OnâOff Mechano-responsive Switching of ESIPT Luminescence in

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

On-Off Mechano-Responsive Switching of ESIPT Luminescence in Polymorphic N-salicylidene-4-amino-2-methylbenzotriazole Fabio Borbone, Angela Tuzi, Barbara Panunzi, Stefano Piotto, Simona Concilio, Rafi Shikler, Shiran Nabha, and Roberto Centore Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01047 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

Crystal Growth & Design

On-Off Mechano-Responsive Switching of ESIPT Luminescence in Polymorphic N-salicylidene-4-amino2-methylbenzotriazole Fabio Borbone,*,† Angela Tuzi,† Barbara Panunzi,‡ Stefano Piotto,§ Simona Concilio,┴ Rafi Shikler,ǁ Shiran Nabha,ǁ and Roberto Centore†

†

Department of Chemical Sciences, University of Napoli Federico II, via Cintia, 80126 Napoli, Italy

‡

Department of Agriculture, University of Napoli Federico II, via Università 100, 80055 Portici NA, Italy

§

Department of Pharmacy, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano SA, Italy ┴

Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano SA, Italy

ǁ

Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, POB 653 Beer-Sheva 84105 Israel

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

Page 2 of 24

ABSTRACT. We report the synthesis of a luminescent N-salicylidene aniline derivative, Nsalicylidene-4-amino-2-methylbenzotriazole (1), and the study of its polymorphism and photophysical properties. Three phases showing yellow (1-Y), orange (1-O) and red (1-R) fluorescence have been isolated and characterized by thermal and single crystal X-ray analysis. The photoluminescence results from excited-state intramolecular proton transfer (ESIPT) process and the quantum yield is strongly dependent on polymorphism (Φ1-Y=0.87, Φ1-O=0.11, Φ1-R=0.028). The poorly emitting 1-R can be easily prepared, converted to the bright 1-Y by grinding, and reverted to 1-R through melting and annealing, giving rise to a luminescence on-off mechano-responsive cycle. The different photophysical properties are explained with the variable π-overlap and molecular conformation changes in the three polymorphs, characterized by a very similar crystal packing. By DFT calculations the absorption properties were explained as dependent on the torsion angle between the two planar portions of the molecule, which affects the equilibrium between enol and keto forms in the ground state.


Page 3 of 24

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


INTRODUCTION Organic luminophores showing strong emission in the solid state are object of interest since decades in basic and applied research. The development of such emitters is of great importance for a number of potential applications, from lighting devices as OLED and LEC,1–4 to sensors,5–8 imaging,9,10 lasers.11,12 Molecular design and structural modifications can be helpful for tuning and predicting the photophysical properties of luminophores in solution, nevertheless their solid state behavior can be greatly influenced by the crystal packing and result more unpredictable. The aggregation of aromatic molecules with rigid conjugated structures can produce a concentration quenching effect on fluorescence, because of typically strong and detrimental π-π interactions arising in the crystal. Conversely, compounds with single-bonded aromatic rings may benefit from the restriction of intramolecular rotations and motions in the solid state and exhibit a fluorescence enhancement effect with respect to solution.13,14 Furthermore, the molecular packing can also affect the position and shape of the emission band, leading to alteration of the luminescence color. Consequently, tuning of solid state luminescence can be achieved when different molecular arrangements and conformations of the same compound are possible, as in polymorphic luminophores.15–18 Polymorphism has been exploited to produce emissive stimuli-responsive materials, in which the phase transition between polymorphs is induced by thermal, mechanical or vapor stimulus.19–24 Particularly, compounds showing mechanoresponsive luminescence (MRL) with on-off switching capabilities are more unconventional and intriguing for practical applications.25,26 A special class of polymorph-dependent luminophores reported in the literature is characterized by an intramolecular hydrogen bond, involved in the emission process through the excited state intramolecular proton transfer (ESIPT).27–33 In this process, during illumination of the fluorophore, the proton of the excited enol form is transferred to the acceptor on a picosecond scale, much more rapidly than the radiative decay process. Therefore, population of an excited zwitterionic or rearranged keto form with lower energy occurs, which relaxes to the ground



