Identifying the Role of Intramolecular Charge Transfer and Excited

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Identifying the Role of Intramolecular Charge Transfer and Excited-State Proton Transfer in Fluorescence Mechanism for an Azido-Based Chemosensor Ningjiu Zhao, Yang Li, Yan Jia, and Peng Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06185 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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The Journal of Physical Chemistry

Identifying the Role of Intramolecular Charge Transfer and Excited-State Proton Transfer in Fluorescence Mechanism for an Azido-Based Chemosensor Ningjiu Zhao,*a Yang Li,a,b Yan Jia,a Peng Lia a

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, P. R. China b University of the Chinese Academy of Sciences, Beijing 100049, P. R. China

Corresponding Email: [email protected]

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ABSTRCT In

the

present

work,

the

fluorescence

enhancement

mechanism

of

H2S

chemosensor

2-(2-azidophenyl)-1-(p-tolyl)-1H-phenanthro[9,10-d]imidazole (P-N3) has been investigated in detail by the density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The excitation/emission energies which were calculated using TDDFT method can simulate the experimental spectra well. The potential energy barrier of ca. 10 kcal/mol for the forward excited-state intramolecular proton transfer (ESIPT) and that of 8000 cm-1) and linear concentration range with good sensitivity and selectivity.23 It was proposed that the nature of the fluorescence-enhancement is based on the excited-state intramolecular proton transfer (ESIPT). Specifically, owing to the reducing ability of H2S, the reduction of the azido group to an amino could turn on the ESIPT, which contributed to restoring the fluorescence properties. However, it should be noted that, despite the dramatic fluorescence enhancement for the sensing product with respect to the chemosensor, their shapes of the fluorescence spectra are highly alike, and



the maximum emission wavelengths in the experiments are also in the quite close range. On the other hand, the dual fluorescence known as a typical feature for the ESIPT is also not observed in the experiments for P-NH2. Therefore, the fluorescence properties for this sensing system are expected to be not dominated by the ESIPT. In addition, the previously proposed mechanism lacks theoretical support and, the experimental exploration cannot provide straightforward information on the photophysical properties of the mentioned fluorescence probe. Aiming to confirm this assumption, we conduct the DFT/TDDFT calculation to study the ground and excited states of these relevant molecules. Because the theoretical calculation based on DFT/TDDFT method can provide the direct excited-state information and molecule configuration, it has been widely used to explore the sensing mechanism for many chemosensors.25-30 Herein, the vertical excitation/emission energies, corresponding oscillator strengths and transition composition and frontier molecular orbitals were presented and analyzed in detail for those molecules involved in this sensing system. The proton transfer process in the ground and the first singlet excited states were also examined. Taken the experimental result in mind, our theoretical calculation supports our assumption that the strengthened fluorescence is not correlated with the ESIPT. Instead, a new fluorescence-enhancement mechanism for this azido-based chemosensor has been unraveled.

COMPUTATIONAL METHODS In this study, all DFT/TDDFT electronic structure and geometry calculations were accomplished by Gaussian program,31 including those of the ground state and the excited state.32-40 For seeking the most proper calculation method, numbers of functionals including B3LYP, wB97XD, M06-2X, and


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CAM-B3LYP were compared. Due to the most satisfactory agreement between the theoretical results at PBE0 and the experimental ones, this functional has been used to investigate the characteristics of the ground and excited states. Besides, In order to select proper basis set, 6-311G(d) and 6-311+G(d) were employed to make comparisons. Due to the similar results calculated under the level of 6-311G(d) and 6-311+G(d) and for saving time, the 6-311G(d) was chosen as the basis set in all calculations. Taken that the experiments were conducted in aqueous state into account, solvent effects were included in all calculations by employing the integral equation formalism version of the polarizable continuum model (IEF-PCM) with the dielectric constant of water (e = 78.4).41-43 Both line-response (LR) and corrected LR (cLR) solvation models were used to calculated vertical emission energies as well as the energy barrier of proton transferring, of which the cLR solvation model was carried out in Gaussian16.31 In order to compare the calculation results derived from Gaussian16 and Gaussian09 in a same accuracy in the two-electron integrals and a same grid, a keyword int=(fine, acc2e=10) was used in Gaussian16. In addition, the electronic structure calculations were completed without constraint, and vibrational frequency analyses were performed to confirm that the optimized structures corresponded to the local minima as well. The Dr index, natural transition orbital (NTO) and electron density difference (EDD) were derived from Gaussian output files and were analyzed by using Multiwfn, in which IOp(9/40=4) keyword was used to get more configuration coefficients.44-46 All calculations were established in Gaussian09 unless otherwise stated.31

