Controlling Excited State Single versus Double Proton Transfer for 2,2

Article pubs.acs.org/JPCA

Controlling Excited State Single versus Double Proton Transfer for 2,2′-Bipyridyl-3,3′-diol: Solvent Effect Jinfeng Zhao, Xiaoyan Liu, and Yujun Zheng* School of Physics, Shandong University, Jinan 250100, China S Supporting Information *

ABSTRACT: In this work, we theoretically investigate the sequential excited state double proton transfer (ESDPT) mechanism of a representative intramolecular hydroxyl (OH)-type hydrogen molecule 2,2′-bipyridyl-3,3′-diol (BP(OH)2). We mainly adopt three kinds of different polar solvents (nonpolar cyclohexane (CYH), polar acetonitrile (ACN), and moderate chloroform (CHCl3)) to explore solvent effects on this system. Two intramolecular hydrogen bonds of BP(OH)2 are testified to be strengthened in the S1 state, which provides possibility for ESDPT process. Explorations of charge redistribution and potential energy surfaces (PESs) reveal ESDPT process. Searching transition state (TS) structures in different polar aprotic solvents, we successfully regulate and control the stepwise ESDPT behaviors of BP(OH)2 through solvent polarity.

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ESIPT process in the biological field, since most photoinduced mutation of DNA or RNA refers to double or multiple protons participated.20−22 Up to this stage, it brings up some fundamental issues regarding stepwise, concerted, or competitive reaction path.23−25 7-Azaindole dimer, a mainly classical system with excited state double proton transfer (ESDPT), opens up this core subject.26−28 2,2′-Bipyridyl-3,3′diol (BP(OH)2) (shown in Figure 1), as another representative case of the ESDPT process, contains two intramolecular hydrogen bonds (O1−H2···N3 and O4−H5···N6). Originally, by

INTRODUCTION It is well-known that the formation of intra- or inter- molecular hydrogen bond could be the prerequistite for occurring proton transfer reaction.1−3 In the past few years, some papers have explored excited state hydrogen bond dynamical process and some photochemical and photophysical properties associated with it, such as charge transfer (CT), fluorescence quenching, photoinduced electron transfer (PET) and so forth.4−6 Excited state inter- or intramolecular proton transfer (ESIPT), as one of the most excited state dynamic processes accompanying interesting changes of a hydrogen bond, has become a topic of much attention recently.7,8 To shed light on the mechanism of the ESIPT process, several molecules have been synthesized strategically to find out their applications as fluorescence probes,9 luminescent materials,10,11 molecular switches,12−14 UV filters, and so on.15−18 Herein, a basically brief ESIPT process has been described as follows: In general, upon the photoexcitation process, the stable ground state structure (enol) can be excited to the corresponding excited state being normal exicted-state form (enol*). In the process of ESIPT, the enol* converts into the excited photoproduct (i.e., the protontransfer phototautomer (keto*)). The keto* could be recognized by fluorescence spectrum largely shifted to longer wavelengths (normally the significant Stokes shift can be as large as 6000−12000 cm−1);1−3 based on this radiation channel coupling with radiationless paths, it relaxes to the ground-state keto tautomer. Then a fast back proton transfer process goes from keto to enol closing this four-level reaction cycle.19 In general, most ESIPT reactions are involved with single proton transfer via the existence of an intra- or inter- molecular hydrogen bond between proton acceptor (nitrogen or oxygen) and proton donor (O−H or N−H). However, such single proton transfer could be not enough to reasonably simulate the © 2017 American Chemical Society

Figure 1. Views of BP(OH)2 molecule with corresponding atom labels involved in two intramolecular hydrogen bonds. Received: February 13, 2017 Revised: May 3, 2017 Published: May 4, 2017 4002

DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008

Article

The Journal of Physical Chemistry A means of experimental approaches,29−31 BP(OH)2 was only attributed to double-proton transfer (DPT) in the S1 state. In 1997, Marks and co-workers found that the absorption and fluorescence properties of BP(OH)2 are expected to be slightly affected by changes in solvent polarity.32 Combining Hartree− Fock (HF) and configuration interaction with single excitation (CIS), Sobolewski et al. investigated the S0-state potential energy surfaces (PESs) and vertical excitation energies.33 A concerted reaction mechanism coupling with a stepwise reaction mechanism was reported by Gelabert et al. using the quantum dynamics method, which was done without considering solvent effect.34 In 2009, Lischka and co-workers, based on time-dependent density functional theory (TDDFT) and RI-CC2 approaches, reported that the double proton transfer process would be sequential in the S1 state rather than concerted.35 Whereafter, Abou-Zied studied spectroscopy properties of BP(OH)2 and examined the hydrophobic environments in several cyclodextrins (CDs), which demonstrates its chemical significances.36 Since then, more attention on BP(OH)2 has changed to its biological probes in octyl-β-Dglucoside (OBG) micelle, vesicles, different bile salt aggregates, and so forth.37−41 In fact, previous papers on BP(OH)2 do not theoretically attach importance to the different excited state processes arising from solvent effects. As a kind of positively charged particle, proton could transfer in solution, which results in a new redistribution of electron density in the studied molecule. This redistribution could not only influence these interactions with the surrounding environment, but also be controlled by surroundings. Particularly, when the ESIPT process occurs in condensed dielectric media (liquid solvents, aqueous-based biological systems, or polymer matrices), the photochemical as well as photophysical properties could be affected by dielectric polarization of surroundings.42 Because of the interactional significance between solute and solvent, different ESPT mechanisms of BP(OH)2 could be largely affected by solvents. In this work, we mainly focus on the sequential ESDPT mechanism and on the effects driving from different solvents. We adopt TDDFT and CIS methods coupling with transition state (TS) theory to investigate the stepwise ESDPT process affected by solvent in detail. A new mechanism of regulating and controlling sequential ESDPT reaction is put forward via different solvent polarities. Stronger polar aprotic solvents can facilitate the stepwise ESDPT process The remainder of this paper is organized such that the next part describes the computational methodology. Then the following section describes and discusses our calculated results organized by subsections that consider structural changes, frontier molecular orbitals (MOs) (i.e., charges redistribution), and last, potential energy surfaces. A final section summarizes and provides the conclusion of this work.

(PCM) using the integral equation formalism variant (IEFPCM),48,49 which considers the solute in a cavity of overlapping solvent (with an average area of 0.4 Å2) that has apparent changes to reproduce the electrostatic potential due to the polarized dielectric within the cavity. To avoid the effects deriving from intermolecular hydrogen bonds, we mainly consider three kinds of aprotic solvent (CYH, CHCl3, and ACN) in the present work. In fact, before designating functional, we test a number of functions, including B3LYP (the percentage of Hartree-Fork exchange is 20%),43−45 PBEPBE (25%),50 MPW1PW91 (42.8%),51 and M062X (54%),52 among which the B3LYP function provides the most satisfactory agreement with experimental results (see Table S1, Supporting Information (SI)). No constrain of bond lengths, bond angles, or dihedral angles is adopted in this work except for additional remarks, and vibrational frequency calculations are analyzed to confirm all the related structures corresponding to the local minima on the S0 and S1 PESs (no imaginary frequency). Vertical excitation energy calculations are performed from the S0-state optimized structure using the TDDFT method with six low-lying absorption transitions. In these three kinds of solvent, the corresponding PESs of both S0 and S1 states are also constructed to further illustrate the ESDPT mechanism. All the stationary points along the reaction coordinate are scanned by constraining optimizations and frequency analyses (no imaginary frequency) to obtain the thermodynamic corrections in the corresponding electronic state. In addition, the CIS/ TZVP method is used to search the TS in the S1 state, which is started from an educated guess based on placing the hydrogen atom being transferred between the hydrogen donor and acceptor and on adjusting the bond lengths affected by the proton transfer. The Hessian calculations are carried out at the guessed TS structure to obtain the force constants. By the Berny optimization method,53 our calculated TS is confirmed to be only one imaginary frequency, and its vibrational eigenvector points to the correct direction. Zero-point energy corrections and thermal corrections to the Gibbs free energy are also performed according to the harmonic vibrational frequencies.

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RESULTS AND DISCUSSION Analysis of Structures. The structures of NT (normal BP(OH)2), SPT (single-proton transfer BP(OH)2) and DPT (dual-proton transfer BP(OH)2) (shown in Figure 1) are obtained within the framework of DFT and TDDFT methods as mentioned above, with a subsequent vibrational frequency analysis to ensure the validity of the stationary points. Since the differences between polar solvent and nonpolar solvent are obvious, herein, we mainly discuss our theoretical results in polar ACN and nonpolar CYH solvents. Some important parameters involved in these two intramolecular hydrogen bonds (O1−H2···N3 and O4−H5···N6) are listed in Table 1. For the NT form, it is noteworthy that our theoretical bond distances of O1−H2 and O4−H5 are lengthened, and those of H2···N3 and H5···N6 are shortened in the S1 state with the concomitant enlargement of bond angles δ(O1−H2−N3) and δ(O4−H5−N6) in both ACN and CYH solvents. That is to say, both intramolecular hydrogen bonds are strengthened in the S1 state.54−59 Differently, in ACN solvent, hydrogen bonds H2··· N3 and H5···N6 are shorter than those in CYC solvent, which not only illustrates that surrounding solvents have effects on

