The effects of amino group meta- and para-substitution on ESIPT mechanisms of amino 2-(2’-hydroxyphenyl) benzazole derivatives

Journal of Luminescence 218 (2020) 116836

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The effects of amino group meta- and para-substitution on ESIPT mechanisms of amino 2-(2’-hydroxyphenyl) benzazole derivatives Yunfan Yang 1, Yong Ding 1, Wei Shi, Fengcai Ma **, Yongqing Li * Department of Physics, Liaoning University, Shenyang, 110036, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Substituent position Hydrogen bond Excited-state intramolecular proton transfer mechanisms Intramolecular charge transfer

The promising amino 2-(20 -hydroxyphenyl) benzazole derivatives have taken extensively attractions in biochemical and photochemical fields because of their strong electron-donor -NH2 group, however little was known about its importance for the excited-state intramolecular proton transfer (ESIPT). In present work, the effects of different -NH2 group positions on the ESIPT mechanisms and its photophysical properties were investigated by using time-dependent DFT calculations. The optimized geometries and infrared (IR) vibration frequencies were analyzed to explore the excited-state intramolecular hydrogen bond (ESIHB) strengthening behaviors. Absorption and emission energies calculated in this work were agreement with the observed results in experiment, reproducing the photophysical phenomena well. To illustrate in detail the effects of different sub­ stituent positions on electronic structures properties, the intramolecular charge transfer (ICT) characters were analyzed by calculating frontier molecular orbitals (FMOs) and electron-hole distribution. The minimum energy pathways explicitly illuminated the complicated ESIPT mechanisms. The calculated Gibbs free energy barriers and reduce density gradient (RDG) scatter diagrams indicated that the order in which ESIPT occurred was consistent with ESIHB intensities. The conclusion was that para-substitution of amino group had a positive in­ fluence on the ESIPT reactions, since it brought the greater effect on ESIHB compared with meta-substitution of amino group.

1. Introduction As one of the most extensive weak interactions in nature, hydrogen bonds widely exist in the biomolecules [1,2] (proteins, amino acids, basic groups, fibrinogen and so on), the natural products [3,4] (myr­ icetin, quercetin, isorhamnetin, curcumin and so on), the pharmaceu­ ticals [5,6] (paracetamol, pyrazinamide, ethionamide and so on) and the synthetic products [7–9] (1-(acylamino)-anthraquinons, 2-(2-hydrox­ yphenyl), 40 -dimethylaminoflavonol, etc), which plays crucial roles in molecular microstructures and physicochemical behaviors. Hydrogen bond (X–H⋅⋅⋅Y) consists of H atom, proton donor group -X-H and acceptor group -Y. Generally, the X and Y atoms have the large elec­ tronegative, such as nitrogen, oxygen and fluorine, etc. [10] The proton transfer is an important hydrogen-bonding dynamic behavior, which gets the great attention. In the beginning, most of experimental and theoretical researches only investigate molecular ground-state due to the limitation of

technological level, which reported that the proton transfer reactions have great impacts on physical and chemical properties [11], but these reactions are usually difficult to occur in the ground (S0) state [12–14]. With the development of science and technology, the investigation of the excited-state intramolecular proton transfer (ESIPT) reactions have become possible. The premier ESIPT species are salicylic acid de­ rivatives, their excited-state proton transfer behaviors were first re­ ported by Weller and co-workers in 1955 [15]. Since then, the revelation of the ESIPT reaction mechanisms have become one of the many contemporary research hotspots [16–26]. The molecular four-level photocycle schematic diagram is presented in Scheme 1. The dual-fluorescence phenomena of ESIPT chromophores can be observed by the steady-state fluorescence spectra technique. Notice that the tremendous Stokes’ shift fluorescence originates from ESIPT emission and the generated isomer form is more stable than normal form, which indicates that the ESIPT reaction is thermody­ namically permissible in electronic excited state [27]. The occurrence of

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Ma), [email protected] (Y. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jlumin.2019.116836 Received 1 September 2019; Received in revised form 8 October 2019; Accepted 22 October 2019 Available online 23 October 2019 0022-2313/© 2019 Published by Elsevier B.V.