RESULTS AND DISCUSSION Excited-state intramolecular proton transfer. The ESIPT phenomenon has been popularly investigated in previous literatures.47-56 In the previous report, the ESIPT was thought to modulate the



fluorescence off-on response for P-N3.23 That is to say, the ESIPT can occur after the azide was reduced to amine for P-N3. To unravel more features of the ESIPT process, the potential energy curves in S1 and S0 states as a function of the N50-H51 bond length in P-NH2 and P-NH2-PT were constructed (see Figure 1). It provides qualitative energetic pathways for the ESIPT process of P-NH2. In the ground state, there is no local minimum on the potential energy curve besides the one with bond length of N50-H51 at around 1.01 angstrom. Therefore, the PT process of P-NH2 cannot take place in the ground state. In the S1 state, however, one local minimum is found at bond length of N50-H51 with 1.83 angstrom. Moreover, the potential energy barrier of 11.05 kcal/mol for the forward ESIPT of P-NH2 is determined, while the potential energy curve exhibits a barrier of 2.29 kcal/mol for the reversed ESIPT, which is much smaller than that for the forward one. To verify the impact of diffusion function, a 6-311+g(d) basis set has been used in the calculation of the excited state potential energy surface (PES) (Figure S1). It can be found that for this system the diffusion function has no important impact on the excited state PES (11.11 vs. 11.05 kcal/mol, 2.92 vs. 2.29 kcal/mol). Besides, in consideration that the calculation at the level of 6-311+g(d) will consume much time, the basis set of 6-311g(d) was used in this study. For the purpose of determining the precise energy barrier in the proton transfer reaction, the energies with/without ZPE of the reactant, product and their transition state (TS) are also given. Without considering the ZPE correction the energy of the TS is minimally higher (by ca. 2.31 kcal/mol) than that of P-NH2-PT, whereas it is prominently higher (by ca. 11.08 kcal/mol) than that of P-NH2. However, in the case of taking the ZPE into account the energy barrier between P-NH2-PT and TS in the first singlet excited state vanishes, and the TS is 0.64 kcal/mol below P-NH2-PT (Figure 1b). This


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phenomenon is common and it means a very small energy barrier.57,58 On the other hand, the TS is 7.91 kcal/mol above P-NH2. According to the above results, one can conclude that the ESIPT reaction is impossible in the present case.

Absorbance/emission spectra and molecular orbital analysis. The calculated vertical excitation energies, the corresponding oscillator strengths and CI values at the TDDFT/PBE0/6-311G(d) level have been listed in Table 1. It can be seen that the theoretical results predict a S0®S1 transition at 317 nm for P-N3 with the oscillator strength of 0.1054. The S0®S3 transition is at 298 nm with the oscillator strength of 0.1609. At the meanwhile, for the sensing product P-NH2, a S0®S1 transition is found at 336 nm accompanied with the oscillator strength of 0.3565. Another nearby transition (S0®S2, 322 nm) with the oscillator strength of 0.1542 was also observed. In addition, other electronic transitions were permitted for both P-N3 and P-NH2 as well. Specifically, in terms of P-N3, the transitions of S0®S7, S0®S9, S0®S11, S0®S12 are located at the range of 262-251 nm with the oscillator strengths of 0.2447-0.2898, while for P-NH2 the excitation energies of 260 nm are dominated by the transitions of S0®S9, S0®S10 and S0®S12. Compared with the main peak and the shoulder peak of the absorption spectra in the experiments, it should be noted that the calculated absorption spectra (Figure S1) are in well agreement with the experimental results. Due to the absence of the local minimum for the ground-state PT tautomer, the excitation energy of P-NH2-PT was not shown. Aiming to investigate the excitation mode for P-N3, P-NH2 and P-NH2-PT, we likewise make



the determination of characteristics of the molecular orbitals (MOs) involved in the electronic transitions. As listed in Table 1, both S0®S1 transitions of P-N3 and P-NH2 are mainly contributed by HOMO®LUMO with the CI values of 0.72 and 0.90. To further verify the reliability of analyzing the excitation mode by MOs, the NTO was also figured out (Figure S3).59 With regard to P-N3, NTO111-->NTO112 transition has a dominant contribution (87.25%) to the first excitation transition (S0®S1), while for P-NH2 the S0®S1 transition predominantly consists of the pair of NTO105®NTO106 with 93.14% contribution. Compared to the patterns of HOMO and LUMO (Figure 2, left panel), it can be found that the NTO111 is very similar to the HOMO and the NTO112 also has the analogous pattern to the LUMO for P-N3. Similarly, the patterns of HOMO®LUMO and NTO105®NTO106 exhibit a high degree of similarity for P-NH2. Due to the distinct p character of HOMO (or NTO111) and p* character of LUMO (or NTO112), the S0®S1 transitions of P-N3 shows a pp* feature. Similarly, for P-NH2 the S0®S1 transitions of P-N3 is subject to a pp* feature as well. To quantify the intensity of ICT, an Dr index that has been previously proposed is used.60 In S0 geometry, the Dr indexes are 1.157 and 2.300 Å for P-N3 and P-NH2, respectively. The large value of Dr of P-NH2 than that of P-N3 indicates that P-NH2 has a more intense ICT character than P-N3 does in terms of S0®S1 transtion. Owing to the small values of the Dr indexes for P-N3 and P-NH2, their S0®S1 transtions are more likely ascribed to the local excitation (LE). To obtain the properties of the first singlet excited states of P-N3, P-NH2 and P-NH2-PT, the qualitative MO representations of HOMO and LUMO and the emission energies are also represented in Table 1 and Figure 2,3 (right panels). For P-N3 and its product P-NH2, the emission energies were