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COMPUTATIONAL DETAILS In this work, all the numerical simulations presented are accomplished based on the density functional theory (DFT) and TDDFT methods with Becke’s three-parameter hybrid exchange function with the Lee−Yang−Parr gradient-corrected correlation functional (B3LYP)43−45 in combination with the triple-ζ valence quality with one set of polarization functions (TZVP)46 basis set by the Gaussian 09 program.47 Cyclohexane (CYH), tetrachloromethane (CCl4), toluene, benzene, chloroform (CHCl3), acetone, acetonitrile (ACN) and dimethyl sulfoxide (DMSO) (according to the solvent polarity from low to high) are selected based on the polarizable continuum model 4003

DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008

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The Journal of Physical Chemistry A Table 1. Primary Bond Lengths (Å) and Bond Angles δ (deg) of NT, SPT, and DPT Structures in the ACN and CYH Solvents NT

ACN

CYH

O1−H2 H2−N3 O4−H5 H5−N6 δ(O1− H2N3) δ(O4− H5N6) O1−H2 H2−N3 O4−H5 H5−N6 δ(O1− H2− N3) δ(O4− H5− N6)

SPT

on the approach, i.e., subtracting the energy of NT-Open form from that of NT form. In fact, this method for calculating hydrogen bond energy is an intrinsic assumption of ignoring the effects of the aforementioned rotations on the overall geometry parameters of the concerned molecule other than the simple cleavage of the hydrogen bond from the contiguous intramolecular hydrogen bond circuit.60 Results show that the S1-state energy of these two hydrogen bonds are 29.19 kcal/ mol in CYH and 30.51 kcal/mol in ACN, which further demonstrates that hydrogen bonds (O1−H2···N3 and O4−H5··· N6) in ACN solvent are stronger. MOs and Orbital Contributions. The six low-lying absorbing transitions and fluorescence of NT chromophore for a reasonable comparison with previous experiment, which are also calculated starting from the optimized NT structure in the S0 state, as determined by the TDDFT method in ACN and CYH solvents. As shown in Table S1, SI, our calculated absorption and fluorescence peaks (332 and 499 nm) of NT form in ACN solvent are in agreement with previous experimental results (341 and 493 nm),39 respectively. Also, in CYH solvent, our theoretical results (335 and 490 nm) are in line with experimental results (340 and 498 nm).35,61 This also confirms the previous statement that electronic spectra of BP(OH)2 are slightly affected by solvents.32 These results demonstrate that the theoretical method we adopted is suitable for the BP(OH)2 system. As it is well-known that charge changes deriving from photoexcitation could reasonably depict the nature of electronically excited state behavior, our calculated frontier molecular orbitals (MOs) of NT structure are displayed in Figure 3. In

DPT

S0

S1

S0

S1

S0

S1

1.006 1.666 1.006 1.666 149.1

1.045 1.547 1.045 1.547 152.0

1.582 1.065 1.013 1.644 145.8

1.832 1.023 1.017 1.619 136.9

1.653 1.052 1.653 1.052 143.9

1.763 1.030 1.763 1.030 139.0

149.1

152.0

149.8

149.7

143.9

139.0

1.003 1.676 1.003 1.676 148.8

1.040 1.563 1.040 1.563 151.7

1.498 1.090 1.011 1.672 148.3

1.806 1.027 0.999 1.706 139.3

1.627 1.058 1.627 1.058 144.6

1.740 1.035 1.740 1.035 140.0

148.8

151.7

149.3

147.5

144.6

140.0

target molecule, but also suggests that hydrogen bonds could be stronger in polar ACN solvent. Furthermore, monitoring the infrared (IR) vibrational spectral shift,4,5 as another effective way to verify the changes in an excited state hydrogen bond, is also adopted in this work. In Figure 2, we show that vibrational spectra of NT form in the

Figure 3. View of HOMO and LUMO for the normal BP(OH)2 form in both ACN (left) and CYH (right) solvents. Herein, transition energies are also listed in ACN, CHCl3, and CYH solvents.

this figure, only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown, since these two orbitals are mainly related to S1 state. It can be clearly seen that the HOMO orbital is π character and the LUMO orbital is π* character, thus the dominant ππ*-type transition could be assigned with the composition (97.72% for ACN and 97.94% for CYH) from HOMO to LUMO. Moreover, it is worth mentioning that the charge densities of hydroxy moieties (O1−H2 and O4−H5) decrease, whereas those of proton acceptors (N3 and N6 atoms) increase through the transition (HOMO to LUMO). In addition, it can not deny that the charge distributions in these two solvents are alike qualitatively, while the quantificational contributions of proton donor and proton acceptor are different. In ACN solvent, the contribution of both O1 and O3 atoms drops from 20.52% (HOMO) to 1.47% (LUMO), while that of N4 and N6 adds

