Effect of the Aggregation on the Photophysical Properties of a Blue

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Effect of the Aggregation on the Photophysical Properties of a BlueEmitting Star-Shaped Molecule Based on 1,3,5-Tristyrylbenzene Andres Garzon, Maria Paz Fernandez-Liencres, Mónica Moral, Tomas PeñaRuiz, Amparo Navarro, Juan Tolosa, Jesus Canales-Vázquez, Daniel HermidaMerino, Ivan Bravo, Jose Albaladejo, and Joaquin C. Garcia-Martinez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00311 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 12, 2017

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The Journal of Physical Chemistry

Effect of the Aggregation on the Photophysical Properties of a Blue-Emitting

Star-Shaped

Molecule

Based

on

1,3,5-

Tristyrylbenzene Andrés Garzóna,*, M. Paz Fernández-Liencresb, Mónica Moralc, Tomás Peña-Ruizb, Amparo Navarrob,*, Juan Tolosad, Jesús Canales-Vázquezc, Daniel Hermida-Merinoe, Iván Bravoa, José Albaladejof and Joaquín C. García-Martínezd

a

Department of Physical Chemistry, Faculty of Pharmacy, University of Castilla-La Mancha, Cronista Francisco Ballesteros Gómez, 1, 02071, Albacete, Spain. E-mail: [email protected] b Department of Physical and Analytical Chemistry, Faculty of Experimental Sciences, University of Jaén, Campus Las Lagunillas, 23071, Jaén, Spain. E-mail: [email protected] c Renewable Energy Research Institute, University of Castilla-La Mancha, Paseo de la Investigación 1, 02071, Albacete, Spain. d Pharmaceutical Organic Chemistry Group, Regional Center for Biomedical Research (CRIB), Faculty of Pharmacy, University of Castilla-La Mancha, Cronista Francisco Ballesteros Gómez, 1, 02071, Albacete, Spain. e Netherlands Organization for Scientific Research (NWO), DUBBLE@ESRF, 6 Rue Jules Horowitz, 38000, Grenoble, France f Department of Physical Chemistry, Faculty of Chemical Sciences and Technologies, University of Castilla-La Mancha, Avenida Camilo José Cela 10, 13071, Ciudad Real, Spain.

ABSTRACT In this work we present a study on the effect of the aggregation on the optical properties of star-shaped molecules. We analyzed the modification of the absorption and fluorescent properties of a 1,3,5-tristyrylbenzene core due to the formation of diverse aggregates. The nature of the aggregates in solution was investigated by different spectroscopic techniques such as electronic absorption, steady-state fluorescence, fluorescence anisotropy, time-resolved fluorescence, small-angle X-ray scattering and 1 ACS Paragon Plus Environment


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dynamic light scattering spectroscopy. In order to simulate the molecular arrangement of the aggregates, the structure and electronic properties of different clusters formed by stacking of star-shaped molecules were studied by means of Density Functional Theory calculations. The theoretical insight was performed in gas phase as well as in solution through the polarizable continuum model and both linear response and state specific polarization schemes were applied. In the solid state, high quantum yields of up to 0.51 were measured for a 1,3,5-tristyrylbenzene derivative. Finally, the morphological properties of different solid samples were analyzed by differential scanning calorimetry, as well as scanning and transmission electron microscopies.

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1. INTRODUCTION Multiple applications in the field of chemistry, biology, biomedicine and materials science are currently being investigated for π-conjugated molecules.1-6 These applications do not only depend on their molecular properties but also on their supramolecular arrangement.1,2,7-9 In solution, self-assembly of π-conjugated molecules used as fluorescent sensors can change their performance in biological systems as live cells.10 Likewise organic semiconductors are used in the fabrication of electronic devices such as organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs) since they exhibit anisotropic electrical conductivity in contrast to inorganic semiconductors.7 Light emitting properties of π-conjugated molecules employed in OLEDs also strongly depend on the molecular packing.11-13 Different effects as aggregation-caused quenching (ACQ), aggregation-induced emission (AIE) and aggregation-induced enhanced emission (AIEE) have been described in the literature evidencing that fact.14 For instance, in some aggregations types, rotational and vibrational molecular motions could be restricted and the excited state decay is driven into a radiative process producing the emission enhancement.15,16 Therefore, understanding the supramolecular interactions between individual π-conjugated molecules and the control of the molecular packing is crucial for the development of these and other applications.1,8 Supramolecular ordering is controlled by the shape and size of the π-conjugated systems and their non-covalent interactions, mainly π-π interactions and hydrogen bonds.7,8 In solution, solvent, concentration and temperature are also important factors affecting the molecular self-assembly.7,8 1D self-assembly is a particular form of supramolecular ordering in which π-conjugated discotic molecules tend to stack on top



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of one another to form columnar mesophases.7,8 The size and symmetry of the conjugated core in addition to the type and length of the side-chains can also control the morphology of the molecular assembly.7,8 The formation of columnar mesophases has been previously studied in π-conjugated discotic molecules such as benzocoronenes,7,17 triphenylenes,18,19 triazatruxenes,18

porphyrins,20,21 phthalocyanines,21 triazines,22

perylene diimides23 and polyphenylbenzenes.24 In the present work, we have chosen the star-shaped molecule 1,3,5-tristyrylbenzene, with C3 symmetry, as a model to shed light on the self-assembly of π-conjugated discotic molecules. Different aggregation trends have previously been observed for a set of 1,3,5-tristyrylbenzenes bearing polyamine or poly(amidoamine) (PAMAM) chains into the peripheral para positions.25 Here, we present a combined experimental and theoretical study on self-assembling processes of the molecule 1 formed by 1,3,5-tristyrylbenzene and hexyloxy pendant chains into the peripheral para positions (see Scheme 1). 1 is an interesting molecule which has already been successfully used to fabricate an experimental light emitting device.26 In that work, the authors detected the presence of aggregates in the film but no additional studies were performed to characterize them.26 Moreover, the aggregation modes of 1 are compared with those of 2, a related compound previously studied in our laboratory, to analyze the effect of the pendant chains on the self-assembly (see Scheme 1).25


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R

R

1

R = O(C6H13)

2

R = CH2NH(CH2)2NH2

R

Scheme 1. Molecular formulae of the studied compound (1) and a related derivative (2)

2. MATERIAL AND METHODS 2.1. General Unless otherwise stated, all reagents were used as received and without further purification. Tetrahydrofuran (THF), dichloromethane, chloroform, acetonitrile, and nhexane were used as CHROMASOLV quality. Ethylenediamine (EDA) was dried over CaH2, filtered, distilled under reduced pressure, and stored over molecular sieves (4 Å). NMR spectra were acquired at 25 ºC. Chemical shifts are given in ppm relative to tetramethylsilane, TMS (1H, 0.0 ppm) or deuterated chloroform, CDCl3 (13C, 77.0 ppm). Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrophotometer equipped with an attenuated total reflection (ATR) accessory.

