Aggregation-Induced Enhanced Emission - ACS Publications

Article Cite This: J. Phys. Chem. C 2017, 121, 27071−27081

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Aggregation-Induced Enhanced Emission (AIEE) from N,N‑Octyl-7,7′diazaisoindigo-Based Organogel Andrés Garzón,⊥ Amparo Navarro,∥ Daniel López,§ Josefina Perles,‡ and Eva M. García-Frutos*,† †

Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Campus de Cantoblanco, 28049 Madrid, Spain Laboratorio de Rayos X de Monocristal, Servicio Interdepartamental de Investigación, Universidad Autónoma de Madrid (UAM), Cantoblanco, 28049 Madrid, Spain § Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva 3, 28006 Madrid, Spain ∥ Departamento de Química Física y Analítica, Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain ⊥ Departamento de Química Física, Facultad Farmacia, Universidad de Castilla-La Mancha (UCLM), 02071 Albacete, Spain ‡

S Supporting Information *

ABSTRACT: A new kind of low molecular-mass organic gelator (LMOG) π-electrondeficient N,N-octyl-7,7′-diazaisoindigo (1) with aggregation-induced enhanced emission (AIEE) phenomenon is described. This organogel is capable of self-assembling through intermolecular H-bonding and π−π interactions between diazaisoindigo molecules. Its rheological properties, X-ray diffraction pattern, optical properties and theoretical calculations were investigated. The AIEE effect is exhibited in fluorescence during the formation of the supramolecular organogel, which persisted in the xerogel state, and the spectral red-shifts suggest the formation of J-type aggregates during the gelation process via π−π interactions in microbelts or 3D networks. Fluorescence lifetime and quantum yield significantly increase from dilute solution to the aggregate state. From a theoretical perspective, the effect of the aggregation of 1 on the photophysical properties was also studied by means of the density functional theory (DFT). In this sense, the lowest energy electronic transitions were calculated for both the single molecule and different size aggregates in order to predict spectral shifts. In addition, the geometry and molecular properties of the excited state were analyzed in different material states.

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the contrary effect to ACQ; that is, the fluorescence emission is induced or enhanced upon the gelation (AIE, aggregationinduced emission, and AIEE, aggregation-induced enhanced emission).14,15 Since 2001, when Tang and co-workers described the AIE phenomenon,16 much attention has been paid due to fascinating photophysical properties.17 AIE active molecules have received great research attention because of the main characteristic of these materials, which are nonemissive in dilute solutions but highly emissive in their aggregated state. Different AIE active gelators have been widely described in the literature such as silole, tetraphenylethene (TPE),18,19 cyanostilbene,20 aromatic compounds based on carbazole, 1,2,4, oxadiazole,21 and 1,3,4 oxadiazole derivatives.13 Moreover, functional organogels based on π-electron-deficient (ntype) aromatic building blocks, including naphthalene bisimides22 and perylene bisimides,23,24 have been described. Herein, we investigate a new kind of low molecular-mass organic gelator (LMOG) π-electron-deficient N,N-octyl-7,7′diazaisoindigo (1) (Figure 1) with induced enhanced emission (AIEE) phenomenon. This organogel is capable of self-

INTRODUCTION In recent years, low molecular mass organic compound based gels (LMOGs) with a wide range of applications, such as optoelectronics, drug delivery, sensors, and biomedical uses, have been reported.1−7 Among the different supramolecular gels, fluorescent organogels formed by π-conjugated low molecular weight gelators have attracted intense attention due to the diversity, flexibility, and its promising applications. The formation of the organogel can bring important changes in the different material properties. The supramolecular organogels may be cross-linked into different 3D self-assembled blocks such as fibers, wires rods, ribbons, or other morphologies by the self-assembling process.8 The physical organogels are built through noncovalent interactions such as hydrogen bonding, π−π stacking interactions, dipole−dipole interactions, van der Waals forces.9,10 On the other hand, a range of external chemical stimuli such as temperature, sonication, or mechanical forces have also induced reversible gel−solution phase transition.11,12 For light-emitting applications, the severe intermolecular interactions that are conducive to selforganization generally consume the excited state energy, inducing the so-called fluorescence aggregation-caused quenching (ACQ) in gel phase.13 Among the different types of supramolecular gels, only a small variety of organogelators show © 2017 American Chemical Society

Received: August 2, 2017 Revised: November 3, 2017 Published: November 15, 2017 27071