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is of 54% Hartree-Fock exchange, which is relatively accurate to evaluate the “long-range” electron correlation in hydrogen bonding interaction [48,49]. DFT-D3 dispersion corrections are also carried out to offer the more accurate interaction energies [50] Transition-state theory has been successful in the standard Gibbs activation energy, which is calculated by Gaussian 16 A.03 version [47] based on Berny arithmetic method [51]. 2. The infrared (IR) vibration frequencies of stable and transition-state structures are calculated with the same computational level as optimizing calculation. The whole positive frequencies confirm that structures are real local minimums. The only imaginary frequency along with hydrogen bond orientation represents the transition-state structure of ESIPT reaction. The validity of transition-state structure is further confirmed by calculating intrinsic reaction coordinates (IRCs) [52]. 3. For the calculation of transition energies, Jamorski and Lṻthi et al. reported that the functional bearing 20%–25% Hartree-Fock ex­ change yields good agreement with experiment for the vertical transition energies of the first few excited states [53]. Herein, the vertical absorption, emission energies can be accurately calculated with B3LYP-D3/6-31 þ G (d, p) level of theory, since the B3LYP functional has 20% Hartree-Fock exchange. 4. To provide molecular-level insight into the effect of different -NH2 group positions, we analyze the frontier molecular orbitals (FMOs), the electrostatic potentials and the reduce density gradient (RDG) isosurfaces, as well as the charge transfer degree. The electrostatic potentials isosurfaces are described by Gaussview 5.0.9 program [54]. The FMOs are visualized by Chemcraft program [55]. The RDG isosurfaces are calculated by Multiwfn program [56] The concepts of RDG are proposed by Yang et al. [57], to distinguish molecular non-covalent interactions. The expression is as follows:

Scheme 1. Molecular four-level photocycle diagram. The red dash line is vertical absorption; the light blue dash lines represent vertical emission; The RPT represents reversed proton transfer in S0 state. Left panel indicates an electron is excited from occupied orbital to unoccupied orbital upon photoexcitation process.

ESIPT reactions and its fluorescence properties could be due to the change of electronic structures [28]. During photo-excitation process the redistribution of electron density changes the acid-base properties of proton donor and acceptor groups. The enhanced acidity and alkalinity of proton donor and acceptor make intramolecular hydrogen bonding interaction strengthen, which activates the ESIPT reactions [29]. In general, the isomerization structure yields a large Stokes shift’s fluo­ rescence of 8000–10000 cm 1, which distinctly differs from the fluo­ rescence of normal structure in the first excited (S1) state. The ESIPT chromophores are extensively used to the many fields due to their dual-fluorescence behaviors. For example, the organic light-emitting diodes (OLED) [30], newer arenas of biotechnology [31], fluorescence imaging [32], fluorescence probing [33], optoelectronic devices [34] and laser dyes [35], etc. [36,37]. The amino 2-(20 -hydroxyphenyl) benzazole derivatives are exten­ sively applied in fluorescence probes for identifying proteins or DNA of biosystems [38], and helpful for the improvements of synthesizing polymer-drugs [39]. Because of the strong electron-donating ability of amino group, the changes of substitutive positions have the great impact on the ESIPT mechanisms, which in turn influence molecular photo­ physical properties [40,41]. Stefani et al. have systematacially investi­ gated the photophysical phenomena of amino 2-(20 -hydroxyphenyl) benzazole derivatives (AHBT (when R3 ¼ S) and AHBO (when R3 ¼ O)) with different -NH2 group positions in experiment [42]. But this experimental work failed to offer fundamental explanations for: (i) why -NH2 group can cause different photophysical phenomena when they are in different positions? (ii) What does the effect on the ESIPT mech­ anisms when -NH2 group locates at different positions? For the above two open questions, the advanced highlights about this research work have been reported by our group. In present research, the effect of -NH2 group positions on ESIPT mechanisms will be uncovered. The effect of amino group meta- and para-substitution on ESIPT behaviors plays leading roles in the highly efficient fluorescence probes [43], the prodrug-mediated and the antineoplaston [44,45].

RDGðrÞ ¼

1

jrρðrÞj

2ð3π2 Þ1=3 ρðrÞ4=3

(1)

Where ρ (r) represents the total electron density, RDG (r) is the reduced density gradient of the exchange contribution. The Lap­ lacian of electron density is defined as r2ρ ¼ λ1þλ2þλ3. The λ2 is the second largest eigenvalue of Hessian matrix of electron density. The relationship of the ρ (r) and λ2 is: ΩðrÞ ¼ Signðλ2 ðrÞÞρðrÞ

(2)

where λ2 is applied to distinguish bonding (λ2<0) and nonbonding (λ2>0) interactions. The hydrogen bond interactions are described by RDG (r) and density ρ (r), which contributes to investigating the effect of different -NH2 group positions on intramolecular hydrogen bonding strength. In addition, according to the conception of eval­ uating the charge transfer degree given by Guido et al. [58], we calculate charge-transfer length (Δr) by using Multiwfn program [56], so as to reveal the fundamental reason for impacting photo­ physical properties. The Δr can be written as: �2 P ðKunocc j〈ϕunocc jrjϕunocc 〉 〈ϕocc jrjϕocc 〉j occ occ;unocc Δr ¼ (3) �2 P ðKunocc occ occ;unocc

Where the occ and unocc represent the molecular occupied and un­ occupied orbitals, respectively. The ϕ is orbital wavefunction. The Kunocc is transition configuration coefficient. The Δr value is used to occ measure the degree of charge transfer. The integral of overlap of hole-electron (S) and the distance between centroid of hole and electron (D) are also calculated to investigate the effect of -NH2 group positions on the charge transfer. The hole and electron are separately defined by the predominantly occupied and unoccupied orbitals of FMOs [56].