2.2 Thermal Analysis The

thermal

decomposition

mechanisms

were

investigated

on

a

thermogravimetric analysis (TGA) Q20 apparatus (TA Instruments) fitted with a standard platinum pan. The conditions of thermogravimetry were as follows: sample



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masses of about 3 mg were heated at a rate of 10 °C min-1 under a nitrogen atmosphere. The differential scanning calorimetry (DSC) experiments were carried out with a model DSC Q50 (TA Instruments) fitted with a standard aluminum pan. The conditions of DSC were as follows: sample masses of about 3 mg were heated at a rate of 10 °C min‒1 under a nitrogen atmosphere. A sample of indium was used as the reference.

2.3. Samples 5 µM solutions of 1 in different solvents were prepared. In addition, water/acetonitrile mixtures with water fractions (fw) from 10 to 90% were used to investigate the formation of molecular aggregates. The solutions of 1 in solvent mixtures (5 µM) were sonicated for 10 min and kept at 4˚C for 12 hours. Thin films were prepared by slow evaporation of 80 µL of concentrated solutions of 1 (3 mM) in tetrahydrofuran and acetonitrile on quartz slides and under a solvent-saturated atmosphere.

2.4. Absorption Spectroscopy Absorption spectra of 1 were acquired in different solvents and mixtures of solvents at 20ºC. Absorption spectra were also recorded within the temperature range of 10 to 80ºC to investigate on the stability of the formed aggregates. Quartz cuvettes (Hellma Analytics) of 10 mm were employed for all the absorption and emission measurements of liquid samples. UV-Vis absorption spectra were acquired in a V-650 (Jasco) spectrophotometer and with a scan rate of 600 nm min‒1. A Peltier accessory was employed to control the temperature of the spectrophotometer measuring cell.


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For solid samples of 1 were used two different spectrophotometers: i.e. V-650 (Jasco) spectrophotometer for thin films and Cary 4000 (Varian) equipped with an integrating sphere for powdered samples. 2.5. Steady-State Fluorescence and Quantum Yield Measurements Spectra of the liquid samples were recorded employing a FLS920 (Edinburgh Instruments) spectrofluorometer equipped with a time correlated single photon counting (TCSPC) detector and a Xe lamp of 450 W as light source. The excitation and emission slits were both fixed at 1 nm. The step and dwell time were 1 nm and 0.1 s, respectively. A temperature-controlled cuvette holder, TLC 50 (Quantum Northwest) was used for the measurements. Emission spectra in powder and thin films were acquired in a FS5 (Edinburgh Instruments) spectrofluorometer equipped with a Xe lamp of 150 W as light source, a TCSPC detector and an integrating sphere. The quantum yield of both liquid and solid samples, Փ F, were also measured using the integrating sphere. For the liquid samples, the step, dwell time, excitation and emission slits were fixed at 1 nm, 0.5 s, 7 nm and 1 nm, respectively. Excitation slit was opened to 9 nm for solid samples. Chromaticity calculations of the emission spectra were done using the F980 Software of Edinburgh Instruments.

2.6. Fluorescence Anisotropy The FLS920 spectrofluorometer (previously described) equipped with two polarizers placed in the excitation and emission light pathways was employed for steady-state fluorescence anisotropy measurements. Polarized emission spectra were recorded at four polarizer directions, i.e. both horizontal (HH), both vertical (VV),



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horizontal on excitation and vertical on emission (HV) and vice versa (VH). The anisotropy spectra were calculated as =

(1)

where Ii is the fluorescence intensity at the corresponding configuration of the polarizers and G is a correction factor given by =

(2)

2.7. Time-Resolved Fluorescence (TRF) Spectroscopy Fluorescence decay profiles were collected at the wavelength of maximum emission using the FLS920 spectrofluorometer (previously described). A subnanosecond pulsed Light-Emitting Diode, EPLED-290 (Edinburgh Photonics) was employed as light source at 291 nm for TRF experiments. The fluorescence intensity decay, I(t), was fitted to the following multiexponential function using an iterative least square fit method = ∑ exp −/

(3)

where αi and τi are the amplitude and lifetime for each ith term. The mean lifetime of the decay was then calculated as =

∑! "#

(4)

∑! "#

2.8. Dynamic Light Scattering (DLS) Measurements DLS measurements were recorded employing a Zetasizer NanoZS (Malvern Instruments). Solutions of 1 in water/acetonitrile mixtures were prepared as those for UV-Vis absorption and emission spectra. Solutions were passed through a nitrocellulose 0.22 µm pore-size filter.


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2.9. Scanning and Transmission Electron Microscopy (SEM and TEM) SEM studies were carried out using a JEOL 6490LV electron microscope operating in the 0.1 – 30 kV range and equipped with secondary and backscattered electron detectors for imaging and an EDS detector (Oxford Link) for microanalysis. Two specimens were observed by SEM: compound 1 as powder and a second sample prepared by evaporation of a dilute acetonitrile solution of 1 (0.1 µM). TEM studies were performed using a 2100 JEM (JEOL) electron microscope operating at 200 kV and equipped with a double tilt (±25°) sample holder and an Orius Gatan CCD camera (2x2MPi). Low dose conditions were necessary to avoid beam damage of the specimens. Specimens for TEM observation were prepared by depositing a dilute acetonitrile solution of 1 (0.1 µM) onto a holey carbon grid (EMS).

2.10. Small-Angle X-Ray Scattering (SAXS) Measurements SAXS experiments have been conducted at the Dutch-Belgian Beamline (DUBBLE) station BM26B of the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). 5 µM solutions of 1 in water/acetonitrile mixtures with different water fractions were measured. The sample to SAXS detector distance was ca. 3477 mm. The samples were measured employing a monochromatic beam with a wavelength of 1.033 Å. A Dectris-Pilatus 1M detector with a resolution of 981 × 1043 pixels and a pixel size of 172 × 172 µm has been employed to collect the 2D SAXS scattering patterns. Standard data corrections for sample absorption and background subtraction have been performed prior data reduction. The data were normalized to the incident beam to correct the primary beam intensity fluctuations. The AgBe scattering patterns were used to calibrate the wavevector scale of the scattering curve. The data fitting was conducted using the MINUITS minimization program developed by the CERN.27,28



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Quantitative structural information about the clusters in solution was obtained by fitting the experimental SAXS measured intensity (I(q)) to non-interacting spheres like model described by: $ =

%&'() ,* ∆. /01 2323 45/ 23

+ 7

236 *

(5)

