Mechanism of Fluorescence Quenching by Acylamino Twist in the

Since the dual fluorescence of methyl salicylate was observed by Weller,(1,2) the proton transfer (PT) or more properly hydrogen transfer (HT) reactio...
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Mechanism of Fluorescence Quenching by Acylamino Twist in the Excited State for 1‑(Acylamino)anthraquinones Yanliang Zhao,†,‡ Meishan Wang,*,† Panwang Zhou,‡ Songqiu Yang,‡ Yan Liu,‡ Chuanlu Yang,† and Yunfan Yang‡ †

School of Physics and Optoelectronics Engineering, Ludong University, Yantai 264025, China State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

S Supporting Information *

ABSTRACT: Nitrogen-containing anthraquinone derivatives are widely applied in vegetable fiber dyes. In this paper, the fluorescence quenching mechanism by an acylamino group twist in the excited state for the 1-(acylamino)anthraquinones (AYAAQs) derivatives in acetonitrile is investigated by density functional theory (DFT) and timedependent density functional theory (TD-DFT) methods. The calculated Stokes shift is in good agreement with the experimental data. The energy profiles show that each AYAAQs derivative reveals a barrierless twist process, indicating that the involvement of acylamino group rotation in addition to proton transfer becomes as another important coordinate in the excited state relaxation pathway. The effects of electron-substituted group promote twist process compared with 1-aminoanthraquinone (AAQ). Then, the cross points are searched by the constructed linearly interpolated internal coordinate (LIIC) pathways for AYAAQs, demonstrating that the potential energy curves of the S1 and T2 states intersect each other and are in accord with the El-Sayed rules. So one can conclude that the acylamino group twist and following intersystem crossing (ISC) processes are important nonradiative inactivation channel for the S1 state of the AYAAQs derivatives, which is more prone to proton transfer process and can explain the low fluorescence efficiency. In addition, we have measured the phosphorescence spectra of AAQ, and on this basis, it can be predicted that the phosphorescence may occur for the AYAAQs derivatives.

1. INTRODUCTION Since the dual fluorescence of methyl salicylate was observed by Weller,1,2 the proton transfer (PT) or more properly hydrogen transfer (HT) reaction has been attracted increasing attention in experiment and theory,3−9 owing to their potential applications in optical switches,10 data storage devices,11 polymers photostabilizers,12 and LED materials.13 As a near ideal system for investigating excited-state intramolecular proton transfer (ESIPT), the 1-(acylamino)anthraquinones (AYAAQs) derivatives, with an infrequent intramolecular hydrogen bond N− H···O, are extensively concerned by researchers. As early as the 1940s, nitrogen-containing anthraquinone derivatives were synthesized and used as vegetable fiber dyes such as cotton.14 The excited-state properties of the AYAAQs derivatives were investigated by transient absorption, static fluorescence, and time-resolved fluorescence techniques by Smith and co-workers.15,16 Their investigations showed that two emission bands were attributed to the normal and proton transfer structures of the first excited singlet state (S1). The relative intensities of these two bands were related to electron-withdrawing groups and solvent polar. 1-(Heptanoylamino)anthraquinone (HPAQ) displayed mostly short wavelength emission (SWE), while 1-(trifluoroacetylamino)anthraquinone (TFAQ) only showed long wavelength emission (LWE). As for 1-(chloroacetylamino)anthraquinone (CAAQ), the main measurement in acetonitrile was SWE, whereas cyclohexane is LWE. Nagaoka and co-workers © 2018 American Chemical Society

explained the substituent and solvent effect on ESIPT of AYAAQs by nodal pattern of the wave function, and studied intramolecular proton transfer in the triplet state by transient absorption spectroscopy.17,18 The proton transfer in the S1 state of CAAQ was confirmed to be ultrafast with a time constant of 110 fs.19 Subsequently, Schmidtke et al. investigated the excited-state dynamics and intramolecular proton transfer in AYAAQs via the intermolecular solvent response by Raman direct probing of solvent response.20,21 One of their interesting findings was that CAAQ began to undergo an ultrafast ESIPT process followed by a transfer to the N* (normal structure in the S1 state) structure within about 1.1 ps, which indicated the solvent reorganizes in response to the charge distribution in S1 state. The measured solvent response showed that TFAQ underwent ballistic and complete proton transfer whereas HPAQ did not undergo proton transfer. Subsequently, Nagaoka et al. studied the ESIPT and 1O2 quenching activities of AYAAQs by the static and laser spectroscopic techniques.22,23 Their work demonstrated that the ESIPT positively and linearly correlated with 1O2 quenching activities. Recently, Ma and Zheng et al. investigated the effects of different substituted groups in intramolecular proton transfer in the S1 state of AYAAQs by the Received: November 27, 2017 Revised: January 25, 2018 Published: January 27, 2018 2864