1.1. Theoretical method and computational detail 1. To better simulate the surrounding dichloromethane microenviron­ ment, the self-consistent reaction field (SCRF) method with the sol­ vation model universal [46] (SMD) has been carried out. The dielectric constant and polarity of dichloromethane is 8.9 and 4.4, respectively. Based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT) method with M06-2X-D3/TZVP level of theory, all geometric structures are full optimized by Gaussian 16 A.03 version [47]. The M06–2X functional 2

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Journal of Luminescence 218 (2020) 116836

Fig. 1. Optimized geometrical configurations of AHBT, AHBO and their isomers in dichloromethane solvent. Key atoms are numbered 1–3. Atom color coding: O, red; N, blue; C, gray; S, yellow; H, white. The molecular electrostatic potential diagrams of all molecules.

5 The minimum energy pathways of ESIPT processes are constructed by using the relaxed potential energy surface scan method [47]. The activation energy between normal and isomerization structures are evaluated by calculating the Gibbs free energies (ΔG) under 1 atm and 298.15 K. The single point energies and Gibbs free energies are calculated with M06-2X-D3/TZVP computational level. Herein, all theoretical calculation methods consider ultrafine integration grid (99, 590 points).

Table 1 Bond lengths (Å) and angles (� ) of S0 and S1 states for the studied molecules AHBT and AHBO in dichloromethane solvent.

2. Results and discussion

Species

O1–H2(Å)

States AHBT-para AHBT-meta AHBO-para AHBO-meta

S0 0.98337 0.98738 0.97961 0.98270

H2⋯N3(Å) S1 1.03008 0.99867 1.00996 0.98889

S0 1.77007 1.74789 1.82713 1.80458

δ(O1–H2–N3)(� ) S1 1.57454 1.69084 1.66160 1.76611

S0 145.17 146.35 144.33 145.54

S1 151.03 149.42 149.45 148.15

confirms that the -NH2 group is strong electron donor group. In addi­ tion, we find that the electrostatic potential differences are larger when the -NH2 group is in para-position (AHBT-para and AHBO-para) than those in meta-position (AHBT-meta and AHBO-meta). It indicates that the different substituent positions have the significant impact on elec­ tronic structures, which in turn affects the intramoleular hydrogen bond interaction. The hydrogen bonding strengths are analyzed and compared by calculating hydrogen-bond structure parameters. As a matter of conve­ nience, the key atoms that constitute hydrogen bonds are numbered 1–3. The parameters of intramolecular hydrogen bond O1–H2⋯N3 are pre­ sented in Table 1. The O1–H2 bond lengths of AHBT-para and AHBTmeta (when R3 ¼ S) are 0.98337 Å and 0.98738 Å in the S0 state, increasing to 1.03008 Å and 0.99867 Å in the S1 state, respectively. The H2⋯N3 distances of AHBT-para and AHBT-meta are 1.77007 Å and 1.74789 Å in the S0 state, decreasing to 1.57454 Å and 1.69084 Å in the S1 state, respectively. Moreover, upon photo-excitation process the hydrogen-bond angles δ(O1–H2–N3) of AHBT-para and AHBT-meta in­ creases from 145.17� to 146.35� –151.03� and 149.42� , respectively. Similarly, for AHBO-para and AHBO-meta molecules (when R3 ¼ O), the O1–H2 bond lengths increase from 0.97961 Å and 0.98270 Å in the S0 state to 1.00996 Å and 0.98889 Å in the S1 state, respectively; the dis­ tances of H2⋯N3 shorten from 1.82713 Å and 1.80458 Å in the S0 state to 1.66160 Å and 1.76611 Å in the S1 state, respectively. The hydrogenbond angles δ(O1–H2–N3) of AHBO-para and AHBO-meta separately

In the following, firstly, we implement the geometry optimization and frequency analysis in different electronic states based on DFT and TDDFT methods. The Cartesian coordinates of the geometries are pre­ sented in section 1-4 of ESIy. Secondly, the FMOs analyses provide fundamental reasons for the effects of different -NH2 group positions on photophysical properties. Thirdly, we reproduce the photophysical phenomena by calculating vertical excitation and emission energies. Afterwards, we construct the minimum energy pathways and calculate the reactive activation energies of ESIPT reactions. Finally, the analyses of RDG explain the correlation between the ESIPT reactions and the hydrogen bonding interactions. 2.1. Geometry optimization As presented in Fig. 1, the AHBT and AHBO structures with different -NH2 group positions are fully optimized based on M06-2X-D3/TZVP level of theory in the S0 and S1 states. Free from imaginary fre­ quencies indicate that these structures are real local minimums. The electrostatic potential isosurfaces are applied to analyze the effect of different -NH2 group positions on electronic structures. The blue and red surfaces represent positive and negative electrostatic potential, respec­ tively. Note that the electrostatic potentials of -NH2 and hydroxyl groups are the highest and lowest, respectively. The direction of elec­ tronic charge migration is dependent on electrostatic potential differ­ ence according to electronegativity equalization principle [59], which 3