3. COMPUTATIONAL DETAILS All the calculations were performed with the Gaussian09 (revision D.01) suite of programs29 using the metahybrid GGA M06-2X functional.30 This functional has shown to yield good results in previous studies on noncovalent interactions and electronic transitions of π-conjugated organic molecules.16,19,31-33 The geometry optimization of both monomer and clusters were accomplished using the ONIOM methodology.34 Thus, two layers were defined: the high accuracy one, involving the core (computed at the M06-2X/6-31G* level), and the low accuracy layer, for the side chains (-OC6H13) (computed at the M06-2X/STO-3G level). The stacked π-conjugated molecules were modeled by a cluster made up of four units. In this way, a previous careful study of three important molecular parameters must be carried out such as azimuthal angle (α), inter-core distance and side displacement (see Figure 1). In order to predict the most favorable orientation between molecules inside the stack, we first calculated the binding energy for a dimer built with the optimized monomer structure as a function of the relative (x,y)-displacement between both molecules put away at 3.5 Å, the typical πstacking distance35 and α = 0º, 30º and 60º (90º is equivalent to 30º) (see Figure 1). During this scanning phase, -OC6H13 chains were substituted by methoxy to reduce the computational cost. The (x,y)-displacement was ranged from zero to 10 Å with an increment of 0.5 Å (z-distance was kept at 3.5 Å). Once the global minima were localized on the energy landscapes, the dimers corresponding to the minima found in 10 ACS Paragon Plus Environment

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the binding energy landscapes were used to build three different tetramer clusters (called t1, t2 and t3), for which full geometry optimizations were performed. Grimme´s D3 dispersion correction was considered with the original D3 damping function as implemented in Gaussian D.01.36 The geometry optimizations for monomers and clusters were performed in the gas phase and solvent environments. The solvent has been described by the polarizable continuum model (PCM) as implemented in the Gaussian package.37

z-axis y-axis R

x-axis

R

R R

α

R R

Figure 1. Two stacked molecules showing the azimuthal angle (α) and (x,y)-displacement directions.

The excitation energies, oscillator strengths and configurations involved in the electronic transitions were evaluated using the ground-state optimized geometries for monomer and tetramer clusters t1, t2 and t3 using Time-Dependent Density Functional Theory (TD-DFT) at the M06-2X/6-31G** level. In addition, the two central molecules of the optimized cluster were extracted as dimers (d1, d2 and d3 from the corresponding tetramers t1, t2 and t3, respectively) and TD-DFT calculations were also performed on those to analyze the effect of the cluster size on the electronic transitions. The influence of the solvent on excited states was analyzed within both the linear response (LR) regime, which is valid in the limit of weak external perturbation, and the state specific



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(SS) approach, which could be more appropriate in order to describe polarizable electronic excitations in solutions.38-41

4. RESULTS AND DISCUSSION 4.1. Synthesis and Thermal Analysis Compounds 142 and 243 were prepared according to a procedure previously developed by us based on the Horner-Wadsworth-Emmons reaction with the appropriated aldehyde over the triphosphonate core 3 (see Figures S1 – S3 in Supporting Information).44 More specifically, compound 1 was obtained from 4hexyloxybenzaldehyde, precipitated by adding water to the reaction crude, and later filtered and purified according to a previously published method.42 The synthesis of compound 2 starts from the trialdehyde core, obtained in a similar procedure that 1 after acidic cleavage of the acetal groups, which was converted to the amine derivative 2 by treatment with a large excess of EDA, followed by reduction of the imine groups in the presence of sodium borohydride. Thermal stability of compounds 1 and 2 were studied by TGA and DSC (see Figures S4 and S5). TGA shows thermal decomposition with loss of weight at 360ºC for compound 1 and 160ºC for compound 2. Three sequential heating and cooling DSC curves were performed for each compound below their decomposition temperature and curves overlap indicating that heat capacities are the same in each scan and compounds are thermally stable till these temperatures. Specifically, compound 1 shows two first order transitions, a characteristic melt endothermic peak at 114 °C and an exothermic peak at 66ºC with enthalpy values (∆H) of 53 and -50 Kcal mol‒1, respectively. Contrary, compound 2 does not show first or second order transitions before thermal decomposition at 160ºC. We have previously shown that compound 2 has


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intramolecular interaction such as hydrogen bonds and π-stacking resulting in the formation of entangled three-dimensional networks.25 It is expected a higher melting temperature, compared with the compound 1, because of these stronger intramolecular interactions but the presence of the amine groups induces decomposition before melting.

4.2. Spectroscopic Studies on Aggregation In a previous work, we observed the formation of polydisperse particles of the compound 2 in basic aqueous solutions.25 For solutions of 1 in water/acetonitrile mixtures, DLS experiments have also revealed the formation of polydisperse aggregates in the range of water fractions fw ≥ 20%. Concretely, the highest values of Zaverage radius were observed for the solutions fw = 30 and 40%, decreasing this value at higher water fractions (see Figure S6). The absorption spectrum of 1 in acetonitrile solution shows a strong band centered at 326 nm (see Figure 2a and Table 1) that does not change with the concentration in the range of 0.05 to 10 µM. Similar spectra were observed in different polarity solvents and, hence, intramolecular charge transfer transitions are not expected (see Figure 2a and Table 1). That band was assigned to S0→S1 (302 nm) and S0→S2 (301 nm) electronic transitions with different components involving the frontier orbitals between HOMO-2 and LUMO+2 (calculated at the SS-PCM/TD-M06-2X/6-31G** level in acetonitrile, see Table 2). These π and π* molecular orbitals are fully delocalized on the conjugated core of the molecule (see Figure S7). It is worth to mention the small differences observed between the lowest vertical transitions computed employing SS formulation compared to their LR counterpart (≤ 5 nm) and, therefore, only minor variations in the electron density of these transitions are expected.



In addition, no significant differences (< 0.5 nm) were calculated for the lowest electronic transitions of 1 in the solvents used for further aggregation experiments, i.e. acetonitrile and water.

a)

b)

0.6

n-hexane tetrahydrofuran dichloromethane acetonitrile

0.5

fw (%)

0.4

Absorbance

0.4

Absorbance

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0.3 0.2

10 20 30 40 50 70 90

0.3

0.2

0.1

0.1 0.0

250

0.0 200

300

350

400

300

450

400

500

600

700

Wavelength (nm)

Wavelength (nm)

Figure 2. Absorption spectra of 1 solved in (a) different solvents (acetonitrile, dichloromethane, n-hexane and tetrahydrofuran); (b) mixes of H2O:CH3CN at different water fractions (fw). Concentration of the samples was 5 µM.

Table 1. Absorption maximum wavelength (λabsmax), molar absorption coefficient (ε), emission maximum wavelength (λemmax) and quantum yield (Փ) found in the UV−Vis absorption and fluorescence spectra of compound 1 in solution b Solvent λabmax (eV [nm]) ε (mM-1 cm-1) λemmax (eV [nm])a ՓF 3.16 [392] n-hexane 3.80 [326] 25.7 0.22 2.99 [415] 3.11 [399] tetrahydrofuran 3.77 [329] 98.7 0.60 2.95 [420] 3.08 [403] dichloromethane 3.77 [329] 78.5 0.40 2.95 [421] (sh) c 3.08 [402] trichloromethane 3.81 [325]c ‒ 0.77 c 2.95 [420]c 3.10 [400] acetonitrile 3.80 [326] 88.1 0.46 2.97 [417] a Two peaks can be distinguished in the emission band. (sh) corresponds to a shoulder. b Fluorescence quantum yield determined using an integration sphere. Measures were performed at room temperature with excitation and emission slits of 9.0 and 0.1 nm, respectively, dwell time of 0.5 s and step of 1 nm. c From reference 26.


