Solid State Luminescence Enhancement in Ï-Conjugated Materials

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On the Solid State Luminescence Enhancement in #-Conjugated Materials: Unraveling the Mechanism beyond the Framework of AIE/AIEE Junqing Shi, Luis E. Aguilar Suarez, Seong-Jun Yoon, Shinto Varghese, Carlos Serpa, Soo Young Park, Larry Lüer, Daniel Roca-Sanjuán, Begoña Milián-Medina, and Johannes Gierschner J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08060 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

On the Solid State Luminescence Enhancement in π-Conjugated Materials: Unraveling the Mechanism beyond the Framework of AIE/AIEE Junqing Shi,1 Luis E. Aguilar Suarez,2 Seong-Jun Yoon,3 Shinto Varghese,1 Carlos Serpa,4 Soo Young Park,3,* Larry Lüer,1 Daniel Roca Sanjuán,2 Begoña Milián-Medina,1,5,* Johannes Gierschner1,* 1

Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Campus

Cantoblanco, 28049 Madrid, Spain. 2

Institute of Molecular Science, University of Valencia, P.O. Box 22085, ES-46071,Valencia

3

Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program,

Department of Materials Science and Engineering, Seoul National University, ENG 445, Seoul 151-744, Korea. 4

CQC, Chemistry Department, University of Coimbra, P3004-535 Coimbra, Portugal.

5

Department for Physical Chemistry, Faculty of Chemistry, University of Valencia, Avenida Dr.

Moliner 50, 46100 Burjassot (Valencia), Spain, * E-mail: [email protected], [email protected], [email protected] Abstract Solid state luminescence enhancement (SLE) of conjugated organic materials has found great impact in materials science, but a deep understanding is rather limited by now. Here, we investigate a prototype example of SLE materials, cyano-substituted distyrylbenzene (DCS) with varying systematically and subtly the substitution pattern (inter alia of the position of the cyanosubstituent) to give largely different photo-response in fluid and solid solution as well in the crystalline state. Combination of quantitative (ultra)fast optical spectroscopic techniques, appropriate quantum-chemical methods, and structural (X-ray) data allow us to elucidate and rationalize all details of the SLE process, including steric vs. electronic factors, radiative vs. nonradiative decay channels and intra- vs. intermolecular contributions, providing a first holistic picture of SLE.

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1. Introduction During the last two decades, luminescent organic micro- and nanocrystals have become an increasingly important subject of current materials research with emerging applications in organic (opto)electronics, such as organic field effect transistors (OFETs), light emitting diodes (OLEDs), lasers, solar cells (OSCs), sensors and luminescent bioprobes.1-16 This interest is driven by the possibility to design not only molecular properties by imaginative chemical synthesis, but by the prospect to design materials by ultimate control of the intermolecular arrangement; the latter was shown to be decisive for the resulting electronic, optical and photophysical materials' properties.11, 13 Within the class of luminescent organic crystals, special attention was paid to materials which are non-luminescent in solution, but becoming luminescent in the solid state. This is a long known phenomenon, firstly described on isocyanines in the 1930s by Jelley and by Scheibe,16 who highlighted the intermolecular (i.e. inter-chromophore) contributions to the luminescence enhancement (so-called 'J-aggregation'). Starting from the 1960s, the intramolecular contributions were thoroughly examined through low temperature and/or high viscosity experiments (e.g. in solid solutions) for various stilbenoid systems, including stilbene itself as well as tetraphenylethylene, TPE.17-23 In the 1990s, Oelkrug and Hanack showed how systematic changes in the substitution pattern of distyrylbenzene (DSB), especially by the introduction of the cyano functionality in the vinylene unit (DCS derivatives), allowed for all combinations of non/-emissive solution/solid state phases.24-32 The authors stressed, probably for the first time, the importance of intra- vs. intermolecular contributions, by concurrently investigating solid solutions and (poly)crystalline phases,25 as recently reviewed.10, 11, 13

Some years later, the phenomenon was popularized as 'aggregation-induced emission'

(AIE),15, 33, 34 being however a somewhat infelicitous label, since this term blurs the differences of the above mentioned intra- and intermolecular contributions; in fact, according to the IUPAC definition,35 aggregation describes a "process whereby dispersed molecules or particles form aggregates"; i.e. not applicable to highly viscous or solid solutions. The introduction of the term 'aggregation-induced enhanced emission' (AIEE)36 reflected this ambiguity, and stressed the synergy of intra- and intermolecular contributions; in particular for DCS-type materials.3 A physically sound, phenomenological term which avoids arguable mechanistic ones (e.g. 'induced' or 'caused'), and includes both intra- and intermolecular factors, might be simply coined as 'solid state luminescence enhancement' (SLE). Indeed, 'solid state' can mean both, molecular dissolved solids, i.e. frozen or solid solutions - for instance poly(methylmethacrylate), PMMA, and other highly viscous matrices - as well as the condensed phase of the compounds, i.e. amorphous, poly- or single-crystalline materials. 2 ACS Paragon Plus Environment

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Intramolecular Factors for SLE. Despite the success story of SLE systems in materials science, the mechanistic aspects of the luminescence enhancement are still a matter of debate due to the above mentioned intra- and intermolecular contributions of excited state deactivation which might widely vary for different SLE-active compounds. In general, for the intramolecular part of SLE, the low fluorescence quantum yields (Φ F) in solution can have two contributions (Scheme 1), i.e. a low radiative rate kr or a high non-radiative rate knr from the first excited singlet state (S1) according to Φ F = k rτ F = k r /(k r + k nr )

(1)

with τF being the fluorescence lifetime. Intramolecular SLE due to low kr can be thus induced by excited state reordering between bright (i.e. large oscillator strength f) and dark (small f) states in different environments, since kr is directly related to f (each in a given solvent of refractive index n) of the emitting state via the Strickler-Berg (SB) relation37 ,38

[

]

k r ,SB = 0.667 cm 2 s −1 ν~F−3 where ν~F−3

−1

= ∫ I (ν~)dν~

−1

−1 n 2 ⋅ ν~A f

∫ν~

−3

(2a)

I (ν~)dν~ and ν~A ,ν~F are the energies of absorption and emission

(in cm-1). For the determination from quantum chemistry, an approximate equation can be used ν~ 3 k r , SB = 0 .667 cm 2 s − 1 ~F , vert n 2 f ν

[

]

(2b)

A , vert

where ν~A,vert , ν~F ,vert are the vertical energies in absorption and emission (in cm-1). Excited state switching can be e.g. provoked by changes in polarity or polarizability of the solvent.39 Intramolecular SLE due to high knr is more complex due to a multitude of non-radiative deactivation pathways such as (S1→S0) internal conversion (IC), intersystem crossing (ISC) to the triplet manifold, or photochemical processes like trans-cis isomerization (as in the case of stilbene).23 The deactivation channel will depend sensitively on the energetics and shapes of the Sn (n = 0,1,2...) potential energy surfaces PES (and possibly further involved triplet states) along different (torsional) coordinates, and on their thermal population; this makes them vulnerable against (subtle) environmental changes such as decreasing temperature and increasing viscosity, which reduce knr. In stilbenoid systems, deactivation scenarios involve combined torsional motions around single and double bonds,20, 40 being much more complex as now popular terms like 'restriction of internal rotation' (RIR) anticipate. Indeed, for DCS compounds, it was suggested that a 'floppy' molecular structure is a necessary but insufficient condition, but further requires twists in S1 with subsequent slow relaxation.25



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Despite intense research over the years, the understanding of systems with ultrafast excited state deactivation via IC is limited; in particular, in systems with an extended π-conjugation. Experimentally, pump-probe experiments with ultrafast time-resolution are needed.41-44 From theory, it requires a reliable description of all involved PES, particular in the state-crossing regions, which give rise to conical intersections (CI), along local coordinates which promote such processes. Importantly, the PES of S0 at such high energies (i.e. 1.5-3.5 eV) becomes very anharmonic, which questions harmonic approaches to tackle this problem at a quantum level,45 in particular employing 2-states models based on normal modes. In general, time-dependent density functional theory (TD-DFT) fails to describe excited state PES in the region of the CI. One way to deal with this problem in a reliable manner is the use of higher correlated methods using multi-configurational wave functions, in particular, the complete-active-space self-consistent field/complete-active-space second-order perturbation theory (CASSCF/ CASPT2),46 which however is limited to rather small molecules (with a current maximum of about 16 π-electrons). In fact, the IC processes of SLE-active molecules (tetraphenylsilole47, B18H20(NC5H5) hybrid composite48) were recently elucidated, which allowed for a qualitative determination of the mechanisms. However, the active spaces were largely reduced, which prohibits a deep understanding of the IC processes. Alternatively, mixed quantum-classical trajectory surface hopping approaches allow for a dynamic description of IC in SLE materials, as recently demonstrated for TPE.49 Even higher approximations are used here in the methodology due to the high cost of the semi-classical dynamics computations.

Scheme 1: What controls SLE? Intra- vs. intermolecular contributions and radiative vs. non-radiative contributions (Eopt = optical gap, lowT = low temperature, AHT = aggregate-type Herzberg-Teller, Z = number of molecules per unit cell).