DOI: 10.1021/acs.jpcc.7b07625 J. Phys. Chem. C 2017, 121, 27071−27081

Article

The Journal of Physical Chemistry C

Oscillatory Rheology. Oscillatory shear measurements were carried out in a TA Instruments ARG2 rheometer using the 40 mm diameter aluminum parallel-plate geometry to determine the storage modulus G′ and the loss modulus G″. Since the evaporation of solvent takes place rapidly after the gel is poured out between the parallel plates, a solvent trap system consisting of an immiscible liquid (water) surrounding the gel and in contact with it was utilized. Samples were put on the rheometer at 30 °C to prevent gelation and then cooled down to 20 °C and kept at this temperature for 30 s before the beginning of experiment to ensure the formation of the gel. The linear viscoelastic zone (LVZ), in which G′ and G″ are independent of strain amplitude, was set with the aid of a strain sweep from 0.01% to 1000% at 20 °C and an oscillatory frequency of 1 Hz. Recovery test from 1000% to 0.01% of strain were subsequently performed. Isothermal frequency sweeps between 100 and 0.1 Hz were carried out at 20 °C and a constant strain amplitude of 0.1% within the LVZ. Finally, temperature ramps from 10 to 40 °C at a constant frequency of 1 Hz and 0.1% of strain amplitude were performed. Powder X-ray Diffraction. Structural information on 1 was provided from powder X-ray diffraction (XDR), which was obtained using a D8-ADVANCE equipment from Bruker with SOLX or scintillation detectors and using copper Kα radiation. The voltage and current source were set at 40 kV and 30 mA, respectively. Diffraction patterns were recorded with a goniometer speed of 2°/min between 2° and 30° (2θ). Single Crystal X-ray Diffraction. The X-ray diffraction data collections were done at 296(2) K on a Bruker Kappa Apex II diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The structure was solved and refined using the Bruker SHELXTL software package.25 The cell parameters were determined and refined by a least-squares fit of all reflections. A semiempirical absorption correction (SADABS) was applied for all cases. All the structures were solved by direct methods and refined by full-matrix leastsquares on F2 including all reflections (SHELXL-97). Relevant data acquisition and refinement parameters are gathered in Tables S2 and S3 in the Supporting Information. The crystal structure has been deposited at the CSD with deposition number CCDC 1520614. Molecular plots and powder X-ray diffraction pattern simulation were made with Mercury.26 The simulated powder pattern was corrected with a preferred (100) orientation, as observed in the ribbon-like single crystals, using a March− Dollase parameter of 0.01 (for plate-like habit).27,28 1 H Nuclear Magnetic Resonance. TD 1H NMR spectra were recorded on a Bruker DRX-500 using cyclohexane-d12. Computational Details. All the calculations were performed with the Gaussian 09 (revision D.01) suite of programs.29 The ground (S0) and first electronic excited (S1) state geometry of the compound 1 was optimized using a set of hybrid, metahybrid, and hybrid exchange−correlation functionals (B3LYP,30,31 PBE0,32,33 M06-2X34 and CAM-B3LYP35) together with the 6-31G** basis set. Vertical electronic transitions were computed at the time-dependent TD-DFT level for the solvated molecule within the polarizable continuum model (PCM) methodology.36,37 The influence of the solvent on the excited state was analyzed within both the linear response (LR) regime, which is valid in the limit of weak external perturbation, and the state specific (SS) approach, which could be more appropriate in order to describe polarizable electronic excitations in solutions.38−41 Vertical

Figure 1. Molecular structure of gelator 1 and photographs of the progressive formation of organogel from 1 in cyclohexane upon heating−cooling by “inverted test tube”.

assembling through intermolecular H-bonding and π−π interactions between diazaisoindigo molecules. Its rheological properties, crystal-packing manners, and optical properties were investigated. From a theoretical perspective, the effect of the aggregation of N,N-dioctyl-7,7′-diazaisoindigo on the photophysical properties was also studied by means of the density functional theory (DFT).

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EXPERIMENTAL METHODS Materials and Methods. All solvents were obtained from commercial sources and used without further purification. UV−Visible Absorption and Emission Spectroscopy. UV−vis absorption spectra studies were carried out on a UV2401 PC Shimazu spectrophotometer. UV−vis of the xerogel was recorded in thin film on a quartz plate. Steady state fluorescence (SSF) and time-resolved fluorescence (TRF) spectra in liquid and gel state were recorded on an FLS920 spectrofluorometer (Edinburgh Instruments) equipped with a time correlated single photon counting (TCSPC) detector. A TLC 50 cuvette holder (Quantum Northwest) was used for temperature-dependent experiments. For SSF spectra, a Xe lamp of 450 W was used as the light source, and the excitation and emission slits were both fixed at 5 nm. The step and dwell time were 1 nm and 0.1 s, respectively. A EPLED 360 subnanosecond pulsed light emitting diode (Edinburgh Photonics) was employed as the light source at 368 nm for TRF experiments. The fluorescence intensity decays, I(t), were fitted to the following monoexponential function using an iterative least-squares fit method, I(t ) = α exp( −t /τF)