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Table 2. Lowest electronic transition energies (E [λ]), oscillator strength (f) and main components of the transition calculated for the compound 1 in gas phase and in different solvents at the TDM06-2X/6-31G** level. Results from Linear Response (LR) and State Specific (SS) approaches are compared Phase / Solvent E [λ] (eV [nm]) f Transition Main components of the transition (% contribution) 4.11 [301.7] 0.01 H-1→L (40); H→L+1 (35) S0→S1 gas 4.14 [299.4] 1.89 S0→S2 H→L (30); H-1→L+1 (30); H-2→L (16); H→L+2 (14) 4.14 [299.2] 1.88 S0→S3 H→L+1 (33); H-1→L (26); H-2→L+1 (17); H-1→L+2 (14) acetonitrile 4.04 [306.9] 1.98 S0→S1 H→L (31); H-1→L+1 (24); H-2→L (18); H→L+2 (14) (LR) 4.05 [305.8] 1.97 H→L+1 (27); H-1→L (27); H-2→L+1 (19); H-1→L+2 (15) S0→S2 4.09 [303.4] 0.01 S0→S3 H-1→L (39); H→L+1 (37) acetonitrile (SS) 4.10 [302.4] 1.87 S0→S1 H-2→L+1 (17); H-1→L (30); H→L+1 (33); H→L+2 (14) 4.11 [301.5] 1.87 S0→S2 H-2→L+1 (19); H-1→L (21); H-1→L+1 (11); H→L+1(34); H→L+2 (14) 4.07 [304.4] 0.28 S0→S3 H-1→L+1 (11); H→L (66) water (LR) 4.05 [306.3] 1.98 S0→S1 H→L (26); H-1→L+1 (21); H-2→L+1 (14); H→L+2 (14) 4.05 [305.9] 1.96 S0→S2 H→L+1 (24); H-1→L (21); H-2→L (14); H-1→L+2 (14) 4.09 [303.1] 0.01 S0→S3 H-1→L (19); H-1→L+1 (25); H→L (20); H→L+1 (18) water (SS) 4.11 [302.1] 1.88 S0→S1 H→L+1 (35); H-1→L (29); H-2→L+1 (18); H→L+2 (15) 4.11 [302.1] 1.88 S0→S2 H→L+1 (35); H-1→L (29); H-2→L+1 (18); H→L+2 (15) 4.08 [303.8] 0.19 S0→S3 H-1→L+1 (16); H→L (63)

In water/acetonitrile mixtures, the intensity of the band centered at 326 nm for the free molecule gradually decreases with the increase of the water fraction, from fw = 10 to 40%, but no shifts are detected (see Figure 2b). Afterward, a broadening of the absorption band is observed for fw = 50% and a new red-shifted band emerges for fw ≥ 60%. For fw = 60 and 70%, the band is still broad and reaches its maximum red-shift of 40 nm. The intensity of the new band increases for fw = 80 and 90% exhibiting two well-defined maxima at 339 and 353 nm which are less shifted (∆λ = 13 and 27 nm) than the corresponding ones observed for fw = 60 and 70%. In contrast to the classical H- and J-type aggregates (cofacial stacking and head-to-tail stacking, respectively), in which significant spectral shifts are expected, no large shifts in the absorption spectra are generally observed for other molecular aggregates such as X-aggregates. According to exciton-coupling theory, for dimers with oblique transition dipoles, the exciton splitting energy (∆ξ) is given by



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∆8 =

|:| 6 3;<

cos @ + 3 cos C

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(6)

where φ is the angle between the transition dipoles, θ is the angle made by the transition dipole of the unit molecule with the line of the molecular centers, ruv is the center to center distance between the monomers u and v and, M is the transition moment for the singlet-singlet transition in the monomer.45-47 Consequently, no large shifts in the absorption spectra are generally observed for aggregates with φ and θ angles near 90º.48,49 This type of molecular arrangement, in which the stacked molecules are also weakly coupled, could correspond to the aggregates detected in water/acetonitrile mixtures with low water fractions (fw ≤ 40%). The increase of the solvent polarity for the solutions with fw ≥ 60% provokes red-shifts that suggest the formation of new aggregates with a stronger intermolecular electronic coupling and an angle between the transition dipoles away from 90°. Consequently, the large broadening of the absorption band observed for fw = 50% should be associated to a transition region in which different aggregation states coexist. In contrast, blue-shifts in the absorption spectrum of 2 were observed in basic aqueous solutions and attributed to H-type aggregation due to the deprotonation of the anime groups of the pendant chains.25 H-bonding between the side chains -CH2NH(CH2)2NH2 of 2 might favor an aggregation in which the monomers are fairly overlapped. In this sense, stronger intramolecular interactions are expected for 2 in comparison with 1, as we described in synthesis and thermal analysis section. The absorption spectrum of a solution of 1 at fw = 80% was also recorded at different temperatures to investigate on the thermal stability of the aggregates (see Figure 3). DSC shows that compound 1 is perfectly stable within the thermal interval of the study. An isobestic point at 330 nm indicates the transformation between the aggregate and the free molecule. Nevertheless, the conversion was not completed after 40 minutes at


Page 17 of 43

80ºC. The transition from aggregated to molecularly dissolved species as a function of temperature has been described for other conjugated systems as perylene diimides.23,50

a)

b) 0.25

0.25 10ºC 30ºC 50ºC 70ºC 80ºC

0.15

0.10

0.15

0.10

0.05

0.05 300

10 min 20 min 30 min 40 min

0.20

Absorbance

0.20

Absorbance

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


320

340

360

380

300

400

320

340

360

380

400

Wavelength (nm)

Wavelength (nm)

Figure 3. Absorption spectra of 1 in a H2O/CH3CN solution (fw = 80%), (a) at different temperatures and (b) at different waiting times and keeping the temperature at 80 ºC. The dashed line corresponds to the maximum of wavelength of 1 in CH3CN solutions (concentration of the sample was 5 µM)

