Article Cite This: Acc. Chem. Res. 2018, 51, 1681−1690
pubs.acs.org/accounts
Unraveling the Detailed Mechanism of Excited-State Proton Transfer Panwang Zhou and Keli Han*
Downloaded via UNIV OF READING on July 17, 2018 at 04:48:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, Liaoning, China CONSPECTUS: As one of the most fundamental processes, excited-state proton transfer (ESPT) plays a major role in both chemical and biological systems. In the past several decades, experimental and theoretical studies on ESPT systems have attracted considerable attention because of their tremendous potential in fluorescent probes, biological imaging, white-lightemitting materials, and organic optoelectronic materials. ESPT is related to fluorescence properties and usually occurs on an ultrafast time scale at or below 100 fs. Consequently, steady-state and femtosecond time-resolved absorption, fluorescence, and vibrational spectra have been used to explore the mechanism of ESPT. However, based on previous experimental studies, direct information, such as transition state geometries, energy barrier, and potential energy surface (PES) of the ESPT reaction, is difficult to obtain. These data are important for unravelling the detailed mechanism of ESPT reaction and can be obtained from state-of-the-art ab initio excited-state calculations. In recent years, an increasing number of experimental and theoretical studies on the detailed mechanism of ESPT systems have led to tremendous progress. This Account presents the recent advances in theoretical studies, mainly those from our group. We focus on the cases where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of excited-state double proton transfer (ESDPT), revealing the sensing mechanism of ESPT chemosensors, illustrating the effect of intermolecular hydrogen bonding on the excited-state intramolecular proton transfer (ESIPT) reaction, investigating the fluorescence quenching mechanism of ESPT systems by twisting process, and determining the size of the solute·(solvent)n cluster for the solvent-assisted ESPT reaction. Through calculation of vertical excitation energies, optimization of excited-state geometries, and construction of PES of the ESPT reactions, we provide modifications to experimentally proposed mechanisms or completely new mechanism. Our proposed new and inspirational mechanisms based on theoretical studies can successfully explain the previous experimental results; some of the mechanisms have been further confirmed by experimental studies and provided guidance for researchers to design new ESPT chemosensors. Determination of the energy barrier from an accurate PES is the key to explore the ESPT mechanism with theoretical methods. This approach becomes complicated when the charge transfer state is involved for time-dependent density functional theory (TDDFT) method and optimally tuned range-separated TDDFT provides an alternative way. To unveil the driving force of ESPT reaction, the excited-state molecular dynamics combined with the intrinsic reaction coordinate calculations can be employed. These advanced approaches should be used for further studies on ESPT systems.
■
ground state, two π-conjugated moieties or molecules are linked by hydrogen bonding X1−H···X2, and the system exhibits normal form (N). Upon photoexcitation, ESPT occurs, and a tautomer form (T) is produced with hydrogen bond = X1···H−X2. Depending on the energy barrier of ESPT, single or dual fluorescence may be observed for the ESPT systems (Scheme 1). Typically, the energy barrier of ESPT process can be influenced by solvent, pH, hydrogen bonding, and interaction with other molecules.6−18 Therefore, determining the energy barrier for the ESPT process under different conditions is crucial, and theoretical studies provide direct approaches to achieve this goal. The most notable photophysical property of the ESPT system is that the fluorescence from proton transfer (PT) tautomer possesses large Stokes
INTRODUCTION Excited-state proton transfer (ESPT) is one of the most fundamental processes that play vital roles in biological systems, such as photosystem II,1 bacteriorhodopsin,2 DNA,3 and green fluorescence protein.4 In addition, an increasing number of new chromophores based on the ESPT have been developed and applied in recent years in laser dyes,5 fluorescent probes,6 white light-emitting materials,7 and organic optoelectronic materials.8 Given the importance of the ESPT processes in chemical and biological systems and their unique photophysical properties and enormous application potential, experimental and theoretical studies on the ESPT systems have received increasing attention and remain active topics among researchers.5−11 Most of the ESPT processes occur through intermolecular or intramolecular hydrogen bonding = X1−H···X2 (where X1 and X2 are electronegative atoms, such as N or O). At the © 2018 American Chemical Society
Received: April 17, 2018 Published: June 15, 2018 1681
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
depth insight into the mechanism of the ESPT reactions. Moreover, owing to limitations, experimental studies usually leave certain outstanding issues that must be addressed by theoretical studies. For example, when new ESPT chromophores are developed, the design strategies and possible mechanisms are proposed based on the experimental results.15−18 However, the suggested mechanism is usually not completely corrected and sometimes problematic, thereby requiring further theoretical studies to validate or invalidate;19−22 this can provide guidance for developing new ESPT chromophores.23 When multiple protons are involved, such as in excited-state double PT (ESDPT), whether the mechanism is stepwise or concerted is usually under debate among experimental scientists;24,25 extensive theoretical studies are necessary to resolve this controversy.26−28 When solvent molecules participate in ESPT as proton acceptor or catalyst, the size of the solute·(solvent)n cluster is difficult to determine experimentally, and theoretical studies are necessary.29−36 Recently, our group has systematically investigated a number of ESPT systems using different excited-state quantum chemistry methods.19−22,27,35−39,41 In this Account, on the basis of the importance of theoretical studies in overcoming the experimental limitations of ESPT mechanism studies, we present an overview of these works and related studies from other laboratories. We show that, the theoretical studies on the ESPT systems can provide modifications to the proposed mechanism based on the experimental results19−22 and may present a completely new mechanism that is in accordance with the experimental results.21,38 Moreover, the theoretical studies on the ESPT systems may inspire experimental scientists to perform investigations to confirm the proposed mechanism based on the theoretical results.38,40 Finally, advantages and disadvantages of different theoretical methods on describing the potential energy surface (PES) of the ESPT reaction are discussed.