(1)

where α and τF are the amplitude and lifetime. Fluorescence spectra in powder and xerogel states were acquired in a FS5 (Edinburgh Instruments) spectrofluorometer equipped with a Xe lamp of 150 W as the light source, a TCSPC detector, and an integrating sphere. The quantum yield of the samples, ΦF, was also measured using the integrating sphere in which the number of absorbed photons of the sample is determined by the reduction of the light scatter compared to a blank measurement. The excitation wavelength was selected at 500 nm, and the excitation and emission slits were opened to 10.0 and 1.0 nm, respectively. The step and dwell time were 1 nm and 0.5 s, respectively. The sample holder for liquids of the integrating sphere was employed to measure ΦF of the xerogel, while the holder for solids was used for the sample in powder state. Scanning Electron Microscopy (SEM). Field emission scanning electron microscopy (FE-SEM) image of the xerogels were recorded on an FE-SEM FEI Nova NanoSEM 230 instrument with a vCD detector and Simatzu S-8000 with fieldemission filament. 27072


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parameters. Essentially, the dynamic storage modulus, G′, must be greater than the loss modulus, G″; and also, G′ must be independent of the oscillatory frequency. That means that gels must possess a solid-like behavior, which is the mechanical definition of gel.47 The gels reported in this study were also found to be thixotropic, as they change from a solid-like gel into a clear solution under vigorous shaking. Upon resting, gel reforms rapidly. Therefore, to undertake the detailed viscoelastic characterization of these systems, rheology tests were carried out. In Figure 2A, the results of the rheological experiments performed at 20 °C at constant oscillatory frequency of 1 Hz and increasing oscillatory strains are depicted. G′, associated with the elastic contribution, and G″, associated with the viscous contribution, are monitored as a function of shear strain. At low strain values, the value of G′ (about 10 Pa) is greater than G″ (about 4 Pa) indicating the predominant elastic character of the sample and hence a solid-like behavior (see up curves). Yet, these low values of G′ and G″ are characteristic of very feeble gels, whose network structure is made of weak interactions. On the other hand, both moduli G′ and G″ remain approximately constant below a critical strain value of 0.5%, which defines the upper limit of the linear viscoelastic regime. Above that critical value, a decrease of G′ and G″ was observed, indicating a partial breakup of the gel structure. Then, a level off of G′ and G″ was obtained at a shear strain of 60% followed by a new decrease leading to the crossover of G′ and G″ at a shear strain of 300%, indicating the loss of the solid-like behavior. These results suggest two types of interactions of different strength, which break at different shear strains, as responsible for the gel formation. This statement should be confirmed by structural determinations. The thixotropic character of the organogels was confirmed by carrying out recovery tests immediately after sample structures were destroyed by applying strains above the yield strain at 300%. The application of decreasing oscillation strains (see Figure 2A, down curves) provokes the recovery of the solid-like behavior (G′ values above G″). Nevertheless, neither G′ nor G″ attains the pristine values of the starting organogel. This should be the case for proper thixotropic materials. Then, the fact that the original values were not attained is an indication that longer times are required for the recovery of the gel to be completed. Dynamic frequency sweeps at 20 °C and a small value of applied strain (0.1%) within the linear viscoelastic region were also conducted, and the results are depicted in Figure 2B. Experimental values of moduli showed a consistently higher value for the storage modulus over the loss modulus for all the frequency range studied, which is typical for viscoelastic soft solids. Additionally, G′ is almost independent of the frequency of oscillation at low frequencies. All these results corroborate that in this study we are dealing with weak physical gels of rather low elastic modulus. The thermal behavior of the organogels was also analyzed through temperature sweeps carried out at constant strain amplitude of 0.1% and oscillatory frequency of 1 Hz. Results are shown in Figure 2C. Unlike it occurred for a previous system studied in our lab,11 where the elastic modulus was independent of temperature at low temperature values, here G′ and G″ continuously diminish with temperature from the lowest temperatures studied. In the aforementioned study, the presence of a pseudoplateau in the evolution of the elastic modulus with temperature was attributed to an elasticity of enthalpic origin characteristic of gels presenting rigid elements as constituents of the network structure.48 In the present case

electronic transitions were also computed for clusters of stacked 1 molecules extracted from the X-ray structure of the xerogel without further optimization to study the effect of the aggregation on the optical properties. In order to gain insight on the AIEE phenomena, the geometry and molecular properties of the excited state were analyzed both in solution and in solid state. In solution, the S1 state was optimized in equilibrium solvation and the fluorescence emission energy was calculated as ΔEem = ES1(GS1) − ES0(GS1)