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Fig. 2. Calculated changes of bond length (O1–H2) (a) and distance of H2 and N3 (b) in different electronic states for the normal structures.

increase from 144.33� to 145.54� in the S0 state to 149.45� and 148.15� in the S1 state. These changes indicate that the intramolecular hydrogen bonding strengths of AHBT and AHBO molecules are reinforced upon photo-excitation process. As shown in Fig. 2 the O1–H2 bond lengths and distances of H2⋯N3 separately increase and shorten during the photo-excitation process, which confirms the excited-state intramolecular hydrogen bonding (ESIHB) strengthening mechanism. In addition, when we investigate the effect of different -NH2 group positions on hydrogen bond interaction, notice from Fig. 2 that the O1–H2 bond lengths of AHBT-para and AHBO-para (para-substitution of -NH2 group) are shorter than that of AHBT-meta and AHBO-meta (meta-substitution of -NH2 group) in the S0 state, respectively; the H2⋯N3 distances of AHBT-para and AHBO-para (para-substitution of -NH2 group) are longer than that of AHBT-meta and AHBO-meta (meta-substitution of -NH2 group) in S0 state, respec­ tively. It indicates that the intramolecular hydrogen bonds are stronger in S0 state as -NH2 group locates at meta-position. On the contrary, the excited-state intramolecular hydrogen bonds of para-substitution are stronger by comparing hydrogen-bond parameters. Upon photoexcitation process, the O1–H2 bond lengths of AHBT-para and AHBOpara molecules increase by 0.04671 Å and 0.03035 Å, respectively; the O1–H2 bond lengths of AHBT-meta and AHBO-meta molecules increase by 0.01129 Å and 0.00619 Å respectively in Fig. 2. Meanwhile, the H2⋯N3 distances of AHBT-para and AHBO-para molecules reduce by 0.19553 Å and 0.16553 Å, respectively; the H2⋯N3 distances of AHBT-

meta and AHBO-meta molecules reduce by 0.05705 Å and 0.03847 Å respectively in Fig. 2. We find that the changes of intramolecular hydrogen-bond parameters are more drastic as -NH2 group locates at para-position. Compared with the meta-substitution, the parasubstitution of -NH2 group has the greater impact on intramolecular hydrogen bond. The analyses of IR vibration frequencies are used as the criterion of bond strength in general as Karas et al. have proposed [60]. In order to further measure the effect of different -NH2 group positions (When R3 ¼ O or S) on intramolecular hydrogen bonding interactions, the IR frequencies of O1–H2 moiety are calculated by M06-2X-D3/TZVP level of theory as shown in Fig. S1 (see section 5 in ESIy). It is confirmed that the para-substitution of amino group has a larger enhancement on ESIHB strength regardless of R3 ¼ S or O. The ESIHB strengthening mechanism can be explained via the geometry optimization and fre­ quency analyses. The different strengthening behaviors of hydrogen bond may be dependent on the electronic structure properties with different positions of -NH2 group. 2.2. Frontier molecular orbitals (FMOS) and charge-transfer degree analyses Upon the photo-excitation process the electron jumps from the mo­ lecular occupied orbital to unoccupied orbital, which causes the redis­ tribution of electron population. The FMOs analyses will reveal the

Fig. 3. Frontier molecular orbitals (FMOs) and their compositions (based on Hirshfeld method) of AHBT and AHBO molecules with M06-2X-D3/TZVP level of theory. AHBT and AHBO molecules are decomposed into fragment 1 and fragment 2, respectively. 4

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Journal of Luminescence 218 (2020) 116836

Table 2 Composition index (CI) (%) of orbital transition and corresponding oscillator strengths for the AHBT and AHBO molecules.

Table 4 Calculated charge-transfer length (Δr), the integral of overlap of hole-electrons (S) and distances between centroid of hole-electrons (D).

Species

Transition

Composition

CI (%)

fa

Species

Transition

Δr (Å)

D (Å)

S

AHBT-para AHBT-meta AHBO-para AHBO-meta

S0→S1 S0→S1 S0→S1 S0→S1

H→L H→L H→L H→L

94.9% 94.7% 93.2% 93.3%

0.33 1.02 0.33 1.13

AHBT-para AHBT-meta AHBO-para AHBO-meta

S0→S1 S0→S1 S0→S1 S0→S1

2.44 1.67 2.29 1.58

1.81 1.67 1.69 1.58

0.391 0.407 0.397 0.402

a

The oscillator strengths greater than 0.01 indicate the nonnegligible transitions.