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The Journal of Physical Chemistry A DFT/TD-DFT/B3LYP level.24,25 Among the previous experimental and theoretical studies, researches mainly focused on the characteristics of excited state fluorescence and the ESIPT reactions of the AYAAQs derivatives. However, their low fluorescence efficiency (Table 1) has not been well explained.

dye were studied by steady-state, time-resolved absorption spectroscopy, and theoretical calculations.26−29 For the AYAAQs derivatives, however, their excited-state acylamino twist and nonradiative processes have been neglected by researchers. For the sake of reasonably explaining the phenomenon of the low fluorescence efficiency for the AYAAQs derivatives, the main motivation for the present work is to investigate the nonradiative inactivation process via the acylamino twist in the relaxing pathway for the S1 state. In addition, we study the effect of electronsubstituted on the twist process by contrasting the AAQ dyes. The paper is organized as follows: The relevant details of our experimental method and ab initio calculations are summarized in sections 2 and 3. In section 4, geometry conformations, excitation characteristics, frontier molecular orbitals (FMOs), energy profiles, and cross points are presented. Finally, the main results of our current contribution are summarized in section 5.

Table 1. Fluorescence Efficiency of the AYAAQs Derivatives and the AAQ Dye Φf × 100 a

HPAQ

AAAQ

CAAQ

DCAQ

TFAQ

AAQ

1.9a

0.91b 0.81c

0.59a

0.31a

0.27a

0.94b 1.0c

From ref 16. bFrom ref 27. cFrom ref 26.

In another aspect, the radiationless processes in the S1 state and the mechanism of amino-group twist and intramolecular charge transfer (TICT) of the 1-aminoanthraquinone (AAQ)

2. EXPERIMENTAL METHODS 1-Aminoanthraquinone (AAQ) was purchased from Shanghai Aladdin Bio-Chem Technology Co. without further purification. It was dissolved in acetonitrile solvent. Absorption spectra were obtained at room temperature using a PerkinElmer Lambda 35 double-beam spectrometer. Fluorescence spectra were recorded at room temperature using a HORIBA Jobin Yvon FluoroMax-4P spectrofluorometer, with a xenon lamp and 10 mm × 10 mm quartz cuvette, and phosphorescence spectra were measured at 77K temperature. 3. COMPUTATIONAL DETAILS In the present work, all electronic structure calculations were carried out using Gaussian 0930 program suite. Six compounds (different substituted group) were chosen, including HPAQ, 1-(acetylamino)anthraquinone (AAAQ),27,31 CAAQ, 1-(dichloroacetylamino)- anthraquinone (DCAQ), TFAQ and their parent molecule AAQ. Conformation optimizatins and energies calculations in the ground and excited states were performed using the density functional theory (DFT) and the timedependent density functional theory (TD-DFT) methods, in which the long-range corrected Coulomb-attenuating functional (CAM-B3LYP/65% HF)32 was chosen in combination with the 6-31G(d,p) basis set. In order to be consistent with the experiment, the acetonitrile (ε = 35.688) polar solvent was considered. At the same time, solvation effects were included using the integral equation formalism33,34 (IEF) version of the polarizable continuum35,36 (PCM) model. The self-consistent field (SCF) convergence thresholds of energy for all optimization were performed by the default setting 10−6, and the geometry optimizations were carried out in assumed equilibrium

Figure 1. Steady-state absorption (red) and fluorescence (blue) spectra of AAQ in room-temperature acetonitrile solvent.

Figure 2. Normal and acylamino group twist conformations of the AYAAQs derivatives.