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 4 of 24

state by emitting radiation at longer wavelengths. The resulting large Stokes shift and limited overlap between the absorption and emission bands can be advantageous in many of the cited applications. To date, few examples of solid state ESIPT luminophores with polymorph-dependent emission have been reported.34 These molecules are mainly based on an imidazole or imidazo-[1,2]pyridine core,27–31 but also compounds containing salicylaldehyde azines (SAA)32 and quinazolinone (HPQ) moieties are known.33 As concerns the class of N-salicylidene anilines (SA), very recently a thermochromic derivative claimed as one of the first cases of polymorphic SA was reported by Carletta et al.,35 while only one case of proved dimorphic SA featuring polymorph-dependent emission was reported later, to our knowledge.36 In this paper, we report a derivative of N-salicylidene aniline containing the 2methylbenzotriazole (BTz) unit, showing three polymorphs, ESIPT fluorescence and polymorphdependent photophysical properties. The compound revealed mechano-responsive luminescence capability because of the reversible transformation between two of the three polymorphs accompanied by a dramatic change of luminescence intensity and color. Significant influence of molecular conformation on absorption properties was observed, while crystalline packing and intermolecular interactions strongly affect the efficiency of solid state photoluminescence. These conclusions are supported by DFT calculations and evaluation of frontier orbitals.


Page 5 of 24

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


RESULTS AND DISCUSSION Compound 1 (Scheme 1) was prepared by reaction of 2-methyl-2H-1,2,3-benzotriazol-4-amine with salicylaldehyde in refluxing toluene or methanol (Scheme S1, see Supporting Information). After solvent evaporation and addition of hexane, yellow/orange crystals were obtained, melting point 122 °C, phase 1-Y. The mother liquor was isolated and a second phase precipitated after cooling. This phase showed melting point of 104 °C and was named 1-R. By melting pure 1-R or 1-Y no recrystallization was observed after rapid or slow cooling. In the presence of small amounts of 1-Y, 1R melts and recrystallizes as 1-Y. This behavior is represented in Figures S1, S2, S3, where DSC diagrams of the pure phases and a mixture of them are reported. After cooling the melt from any of the two phases to room temperature and following heating or annealing at 80°C in the aluminum capsule of DSC, the liquid always crystallized as 1-R. This behavior was reproduced when the sample was placed between a glass or quartz slide and an aluminum foil and subjected to the same thermal cycle. We observed that 1-R could be easily and quantitatively transformed into 1-Y by applying mechanical stress such as grinding or rubbing between slides. When the stress was applied to the melt, the behavior was not reproducible and any of the phases or a mixture of them was obtained. For this reason, pure 1R could only be prepared in absence of dynamic mechanical stress. Experimental PXRD spectra of 1-Y and (unground) 1-R powders were compared with the PXRD profiles calculated from single crystal Xray data (Figures S4, S5). The matching is very good and confirms that the two phases can be selectively prepared as pristine powders. Stick shaped single crystals of 1-Y were grown as the main fraction after recrystallization from dichloromethane, along with a small amount of 1-R as thinner stick shaped crystals. After several recrystallization attempts from diethyl ether, few crystals of a third phase were obtained, showing melting point at 93°C, named 1-O. Unfortunately, any attempt to reproduce this phase was unsuccessful. After melting 1-O, no crystallization was observed, as in the case of 1-R



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 6 of 24

and 1-Y. No phase transformation was observed by grinding crystals of 1-O. All the above observations may suggest that 1-Y is the most thermodynamically stable polymorph of 1 among the observed phases.

Scheme 1. Chemical structure of 1 and definition of molecular conformations.

Molecular structure. The three crystal phases of 1 show a different number of molecules in the crystallographically independent unit: Z′=1 for 1-R, Z′=2 for 1-O and Z′=4 for 1-Y. The asymmetric units of 1-Y and 1-O are shown in Figures S6 and S7 respectively. A common feature of the molecules in all the crystal phases is the presence of a strong intramolecular H bond between the ortho O-H donor and the imino N acceptor (O···N distances between 2.590(3) and 2.640(3) Å). As a consequence of the intramolecular H bond, the only actual conformational degree of freedom observed in the conformers of molecule 1 is the rotation around the bond from the benzotriazole (BTz) moiety to the imino N atom, inducing a dihedral angle θ between the average planes of the BTz and salicylaldimine moieties. In phase 1-Y (Figure 1) the four crystallographically independent molecules have a similar flat conformation, with a very small θ angle, within 7.5°. In 1-O, one molecule is planar (θ=1.8°) while the other shows a dihedral angle of 17°. In the case of 1-R the structure is disordered, with two split positions differing by 180° rotation of the salicylaldimine moiety around the bond from the benzotriazole to the N imine atom. The two split positions are shown in Figures S8 and S9.


