Impact of Molecular Arrangement and Torsional Motion on the

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Impact of Molecular Arrangement and Torsional Motion on the Fluorescence of Salophen and Its Metal Complexes Tuhin Khan, and Anindya Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11903 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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Impact of Molecular Arrangement and Torsional Motion on the Fluorescence of Salophen and its Metal Complexes Tuhin Khan and Anindya Datta* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076. ABSTRACT: Salophen is a weakly emissive molecule with a flexible structure. The decrease in the flexibility of the molecule, which can be achieved by chemical or physical means, causes a significant increase in the emissivity and fluorescence lifetime. This phenomenon has been observed upon incorporation of salophen in the solid polymer matrix of polymethylmethacrylate (PMMA). The enhancement in emission is even more prominent in the pure solid form of salophen. An enhancement of emission is also observed in the case of the zinc complex of salophen, SalZn, which is inherently more emissive than free salophen in solution. However, the enhancement in emission is greater in the PMMA matrix for the complex, than in its solid form. Interestingly, a quenching of fluorescence is observed in the crystals of the aluminum complex of salophen (SalAl+), which is strongly emissive in solution phase. These apparently conflicting trends have been rationalized in the light of the molecular arrangement of salophen and its complexes in a solid matrix and in the pure solid forms. In the case of salophen, torsional motion provides major nonradiative channels of depopulation of its excited state in solution. These channels are blocked in the rigid environment provided of the polymer matrix and of the crystal, giving rise to aggregation induced enhancement of emission (AIEE). In the case of SalAl+, the torsional motion is restricted anyway due to complexation. The X-ray crystal structure indicates the possibility of π-π interaction between the planar ligands of two neighboring complex molecules, which could lead to aggregation-caused quenching (ACQ). This provides a justification for the lower emissivity of SalZn, as compared to SalAl+. SalZn is likely to exist as a dimer, in which intramolecular π-π interaction is possible. Thus, the emissivity of salophen and its complexes is found to be governed by interplay of torsional motion and intermolecular interaction. Experiments have been performed at liquid nitrogen temperature, whereby conformational motion is arrested, but additional intermolecular interactions are not brought in. Maximal fluorescence of each of the three species studied is observed in this condition.

INTRODUCTION The last decade has seen the rise of significant interest in emissive solids, due to their potential optoelectronic applications.1–4 The appeal of organic molecules in this context stems from the relative ease of their synthesis, readily available starting material and facile fabrication.5 Aggregation caused quenching (ACQ) of conventional organic fluorophores poses a major problem in this field.6 However, in recent times, aggregation-induced enhancement of emission (AIEE)7,8 and crystallization-induced enhancement of emission (CIEE)9,10 have been exploited extensively to fabricate emissive aggregates and organic solids. These strategies generally target weakly emissive molecules with free rotatable bonds and twistable angles.5,7 Torsional motion about these bonds provide efficient nonradiative pathways for deactivation of the excited states of these molecules. Restriction of such motion brought about by aggregation or crystallization, cuts off these non-radiative pathways to a great extent, thereby causing enhancement in emission.5 In addition to simple organic compounds, complexes have also been reported to be emissive in the solid state.11–15 Molecular packing plays a crucial role in this context. Herringbone structure with intermolecular interaction usually facilitates AIEE while π-π stacking often leads to ACQ through mechanisms like exciplex formation. Excited state intramolecular proton transfer (ESIPT) coupled AIEE has emerged as an interesting recent development in this context.16,17 ESIPT leads to a significantly Stokes-