(2)

where ES1(GS1) is the energy of the S1 state at its equilibrium geometry (in the SS solvation approach) and ES0(GS1) corresponds to the energy of the S0 state at the S1 state geometry and with the static solvation from the excited state. To calculate the electronic transitions in the solid state, a cluster of nine molecules was first extracted from the X-ray structure of the xerogel. Then, the ONIOM (QM:MM) method was employed to optimize the molecular geometry of the S0 and S1 states of the central molecule in the cluster.42 Two layers were defined: the high accuracy one, involving the central molecule (computed at the PBE0/6-31G** level), and the low accuracy layer, for the eight surrounding molecules, computed at the UFF level43 and using the QEq method for the charge assignation.44 The atomic positions in the low accuracy layer were kept fixed for all the ONIOM calculations. The fluorescence emission energy in solid state was also calculated following eq 2, but in this case, ES1(GS1) corresponds to the energy of the cluster with the central molecule in S1 state at its equilibrium geometry and ES0(GS1) is the energy of the cluster with the central molecule in S0 state at the S1 state geometry. The normal modes were computed for S0 and S1 optimized states both for the molecule in solution and for the cluster to check the absence of imaginary vibrational frequencies. The reorganization energy (λj) of the normal modes involved in the excited state relaxation was determined using the DUSHIN program.45

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RESULTS AND DISCUSSION Molecular Synthesis, Organogelation Behavior, and Morphology of Organogel. The synthesis of the gelator 1, N,N-dioctyl-7,7′-diazaisoindigo (see Figure 1), was previously described.46 It was carried out by alkylation of 7,7′diazaisoindigo with 1-iodooctane in the presence of K2CO3 as base in dimethylformamide (DMF), providing 1 with moderate yields. Description of the synthesis and characterization of this compound can be found in our previous work. The gelation capability of 1 was studied in different solvents by the typical heating−cooling method (Table S1 in Supporting Information). No gelation was observed in toluene, dichloromethane, acetone, ethyl acetate, or tetrahydrofuran. However, it was found that compound 1 can gelate in two different solvents, cyclohexane and hexane. The critical gelation concentration (CGC) in these solvents was further studied. Cyclohexane was one of the best solvents for the gelation with minimum gelation concentration of 2.75 wt %. The morphology of the xerogel was investigated by FE-SEM. The SEM images obtained from the xerogel in cyclohexane show belts aggregates (see Figure S1). Rheological Properties. Two important parameters obtained from dynamic rheology are the storage modulus (G′) and the loss modulus (G″). To be considered as a gel, a system must fulfill some requirements in relation to these two 27073


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the TD 1H NMR experiments of compound 1 in gel state, the aromatic signals are slowly shifted upfield upon the increase of the temperature from 298 to 343 K (Figure S2). This observation suggests the existence of π-stacking arrangements between aromatic molecules being the force that drives the association process. The structure in the solid state of compound 1 was solved by single crystal X-ray diffraction. It crystallizes in the monoclinic P21/c space group with one-half of the molecule in the asymmetric unit and an inversion center located in the middle of the C6−C6′ bond (see Supporting Information, Figure S3). The four-ring core of the molecule is completely planar, and the packing of the molecules in the crystal is achieved by π−π interactions that give rise to stair-like columns parallel to the [010] direction, where adjacent molecules are located at 3.355(4) Å, with a slip angle of 43.3(2)° (Figure 3). This value

Figure 3. View of the stacking of the molecules in columns in 1 with the value of the slip angle. Hydrogen atoms have been omitted for clarity.

of the angle (calculated as the angle between the long axis of one molecule, C2−C2′ (Figure S3) and the line of centers of adjacent molecules in the column) is indicative of a J-aggregate behavior. The packing of the columns in the crystal showed that neighbor columns in the [100] direction are slanted at the same angle, while cores of molecules in adjacent columns on the [001] direction displayed an almost perpendicular angle of 86.5(2)° (Figure S4). There is also a C−H···O hydrogen interaction that joins the molecules located in these neighbor columns in the c direction (see Figure S5 and Table S4). Moreover, to investigate the molecular packing of this compound in the gel phase, the X-ray diffractogram of the xerogel of gelator 1 prepared in cyclohexane was analyzed for elucidating the molecular structure of organogel (Figure 4). The pattern clearly indicated an ordered structure in the xerogel (Figure 4A) of a lamellar packing. In order to investigate if the arrangement of the molecules in the xerogel was similar to the molecular packing found in the crystal structure, the simulated powder pattern from single crystal data was corrected with a preferred (100) orientation, using a March−Dollase27,28 parameter of 0.01 to take into account the expected plate-like habit (Figure 4B and Figure 4C), previously observed in the crystalline sample (Figure 5A). The fact that the peaks in the experimental powder diffractogram of the xerogel correspond to the (h00) planes proves the preferential orientation of the molecules in the xerogel, identical to the one found in the crystals (see Figure 5). Photophysical Properties. UV−vis absorption and emission spectra of the compound 1 were recorded in different