Scheme 1. Schematic Illustration of the Relation between the Energy Barrier of the ESPT Process and the Fluorescence of the ESPT System
shift, which makes determination of whether the ESPT process can occur from steady-state absorption and fluorescence spectra easy. However, the overall ESPT process is complicated, and unraveling the detailed mechanism of ESPT process requires further experimental and theoretical studies. ESPT, especially the excite-state intramolecular PT (ESIPT), typically occurs on ultrafast time scale at or below 100 fs. Experimental studies on these ultrafast processes requires the femtosecond time-resolved spectroscopic techniques, where the femtosecond stimulated Raman spectroscopy (FSRS) has been demonstrated as a powerful tool to investigate the ESPT processes by revealing the skeletal motions involved in PT.12−14 However, direct information on geometrical relaxation upon photoexcitation and the transition state geometries is difficult to obtain from the time-resolved spectroscopic experiments. State-of-the-art ab initio excitedstate calculations can provide this missing information and inScheme 2. Three Types of ESDPT Reaction Mechanism
Reproduced with permission from ref 27. Copyright 2009 The Royal Society of Chemistry. 1682
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
■
ESDPT: STEPWISE OR CONCERTED?
One well-known typical system that undergoes ESDPT is the dual hydrogen-bonded 7-azaindole dimer (7AI2). The issue between stepwise and concerted mechanism of 7AI2 remains unclear because the ESDPT of 7AI2 is essentially a multidimensional process.24 Another typical system that involves the ESDPT process is the photoinduced amino−imino tautomerization of 2-aminopyridine (2-AP), and the acetic acid molecule can act as a catalyst for this process.25−27 The three types of possible ESDPT processes of 2-AP/acid system are shown in Scheme 2. In the concerted mechanism, two protons simultaneously transfer along the intermolecular hydrogen bonds O−H···N and N−H···O. In the stepwise 1 mechanism, one proton first transfers along the O−H···N to form an ion-pair of the protonated 2-AP cation and the acid anion. Then the other proton transfers from the amino part to the acetic acid to complete the amino−imino tautomerization. The stepwise 2 refers to the opposite sequence of stepwise 1. In the picosecond time-resolved fluorescence spectroscopic experiments, Ishikawa et al. suggested that the ESDPT of 2AP/acid proceeds in the stepwise 1 mechanism.25 The first PT is too fast to be detected by the picosecond time-resolved experiments, and the second proton is transferred with a lifetime of 5 ps.25 Although subsequent theoretical studies with CIS method on the 2-AP/ACID system also support the stepwise 1 mechanism, the calculated energy barrier for the first PT step is approximately 9.48 kcal/mol, whereas that of the second step is negligible.26 This finding supports the slow first-step followed by the fast second-step PT, which contradicts the experimental results.25 Given the controversy between the experimental25 and theoretical26 studies, we reinvestigate the ESDPT reaction of 2AP/acid system by using a theoretical study.27 The TDDFT calculated potential energy curves along the PT coordinates for the three possible mechanisms (Figure 1a) reveal that the first PT step in stepwise 1 mechanism was nearly barrierless, whereas a high barrier was observed in both the stepwise 2 and concerted mechanisms. Figure 1b shows the additional potential energy curves along the second PT coordinate, and a small barrier was observed in the second PT step in stepwise 1 mechanism. Therefore, our calculation results support the stepwise 1 mechanism for the ESDPT reaction of 2-AP/acid system and are in accordance with the experimental results by Ishikawa et al.25 This finding may indicate that the CIS method26 is not a good option for studying ESPT. To the best of our knowledge, this work27 is the first to apply the TDDFT method to investigate the ESPT process with the optimization of excited-state geometries and the PES along the PT coordinate provided. To date, this method has become a conventional approach to investigate the ESPT process.
Figure 1. TD-B3LYP/TZVP Calculated potential energy curves along the PT coordinate with different mechanisms. Reproduced with permission from ref 27. Copyright 2009 The Royal Society of Chemistry.
after interaction with the target analyte plays a key role in unraveling the sensing mechanism and can provide guidance for developing new chemsensors, as demonstrated in our recent studies.19−22,41 In 2008, Lee et al. developed a fluorescent chemodosimeter, 8-formyl-7-hydroxycoumarin (Scheme 3, compound 1) for the selective detection of cyanide.16 Compound 1 is nonfluorescent and adding the cyanide anions will induce a significant fluorescence enhancement.16 Figure 2 shows our proposed detailed sensing mechanism of compound 1 for cyanide detection based on TD-B3LYP/TZVP calculations.20 The ESPT of compound 1 is barrierless, and the formed PT tautomer is nonfluorescent. The addition of cyanide anions to compound 1 induces PT in the ground state, and the nucleophilic addition product (compound 2, Scheme 2) is formed. The calculated energy profiles in the S1 state of compound 2 show that the ESIPT reaction must overcome a barrier approximately 30 kJ/mol, indicating that the photoexcited normal form of 2 (denoted as 2a* in Figure 2) prefers to return to the S0 state through the fluorescence rather than by surmounting the potential barrier in the potential energy curve.20 This calculated sensing mechanism provides a reasonable explanation for the observed experimental results.16 On the basis of this mechanism, Bera and co-workers designed and synthesized a novel chemdosimetric probe FSal (Scheme 4), that can be used for detecting cyanide with high selectivity and low detection limit.23 Two model compounds (FBal and FMBal, Scheme 4) were also synthesized, and both showed strong emission because the phenolic−OH group was missed or protected; therefore, ESIPT was hindered.23 This finding further confirms our proposed mechanism. We have theoretically investigated the sensing mechanisms for a series of fluoride chemosensors.19,21,22 Our theoretical results provide detailed sensing mechanism for compounds phenyl-1H-anthra(1,2-d)imidazole-6,11-dione19 and N-(3-
■
SENSING MECHANISM: MODULATION OF ESPT WITH THE TARGET ANALYTE ESPT has been widely utilized as a strategy to design the fluorescent chemosensor. The interaction with the target analyte facilitates or precludes the ESPT of the chemosensor, thereby inducing a distinct fluorescence change, such as enhancement, quenching or a large shift of the fluorescence band. Most experimental studies on chemosensors focus on the performance of chemosensor, and the detailed sensing mechanism usually remains unclear. Theoretically determination of the ESPT energy barrier of chemosensor before and 1683
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research Scheme 3. Structure of the Dosimeter and Its Proposed Cyanide-Sensing Mechanism
Reproduced with permission from ref 16. Copyright 2008 American Chemical Society.
