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Role of Solvent Polarity and Hydrogen Bonding on Excited-State Fluorescence of 3-[(E)-{4-(N,N-Dimethylamino-Benzylidene}-Amino)-2Naphthoic Acid (DMAMN): Isomerization Versus Rotomerization Ibrahim A. Z. Al-Ansari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11623 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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The Journal of Physical Chemistry

Role of Solvent Polarity and Hydrogen Bonding on Excited-State Fluorescence of 3-[(E)-{4-(N,N-Dimethylamino-benzylidene}-amino)-2-naphthoic acid (DMAMN): Isomerization versus Rotomerization

Ibrahim Ahmed Z. Al-Ansari Department of Chemistry & Earth Sciences, College of Arts & Sciences - Qatar University, P.O. Box: 2713 - Doha-Qatar, E-mail: [email protected], [email protected]

ABSTRACT: The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethyl-aniline, π = imine, pull = 2-naphthoic-acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change, depend on the polarity and hydrogen-bonding environment. In highly-polar solvents, the emission is weak and red-shifted compared to cyclohexane, but it is more red- shifted in moderate aprotic polar solvents. In hydroxylic solvents, a new weak lowenergy emission band appears at ~ 525 nm, and attributed to the inter-molecularly H-bonded openconformer. Based on the generated potential energy landscapes of the ground- and low-lying excited states in the gas-phase and solution, we found that selective photon absorption, brings this molecule to a “bright” state, from which N=C isomerization ZE, takes place. This isomerization in gasphase and low-polarity solvents leads to two minima with a barrier, while in highly-polar protic media, gives one minimum on the S1 surface with low ∆ES1/T1 (0.17 eV) facilitating deactivation via ISC.

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1.

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Introduction Small photoactive Schiff base molecules undergo trans ∏ cis photo-isomerization as a first

step in many biological processes .1 Imine (C=N) Schiff base, is a photo-sensitive functional group, which switch under certain conditions. Photo-isomerization about this bond, was firstly inferred by Kuhn and Weitz, for the explanation of a reversible color change due to irradiation of triphenylformazan .2 In 1957, Fischer and Frei, were the first to spectroscopically investigate the trans ∏ cis photo-isomerization on imines. They studied the absorption spectral change with photoirradiation at -100°C ,3 followed by restoring the original isomer with heating. Many spectroscopic methods followed this original work, such as NMR spectroscopy,4 UV/Vis spectroscopy5 and flash photolysis.6 Accompanying these experimental techniques, theoretical methods, were also used for better understanding of the photochemical behavior of this functional group.7-9 The outcome conclusion reached in many systems, was that photochemical trans ∏ cis isomerization was possible by out-ofplane rotation about the C=N double bond. Lately, these neutral aromatic10-11 and protonated12 Schiff bases, gained interest for their potentials in designing “molecular machines” (motors). The core of this design is the unidirectional photo-isomerization of the rotatable unit.13-15 Mechanistically, the torsion about the C=C or C=N dihedrals, is considered an important non-radiative pathway in many compounds possessing these functional groups. Studies, show that the twist around these double bonds following light absorption, can induce S1/S0 crossing via conical intersection.16-17 Intermolecular coordination between the solute and solvent, plays a vital role in determining the fate of the exciton in many molecules. For instance, it was shown that intermolecular interactions by hydrogen-bonding, regulate electronic states, leading to quenching or enhancing fluorescence.

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Moreover, a recent study on oxazine 750, has demonstrated that electron transfer (ET) from alcoholic solvents to this molecule mediated by H-bonding.19 Theoretically 20, H-bonded complexes of Pchilide a with methanol, has been shown to increase in strength in its electronically excited state due to sitespecific solvation enhancement. In our previous publication, we presented a through steady-state absorption spectroscopic investigation and theoretical calculations on a novel D-π-A chromophore DMAMN, where the donor (N,N-dimethyl-aniline) and acceptor (2-naphthoic acid) are bridged by imine moiety.21 The design of this compound, makes the imine entity, confined to the plane of the acceptor, by virtues of the intramolecular H-bond formed by the carboxyl –OH arm with the imine. Herein, we give our findings of the steady-state fluorescence emission of this compound.

We observed that this

compound, despite of its broad band absorption and normal moderate red-shifted solvatochromism in all polar solvents, is extremely weakly fluorescent with slight low-energy shift in highly polar media ACS Paragon Plus Environment

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e.g., acetonitrile (ACN), dimethyl-sulfoxide (DMSO) and normal alcohols, compared to the less polar solvents e.g., chloroform (CHCl3) and dichloromethane (DCM). Based on our experimental findings, we expect efficient competing non-radiative channels to be operative in these polar solvents. This unusual behavior, provided our motivation, to theoretically, investigate the photo processes in the excited-state of this fluorophore. To this end, we attempted to explore the photochemical reaction paths via mapping the relevant ground (S0) and the low-lying energy states potential energy profiles from the Frank-Condon region, along important reaction coordinates responsible for efficient energy dissipation, which mainly involve out of plane modes. 2.