AHBO molecules (when R3 ¼ O) can be obtained. Given the effect of different -NH2 group positions on molecular ICT degrees, the length of charge transfer (Δr), the distances between centroid of hole-electrons (D) and the integral of overlap of holeelectrons (S) are calculated in Table 4. Herein, the electron represents the LUMO, the hole represents the HOMO. According to Guido et al., the electron and hole distributions can show the distinct ICT character when Δr is larger than 2.0 Å [58]. The calculated Δr of AHBT-para and AHBO-para are 2.44 Å and 2.29 Å, respectively. The calculated Δr of AHBT-meta and AHBO-meta are 1.67 Å and 1.58 Å, respectively. However, we cannot accurately esti­ mate the charge transfer degree only with Δr index. In addition to Δr index, the measurements of the distances between centroid of hole-electrons (D) and the integral of overlap of hole-electrons (S) are also analyzed. The D of AHBT-para and AHBO-para are 1.81 Å and 1.69 Å, respectively. The D of AHBT-meta and AHBO-meta are 1.67 Å and 1.58 Å, respectively. It indicates that the ICT distances are larger with para-position amino substituent. In addition, the S of AHBT-para and AHBO-para are 0.391 and 0.397, which are lower than that of AHBT-meta (0.407) and AHBO-meta (0.402), respectively. It indicates that the electron densities of para-position amino substituent have lower overlap level. Therefore, the para-position -NH2 group has a greater electron-donating capacity, generating a more drastic ICT degree compared with meta-position -NH2 group. The greater electron-donating capacity of -NH2 group activates molecular bio­ activities in prodrug-mediated and antineoplaston [44,45]. Moreover, the different ICT degrees will probably bring in different photophysical and photochemical properties, promoting the applications of ICT chro­ mophores in the fluorescent materials [19] and pigments [63].

Table 3 Calculated electron density components (%) of O1 and N3 atoms based on Hirshfeld method of AHBT and AHBO molecules with M06-2X-D3/TZVP level of theory. Atoms O1 N3

LUMO HOMO LUMO HOMO

AHBT-para

AHBT-meta

AHBO-para

AHBO-meta

1.65% 10.28% 10.89% 1.96%

1.47% 1.80% 10.34% 5.95%

1.57% 9.92% 8.95% 2.12%

1.40% 1.58% 8.21% 7.28%

molecular physical and chemical properties. In Fig. 3 the characteristic ππ* transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) are presented. The composition index (CI) of orbital transitions in Table 2 are 94.9% (AHBT-para), 94.7% (AHBT-meta), 93.2% (AHBO-para) and 93.3% (AHBO-meta) respectively, which indicates that the electron transitions from HOMO to LUMO are dominant. The FMOs isosurfaces present the distinct redistributions of electron densities in Fig. 3, in which the electron densities of proton donor and acceptor group decrease and increase from HOMO to LUMO, respec­ tively. In order to quantitatively illustrate the electron density changes of proton donor and acceptor groups, their components percentages of electron density are calculated with Hirshfeld method [61] shown in Table 3. For AHBT and AHBO molecules, the components percentages of donor O1 atom and acceptor N3 atom are decrease and increase, respectively. For example, for AHBT-para the electron density components of N3 atom increase from 1.96% of HOMO to 10.89% of LUMO; the compo­ nents of O1 atom decrease from 10.27% of HOMO to 1.65% of LUMO. For AHBT-meta the components of N3 atom increase from 5.95% of HOMO to 10.34% of LUMO; the components of O1 atom decrease from 1.80% of HOMO to 1.47% of LUMO. It indicates that the hydrogen bonding interactions are enhanced upon photoexcitation process where the hydrogen proton is more likely to transfer from the donor to acceptor group. In addition, the obvious intramolecular charge transfer (ICT) char­ acters could generate extreme influences on the molecular photo­ physical and photochemical properties. In order to investigate the ICT degree of different molecules, we divide four molecules into fragment 1 and fragment 2 as shown in Fig. 3. The components percentages of electron density are calculated based on Hirshfeld method [62]. When R3 ¼ S, for AHBT-para molecule, it should be noted that the fragment 1 and fragment 2 have about 91.65% and 8.35% electron densities on the HOMO, respectively. On the LUMO, the electron densities of fragment 1 decrease to 41.10%, while the electron densities of fragment 2 increase to 58.90%, about 50.55% electron densities transfer from fragment 1 to fragment 2 in the transition process. For AHBT-meta molecule, about 69.16% and 30.84% electron densities locate at fragment 1 and frag­ ment 2 on the HOMO, about 37.13% and 62.87% electron densities located at fragment 1 and fragment 2 on the LUMO, respectively. About 32.03% electron densities transfer from fragment 1 to fragment 2 in the transition process. It concludes that the ICT degree is stronger when the -NH2 group locates at para-position. Similarly, the same conclusion of