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determined to be 410 nm and 416 nm, corresponding to the HOMO¬LUMO transition with the CI values of 0.99 and 0.98, respectively. The results are in good accordance with the experimental value (~435 nm). The respective Stokes-shift in experiment and theory is 1.092 eV and 0.887 eV, in which the latter agrees approximately with the former. Furthermore, the slightly red-shifted emission for P-NH2 compared with that of P-N3 are also the same as the experimental spectra. However, the calculated emission energy of P-NH2-PT is largely shifted to 521 nm, which obviously does not coincide with the experimental phenomenon. The cLR solvation model was also employed to determine the fluorescence energies of P-N3, P-NH2 and P-NH2-PT. The corresponding fluorescence energies are 3.176 eV (ca. 390 nm), 3.122 eV (ca. 397 nm) and 2.487 eV (ca. 499 nm), respectively. Besides, in order to verify whether the ESIPT process can be affected by the cLR effect, the S1 PES with cLR solvation model was also constructed (Figure S9). It is found that the energy barriers of forward ESIPT and back one are nearly identical to the result derived from the LR solvation model, i.e. 10.35 vs. 11.05 kcal/mol and 4.22 vs. 2.29 kcal/mol. Herein, we note that, regardless of how the energy barrier of ESIPT is obtained (LR or cLR), the ESIPT reaction cannot occur. As a consequence, it can be concluded that P-NH2-PT would not be the dominant emission species. In addition, all the S0¬S1 transitions in S1 geometry for P-N3, P-NH2 and P-NH2-PT as shown in Figure 2 and 3 (right panels), predominantly exhibit a pp* feature. The respective Dr index is 1.794, 0.910 and 2.657 Å for P-N3, P-NH2 and P-NH2-PT. More importantly, in order to quantitatively make comparison of the charge transfer character between the first singlet excited state and ground state, the electron density difference is used for P-N3, P-NH2 and P-NH2-PT in S0 and S1 geometries.61 As shown in Figure 4a,b, the green and blue regions



represent the increase and decrease in electron density upon photoexcitation in S0 geometry. In order to further quantify the charge transfer character, C+ and C- functions were also plotted (Figure 4c,d). It is evident that P-N3 has a smaller distance (1.442 vs. 2.057 Å) between the barycenters of positive and negative parts than P-NH2 does, which shows that P-NH2 possesses a larger extent of ICT character in S0 geometry, corresponding to the S0®S1 transition.

In the case of S1 geometry, corresponding to the S0¬S1 transition (Figure 5), the amount and direction of ICT have changed from P-N3 to P-NH2. Relative to P-N3 and P-NH2-PT, P-NH2 has the smallest distance between the barycenters of positive and negative parts, indicating a smaller extent of ICT character. When compared with the case of S0 geometry, it can be found that the ICT character of P-NH2 varies as the geometry transforms from S0 geometry to S1 geometry. On the other hand, it is the same case for P-N3, showing a large extent of variation on ICT feature from S0 geometry to S1 geometry as well. Bearing this in mind, we turn to analyzing the large Stokes-shift between the absorption and emission bands. The nature of large Stokes-shift. It is well known that the Stokes-shift is generally the resultant of the energy loses and any process which consumes energy will lead to the presence of Stokes-shift. Herein, it is noted that the dihedral angle of ÐN49-C23-C39-C41 (or ÐN49-C23-C39-C40) of P-N3 becomes more planar in the excited state than in the ground state (Table 2). For P-NH2, the changes of dihedral angle of ÐN49-C23-C39-C40 (or ÐN49-C23-C39-C41) also indicate the rearrangement of molecular structure. As both P-N3 and P-NH2 have multiple structural degrees of freedom, we