Figure 2. Entire sensing processes for the chemosensor for cyanide anion. Reproduced with permission from ref 20. Copyright 2010 Wiley Periodicals, Inc.
nearly no free hydrogen ions were observed. Thus, the chance of DBMOHF formation is small. Instead, we proposed that the anionic form DBMO is formed when the DBM is reacted with fluoride.21 The calculated absorption spectra of DBMO are in good agreement with the experimental results for those of DBM with the addition of [(Bu)4N]F, whereas the calculated two permitted transitions of DBMOHF exhibit a blue shift (Figure 3B).21 In addition, the calculated 1H NMR spectra of DBMOHF show a distinct downfield signal at δ = 15.47 and can be assigned to the H4 between fluoride and oxygen (Figure 3C). However, this observation was not found in the experiments, confirming that DBMOHF may not exist.21 The calculated emission band of DBMO is located at 520 nm and is in accordance with the experimentally measured emission spectra of DBM with the addition of fluoride anion.21 Therefore, the sensing mechanism of DBM for detection of fluoride involves DBMO formation, and the red shift of DBMO in the emission is attributed to the intramolecular charge transfer instead of the ESIPT according to the molecular orbital analysis (Figure 3A).21
Scheme 4. Structures of the Fluorene-Based Probe, FSal, and Two Model Compounds, FBal and FMBal
Reproduced with permission from ref 23. Copyright 2014 The Royal Society of Chemistry.
(benzo[d]thiazol-2-yl)-4-(tertbutyldiphenylsilyloxy)phenyl)benzamide (BTTPB)22 and support the experimentally proposed ones.15,18 However, for 5,7-dibromo-8-tert-butyldimethylsilyloxy-2-methylquinoline (DBM, Figure 3A),21 we demonstrated that the experimentally proposed mechanism was incorrect.17 It has been proposed that the Si−O bond of DBM is broken upon addition of fluoride anion and that a fluoride-hydrogen-bond complex (DBMOHF, Figure 3A) is formed. Photoexcitation of the DBM to the S1 state induces ESIPT, which is responsible for the red shift of the fluorescence spectra.17 However, we noted that the experiment was carried out in the organic solvent tetrahydrofuran, and
■
EFFECT OF INTERMOLECULAR HYDROGEN BONDING ON ESIPT For most conventional ESIPT chromophores in aqueous solution, the ESIPT process is typically disrupted due to the formation of intermolecular hydrogen bonds.6,8,10 However, the detailed mechanism of this phenomena and the effect of 1684
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
Figure 3. (A) Structures of DBM, DBMOHF, DBMO, and SiCH3F. (B) Comparison of experimental and calculated UV−vis absorption spectra. (a) Experimental UV−vis spectra in tetrahydrofuran with the addition of [(Bu)4N]F. (b) Calculated absorption bands of DBM, black line; DBMO, red line; and DBMOHF, green line, obtained at the TDDFT/B3LYP/TZVP level. (C). Calculated 1H NMR spectra of DBM (bottom); DBMO and SiCH3F (middle); DBMOHF and SiCH3F (top), at the DFT/B3LYP/6-311+G(2d,p) level. Tetramethylsilane is chosen as the standard substance. Reproduced with permission from ref 21. Copyright 2013 The Royal Society of Chemistry.
Figure 4. (A) PBE0/aug-cc-pVTZ optimized geometries of ketoA, ketoB, and enol forms of MS. (B) Our proposed mechanism for the dual fluorescence of MS in alcoholic solvents. The ground and first excited state geometries are calculated by the PBE0/aug-cc-pVTZ and TD-PBE0/ aug-cc-pVTZ methods, respectively. The bond lengths are given in angstroms. The computed excitation and emission energies by the TDDFT (black), ADC(2) (red), and RI-CC2(blue) methods are also provided. Reproduced with permission from ref 38. Copyright 2015 American Chemical Society.
form a hydrogen-bonded complex with methanol at two sites: with the hydroxyl group and with the carbonyl group. We denote these two hydrogen bonded complexes as ketoB− methA and ketoB−methB (Figure 4B), respectively. Optimization of the first excited-state geometries of ketoB and ketoB−methA directly yields the corresponding enol form, indicating that the ESIPT processes of ketoB and ketoB− methA are either barrierless or present a negligible barrier. By contrast, for ketoB−methB, the enol form cannot be obtained by using either S0 geometry or the corresponding enol form as the initial structure, thereby confirming that the ESIPT of ketoB−methB is precluded by inter-HB.38 Therefore, the ESIPT of MS can be modulated by inter-HB. Inspired by our proposed new mechanism, Ghosh and co-workers recently
intermolecular hydrogen bonding (inter-HB) on ESIPT has rarely been investigated theoretically. Recently, we have focused on this topic by studying the dual fluorescence of methyl salicylate (MS).38 The experimental studies revealed that the dual fluorescence of MS is solvent dependent, which can be clearly observed in alcoholic solvents and not in nonpolar solvents.38 These findings indicate that the inter-HB may play an important role in determining the dual fluorescence of MS. In the ground state, two tautomers of MS may exist: ketoA and ketoB (Figure 4A). Theoretical studies by Massaro et al.42 and us38 confirmed that the ketoB → ketoA isomerization reaction cannot occur spontaneously at room temperature, indicating that the dual fluorescence of MS may only result from ketoB. The ketoB can 1685
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
Figure 5. (A) Geometries and numbering scheme of SMA tautomers in the S0 state. (B) Geometries of the five MECIs optimized at the SA2CASSCF(14,12)/6-31G** level. (C) Linearly interpolated pathways connecting the S1 min-α and the ESIPT (left) and Twin1 (right) constructed at the CASPT2/SA2-CASSCF(14,12)/6-31G** level. (D) Diagram (left) for dihedral angles C4C7N8C9 (ordinate) and NH bond length (abscissa), and (right) for dihedral angle C4N3O10H11 (ordinate) and bond length NH (abscissa) at hopping points from the S1 to S0 state. The balls are five MECIs we located, and the red stars are the hopping structures of the trajectories. Reproduced with permission from ref 37. Copyright 2016 American Chemical Society.