2.3. Photophysical properties analyses The absorption and emission spectra with different -NH2 positions have been observed in experiment [42], however little is known about the reason for influencing on photophysical properties. In this research, we calculate molecular vertical excitation and radiation energies. The relevant explanation for calculating absorption and emission energies with B3LYP functional has been shown in section 6 of ESIy. The ab­ sorption spectra of four species are presented in Fig. 4 (a). For meta-position -NH2 group substituent the absorption peaks of AHBO-meta (R3 ¼ O) and AHBT-meta (R3 ¼ S) are 325 nm and 346 nm, respectively. For para-position -NH2 group substituent the red-shifted absorption peaks of AHBO-para (R3 ¼ O) and AHBT-para (R3 ¼ S) locate at 378 nm and 402 nm, respectively. The emission spectra of enol and keto forms are presented in Fig. 4 (b) and (c), respectively. In Fig. 4 (b), for meta-position -NH2 group substitution the enolic emission peaks of AHBO-meta (R3 ¼ O) and AHBT-meta (R3 ¼ S) are 368 nm and 391 nm, and the Stokes’ shifts are 43 nm and 45 nm, respectively. For para-position -NH2 group substitution the enolic emission peaks of AHBO-para (R3 ¼ O) and AHBT-para (R3 ¼ S) are 472 nm and 512 nm, and the Stokes’ shifts are 94 nm and 110 nm, respectively. In Fig. 4 (c), for meta-position -NH2 group substituent the ketonic emission peaks of AHBO-meta (R3 ¼ O) and AHBT-meta (R3 ¼ S) are 432 nm and 445 nm, and the Stokes’ shifts are 107 nm and 99 nm, respectively. For para-position -NH2 group substituent the ketonic emission peaks of AHBO-para (R3 ¼ O) and AHBT-para (R3 ¼ S) are 565 nm and 595 nm, 5

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Journal of Luminescence 218 (2020) 116836

Fig. 4. Calculated absorption and fluorescence spectra for AHBT and AHBO molecules. (a) Absorption spectra; (b) Emission spectra of enol forms; (c) Emission spectra of keto forms. Table 5 DFT-simulated and observed in experiment electronic spectral data (nm) of AHBT and AHBO molecules in dichloromethane solvent. The deviations (Δe) between theoretical and experimental data (eV) are presented. The Stokes Shifts (nm) are also listed. Species

Abs. of enol Theor. (Exp.)

Δe

Flu. of enol Theor. (Exp.)

Δe

Flu. of keto Theor. (Exp.)

Δe

AHBT-meta AHBT-para AHBO-meta AHBO-para

346 402 325 378

0.05 0.14 0.07 0.08

391 (389) 512 (437) 368 (376) 472 (442)

0.02 0.42 0.07 0.18

445 (500) 595 (606) 432 (467) 565 (565)

0.31 0.04 0.22 0.00

(351) (384) (331) (369)

and the Stokes’ shifts are 207 nm and 193 nm, respectively. The above analyses reveal that the absorption and emission of para-position sub­ stituents have larger red-shift than that of meta-position substituents. The different photophysical properties are due to the controllable ICT characters with different substituent group positions. Therefore, the AHBT derivatives are ideal organic materials that can be applied to laser dyes by varying substituent positions of -NH2 group. In order to compare the calculated spectra with observed spectra values in experi­ ment, the relevant spectra data are exhibited in Table 5. It is found that the errors of spectra data between theory and experiment are in a general range of 0.30 eV, indicating that the experimental spectra are well reproduced theoretically. It follows that the optimized geometry structures are correct, and the functional M06–2X is reliable to further investigate the ESIPT mechanisms.

Stokes Shift enol

keto

45 110 43 94

99 193 107 207

2.4. ESIPT reaction mechanisms and hydrogen bonding interaction ESIPT reaction is one of the most significant hydrogen-bond dynamic behaviors, which has a great impact on molecular photophysical prop­ erties. The complicated ESIPT reaction mechanisms need to be explained and explored in detail. In this research, the molecular-level explanation of different substituent positions effects on ESIPT re­ actions will be offered. The minimum energy pathways are constructed to simulate proton transfer reaction paths in the S0 and S1 states, the corresponding electronic and activation energies are also calculated based on M06-2X-D3/TZVP computational level. In Fig. 5 (a), notice that all proton transfer reactions are the endo­ thermic processes in the S0 state. When R3 ¼ S, the keto form of AHBTpara is not the local minimum, which indicates that the keto form is 6

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Journal of Luminescence 218 (2020) 116836

Fig. 5. Constructed proton-transfer potential energy curves as function of hydroxyl bond length O1–H2 in S0 (a) and S1 (b) states for AHBT and AHBO molecules.

