Insights into Excited State Intramolecular Proton Transfer: An

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Insights into Excited State Intramolecular Proton Transfer: An Alternative Model for Excited State Proton Transfer of Green Fluorescence Protein Yi-Hui Chen, Robert Sung,† and Kuangsen Sung* Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan

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ABSTRACT: The excited state intramolecular proton transfer (ESIPT) that occurs in the o-sulfonamide analogue (o-TsABDI) of the green fluorescent protein (GFP) chromophore provides an alternative model to get insights into the excited state proton transfer (ESPT) related photophysics of GFP. In this article, we explored the ESIPT-related photophysics of o-TsABDI by electronic absorption and fluorescence emission spectra in a wide polarity range of solvents, cis−trans photoisomerization experiment, and Coulombattenuating method (CAM)/time-dependent (TD) density functional theory (DFT) calculations. We found that the whole ESIPT process involves four steps. The first step is photoexcitation of o-TsABDI, which does not involve charge transfer (CT). The second step is ESIPT and accompanying electron transfer from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety. The third step is fluorescence emission of the zwitterionic o-TsABDI and accompanying CT from the π* orbital of the 4-benzylideneimidazolone moiety to the half-filled n orbital of the sulfonamide nitrogen. The last step involves irreversible and barrierless proton recombination. In contrast to the isolated GFP chromophore and its p- and m-amino analogues, the S1 excited state of o-TsABDI does not relax by way of cis−trans photoisomerization through the S1/S0 conical intersection CI(I) by rotating around the I-bond, but follows the ESIPT pathway. The low fluorescence quantum yield of the zwitterionic o-TsABDI might be due to (1) the fluorescence that involves the low-probability π* → n charge transfer and (2) nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond.

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INTRODUCTION Green fluorescent protein (GFP) and its mutants have attracted interest as fluorescent biological labels in the past two decades.1,2 Wild-type GFP (wtGFP) is a globular protein of 238 amino acids that are folded into an 11-stranded β-barrel.3−5 Its chromophore, p-hydroxybenzylideneimidazolinone (p-HBDI), is located at the center of GFP, and rotation around the P-bond and the I-bond of p-HBDI is restricted by its surrounding protein, leading to a high fluorescence quantum yield (80%) of GFP.3−7 The major electronic absorption of wtGFP at 395 nm is attributed to the electronic absorption of neutral p-HBDI, which is called the A form, and its minor electronic absorption at 475 nm is assigned to the electronic absorption of the p-HBDI anion, which is called the B form.6,7 Upon excitation at 395 nm, neutral p-HBDI (A form) undergoes a rapid excited-state proton transfer (ESPT)8−11 to form the anionic I* state, which emits dominant GFP fluorescence at 508 nm.6,7 After proton recombination, the ground-state I form quickly reforms neutral p-HBDI (A form). Upon excitation at 475 nm, the p-HBDI anion (B form) emits fluorescence at 503 nm.6,7 Conversion between the I state and the p-HBDI anion (B form) might involve reorganization of the surrounding protein matrix, but it occurs on a much slower time scale.6,7,12−15 © XXXX American Chemical Society

The hydrogen-bonding network that involves neutral p-HBDI, the internal water molecule, Ser205, and Glu222 was proposed to promote the rapid ESPT of the neutral chromophore (p-HBDI) of wtGFP according to the crystal structures of wtGFP and GFP S65T.3−5,13,16,17 The double mutant S205V/T203V of wtGFP, of which the blue fluorescence emission confirms no ESPT occurs, was designed to indirectly prove the ESPT of wtGFP and the hydrogen-bonding network.18 However, the ESPT-related photophysics of wtGFP such as charge transfer (CT) is still not clear, but understanding the photophysics of the GFP chromophore and its analogues is one of the important issues. To study the ESPT-related photophysics of wtGFP, one needs to build a model that involves p-HBDI, H2O, Ser205, and Glu222 in the right positions, but it is hard to do that. Use of the synthetic GFP chromophore, p-HBDI, was attempted to reproduce the ESPT of wtGFP, but it failed to display any ESPT because its S1 excited state undergoes other much faster radiationless transitions and a suitable proton acceptor is also critical.19 Received: February 21, 2018 Revised: June 16, 2018 Published: July 2, 2018 A