shifted emission, which is less prone to self-quenching. ESIPT molecules have application and potential in biological systems and for laser action.2,18 Schiff bases like N-salicylideneaniline (salan), for example, undergo ESIPT and subsequently undergoes cis-trans isomerization to exhibit photochromic property.19 The emission of this class of molecules, attributed to their cis-keto form,19,20 is generally feeble due to the presence of efficient non-radiative deactivation pathways like cistrans isomerization,19 which leads to the formation of trans photoproduct.19,21 This process is expected to be hindered strongly by viscous and rigid environments. Torsional motions around σ bonds, which provide additional channels of nonradiative deactivation, are also hindered in these environments. Aggregation and rigidification are likely to suppress these nonradiative pathways, making Schiff bases potential candidates for AIEE. An example is available in N,N′bis(salicylidene)-p-phenylenediamine (salpphen), which undergoes 60-fold enhancement of emission upon aggregation.22 Another Schiff base, which finds application in fluoride sensing, has also been reported to undergo AIEE.23 In recent times, we have initiated spectroscopic studies on a Schiff base, N,N′-bis- (salicylidene)-o-phenylenediamine (salophen, SalH2).24 It exists predominantly in its enol form in the ground state. Upon photoexcitation, an ultrafast ESIPT leads to the formation of the excited state in the cis-monoketo form,25 which is weakly emissive with a maximum around 550 nm and fluorescence quantum yield of about 10–4. The fluorescence quantum yield increases by an order of magnitude upon

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complexation with Zn2+ and Al3+, due to rigidification and a consequent hindrance to torsional motion in the Schiff base moiety in these complexes. We have also made a preliminary observation on the increase of lifetime of salophen in its crystalline form.24 These observations have served as the motivation to explore the prospect of salophen in AIEE and CIEE. In the present work, we have attempted to achieve the enhancement of emission by incorporation of salophen in solid matrices and in pure solid form, in order to understand if rigidification by these means has a similar effect as complexation. To this end, time-resolved fluorescence studies have been performed on salophen and its metal ion complexes in pure solid form. In the case of salophen and its Al3+ complex (SalAl+), studies have been performed on crystalline form as well. Torsional motion can also be minimized in a rigid polymer/glassy matrices. In the present work, polymethylmethacrylate (PMMA) has been used as a solid host for salophen. This matrix provides a rigid environment for the molecule while maintaining its concentration at significantly lower levels than in pure solid form. Finally, we have studied the photophysics of salophen at cryogenic temperature. The motivation for this part of the study is the well–known temperature dependence of activated non-radiative processes like torsional motion. Upon lowering of temperature, such motion is expected to be hindered, giving rise to augmented emission like in solid state/ solid matrices. In addition, it will minimize additional temperature-dependent processes. Like in PMMA, the concentration of the fluorophore is pretty low (~10 µM) in the glassy matrix provided by the solvents at liquid nitrogen temperature and so, concentration-dependent quenching is minimized. MATERIALS AND METHODS Synthesis. Salophen (Figure 1) is synthesized by a condensation of salicylaldehyde with o-phenylenediamine (1:2 mole ratio). Its metal complexes have been synthesized by the reaction between salophen and metal salt. Synthesis and characterization have been performed in the same method as discussed earlier.24 The axial positions of the complexes are occupied by solvents. NO3– is the counterion for SalAl+.

Figure 1. Chemical structure of salophen (SalH2) and its complexes with Zn2+ and Al3+ (SalZn and SalAl+, respectively). Preparation of sample for experiments in solid state and PMMA matrix. For the study of the compounds in their solid form, either the samples are packed between two quartz slides or a solution of the samples is drop-casted on a quartz slide and air dried to form a film. For the measurement of solidstate emission of salophen in its crystalline form, the sample is crushed and mixed with NaCl to dilute it and thereby minimize the effect of self-quenching. Salophen is incorporated into PMMA film in the following way: A solution of salophen