Figure 3. Normal and amino group twist conformations of the AAQ dye. 2865

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The Journal of Physical Chemistry A solvation and under Cs symmetry. There were no imaginary vibrational frequencies indicating that all optimized geometries were at local energy minima. The vertical excitation and

emission energies were separately calculated in the assumed nonequilibrium and equilibrium solvation. Linearly interpolated internal coordinate (LIIC) pathways were constructed to search the cross points. The graph of frontier molecular orbitals isosurfaces was drawn by VMD.37

Table 2. Primary Structure Parameters of the AYAAQs Derivative and the AAQ Dye in the S0 and S1 States (Dihedral: C4−C5−N1−C6 for AYAAQs; C4−C5−N1−H6 for AAQ) species HPAQ AAAQ CAAQ DCAQ TFAQ AAQ

S0(N) S1(T) S0(N) S1(T) S0(N) S1(T) S0(N) S1(T) S0(N) S1(T) S0(N) S1(T)

H2···O3 (Å)

dihedral (deg)

C5−N1 (Å)

μ (D)

1.797 − 1.813 − 1.809 − 1.835 − 1.792 − 1.877 −

−152.4 −97.9 −150.7 −98.9 −146.5 −97.6 −144.5 −93.5 −148.2 −100.7 −179.7 −104.9

1.385 1.417 1.392 1.417 1.397 1.420 1.402 1.421 1.404 1.423 1.349 1.413

3.700 2.998 3.878 2.810 6.071 2.906 6.575 2.737 4.814 2.558 2.865 1.486

4. RESULTS AND DISCUSSION 4.1. Steady-State Spectra and Geometry Conformations. The absorption and fluorescence spectra of AAQ at room-temperature in acetonitrile solvent are shown in Figure 1. A relatively wide absorption band corresponds a peak of 466 nm and is relevant to the transition of S0 → S1. The fluorescence emission peak is around 578 nm. The obviously large Stokes shift reveals that the AAQ compound undergoes a significant rearrangement upon photoexcitation. The normal and twist conformations from the S0 and S1 states of the AYAAQs derivatives (Figure 2) and the AAQ dye (Figure 3) are optimized using the CAM-B3LYP/TD-CAMB3LYP/6-31G(d,p)/PCM/acetonitrile level. There is no discussion of the proton transfer conformations (Supporting Information) in this article, owing to that the intramolecular proton transfer

Table 3. Calculated Electron Excitation Energies (Ecal exc), Oscillator Strengths ( f) of the AYAAQs Derivatives and the AAQ Dye in Acetonitrile by the TD-CAM-B3LYP/6-31G(d,p) Level species

HPAQ

AAAQ

CAAQ

DCAQ

TFAQ

AAQ

Eexp exc (eV) Ecal exc (eV) crucial stated f

3.054a 3.390 S1 0.046

3.100b 3.392 S1 0.047

3.163a 3.402 S1 0.020

3.212a 3.405 S1 0.011

3.280a 3.420 S1 0.007

2.661c 3.137 S1 0.201

a

Electronic absorption data for AYAAQs from ref 16. bElectronic absorption data for AAAQ from ref 26. cSteady-state absorption data in the present work, shown in Figure 1. dThe crucial state is defined as the lowest optically allowed excited state with relatively large oscillator strength in this work.

Figure 4. Schematic diagram of frontier molecular orbitals for the AYAAQs derivatives and the AAQ dye. The orbital wave functions are positive in the red regions and negative in the blue (L, LUMO; H, HOMO; isovalue = 0.03 au). 2866

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Table 4. Calculated Electron Emission Energies (Ecal exc), Oscillator Strengths (f), and Stokes Shift of the AYAAQs Derivatives and the AAQ Dye in Acetonitrile by the TD-CAM-B3LYP/6-31G(d,p) Level stokes shift (eV)a Eexp emi (eV)

species HPAQ AAAQ CAAQ DCAQ TFAQ AAQ

S1(N) S1(T) S1(N) S1(T) S1(N) S1(T) S1(N) S1(T) S1(N) S1(T) S1(N) S1(T)

2.340b 2.340c 2.384b ∼2.48d ∼2.53d 2.145e

Ecal emi (eV)

f

exptl

calcd

2.766 2.118 2.780 2.138 2.820 2.283 2.819 2.429 2.830 2.504 2.580 1.206

0.315 0.000 0.309 0.000 0.321 0.000 0.338 0.000 0.323 0.000 0.274 0.000

0.714

0.624

0.760

0.612

0.779

0.582

∼0.73

0.586

∼0.75

0.590

0.516

0.557

a

The left is the experimental data; the right is the calculated results. bElectronic emission data from ref 16. cElectronic emission data for AAAQ from ref 26. dEstimated values in ethanol from ref 22. eFluorescence data in the present work, shown in Figure 1.

