In the site-selective experiments of ref 48, the 33 and 63 cm−1 signals of the fluorescence spectrum remained unassigned. Taking into account our (TD)DFT results on the torsional modes and applying the model for distorted, undisplaced oscillators, where excitations to even

quanta are allowed, we tentatively assign them to signatures of the torsional modes (2·θ1, 2·θ2), thus giving θ1 = 16 cm−1 and θ2 = 32 cm−1. Accordingly, we tentatively assign the signatures at 51 and 172 cm−1 in the fluorescence excitation spectrum to the respective torsional modes in the first excited state, θ1 = 26 cm−1 and θ2 = 86 cm−1. (50) For the precise determination of vertical and adiabatic transition energies from the experimental spectra, see: Gierschner, J.; Cornil, J.; Egelhaaf, H.-J. Optical Bandgaps of π-Conjugated Organic Materials at the Polymer Limit: Experiment and Theory. Adv. Mater. 2007, 19, 173−191. (51) Aloshyna, M.; Milián Medina, B.; Poulsen, L.; Moreau, J.; Beljonne, D.; Cornil, J.; Di Silvestro, G.; Cerminara, M.; Meinardi, F.; Tubino, R.; Detert, H.; Schrader, S.; Egelhaaf, H.-J.; Botta, C.; Gierschner, J. Oligophenylenevinylenes in Spatially Confined Nanochannels: Monitoring Intermolecular Interactions by UV/Vis and Raman Spectroscopy. Adv. Funct. Mater. 2008, 18, 915−921. (52) Rumi, M.; Barlow, S.; Wang, J.; Perry, J. W.; Marder, S. R. TwoPhoton Absorbing Materials and Two-Photon-Induced Chemistry. Adv. Polym. Sci. 2008, 213, 1−95 , and citations therein. (53) For ortho-(2,2′)dimethyl-DSB, the vertical 11Ag → 21Ag is seen as a broad band with a maximum at around 4.3 eV (ref 96), thus about 0.8 eV above 11Bu of DSB (3.53 eV in hexane). At the same time, the TD-DFT-calculated energy difference (vertical, in vacuo) for both odimethyl-DSB and DSB is 0.70 eV, while 11Bu is located at 3.16 and 3.18 eV, respectively. (54) Sandros, K.; Sundahl, M.; Wennerström, O.; Norinder, U. Cis− Trans Photoisomerization of a p-Styrylstilbene, a One- and Twofold Adiabatic Process. J. Am. Chem. Soc. 1990, 112, 3082−3086. (55) Measurements in different solvents give qualitatively similar numbers; see the SI. (56) Sundahl, M.; Wennerströ m, O.; Sandros, K.; Arai, T.; Tokumaru, K. Triplet State Z/E Isomerization of a p-Styrylstilbene, a Partly Adiabatic Process. J. Phys. Chem. 1990, 94, 6731−6734. (57) Takeuchi, S.; Ruhman, S.; Tsuneda, T.; Chiba, M.; Taketsugu, T.; Tahara, T. Spectroscopic Tracking of Structural Evolution in Ultrafast Stilbene Photoisomerization. Science 2008, 322, 1073−1077. (58) Kovalenko, S. A.; Dobryakov, A. L.; Ioffe, I.; Ernsting, N. P. Evidence for the Phantom State in Photoinduced cis−trans Isomerization of Stilbene. Chem. Phys. Lett. 2010, 493, 255−258. (59) Bhongale, C. J.; Chang, C.-W.; Lee, C.-S.; Diau, E. W.-G.; Hsu, C.-S. Relaxation Dynamics and Structural Characterization of Organic Nanoparticles with Enhanced Emission. J. Phys. Chem. B 2005, 109, 13472−13482. (60) Hsu, F.-C.; Lin, S. H.; Wang, J.-K. Excited-State Dynamics of trans,trans-Distyrylbenzene: A Femtosecond Transient Absorption Study. Chem. Phys. Lett. 2005, 411, 103−107. (61) Hsu, F.-C.; Hayashi, M.; Wang, H.-W.; Lin, S. H.; Wang, J.-K. Excited-State Dynamics of trans,trans-Distyrylbenzene: Transient Anisotropy and Excitation Energy Dependence. J. Phys. Chem. A 2007, 111, 759−763. (62) Hsu et al. base their arguments against kCR as the responsible channel for the 10 ps component on the similarity of their transient studies at two pump wavelengths (350 and 380 nm), assuming that the latter represents the bottom of the S1 state (refs 60 and 61). However, in n-hexane, the onset of absorption onset is at 390 nm, so that at 380 nm, already 22% of the maximum absorbance is reached, so that their assumption is not correct. Furthermore, they claim that thermal relaxation should be faster than their time resolution but without giving further proof for this. This should be correct for the (in-plane) high-frequency modes but might be very different for the lowfrequency torsional modes discussed here. (63) Seixas de Melo, J.; Pina, J.; Burrows, H. D.; Di Paolo, R. E.; Maçanita, A. L. Electronic Spectral and Photophysical Properties of some p-Phenylenevinylene Oligomers in Solution and Thin Films. Chem. Phys. 2006, 330, 449456. (64) Di Paolo, R. E.; Seixas de Melo, J.; Pina, J.; Burrows, H. D.; Morgado, J.; Maçanita, A. L. Conformational Relaxation of pPhenylenevinylene Trimers in Solution Studied by Picosecond Time-Resolved Fluorescence. ChemPhysChem 2007, 8, 2657−2664. 2696



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