energy CIs of SMA. One CI is characterized by ESIPT process, and the other four CIs correspond to the twisting process around C7N8 or C3O10 bonds. Figure 5C shows the calculated reaction pathways for the internal conversion of SMA at the ESIPT and Twin1 minimal energy CIs (MECIs).37 The results revealed a high energy barrier (2.2 eV) along ESIPT, whereas only a slight energy barrier (0.2 eV) was observed along the twisting process. The results of surfacehopping dynamic simulations (Figure 5D) also show that the hopping structures are centralized around the four twisting MECIs but are distant from MECI related to the ESIPT process. Consequently, our results support that SMA in gas phase prefers to decay to the ground state through the fourrotational motion related MECIs rather than the ESIPT related MECI.37 This finding agrees with the previous experimental results from femtosecond pump−probe photoionization spectroscopy.43
detected two distinctly different hydrogen-bonded MS−water complexes by chirped pulse and cavity-based Fourier transform microwave spectroscopy.40 This observation provides direct experimental evidence for our proposed mechanism.
■
COMPETITION BETWEEN ESIPT AND TWISTING PROCESSES The twisting motion between proton-donating and protonaccepting units is an important nonradiative decay route in the ESIPT system; this motion typically leads to an ultrafast internal conversion and quenches the fluorescence of ESIPT tautomer.37,43−48 The competition between ESIPT and twisting complicates the excited state dynamics of these systems compared with the general ESIPT systems; and determining the energy barrier for both ESIPT and twisting processes is crucial. Moreover, given the involvement of the conical interactions (CIs), multireference methods, such as CASSCF, should be used to theoretically investigate these systems. As one of the simplest aromatic Schiff bases, the photochromic process of salicylidene methylamine (SMA) (Figure 5A) involves ESIPT and twisting.37,43−47 An experimental study by Grzegorzek and co-workers43 suggests that the primary decay process of SMA upon photoexcitation is the twisting motion around the CN bond because only the enol tautomers related to this process were observed, and no keto tautomers corresponding to ESIPT was observed. However, the theoretical studies44−47 proposed that the excited state of SMA preferentially decays to the ground state along an ESIPT pathway because the computed energy barrier for ESIPT is lower than that of twisting. Motivated by this controversy, we conducted a theoretical study on the photochemistry behavior of SMA at a high calculation level CASPT2//CASSCF.37 Figure 5B shows the five located important S1/S0 minimum
■
SOLVENT-ASSISTED ESPT: DETERMINING THE SIZE OF THE SOLUTE·(SOLVENT)n CLUSTER The solvent can affect the ESPT process in three aspects. First, the ESPT process can be fine-tuned by solvent polarization when it is accompanied by strong redistribution of the electronic density.9,10 Our recent theoretical studies demonstrated that the TDDFT method with polarizable continuum model can partially reproduce the solvent-dependent energy barrier of ESPT.38,39 Second, the solute−solvent inter-HB can modulate ESIPT6,8,10,38 as discussed in previous sections. Third, the solvent molecules can act as proton-acceptors or catalysts when proton-donating and proton-accepting groups are not directly linked by hydrogen bonding. In this situation, determination of the size of the solute·(solvent)n cluster unravels the detailed mechanism of ESPT process.29−36 1686
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
yquinolinium (NM6HQ+) shows that the energy barrier of ESPT of NM6HQ+(H2O)3 is only about 0.5 kcal/mol, where only the proton-accepting water molecule and its first solvation shell (two water molecules) are involved.33 Determining the cluster size and configuration simultaneously can be achieved by means of molecular dynamics simulation and this should be done first before further quantum chemistry calculations. When the solvent molecule act as the catalyst of ESPT of solute, determination of the number of the involved solvent molecules and identification of which proton is transferred first from the solvent to solute or from the solute to solvent are both important.29,34−36 Chung and co-workers recently designed and synthesized 2,6-diazaindole (2,6-DAI) to probe water molecule catalyzed ESPT in aqueous solution.34 They proposed that two water molecules are involved, and three protons are transferred in a concerted asynchronous manner.34 However, in our optimized geometries of the 2,6-DAI·(H2O)2 cluster, the distance between the two water molecules is too large to form a hydrogen-bonded wire, and an additional water molecule is needed.35 We then constructed the PE curves of 2,6-DAI·(H2O)3 along the H2−O3 and O7−H8 bonds (Figure 7) to consider the two possible mechanisms, that is, the proton is initially transferred from solute to solvent (Type A) and from solvent to solute (Type B). The computed energy barriers for the ESPT of Type A and B are 13.16 and 9.69 kcal/ mol, respectively. However, only one proton is transferred in Type B, and we cannot obtain the desired PT tautomer 2,6DAI·(H2O)3-PT. Therefore, the water-catalyzed ESPT of 2,6DAI can only proceed along the N 1 −H 2 ···O 3 bond accompanied by other proton transfers. The computed energy barrier for the ESPT of 2,6-DAI·(H2O)4 is essentially identical to that of 2,6-DAI·(H2O)3, indicating that three water molecules is sufficient to assist the PT.