DOI: 10.1021/acs.jpca.8b01799 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A The isolated GFP chromophore, p-HBDI, significantly loses fluorescence after it is stripped of the protein environment of GFP.19 Low fluorescence quantum yields of p-HBDI and its p- and m-amino analogues were attributed to nonradiative relaxation by way of cis−trans photoisomerization through rotation around the I-bond,20−23 while another nonradiative decay pathway through solvent−solute hydrogen bonding plays an important role in the low fluorescence quantum yields in protic solvents.20−22 This is consistent with another experimental result that the excited-state lifetime of the GFP chromophore (p-HBDI) substantially increases in a solvent with high viscosity.24 Although cis−trans photoisomerization was suggested to be the major decay pathway for the S1 excited states of p-HBDI and its p- and m-amino analogues in an aprotic solvent, ab initio calculations at the CASPT2//CASSCF level suggested that the fluorescence state of the anionic form of p-HBDI decays mainly through barrierless torsional deformation around the P-bond rather than through cis−trans photoisomerization.25 The o-H2N analogue of GFP chromophore (o-ABDI)26,27 may have a different story from p-HBDI and its p- and m-amino analogues because the former has intramolecular hydrogen bonding. The fluorescence state of the o-Me2N analogue of GFP chromophore (o-DMABDI), which does not have intramolecular hydrogen bonding, was confirmed to be a 1(π, π*) charge-transfer state.26−28 Its fluorescence quantum yield is higher than that of o-ABDI, which has intramolecular hydrogen bonding but does not undergo excited-state intramolecular proton transfer (ESIPT).26−28 This suggests that intramolecular hydrogen bonding in o-ABDI decreases its fluorescence quantum yield. The o-HO analogue of GFP chromophore (o-HBDI) has intramolecular hydrogen bonding and undergoes ESIPT, which is ultrafast and finishes within 25 fs.29 According to the results from time-resolved fluorescence spectroscopy, after ESIPT, less than 1% of the zwitterionic o-HBDI emits fluorescence and the rest of it decays radiationlessly through the S1/S0 conical intersection by cis−trans isomerization around the I-bond.29 However, time-dependent (TD) density functional theory (DFT) and ab initio calculations suggested that, in addition to fluorescence, the zwitterionic o-HBDI decays radiationlessly through the S1/S0 conical intersection by rotating around the P-bond.30 To explore or tune the photophysical properties of wtGFP, chemists prepared various analogues of the p-HBDI chromophore.26−33 In our lab, we first prepared the o-sulfonamide analogue (o-TsABDI) of the GFP chromophore, obtained its single crystal X-ray diffraction structure, and found that it undergoes ESIPT according to its anomalously large Stokes shift of 10 666 cm−1.34,35 Later on, its ESIPT was confirmed by time-resolved ultraviolet−visible spectroscopy.36 The next interesting thing is the ESIPT-related photophysics of o-TsABDI. In this article, we explored the ESIPT-related photophysics of o-TsABDI by electronic absorption and fluorescence emission spectra in a wide polarity range of solvents, the cis−trans photoisomerization experiment, and the Coulomb-attenuating method (CAM)/TD-DFT calculations.37−44 The ESIPT that occurs in o-TsABDI might provide an alternative model to get insights into the ESPT-related photophysics of wtGFP.