nonexistent. The proton-transfer energy barrier of AHBT-meta is 9.71 kcal/mol in the S0 state, the negligible reversed proton-transfer energy barrier is 0.03 kcal/mol. When R3 ¼ O, the proton-transfer en­ ergy barrier of AHBO-para molecule is 14.01 kcal/mol in the S0 state, its reversed energy barrier is 0.18 kcal/mol. For AHBO-meta molecule, the energy barrier of proton transfer is 11.85 kcal/mol in the S0 state, its reversed energy barrier is 0.14 kcal/mol. It follows that all proton transfer processes are thermodynamic infeasible in the S0 state. The ESIPT reaction pathways are presented in Fig. 5 (b). When the -NH2 group locates at para-position, the calculated ESIPT energy barrier of AHBT-para (R3 ¼ S) is 1.31 kcal/mol, its reserved reaction energy barrier is 4.54 kcal/mol. The calculated ESIPT energy barrier of AHBOpara (R3 ¼ O) is 3.08 kcal/mol, its reserved reaction energy barrier is 5.19 kcal/mol. The electronic energies of keto products are more stable than enol reactants, indicating that the ESIPT reactions are exothermic processes. It concludes that the ESIPT reactions are spontaneous in S1

state. However, when the -NH2 group locates at meta-position, the calculated ESIPT energy barrier of AHBT-meta (R3 ¼ S) is 5.36 kcal/mol, its reserved reaction energy barrier is 2.80 kcal/mol. The calculated ESIPT energy barrier of AHBO-meta (R3 ¼ O) is 7.99 kcal/mol, its reserved reaction energy barrier is 5.07 kcal/mol. The electronic en­ ergies of enol reactants are more stable than keto products, indicating that the ESIPT reactions are endothermic processes. More importantly, the energy barriers of ESIPT reactions are larger when the -NH2 group locates at meta-position than that locates at para-position. Therefore, we can conclude that the ESIPT reactions are easier to occur when the -NH2 group locates at para-position. Based on calculated reactions energy barriers, the ascending order of ESIPT reactions occurring is AHBTpara > AHBO-para > AHBT-meta > AHBO-meta. To confirm the above conclusion, we optimized the reactive transition-state structures based on Berny arithmetic method [51], the accuracy of transition-state ge­ ometries is further confirmed by calculating IRCs. Fig. S2 and Fig. S3

Table 6 Gibbs free energy values (Hartree) of normal, TS and isomer structures of AHBT and AHBO molecules, as well as the proton-transfer activation energy barriers (Kcal/ mol) in the S0 and S1 states. Species ES

GS

AHBT-meta AHBT-para AHBO-meta AHBO-para AHBT-meta AHBT-para AHBO-meta AHBO-para

Normal (Hartree) 1084.038775 1084.042014 761.060266 761.068231 1084.117390 1084.109101 761.149879 761.141707

TS (Hartree)

Barrier (kcal/mol)

1084.030404 1084.043485 761.049810 761.066573 1084.104892 ─

5.25 0.92 6.56 1.04 7.84 ─ 9.38 11.91

761.134929 761.122731

7

Isomer (Hartree) 1084.034805 1084.04813 761.054656 761.073914 1084.103600 ─

761.132867 761.121125

Reversed barrier (kcal/mol) 2.76 2.91 3.04 4.61 0.81 ─ 1.29 1.01

Y. Yang et al.

Journal of Luminescence 218 (2020) 116836

Fig. 6. Reduce the density gradient (RDG) isosurfaces of AHBT and AHBO molecules in different electronic states. The color gradient scale maps corresponding to the diverse types of the interaction locate at right edge. Atom color coding: O, red; N, blue; C, gray; S, yellow; H, cyan.

show the IRCs curves for AHBT and AHBO molecules in different elec­ tronic states, respectively. (see section 7 in ESIy) The Gibbs free energies of ESIPT reactions are calculated under the condition of 1 atm and 298.15 K. In Table 6, we find that the transition state of AHBT-para (R3 ¼ S) is nonexistent in S0 state. The proton-transfer activation en­ ergy barrier of AHBT-meta (R3 ¼ S) is 7.84 kcal/mol, its reversed reac­ tion barrier is 0.81 kcal/mol in S0 state. The proton-transfer activation energy barrier of AHBO-para and AHBO-meta (R3 ¼ O) is 9.38 kcal/mol and 11.91 kcal/mol, their reversed reaction barrier is 1.29 kcal/mol and 1.01 kcal/mol in S0 state, respectively. Herein, the negative acti­ vation energy barriers are due to the overcorrection of Gibbs free energy [64]. The calculation of Gibbs free energies indicates that the keto forms of S0 state are unstable and the proton transfers are infeasible. However, under photo-induced process the ESIPT activation energy barriers of AHBT-para and AHBT-meta (when R3 ¼ S) are 0.92 kcal/