Intermolecular Factors for SLE. In aggregates or crystals, IC is usually small due to dense packing, and kr might be enhanced due to planarization, which (somewhat) increases f. The main 4 ACS Paragon Plus Environment

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intermolecular part of SLE however can be treated in the framework of molecular excitons as introduced by Kasha in the 1950s,50 who considered the interactions of electronic transitions in r

molecular assemblies, i.e. the relative position of their constituting transition dipole moments µ01 , quantified by their strength and energies. There, two cases can be distinguished, H-aggregates, characterized by a 'side-by-side' arrangements with blue-shifted absorption and (for perfect alignment) forbidden emission, and the 'head-to-tail' oriented J-aggregates with red-shifted, allowed emission, where the emission enhancement scales with the exciton coherence in the aggregate.51,52 Since Kasha's original work is still largely cited, it should be reminded that his model is based on strong approximations, i.e. using dimers with a constant center-to-center distance treated by point dipoles; only in this case the inversion point between H/J-aggregates is found at the 'magic angle' value of θi = 54.7º. All these approximations break down in molecular crystals, and Kasha's model thus has to be replaced by a quantum-chemical model to properly account for Coulombic interactions53 (and possibly exchange contributions54), where neighboring molecules are characterized rather by a constant inter-plane separation and shifted against each other along their long molecular axis (x-slip; see Scheme 1),11, 13 and θi depends on nature and intermolecular separation of the molecules; further, quantitative results have to take into account non-nearest neighbor interactions.55 Finally, the exciton model tells only something about radiative rates kr, and does not yield ΦF. Therefore, H-aggregates do not necessarily 'notoriously' quench the emission as often claimed in the frequently used 'aggregation-caused quenching' (ACQ) terminology, but are usually highly emissive under practically trap-free conditions, e.g. single crystals.11,

13

Emission quenching is instead often observed in

polycrystalline samples (films or nanoparticle suspensions) as an effect of trapping at surfaces and/or grain boundaries,10 due to high surface trap concentrations, large surface: volume ratios and small path lengths towards the surface/interface. Having properly defined the principle parameters which control SLE, the present work aims at disentangling radiative and non-radiative decay channels, and intra- and intermolecular contributions of the SLE phenomenon, examined for a family of structurally closely related DCS compounds. Following the pioneering work in the groups of M. Hanack and D. Oelkrug,24-32 DCS-type materials were intensively investigated for optoelectronic applications as reviewed over the years.3, 9, 11-13, 56 Choice of the substitution position of the cyano-group in the vinylene unit and additionally alkoxy-substitution in the terminal or/and central benzene groups generates a family of DCS compounds with very different photophysical properties in fluid solution, solid solution (PMMA matrix etc.), single crystals and polycrystalline samples (nanoparticles & thin films).24, 25, 43, 57--77 The materials are investigated here by quantitative steady-state and time5 ACS Paragon Plus Environment


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resolved fluorescence, transient absorption spectroscopy and photoacoustic calorimetry (to quantify the multiple pathways of nonradiative deactivation), where the combination with TDDFT calculations and exact structural information through single crystal X-ray analysis allows for a unique insight into the SLE mechanism, and to identify true AIEE compounds. Due to the complexity of the current study, we will start with an overview of the materials in the Results part (Section 3), detailing the structure variation, and spectra and photophysics in fluid and solid solution as well as in the single crystalline state. The Discussion part (Section 4) will then systematically elucidate geometries, spectral positions, intensities and shapes in liquid and solid solution and matrix-dependent photophysics, to finally turn to the crystalline state.

2. Experimental and Computational Details 2.1 Materials Preparation. Synthesis and characterization of the DCS-type molecules have been described elsewhere.57-61 Handling of the samples (storing, preparation of the solutions and crystal growth) was done in the dark to avoid photoisomerization (and possible photo-oxidation). Fluid solution measurements were carried out in chloroform (CHCl3; Sigma-Aldrich, spectroscopic grade) with an absorbance of typically E ≈ 0.1 except for pump-probe spectroscopy (E ≈ 1 in 1,4-Dioxane, Sigma-Aldrich, spectroscopic grade). Solid solution experiments were performed in PMMA (Sigma-Aldrich) matrix at low concentration (cM ≈ 10-4 M); vigorous vibration (done by platform shaker Vibramax 100) was performed during mixing CHCl3 solution with PMMA powder to further prevent aggregation. For steady state and time resolved optical spectroscopy measurements, PMMA films were fabricated on glass slides via drop casting; for PAC experiments, the PMMA slides were prepared by doctor blading through a Cube Film Applicator (Elcometer 3505). Single crystals were prepared from CHCl3/hexane mixture solutions.

2.2 Spectroscopy. UV-Vis absorption spectra were recorded on a Varian Cary 50 UV-VIS spectrophotometer in 1 cm quartz Suprasil cuvettes. Light exposure, which induces photoisomerization in some of the compounds (vide infra), was minimized by sample handling in the dark, and rapid spectral sampling. Room-temperature photoluminescence emission (PL) and excitation (PLE) spectra were acquired in a Horiba FluoroLog 3 spectrofluorimeter equipped with a high pressure Xenon lamp, and a Hamamatsu R928P photomultiplier tube; the PLE and PL spectra were corrected for the characteristics of the lamp source and of the detection system, respectively. Relative fluorescence quantum yields of CHCl3 solutions were measured using quininesulfate in 0.5 M sulfuric acid as a standard reference (ΦF = 0.54),78 and absolute fluorescence ΦF of PMMA films and (single) crystals were determined in an integrating sphere 6 ACS Paragon Plus Environment

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setup (Hamamatsu C9920) equipped with a xenon high-pressure lamp and a multichannel analyzer at 350 nm excitation wavelength. For the crystal measurements, we utilized the smallest possible sizes (< 1 mm) to reduce reabsorption, and at the same time allow for enough intensity, due to the limited sensitivity of the integrating sphere. Anyway, it should be noted that even under these conditions, reabsorption is unavoidable; for that reason the reported values for ΦF of the crystals represent the lower limit of the intrinsic PL quantum yields. It should be further noted that the sensitivity of the integrating sphere is not high, giving rise for a high absolute error; this is in particular problematic for PMMA samples with low ΦF. PL lifetime experiments were performed by the time-correlated single photon counting (TCSPC) technique with an Acton SP2500 spectrometer (PicoQuant) equipped with a PicoHarp300 TCSPC board (PicoQuant) and a PMA 06 photomultiplier (PicoQuant). A HydraHarp-400 TCSPC event timer with 1 ps time resolution was used to measure the PL decays. The excitation source was a 405 nm picosecond pulsed diode laser (LDH-D-C-405, PicoQuant) driven by a PDL828 driver (PicoQuant) with FWHM < 70ps. The decay time fitting procedure was carried out by deconvolution with the IRF using the Fluofit software (PicoQuant). To reduce reabsorption in the crystal measurements, we utilized the smallest possible sizes, detected under front face conditions and irradiation of the crystal edge only; like this, the very non-exponential PL lifetimes observed at strong reabsorption of the crystal become increasingly exponential at low reabsorption. Femtosecond TA spectroscopy measurements were carried out on a probe wavelength scale ranging from 470 nm-730 nm, from 100 fs to 400 ps, at different excitation intensities. Optical pulses centered at 775 nm were generated from a Ti:sapphire laser (Clark-MXR, CPA-2001) driven at 1 kHz by a regenerative amplifier, and split into two parts. A fraction of one passed through an optical delay line, which was controlled via a mechanical translation stage, and directed to a sapphire plate in order to obtain a white light continuum, which we used as the probe beam. One part of the probe beam was focused onto the sample (about 134 µm spot size) with a spherical mirror. After passing through the sample, the probe beam was focused onto the slit of our prism spectrometer (Entwicklungsbüro Stresing GmbH), which consisted of a dual channel CCD array (2 X 256 pixels, VIS-enhanced InGaAs, Hamamatsu Photonics Inc). The other part of the probe beam was used as a reference to reduce laser fluctuation induced noise. The second part of the fundamental 775 nm pulses was sent to a second harmonic generator to achieve the pump pulse centered at 387 nm, and chopped at 500 Hz. Then the pump beam was focused onto the sample (about 260 µm spot size) to overlap with the probe beam, and blocked after the sample. Intensities of both pump and probe beams were controlled via neutral density 7 ACS Paragon Plus Environment


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filters. In this work, all the measurements were carried out on the magic angle (otherwise stated), by setting the polarization angle of both pump and probe beams with 2/λ plates. Photoacoustic calorimetry (PAC) experiments were performed using a homemade front-face PAC cell design described by Arnaut et al79. The instrumentation and procedure employed in the measurements, and the data analysis protocol, were reported in detail elsewhere.80-82 In short, The PAC data were acquired by a setup consisting in a unfocused Nd:YAG laser (EKSPLA PL2143A, 35 ps pulse duration, 10 Hz, ~8 mm beam diameter), for sample excitation and a 2.25 MHz Panametrics transducer (model A106S) pre-amplified with a Panametrics ultrasonic preamplifier (model 5676) for detecting the acoustic waves generated by non-radiative processes All the samples were irradiated at 355 nm and 2-hydroxybenzophenone was used as photoacoustic reference. The data analysis was based on excitation energy balance; for the details of multifold nonradiative rate constants calculation see Supporting Information (SI).