When the proton is transferred from solute to solvent, the well-known Eigne−Weller model49,50 is frequently utilized to depict the ESPT as a two-step process: formation of a contact ion pair (CIP) through short-range PT and diffusioncontrolled separation of the ions. As shown in Figure 6A, 6-
■
CONCLUDING REMARKS AND PROSPECTS In this Account, we summarized our recent research progress on unravelling the detailed ESPT mechanism of a range of typical systems with state-of-the-art ab initio excited-state calculations. Modulations of ESPT with solvent, inter-HB, and interaction with target molecules were illustrated. In addition, the stepwise and concerted mechanism of ESDPT reaction, competition between ESIPT and twisting processes, and the mechanism of solvent-assisted ESPT reaction were discussed. The significance and advantages of the theoretical studies on revealing the detailed mechanism of the above-mentioned ESPT systems were shown. Although most theoretical studies in this account were carried out using TDDFT, this method, with certain conventional functional, typically underestimates the energies of CT states or yield artificially low energy CT states in general.51−55 However, the ESPT reaction is frequently coupled with CT. TDDFT calculation using a range-separated functional can provide an accurate excitation energy for the CT state but overshoots the excitation energy of the local excited state.52,53 Our recent study on an ESIPT chromophore showed that the optimally tuned range-separated TDDFT can provide an accurate description of PES for ESIPT reactions where electronic states of different natures are involved in different tautomers.39 Notably, the location of the transition state (TS) for the ESIPT reaction previously remained a challenge owing to its complexity and large time consumption entailed by TDDFT method; few theoretical studies involve the optimization of the
Figure 6. (A) Model structure we used to investigate the 6HQc: (H2O)n. (B) TD-B3LYP optimized energy profiles for S1 state of 6HQc:(H2O)n complexes along the phenolic O−H bond. The blue line denotes the energy profiles for the case of n = 4 by using CAMB3LYP functional. Reproduced with permission from ref 31. Copyright 2011 Elsevier B.V.
hydroxyquinolinium (6HQc) is a photoacid that can easily transfer a proton to solvents upon photoexcitation. Pérez− Lustres and co-workers proposed that two water molecules are necessary to accept the photodetached proton to correlate the two time constants obtained from femtosecond transient absorption spectroscopy.30 Liu and co-workers computed the energy profiles of the S1 state of 6HQc:(H2O)n (n = 1, 2, 3, 4) complexes along the phenolic O−H bond (Figure 6B).31 The results reveals that, when n is 1 or 2, the energy along the phenolic O−H bond always increase, and stable CIP 6HQz: (H2O)n+ cannot be formed. When n is 3, although the PT can proceed, the formed CIP 6HQz:(H2O)3+ can easily return to its cationic form due to an extremely low energy barrier of back PT (Figure 6B). When n is 4, the proton can transfer to water and then form a stable CIP 6HQz:(H2O)4+. Therefore, the determined value of n by Liu and co-workers is 4 for the ESPT between 6HQc and water,31 not the experimentally proposed 2.30 Besides the cluster size, cluster configuration is also important. A later theoretical study on N-methyl-6-hydrox1687
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
calculations have made significant contributions to this field.58,59,63−65 One of the biggest advantages of this approach is that it can unveil the driving force of the ESPT reaction. Moreover, analysis of the AIMD trajectories based on wavelet transform allows a direct comparison between the theoretical results and FSRS signals. Eventually, we anticipate that further studies on the ESPT mechanism should make progress on the following aspects: (1) developing and searching new functionals that can provide more accurate PES for ESPT reactions; (2) extending the studies to systems involving multiple protons transfer;66 (3) constructing the PES by optimization of the TS and performing IRC calculations; (4) accounting for the solvent effects by QM/MM; (5) performing AIMD calculations and then comparing with ultrafast time-resolved spectra. These strategies will provide deeper insights into the ESPT mechanism.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Panwang Zhou: 0000-0002-9618-7038 Keli Han: 0000-0001-9239-1827 Notes
The authors declare no competing financial interest. Biographies Panwang Zhou was born in 1981 in Shanxi, China. He received his doctorate in chemical physics in 2010 and subsequently became an assistant professor at the Dalian Institute of Chemical Physics (DICP). He is currently an associate professor of the State Key Laboratory of Molecular Reaction Dynamics at DICP. His current research interests are focused on excited-state dynamics of organic and biological molecules.
Figure 7. Potential energy curves of the S0 and S1 states for the 2,6DAI·(H2O)3 cluster along with N1−H2···O3 (Type A) and O7··· H8−N9 (Type B). Reproduced with permission from ref 35. Copyright 2018 American Chemical Society.
Ke-Li Han was born in 1963 in Shandong, China. He received his doctorate in 1990 from the State Key Laboratory of Molecular Reaction Dynamics at DICP and subsequently became an assistant professor. He pursued postdoctoral studies at University of California at Davis and Emory University in the years 1993−1995. In 1995, he became a full professor of Chemical Physics at DICP. Professor Han’s current research interests involve experimental and theoretical chemical dynamics.
TS geometry for the ESIPT reactions.11,28,39,56−59 Usually, a relaxed PES scan along the PT coordinate is performed, and the energy barrier for the ESIPT reaction is estimated.19−22,27,29,31,35−38,41,48,60,61 However, the results are not sufficiently accurate. Given the implementation of the analytical second derivatives of TDDFT in quantum chemistry packages such as Gaussian and Q-Chem, the optimization of the TS geometry in the excited state has become feasible, and an increasingly accurate excited-state PES for the ESIPT reactions can be obtained from TDDFT calculations. In future theoretical studies on the ESPT systems, the location of the TS geometry will become a routine pathway, and the PES scan will be depreciated. Moreover, with the gradual improvement of computing capacity, the accurate methods, such as ADC(2),11,38 CC2,38 and SAC-CI,62 can be applied to investigate the ESPT reactions. The time evolution of the ESPT reaction cannot be obtained from static calculations; thus, on-the-fly dynamic simulations37,63,64 should be performed, and the results should be compared with those of femtosecond time-resolved spectra. Recent theoretical studies by means of ab initio molecular dynamics (AIMD) and intrinsic reaction coordinate (IRC)
■
ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Grant No: 21533010 and 21773238), DICP DMTO201601, DICP ZZBS201703, the Science Challenging Program (JCKY2016212A501).