Fluorescence emission spectra of 1 in different solvents are shown in Figure 4a. It is a blue-emitting molecule and only small shifts in the emission spectrum were observed as a function of the polarity of the solvent. Quantum yield values in the range of 0.22 – 0.60 were measured in the different solutions (see Table 1). In addition, in some water/acetonitrile mixtures (fw = 10 to 40%), an increase of Փ F with respect to pure acetonitrile solutions was observed (see Fig. 5(a)), reaching its maximum value for fw = 30% (Փ F = 0.71). The phenomenon of aggregation-induced enhanced emission (AIEE) seems be related to those high Փ F values, although the reduction of the oxygen quenching inside the aggregate can also favor the fluorescence intensity increase. As discussed before, both absorption and emission spectra of these samples are similar to those recorded in pure acetonitrile. This behavior is typical of aggregation with orthogonal transition dipole moments of the molecular units where the electronic coupling is generally weak.48,51 In this sense, recent works on AIEE effect based on Xaggregation of π-conjugated molecules have been recently reported.48,49,52 In contrast, a 17 ACS Paragon Plus Environment


strong decrease of the emission intensity along with a small red-shift of the fluorescence maximum were observed when fw increased beyond 40% (see Figures 4b, 4c and 5a). A transition region appeared between fw = 50 and 70% in which a slight broadening of the emission band, with a single emission peak at 428 nm, is produced. Here, it must be remembered the broadening of the absorption band, occurred in the same range of fw, which was attributed to the coexistence of different aggregation states. A well-defined emission band with two peaks at 409 and 425 nm appears at fw = 80 and 90%. As commented before, the increase of the solvent polarity could induce a closer approach and higher electronic coupling of the aggregated molecules and, thus, the quenching of fluorescence emission. The red-shift observed in both absorption and emission spectra at high water fractions, in turn, could be related to variations in the angle φ between the transition dipole moments of the monomers. Figure 5b shows how the quantum yield also decreases when the solvent polarity increases due to the adding of NaCl.

b) 1.0

n-hexane tetrahydrofuran dichloromethane acetonitrile

0.8

0.6

0.4

0.2

0.0 350

400

450

500

550

600

Fluorescence intensity (104 counts)

a) Normalizaed fluorescence intensity

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

Page 18 of 43

fw (%) 0 10 20 30 40 50 70 90

15

10

5

0

650

400

Wavelength (nm)

500

Wavelength (nm)

c)


600

1.0

fw (%)

0.8

10 50 70 90

0.6

0.4

0.2

0.0 400

500

600

Wavelength (nm)

Figure 4. Normalized emission spectra of 1 in different solvents (the excitation wavelengths, λexc, correspond to their absorption maximum wavelengths, λabsmax) (a). Emission spectra (b) and normalized emission spectra (c) of 1 at different fw values (concentration of the samples was 5 µM and λexc = 326 nm).

a)

b) 0.8

0.7 0.7

0.6

0.6

0.5

0.5

0.4 0.3

0.4

ΦF

ΦF

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


Normalized fluorescence intensity

Page 19 of 43

0.2 0.3

0.1

0.2

0.0

0 0.1 0

20

40

60

80

[N

fw (%)

aC

30

5

50

10

l] (

15

mM )

20

80

fw

) (%

Figure 5. Quantum yield (Փ F) measured as a function of (a) the water/acetonitrile fraction (fw) and (b) the concentration of NaCl.

Table 3 shows the steady state fluorescence anisotropy (r) values measured for samples of 1 with fw = 0, 30, 50 and 80%. The increase of the anisotropy as function of fw is generally attributed to the smaller displacement of the emission dipole of the fluorophores which tend to aggregate. In our anisotropy experiments, two different groups of samples can be differentiated, i.e. a first group with low anisotropy values (r ≤ 0.04) for samples at fw = 0 and 30%, and a second one in which r increases at values ≥ 0.15 for samples fw = 50 and 80%. Hence, the molecules seem to have higher mobility in the aggregates formed at low fractions of water than in those aggregates observed at higher fw values. This can be explained on the basis of the weaker intermolecular coupling expected for the first type of aggregates. Nevertheless, the increase of the 19 ACS Paragon Plus Environment


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

polarity of the medium could also play a role on the mobility reduction of the fluorophores. Similar mean fluorescence lifetimes (τm) were measured at 400 and 500 nm for 1 in acetonitrile solution indicating that emission at both wavelengths originate from the same specie (see Table 3 and Figure 6). Likewise, it was observed in the samples fw = 30 and 50%. Nevertheless, an increase of about 2 ns in the fluorescence lifetime was observed from fw = 0 to 30%. This fact seems to be in agreement with the formation of weakly coupled aggregates. In contrast, τm strongly decreased for the samples with higher water fractions. It is known that the molecular organization has a large effect on the efficiency of the exciton diffusion and in turn on the fluorescence lifetime.53-55 For instance, large tilt angles of the molecular planes in self-assembled stacks of phthalocyanine molecules causes strong electronic coupling, resulting in efficient exciton motion from one stack to another and shorter fluorescence lifetimes.53 Thus, the exciton diffusion is generally faster in J-type aggregates than in H-type ones. The closer approach between the molecules, and the change in their relative orientations, at fw = 50 and 80% can lead to a more efficient exciton diffusion and shorter fluorescence lifetimes. More than one aggregate type could coexist at fw = 80% due to the difference of about 2 ns in the lifetimes measured at 400 and 500 nm.


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Page 21 of 43

10 4

10 3 Counts Counts (a.u.)

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


10 2

10 1

10 0 0

10

20

30

40

50 60 Time/ns Time (ns)

70

80

90

Figure 6. Fluorescence emission decays of 1 recorded at 400 nm in different water/acetonitrile mixtures. Black, red and blue lines correspond to fw = 0, 30 and 50%, respectively. Excitation wavelength was 291 mn. Table 3. Steady state fluorescence anisotropy (r) and mean fluorescence lifetime (τm) of 1 in different water/acetonitrile mixtures a τm (ns) c fw (%) rb λem = 400 nm λem = 500 nm 0 0.022 8.09 7.93 30 0.044 9.69 10.11 50 0.147 6.68 6.99 80 0.217 3.71 5.65 a The concentration of the samples was 5 µM and the temperature was kept at 20 ºC. b Steady state anisotropy was measured at λem = 400 nm. c The samples were excited at 291 nm and the fluoresce decays were recorded at 400 and 500 nm.