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tentatively employ the intensity change of ICT between S0 and S1 to characterize their overall geometry change upon photoexcitation since the movement of the atomic nucleus follows that of electron flow. As mentioned above, it should be noted that whether for P-N3 or P-NH2 a large change of the ICT extents between the ground state geometry and the first singlet excited state geometry occurs, which indicates that both P-N3 and P-NH2 undergo a significant geometry rearrangement upon photoexcitation. Consequently, it is not surprising to observe that the large Stokes-shifts for P-N3 and P-NH2 take place. In addition, it also should be noted that the maximum absorption peak does not correspond to the S0®S1 transition. Therefore, according to the Kasha’s rule, the nonradiative transition between Sn (n>1) and S1 will also consume the excitation energy, which is another origin of the large Stokes-shift observed here. Fluorescence mechanism. We now discuss the fluorescence-enhancement mechanism for P-N3 sensing H2S in the case of excitation at l=300 nm as used in the experiment.23 It is well known that ESIPT can probably lead to the dual fluorescence and large Stokes-shift.62-64 Previously, the ESIPT process has been proposed to be the origin of the large Stokes-shift and the fluorescence-enhancement for the azido-based chemosensor P-N3. However, it should be noted that, despite the dramatic fluorescence intensity enhancement for P-NH2 with respect to P-N3, the shapes of the fluorescence spectra are highly alike, and the maximum emission wavelengths regardless of observing in the experiment or in the calculation are also located in the quite close range. In addition, the dual fluorescence is also not observed in the experiments for P-NH2. Based on the potential energy curve shown in Figure 1, for P-NH2, it is likely that the ESIPT could take place with an energy barrier of 11.05 kcal/mol. However, the reversed ESIPT of P-NH2-PT possesses a much lower potential energy



barrier with 2.29 kcal/mol than the forward one does. That is to say, the ESIPT product (P-NH2-PT) is not the major species in this system. Instead, P-NH2 is assumed to be the dominant product. Besides, the fluorescence feature of P-NH2-PT is theoretically determined to be at ~520 nm. However, after sensing H2S the fluorescence of the system shows no distinct emission feature around this wavelength.23 Taken together, it can be derived that the experimentally observed Stokes-shift and fluorescence-enhancement cannot be attributed to the happening of ESIPT. On the other hand, it is noticed that the emission process of P-N3 is dominated by the HOMO¬LUMO transition (Table 1), which is a locally excitation over the whole molecule except the benzyl moiety (Figure 2). As a consequence, the fluorescence quenching owing to the electron transfer from the electron-rich

a-nitrogen in azido group to other moiety could also be excluded. We finally make a discussion on the effect of unit twisting on the fluorescence quenching. By comparing the geometries of P-N3 in the ground state and the first singlet excited state, it can be found that the dihedral angle of ÐN49-C23-C39-C41 (or ÐN49-C23-C39-C40) varies most significantly than other twist angles. Thus the dihedral angle of ÐN49-C23-C39-C41 was expected to be a key structural factor which can affect the fluorescence property of P-N3. To substantiate the proposed suppose, the potential energy surface along the dihedral angle of ÐN49-C23-C39-C41 was constructed. As shown in Figure S7, it can be found that the optimized P-N3 in S1 (ca. -24°) can easily transform to tautomer1 (ca. -150°) and tautomer2 (ca. 24°) with quite small energy barriers (6.42 and 3.72 kcal/mol). Furthermore, the energy profiles of S0, S1, T1, T2 and T3 in P-N3 were investigated to verify whether the cross point between the potential energy surfaces of the involved excited states is present, along the constructed pathway by linear interpolations of internal coordinates


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(LIIC) from tautomer1 to tautomer2 (Figure 6). It can be seen that there exist a cross point for the potential energy curves of S1 and T3 nearby the last structure, which suggests that the intersystem crossing (ISC) is an significant deactivation pathway in S1 state for P-N3. As a result, the competition between ISC and fluorescing can explain the weak fluorescence of P-N3. In contrast, an energy barrier of >17 kcal/mol demonstrated that the rotation of aminophenyl is energetically unfavorable for P-NH2 in S1 state (Figure S8). Similarly, the amino group cannot rotate due to the relatively large energy barrier as well (Figure S4). Thus, without the twisting there is no ISC channel competing with the fluorescence for P-NH2, leading to the emission enhancement.

CONCLUSSION In summary, a new insight on the fluorescence-enhancement mechanism for an azido-based chemosensor has been determined by DFT/TDDFT methods. The theoretical investigation suggests a different fluorescence-enhancement mechanism for the H2S sensor from that proposed in the experiment. The UV-vis absorption spectra and fluorescence spectra are well simulated by the vertical excitation/emission energies calculated from the optimized geometries of S0 and S1 states for P-N3 and its product (P-NH2). The potential energy barriers in the forward ESIPT and the reversed one of P-NH2 are determined to be ca. 10 kcal/mol and

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Identifying the Role of Intramolecular Charge Transfer and Excited

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