Experimental Details

2.1

Synthesis of the compound 3-[(E)-{4-[dimethylamino]benzylidene}amino]-2-naphthoic acid

(DMAMN). The complete procedure for the synthesis and characterization of the title compound, are given in our earlier work.21 The solid Infra-red (IR) in KBr and the 1H spectra of the title compound, are given in the supporting information (see Figure S1, Figure S2 and Figure S3). In IR spectra, the 3300-3500 cm-1 band corresponds to the O-H stretch of the carboxylic acid H-bonded to the nitrogen. This is obvious due to broadening shape, which comes due to the hydrogen pulls on the O-H bond from both nitrogen and oxygen, facilitated by intramolecular charge transfer from the dimethyl aniline, thus giving rise to a variation in O-H…N distance. In effect, the recorded IR absorption of this bond, resembles an ensemble of many structures, with the H atom close to the donor (oxygen), others with H atom close to the acceptor (nitrogen). 1H NMR spectra of DMAMN in Ethanol and other (not shown) solutions, were measured in 5 mm o.d. NMR tubes at 25°C with the use of VNMRS NMR spectrometer working at 11.7 T magnetic field using standard measurement procedures. All reagents used in synthesis and the Spectroscopic grades solvents cyclohexane (CHX), chloroform (CHCl3), dichloromethane (DCM), tetrahydrofuran (THF), 1,4dioxane (DXN), dimethyl-sulfoxide (DMSO), acetonitrile (ACN), methanol (MeOH), ethanol (EtOH), 1-propanol (1-ProOH) and 1-butanol (1-ButOH), used in measurements of fluorescence emission, were purchased from Fluka chemical Co. and used as received. 2.2

Computational Methods All calculations have been performed with the latest version of Gaussian 16 program,22 and

molecular orbitals visualization were carried by Gauss View 6.0.16.23 Ground state geometry optimization for the isolated and DMAMN-solvent cluster in the gas-phase, were carried out at the density functional theory (DFT) level, using the B3LYP hybrid functional set.

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24-25

, and 6-31G(d) basis

Here, B3LYP is applied due to its typically excellent performance in the prediction of

geometries and vibrational frequencies for simple organic molecules. Also, the choice of 6-31G(d), is



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justified by many studies on similar systems, that have shown, that this atomic basis set is sufficient for geometries of both ground and excited-states, and is a good compromise between accuracy and efficiency.27-29 Non-specific solvent effects were considered using the Tomasi’s polarizable continuum model,30-32 PCM. Default parameters have been used for the PCM cavity and have considered methanol (MeOH), ethanol (EtOH), 1-propanol (1-ProOH) and 1-butanol (1-ButOH) as medium. Solvation through specific solvent effects, were conducted using one MeOH molecule Hbonded to the C=O of the carboxyl group. Excited state geometry optimization in the gas phase, has been performed using Timedependent density functional theory (TD-DFT)33-34 method, to observe the geometrical change on relaxation from the Franck-Condon state as well as the emission characteristics. This, is done with the analytical gradients implemented in Gaussian 16, and the same used for the LR-PCM excitedstate calculations. Recent studies on many systems, have shown that LR-PCM-TDDFT provides a reliable treatment of solvation effects in electronic transitions with large oscillator strength and reasonably excited state geometries are obtained.35-38 The structures of DMAMN in the ground- and first excited singlet state (S1), and the DMAMN-MeOH complex also have been optimized using coulomb attenuated method hybrid exchange correlation functional (TD-CAM-B3LYP).39 The CAM-B3LYP functional comprises 0.19 Hartree-Fock (HF) plus 0.81 Beck 1988 (B88) exchange iteration at short-range, and 0.65 HF plus 0.35 B88 at long-range.40 The transition energies between states, have been computed on the ground state optimized structures utilizing the same functional, but with a more extended atomic basis set, namely 6-31+G(d). The NBO charge analysis was performed at B3LYP/6-31+G(d) level of theory on the optimized structures in the gas-phase and solution. This has been achieved using NBO 6.0

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as

implemented in Gaussian 16. Atoms in molecule (AIM) calculations42, were carried out using the AIM2000 program to find true hydrogen bond. Potential energy surfaces of the ground-state (S0), were generated in the gas-phase at the S0 state geometry adopting a relaxed scan, where at each value of the dihedral angle (φ) the other nuclear coordinates were optimized at B3LYP/6-31G(d). For the excited-state curves TDDFT/B3LYP or LR-PCM-TDFT/B3LYP, were used according to the variation of the rotational angle from its optimized ground-state value both in the gas-phase and in solutions. All computations, were performed by imposing no symmetry restrictions (Ci point group). 3.