Page 7 of 24

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


Particularly, molecules A and B correspond to the s-cis (1-Rc) and s-trans conformations (1-Rt) around the C8-N1 bond, respectively. Only s-trans conformations are present in the crystals of 1-Y and 1-O. The dihedral angle θ for both split positions of 1-R is higher as compared to 1-O and 1-Y, with values of 32.6° and 30.1° respectively for molecule A and B.

Figure 1. Dihedral angle θ between BTz and salicylaldimine groups in 1-Y (a), 1-O (b) and 1-R (c).

Photophysical properties. Figure 2a shows the fluorescence excitation and fluorescence spectra of 1 in diluted dichloromethane solution. The absorption maximum is at 364 nm, while the maximum in the fluorescence spectrum excited at 364 nm is at 545 nm. The high Stokes shift (181 nm) is an indication of ESIPT occurring when the enol form of 1 is excited and rapidly rearranges to the excited keto form, which is responsible for the strongly red-shifted emission (Figure 2b). The ESIPT process involves the proton transfer from the OH donor to the nitrogen acceptor of the salicylaldimine moiety, as known for SA compounds.37 Only a weak emission between 400 and 500 nm was observed, due to the radiative decay of the excited enol form. The emission intensity of solution resulted very low (Φ=0.0013), due to typical fluorescence deactivating processes connected with the structural flexibility of the N-



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 8 of 24

salicylidene anilines and involving the formation of twisted intramolecular charge transfer (TICT) states and rotoisomerization around the C=N bond.38 Solid state fluorescence excitation spectra were recorded on crystalline samples deposited on quartz slides (Figure 3).

Figure 2. Excitation and fluorescence spectra of 1 (a). Scheme of ESIPT process (b).

The fluorescence was monitored at 550 nm and 590 nm and the spectra did not show remarkable differences at these two wavelengths. The spectrum of 1-Y resembles the absorption spectrum of the solution with a peak at 368 nm; hence, in this case, solid state interactions do not seem to significantly alter the molecular photophysics. Since the molecule is fixed in the s-trans (1-Yt) conformation around the C-N single bond, the electronic transition is from the ground state (S0) to the excited state (S1) of


Page 9 of 24

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


the 1-Yt enol form. The spectrum shows a more vibronic structure if compared to solution due to the fixation of the four slightly different conformations of molecule 1 (Figure 1). The fluorescence spectrum of 1-Y resembles that of the solution as well, with the maximum at 547 nm. For this reason, the emission of both solution and 1-Y can be ascribed to the S1 → S0 transition of the 1-Yt keto forms, while the very small peaks at 424 and 456 nm are due to the emission of excited enol conformers. The solid sample shows a very bright yellow luminescence (Φ=0.87 ± 3%) when illuminated under a 365 nm UV lamp (Figure 4). To our knowledge, the exceptionally high quantum yield of 1-Y was never reported for any SA or polymorphic ESIPT fluorophore and can be placed among the highest values when compared to those of single molecule ESIPT emitters in solid state.34 This value and the strong difference with the very low emission intensity in solution can be attributed to the fixation of the molecules in highly planar conformations by the crystal environment and to the lack of fluorescence quenching strong π-π interactions (vide infra).

Figure 3. Normalized excitation and fluorescence spectra of 1-Y (a, d), 1-O (b, e) and 1-R (c, f).

The excitation spectrum of 1-R shows a quite different aspect. Beside the absorption at 370 nm, a strong red-shifted and vibronic band in the 450-500 nm range arises, with well discernible peaks at 465,



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 10 of 24

479 and 488 nm. Again, spectra did not show remarkable differences with changing the monitoring wavelength from 550 to 610 nm. These absorptions can be ascribed to the transition from the ground state to the excited state of the keto form, as also reported for N-salicylidene anilines and similar compounds featuring intramolecular proton transfer.39,40

Figure 4. Photograph of 1-Y, 1-O and 1-R powder under UV lamp and scheme of mechanoresponsive behavior.