■
REFERENCES
(1) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112, 4016−4093. (2) Hayashi, S.; Tajkhorshid, E.; Schulten, K. Structural Changes during the Formation of Early Intermediates in Bacteriorhodopsin Photocycles. Biophys. J. 2002, 83, 1281−1297. (3) Jacquemin, D.; Zúñiga, J.; Requena, A.; Ceron-Carrasco, J. P. Assessing the Importance of Proton Transfer Reactions in DNA. Acc. Chem. Res. 2014, 47, 2467−2474.
1688
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research (4) Tonge, P. J.; Meech, S. R. Excited State Dynamics in Green Fluorescent Protein. J. Photochem. Photobiol., A 2009, 205, 1−11. (5) Chou, P.; McMorrow, D.; Aartsma, T. J.; Kasha, M. The protontransfer laser. Gain spectra and amplification of spontaneous emission of 3-hydroxyflavone. J. Phys. Chem. 1984, 88, 4596−4599. (6) Zhao, J. Z.; Ji, S. M.; Chen, Y. H.; Guo, H. M.; Yang, P. Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials. Phys. Chem. Chem. Phys. 2012, 14, 8803−8817. (7) Tseng, H.-W.; Liu, J.-Q.; Chen, Y.-A.; Chao, C.-M.; Liu, K.-M.; Chen, C.-L.; Lin, T.-C.; Hung, C.-H.; Chou, Y.-L.; Lin, T.-C.; Wang, T.-L.; Chou, P.-T. Harnessing Excited-State Intramolecular ProtonTransfer Reaction via a Series of Amino-Type Hydrogen-Bonding Molecules. J. Phys. Chem. Lett. 2015, 6, 1477−1486. (8) Kwon, J. E.; Park, S. Y. Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process. Adv. Mater. 2011, 23, 3615−3642. (9) Hsieh, C.-C.; Jiang, C.-M.; Chou, P.-T. Recent Experimental Advances on Excited-State Intramolecular Proton Coupled Electron Transfer Reaction. Acc. Chem. Res. 2010, 43, 1364−1374. (10) Demchenko, A. P.; Tang, K.-C.; Chou, P.-T. Excited-state proton coupled charge transfer modulated by molecular structure and media polarization. Chem. Soc. Rev. 2013, 42, 1379−1408. (11) Azarias, C.; Budzák, Š .; Laurent, D.; Ulrich, G.; Jacquemin, D. Tuning ESIPT fluorophores into dual emitters. Chem. Sci. 2016, 7, 3763−3774. (12) Fang, C.; Frontiera, R. R.; Tran, R.; Mathies, R. A. Mapping GFP structure evolution during proton transfer with femtosecond Raman spectroscopy. Nature 2009, 462, 200−205. (13) Han, F.; Liu, W.; Fang, C. Excited-state proton transfer of photoexcited pyranine in water observed by femtosecond stimulated Raman spectroscopy. Chem. Phys. 2013, 422, 204−219. (14) Liu, W.; Wang, Y.; Tang, L.; Oscar, B. G.; Zhu, L.; Fang, C. Panoramic portrait of primary molecular events preceding excited state proton transfer in water. Chem. Sci. 2016, 7, 5484−5494. (15) Peng, X.; Wu, Y.; Fan, J.; Tian, M.; Han, K. Colorimetric and Ratiometric Fluorescence Sensing of Fluoride: Tuning Selectivity in Proton Transfer. J. Org. Chem. 2005, 70, 10524. (16) Lee, K.-S.; Kim, H.-J.; Kim, G.-H.; Shin, I.; Hong, J.-I. Fluorescent Chemodosimeter for Selective Detection of Cyanide in Water. Org. Lett. 2008, 10, 49−51. (17) Bao, Y.; Liu, B.; Wang, H.; Tian, J.; Bai, R. A ‘‘naked eye’’ and ratiometric fluorescent chemosensor for rapid detection of F− based on combination of desilylation reaction and excited-state proton transfer. Chem. Commun. 2011, 47, 3957−3959. (18) Hu, R.; Feng, J.; Hu, D.; Wang, S.; Li, S.; Li, Y.; Yang, G. A Rapid Aqueous Fluoride Ion Sensor with Dual Output Modes. Angew. Chem., Int. Ed. 2010, 49, 4915−4918. (19) Li, G.-Y.; Zhao, G.-J.; Liu, Y.-H.; Han, K.-L.; He, G.-Z. TDDFT Study on the Sensing Mechanism of a Fluorescent Chemosensor for Fluoride: Excited-State Proton Transfer. J. Comput. Chem. 2010, 31, 1759−1765. (20) Li, G.-Y.; Zhao, G.-J.; Han, K.-L.; He, G.-Z. A TD-DFT Study on the Cyanide-Chemosensing Mechanism of 8-Formyl-7-hydroxycoumarin. J. Comput. Chem. 2011, 32, 668−674. (21) Chen, J.-S.; Zhou, P.-W.; Yang, S.-Q.; Fu, A.-P.; Chu, T.-S. Sensing mechanism for a fluoride chemosensor: invalidity of excitedstate proton transfer mechanism. Phys. Chem. Chem. Phys. 2013, 15, 16183−16189. (22) Chen, J.-S.; Zhou, P.-W.; Zhao, L.; Chu, T.-S. A DFT/TDDFT study of the excited state intramolecular proton transfer based sensing mechanism for the aqueous fluoride chemosensor BTTPB. RSC Adv. 2014, 4, 254−259. (23) Bera, M. K.; Chakraborty, C.; Singh, P. K.; Sahu, C.; Sen, K.; Maji, S.; Das, A. K.; Malik, S. Fluorene-based chemodosimeter for “turn-on” sensing of cyanide by hampering ESIPT and live cell imaging. J. Mater. Chem. B 2014, 2, 4733−4739.