The structure of the aggregates of 1 in water/acetonitrile mixtures was finally investigated by Small Angle X-ray Scattering (SAXS) in order to correlate with the rest of spectroscopic measurements (see Figure 7 and Table 4). It has not been observed the presence of ordered clusters at low water fractions. However, spherical aggregates of an average radius (rave) of 1.91 ± 0.04 nm were found at critical fw = 30% which increased slightly in number and size at fw = 40% (rave = 3.11 ± 0.12 nm). Those spherical clusters could correspond to small columnar aggregates formed by π-stacked 1 molecules. The SAXS profile of sample corresponding to fw = 50% suggests also the existence of a network that is subsequently disrupted at higher fw values. The spherical clusters decrease in number and in size as water was further added into the system. In addition, 21 ACS Paragon Plus Environment


the formation of smaller clusters at high fw values seems consistent with the closer approach between the molecules and the change in their relative orientations previously suggested as a response of the aggregate to the increase of the polarity of the medium.

fw (%) 30 40 50 60 70

0

10

-1

I (cm-1)

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

Page 22 of 43

10

-2

10

-3

10

0.1

1

q (nm-1\+)

Figure 7. SAXS profiles of 1 in different water/acetonitrile mixtures

Table 4. Average radius determined for the aggregates at different water/acetonitrile mixtures by means of SAXS fw (%) rave (nm) 30 1.91±0.04 40 3.11±0.12 50 2.47±0.02 60 1.66±0.02 70 0.89±0.01

4.3. Theoretical Studies on the Aggregates The binding energy landscapes calculated for stacking dimers, with different azimuthal angles, as a function of the relative (x,y)-displacement are shown in Figure 8. 22 ACS Paragon Plus Environment

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For α = 0º, the global minimum is reached for the fully overlapped dimer. Broad minima appear for the dimer with α = 30º being the global minimum placed at x ≈ 4−5 Å and y < 1 Å. In the case of the dimer with α = 60º, a narrow minimum is located at x ≈ 5−6 Å and y ≈ 5−6 Å. Subsequently, the tetramer clusters t1, t2 and t3 were optimized both in gas phase and in water solution (see Figures 8). In gas phase, the cluster t1, formed by overlapped molecules, is the lowest in energy (see Table S1). Substantially higher energies (≥ 20 kcal mol-1) were calculated for t2 and t3 while the lowest relative energy was calculated for t2 in water solution. These results corroborate that the solvation plays a role in the type of aggregation. Tables 5 and S2 collect the lowest electronic transition energies calculated for the tetramers (t1, t2 and t3) and for the dimers (d1, d2 and d3) extracted from the central part of the corresponding tetramers (see Figure S8). The differences in energy calculated for the most relevant electronic transitions of d1 and t1 with respect to the energy of the lowest electronic transition of the monomer are higher than zero. Therefore, blue-shifts should be expected for the electronic absorption spectra of those clusters formed by overlapped molecules. This is in agreement with the behavior generally observed for H-aggregates. Concretely, the lowest energy transitions S0→Sn (being n = 1 ‒ 3) calculated for d1 and t1 are forbidden but some transitions such as S0→S5 and S0→S6 for d1, and S0→S14 and S0→S15 for t1, were predicted with a significant oscillator strength (f = 3.1 ‒ 5.6) showing ∆λ < 0. Hence, the formation of this type of aggregates can be discarded due to hypsochromic shifts have not been observed in our experimental study and t1 is not the most energetically favored cluster in water solution. According to our calculations, the lowest energy transition is predicted at 307 nm (f = 2.0) for the monomer in aqueous environment and LR regime. The evolution of the electronic excitations, as the system size increases, reveals that in d2 the lowest



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

energy transitions with a significant value of f were calculated at 316 nm (S0→S2; f = 0.6), 315 nm (S0→S3; f = 0.6) and 311 nm (S0→S4; f = 1.7), all of them with a positive value of ∆λ (within LR approach). For the corresponding tetramer, t2, the analysis of the transitions is more complex. However, two red-shifted transitions of significant intensity are predicted at 309 nm (S0→S9; f = 1.7) and 306 nm (S0→S11; f = 1.3). As it can be seen in Tables 5 and S2, similar bathochromic shifts are observed for d3 and t3 from both LR and SS regimes. The electronic transitions calculated for the most energetically favorable clusters in water solution, d2 and t2, are consistent with the red-shift observed in our experiments where the absorption maximum of 326 nm (in acetonitrile) is shifted to 339 and 353 nm (in water/acetonitrile mixtures with fw ≥ 80). In addition, we calculated an angle φ = 150º between the transition dipole moment vectors of the monomers of d2 in water solution. Therefore, clusters d2 and t2 seem to correspond to an aggregate with oblique transition dipoles which exhibits red-shifted absorption bands with respect to the isolated monomer. A substantial planarization of the two central molecules of cluster t2 is also predicted. Thus, the three torsion angles between the central benzene ring and the peripheral phenyl substituents, ω, take absolute values of 20‒19‒18º for the isolated molecule. Those angles generally decrease for the two central molecules of t2 (ω = 3‒ 19‒10º and 4‒19‒21º; and, also, for t3, ω = 8‒2‒2º and 7‒22‒33º). In this sense, the Restriction of the Intramolecular Rotation (RIR) principle postulates that the restriction of rotation of conjugated rotors can favor the excited state decay via radiative channels causing AIEE. On the contrary, ω angles increase in the case of t1 cluster (cofacial arrangement) taking values of 26‒27‒26º for the two central molecules.


Page 24 of 43

Page 25 of 43

Y-Displacement / Å

10

8

6

4

2

0 0

2

4

6

8

t1

10

X-Displacement / Å α = 0º

Y-Displacement / Å

10

8

6

4

2

t2

0 0

2

4

6

8

10

X-Displacement / Å

α = 30º 10 -0.560 -0.500

Y-Displacement / Å

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


-0.300

8

0 0.500 1.00

6

2.00 3.00 3.50

4

2

t3

0 0

2

4

6

8

10

X-Displacement / Å

α = 60º Figure 8. (Left) Binding energy (eV) landscapes calculated at the M06-2X/6-31G** level of theory for dimers with different azimuthal angles (α) and stacking distance of z = 3.5 Å. (Right) Tetramer clusters of 1 optimized in water solution at the M06-2X/6-31G*//M06-2X/STO-3G level of theory.

Table 5. Lowest electronic transition energies (E [λ] in eV and [nm]), oscillator strength (f) and main components of the transition (%) calculated for tetramers of the compound 1 in gas phase and water solution at the LR-PCM/TD-M06-2X/6-31G** level. ∆E (eV) corresponds to the difference in energy of the calculated transition with respect to the energy of the lowest electronic transition calculated for 1 in the same conditions. gas

Spc. E [λ]

∆E

f

Trans.

%

E [λ]

∆E


f

water solution LR Trans.