Fluorescence spectrum of 1-R is significantly red-shifted as well, with a maximum at 592 nm when excited at 370 nm. A very similar spectrum is recorded when 1-R is excited at 465, 479 or 488 nm. This behavior indicates that the same excited species are involved in the radiative decay, namely the twisted 1-Rt and 1-Rc keto forms. Also, the intensity of emission resulted drastically reduced with respect to 1-Y, as can be visibly appreciated in Figure 4, and a photoluminescence (PL) quantum yield as low as 0.028 ± 0.5% was recorded. By grinding the poorly red-emitting 1-R, switching to the bright yellow emitting 1-Y can be performed, followed by reconvertion to 1-R by melting and annealing at 80°C. Thus, thanks to the combination of mechanical stress induced phase transition, distinct color modulation and particularly strong PL intensity variation, compound 1 can be exploited as an efficient mechano-responsive fluorophore for stimuli-responsive and optoelectronic applications. The photophysical behavior of 1-O resulted intermediate between that of 1-Y and 1-R. The excitation spectrum still shows the absorption maximum at 370 nm, with a further peak at 408 nm. As in 1-R, a ACS Paragon Plus Environment

Page 11 of 24

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


second peak is evident at about 490 nm, although its intensity is quite lower than that at 370 nm. Powder of 1-O shows an orange luminescence under the UV lamp (Figure 4). The emission band excited at 370 nm is between those of 1-Y and 1-R, and the spectrum shows a peak at around 562 nm, attributed to the radiative decay of the excited keto form of the 1-Ot conformers. This phase exhibits a higher fluorescence intensity than 1-R (Φ = 0.11 ± 0.6 %), but much lower than 1-Y. Fluorescence lifetimes were measured on thin film samples of the three phases by Time-Resolved Photoluminescence. The 1-Y and 1-O samples showed a monoexponential decay with τ = 2.21 ns and τ = 1.69 ns respectively, while 1-R showed a biexponential decay with τ1 = 3.18 ns (10%) and τ2 = 0.26 ns (90%) and a calculated mean lifetime of 0.55 ns. These results indicate a singlet emission and confirm the increasing contribution of nonradiative relaxation pathways along the series 1-Y → 1-O → 1-R. Structure-properties relationship. X-Ray structural analysis shows that all the three phases feature a similar crystal packing, with chains of molecules close-packed with face-to-face contacts by translation along the short crystallographic axis (b for 1-Y and 1-R, a for 1-O, Figure 5) and a lateral packing of chains through weak interactions. Consequently, the factors that mainly affect the photophysical properties and are responsible for the observed modulation of fluorescence color and intensity are the type of intermolecular interactions within a chain and the different molecular conformations in the three polymorphs. Figure 6 shows the view of two adjacent molecules in a packing chain, in the direction perpendicular to the plane of salicylaldimine moiety. Among the four independent molecules of 1-Y (Figure 6a), pairs of them show a very similar slip-stack, in a direction of about 90° and 45° with respect to the main conjugated backbone direction. The slipping angle α of the molecular centroid, measured with respect to the salicylaldimine plane, is 58.9°, 49.4°, 57.0° and 54.9° for molecules A, B, C and D, respectively (Figure 6d). The overlapping of aromatic systems is always poor and virtually no π-π interaction can be observed. Therefore, the S1→S0 transition is not



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 12 of 24

greatly affected by the molecular packing, and the emissive properties of 1 are thus similar in 1-Y and in solution. In the case of 1-O (Figure 6b), the two independent molecules A and B show a slip, respectively, in a 45° and parallel direction with respect to the adjacent molecule.

Figure 5. Partial crystal packing of 1-Y (a), 1-O (b) and 1-R (c).