(24) Sekiya, H.; Sakota, K. Excited-state double-proton transfer in a model DNA base pair: Resolution for stepwise and concerted mechanism controversy in the 7-azaindole dimer revealed by frequency- and time-resolved spectroscopy. J. Photochem. Photobiol., C 2008, 9, 81−91. (25) Ishikawa, H.; Iwata, K.; Hamaguchi, H. Picosecond Dynamics of Stepwise Double Proton-Transfer Reaction in the Excited State of the 2-Aminopyridine/Acetic Acid System. J. Phys. Chem. A 2002, 106, 2305−2312. (26) Hung, F.-T.; Hu, W.-P.; Li, T.-H.; Cheng, C.-C.; Chou, P.-T. Ground and Excited-State Acetic Acid Catalyzed Double Proton Transfer in 2-Aminopyridine. J. Phys. Chem. A 2003, 107, 3244−3253. (27) Chai, S.; Zhao, G.-J.; Song, P.; Yang, S.-Q.; Liu, J.-Y.; Han, K.-L. Reconsideration of the excited-state double proton transfer (ESDPT) in 2-aminopyridine/acid systems: role of the intermolecular hydrogen bonding in excited states. Phys. Chem. Chem. Phys. 2009, 11, 4385− 4390. (28) Su, Y.; Chai, S. A TDDFT study of the excited-state intramolecular proton transfer of1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole. J. Photochem. Photobiol., A 2014, 290, 109−115. (29) Liu, Y. H.; Mehata, M. S.; Liu, J. Y. Excited-State Proton Transfer via Hydrogen-Bonded Acetic Acid (AcOH) Wire for 6Hydroxyquinoline. J. Phys. Chem. A 2011, 115, 19−24. (30) Pérez-Lustres, J. L.; Rodriguez-Prieto, F.; Mosquera, M.; Senyushkina, T. A.; Ernsting, N. P.; Kovalenko, S. A. Ultrafast Proton Transfer to Solvent: Molecularity and Intermediates from Solvationand Diffusion-Controlled Regimes. J. Am. Chem. Soc. 2007, 129, 5408−5418. (31) Liu, Y. H.; Chu, T. S. Size effect of water cluster on the excitedstate proton transfer in aqueous solvent. Chem. Phys. Lett. 2011, 505, 117−121. (32) Simkovitch, R.; Shomer, S.; Gepshtein, R.; Huppert, D. How Fast Can a Proton-Transfer Reaction Be beyond the Solvent-Control Limit? J. Phys. Chem. B 2015, 119, 2253−2262. (33) Cimino, P.; Raucci, U.; Donati, G.; Chiariello, M. G.; Schiazza, M.; Coppola, F.; Rega, N. On the different strength of photoacids. Theor. Chem. Acc. 2016, 135, 117. (34) Chung, K.-Y.; Chen, Y.-H.; Chen, Y.-T.; Hsu, Y.-H.; Shen, J.-Y.; Chen, C.-L.; Chen, Y.-A.; Chou, P.-T. The Excited-State Triple Proton Transfer Reaction of 2,6-Diazaindoles and 2,6-Diazatryptophan in Aqueous Solution. J. Am. Chem. Soc. 2017, 139, 6396−6402. (35) Tang, Z.; Qi, Y.; Wang, Y.; Zhou, P. W.; Tian, J.; Fei, X. Excited-State Proton Transfer Mechanism of 2,6-Diazaindoles·(H2O) n (n = 2−4) Clusters. J. Phys. Chem. B 2018, 122, 3988−3995. (36) Qu, Z. J.; Li, P.; Zhang, X. X.; Wang, E. D.; Wang, Y. N.; Zhou, P. W. Excited-state proton transfer of 4-hydroxyl-1,8-naphthalimide derivatives: A combined experimental and theoretical investigation. J. Lumin. 2016, 177, 197−203. (37) Zhao, L.; Liu, J. Y.; Zhou, P. W. New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum. J. Phys. Chem. A 2016, 120, 7419−7426. (38) Zhou, P. W.; Hoffmann, M. R.; Han, K. L.; He, G. Z. New Insights into the Dual Fluorescence of Methyl Salicylate: Effects of Intermolecular Hydrogen Bonding and Solvation. J. Phys. Chem. B 2015, 119, 2125−2131. (39) Zhou, P. W.; Zhao, L. Accurate Description of Excited State Intramolecular Proton Transfer that Involves Zwitterionic State Using Optimally Tuned Range-Separated Time-Dependent Density Functional Theory. Int. J. Quantum Chem. 2018, 118, e25618. (40) Ghosh, S.; Thomas, J.; Huang, W.; Xu, Y.; Jäger, W. Rotational Spectra of Two Hydrogen-Bonded Methyl Salicylate Monohydrates: Relative Stability and Tunneling Motions. J. Phys. Chem. Lett. 2015, 6, 3126−3131. (41) Li, G. Y.; Han, K. L. The sensing mechanism studies of the fluorescent probes with electronically excited state calculations. WIREs Comput. Mol. Sci. 2018, 8, e1351. (42) Massaro, R. D.; Dai, Y.; Blaisten-Barojas, E. Energetics and Vibrational Analysis of Methyl Salicylate Isomers. J. Phys. Chem. A 2009, 113, 10385−10390. 1689
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
Article
Accounts of Chemical Research
(62) Savarese, M.; Raucci, U.; Fukuda, R.; Adamo, C.; Ehara, M.; Rega, N.; Ciofini, I. Comparing the Performance of TD-DFT and SAC-CI Methods in the Description of Excited States Potential Energy Surfaces: An Excited State Proton Transfer Reaction as Case Study. J. Comput. Chem. 2017, 38, 1084−1092. (63) Donati, G.; Petrone, A.; Caruso, P.; Rega, N. The mechanism of a green fluorescent protein proton shuttle unveiled in the timeresolved frequency domain by excited state ab initio dynamics. Chem. Sci. 2018, 9, 1126−1135. (64) Chiariello, M. G.; Rega, N. Exploring Nuclear Photorelaxation of Pyranine in Aqueous Solution: an Integrated Ab-Initio Molecular Dynamics and Time Resolved Vibrational Analysis Approach. J. Phys. Chem. A 2018, 122, 2884−2893. (65) Heo, W.; Uddin, N.; Park, J. W.; Rhee, Y. M.; Choi, C. H.; Joo, T. Coherent intermolecular proton transfer in the acid−base reaction of excited state pyranine. Phys. Chem. Chem. Phys. 2017, 19, 18243− 18251. (66) Tu, T.-H.; Chen, Y.-T.; Chen, Y.-A.; Wei, Y.-C.; Chen, Y.-H.; Chen, C.-L.; Shen, J.-Y.; Chen, Y.-H.; Ho, S.-Y.; Cheng, K.-Y.; Lee, S.L.; Chen, C.-H.; Chou, P.-T. The Cyclic Hydrogen-Bonded 6Azaindole Trimer and its Prominent Excited-State Triple-ProtonTransfer Reaction. Angew. Chem., Int. Ed. 2018, 57, 5020−5024.