%


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

S0→S1

H→L (19) H-1→L (10)

3.56 [348.7]

-0.49

0.00

S0→S1

0.00

S0→S2

H→L+1 (19) H→L (15) H→L+2 (11)

3.56 [347.9]

-0.48

0.00

S0→S2

-0.61

0.00

S0→S3

H-1→L+1 (26) H-1→L (15)

3.62 [342.9]

-0.43

0.00

S0→S3

0.05

1.11

S0→S16

5.29

S0→S14

3.69

S0→S17

4.14 [299.3] 4.15 [298.8]

0.09

0.07

H-1→L+1 (8) H-1→L+8 (8) H-1→L+7 (20) H-1→L+8 (15)

0.10

5.56

S0→S15

0.08

3.11

S0→S18

H-1→L+7 (15)

-0.44

0.01

S0→S1

H→L+1 (43) H-1→L (11)

-0.30

0.08

S0→S1

-0.38

0.03

S0→S2

-0.27

0.06

S0→S2

3.49 [355.7]

-0.66

0.00

3.50 [354.6]

-0.65

3.53 [351.1]

t1

t2

t3

4.19 [295.6] 4.22 [294.1] 4.22 [293.8] 3.70 [335.1] 3.76 [329.8] 3.88 [319.6] 4.08 [303.7] 4.15 [299.1] 4.16 [297.7] 4.21 [294.3]

3.85 [322.2] 3.92 [316.3] 4.01 [308.9] 4.11 [302.1] 4.16 [297.9] 4.17 [297.1] 4.30 [288.3] 4.32 [286.9]

Page 26 of 43

-0.26

0.04

S0→S3

-0.06

1.94

S0→S11

0.00

3.71

S0→S13

0.02

1.29

S0→S14

0.07

0.96

S0→S15

-0.29

0.01

S0→S1

-0.22

0.01

S0→S2

-0.13

0.04

S0→S3

-0.04

1.81

S0→S6

0.02

1.52

S0→S9

0.03

2.20

S0→S10

0.16

1.06

S0→S12

0.18

2.74

S0→S13

H→L (59) H-1→L+1 (18) H-1→L+5 (8) H-4→L+2 (10) H-2→L+5 (7) H-2→L+4 (9) H-7→L+1 (7) H-1→L (16) H-4→L (8) H→L+6 (25) H-1→L+4 (8)

H-3→L (13) H-1→L+2 (9) H-5→L (23) H-1→L+2 (8) H→L+7 (13) H-5→L (12) H-6→L (8) H-7→L+6 (8) H-7→L (13) H-6→L+1 (6) H-3→L+4 (5) H-3→L+4 (11) H-4→L+3 (9) H-2→L+7 (6) H-3→L (5)

3.75 [330.3] 3.78 [328.1] 3.85 [321.9] 4.02 [308.6] 4.05 [306.3] 4.08 [304.1] 4.12 [301.2] 4.14 [299.5] 3.81 [325.7] 3.88 [319.4] 3.93 [315.8] 4.01 [309.5] 4.03 [307.9] 4.07 [304.9] 4.08 [304.2] 4.14 [299.6] 4.17 [297.7]

H-1→L (16) H-1→L+1 (11) H→L (12) H→L+1 (14) H-1→L+1 (11) H→L (19) H→L+1 (18) H-1→L (35) H-1→L+1 (10) H→L+1 (23) H-1→L+7 (22) H→L+6 (12) H→L+7 (18) H-1→L+6 (15)

H-1→L (28) H-1→L+1 (18) H→L (46) H-2→L (22) H→L+1 (12) H-3→L+3 (8) H-1→L+1 (8)

-0.20

0.00

S0→S3

-0.03

1.69

S0→S9

0.00

1.34

S0→S11

0.03

1.98

S0→S12

0.07

1.07

S0→S13

0.09

2.83

S0→S14

-0.29

0.04

S0→S1

-0.17

0.17

S0→S2

-0.12

0.02

S0→S3

-0.04

1.11

S0→S6

-0.02

1.68

S0→S8

H→L+5 (11)

0.02

2.38

S0→S10

H-7→L+4 (14)

0.03

1.25

S0→S11

H-6→L+1 (12)

0.09

3.18

S0→S12

H-7→L+4 (11)

0.12

0.98

S0→S13

H-6→L (10) H→L+1 (19) H→L+4 (10)

H-2→L+5 (9) H-5→L+1 (12) H-2→L+3 (11) H-4→L+3 (8) H→L (10) H-5→L+4 (5) H→L+7 (6) H-1→L+4 (15) H→L+7 (17) H-1→L+7 (12) H→L+4 (9) H-5→L+2 (7) H→L (6) H→L (16) H-6→L+1 (10)

4.4. Spectroscopic, Microscopic and Calorimetric Studies in Solid Phase Two thin films of 1 were prepared by drop casting on quartz slides and slow evaporation of concentrated solutions in acetonitrile and tetrahydrofuran. In these samples, the lowest energy absorption band appears red-shifted (343 – 345 nm) with 26 ACS Paragon Plus Environment

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respect to that in the spectra in solution (325 – 329 nm) (see Figure 9 and Table 6). A strong broadening of those bands can be also observed which might indicate the coexistence of various molecular arrangements. Nevertheless, not all solid aggregates exhibit the same fluorescence response, and the maximum excitation wavelengths (λexcmax) is even more red-shifted (about 40 – 60 nm) than the corresponding λabsmax. Coya et al. also observed broad excitation bands in thin films fabricated by spin coating using different solutions.26 This fact was attributed to the presence of different intermolecular interactions related to the non-uniformity of the film and the tendency to form molecular islands. The shortest shift in the absorption band was observed for the thin film fabricated from a solution of 1 in chloroform with respect to the corresponding solution.26 In powder, an intense and strongly red-shifted absorption band emerges at 423 nm while a weak peak can be also observed at 357 nm. Hence, the optical properties of the sample seem to be very conditioned to the aggregation and the solid forming process. Here it must be remembered that powder was obtained by adding water to an organic solvent and after drying, and showing some crystallinity in DSC experiments. For the powder, λexcmax (419 nm) is close to the corresponding λabsmax (423 nm) and it can be associated to a higher fluorescence of these aggregates than those obtained by drop casting and slow evaporation of acetonitrile. Similar emission spectra were recorded for both thin films with maxima within 440 – 441 nm, which is red-shifted with respect to values determined in solution, whereas the highest shift was observed for the sample in powder which exhibits blue emission with a maximum of 448 nm and Commission International de l’Eclairage (CIE) coordinates of (0.15, 0.10) (see Figure 10). High quantum yields of 0.50 – 0.51 were determined for the solid sample in powder and in thin film from a tetrahydrofuran solution. Those values are within the range of the recently reported Փ F for butterfly-



shaped molecules (0.48 – 0.75) which produce X-aggregation in solid state.56,57 Different studies have shown that X-aggregation is the most effective in preventing quenching in the solid state.48,56-59 On the contrary, Փ F decreases to 0.29 for the thin film obtained from the acetonitrile solution and, hence, the slower evaporation of this solvent has an influence on the molecular ordering of the sample and its fluorescence behavior. Further electron microscopy experiments were carried out to shed light on this issue.

0.9

0.9

0.6

0.6

0.3

0.3

0.0 300

400

500

0.0 600

Normalized emission intensity

Absroption in powder Absorption in thin film (from an acetonitrile solution) Absorption in thin film (from a THF solution) Excitation in powder Excitation in thin film (from an acetonitrile solution) Excitation in thin film (from a THF solution) Emission in powder Emission in thin film (from an acetonitrile solution) Emission in thin film (from a THF solution)

Normalized absorption intensity

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

Page 28 of 43

Wavelength (nm)

Figure 9. Absorption, excitation and emission spectra of 1 in powder and in thin film. For the emission spectra, λexc corresponds to the absorption maximum wavelength (λabsmax). Excitation spectra were collected at their emission maximum wavelengths (λemmax).