Results and Discussion

3.1.

Ground State Conformational Analysis To gain a better insight into the conformational energy changes of all available structures

associated with all degrees of freedom, mainly the C-N, C-C, C-C, C-O and C=N bonds (cf. Figure


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1), a potential energy surfaces (PES) scans in the ground state, were obtained at the B3LYP/631G(d) level of theory for the various torsion angles. The title compound, has six internal axes of rotation, which gives rise to different structural isomers. Two of these axes comes from the benzylidene and phenyl fragments, corresponding to rotation about the C=N imine bond and the C-C bond connecting the phenyl ring to the imine central moiety. The other four internal rotations available in the carboxy-naphthyl fragment [(C10H6)COOH], two rotations around C-C and C-N involves the naphthyl arm, while the other remaining rotations are defined around the C-C and C-O bonds of the carboxylic moiety. The results of the PES scans, given in Figure 2, show that the patterns of energy changes associated with rotations about these bonds, are different. The (CH3)2-N-C-C/C torsion (angle φ1) is the lowest (ca. 7.25 k cal mol-1), whereas the rotation about (CH3)2-N-C-C/C bond angle φ2 is a little higher (ca. 11.0 kcal mol-1, and involves the dimethyl-aniline ring), followed by angle φ3 (ca. 12-15 kcal mol-1) with the rotation about the -Np-COOH dihedral angle, due to the steric effect between the hydroxyl hydrogen and the vicinal naphthyl hydrogen. The rotation about angle φ4 (Np-N=CHis the highest (ca. 16 kcal mol-1) among these rotations, because of involvement of the whole substituted-naphthyl ring and requirement of breaking the intra-molecular hydrogen bond between the hydroxyl-hydrogen and the imine-nitrogen. The full scan rotation about both the dimethyl-amino and the dimethyl-aniline groups, each give two identical iso-energetic stereo-isomers with opposite alkyl/phenyl plane orientation. On the other hand, the energy profile for rotations that involve the naphthalene-imine and naphthalene-carboxyl fragments are more complex, and are the highest in energy with multiple stable high-energy structures (cf. Figure 2, and Table 1). Hence, the rotation about the carboxyl-group, produced three minima (conformers B, C and D) starting from the global one at ~4.6° (conformer A), these are higher in energy at about ca. 1.8 kcal mol-1, 11.0 kcal mol-1and 12.5 kcal mol-1. The activation energies for these four transitions separating these stable structures are ca. 2.0 kcal mol-1, 10.5 kcal mol-1and 3.61 kcal mol-1. As mentioned earlier, the rotation about the bond C-C connecting the naphthyl unit with the imine-dimethyl-aniline requires the highest energy in DMAMN compound. Three minimum energy conformers E, F and G, were located upon the geometry search about this bond from the lowest point. E is higher in energy by 12.72 kcal mol-1, while the barrier separating structures E and F is ca. 3.51 kcal mol-1and F G cost only ca. 1.22 kcal mol-1 of energy. Structure G requires 13.8 kcal mol-1 to travel on the opposite direction to conformer F, which reached by 360° rotation about angle φ2 as well. Finally, we scanned the twist about the N=C bond (isomerization) starting from the lowest closed-conformer, where the imine-hydrogen in this structure takes a trans position with respect to ACS Paragon Plus Environment