Figure 2. Theoretical IR spectra of the normal BP(OH)2 structure in the spectral region of the O−H stretching bonds in both S0 and S1 states. The inset provides the amplifying IR peak.

conjunct vibrational region of both O−H stretching modes. It is worth mentioning that large red shifts from S0 state to S1 state can be found in both ACN and CYH solvents, which is ascribed to the effect of excited-state hydrogen bonds (O1−H2···N3 and O4−H5···N6). Also, analysis of IR spectra further confirms the strengthened hydrogen bonds in the S1 state. Also, we twist both hydroxyl moieties of NT to around 180° forming NT-Open and optimize the NT-Open structure confirming it is stable with the rotation dihedral of twisting hydroxyl moieties really close to 180°. In addition, no significant changes exist other than two hydroxyl moieties. The excited state hydrogen bond energies are calculated based 4004

DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008

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The Journal of Physical Chemistry A

Figure 4. PESs on both S0 and S1 states as functions of O1−H2 and O4−H5 lengths for all the relative structures. (a) The S1-state PES in ACN; (b) the S0-state PES in ACN; (c) the S1-state PES in CHCl3; (d) the S0-state PES in CHCl3; (e) the S1-state PES in CYH; (f) the S0-state in CYH. Herein, all the projective planes of the S0-state PESs are shown on the undersurface.

kinetic stability.62 Herein, even though the differences of orbital contributions and frontier orbital gap are not obvious in both CYH and ACN solvent, results stemming from different solvent polarities do have important effects on excited state dynamical behaviors, which we will discuss in the following section. Mechanism Analysis. Based on the discussions above, it is an undeniable fact that solvent effects affect the excited state properties of BP(OH)2 system. To further reveal the detailed and different ESIPT mechanisms brought by different solvents, all the S0 and S1 state geometrical structures with fixed O1−H2 and O4−H5 bond distances are optimized using the IEF-PCM model (ACN, CYH, and CHCl3). The PESs in these three kinds of solvents are shown in Figure 4, which are shown as functions of O1−H2 and O4−H5 bonds from 0.9 to 2.1 Å in steps of 0.1 Å. The reason this range was chosen is that all the

from 0.77% to 23.09%. Similarly, in CYH solvent, contribution of O1 and O3 atoms also decreases from 21.26% (HOMO) to 0.78% (LUMO) and that of N4 and N6 increases from 1.49% (HOMO) to 23.04% (LUMO). The coherence of the changing trend is undeniable, while the specific distribution is different, which derives from different solvent surroundings. From the viewpoint of valence bond theory, the interactions between the lone pair N3 and N6 atoms and the corresponding σ* orbital for O1−H2 and O4−H5 are mainly responsible for the ESPT from oxygen atoms to nitrogen atoms. That is to say, the intramolecular charge transfer process provides the possibility for the ESIPT process. The energy gap between HOMO and LUMO reflects the biological activity, chemical hardnesssoftness, and optical polarizability of molecule; that is to say, frontier orbital gap is associated with chemical reactivity and 4005

DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008

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The Journal of Physical Chemistry A NT, SPT, and DPT structures could be included in it. In fact, even though the precise values and details of surfaces may not be completely reproduced by DFT and TDDFT methods, some previous calculations have shown that this method is reliable to provide qualitatively or even semiquantitatively accurate pathways for discussing the proton transfer process.63,64 That is to say, the main trends of changes could be provided by TDDFT. It can be found clearly that almost no changes exist in S0-state PESs for these three solvents in Figure 4; however, obvious changes could be noticed in S1-state PESs. Clearly, the ESIPT process could occur in the S1 state, which is consistent with the conclusion that the ESPT process can result due to the strengthening of O1−H2···N3 and O4−H5··· N6 in the excited state.54−59 Herein, we mainly focus on the stepwise path. For the convenience of descriptions, we separate the projective plane in these three solvents and show the S1-state contour map of ACN solvent in Figure 5. Herein, four stable