Slipping angles are strongly reduced as compared to 1-Y (30.8° and 27.0° respectively), so that a minor lateral displacement leads to an increased overlap of conjugated backbones. The distance between salicylaldimine planes of adjacent molecules is 3.44 Å and 3.57 Å, while that between BTz


Page 13 of 24

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


planes is 3.50 Å and 3.48 Å respectively for molecules A and B. Consequently, significant intermolecular π-π stacking interactions can be expected for 1-O, and their effect on the excited state is responsible for the observed red shift of the emission. In 1-R (Figure 6c), the slip is again in an almost parallel direction and the α value of average centroids decreases to 24.7°, while the π-π stacking distances are still in a range of strong interactions (3.61 Å between salicylaldimine planes and 3.50 Å between BTz planes). Thus, the increasing overlap between the aromatic systems leads to a further increase of π-π stacking and exciton interaction among nearest neighbor molecules in the close-packed organization, resulting in a more red-shifted emission. The expected decrease of the HOMO-LUMO gap and the increase of vibronic coupling from a non-stacked to an infinite π-stacked structure can also be considered as a cause of the growing contribution by non-radiative decay pathways leading to the drop of PL quantum yield from 1-Y to 1-O and 1-R.

Figure 6. Stacking of two adjacent molecules in 1-Y (a), 1-O (b) and 1-R (c). Slipping angle (α) of molecular centroids (d).



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 14 of 24

These observations are confirmed by the analysis of the Hirshfeld fingerprint plots (Figure 7). In phases 1-O and 1-R there is a predominance of parallel slipped face-to-face contacts between translated molecules. This is clearly evident in the plots of Figure 7b and 7c, showing a green area centered at di+de= 3.6 Å as a common feature of 1-O and 1-R, respectively. In the phase 1-Y there are also face-toedge intermolecular contacts, as evidenced by the presence of “wings“ at the upper left (di, de) =(1.2, 1.9) and lower right (di, de) = (1.9, 1.2) corresponding to the C–H···π interactions. In 1-O and 1-R the packing is also stabilized by weak H bonding interactions involving aromatic (or aliphatic) C-H donors and N of benzotriazole or O acceptors. These interactions are evidenced by the small spikes of the fingerprint plots at de+di≈ 2.6 Å, placed above and/or below the diagonal.

Figure 7. Fingerprint plots for 1-Y (a), 1-O (b) and 1-R (c).

Theoretical calculations. The absorption spectra of the enol and keto forms of 1-Y, 1-O and 1-R were calculated following the protocol recently presented41–43 and reported in Figure 8. Due to the disordered structure of 1-R caused by the rotation of salicylaldimine group, the spectra were calculated


Page 15 of 24

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


by considering the two distinct ordered arrangements with 1-Rt and 1-Rc. All the s-trans enol forms show a similar spectrum with a strong absorption around 370 nm, according to experimental results (Figure 3). Less intense absorptions at higher wavelengths (420-440 nm) are consistent with the shoulders observed in the excitation spectra. The s-trans keto forms of all phases show a similar spectrum as well, with red shifted absorptions respect to the enol around 390 nm and 500 nm. Since no signals were observed beyond 430 nm in the experimental excitation spectrum of 1-Y, it can be concluded that no significant population of the keto form in the ground state exists in this phase. As concerns 1-O, the contribution of keto forms become more relevant, because the calculated absorptions at 390 nm and 500 nm are clearly visible in the experimental spectrum, respectively at 408 nm and 490 nm. In 1-R, the 1-Rc arrangement generates a sensibly different spectrum in both enol and keto form, therefore the coexistence of 1-Rt and 1-Rc in the crystal leads to a spectrum with a more pronounced vibronic structure, as experimentally observed. Furthermore, the long wavelength peaks associated with the keto forms becomes prevalent, suggesting that the ground state of these forms is sufficiently populated at room temperature. The different absorption behavior of the three polymorphs was investigated by considering that basically a single degree of freedom was found in all the conformations of compound 1 among its phases, that is the rotation around the C-N bond connecting the planar BTz unit and the mostly coplanar salicylaldimine moiety, as previously described. This feature permitted a complete investigation of the total energy of the isolated molecule 1 in the enol and keto form (Figures 9), as well as their frontier orbitals, as a function of the angle between the two planar moieties. The energy evolution of the keto form has two minima, one relative at 0/360° and one absolute at 180°. The energy profile of the enol follows a similar trend in the range 0-130° and 230360°, while in the range of s-cis conformations the energy reaches its maximum value. Interestingly, the enol form appears to be favorite for planar s-trans conformations with a θ angle less than about 12°, whereas for larger torsion angles the keto form is always more stable. All the four s-trans conformers



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 16 of 24

of 1-Y are mostly planar and lie in a region of enol stabilization, which is consistent with the observed absorption. As for 1-O, one of the two s-trans conformers is planar, while the other (17°) lies in a region where the energies of the enol and keto are comparable, and thus the contribution of the keto form to the overall spectrum becomes significant. In 1-R both conformers lie in a range of keto stabilization, and, therefore, a higher population of this form in the ground state with respect to 1-Y and 1-O is expected, as experimentally observed.