the nitrogen (Z-isomer). The torsion about this bond led to a stable E-isomer with 7.26 kcal mol-1 higher than the Z-isomer. These two isomers, are separated by 25.9 kcal mol-1 (1.123 eV) barrier, so it is unlikely that the E-isomer exist at ambient. The rotation about the hydroxyl O-H arm (φ φ5, produces two stable structures with different orientation of the hydroxyl group. In the lowest structure (A) the hydroxyl points towards the molecule forming a six-membered ring in which the hydrogen is intra-molecularly bonded to the central nitrogen atom (named hereon closed-conformer); while for the other higher energy structure the O-H is in syn position to the carboxyl C=O group, and exposed to specific inter-molecular interaction with nearby solvent molecules through hydrogen bonding (named open-conformer). The different conformers associated with these structural transformations, are given in Figure S4 (supporting material), while Table S5 collects their minimized energies and relative energies to the most stable lowest energy structure (A), in which the carboxyl in DMAMN forms intramolecular hydrogen bond (chelated) with the central nitrogen along (closed-conformer) with some important structural parameters. Table S5 (supporting information) also shows the corresponding values for the transition energies found along these stable isomers, whereas, Figure S6 in the supporting information, displays the transition energies structures found along these coordinate scans. Careful inspection of the calculated data, reveal the following. (1) In scanning along angle φ1 in the transition state (TS1), we notice that the dimethyl amino group is tilted by ca. 64.180° with respect to the phenyl ring (i.e, it is decoupled). This angle is increased to ca. 90° in the transition state TS2 during rotation of the whole amine-phenyl fragment. (2) Rotation about angle φ3, produces four transition states. The first transition state is unstable and collapses to the lowest structure during minimization, whereas, in TS2 the phenyl is at -48.9° to the naphthyl plane ring. On the other hand, the carboxyl motif is orthogonal to the naphthyl ring ca. -82.3°. The phenyl ring is further twisted (ca. -63.0°) in TS3, and the carboxyl is almost in the same plane (ca. 2.6°) to the naphthyl ring with C=O in cis position to the central imine-nitrogen. In TS4, the phenyl is twisted by ca. -45.2°, whereas, the carboxyl is almost orthogonal (ca. 81.7°) to the naphthyl plane. The O-H here is pointing to the longitudinal axis of the naphthyl ring making 94.7° angle. (3) Rotation about angle φ4, produces three transition states. In the first (TS1), the phenyl and the naphthyl ring are 7.25° a part, however, the carbonyl is at 97.55° angle to the naphthyl plane, whereas, the O-H group is pointing inside the naphthyl pocket. In TS2, the naphthyl makes 100° to the rest of the molecule. The carboxyl arm here makes 36.29° to the naphthyl ring. The phenyl and the naphthyl rings are almost in the same plane (only ca. 8.0 difference). On contrary, the carboxyl makes a 56.29° tilt with respect to the naphthyl ring, with O-H pointing inside the pocket (forming six-membered ring). (4) Isomerization about the imine-bond (N=C), goes through a transition state, where the DH angle is 94.45°. ACS Paragon Plus Environment

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3.2.

Steady State Absorption and Emission DMAMN absorption spectra in non-polar solvent cyclohexane (CHX) is intense and

slightly structured with two main peaks at 319 nm and 326 nm, with a shoulder appearing from 343 nm and extending to 432 nm. We see some vibrational structure at ~ 355 nm and ~ 373 nm (cf. the expanded spectra in Figure 3a-b). The absorption in highly-polar aprotic solvents, such as ACN, consist of two bands at 333 nm and 402 nm, with the later peak is lower in intensity. The emission, however, is unstructured and broad centering at 485 nm (cf. Figure 4a-b). For better understanding of the dynamics of the excited-states in different kinds of solvents, we have compared and analyzed the characteristics of the absorption in our previous work. 21

. In this study, we focus on the fluorescence behavior in a set of solvents of different polarity and

proticities, namely non-polar aprotic solvents (CHX), intermediate-polar aprotic solvents (DXN, THF, CHCl3 and DCM), highly polar aprotic solvents (ACN and DMSO) and highly polar protic solvents (MeOH, EtOH, 1-ProOH and 1-ButOH). Contrary to the absorption, which showed slightly solvents dependence (cf. our previous publication for details), important effects of solvent’s polarity and proticities, were seen on the fluorescence spectral shapes and maxima positions. Figure 5, shows the normalized fluorescence emission spectra in some aprotic solvents collected according to their polarity strength, whereas, Figure 6, gives the spectra in both aprotic and protic media. On the other hand, Figure S7 in the supporting information, depicts the normalized spectra for the same. Inspection of the normalized fluorescence emission given in Figure S7, show that the fluorescence emission exhibit significant solvent effects. The fluorescence in non-polar solvent CHX, when excited at the 350 nm (the bright state of the closed-conformer, see our discussion in section 3.6), does not show the mirror image rule implying that the emitting state processes different geometry from the ground-state. This emission is broad with a clear peak at 426 nm and a shoulder around 400 nm. On the other hand, fluorescence emission in the intermediate polarity solvents, is strong and markedly red-shifted with respect to CHX with band maxima in the 517-523 nm wavelength region (cf. Figure 5). Contrary to our expectation, the fluorescence in the highly polar aprotic solvents (e.g., ACN and DMSO) is weak, observed at a higher energy compared to the less-polar solvents. These emissions were found at ~485 nm and 490 nm, respectively. In polar protic normal alcohols, the emission is the weakest among the solvents studied and are very broad at 462-485.5 nm domain, with extra shoulder at ~550 nm. The intensity of this lower-energy band increases significantly, and becomes broader with increase in the alcohol series and follow the order MeOH

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