2.3 Computational Details. The geometries of the DCS-type compounds were optimized under Ci symmetry restriction (for DSB, C2h symmetry was used) in vacuum at the density functional theory (DFT) level of theory. Thus, only anti-rotamers (as defined earlier)10 were considered, in agreement with the available x-ray data. For compounds with MeO groups in the central ring, the calculations were performed for two rotational conformers indicated by (P) and (S) in Scheme 2; for details see Section 3.1. To reduce the computational effort, in the compounds with butoxy groups the latter were replaced by methoxy groups. Vertical singlet and triplet excited states were calculated using time-dependent (TD-)DFT in vacuum and in CHCl3 using the polarizable continuum model (PCM). Adiabatic singlet and triplet transitions were calculated by relaxing the S1 and T1 geometries respectively. According to group theory, the symmetry allowed π-π* type singlet transitions of the DCS-type compounds in the Ci point group from the ground state (11Ag) take place to the bright n1Au (n = 1,2...) states, while the n1Ag (n = 2,3...) are essentially dark; for

DSB (C2h) the corresponding states are the n1Bu and n1Ag, respectively. The ground-state barriers for the torsions around specific single bonds indicated in Scheme 2 were monitored by DFT calculations via scanning frozen bonds in steps of 15° with full geometry relaxation under this restriction. Excited-state barriers for the torsion around the double bond shown in Scheme 2 were explored by performing TD-DFT computations along twisted structures differing 15° from each other. Torsional angles around phenylene-vinylene single bond (φi/ φo) ranges from 0 to 90° due to symmetry reason. For the vinylene double bond (φDB), the torsion between 180º (trans isomer) and 75º were considered in the calculations. In all calculations, the B3LYP functional and the 6311G* basis set were employed as implemented in the Gaussian09 program package.83 Molecular orbitals (MOs) were visualized with GaussView 5.0. 8 ACS Paragon Plus Environment

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For the monomer and dimer calculations of the crystal, molecular geometries and nearest neighbor arrangements were extracted from the available X-ray analysis.57-60, 70, 84 The molecules were then B3LYP-optimized, however imposing the torsional angles found in the X-ray analysis. Replacing the monomers/dimers of the X-ray analysis by these B3LYP-optimized molecules, single point TD-DFT calculations were then performed at the CAM-B3LYP level (6-311G* basis set);83 For comparison, the same procedure was applied to the fully relaxed monomers. CASSCF computations were performed to determine the electronic-structure properties of the ground and lowest-lying singlet excited states at the Franck-Condon and CI regions of the PES. The atomic natural orbital S-type valence double-ζ plus polarization (ANO-S-VDZP) basis set was used.85 The active space comprises the 12 most relevant π-electrons distributed in 12 natural orbitals; hereafter, CASSCF(12-in-12). The MOLCAS 8.1 quantum-chemistry package of software was used for these computations.86

Scheme 2: The DCS family under study: substitution pattern (Rα, Rβ, Rt, Rc) of the cyano (CN), methoxy (MeO) and butoxy (BuO) substituents; torsional angles around the inner and outer vinyl-phenyl bonds (φi, φo,) and the vinyl double bond (φDB); MeO orientation at the central ring (P, S).

3. Results 3.1. The DCS family under study We study a specific library of compounds obtained via functionalizing DSB with cyano groups (DCS) either in the inner (α α) or in the outer (β β ) position of both vinylene units (α α-/β β -series), optionally adding alkoxy groups; the latter are either placed as di-methoxy (MeO) groups in the central aromatic ring (MO-compounds), and/or as di-butoxy (BuO) groups in the terminal aromatic rings (DB-compounds), providing in all a library of eight different DCS compounds. The MeO-groups in the central rings can be oriented parallel (P) to the vinylene unit or perpendicular ('senkrecht'; S), see Scheme 2: Here, the S-form was identified as the rotamer present in the crystal structure of α-/β β-MODCS and β-MODBDCS, while the P-rotamer was found in α-MODBDCS.60 It will be shown in the following that the systematic variation of the 9 ACS Paragon Plus Environment


α/β β position as well as of MeO/BuO substitution significantly affects the optical and photophysical properties in solution and in the solid state. It should be mentioned that the optical and photophysical properties in fluid solution and single crystals were partially published before;57-61,70 however, neither the data sets were not complete, nor a systematic comparative analysis in different environments was done by now. This will allow for a unique insight to structure-property relationships in this and related classes of compounds. 3.2. UV/Vis Absorption and Fluorescence Properties a) Fluid Solution The UV-Vis absorption and PL spectra in CHCl3 solution of all the compounds are shown in Figure 1 and the corresponding maxima values are listed in Table 1; PL quantum yields (ΦF) and lifetimes (τF) are displayed in Figure 2. The parent compound DSB shows a vibronically structured PL spectrum with 'apparent' 0-0 and 0-1 bands at 3.18 eV (390 nm) and 3.01 eV (412 nm);87 for a definition of 'apparent' modes, see SI. Differently, the absorption spectrum is little structured with a maximum at 3.48 eV (356 nm); a small shoulder is recognizable at about 4.1 eV (302 nm). DSB is highly emissive with ΦF = 0.87, showing a lifetime of τF = 1.16 ns, in all very similar to earlier reports. 10, 87, 88 The DCS series exhibit a general red-shift of the PL and absorption spectra (except for the α-DCS absorption) and a loss of vibronic feature relative to that of DSB, however varying largely between the different compounds. The red-shift is more pronounced in the β-series compared to the α-series, and increases with subsequent substitution from DCS → DBDCS → MODCS ≈ MODBDCS; thus, the PL spectrum of β-MODBDCS is red-shifted by 0.7 eV against DSB, i.e. to 2.46 eV (505 nm). 0 0 3

wavelength / nm 0 0 4

0 0 5

0 0 6

0 0 3

0 0 4

0 0 5

0 0 6

0-1 0-0

CHCl3 PMMA

α-series

β -series

DSB d e z i l a m r o n

)

DCS )

. u . a e c n a b r o s b a

(

(

DBDCS

y t i s n e t n i L P

MODCS

A1

A2

MODBDCS 5 . 4

0 . 4

5 . 3

0 . 3

5 . 2

0 . 2 5 . 4

0 . 4

5 . 3

0 . 3

5 . 2

0 . 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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energy / eV

Figure 1: Absorption and PL spectra of the α- (left) and β -series (right) in solution (CHCl3: solid lines; PMMA: dashed lines); the spectra of DSB (top) are shown for comparison.


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Table 1: Optical and photophysical data of DSB and the DCS series in CHCl3: Peak position of absorption (Eabs) and PL (Eem), Stokes Shift EStokes = Eabs(A1) - Eem, adiabatic energy E00, relative intensity ratio f1/f2 of the absorption subbands A1 and A2 (see Figure 1). Eabs (A1, A2) f1/f2 Eem / eV a / eV b 3.48 3.01 (3.18) DSB 3.61 2.93 (3.02) α-DCS 3.34 2.70 (2.81) α-DBDCS 3.45, 4.37 0.65 2.60 α-MODCS 3.43, 3.84 0.58 2.62 α-MODBDCS 3.46 2.77 (2.92) β-DCS 3.19 (2.54) 2.68 β-DBDCS 2.95, 3.60 1.11 (2.37) 2.47 β-MODCS 2.87, 3.40 1.19 (2.32) 2.46 β-MODBDCS a absorption maxima. b PL maxima with apparent (sub)bands from the intersection of absorption and PL spectrum.

EStokes E00 / eV / eV c 0.47 3.26 0.68 3.24 0.68 2.99 0.85 2.96 0.81 2.97 0.79 3.05 0.51 2.84 0.48 2.62 0.41 2.57 in parenthesis. c

s n 1 10/

10

10

-2

10

-3

10

-4

Φ τ

F

-1

F

10

l a t s y r c A M M P

F

s e 0 m 10i t e f i l e c -1 10n e c s e r o -2 u 10l f

l3 C H C

s d l e i y m u t n a u q e c n e c s e r o u l f

0

τ

Φ

F

e l g n i s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B S S S S S S S S S D DC DC DC DC DC DC DC DC B B B B D MO D D MO D O O M M

-3

10

β-series

α-series

Figure 2: Fluorescence quantum yields (solid symbols) and lifetimes (open symbols) of the compounds in CHCl3 (blue), PMMA (red), and single crystals (green).

The loss of vibronic structure in the PL spectra is pronounced in the α-series, in particular upon MeO substitution in the central ring (α α-MODCS, α-MODBDCS). In the β-series, the PL spectra are only slightly broadened against DSB, however a pronounced decrease of the 0-1/0-0 band ratio (i.e. of the 'apparent' Huang-Rhys factor)78 is observed upon subsequent substitution with BuO and MeO substituents. The absorption spectra are, differently to DSB, completely unstructured. The introduction of MeO at the central ring in addition to CN and BuO provokes a splitting of the absorption band, giving the spectra a characteristic 'camel hump' shape.25 11 ACS Paragon Plus Environment


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The PL quantum yields and lifetimes are sharply changed upon CN substitution, see Figure 2: in the α-series ΦF becomes practically zero, hardly affected by extra alkoxy-substituents. In the β series the behavior depends strongly on the substituent pattern; β-DCS itself is very low emissive (ΦF = 1%), while additional alkoxy substituents enhance the fluorescence strongly to values of 20% (β β -MODCS), 31% (β β-MODBDCS) and 54% (β β-DBDCS). The PL lifetimes of the luminescent compound are around 1 ns, while for the practically non-fluorescent compounds, the lifetimes are in the (low) ps range. In general, changes in the PL lifetimes correlate with changes in the quantum yields, Figure 2. b) Solid Solution (PMMA Film) PMMA provides a relatively rigid environment which largely restricts the intramolecular motions of the molecules, and therefore exerts significant influence on the optical spectra (position and bandshape) but in particular on the photophysics (PL quantum yields and lifetimes; see eq. 1). Low concentrations of the molecules in the PMMA matrix (cM ≈ 10-4 M) were used to avoid aggregation, which would significantly impact the photophysics. As UV/Vis absorption spectra could not be recorded for the PMMA samples, we recorded PL excitation (PLE) spectra instead. Figure 1 compares the PL and PLE spectra for all compounds; for DSB the spectral positions and shapes are practically identical to those in CHCl3. This is to a less extent also true for α/β β-DCS as well as α-DBDCS. For the MeO-substituted compounds (as well as for β DBDCS), a significant blue-shift of the PL spectra was observed in PMMA vs. CHCl3, while the onset of the absorption spectra are little changed; furthermore, alkoxy-substituted α-compounds suffer from significant changes of the absorption bandshapes. The vibronic features of the PL spectra in the β -series tend to be more blurred in PMMA compared to CHCl3. As seen in Figure 2, PMMA (substantially) enhances the quantum yield for the DCS compounds, while DSB shows similar (high) quantum yield and lifetime as in fluid solution; the increase of ΦF is especially pronounced in β -DCS (from 1% to 29%), while the increase in ΦF for α-series, which are particularly low emissive in fluid solution, is less pronounced. The already emissive alkoxy-substituted β-DCS compounds become even more emissive in PMMA, with ΦF up to 88% in β-DBDCS. c) Single Crystals To investigate the aggregated state, we utilize monolithic (single) crystals instead of polycrystalline samples (such as vapor-deposited or spin-coated films or nanoparticle suspensions) in order to avoid trapping of the originally created exciton by surface or interface states, which can completely mask the SLE effect as discussed earlier.10 While the photophysical 12 ACS Paragon Plus Environment