and NEt3 (0.3 mL) in 3 mL of dried THF was added p-toluenesulfonyl chloride (191 mg, 1.2 mmol). The mixture was stirred in a nitrogen atmosphere at room temperature for 15 h. The solution was extracted with ethyl acetate, and the extracted solution was dried with anhydrous sodium sulfate and concentrated by a rotary evaporator. The crude product was purified by column chromatography with hexane/ethyl acetate (1:1) as a mobile phase to get o-TsABDI, and the yield was 65%: 1H NMR (CDCl3, 300 MHz) δ 2.33 (s, 3H, CH3), 2.48 (s, 3H, CH3), 3.24 (s, 3H, CH3), 6.88 (s, 1H, CH), 7.06 (m, 3H, PhH), 7.28−7.36 (m, 2H, PhH), 7.53 (d, J = 8.2 Hz, 2H, PhH), 7.63 (d, J = 8.2 Hz, 1H, PhH), 13.09 (s, 1H, NH); 13 C NMR (CDCl3, 300 MHz) δ 15.5, 21.5, 26.8, 122.8, 124.3, 125.2, 126.9, 127.5, 129.3, 132.2, 134.9, 136.1, 137.1, 137.2, 143.3, 161.1, 168.7; HRMS (EI, M+) m/z calcd for C19H19N3O3S, 369.1147, found 369.1143. cis-4-(2-(Ditosylamino)benzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-DTsABDI). To o-ABDI (215 mg, 1 mmol) and NEt3 (0.6 mL) in 3 mL of chloroform was added p-toluenesulfonyl chloride (420 mg, 2.2 mmol). The mixture was stirred in a nitrogen atmosphere at room temperature for 15 h. The solution was extracted with chloroform, and the extracted solution was dried with anhydrous sodium sulfate and concentrated by a rotary evaporator. The crude product was purified by column chromatography with hexane/ethyl acetate (1:1) as a mobile phase to get o-DTsABDI, and the yield was 85%: 1H NMR (CDCl3, 300 MHz) δ 2.33 (s, 3H, CH3), 2.41 (s, 6H, CH3), 3.12 (s, 3H, CH3), 6.75 (s, 1H, CH), 7.04 (d, J = 8.0 Hz, 1H, PhH), 7.26−7.34 (m, 5H, PhH), 7.49 (t, J = 7.5 Hz, 1H, PhH), 7.80 (d, J = 8.0 Hz, 4H, PhH), 8.73 (d, J = 8.0 Hz, 1H, PhH); 13C NMR (CDCl3, 300 MHz) δ 15.7, 21.7, 26.5, 121.1, 128.9, 129.7, 129.9, 130.4, 132.6, 133.2, 134.3, 135.8, 135.9, 139.7, 145.1, 163.8, 169.4; HRMS (EI, M+) m/z calcd for C26H25N3O5S2 523.1236, found 523.1235. cis−trans Photoisomerization. A solution for the cis−trans photoisomerization experiment was prepared by dissolving o-TsABDI or p-HBDI (0.1 mmol) in 1 mL of CD3CN. The solution was placed in a NMR tube, which was then purged with nitrogen and capped with a plastic cap. The solution was then irradiated with 350 nm UV light in a photoreactor at room temperature for 20 min. The conversion of cis−trans photoisomerization was calculated by proton NMR spectroscopy with fixed concentration of DMF as an internal standard. Fluorescence Quantum Yield Measurement. The fluorescence quantum yields of the compounds in this article were determined by the comparative method45 and measured by comparing the wavelength-integrated intensity of the test samples to a standard. The standard used in this article is quinine sulfate in 0.1 M H2SO4 with the known fluorescence quantum yield of 0.577 under the excitation wavelength of 350 nm.46 The fluorescence quantum yields of the compounds in this article were calculated using ϕ = ϕR(I/IR)(ODR/OD)(n2/nR2), where ϕ is the fluorescence quantum yield, I is the integrated fluorescence intensity, n is the refractive index of the solvent, and OD is the optical density. The subscript “R” refers to the reference standard (quinine sulfate in 0.1 M H2SO4). In this method, solutions of quinine sulfate and the compounds in this article were illuminated with the light of the same excitation wavelength (350 nm), where quinine sulfate and the compounds in this article have significant absorption. Solutions of the compounds in this article were prepared with optical densities in the range 0.1−0.01 for the sake of accuracy.

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EXPERIMENTAL SECTION Materials. o-ABDI26 was prepared according to the literature. cis-4-(2-(Tosylamino)benzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-TsABDI). To o-ABDI (215 mg, 1 mmol) B


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The Journal of Physical Chemistry A

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Scheme 1. Synthesis of o-TsABDI and o-DTsABDI