structures can be found along with stepwise ESDPT path in our calculations, which is consistent with previous conclusion that the ESDPT mechanism of BP(OH)2 is sequential rather than concerted in the S1 state.35 Furthermore, we do not find TS structures along with concerted path in these eight kinds of solvents. Nevertheless, we calculate the potential energy curves along the synchronous O1−H2 and O4−H5 bond length fixed from 1.0 to 1.34 Å in steps of 0.02 Å (see Figure S2, SI). Comparing with the barriers of TS1 structures for BP(OH)2 in the S1 state, the ESDPT reaction is likely to occur along with the stepwise process. In addition, theoretical results show that the O−H length of all the TS1 structures are around 1.14 Å, while the O−H length of maximum points along with the concerted path are around 1.28 Å in all the solvents mentioned above. That is to say, comparing with the longer bond length (1.28 Å), the ESPT reaction is more likely to happen via the path of shorter O−H length (1.14 Å). Accordingly, in this work, we are mainly concerned with the stepwise path. That is to say, the stepwise ESDPT path (A-B-D) of the BP(OH)2 system depends on solvent polarity, and stronger polar aprotic solvent promotes the sequential ESDPT process. Therefore, a new mechanism is provided in which the excited state behavior between single proton transfer and double proton transfer could be regulated and controlled via different solvent polarities for the BP(OH)2 system.

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CONCLUSIONS In this work, we theoretically investigate the stepwise ESDPT process of BP(OH)2 controlling by solvent polarity. Three kinds of representative solvents (CYH, CHCl3, and ACN), according to the polarity order from low to high, are mainly considered described by the IEFPCM solvent module. Our theoretical results demonstrate that different solvent surroundings do bring different influences for excited state hydrogen bond dynamical process, which could be confirmed in the aspect of bond lengths, bond angles, and energies of hydrogen bonds. Charge redistribution further shows that the increased charge densities on proton acceptor moiety could promote the ESDPT process, and different solvent environments have various effects on charge transfer tendency. The S1-state PESs in these three solvents affirm this viewpoint once again. Finally, combining CIS and TS theory, we conclude that strong polar aprotic solvent could facilitate the stepwise ESDPT process for the BP(OH)2 system. As far as we know, the studies on solvent controlling the ESDPT process are limited theoretically. This work successfully presents the stepwise ESDPT mechanism of BP(OH)2 and supplements the shortages resulting from different solvents in previous work. Even though Marks and co-workers mentioned that the excited state behavior of BP(OH)2 might not be influenced by solvent properties,32 their experimental results are outdated more or less. We hope that this study will encourage additional experimental work to re-examine or refute our results.

Figure 5. S1-state projective plane with four stable points (A, B, C, and D). The energy between every contour is 0.9 kcal/mol. Herein, transition states (TS1 and TS2) along with the stepwise path (A-B-D) are marked.

points (A corresponding to NT; B and C corresponding to SPT; D corresponding to DPT) are found; in fact, the PES is symmetrical along a diagonal line (i.e., point B equal to point C). Since the three projective plane are almost the same, we only provide one figure. In order to show the effects resulting from solvent polarity, herein we compare the potential energy barriers of stepwise and concerted ESDPT in eight kinds of solvents (CYH, CCl4, toluene, benzene, chloroform, acetone, ACN, and DMSO). To obtain more accurate potential energy barriers, the CIS/TZVP method is used to search the TS structures on the S1-state PESs, and all the TS structures are shown in Figure S1, SI. Within the framework of the CIS/TZVP method, we calculate all the S1-state NT, SPT, and DPT configurations in eight kinds of solvents to ensure the consistency of theoretical level with TS structures. By the Berny optimization method,53 our calculated TS is confirmed to be only one imaginary frequency, and its vibrational eigenvector points to the correct direction. Zero-point energy corrections are also performed according to the harmonic vibrational frequencies. All the potential energy barriers in eight kinds of solvents (solvent polarity from low to high) are shown in Table S2, SI. The corresponding imaginary frequencies for TS structures are listed in Table S3, SI. Obviously, potential energy barriers decrease from point A to point B and from point B to point D with the increase of solvent polarity, respectively. In fact, only TS1 and TS2

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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.7b01404. The comparisons of different functionals for BP(OH)2 in ACN, CHCl3, and CYH solvents (Table S1); values of potential barriers for BP(OH)2 along with stepwise ESDPT path in eight kinds of solvents (Table S2); the 4006

DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008

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The Journal of Physical Chemistry A

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imaginary frequencies for TS1 and TS2 structures (Table S3) and corresponding TS1 and TS2 structures structures (Figure S1); and potential energy curves of BP(OH)2 in concerted ESDPT path (Figure S2) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yujun Zheng: 0000-0001-9974-2917 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (No. 11374191 and 11604333) and the National Basic Research Program of China (No. 2015CB921004).

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DOI: 10.1021/acs.jpca.7b01404 J. Phys. Chem. A 2017, 121, 4002−4008