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data of some of the crystals were taken from literature, we additionally grew some single crystals from CHCl3 and hexane mixture solution to measure quantum yields and lifetimes; the report on the β -DCS crystal structure is found elsewhere.70 As Figure 2 shows, the single crystals of all compounds are highly emissive with ΦF between 42% and 90%, independently on their specific intermolecular arrangement (in fact ranging from J- to H-aggregates; vide infra);60, 11 here, the yields are always higher than in the corresponding liquids solutions. Thus, no 'aggregationinduced quenching' (ACQ) is seen for the DCS compounds, but instead SLE is observed for all compounds under study independently on intra- and intermolecular factors. On the contrary, the latter are reflected in largely different PL lifetimes, ranging from ca. 2 ns to about 20 ns, see Figure 2. 4. Discussion 4.1. Ground State Geometries Crucial for the understanding of the spectral and photophysical properties of the DCS family are, as we will see later, the geometries of the compounds. The geometries were elucidated by DFT, giving strongly twisted structures upon free optimization; see Figure 3 and Table 2. It should be noted that a comparison with experiment is however difficult, since the only available experimental data are single crystal x-ray analyses.57-60, 67-69 In the solids however, the twisted DCS structures are sometimes (partially or completely) planarized due to packing constraints; this is enabled by the 'twist elasticity' when cyano is introduced in the vinylene unit of DSB.3 The 'twist elasticity' concept was recently exemplified at a DFT level for β -DCS;70 we summarize the discussion here shortly, since this in an essential point for the understanding of structure-property relationships in the DCS compounds. DSB exhibits a planar equilibrium geometry in vacuum, in solution,10,

87

and in single

crystals.84,89 The average structure though is non-planar due to thermal population of the shallow torsional potentials around the vinyl-phenyl single bonds.87, 90 In α-DCS, the steric hindrance with the neighboring H-atoms (Hi, Ho; Figure 3) significantly increases compared to DSB. Upon free optimization (Ci symmetry) of α-DCS, the inner vinyl-phenyl single bond reacts with a substantial twist of φi = 30º and an increase of the bond length from 1.461 Å in DSB to 1.486 Å (Table S1), while the outer twist is only φo = 9º (Table 2) since here the bond angle can strongly release to αo = 132º. Forced planarization though (C2h symmetry) also reveals a favorable situation with CCN⋅⋅⋅Ho = 2.42 Å and CCN⋅⋅⋅Hi = 2.43 Å, which rationalizes the 'twist elasticity' concept. An analogue situation is found for β-DCS with a substantial twist around the outer 13 ACS Paragon Plus Environment


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vinyl-phenyl bond φo = 32º, while φi = 7º. Qualitatively, the situation does not change by BuOsubstitution in the terminal rings (α α-/β β -DBDCS), however with somewhat varying twists due to slight changes in the electronic structure (i.e. frontier MO topologies, see Figure S1).

Figure 3: DFT-optimized structures of DSB and α-DCS in the electronic ground state (S0); relevant atoms, angles and torsions are indicated, as well as the substitution positions (S, P) of the MeO groups at r the central ring, and the TD-DFT calculated S0→S1 transition dipole moment µ01 (red-green bi-colored line).

The effect of MeO substitution in the central ring on the resulting geometry depends sensitively, whether the α- or the β-series,91 and whether P- or S-rotamers (Scheme 1) are considered; see Table 2. In the α-series, the torsional angles are hardly changed upon MeO substitution for the P-rotamers, i.e. α-(DB)MODCS; on the other hand, for S-rotamers, CCN⋅⋅⋅OMeO interactions between the CN and MeO groups lead to a significant stronger twist of the inner vinyl-phenyl bonds, i.e. φi ≈ 49º (it should be stressed at this point that alkoxy-substitution in the central ring does not provide substantially different geometries than alkyl-substitution as previously suggested24). Energetically however, the P-rotamers are only somewhat favored against S by 0.06 eV. This is the apparent energetic reason why the P-rotamer can be realized in the crystal structure of α-DBMODCS: obviously, the P-rotamer allows for favorable packing, while for the other MO-compounds, the S-rotamer is favored. In the β-series, the P-rotamers are subject to considerable CCN⋅⋅⋅OMeO interactions, widening the twist of the inner angle to φi = 39º, while in the S-rotamers, φi increases only slightly to about 13-15º. The unfavorable CCN⋅⋅⋅OMeO interactions in the P-rotamers manifest themselves in a substantial energetic favor for the Srotamers (-0.34 eV).


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Table 2: (TD)DFT calculated torsional angles around the inner/outer vinyl-phenyl bonds (φi/φo) in the S0 (11Ag) and S1 (11Au) electronic states. compound

α-series

rotamer

β-series

S0

S1

S0

S1

i / o (º)

i / o (º)

i / o (º)

i / o (º)

DCS

30 / 9

1/1

7 / 32

5 / 15

DBDCS

29 / 5

0/0

6 / 30

5 / 16

P

28 / 13

26 / 7

39 / 32

29 / 23

S

49 / 6

36 / 7

15 / 30

4 / 17

P

29 / 5

22 / 7

39 / 30

26 / 21

S

48 / 4

31 / 8

13 / 26

3 / 13

MODCS MODBDCS

4.2. Spectral Positions and Intensities in Solution The spectral (and photophysical properties) of the parent compound DSB in solution are well understood, as recently reviewed.10 The absorption in the visible range is dominated by only one strong band (Figure 1),92 which arises from the allowed S0(11Ag)→S1(11Bu) electronic transition.87 The latter is mainly described by a promotion between the highest occupied to the lowest unoccupied MO (HOMO→LUMO). The shoulder at 4.1 eV in the absorption spectrum was assigned to the 21Bu state by spectral decomposition.88 The S0→S1 vertical transition energy and oscillator strength are reasonably well reproduced by TD-DFT, see Tables S2 and S3; the r

calculations further reveal that the transition dipole moment µ01 is oriented along the long molecular axis of DSB, see Figure 3.88 The vibronically resolved PL and the less structured absorption spectrum was shown to be due a situation with planar equilibrium geometries both in S0 and S1, but a much more shallow torsional potential in S0 compared to S1 due to the pronounced single-bond character of the vinyl-phenyl bond in S0, and a consecutive shortening in S1.87,

90

The very small spectral shift in PMMA compared to CHCl3 reveals similar

polarizabilities of both matrices.93 Furthermore, very similar spectral shapes for PL but in particular for absorption are observed in PMMA and CHCl3. This demonstrates that the shape of the shallow torsional potential in S0 is hardly affected in PMMA. Therefore, although the PMMA matrix is solid, the free volume is quite considerable (a point to which we will return later); differently, more rigid (and highly ordered) environments (like perhydrotriphenylene) strongly sharpen the vibronic features in the absorption spectrum of DSB.90 All compounds of the α- and β -series exhibit (significant) red-shifts of the PL spectra as well as their electronic origin (adiabatic energy E00) against DSB, see Figures 1, 4 and Table 1; E00 is here best approximated by the intersection between PL and absorption (rather than the onset or 15 ACS Paragon Plus Environment


the apparent PL 0-0 band,94 as the shape of the spectra vary largely between the different compounds. As described in Section 3, the red-shift increases in the sequence DSB < DCS < DBDCS < MODCS ≈ MODBDCS, and α < β, see Figures 1, 4. The TD-DFT calculations reproduce reasonably well the trend for E00 upon substitution for both series (although somewhat varying in the absolute position compared with experiment; see Figure 4); thus recommending TD-DFT for the analysis of the substituent effects. Anyway, it should be already noted here that the deviations between theory and experiments are much larger for the vertical transitions, in particular for emission; these effects will be discussed in Section 4.3. Very similar to DSB, the main absorption band is described by the symmetry-allowed S0(11Ag)→S1(11Au); the transition for all compounds is almost exclusively described by a HOMO→LUMO transition, i.e. with configuration interaction coefficients > 95% (for details see Table S2). Therefore, an understanding of the substituent effects can essentially rely on simple r

MO analysis. The orientation of the transition dipole moment µ01 of the entire DCS family turned out to be essentially the same as in DSB (Figure 3), i.e. along the long axes of the molecule. α-DCS series 3.6

3.2 3.0 2.8

3.0 E vert,em

2.6

2.8

2.4

2.6

2.2

2.4

3.2

E00

3.0

3.4 experiment / eV

E00

3.4

Evert,abs

3.2 3.0

2.8

Evert,em

2.8

2.4

2.6

2.2

2.4

2.0 S S S S B DS DC BDC ODC BDC D M OD M

2.6

calculation / eV

3.4

Evert,abs

3.4 3.2

β-DCS series

calculation / eV

3.6

experiment / eV

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2.0 B S S S S DS DC BDC ODC BDC D M OD M

Figure 4: Experimental (black) and calculated (red) adiabatic and vertical transition energies in absorption and emission (E00, Evert,abs, Evert,em); for the TD-DFT data set, see Table S3.