Figure 2. (A) Variation of G′ (circles) and G″ (squares) with oscillatory strain for a 2.75 wt % gel of 1 in CH at 20 °C and frequency of 1 Hz. Full symbols correspond to curves obtained at increasing strains, and empty symbols correspond to curves obtained at decreasing strains. (B) Variation of G′ (circles) and G″ (squares) with oscillatory frequency for a gel of 1 at a concentration of 0.046 M in cyclohexane. Experiment was performed at 20 °C and constant oscillatory strain amplitude of 0.1%. (C) Variation of G′ (circles) and G″ (squares) with temperature for a gel of 1 at a concentration of 0.046 M in cyclohexane. Experiment was performed at a constant frequency of 1 Hz and constant oscillatory strain amplitude of 0.1%.

this conclusion cannot be straightforwardly obtained, as the pseudoplateau in the elastic modulus was not observed. However, the constant decrement of the elastic modulus with temperature can be attributed to the continuous breakup of the weak links responsible for the network structure in these gels. Driving Force of the Organogelation. We investigated the driving force by temperature dependent (TD) 1HNMR, powder X-ray diffraction and single crystal X-ray diffraction. In 27074


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Figure 4. (A) Experimental diffractogram of the xerogel. (B) Simulated powder diffractogram taking into account a (100) preferential orientation and a plate-like crystalline shape. (C) Simulated powder diffractogram of compound 1 with a random crystallite orientation.

Figure 6. (A) Absorption spectra of 1 in cyclohexane solution (2 × 10−5 M, 25 °C) and in powder, gel, and xerogel states. (B) Calculated theoretical spectra for different size aggregates of compound 1. The vertical bars correspond to the oscillator strengths ( f) calculated for the electronic transitions of clusters of n stacked molecules (n = 1−4) at the TD-PBE0/6-31G** level.

Figure 5. (A) Photograph of a ribbon-like single crystal with its faces indexed. (B) View of the orientation of the molecules both in the crystals and in the xerogel.

state, the lowest energy absorption maximum appears centered at 506 nm and the highest energy bands are not clearly defined. Those spectral red-shifts suggest the formation of J-type aggregates during the gelation process via π−π interactions in microbelts or 3D networks.50,51 Vertical electronic transitions calculated for clusters of several molecules extracted from the X-ray structure agree with the experimental observations, showing the mentioned spectral red-shifts (see Figure 6B). The bathochromic shift is especially obvious in the S0 → S1 transition, Δλ being 12 and 18 nm for trimer and tetramer clusters, respectively, with respect to the single molecule (see Table 2). Two bands centered at 393 and 619 nm were observed in the fluorescence emission spectrum recorded for 1 in cyclohexane solution (2 × 10−4 M) when it was excited at 328 nm (see Figure S6). We did not find the same level of agreement between experimental and theoretical values as in the case of the vertical excitation transitions. In this case, the difference in energy between both values was of 0.41 eV, the fluorescence emission wavelength being calculated for 1 in cyclohexane solution of 779 nm at the PBE0/6-31G** level of theory. The emission of 1 was partially quenched upon increasing the concentration from 10−4 to 10−2 M in the sol state. At this concentration during 10 min the emission was gradually increased while it is accompanied by a gelation process. Figure 7B shows the red-shift and AIEE effect observed during the gelation process upon cooling the hot solution (see also the normalized emission spectra in Figure S7). At 20 °C, the

conditions, and time dependent TD-DFT calculations were used in their analysis. UV−vis spectra of 1 were collected in a dilute cyclohexane solution (2 × 10−5 M) as well as in gel and xerogel states (Figure 6A). The UV absorption spectrum in solution showed three absorption bands centered at λabsmax = 280 nm (ε = 37 770 dm3 mol−1 cm−1), 328 nm (ε = 15 405 dm3 mol−1 cm−1), and 470 nm (ε = 6527 dm3 mol−1 cm−1). The lowest energy transitions of the compound 1 were calculated with different TD-DFT methods. As shown in Table 1, the best agreement between experimental and theoretical results was obtained with TD-PBE0/6-31G** (|Δλ| ≤ 14, 3, and 23 nm for the S0 → S1, S0 → S6, and S0 → S9 transitions, respectively). TD-PBE0 has been recommended to calculate the lowest energy transitions of organic dyes in the extensive TD-DFT benchmark reported by Jacquemin et al., with a mean absolute error of 22 nm.49 In addition, small differences were observed between the wavelengths computed for the S0 → S1 transition employing the SS approach compared to their LR counterpart (|Δλ| ≤ 5 nm), and therefore, only minor variations in the electron density of these transitions are expected (see frontier molecular orbital shapes in Figure S10). For these reasons, LR-TD-PBE0 was the choice method for the following calculations on the effect of the aggregation on the photophysical properties. Figure 6A shows the red-shift and broadening undergone by the absorption spectrum of 1 upon the gelation. In xerogel 27075


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Table 1. Maximum Absorption Wavelength, Oscillator Strength (f), and Main Component of the Transition (% Contribution) of the Most Relevant Electronic Transitions Computed for Monomer in Cyclohexane Solution at LR-PCM/TD-DFT/6-31G** and SS-PCM/TD-DFT/6-31G** Levels of Theory method B3LYP