→ LUMO (66%) as well as HOMO → LUMO (13%). The electron cloud of HOMO−3 is clearly the lone pair orbital of carbonyl, so that the electron transition from HOMO−3 to LUMO shows a n−π* feature. Furthermore, the CT properties could also be easily noticed for HOMO → LUMO due to that the electron cloud of HOMO concentrates on the acylamino group, whereas the LUMO is largely localized at the whole anthraquinone group. Similar property can be revealed for other AYAAQs derivatives (Figure 4a,b,d,e). Obviously, the transition contribution of AAQ can be presented by the single transition orbital of HOMO → LUMO, and its schematic diagram of FMOs (Figure 4f) verifies well the CT excitation type.26−28 The calculated electron emission energies (Ecal emi) of the AYAAQs derivatives and the AAQ compound are summarized in Table 4. The Ecal emi calculated by the TD-CAM-B3LYP method overestimates the corresponding experimental value;

reaction of AYAAQs has been widely investigated before.15−25 The corresponding primary structural parameters of AYAAQs and AAQ are summarized in Table 2, in which the stable normal conformations in the S0 state and the twist conformation in the S1 state are abbreviated as S0 (N) and S1 (T), respectively. The dihedral angles of C4−C5−N1−C6 (around −150°) and C4−C5−N1−H6 (around −180°) show that the optimized geometries of AYAAQs and AAQ are near planar in the S0 state, and the distance of H2···O3 (around 1.8 Å) indicates that the intramolecular hydrogen bond N1−H2···O3 of AYAAQs is relatively weak. Interestingly, there is a twist structural form for each compound in the S1 state, in which the acylamino (or amino) group is twisted and almost perpendicular to the anthraquinone plane, and has the dihedral angles C4−C5−N1−C6 (or C4−C5−N1−H6) about to −100°. Besides this, an interesting feature is found that the C5−N1 length has an increasing trend. For example, the C5−N1 bond of HPAQ increases from 1.385 Å in S0 (N) to 1.417 Å in S1 (T). The feature stems from the relatively strong donating character of the acylamino group, which induces a charge transfer localization around the acylamino portion. 4.2. Excitation Energy, Emission Energy, and FMOs. The electron vertical excitation energy and excitation characteristics of AYAAQs and AAQ are calculated by the TD-CAMB3LYP/6-31G(d,p) level, and their results are listed in Table 3. The calculated excitation energies (Ecal exc) overestimate the experimental (Eexp exc) values with the ΔE values decreasing from +0.476 eV of AAQ to +0.140 eV of TFAQ. In order to thoroughly figure out the excitation mode (local excitation (LE) or charge transfer excitation (CT)) of the AYAAQs derivatives, we calculated and analyzed the FMOs. Upon photoexcitation, the electron cloud in the AYAAQs derivatives and the AAQ compound rearrange. In general, the contributions of electron excitation are primarily derives from the FMOs transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Surprisingly, the major orbital contributions (Figure 4) of FMOs for each AYAAQs derivative are provided by multiple molecular orbital pairs, which should not be expressed only by HOMO → LUMO.25 Herein, all FMOs with a transition contribution greater than 1% are used to visualize and analyze the electron excitation characteristics. As shown in the Figure 4c, the electron transition of CAAQ is mainly contributed by HOMO−3

Figure 5. Energy profiles of the AYAAQs derivatives and the AAQ dye, along the proton transfer and the twist coordinates in the S1 state, respectively. The proton transfer processes only exist in CAAQ, DCAQ, and TFAQ in acetonitrile. The Franck−Condon, normal, proton transfer, and twist structures in the S1 state were denoted as FC, S1(N), S1(PT), and S1(T), respectively. The energy of the S1(N) and FC-structures per molecule was set to zero, respectively. 2867

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Figure 6. Energy profiles computed by the DFT/TD-DFT/CAM-B3LYP/6-31G(d,p) level, along the constructed LIIC pathway between the optimized S0(N) and S1(T) conformations: (a) AAQ; (b) HPAQ; (c) AAAQ; (d) CAAQ; (e) DCAQ; (f) TFAQ. For each compound, the energy of the S0(N) conformation was set to zero and the symbols of S0−S2 and T1−T2 corresponded to single and triplet electronic states, respectively.

calculations for the transition state conformations in the twist pathway. The electron energies of the Franck−Condon (FC), transition state (TS), and twist (S1(T)) conformations for AYAAQs and AAQ in the S1 state are shown in Figure 5, and the energy of FC structure per molecule was set to zero. One

however, the calculated Stokes shift agrees well with the experiment. 4.3. Energy Profiles. In order to confirm the existence of twist process for AYAAQs and AAQ, the TD-CAM-B3LYP functional is employed for optimization and frequency 2868