The most striking substituent effect on the spectral properties is the much larger bathochromic shift of the β - vs. the α-series. For instance, while the experimental red-shift in E00 of α-DCS against DSB is only -0.03 eV, it is -0.21 eV for β-DCS. Generally, the introduction of the cyanogroup in the vinylene units stabilizes the frontier MOs due to the negative inductive (-I) effect of CN, see Figure S1. In most cases, the LUMO is stronger stabilized than the HOMO in the α- and β-series due to the prevailing positive mesomeric (+M) effect of the cyano-group (see Figure S1), compensating the opening of the HOMO-LUMO gap vs. DSB due to strong torsional twists in 16 ACS Paragon Plus Environment

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the DCS compounds (see Section 4.1.); an exception is α-DCS, where the steric effect predominates. The stabilization of the LUMO is stronger when the cyano-group is introduced in the β-position of DSB compared to the α-position; this results in a stronger bathochromic shift of the electronic transition for the β - vs. the α-series in agreement with experiment. Comparing the S1 state energies for planar and non-planar ground state geometries by TD-DFT (see Table S3), a small steric contribution is found; however (and in contrast to what was previously suggested91) the main effect is of electronic nature. For instance, the calculated energy difference ∆Evert between α-and β -DBDCS for the relaxed ground state geometries is 0.29 eV, whereas for planar geometries it is still 0.23 eV. The prevalence of the electronic factor is readily explained through a resonance effect, which could be termed 'enhanced resonance stabilization' (in the following abbreviated by ERS), which is analogous to the well-known para-/ortho- vs. meta-effect in aromatic compounds.95 While zwitterionic resonance structures stabilize S1 of the α- and β compounds equally when the negative charge resides on the carbon atoms, this is different if the charge resides on nitrogen; as shown in Scheme 3, eight resonance structures can then be formulated for β -DCS, while it's only four in α-DCS, in all stabilizing S1 in the β-series. enhanced resonance stabilization (ERS) α-DCS

positive negative

β -DCS Scheme 3: Illustration of enhanced resonance stabilization (ERS): given in red are the possible positions

of the positive charge for α-DCS and β-DCS when the negative charge (blue) is located at the nitrogen atom.

For the same reason, the introduction of alkoxy-groups in the para-position of the terminal rings as well as the ortho-positions of the central ring additionally red-shifts the spectra due to the +M effect, while a general destabilization of the frontier MOs is observed due to the +I effect of the alkoxy groups. Noteworthy, methoxy-substitution in the central ring splits the HOMO (see Figure S1) as discussed earlier,96 to generate two electronic transitions (A1, A2; corresponding to the 11Au, 21Au states according to TD-DFT), see Figure 1, with comparable (however varying) oscillator strength (f1, f2) which gives the absorption spectra of the MO-compounds the characteristic 'camel hump' shape, see Figure 1.25 The electronic situation differs depending on 17 ACS Paragon Plus Environment


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the presence of the butoxy substituents in the terminal ring; in fact a higher f1/f2 ratio is observed in the DB compounds (Figure 1) due to larger LCAO coefficients of the terminal rings in the frontier MOs as induced by the presence of the BuO groups. The f1/f2 ratio is also higher upon β vs. α-cyano substitution, see Figure 1. As the TD-DFT calculations clearly reveal, this is mainly a geometrical effect: calculations on planar geometries gave only small differences in f1/f2 for α vs. β (and also hardly any difference for P- against S-rotamers), while those on fully optimized ground states gave significant differences, Table S3. Thus, the low f1,/f2 ratio for αMO(DB)DCS found in experiment is as strong indication for significant twists compared to the β analogues. This gives further evidence for the prevalence of the S-rotamer, which shows a much more distorted structure for α- vs. β-MO(DB)DCS compounds if compared to the β analogues, see Table 2. 4.3. Spectral Shapes in Solution While the trends of substitution for the pure electronic transitions E00 are quite well reproduced by TD-DFT, there is an apparent mismatch in the vertical transition energies Evert both for absorption and emission for some of the compounds, so that the reorganization energies Ere = Evert-E00are systematically underestimated by TD-DFT, see Figure 4. It should be noted in this context that Evert from experiment cannot be identified with the PL and absorption maxima due to the asymmetric spectral shapes.94 Instead, experimental Evert were extracted from the experimental PL spectrum by87 Evert = ∫E⋅I(E)dE / I(E)dE

(3)

For the absorption spectrum, to avoid integration over higher excited states (i.e. Sn with n > 1), we followed a procedure introduced earlier.88 For this, the PL spectra are mirrored at E00 and convoluted with an exponential function (to account for thermal population of low frequency torsional modes),87 in such a way that the convoluted spectrum follows closely the low frequency part of the absorption spectrum; integration with eq. (3) thus gives Evert for the S0→S1 absorption. This mismatch of theory with experiment for the prediction of Evert is ascribed to the thermal population of low frequency modes as pointed out for the case of the absorption spectrum of DSB (vide supra); in fact, such thermal effects are not considered in the computational scheme applied here. Compared to the DSB case, these effects become even more important in the absorption spectra of the DCS compounds, giving rise to entirely featureless and hypsochromically shifted spectra, see Figure 1. This effect can be ascribed to the flattening of the torsional potentials around the vinyl-phenyl bonds upon substitution as discussed previously for substituted oligothiophenes and polyphenylenevinylenes.97,98 In fact, as discussed in Section 4.1, 18 ACS Paragon Plus Environment

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considerable twists in the ground state torsional angles are observed upon cyano-substitution in the DCS family (Table 2), along with very shallow torsional potential hypersurfaces of S0 compared to the already shallow potential in DSB, see Figure 5. The majority of the DCS emission spectra are somewhat vibronically structured, which is ascribed to largely reduced twists in S1 (φi/o = 0-17°, Table 2), together with steepened torsional potentials (Figure 5) compared to S0.87, 90, 97 The S1 torsional potential is however somewhat steeper for DSB, being responsible for the sharper vibronic features in the PL spectrum of DSB. Blurring of the DCS PL spectra (and a concomitant bathochromic shift) becomes especially pronounced for α-MODCS and α-MODBDCS; indeed, here the calculated twists of the Srotamer remain particularly high in S1 with φi = 31-36° (Table 2), along with flat torsional potentials, see Figure 5 and Figure S2. Due to the hypso- and bathochromic shifts induced by the flat torsional potentials in S0 and S1, respectively, the Stokes shift EStokes = Eabs(A1) - Eem becomes a sensitive measure of the overall molecular distortion. Thus, while DSB shows a Stokes shift between PL and absorption of ∆EStokes ≈ 0.5 eV, it increases to 0.7 eV in α-DCS and 0.8 eV in α-MO(DB)DCS (Table 1); this is also true for β -DCS, while for the other β compounds EStokes ≤ 0.5 eV is observed.

Figure 5: Torsional potentials around the inner vinyl-phenyl bonds φi for DSB, -α-DBDCS and αMODCS in the electronic ground state S0 (11Ag), and the first excited singlet state S1 (11Au; 11Bu for DSB), calculated by TD-DFT (in vacuum).



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In solid solution (PMMA), onset and absorption maxima agree largely with those in fluid solution, confirming our statement on DSB on similar polarizabilities and torsional potentials in both environments. In emission however, a remarkable hypsochromic shift is observed, which becomes particularly strong for α-MO(DB)DCS, which also significantly enlarges E00, see Figure 1. This gives evidence of a substantial flattening of the torsional twist in S1, which we ascribe to some specific interactions with the polar environment of PMMA. The DCS compounds with the both present both alkoxy and cyano functionalities might be more prone to such interactions due to the stabilization of polar resonance structures in S1 state compared to S0. It should be pointed out that in the higher energy region of the absorption spectra of the α-series, i.e. 3.5-4.3 eV, (much) lower absorbance is observed in PMMA compared to the spectra in CHCl3. Furthermore, these bands were found to increase in CHCl3 with the time under illumination, clearly pointing to photochemical processes. In fact, trans-cis isomerization was observed in compounds with the same principal structural motif of α-MODCS29 and αDBDCS,99 in agreement with our observations. The assignment is further supported by (TD)DFT calculations (Tables S4, S5), which predict a hypsochromic shift of the main absorption band by ca. 0.3 eV for the cis-isomer vs. trans (Table S5), in reasonable agreement with experiment (Figure 1). In PMMA on the other hand, such isomerization processes are inhibited due to the restriction of large amplitude motions by the environment. It should be stressed in this context that the formation of the cis-isomer is not very effective under short-time illumination, as equilibrium is reached only after 15 mins of mercury lamp irradiation.29 Remarkably, we didn't find indications for effective isomerization for the β-series; this indicates a directive functionality of the cyano-group on the isomerization process. In this context it should be reminded that trans-cis photoisomerization was in particular observed in stilbene,100 as well as its differently substituted 1-cyano and 1,1'-dicyano-functionalized derivatives.101-106 On the other hand, it doesn't play a role in DSB,21,22 indicating that extended conjugation is an important parameter which limits isomerization. This gives a plausible explanation for the differences in the α-/β β -series, where the ERS effect of the β -compounds as discussed above, might effectively inhibit isomerization. Evidence for this scenario is found in the ground state energies, where the trans-isomers of the β-series, in particular for the MO-compounds, are significantly stabilized against the α-series; we will return to this point in the following section.