COMPUTATIONAL DETAILS

All the calculations were performed with the Gaussian 09 program.47 Geometry optimization of the ground state of o-TsABDI and its zwitterion was tried with the DFT method and the correlation-consistent polarized valence double-ζ basis set of Dunning (cc-pVDZ)48 without any symmetry restriction. After the geometric optimization, analytical vibration frequencies were calculated at the same level to determine the nature of the located stationary point. Geometry optimization of the S1 excited state of o-TsABDI and its zwitterion was tried with CAM-TD-DFT,37,38 which combines the hybrid qualities of TD-DFT39−44 and the long-range correction, and the cc-pVDZ basis set without any symmetry restriction. Simulation of the electronic absorptions of o-TsABDI was carried out at the level of CAM-TD-B3LYP/cc-pVDZ//B3LYP/cc-pVDZ, and the molecular orbitals that involve the electronic absorptions were visualized by GaussView. Simulation of the fluorescence emission of the S1 excited state of the zwitterionic o-TsABDI was carried out at the level of CAM-TD-B3LYP/cc-pVDZ, and the molecular orbitals that involve the fluorescence emission were also visualized by GaussView. The S1 excited-state and S0 ground-state rigid potential energy surfaces (PESs) for the ESIPT of o-TsABDI were performed by single-point energy calculation of the molecular structures along the bond length coordinate of SO2N−H from 104.7 to 192 pm and along the dihedral angle coordinate (φ) around the P-bond from 1.1° to 19.4° at the CAM-TD-B3LYP/cc-pVDZ level. The S1 excitedstate and S0 ground-state relaxed PESs49 of o-SABDI (o-HSO2NH analogue of GFP chromophore) and the zwitterionic o-SABDI were calculated by constrained geometry optimization of the molecular structures along the bond length coordinate of SO2N−H or along the dihedral angle coordinate (τ or φ) at the level of CAM-TD-B3LYP/cc-pVDZ. The barrier of the ESIPT of o-SABDI and the S1 excited-state rotational barriers of o-SABDI and the zwitterionic o-SABDI were calculated by minimum energy path (MEP) computation between the given initial and final states.49 The MEP computation involves constrained geometry optimization of a series of molecular structures along the reaction coordinate.49 In the constrained geometry optimization of each of molecular structures along the reaction coordinate, the reaction coordinate is fixed but the rest are relaxed. This is the same concept as computation for a relaxed PES.49 The structures of the S1/S0 conical intersections in the S1 excited-state and S0 ground-state PESs of o-SABDI and the zwitterionic o-SABDI were calculated by using the complete active space self-consistent field (CASSCF) method with the 4-31G basis set and an active space of 6 electrons in 6 π and n orbitals.47

(Table 1 and Figure 1). These electronic absorptions are not regularly shifted with solvent polarity, indicating that photoexcitation of o-TsABDI does not involve CT. Its electronic absorption in water is a little bit more blue-shifted. It is likely caused by ground-state stabilization of o-TsABDI through hydrogen bonding with water. The fluorescence emission of o-TsABDI is located at 590 nm (ϕF = 3.0 × 10−5) in DMSO, 595 nm (ϕF = 2.0 × 10−5) in acetonitrile, 597 nm (ϕF = 2.0 × 10−5) in THF, and 598 nm (ϕF = 4.0 × 10−5) in cyclohexane, which has an anomalously large Stokes shift of 10 666 cm−1 (Figure 1 and Table 1). This indicates that these fluorescence emissions are from the S1 excited state of the zwitterionic o-TsABDI that is generated by ESIPT of o-TsABDI, but the ground state of o-TsABDI still exists in the neutral structure with the proton on the sulfonamide nitrogen, instead of the zwitterionic structure. These fluorescence emissions are blue-shifted in a more polar solvent, indicating that the S1 excited state of the zwitterionic o-TsABDI is less polar than its Franck−Condon ground state. Besides, in water, both o-TsABDI and the zwitterionic o-TsABDI do not display any fluorescence. It is likely due to much faster nonradiative decay pathway through solvent−solute hydrogen bonding.20−22 The lowest-energy electronic absorption of o-DTsABDI is located at 367 nm (ε = 7.3 × 103 M−1 cm−1) in DMSO, 367 nm (ε = 1.05 × 104 M−1 cm−1) in acetonitrile, 366 nm (ε = 1.07 × 104 M−1 cm−1) in THF, and 365 nm (ε = 7.4 × 103 M−1 cm−1) in cyclohexane (Figure 2 and Table 1). These electronic absorptions change very little in solvents with a wide range of polarity, indicating that photoexcitation of o-DTsABDI, like photoexcitation of o-TsABDI, does not involve CT. On the other hand, the fluorescence emission of o-DTsABDI is located at 446 nm (ϕF = 2.1 × 10−2) in DMSO, 441 nm (ϕF = 1.7 × 10−2) in acetonitrile, 438 nm (ϕF = 3.0 × 10−2) in THF, and 427 nm (ϕF = 8.0 × 10−3) in cyclohexane. These fluorescence emissions are red-shifted in a more polar solvent. This indicates that the S1 excited state of o-DTsABDI is more polar than its Franck−Condon ground state. However, this behavior of the S1 excited state of o-DTsABDI is completely different from that of the S1 excited state of the zwitterionic o-TsABDI, of which the fluorescence emission is blue-shifted in a more polar solvent, even though both o-DTsABDI and the zwitterionic o-TsABDI have similar chromophores. Hence, the electronic and molecular structures of the S1 excited state of the zwitterionic o-TsABDI are expected to be quite unusual and interesting. The o-TsABDI anion was generated by titration of o-TsABDI in acetonitrile with NaOH(aq) with 3 μL of water added. Its electronic absorption is located at 450 nm (ε = 9.4 × 103 M−1 cm−1),