Figure 8. Theoretical absorption spectra of the three phases in enol (a) and keto (b) forms. For 1-R two distinct ordered arrangements with 1-Rt and 1-Rc are reported.


Page 17 of 24

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


The analysis of the frontier orbitals in the isolated molecule (Figure S10) shows that in planar structures both HOMO and LUMO are distributed on the entire molecule, due to effective delocalization of electrons in the conjugated system. An evident decrease of electronic density around the hydroxyl oxygen from the HOMO to the LUMO of the enol form indicates an increase of the hydroxyl acidity after excitation, which favors the proton transfer in the excited state.

Figure 9. Diagram of energy for the enol and keto forms of the isolated molecule as a function of the torsion angle θ.

Contrarily, a poor and rich electron density around, respectively, the protonated nitrogen and the carbonyl oxygen in the keto form ground state, drives the reverse proton transfer. Molecular conformation does not seem to affect much the distribution of frontier orbitals in the observed conformers of the three phases, since they remain almost unchanged by varying the θ angle (Figure S11) in both enol and keto forms. Only for θ angles next to 90°, HOMO and LUMO separate and the LUMO remains confined on the benzotriazole portion (Figure S11). Even the HOMO-LUMO energy 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

Page 18 of 24

levels are similar (Table S6), with a slight increase of energy gap on going from the planar to the twisted conformations. As far as the emissive excited keto form is concerned, this behavior results opposite to the observed red shift of emission along the series 1-Y→1-O→1-R, thus confirming the strong contribution of intermolecular interactions in the solid state, particularly the increasing π-π interactions, which also lead to deactivation of radiative pathways and drop of quantum yield. The Density of States (DOS) and projection of DOS on atom-centered orbitals (PDOS) were obtained from the single point calculation on the experimental cell unit of the three crystals (Figures S13-S15). The analysis showed similar densities of states for each phase. Although the energy gaps are systematically smaller than the experimental values and smaller than the values predicted in vacuum, the DOS analysis provides some hints on the composition of the highest occupied crystal orbital (HOCO) and lowest unoccupied crystal orbital (LUCO). Notably, above the Fermi level, there is a clear difference in the density of p orbitals among the three phases, ranging from a single band in the LUCO of 1-Y, which is composed by substantially planar conformers, to two bands in 1-O and three bands in 1-R, which is made of two out-of-plane conformers. The DOS composition is coherent with the enol absorption profiles in Figure 3.

CONCLUSIONS By reacting 4-amino-2-methylbenzotriazole with salicylaldehyde, an unusual case of trimorphic Nsalicylidene aniline derivative was synthesized, which revealed an efficient solid state ESIPT fluorescence. The polymorphism of 1 was proved to strongly affect its photophysical properties through different situations of conformational fixation and π-overlap of adjacent molecules induced by molecular packing. A significant bathochromic shift of the emission among the three similarly closepacked structures and a parallel dramatic drop of photoluminescence quantum efficiency follows a


Page 19 of 24

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


progression from the non-stacked packing of the thermodynamically stable 1-Y to the substantial infinite π-stacked arrangement of 1-O and 1-R, as a consequence of the increasingly strong exciton interaction, as also suggested by the evaluation of HOMO-LUMO energy gaps for the different conformers. The progressive change of 1 from mostly planar to sensibly twisted conformations affects the equilibrium between enol and keto forms and determines a different absorption behavior of the three polymorphs. The poorly red emitting disordered phase 1-R can be easily obtained from melt, and its luminescence can be switched to the bright yellow of 1-Y by mechanical grinding. These findings offer an advantageous possibility to control the luminescence output for stimuli-responsive applications.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental section. Synthesis scheme of compound 1. DSC thermograms. Calculated and experimental PXRD spectra. Crystal, collection and refinement data. ORTEP view of molecules. Relevant bond lengths. Strong and weak hydrogen bonds. HOMO-LUMO figures, energies and gaps for the enol and keto forms. Density of states and optoelectronic calculations. Accession Codes CCDC 1564538 (1-R), 1564539 (1-Y), 1564540 (1-O) contain the supplementary crystallographic data

for

this

paper.