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4.4. Photophysics in Solution The excited state deactivation of DSB is rather simple,10 showing strong PL (Φ F = 0.87 in CHCl3) with a lifetime of τF = 1.16 ns, so that kr = 0.75 ns-1 and knr = 0.11 ns-1 is obtained via eq. (1), see Table 3. Calculating kr through the SB relationship, see eq. (2a), i.e. via integration over the first intense band of the absorption spectrum (A1; see Figure 1), a very similar value kr,SB is obtained compared to the measured (Table 3), indicating that emission indeed originates from the bright A1 state. The excited state kinetics include a short component of ca. 10 ps,107-109 which reflects S1 conformational relaxation from the initially excited Franck-Condon (FC) region.107, 108 The nonradiative decay takes place mainly via internal conversion IC1-0 to the ground state, while ISC to the triplet manifold108, 110 and trans-cis isomerization21,22 are very minor pathways.

Table 3: Photophysical data of the DSB and the DCS compounds in solution (sol.; CHCl3) and single crystal (SC): fluorescence quantum yield (ΦF), fluorescence lifetime (τF), radiative rate constant (kr; from eq. 1), the ratio of radiative rates Rr = kr(SC)/kr(sol), radiative rates kr,SB according to eq. 2 (Strickler-Berg; SB) from the experimental spectrum, and from quantum chemistry (kr,QC; eq. 2a), nonradiative rate constant in total (knr; from eq. 1) and the contribution from internal conversion part (ϕIC) and intersystem crossing part (ϕISC) according to the PAC analysis (see Table S7). ΦF a

Class DSB α-DCS α-DBDCS α-MODCS

C A A B

α-MODBDCS β-DCS β-DBDCS β-MODCS β-MODBDCS

B C C C

kr

sol.

0.87

1.16

0.75

SC

0.78 c

3.6 c

0.22

sol.

2⋅10-3

5.7⋅10-3 h

0.35

2.1 d

0.43

d

SC

0.90

sol.

2⋅10-3

4.0⋅10-3 h

0.50

SC

0.70

13.7

0.05

sol.

0.02

0.18

0.11

0.66

e

3⋅10

-3

SC

0.42

e

sol.

0.01

SC B

τF

sol.

SC

0.69

sol.

0.54

SC

0.84

sol.

0.20

SC

0.73

sol.

0.31

SC

0.46

f

3.5

e

0.19 -3 h

0.23

e

0.09

22⋅10-3 h

0.45

13⋅10 4.8 9.5

f

0.07

1.19 i g

e

e

0.14

1.33

0.15 e

0.04

1.38 i

0.22

e

0.02

17.8 24.2

H/J b

0.29

Hm

1.23

Hw

0.10

Hs

kr,SB

kr,QC

0.67

0.59

0.39 0.15 0.31 0.26 0.09

ϕIC

ϕISC

0.63

0.37

0.57

0.43

0.61

0.39

0.46

0.54

0.62

0.38

0.85

0.15

0.37

0.63

0.50

0.50

0.11

0.34

175 0.05

0.59

1.72

knr 0.06

0.51

0.39

250 0.02

0.07

0.09

0.21

0.15

0.35

0.36

0.42

0.33

0.17

0.18

0.28

0.25

Jw

5.4 0.10

Hm

77 0.12

Hs

0.45

g

5.9

Rr

45 0.03

Hm

0.39 0.03

Hm

0.60 0.02

Hs

0.50 0.02

Φ F was measured by the relative method for solution and by the absolute method for SCs. b w = weak, m = medium, s = strong. c Ref. 84. d Ref. 58. e Ref. 60. f Ref. 70. g Ref. 59. h From ultrafast TA experiments (in dioxane); for details see Table S10. i Intensity-averaged, from bi-exponential fits of transient PL experiments; see Table S9 for details. a



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The photophysics of the DCS compounds in solution depend drastically on the position of cyano-substitution (α, β), and on the presence (and position) of alkoxy-substituents; furthermore, switching from fluid to solid solution generally enhances Φ F, i.e. showing a SLE effect, however, to a distinctively different degree, see Figure 2 and Table 3. Setting the limit for 'emissive' materials at about ΦF ≈ 0.1 (and 'highly emissive' at about Φ F ≈ 0.3), a classification of the different behavior can be made: Class A: low emissive in fluid and solid solution; i.e. α-DCS ≈ α-DBDCS Class B: low emissive in fluid solution, (highly) emissive in solid solution; i.e. α-MODBDCS < α-MODCS < β -DCS Class C: (highly) emissive; i.e. β-MODCS < β-MODBDCS < β-DBDCS Radiative and non-radiative rates kr, knr from ΦF and τF via eq. (1) allows for a deeper understanding of DCS photophysics. The extracted radiative rates are all in the range of 0.110.50 ns-1, and agree well with the rates obtained from the SB relationships (both from the experimental spectra as well as from quantum chemistry), see Table 3. The result proves that for all compounds emission occurs from the bright 11Au state (responsible for the strong absorption features). The analysis further agrees with the observation of only moderate Stokes shifts (vide supra), as well as with the TD-DFT analysis which predicts the first dark state (21Ag) well above 11Au; see Figure S3. Thus, the low-emissive character of class A and B compounds in fluid solution is clearly due to effective non-radiative deactivation. To further investigate the nature of the non-radiative deactivation we performed photoacoustic calorimetry (PAC) experiments, which give clear evidence that both IC1-0 and ISC are of similar importance for knr for all DCS compounds (Tables 2 and S7), i.e. very different from DSB. In solid solution (PMMA), both Φ F and τF largely increase (Figure 2) indicating a strong reduction of knr. In any case, focusing at the α-series, it might surprise at a first glance that in Btype compounds (α α-MODCS, α-MODBDCS) the Φ F in PMMA are only moderately high (below 30%), and in particular A-type compounds (α α-DCS, α-DBDCS) are still low emissive, although the solid environment is expected to effectively prevent the non-radiative decay. The reason might be found in the free volume of PMMA, which in fact provides cavity diameters of about 5.4 Å.111 This doesn't allow for large amplitude motions of the molecules; however, motions which are more restricted in space should be still operative. The particular difference in the behavior of A- and B-type components gives thus a clear hint on the operative modus of nonradiative decay: since the difference between B- vs. A-type compounds is the presence of the


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methoxy-group at the central ring, the deactivation process should involve partial rotational motion of the central ring, vide infra. To understand the nonradiative deactivation, one might recall the trans-stilbene (TS) case, which is one the most intensively studied systems in photochemistry.40 After photoexcitation in the bright S1, the primary step is a incent from the FC point along the torsional coordinate around the double bond φDB112 with a time constant of a few ps,113 which diabatically intersects with a doubly excited state. The latter descents with φDB,114 forming the main coordinate towards the 'phantom state',115 i.e. a kink-type conical intersection (CI), which might further involve a torsion around the singlet bond via a 'hula-twist' mechanism,112 to finally return to the trans or cis ground state A similar ultrafast process in the excited state of ~1 ps was observed earlier in the excited state kinetics for β-DCS,43 being significantly shorter than in DSB (10 ps) 107-109 which indicates a process rather similar to TS than to DSB. In our fs TA experiments, we found a very short time constant of 0.4-0.9 ps for all DCS compounds (Table S10), besides the main component (which widely varies among the compounds to give very different PL quantum yields; vide supra and Table 3). The similar sub-ps time constant indicates that the initial step of deactivation, i.e. presumably a twist around the φDB in analogy to TS, is essentially the same in the whole DCS family, independently on the consecutive step. In fact, the double bond length for the DCS compounds in S1 is quite labile and of similar length with rDB ≈ 1.40 Å (TD-DFT; Table S1); this is longer than in DSB, rDB = 1.38 Å due to the presence of the cyano-group, while in TS rDB = 1.42 Å. To test the initial steps of such scenario for the current case, we performed a comparative computational study for α-DBDCS (non-luminescent) with β -DBDCS (luminescent); for this we did a rigid torsional scan around one of the two double bonds (φDB) using the geometry optimized for the ground-state in CHCl3, and calculating the energies of the S0, S1, and T1 states for each data point. Here, it should be reminded that although (TD)DFT does not allow for a detailed description of the CI, trends might be however correctly reproduced as shown for TS;114 thus, comparison between structurally similar systems should be possible. In fact, the calculations in Figure 6 give strong evidence for a CI between S1 and S0 at φDB = 90º and at an energy of 2.78 eV for both compounds. The reason for the identical electronic situation is found in the electronic nature of the CI; the latter was computed by CASSCF, which shows a fundamental difference with respect to the FC region (i.e. φDB = 180º); while the latter is of π-π∗ type, the former is largely localized in the vinylene unit (Figure 6), thus it is estimated to exhibit 23 ACS Paragon Plus Environment


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a very similar electronic situation for all DCS-type compounds, independently on the position of the cyano group. Access to the CI is controlled by the activation barrier from the FC state towards the CI. This barrier is considerably higher for β -DBDCS compared to α-DBDCS according to the present results, which fully rationalizes the different excited state kinetics. This consideration agrees well with the expectation from Hammond's postulate, i.e. assuming that the thermodynamic stability of the primarily formed FC state (EFC = Evert,abs) will determine the barrier height of the reaction towards the CI and thus the rate. As discussed in Section 4.2, EFC (as well as the relaxed S1 state) is essentially controlled by ERS, which lowers EFC for the β series compared to α. As shown in Figure 6, in all compounds of the α-series indeed EFC ≥ ECI is found, whereas the high emissive compounds of the β -series (β β-MODCS, β -MODBDCS, β DBDCS) are low in energy with EFC « ECI; only in β -DCS the stabilization by ERS is insufficient due to missing further stabilizing substituents, making β -DCS the sole low-emissive compound within the β -series. It is worth to point out here that the torsional motion around the double bonds requires substantial reorganization of the phenyl rings, which rationalizes the differences between the MeO-substituted compounds vs. their non-substituted counterparts in solid solution (PMMA; vide infra).