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in chloroform is added to a saturated solution of PMMA in chloroform. The mixture is homogenized by stirring. In the case of complexes, the solvents are acetonitrile for PMMA and methanol for the complexes. In this case, the choice of acetonitrile is driven by the fact that the complexes show a tendency of demetallation in chloroform and PMMA is insoluble in methanol. The homogenized solution is poured into a Petri dish and the solvent is allowed to evaporate to form the PMMA film. The polymer film impregnated with SalH2 is transparent, but those containing SalZn and SalAl+ appear to be spatially heterogeneous (in terms of opacity and visual patchiness). However, fluorescence decays recorded from different spots of SalAl+ film are found to be superimposable with each other. Similar heterogeneity is observed for salophen films prepared from acetonitrile as well. It appears that the quality of films is dependent on the procedure of prepartion of the film and not on the samples. Room temperature fluorescence. Room temperature fluorescence spectra have been recorded on Varian Cary Eclipse spectrofluorimeter with an excitation wavelength (λex) of 360 nm. Bandpass of 5 nm is used for both excitation and emission monochromators. Fluorescence lifetime is measured using time correlated single photon counting (TCSPC) spectrometer from IBH Horiba Jobin Yvon (FluoroCube) with excitation at 375 nm by a diode laser (Horiba NanoLED). For steady state as well as time-resolved experiments, the samples (solid/film) are kept at the center of the cell holder at a right angle to the direction of propagation of the incident light. This orientation minimizes reflection and scattering. For a concentrated milliMolar solution, the emission is collected from the front face. The decay is collected at magic angle (54.7o) polarization with respect to vertically polarized excitation light. The full width at half maximum (FWHM) of the instrument response function (IRF) is approximately 250 ps. The decays are fitted to one or more exponential function by iterative reconvolution using IBH DAS 6.2 software. The goodness of fit is judged by reduced χ2. Low-temperature fluorescence. Studies at liquid nitrogen temperature (actual temperature = 80 K) have been performed using an Oxford OptistatDN-V2 coupled with a temperature controller in IBH Horiba Jobin Yvon (FluoroCube) TCSPC setup. The emission spectra are corrected using a correction file prepared by recording the emission of coumarin 30 in methanol, taking its emission spectrum recorded on the Varian Cary Eclipse fluorimeter as the standard. Spectra and decays are recorded after allowing the system to be stable at liquid nitrogen temperature. Two different glass forming solvents have been used: the polar solvent, ethanol and the non-polar solvent, 3-methylpentane (3MP).26 Ethanol is distilled twice before use, in order to eliminate fluorescent impurities. 3MP is used as received. None of the solvents exhibit any impurity emission at low temperature during control experiments. Quantum chemical computations. Calculations are performed using Gaussian 09 package.27 GaussView05 is used for drawing and visualization purpose. Density Functional Theory (DFT) calculations have been carried out at the B3LYP level of theory with 6-311++G(d,p) as the basis set. Frequency calculation during geometry optimization ensures that the structure is at an energy minimum. Excited state energy of the singlet and triplet states are calculated using time-dependent DFT. Solvent effect (acetonitrile) is included by using the polarization continuum model. Constrained geometry optimi-