mol and 5.25 kcal/mol, their reversed activation energy barriers are 2.91 kcal/mol and 2.76 kcal/mol in S1 state, respectively. For AHBOpara and AHBO-meta (when R3 ¼ O) the ESIPT activation energy bar­ riers are 1.04 kcal/mol and 6.56 kcal/mol, their reversed reaction bar­ riers are 4.61 kcal/mol and 3.04 kcal/mol in S1 state, respectively. The negative activation energy barriers are due to the overcorrection of Gibbs free energy [64]. It concludes from calculated activation energy barriers that the ESIPT reactions of AHBT and AHBO are easier to occur when the -NH2 group locates at para-position. The ascending order of ESIPT reaction occurring can be confirmed as AHBT-para > AHBO-­ para > AHBT-meta > AHBO-meta. As is well known the hydrogen bond interactions offer the driving force for the ESIPT processes. Therefore, the analyses of hydrogen bonding interaction can be applied to illumi­ nate the ESIPT mechanisms. The intramolecular hydrogen bonding strength can be analyzed by 8

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Journal of Luminescence 218 (2020) 116836

Fig. 7. Scatter graphs of the reduced density gradient (RDG (r)) versus Sign(λ2)ρ in different electronic states. The spikes referred to hydrogen-bond interaction are in the dotted circles.

calculating RDG isosurfaces, which will provide a systematically explanation for the relationship between ESIPT reactions and ESIHB strengths. The RDG isosurfaces of the S0 and S1 states are depicted in Fig. 6. In accordance with the color gradient scale map, the bluer iso­ surfaces are, the stronger hydrogen bonding strengths will be. The green isosurfaces represent Van der Waals interaction; the red isosurfaces represent repulsion interactions. It is clear that ESIHB isosurfaces are bluer than that of S0 state in Fig. 6, which indicates that the intra­ molecular hydrogen bonds are strengthened in S1 state. However, we find that the intramolecular hydrogen bonding strengths with different substituent positions are difficult to distinguish via the RDG isosurfaces graphs at the same electronic state. Therefore, the molecular RDG scatter graphs are drawn to solve this problem. The RDG scatter diagrams of S0 and S1 states are presented in Fig. 7. The spikes near 0.04 of the X-axis represent intramolecular hydrogen bond interaction as shown in Fig. 7 (a). The larger Sign(λ2) ρ absolute value are, the stronger intramolecular hydrogen bonding interaction will be [57]. The ascending order of hydrogen bonding strengths is AHBT-­ meta > AHBT-para > AHBO-meta > AHBO-para in S0 state. The RDG scatter diagram of S1 state is presented in Fig. 7 (b), the order of ESIHB intensities is different compared with that of hydrogen-bond intensities in S0 state. The ascending order of ESIHB strengths is AHBT-­ para > AHBO-para > AHBT-meta > AHBO-meta as shown in Fig. 7 (b), which is in agreement with the order of ESIPT reaction occurring. It follows that the ESIPT reaction is most likely to occur when the -NH2 group locates at para-position and R3 ¼ S.

difficulty order of ESIPT reactions is obtained by calculating the ESIPT electron and activation energy barriers. Moreover, the analyses of RDG isosurfaces and scatter diagrams indicate that the ESIPT reactions are more likely to occur from AHBT-para, AHBO-para, AHBT-meta to AHBO-meta. The order that ESIPT reaction occurs is in agreement with the sequence of ESIHB strengths, which further validate that ESIHB in­ teractions offer driving force for ESIPT reactions. With the timedependent DFT calculations, we find that amino group in parasubstitution has more prominent impacts on molecular photophysical properties. In addition, the para-substitution of amino group has a positive influence on the ESIPT reactions, since it brings the greater effect on ESIHB compared with meta-substitution of amino group. This research will promote the applications of amino 2-(20 -hydroxyphenyl) benzazole derivatives in fluorescence sensors, laser dyes, prodrugmediated and antineoplaston, etc. Notes The authors declare no competing financial interest. Acknowledgement This work was supported by the Scientific Research Fund of the Educational Department of Liaoning Province (Grant No. LJC201903), the High-level Innovative Talents Program of Shenyang City (Grant No. RC180230), the Youth Program of the Educational Department of Liaoning Province, China (Grant No. LQN201703).

3. Conclusion

Appendix A. Supplementary data

In this research, the AHBT and AHBO structures are fully optimized by using the M06-2X-D3/TZVP level of theory. The analyses of hydrogen-bond parameters and IR vibration frequencies confirm that the hydrogen bonding interactions are enhanced in S1 state. In addition, the calculated absorption and emission spectra data coincides with spectra values observed in experiment. The photophysical phenomena are well reproduced theoretically. To give a molecular-level insight into substituted effect on photophysical properties, the intriguing ICT char­ acters and FMOs isosurfaces are analyzed by calculating the electronhole distributions. It is found that the photophysical properties are dependent on ICT degrees with different -NH2 group positions (when R3 ¼ S or O). In order to unveil the ESIPT mechanisms, the minimum energy pathways of proton transfer are constructed in S0 and S1 states, indicating that the ESIPT reactions are only feasible in S1 state. The

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