S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0

PBE0

CAM-B3LYP

M06-2X

a

λabsmax (nm [eV])a

transition → → → → → → → → → → → →

S1 S6 S9 S1 S6 S9 S1 S5 S9 S1 S5 S9

501.5[2.47] 337.1[3.68] 265.6[4.67] 484.2[2.56] 325.2[3.81] 257.2[4.82] 411.6[3.01] 286.1[4.33] 242.4[5.12] 416.1[2.98] 287.3[4.32] 241.4[5.14]

f

{497.1[2.50]}

{479.6[2.59]}

{406.8[3.05]}

{411.6[3.01]}

0.172 0.538 0.426 0.192 0.551 0.487 0.219 0.488 0.663 0.269 0.516 0.681

contribution (>10%)

{0.127}

{0.142}

{0.285}

{0.207}

H → L(97%) H-2 → L(95%) H → L + 1(68%); H-7 H → L(98%) H-2 → L(95%) H → L + 1(69%); H-8 H → L(98%) H-2 → L(94%) H-1 → L + 2(25%); H H → L(98%) H-2 → L(94%) H-1 → L + 2(25%); H

→ L(19%)

→ L(13%)

→ L + 1(56%)

→ L + 1(58%)

Data in { } are for state specific regime.

Table 2. Maximum Absorption Wavelength (λabsmax), Oscillator Strength (f), and Main Component of the Transition (% Contribution) of the Most Relevant Electronic Transitions Computed for Clusters at the TD-PBE0/6-31G** Level of Theory species

transition

dimer

S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0

trimer

tetramer

→ → → → → → → → → → → → → → → → → → → → → → → →

S1 S14 S20 S22 S32 S23 S1 S10 S27 S39 S41 S67 S70 S74 S1 S14 S42 S44 S56 S62 S64 S113 S116 S123

λabsmax (nm [eV])

f

contribution (>10%)

483.2[2.57] 319.4[3.88] 300.7[4.12] 298.2[4.16] 254.8[4.87] 253.0[4.90] 496.4[2.50] 425.7[2.91] 320.4[3.87] 299.3[4.14] 297.5[4.17] 256.2[4.84] 253.7[4.89] 250.6[4.95] 501.9[2.47] 427.6[2.90] 328.5[3.77] 320.9[3.86] 302.4[4.10] 299.0[4.15] 297.7[4.15] 256.6[4.83] 252.2[4.87] 251.5[4.93]

0.155 0.358 0.229 0.103 0.196 0.522 0.188 0.106 0.488 0.154 0.251 0.216 0.649 0.139 0.247 0.129 0.102 0.593 0.113 0.213 0.296 0.202 0.633 0.397

H → L(96%) H-5 → L(90%) H-7 → L + 1(23%); H-6 → L (14%); H-5 → L + 1(55%) H-13 → L(27%); H-11 → L + 1 (10%); H-9 → L(16%); H-7 → L + 1(10%); H-5 → L + 1(16%) H → L + 4 (35%); H-L + 5 (47%) H-1 → L + 2 (55%); H → L + 4 (17%) H → L(93%) H-2 → L + 2(31%); H-1 → L + 1(39%) H-8 → L(64%); H-6 → L(17%) H-20 → L(13%); H-14 → L(12%); H-6 → L + 2(29%) H-20 → L(13%); H-6 → L + 2(36%) H → L + 6(17%); H → L + 7(58%) H-21 → L + 1(11%); H-2 → L + 3(20%); H-1 → L + 4(25%); H → L + 6(16%) H-23 → L(24%); H-21 → L + 1(22%) H → L(89%) H-3 → L + 3(12%); H-2 → L + 2(28%); H-1 → L + 1(27%) H-8 → L(69%) H-11 → L + 1(19%); H-10 → L(56%) H-11 → L + 1(18%); H-10 → L + 2(35%) H-27 → L + 1(10%); H-9 → L + 3(15%); H-8 → L + 2(15%) H-1 → L + 1(10%); H-9 → L + 3(12%); H-8 → L + 2(17%) H-1 → L + 7(15%); H → L + 9(24%); H → L + 11(17%) H-28 → L + 1(17%); H-2 → L + 5(16%); H → L + 8(13%) H-31 → L + 1(10%); H-28 → L + 1(24%); H-1 → L + 6(14%)

absorption maximum previously reordered for 1 in solution, and a second band centered at 592−600 nm, which significantly increases during gelation. This band was attributed to aggregate formation that is favored by the increase of the solvent viscosity during the cooling. Consequently, monomer and aggregate show different emission spectra and the emission signal is significantly higher when the sample is excited at 600 nm than at 470 nm due to the AIEE effect (see Figure S8). In a previous work, it was discussed that the main deactivation pathway of the excited state in this type of compounds seems to be a nonradiative process via a conical intersection (CI) that involves the twisting of the double bond.46 Hence, the fluorescence lifetime increases upon the solvent viscosity due to the slowing down of the twisting of the double bond after the excitation. In this work, we have corroborated that the