These

data

can

be

obtained

free

of

charge

via

www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 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

Page 20 of 24

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ORCID Fabio Borbone: 0000-0001-7433-9267

REFERENCES (1)

Sasabe, H.; Kido, J. Chem. Mater. 2011, 23, 621–630.

(2)

Dini, D. Chem. Mater. 2005, 17, 1933–1945.

(3)

Mindemark, J.; Edman, L. J. Mater. Chem. C 2016, 4, 420–432.

(4)

Su, H.-C.; Cheng, C.-Y. Isr. J. Chem. 2014, 54, 855–866.

(5)

Luo, X.; Li, J.; Li, C.; Heng, L.; Dong, Y. Q.; Liu, Z.; Bo, Z.; Tang, B. Z. Adv. Mater. 2011, 23, 3261–3265.

(6)

Mukherjee, S.; Thilagar, P. J. Mater. Chem. C 2016, 4, 2647–2662.

(7)

Varughese, S. J. Mater. Chem. C 2014, 2, 3499–3516.

(8)

Yanai, N.; Kitayama, K.; Hijikata, Y.; Sato, H.; Matsuda, R.; Kubota, Y.; Takata, M.; Mizuno, M.; Uemura, T.; Kitagawa, S. Nat. Mater. 2011, 10, 787–793.

(9)

Geng, L.; Yang, X.-F.; Zhong, Y.; Li, Z.; Li, H. Dyes Pigm. 2015, 120, 213–219. ACS Paragon Plus Environment

Page 21 of 24

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)

Lv, H.-M.; Chen, Y.; Lei, J.; Au, C.; Yin, S.-F. Anal. Methods 2015, 7, 3883–3887.

(11)

Kuehne, A. J. C.; Gather, M. C. Chem. Rev. 2016, 116, 12823–12864.

(12)

Samuel, I. D. W.; Turnbull, G. a. Chem. Rev. 2007, 107, 1272–1295.

(13)

Wang, Y.; Liu, T.; Bu, L.; Li, J.; Yang, C.; Li, X.; Tao, Y.; Yang, W. J. Phys. Chem. C 2012, 116, 15576–15583.

(14)

Liu, W.; Wang, Y.; Sun, M.; Zhang, D.; Zheng, M.; Yang, W. Chem. Commun. 2013, 49, 6042– 6044.

(15)

Zhang, H. Y.; Zhang, Z. L.; Ye, K. Q.; Zhang, J. Y.; Wang, Y. Adv. Mater. 2006, 18, 2369– 2372.

(16)

Abe, Y.; Karasawa, S.; Koga, N. Chem. - Eur. J. 2012, 18, 15038–15048.

(17)

Mikhlina, Y. A.; Bolotin, B. M.; Kuz’mina, L. G. Crystallogr. Rep. 2013, 58, 687–691.

(18)

Kohmoto, S.; Tsuyuki, R.; Masu, H.; Azumaya, I.; Kishikawa, K. Tetrahedron Lett. 2008, 49, 39–43.

(19)

Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685–687.

(20)

Xue, S.; Qiu, X.; Sun, Q.; Yang, W. J. Mater. Chem. C 2016, 4, 1568–1578.

(21)

Krishna, G. R.; Kiran, M. S. R. N.; Fraser, C. L.; Ramamurty, U.; Reddy, C. M. Adv. Funct. Mater. 2013, 23, 1422–1430.

(22)

Chen, L.; Yin, S.-Y.; Pan, M.; Wu, K.; Wang, H.-P.; Fan, Y.-N.; Su, C.-Y. J. Mater. Chem. C 2016, 4, 6962–6966.



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

(23)

Page 22 of 24

Luo, X.; Zhao, W.; Shi, J.; Li, C.; Liu, Z.; Bo, Z.; Dong, Y. Q.; Tang, B. Z. J. Phys. Chem. C 2012, 116, 21967–21972.

(24)

Dong, Y.; Xu, B.; Zhang, J.; Tan, X.; Wang, L.; Chen, J.; Lv, H.; Wen, S.; Li, B.; Ye, L.; Zou, B.; Tian, W. Angew. Chem., Int. Ed. 2012, 51, 10782–10785.