(43) Grzegorzek, J.; Filarowski, A.; Mielke, Z. Photophysics of Schiff bases: theoretical study of salicylidene methylamine. Phys. Chem. Chem. Phys. 2011, 13, 16596−16605. (44) Zgierski, M. Z.; Grabowska, A. Theoretical approach to photochromism of aromatic Schiff bases: A minimal chromophore salicylidene methylamine. J. Chem. Phys. 2000, 113, 7845−7852. (45) Ortiz-Sánchez, J. M.; Gelabert, R.; Moreno, M.; Lluch, J. M. Theoretical Study on the Excited-State Intramolecular Proton Transfer in the Aromatic Schiff Base Salicylidene Methylamine: an Electronic Structure and Quantum Dynamical Approach. J. Phys. Chem. A 2006, 110, 4649−4656. (46) Jankowska, J.; Rode, M. F.; Sadlej, J.; Sobolewski, A. L. The origin of radiationless conversion of the excited state in the kindling fluorescent protein (KFP): femtosecond studies and quantum modeling. ChemPhysChem 2012, 13, 4287−4294. (47) Spörkel, L.; Jankowska, J.; Thiel, W. Photoswitching of salicylidene methylamine: a theoretical photodynamics study. J. Phys. Chem. B 2015, 119, 2702−2710. (48) Zhao, Y. L.; Wang, M. S.; Zhou, P. W.; Yang, S. Q.; Liu, Y.; Yang, C. L.; Yang, Y. F. The Mechanism of Fluorescence Quenching by Acylamino Twist in the Excited State for 1-(Acylamino)anthraquinones. J. Phys. Chem. A 2018, 122, 2864−2870. (49) Weller, A. Fast reactions of excited molecules. Prog. React. Kinet. 1961, 1, 187−214. (50) Eigen, M.; Kruse, W.; Maass, H. J.; Maeyer, L. De. Rate constants of protolytic reactions in aqueous solution. Prog. React. Kinet. 1964, 2 (285), 287−318. (51) Autschbach, J. Charge-Transfer Excitations and Time-Dependent Density Functional Theory: Problems and Some Proposed Solutions. ChemPhysChem 2009, 10, 1757−1760. (52) Zhou, P. W.; Liu, J. Y.; Yang, S. Q.; Chen, J. S.; Han, K. L.; He, G. Z. The invalidity of the photo-induced electron transfer mechanism for fluorescein derivatives. Phys. Chem. Chem. Phys. 2012, 14, 15191−15198. (53) Zhou, P. W.; Ning, C.; Alsaedi, A.; Han, K. L. The Effects of Heteroatoms Si and S on Tuning the Optical Properties of Rhodamine- and Fluorescein-Based Fluorescence Probes: A Theoretical Analysis. ChemPhysChem 2016, 17, 3139−3145. (54) Jacquemin, D.; Perpète, E. A.; Scuseria, G. E.; Ciofini, I.; Adamo, C. TD-DFT Performance for the Visible Absorption Spectra of Organic Dyes: Conventional versus Long-Range Hybrids. J. Chem. Theory Comput. 2008, 4, 123−125. (55) Jacquemin, D.; Perpete, E. A.; Ciofini, I.; Adamo, C. Accurate Simulation of Optical Properties in Dyes. Acc. Chem. Res. 2009, 42, 326−334. (56) Yang, Y.; Ma, Y.; Zhao, Y.; Zhao, Y.; Li, Y. Theoretical Investigation of the Reaction Mechanism of Photodeamination Induced by Excited-State Intramolecular Proton Transfer of Cresol Derivatives. J. Phys. Chem. A 2018, 122, 1011−1018. (57) Houari, Y.; Charaf-Eddin, A.; Laurent, A. D.; Massue, J.; Ziessel, R.; Ulrich, G.; Jacquemin, D. Modeling optical signatures and excited-state reactivities of substituted hydroxyphenylbenzoxazole (HBO) ESIPT dyes. Phys. Chem. Chem. Phys. 2014, 16, 1319−1321. (58) Raucci, U.; Savarese, M.; Adamo, C.; Ciofini, I.; Rega, N. Intrinsic and Dynamical Reaction Pathways of an Excited State Proton Transfer. J. Phys. Chem. B 2015, 119, 2650−2657. (59) Petrone, A.; Cimino, P.; Donati, G.; Hratchian, H. P.; Frisch, M. J.; Rega, N. On the Driving Force of the Excited-State Proton Shuttle in the Green Fluorescent Protein: A Time-Dependent Density Functional Theory (TD-DFT) Study of the Intrinsic Reaction Path. J. Chem. Theory Comput. 2016, 12, 4925−4933. (60) Ma, Y.; Yang, Y.; Lan, R.; Li, Y. Effect of Different Substituted Groups on Excited-State Intramolecular Proton Transfer of 1(Acylamino)-anthraquinons. J. Phys. Chem. C 2017, 121, 14779− 14786. (61) Li, Y.; Ma, Y.; Yang, Y.; Shi, W.; Lan, R.; Guo, Q. Effects of different substituents of methyl 5-R-salicylates on the excited state intramolecular proton transfer process. Phys. Chem. Chem. Phys. 2018, 20, 4208−4215. 1690
DOI: 10.1021/acs.accounts.8b00172 Acc. Chem. Res. 2018, 51, 1681−1690