organogel exhibits a broad emission band with a maximum centered at about 665 nm. It is worthy to mention that the organogel also emits near-infrared (NIR) fluorescence which can be interesting for different applications such as biological and biomedical sensing because NIR radiation reduces the photodamage to biological samples, leads to deeper penetration, and does not interfere with the autofluorescence of living systems.52−54 In this sense, different fluorescent probes based on AIEE and NIR-AIEE effect for biological sensing have been recently reported.55−57 Diverse examples of NIR-emitting organogels based on BODIPY and bis(phenylethenyl)benzene can also be found in literature.58,59 Figure 7A shows the evolution of the excitation spectrum upon the cooling. In those spectra, two excitation bands can be observed, i.e., a band centered at 468 nm, which corresponds to the lowest energy 27076


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Figure 8. Excitation and emission spectra of 1 in xerogel and powder states (λex = 500 nm and λem = 660 nm for emission and excitation spectra, respectively).

of intramolecular vibrations (RIV), and the restricted access to conical intersections (RACI). These mechanisms have been applied to elucidate the origin of the AIEE effect in several luminogens through rigorous studies by Tang et al.,60−63 Shuai et al.,64−66 and Blancafort et al.,67,68 among others, combining both experimental and computational approaches. AIEE phenomenon has also been the object of different studies in organogel materials.69,70 In order to investigate this issue further, the main changes in the molecular structure of S0 and S1 states of 1 were analyzed (see Table S5). In the S0 → S1 transition, the largest change is predicted for the molecule in solution, which is almost planar in S0 state (the central dihedral angle, ϕ, is 0.1°) but significantly twisted in S1 state (ϕ ≥ 16°) (see Table S5). A slight elongation of the C6−C6′ distance was also observed in going from S0 to S1 state (see Table S5). As previously discussed, the predicted twisting of the S1 state through ϕ is in agreement with the nonradiative deactivation pathway proposed for compound 1.46 The aggregated state was mimicked by a cluster of nine molecules extracted from the X-ray structure (see Figure 9). A fluorescence emission wavelength of 725 nm was calculated for the central molecule of the cluster shown in Figure 9. In this context, our theoretical results show a different scenario that may justify the fluorescence enhancement upon gelation. The planarity of the molecule 1 is now preserved in the S0 → S1 transition in which the central dihedral angle, ϕ, varies less than 1°. The higher molecular rigidity of 1 in aggregated state should hinder the nonradiative deactivation pathways of the S1 state via the conical intersection and favor the fluorescence decay. RACI effect has also been employed to explain the AIE phenomena observed in other luminogens such as diphenyldibenzofulvene and dimethyltetraphenylsilole both in solution and in solid state.67,68 It is worth mentioning that the geometrical changes in the S0 state when going from the isolated molecule to the molecule embedded in the cluster are very small and, therefore, the main differences are observed in the S1 state. The reorganization energy is a key parameter to take account in the excited state relaxation dynamics. Several works have recently been published analyzing the effect of the aggregation on the in-plane or/and out-plane vibrational modes responsible

Figure 7. Excitation (A) and emission (B) spectra of 1 during the cooling of the sample to induce its gelation in cyclohexane solution (concentration of the sample was 4.5 × 10−2 M) (λexc = 470 nm).

fluorescence lifetime significantly increases from τF ≤ 117 ps 46 for diluted solutions of 1 to 502 ps in gel state (λem = 650 nm and fitted to a monoexponential equation) where the twisting movements are more restricted than in solution (see Figure S9). The fluorescence spectra of the sample in solid state (xerogel and powder) are pointed out in Figure 8. The emission spectra with the emission bands centered at about 650 nm are similar to that recorded for the organogel (see Figure 7) while the excitation spectra do not show well-defined bands indicating the presence of various types of molecular arrangements. Quantum yields, ΦF, of 4.32% and 6.72% were measured for xerogel and powder, where the latter has a crystalline aspect suggesting an ordered supramolecular structure. These quantum yields are significantly higher than those reported for a 7,7′-diazaisoindigo derivative similar to 1 in solution (from ΦF = 0.023% to 0.28% for acetonitrile and carbon tetrachloride solutions, respectively).46 Again, the enhanced quantum yields measured in the solid state could be related to the restriction of molecular movements. To shed light on the AIEE effect observed in gel and solid state, we have carried out a theoretical study on the impact of the aggregation on the molecular and the electronic properties of the ground and excited states. Different mechanisms have been proposed to explain the AIEE phenomenon such as the restriction of intramolecular motions (RIM), which includes the restriction of intramolecular rotations (RIR) and restriction 27077