(25)

Jin, M.; Seki, T.; Ito, H. J. Am. Chem. Soc. 2017, 139, 7452–7455.

(26)

Sagara, Y.; Yamane, S.; Mitani, M.; Weder, C.; Kato, T. Adv. Mater. 2016, 28, 1073–1095.

(27)

Mutai, T.; Tomoda, H.; Ohkawa, T.; Yabe, Y.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 9522–9524.

(28)

Mutai, T.; Shono, H.; Shigemitsu, Y.; Araki, K. CrystEngComm 2014, 16, 3890–3895.

(29)

Shigemitsu, Y.; Mutai, T.; Houjou, H.; Araki, K. J. Phys. Chem. A 2012, 116, 12041–12048.

(30)

Shida, T.; Mutai, T.; Araki, K. CrystEngComm 2013, 15, 10179–10182.

(31)

Konoshima, H.; Nagao, S.; Kiyota, I.; Amimoto, K.; Yamamoto, N.; Sekine, M.; Nakata, M.; Furukawa, K.; Sekiya, H. Phys. Chem. Chem. Phys. 2012, 14, 16448–16457.

(32)

Wei, R.; Song, P.; Tong, A. J. Phys. Chem. C 2013, 117, 3467–3474.

(33)

Anthony, S. P. Chem. - Asian J. 2012, 7, 374–379.

(34)

Padalkar, V. S.; Seki, S. Chem. Soc. Rev. 2016, 45, 169–202.

(35)

Carletta, A.; Dubois, J.; Tilborg, A.; Wouters, J. CrystEngComm 2015, 17, 3509–3518.

(36)

Zhang, Z.; Song, X.; Wang, S.; Li, F.; Zhang, H.; Ye, K.; Wang, Y. J. Phys. Chem. Lett. 2016, 7, 1697–1702.


Page 23 of 24

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


(37)

Hadjoudis, E.; Mavridis, I. M. Chem. Soc. Rev. 2004, 33, 579–588.

(38)

Knyazhansky, M. I.; Metelitsa, A. V.; Bushkov, A. J.; Aldoshin, S. M. J. Photochem. Photobiol., A 1996, 97, 121–126.

(39)

Safin, D. A.; Robeyns, K.; Garcia, Y. RSC Adv. 2012, 2, 11379–11388.

(40)

Staehle, I. O.; Rodríguez-Molina, B.; Khan, S. I.; Garcia-Garibay, M. A. Cryst. Growth Des. 2014, 14, 3667–3673.

(41)

Borbone, F.; Caruso, U.; Concilio, S.; Nabha, S.; Piotto, S.; Shikler, R.; Tuzi, A.; Panunzi, B. Inorg. Chim. Acta 2017, 458, 129–137.

(42)

Borbone, F.; Caruso, U.; Concilio, S.; Nabha, S.; Panunzi, B.; Piotto, S.; Shikler, R.; Tuzi, A. Eur. J. Inorg. Chem. 2016, 818–825.

(43)

Concilio, S.; Bugatti, V.; Neitzert, H. C.; Landi, G.; De Sio, A.; Parisi, J.; Piotto, S.; Iannelli, P. Thin Solid Films 2014, 556, 419–424.



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 24 of 24

For Table of Contents Use Only On-Off Mechano-Responsive Switching of ESIPT Luminescence in Polymorphic N-salicylidene-4amino-2-methylbenzotriazole Fabio Borbone, Angela Tuzi, Barbara Panunzi, Stefano Piotto, Simona Concilio, Rafi Shikler, Shiran Nabha, and Roberto Centore

Synopsis N-salicylidene-4-amino-2-methylbenzotriazole

was

synthesized

and

three

polymorphs

were

crystallized and characterized, exhibiting yellow (Φ=0.87), orange (Φ=0.11) and red (Φ=0.028) excited-state intramolecular proton transfer (ESIPT) luminescence. Crystal-to-crystal transition from dim red to bright yellow polymorph through grinding gives rise to mechano-responsive emission turnon.


OnâOff Mechano-responsive Switching of ESIPT Luminescence in

Recommend Documents

OnâOff Mechano-responsive Switching of ESIPT Luminescence in