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important deactivation channel for the S1 state, which can be used to explain the fluorescence quenching. Because the phosphorescence spectra of AAQ can be observed, the similar theoretical results illustrate that there may be phosphorescence for the AYAAQs derivatives.

can see that the energy barrier of the amino group twist for AAQ is just 0.043 eV, which illustrates that the amino twist is easy to occur.28 The acylamino group twist for each AYAAQs derivative is barrierless, which can be concluded that the effects of electron-substituted group promote twist process. It is interesting to have a discussion on the possible competition between ESIPT and TICT mechanisms, so the transition state along the proton transfer pathway was performed by the same level. As shown in Figure 5, the normal, proton transfer structures in the S1 state are denoted as S1(N) and S1(PT), and the proton transfer processes only exist in CAAQ, DCAQ, and TFAQ in acetonitrile with the energy barrier of 0.059, 0.010, and 0.005 eV, respectively. Obviously, the TICT process is easier to occur which is the main deactivation process. 4.4. Cross Point. The oscillator strength in the S1 state of AYAAQs and AAQ exhibits a decreasing trend via the twist pathway (Figure S1) and tends to zero when the dihedral angle C4−C5−N1−C6 or C4−C5−N1−H6 approaches nearly −100° (Table 4). It is likely that no fluorescence can be observed via the twisting pathway so that the investigation of cross point is a must. The computed energy profiles of AYAAQs and AAQ, including the ground state (S0), the excited single (S1 and S2) and triplet (T1 and T2) states, are displayed in Figure 6, along the constructed pathway by LIIC from the S0 (N) conformation to S1(T). Similar to previous research, the potential energy curves of the S1 and T2 states for AAQ (Figure 6a) become isoenergetic nearby the fifth structure (twist angle around −140°).28 Furthermore, the phosphorescence spectra (Figure S2) of the AAQ dye are found at 77 K temperature in acetonitrile. Therefore, the DFT/TD-DFT methods and the constructed LIIC pathway are reasonable for searching the cross point of the aminoanthraquinone compound, and the same approaches are used to seek the cross points of the AYAAQs derivatives. As shown in Figure 6b−f, the energies of the S0, T2, and S2 states increase with the acylamino twist process, while the energies of the T1 and S1 states first increase and then decrease. A similar conclusion can be obtained for AYAAQs that the potential energy curves of the S1 and T2 states intersect each other. In addition, the electronic character of the S1 state is n-π*, whereas the T2 state is π−π*, which is in accord with the El-Sayed rules. Hence, the intersystem crossing (ISC) process is an important deactivation channel in the S1 state for each AYAAQs derivative, which can explain the fluorescence quenching phenomenon. Besides, it can be predicted that phosphorescence of AYAAQs may occur based upon the analogous study on AAQ.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b11675. Oscillator strength variation in twist pathway for the AYAAQs derivatives and the AAQ dye, phosphorescence spectra of the AAQ dye in 77 K temperature acetonitrile solvent, and oordinates of all geometry conformations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/fax: +86 535 6672142. E-mail: mswang1971@163. com (M.W.). ORCID

Meishan Wang: 0000-0001-8085-2657 Panwang Zhou: 0000-0002-9618-7038 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 11474142), the Natural Science Foundation of Shandong Province (Grant No. ZR2014AM 0005), and the Taishan Scholarship Project of Shandong Province (ts201511055). All calculation data were carried out in the Tiansuo Super Computer Center (TSCC) of Ludong University.

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REFERENCES

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5. CONCLUSIONS In summary, we theoretically investigate the mechanism of the acylamino-group twist and following radiationless inactivation process of the excited state for the AYAAQs derivatives. The calculated Stokes shift is in agreement with the experimental data. The results of FMOs demonstrate that the excitation mode of AYAAQs is CT excitation. The stable twist conformation in the S1 state of each AYAAQs derivative is optimized successfully, in which the acylamino is almost perpendicular to the anthraquinone plane. Electron energy profiles show that the effect of electron-substituted group promotes twist process: the acylamino twist of AYAAQs is a barrierless process, whereas the energy barrier of the amino twisting for AAQ is about 0.043 eV. Then, the ISC process from the S1 state to the T2 state for the AYAAQs derivatives is successfully demonstrated by the cross points. So, the ISC process is an 2869

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DOI: 10.1021/acs.jpca.7b11675 J. Phys. Chem. A 2018, 122, 2864−2870