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the atomic displacement vectors for some selected normal vibrations). Tang et al. also reported that the main contribution to the deactivation of the excited state of 1-methyl-1,2,3,4,5pentapheniylsilole (MPPS) and the phosphorus analogue compound (PPPO) corresponds to stretching motions, and this behavior was catalogued as “abnormal” by the authors.60 A similar behavior was also found by Shuai et al. for 3-(2-cyano-2phenylethenyl-Z)-NH-indole (CPEI).66,71 Other frequency modes of the molecule 1 also have a significant contribution to the deactivation of the S1 state as for instance hydrogen rocking at 1411 cm−1 (λj = 24 meV) and 1412 cm−1 (λj = 28 meV) (see Table S6). Table S7 lists the vibrational wavenumbers for S0 and S1 states calculated for the isolated molecule and the molecule in the cluster. In general, the normal modes are blue-shifted when the molecule is embedded in the cluster since these vibrations should be more hindered compared to the isolated molecule. Therefore, as previously mentioned, the restriction of intramolecular vibrations should favor the fluorescence decay in detriment of the nonradiative deactivation mechanism.

Figure 9. Cluster of nine molecules mimicking the solid state. The central molecule (in ball and tube style) corresponds to the high accuracy layer computed at QM level. The molecules in wireframe style correspond to the low accuracy layer computed at the MM level.

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for the excited state relaxation.60,61,65,66 Figure 10 shows a representation of the calculated reorganization energy, λj, versus

CONCLUSIONS A new LMOG π-electron-deficient compound based on a diazaisoindigo derivative, exhibiting AIEE phenomenon, is described in the present work. This organogel is capable of selfassembling through intermolecular H-bonding and π−π interactions between diazaisoindigo molecules. The analysis of the crystal structure of this compound allowed the identification of the peaks present in the experimental PXRD pattern of the xerogel and the confirmation of the spontaneous supramolecular organization of the molecules. The AIEE phenomenon occurs during the formation of the supramolecular organogel. A red-shift was observed in the absorption and fluorescence emission spectra upon the gelation, suggesting the formation of J-type aggregates via π−π interactions in microbelts or 3D networks. In the fluorescence excitation spectrum, it was observed a band corresponding to the excitation of the isolated molecule and a second band assigned to the aggregated species, which significantly increases during cooling and gelation process. Fluorescence lifetime and quantum yield also increase from dilute solution to the aggregate state. The AIEE phenomenon was also interpreted by DFT calculations which predicted the twisting of the central dihedral angle, ϕ, during the S0 → S1 transition in the isolated molecule. This molecular twisting could result in a nonradiative decay via the conical intersection. However, when the molecule is in aggregate state, that twisting is hindered and the radiative decay is favored. Reorganization energy calculations also predicted that the central high frequency C−C stretching vibration, connecting the two rings of the platform, is one of the main contributions to the deactivation of the excited state rather than low frequency vibrations.

Figure 10. Reorganization energy versus the normal mode wavenumbers for molecule 1 in cyclohexane solution.

the wavenumbers of the normal modes j involved in the relaxation for the molecule in cyclohexane solution at the PBE0/6-31G** level of theory (see Figure S11 for B3LYP/631G**). In addition, the calculated λj values are collected in Table S6. The main components of the reorganization energy correspond to high frequency modes (1000−1600 cm−1). The fundamental mode at 1636 cm−1, assigned to stretching vibration of the C6−C6′ bond connecting the two rings, presents the highest reorganization energy value (λj = 37 meV), along with the mode at 1175 cm−1 (Figures 11 and S12 show

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07625. Experimental procedures, FE-SEM, 1H NMR temperature experiments, single crystal X-ray diffraction data for 1, emission spectra, theoretical calculations (PDF)

Figure 11. Atomic displacement vectors of some selected normal modes for molecule 1 in cyclohexane at the PBE0/6-31G** level of theory. 27078


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrés Garzón: 0000-0002-0077-4562 Daniel López: 0000-0001-6386-5798 Josefina Perles: 0000-0003-0256-0186 Eva M. García-Frutos: 0000-0001-6270-1126 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the MICINN of Spain (Grant CTQ2013-40562-R), by Consejeriá de Economiá y Conocimiento, Junta de Andaluciá (Grant FQM-337), Consejeriá de Educación y Ciencia de la Junta de Comunidades de Castilla-La Mancha (Project PEII11-0279-8538), and Diputación de Albacete (DIPUAB-16-GARZONRUIZ). The authors thank Centro de Servicios de Informática y Redes de Comunicaciones (CSIRC), Universidad de Granada (Spain), and Servicio de Supercomputación de la Universidad de Castilla-La Mancha for providing the computing time. J.P. thanks Dr. M. Ramı ́rez for his help in the PXRD pattern ́ representation and interpretation and E. Rodriguez and P. Ciria for their support. The authors thank Prof. J. Reimers, University of Technology Sydney, for allowing us to use the DUSHIN program.

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