Table 6. Absorption, excitation and emission maximum wavelengths (λabsmax, λexcmax and λemmax) and quantum yield (Փ) found for the compound 1 in solid state Sample λabsmax (eV [nm]) λexcmax (eV [nm]) λemmax (eV [nm]) Փ Fd a thin film 1 3.59 [345] 3.40 [365] 2.82 [439] 0.51 thin film 2b 3.61 [343] 3.26 [380] 2.81 [442] 0.29 thin film 3c 3.81 [325] 3.81 [325] 2.83 [438] – powder 2.93 [423] 2.96 [419] 2.77 [448] 0.50


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a

Sample was prepared by drop casting from a concentrated tetrahydrofuran solution. Sample was prepared by drop casting from a concentrated acetonitrile solution. c From reference 26. Sample was deposited by spin coating from a chloroform solution. d Fluorescence quantum yield was determined using an integration sphere at the excitation wavelength of 370 nm. b

Figure 10. CIE1931 chromaticity diagram of 1 in powder (A) and in acetonitrile solution (B). Photographs in thin film on quartz slide from acetonitrile and tetrahydrofuran solutions (C and D, respectively).

SEM observation revealed the presence of different microstructures such as fibers and plates, which indicates that there is a certain molecular order in the solid state (see Figure 11). No significant differences between the sample in powder and for that prepared from a dilute acetonitrile solution were found. Nevertheless, microscopic structures observed in the powder sample are more agglomerated than those corresponding to the specimen obtained by slow solvent evaporation. In TEM experiments, fibers composed by strips with nanoscale dimensions (typically 20 ‒ 500 nm) and exhibiting a high degree of internal order were observed for a sample obtained 29 ACS Paragon Plus Environment


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by evaporation of a dilute acetonitrile solution. Lattice fringes with a periodicity of 0.88 ‒ 0.92 nm are shown in Figure 11 (such oscillation in d-spacing can be attributed to surface relaxation, which is rather common in layered nanomaterials or stacked molecules as is the present case). As observed in other π-conjugated discotic molecules, that periodic structure may correspond to columnar mesophases formed by stacked molecules.8 We have recently reported similar micros structures for 2 but showing a shorter spacing between lattice fringes (0.34 nm).25 Hydrogen bonds involving the amines of the peripheral chains of 2 could explain the shortening of the space between lattice fringes compared to 1. In addition, the higher spacing observed for 1 is consistent with a cross dipole stacking instead of H-type aggregation suggested for 2.25

a)

b)


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c)

d)

Figure 11. (a,b) SEM images of 1 from a dilute acetonitrile solution; (c) SEM image of 1 in powder; and (d) TEM image of 1 from a dilute acetonitrile solution.

5. CONCLUSIONS The optical absorption and emission properties of 1 in different solutions and solvent

mixtures

has

been

investigated.

The

samples

solved

in

different

water/acetonitrile mixtures follow three distinct patterns. First, at low water fractions, the intensity of the absorption band gradually decreases with the increase of fw and at the same time the quantum yield reached its maximum value (0.71) for fw = 30%. DLS, SAXS and fluorescence anisotropy measurements suggested the presence of spherical aggregates with nanometric size. In addition, the non-displacement of the absorption and emission bands can be attributed to the formation of aggregates with oblique transition dipoles and weak electronic coupling between molecules. A transition-like region, in which a broadening of the absorption band is observed, appears for fw = 50%. 31 ACS Paragon Plus Environment


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The SAXS profile of that sample suggests also the existence of a network that is subsequently disrupted at higher fw values. Finally, a new red-shifted absorption band emerge for more polar solvent mixtures (fw ≥ 60%) being especially intense for fw = 80 and 90%. In this case, the significant red-shifts observed for the absorption band suggests the formation of aggregates with stronger intermolecular electronic coupling than those observed for fw ≤ 40%. These new aggregates also have certain thermal stability since the conversion to molecularly dissolved species was not completed after 40 minutes at 80ºC. The quenching of the fluorescence intensity and the red-shift observed in the emission spectra were attributed to the stronger intermolecular coupling as well as to variations in the angle between the transition dipole moments of the monomers. The highest anisotropy values were measured for the aggregates with stronger intermolecular coupling. In parallel, the fluorescence lifetime also strongly decreased and, hence, the changes in relative orientations of the monomers could allow a more efficient exciton diffusion in the aggregate. Different clusters formed by stacking of dimers and tetramers of 1 with different azimuthal angles were calculated at the DFT level. In water solution, red-shifts were predicted for the electronic transitions of the lowest energy clusters, in agreement with the experimental observations. In this sense, the transition dipole moment vectors of the central molecules of the cluster t2 are expected to be oblique in water solution. The optical and morphological properties of the solid state were also investigated. DSC experiment revealed that 1, in powder form, has a certain degree of crystallinity. Nevertheless, the type of solid sample and the different intermolecular interactions in the solid have a strong influence on the absorption and excitation spectra. High quantum yields of up to 0.51 were measured for the different solid samples which are comparable to those recently reported for butterfly-shaped molecules which produce


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X-aggregation in solid state. Micrometric structures such as fibers and plates were observed by SEM samples both in powder and prepared from a dilute acetonitrile solution of 1. Fibers composed by strips with nanoscale dimensions and lattice fringes with a periodicity of 0.88 ‒ 0.92 nm were observed by TEM.

SUPPORTING INFORMATION Supplementary Figures S1−S8: synthesis mechanism, RMN spectra, TGA and DSC curves, Zaverage radius from DLS experiments, molecular orbital pictures and dimer clusters d1, d2 and d3. Tables S1−S2: relative energies calculated for the tetramers t1, t2 and t3, and information on the lowest electronic transitions calculated for the dimers d1, d2 and d3.

ACKNOWLEDGEMENTS The authors would like to thank the `Consejería de Educación y Ciencia de la Junta de Comunidades de Castilla-La Mancha´ (Project PEII11-0279-8538) for supporting the research described in this article and the Ùniversidad de Castilla-La Mancha´ for additional support of the research group (grants GI20152958, GI20163548, GI20163441 and GI20163569). Likewise, authors are grateful for the project DIPUAB-16GARZONRUIZ financed jointly by the `Diputación de Albacete´ and the Ùniversidad de Castilla-La Mancha´. The authors also thank the `Centro de Servicios de Informática y Redes de Comunicaciones´ (CSIRC), Ùniversidad de Granada (Spain)´ for providing the computing time and `Junta de Andalucía´ (FQM-337). M. Moral thanks the E2TPCYTEMA-SANTANDER program for the financial support.



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