Figure 6: Left: TD-DFT rigid torsional scans of one double bond φDB for α-DBDCS and β-DBDCS using the optimized S0 state in CHCl3; S0, S1, and T1 energies were computed for each data point. CASSCF calculated frontier HOMO- and LUMO-like orbitals which characterize the electronic structure for the CI and FC regions. Right: TD-DFT calculated FC energies (Evert, abs) in CHCl3, and DFT calculated ground state energies of trans-trans vs. cis-trans-isomers.


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Similar to TS, the DCS compounds will relax from the CI towards the cis or trans ground states. Also here, Hammond's postulate can be used approximately to rationalize the effectiveness of the trans-cis photoisomerization by comparing the relative stability on the final (cis-trans; ct, and trans-trans; tt) isomers, see Figure 6: in the β-series the tt-isomer is strongly stabilized against ct by ≥ 0.13 eV which inhibits isomerization (see Table S5). Differently, the stabilization in αMODCS and α-MODBDCS is only 0.06 eV, which opens the channel for isomerization. In any case, some ambiguity is observed for α-DCS and α-DBDCS (0.12 vs. 0.13 eV), indicating that additional kinetic factors have to be considered in some borderline cases. Finally, we investigated the T1 energy along the φDB coordinate to further understand the high efficiency of ISC. As can be seen in Figure 6 the PES effectively intersects with S0 and S1 exactly in the region of the CI; this is due to the strong charge-transfer (CT) character of the CI as can be seen in the frontier MO topologies in Figure 6, which diminishes the exchange integral and thus the singlet-triplet (S-T) splitting. This peculiar electronic situation of the CI rationalizes the experimentally observed about equal contributions of IC and ISC in the DCS series. Summing up the excited state deactivation processes of the DCS compounds in solution, we were able to rationalize qualitatively the experimental results on the effectiveness of nonradiative deactivation, trans-cis isomerization and IC1-0 vs. ISC for non-emissive and emissive DCS compounds based on a simple TD-DFT scheme, and applying Hammond's postulate; the decisive factor for the process turned out to be ERS, which controls both color and photophysics of the compounds. The occurrence of a conical intersection is in line with former studies on stilbene-like compounds, and underlines the importance of the lability of the vinylene double bond in the excited state, which is not the case in the parent DSB compound; it should be stressed that the precise theoretical description of the CI requires a local, multi-reference representation, which for such large molecules are beyond reach at the moment, but will be subject of future investigations. Anyway, it is apparent from our study that simple 2-state normal mode representations based on harmonic oscillators are incapable of capturing these ultrafast processes. Evidently, simple qualitative considerations as RIR cannot explain the differences between the α- and β-series; the suppression of knr in PMMA is due to restricted access to the CI, as described earlier for related systems.47, 48 4.5. Spectral Properties in the Single Crystalline State The emission colors of the DCS compounds in solution (expressed via Evert,em) are, as we have seen in Section 4.2 (Figure 4), significantly influenced by intramolecular geometrical and electronic factors, i.e. α- vs. β -substitution as well as number and position of the alkoxy25 ACS Paragon Plus Environment


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substituents; in all, the colors range from blue to green, see Figure 7. Upon crystallization, bathochromic shifts are observed, which can be defined as ∆Ecryst = Evert,em(crystal) - Evert,em(solution)

(4a)

which can be negligibly small (e.g. α-MODCS, Figure 7), but can amount up to 0.35 eV (α αDBDCS, β-MODBDCS) so that the emission color of the crystals extends into the orange region, see Figure 8. The crystal shift can be partly of intramolecular origin, due to planarization of the molecular backbone enabled through the 'twist-elasticity' of the DCS compounds as discussed in Section 4.1.70 To investigate this effect by TD-DFT, vertical transition energies were first obtained for the geometry found in the crystal (i.e. as determined from x-ray analysis), and then for the fully relaxed geometry (for details on the computational procedure see Section 2.3); the energy difference amounts then to ∆Egeo = Evert(monomer, crystal) - Evert(monomer, relaxed)

(5)

Figure 7: Crystal shifts of the compounds. Top: vertical emission energies Evert in CHCl3 (green circles) and in the single crystals (violet squares); references for compounds are 1: Ref. 84, 2: Ref. 58, 3: Ref. 57, 4, 5, 8, 9: Ref. 60, 6: Ref. 70, 7: Ref. 59. Inset: emission spectra of β-DCS in CHCl3 (green) and in single crystal (violet). Bottom: experimental (red) and calculated (blue) crystal shifts ∆Ecryst, and of the geometrical shift ∆Egeo; see eqs. (4, 5). The calculated values were obtained from monomer and dimer arrangements in the crystal vs. the fully optimized monomer; for details see Section 2, for the data sets, see Table S6).


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α-DBDCS, α-MODCS, α-MODBDCS, β As seen in Figure 7, ∆Egeo is negative in most cases (α DCS, β-MODBDCS) reflecting (partial) planarization upon crystallization, as indeed seen in the nearest-neighbor arrangements extracted from the X-ray analyses, see Figure 8. On the contrary, ∆Egeo is positive for α-DCS, α-MODCS, β-DBDCS) due to stronger twists in the crystal compared to the free molecules; for comparison, ∆Egeo = 0 for DSB, exhibiting planar equilibrium structures in solution and in the crystal.84,87 Because of frequent misconceptions in the community, It has to be stressed in this context that the calculated geometrical effect can only partially explain the experimentally observed ∆Ecryst, emphasizing the concurrent importance of intermolecular effects

Figure 8: Overview on luminescent single crystals: nearest neighbor arrangements with molecules per unit cell (Z) from X-ray analysis, i.e. 1: Ref. 84, 2: Ref. 58, 3: Ref. 57, 4, 5, 8, 9: Ref. 60, 6: Ref. 70, 7: Ref. 59, type of arrangement (π-stacking, micro-/herringbone; µ-/HB), H vs. J-type coupling, fluorescence microscope images and quantum yields ΦF (%).



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Intermolecular effects generally comprise excitonic,10 polarizability,93,

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116

and excimeric

contributions; for more details on the latter, see SI.56, 88 For the sake of computational simplicity, we concentrate here on the excitonic effect, which we treat at a TD-DFT level in a simple nearest-neighbor approach.109 For this, the arrangement taken from the X-ray analysis is compared to the fully relaxed monomer (in vacuum); i.e., geometrical effects are implicitly included: ∆Ecryst = Evert(S1, dimer) - Evert(S1, monomer)

(4b)

The results according to eq. (4a) in Figure 7 reproduce well the experimental trends obtained via eq. (4a) among the different compounds (as well as for further available polymorphs; see Figure S4), recommending this simple method for fast screening of crystal shifts. The reasonable agreement of the absolute values of the calculated ∆Ecryst indicates that (the surely present) nonnearest neighbor effects are partly compensated by polarizability changes when going from solution to the crystalline phase, which are in all not considered in the calculations. In any case, the polarizability can be estimated to not more than ~0.1 eV,93 i.e. relatively small against the calculated ∆Ecryst. The electronic effect (i.e. ∆Ecryst - ∆Egeo) is particular large for β-DCS and β MODCS (Figure 7), which correlates directly with the amount of exciton coupling ∆EEC in the dimer arrangement. In H-aggregates, ∆EEC corresponds to half of splitting between the lowest excited state S1 and the Sf state which carries the oscillator strength (Table S6) ∆EEC = {E(Sf) - E(S1)}/2

(6)

It is large mainly if the x-slip of neighboring molecules (i.e. along the long molecular axis; Scheme 1) is small, see Figure 8. The results on the compounds with large ∆EEC agrees partly with the kinetic analysis on H-aggregate formation (see Section 4.6), although we will see there that ∆EEC is not the only decisive factor for the resulting radiative rate 4.6. Photophysics in the Single Crystalline State As we have seen in Section 4.4, all DCS compounds showed solid state luminescence enhancement (SLE) when going from fluid to the solid environment due to a reduction in nonradiative deactivation by IC a/o ISC, although the effects were not exceedingly pronounced in PMMA due to free volume in the matrix. Thus, in general a further increase of luminescence is expected in more densely packed environments. This is particularly true in the poly- or single crystalline phase with usually dense packing of the molecules due to enthalpic arguments. At the same time dense packing might lead to planarization enabled by 'twist elasticity' as discussed in Setion 4,1,70 which (somewhat) further increases kr. However, the dense environment doesn't 28 ACS Paragon Plus Environment

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necessarily lead to an increase of ΦF since two factors might counteract the decrease of IC/ISC. One factor is possible exciton trapping (kq⋅ctrap) which increases the overall knr.10 Trapping might become pronounced in polycrystalline samples such as films and nanoparticles due to high trap concentrations ctrap at surfaces and grain boundaries, combined with small average distances to reach those, as well as high surface:volume ratios.10 For this reason, only single crystals were considered in the current study. Indeed, as seen in Table 3 and Figure 2, knr decreases in the single crystals compared with fluid and solid solution to just 0.02 - 0.10 ns-1. The other factor which might counteract the decrease of IC/ISC is a possible decrease of kr caused by Haggregation.11, 13 In Kasha's original exciton model the oscillator strength of the lowest excited state becomes zero for perfectly side-by-side oriented molecules, and thus, concomitantly, kr. In reality, non-zero kr are observed even in strong H-aggregates,11 which are caused by (small) inclinations between the transition dipole moments in crystal arrangements with more than one molecule per unit cell, and/or aggregate-type Herzberg-Teller (AHT) coupling.117-119 In thin films or nanoparticle suspensions where the optical density is small, assignment of H-/J-aggregation can be done via blue-/red-shifts of the absorption spectra of the aggregates vs. solution.10 However, in samples with high optical density (in particular powders or crystals), a proper assignment is generally difficult: in the absorption spectra shadowing effects are observed as described by Mie theory, while in the PLE spectra inner filter effects are operative, which in all mask the intrinsic absorption properties, so that assignment on absorption or PLE shifts cannot be done.10 For that reason, a proper assignment has to rely on the analysis of the radiative rate; i.e. a decrease (increase) of kr, when going from solution to the single crystal indicates H- (J-) aggregation. We will thus use the ratio Rr = kr(crystal)/kr(solution)