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RESULTS AND DISCUSSION Synthesis. We prepared o-TsABDI by treating o-ABDI26 with 1.2 equiv of tosyl chloride in the presence of NEt3 (Scheme 1). In comparison with o-TsABDI for the ESIPT-related photophysics, o-DTsABDI was prepared by treating o-ABDI with 2.2 equiv of tosyl chloride in the presence of NEt3. Electronic Absorption, Fluorescence Emission, and Solution Structures. The lowest-energy electronic absorption of o-TsABDI is located at 362 nm (ε = 7.1 × 103 M−1 cm−1) in dimethyl sulfoxide (DMSO), 364 nm (ε = 5.5 × 103 M−1 cm−1) in acetonitrile, 367 nm (ε = 5.3 × 103 M−1 cm−1) in tetrahydrofuran (THF), 366 nm (ε = 4.1 × 103 M−1 cm−1) in cyclohexane, and 347 nm (ε = 6.4 × 103 M−1 cm−1) in water C


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The Journal of Physical Chemistry A Table 1. Photophysical Properties of Neutral, Zwitterionic, Anionic, and Cationic o-TsABDI, and of o-DTsABDI

compound neutral o-TsABDI

zwitterionic o-TsABDI

anionic o-TsABDI

cationic o-TsABDI o-DTsABDI

solvent (ηa/cP)

λabs/nm

λf/nm (ϕF × 10−3)

DMSO (1.99) CH3CN (0.37) THF (0.46) C6H12 (0.89) H2O (0.89) DMSO (1.99) CH3CN (0.37) THF (0.46) C6H12 (0.89) H2O (0.89) DMSO CH3CN H2O CH3CN DMSO CH3CN THF C6H12

362 364 367 366 347 b b b b b 463 450 362 364 367 367 366 365

no fluorescence no fluorescence no fluorescence no fluorescence no fluorescence 590 (0.03) 595 (0.02) 597 (0.02) 598 (0.04) no fluorescence 571 (6) 578 (5) no fluorescence 412 (0.02) 446 (21) 441 (17) 438 (30) 427 (8)

Viscosity at 25 °C from CRC Handbook of Chemistry and Physics, 85th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2004. bThe ground state of the zwitterionic o-TsABDI is not a global minimum in its potential energy surface, so its steady-state electronic spectrum cannot be measured.

a

Figure 2. Normalized electronic absorption (dotted lines) and fluorescence emission spectra (solid lines; excitation wavelength, 350 nm) of o-DTsABDI in acetonitrile (dark blue), THF (red), or cyclohexane (lime green).

Figure 1. Normalized electronic absorption (dotted lines) and fluorescence emission spectra (solid lines; excitation wavelength, 350 nm) of o-TsABDI in water (cyan), DMSO (purple), acetonitrile (dark blue), THF (red), or cyclohexane (lime green).