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zation is performed at a fixed dihedral angle with intervals of 10° at the desired position of salophen. RESULTS AND DISCUSSION Fluorescence of salophen in solid state/ polymer matrix at room temperature. Salophen in its solid form has a broad enol absorption around 330 nm and 390 nm and keto absorption at 480 nm (Figure S1). TD-DFT calculated transitions of the enol form of the crystal geometry matches well with the experimentally obtained spectrum (Figure S1). The presence of strong keto absorption, indicating keto-enol equilibrium, is the reason behind the deep yellow colour of salophen in its solid form.28 This observation is reminiscent of the strong ground state keto absorption in salan derivative salts, reported earlier.29 Salophen exhibits strong, structured emission corresponding to the cis-keto form having a maximum at 530 nm and at 560 nm in its solid crystalline form and as a drop cast film (Figure 2A). The spectrum is similar to that reported by Kotova et al.30 The emission intensity is approximately two orders of magnitude greater than that for salophen in solution. No significant anisotropy is observed for salophen crystal. This may be due to concentration depolarization.31 We refrain from trying to obtain a precise value for the fluorescence quantum yield, as the uncertainty involved is likely to be rather high in our experimental condition. Instead, we focus on the fluorescence lifetime. Fluorescence decay profiles of the drop cast film depend upon the spot from which emission is collected (Figure S2). This is likely to be an indication of the spatial heterogeneity of the film. The modal lifetime is 1.3 ns, which are longer than that obtained in the PMMA matrix (vide infra, Figure 2B). The decay in the crystalline form is single exponential (reported in recent publication),24 and is independent of the emission wavelength. The associated lifetime is 1.7 ns, which is longer than the longest lifetime obtained for the drop cast films (Figure 2B, Table 1). The long lifetime of salophen in solid form is similar to the lifetime of SalAl+ in solution (Table 1).24 A similar increase in lifetime is observed in salan and salpphen, which are Schiff bases of the same class as salophen. They have picosecond lifetimes in solution,32,33 which, upon crystallization, increase to 1.0 ns and 2.2 ns for salan and salpphen respectively (Figure S3). The significant increase in emissivity and lifetime may be attributed to the close intermolecular proximity in solid form and resultant rigidity imposed on the molecule. Enhanced emission in solid form makes salophen a potential candidate for applications in photoluminescent devices, like in encryption and optical waveguides.34,35 However, it is clear that many short range interactions might take place in the pure solid form. This has motivated us to study the emission in PMMA matrix with much lower fluorophore concentration. The emission spectrum of salophen in PMMA matrix is less structured than in solid form (Figure 2A) but more structured than that in solution.24 Rigidification of salophen in PMMA matrix is manifested in steady-state fluorescence anisotropy value of 0.15-0.31 (Figure S4). Scattered light is a persistent problem in this experiment, though. The emission intensity appears to be 20–30 times more than that of salophen in solution. Perceptible fluorescence decay is observed in the TCSPC experiment on salophen in PMMA (Figure 2B), unlike the case of salophen in solutions reported recently.24 The decay in PMMA multiexponential, similar to those of a salan derivative in a polymer matrix.36 They are faster than the decays from

dropcast film and pure crystalline form of salophen. Thus, encapsulation in polymer matrix appears to bring in a sufficient degree of rigidification of salophen so as to make it a much stronger fluorophore than it is in solution, but not quite as strong as in its crystalline form. On a different note, the decay is significantly slower than that of SalZn but faster than that of SalAl+ in solution (Table 1).24

Figure 2. (A) Normalized fluorescence emission spectra and (B) decays of salophen in different media, at λem = 550 nm. The decay of solid crystalline form of salophen has been reported in our recent publication.24 Salophen at 77 K. Conformation changes of molecules, which are suspected to be key players in the nonradiative processes of salophen, are associated with energy barriers and so, can usually be slowed down significantly at low temperature,37,38 without increasing the local concentration of the fluorophore. The emission intensity of salophen in 3MP as well as in ethanol at 77 K is greater than that at room temperature by two orders of magnitude in the same solvent. The emission spectra are structured, with maxima at 530 nm and 560 nm (Figure 2A), similar to features observed earlier in SA39 and for salophen itself at 77 K in its solid form.30 Notably, no characteristic emission from the enol form is observed even at this temperature in 3MP, indicating that enol-keto excited state proton transfer is ultrafast in nature and is associated with a very small barrier, if at all. Tunneling has also been proposed to be a feasible mechanism in such cases.40 However, in molecules that are structurally very similar to salophen, intramolecular excited state proton transfer has been determined to be barrierless. The absence of deuterium isotope effects has been interpreted as the evidence against tunneling.32 The wavelength– independent fluorescence decays (Figure S5) in both the glasses are fitted to a sum of two functions with lifetimes of 1.5 ± 0.2 and 3.2 ns, with almost equal contributions (Table 1). Thus

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Table 1. Temporal parameters of salophen and its complexes in different media.

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Species

λem (nm)

Medium

τ1 (ns)

A1*

τ2 (ns)

A2

χ2

SalH2

550

3MP at RT PMMA Drop cast film Crystal 3MP at 77 K EtOH at 77 K