(7)

as a measure for the excitonic effect. It should be noted however, that kr is a function of the polarizability, and in a first approximation scales with n 2 where n is the refractive index of the environment, see eq. (2). Since the effective n is higher in the crystal compared to solution,93 a general increase of kr by ~50% (Rr = 1.5) simply because of the polarizability effect is expected. Thus, while assignment of H-aggregation (decrease of kr; Rr < 1) is unproblematic with respect to polarizability, it becomes a critical issue for the proper assignment of J-aggregates (increase of kr; Rr > 1); here a proper statement on J-aggregates should only be done if the increase of kr is clearly larger than 50% (Rr > 1.5).120



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According to the radiative rate constant analysis (Table 3), out of the eight DCS compounds, six show a decrease of kr in the monolithic crystals against (fluid) solution). Strong H-aggregation (Rr < 0.2) is in particular observed for α-DBDCS, β-DCS, and β -MODBDCS, while medium Hcoupling (0.5 > Rr > 0.2) is observed for α-MODBDCS, β -DBDCS and β-MODCS (and DSB); weak H-coupling (1.5 > Rr > 0.5) is seen for α-DBDCS, while α-MODCS is the only compound showing (weak) J-coupling (Rr > 1.5). This is in fact confirmed by TD-DFT nearest-neighbor dimer calculations, which predicts J-aggregate formation (i.e. with a strongly allowed S1 state) only for α-MODCS, while the other compounds all give H-aggregates (Table S6). We would like to stress once more that H-/J-aggregation in molecular crystals cannot be decided solely based on X-ray data by comparing the pitch angle with the 'magic angle' (θi = 55º) as frequently done in literature, as the latter is derived from the point dipole approximation, which is inapplicable for molecular crystals.11, 13 It should be reminded in this context, that strong H-aggregation requires the concurrent appearance of two effects.11 One factor is large exciton coupling ∆EEC as discussed in Section 4.5, being especially pronounced for β-DCS and β-MODCS; for smaller ∆EEC, effective AHT coupling is expected, which enhances the oscillator strength of the emissive state,117 and with this kr via eq. (2). The other factor is a perfect parallel alignment of the transition dipole r

moments µ01 , which, strictly speaking, is only realized for crystals with one molecule per unit cell (Z = 1); Among the current compounds, Z = 1 is only observed for α-DBDCS, α-MODCS and β -MODBDCS, see Figure 8. Thus, low radiative rates require large ∆EEC and/or AHT coupling, which might equally contribute, as indeed found for α-DBDCS, β-DCS, and β MODBDCS. We would like to stress once more that even for these compounds SLE is observed, however driven here solely by intramolecular factors; i.e. a reduction of knr. Synergistic lowering of knr and enhancement of kr in the crystalline phase is only observed for α-MODCS, thus being the only AIEE-active compound in the current library. 5. Conclusions Solid state luminescence enhancement (SLE) has evolved to a key research area in the field of conjugated organic materials; however, careful photophysical studies are still very limited to elucidate the complex interplay of different radiative and non-radiative deactivation processes, as well as intra- and intermolecular contributions. To resolve these issues, we have investigated here one of the prototype examples of SLE materials, i.e. distyrylbenzenes with cyanosubstituents in the vinylene unit (DCS compounds), with varying bi-cyano-substitution in the 30 ACS Paragon Plus Environment

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inner (α α) or outer (β β ) position, and additional bi-alkoxy-substituents in the central (MO) a/o terminal (DB) phenyl-rings. This provides a library of eight different compounds, which show distinctively different photo-responses in fluid and solid solution, as well as in monolithic crystals, and thus offer a unique chance to disentangle all contributions to the SLE phenomenon. For this, we utilized a variety of spectroscopic techniques (quantitative steady-state and ps time resolved fluorescence, fs transient absorption, photoacoustic calorimetry) combined with appropriate quantum-chemical methods (TD-DFT, CASSCF) and available structural (X-ray) data. In fluid solution, significant bathochromic shifts of absorption and emission spectra are observed for the β -series vs. the α-series as well as upon MO- and DB-substitution, which could be traced back mainly to enhanced resonance stabilization (ERS) and minor geometrical contributions. Alkoxy-substitution in the central ring (MO-) compounds reveals a characteristic 'camel hump' shape with significant differences in the intensity ratio of the 'humps' for the different compounds, due to equally important electronic and geometrical contributions. Geometrical factors were also found to be the main reason for the different PL band shapes in solution. While the α-series is throughout very low-emissive in fluid solution (Φ F ≤ 2%), this is not true for the β -series. Here, only β -DCS is low emissive (ΦF = 1%) whereas upon MO- a/o DBsubstitution, the compounds become (highly) emissive (Φ F = 20-54%). It is demonstrated that this is driven by non-radiative deactivation (knr), which cannot be rationalized in simple pictures as 'restricted intramolecular rotation (RIR). As a first step, torsional relaxation around the vinyl double bond is observed (0.4-0.9 ps) due to the bond's lability compared to the parent DSB, followed by internal conversion (IC) and inter-system crossing (ISC) with similar contributions. In the low-emissive components the time constant of the non-radiative decay is in the lower ps range, and involves a conical intersection (CI). The CI is of localized electronic character and is therefore very similar in energy for all compounds. For this reason, the path towards the CI is effectively prevented in the β-series by ERS; the latter is however insufficient for β-DCS due to missing further stabilizing substituents, making β-DCS the only low-emissive compound within the β-series. The strong CT character of the CI leads to small S-T splitting and thus rationalizes the experimentally observed similar contributions of IC and ISC. Significant changes in the emission color are observed when going from solution to the crystalline phase, covering the whole visible range. Combined spectral and quantum-chemical analysis reveals that both intramolecular contributions (arising from geometrical non-/twisted



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structures) and intermolecular contributions (due to excitonic coupling) in the solid state are of about equal importance. All compounds show SLE when going to monolithic crystals but also to solid solutions (PMMA) of the materials, proving that 'aggregation' (AIE) is not a necessary condition for SLE. For the compounds already emissive in fluid solution the effect is self-evidently not pronounced. For the non-emissive compounds, inclusion in PMMA leads to a significant increase however not reaching unity. This is ascribed to the free volume in PMMA, which only partly inhibits the nonradiative process. Differently, in the densely packed monolithic crystals of the compounds, IC/ISC are effectively suppressed (knr = 0.02- 0.12 ns-1) and the crystals are brightly emissive with ΦF = 46-90%. The variations are thus mainly due to differences in the radiative rates kr as induced by H- vs. J-aggregation. The impact of H-aggregation on the resulting rate kr is shown to depend both on the strength of exciton coupling as well as on the amount on aggregate-type Herzberg-Teller coupling. Even for strong H-aggregation the PL quantum yields are high in the monolithic crystals, demonstrating once more that under these low-trap and low surface:volume conditions, 'aggregation-caused quenching' (ACQ) is not operative (while effective trapping is often observed in polycrystalline samples like films and nanoparticle suspensions).10 In only one out of the eight compounds a synergetic effect of suppression of knr and, at the same time, enhancement of kr by J-aggregation (AIEE effect) was found. In all, we represented an in-depth combined photophysical and computational study on structurally very similar compounds which however show very different photo-response in solution and in the crystalline phase; this revealed a detailed insight to the SLE phenomenon, disentangling steric and electronic factors, radiative and non-radiative channels as well as intraand intermolecular contributions, providing an important step towards the urgently required rational materials design. Supporting Information Supporting Information (details of the experimental and computational results) is available free of charge on the ACS Publications website at DOI: 10.1021/XXX. Acknowledgements: The team at IMDEA Nanoscience acknowledges support from the 'Severo Ochoa' program for Centers of Excellence in R&D (MINECO, Grant SEV-2016-0686). Financial support at IMDEA and University of Valencia (UV) was further provided by the Spanish Ministry for Science (MINECO-FEDER project CTQ2014-58801), and at IMDEA as well by the Comunidad de 32 ACS Paragon Plus Environment

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Madrid (Project Mad2D, Grant No. S2013/MIT-3007, and PhotoCarbon, S2013/MIT-284) and by the Campus of International Excellence (CEI) UAM+CSIC. The work at IMDEA and UV was performed in the context of the European COST Action Nanospectroscopy, MP1302. The work at Seoul National University was supported by the National Research Foundation of Korea (NRF)

through

a

grant funded

by

the

Korean

Government

(MSIP;

No.

2009-

0081571[RIAM0417-20150013]). Funding for the PAC measurements was provided by Laserlab-Europe (grant agreement n°284464, EC's Seventh Framework Program). D. R.-S. acknowledges the “Ramón y Cajal” grant (Ref. RYC-2015-19234) of the MINECO. J. S. acknowledges a PhD grant of the Chinese Scholarship Council (CSC). The authors thank H. Bolink (Valencia) for access to the integrating sphere. References 1

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Zhang, W.; Yao, J.; Zhao, Y. Organic Micro/Nanoscale Lasers. S. Acc. Chem. Res. 2016, 49, 1691-1700.

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Ng, K. K.; Zheng, G. Molecular Interactions in Organic Nanoparticles for Phototheranostic Applications. Chem. Rev. 2015, 115, 11012-11042.

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Solid State Luminescence Enhancement in Ï-Conjugated Materials

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