higher than that of the zwitterionic o-TsABDI, it is still a little bit smaller than that of o-DTsABDI. Just like the fact that the fluorescence quantum yield of o-DMABDI is higher than that of o-ABDI,26−28 we suggest that one of reasons why both the o-TsABDI anion and o-DTsABDI have higher fluorescence quantum yields is because they do not have intramolecular hydrogen bonding. The o-TsABDI cation was prepared by titration of o-TsABDI in acetonitrile with HCl(aq) with 3 μL of water added. Its fluorescence emission is significantly blue-shifted to 412 nm with the same fluorescence quantum yield (ϕF = 2.0 × 10−5), indicating that a proton is attached to the amidine nitrogen of o-TsABDI during the acidification (Table 1). However, its electronic absorption is located at 364 nm (ε = 5.5 × 103 M−1 cm−1), which does not change in comparison with o-TsABDI. This indicates o-TsABDI in acetonitrile exists in the neutral structure,

which is significantly red-shifted in comparison with o-TsABDI (Figure 3 and Table 1). This is consistent with another similar experimental result where the electronic absorption of the p-HBDI anion is around 75 nm red-shifted in comparison with that of p-HBDI.31 This result indicates that most of the sulfonamide N−H acids of the ground-state o-TsABDI are not dissociated. However, its fluorescence emission is blue-shifted to 578 nm (ϕF = 5.0 × 10−3) with a 250-fold increase in the fluorescence quantum yield in comparison with the fluorescence emission from the zwitterionic o-TsABDI. This result confirms that the fluorescence emission of o-TsABDI is not directly from the S1 excited state of o-TsABDI but is from the S1 excited state of the zwitterionic o-TsABDI. Although the fluorescence quantum yield of the o-TsABDI anion is much D


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Figure 3. Changes in electronic absorption (left) and fluorescence emission spectra (right; excitation wavelength, 350 nm) for titration of o-TsABDI in CH3CN with NaOH(aq) with 3 μL of water added.

level. The predicted electronic absorption (S0 → S1) energy of o-TsABDI is 3.46 eV with an oscillator strength of 0.42, which is quite close to the experimental value of 3.38 eV (366 nm, f = 0.12) (Table 2). The Franck−Condon S1 excited state to

instead of the zwitterionic structure. If o-TsABDI in acetonitrile exists in the zwitterionic structure, titration of o-TsABDI in acetonitrile with HCl(aq) would make its electronic absorption blue-shifted. In order to understand the properties of neutral o-TsABDI and the o-TsABDI anion even better, their electronic absorption spectra in various ratios of mixed CH3CN−H2O solvents were analyzed (Figure 4). Upon increasing the water percentage

Table 2. Calculated Electronic Vertical Transition Energies ΔE (in eV) and Oscillator Strengths f of the Ground State of o-TsABDI in the Gas Phase at CAM-TD-B3LYP/cc-pVDZ Level CAM-TD-B3LYP compound

electronic transition

o-TsABDI

S0 → S1

ΔE

f

a

3.46

0.42

expt ΔE b

3.38

f 0.12

The excited state is described mostly by 0.70[MO 97 → MO 98]. b Solvent for the electronic absorption is cyclohexane. a

where electronic excitation takes place is described mostly by an excited Slater determinant where a molecular orbital (MO) 97−MO 98 excitation is considered. Hence, it is practically possible to understand the electronic structure of the Franck− Condon S1 excited state at the MO level.51 Both MO 97 (highest occupied molecular orbital, HOMO) and MO 98 (lowest unoccupied molecular orbital, LUMO) are characterized by the uniform spread of electron density around the 4-benzylideneimidazolone moiety (Figure 5), so the electronic vertical transition of o-TsABDI from its ground state (S0) to its Franck−Condon S1 excited state during electronic absorption does not involve CT (Scheme 2). This is consistent with the experimental result that the lowest-energy electronic absorption of o-TsABDI is not regularly shifted with solvent polarity. Thus, the Franck−Condon S1 excited state of o-TsABDI is assigned to a 1(π, π*) excited state. The ground-state structure of o-TsABDI was also optimized at the B3LYP/cc-pVDZ level (Figure 5). The cis-isomer is more stable than the trans-isomer. The calculated torsional angles τ and φ around the I-bond and P-bond are 0.2 and 1.1°, respectively, which are close to the values of 3.84 and 8.17° in its single crystal structure,34,35 respectively. These torsional angles indicate that the imidazolinone and aniline rings of o-TsABDI almost stay in the same plane. The calculated bond lengths of the I-bond and the P-bond are 136.3 and 145.7 pm, respectively, which are quite consistent with the values of 136.7 and 146.0 pm in its single crystal structure,34,35 respectively. These bond lengths and torsional angles indicate that both P-bond and I-bond linking the aniline and imidazolinone rings stay in a fully conjugated resonance. The Mulliken charges of o-TsABDI and its Franck−Condon S1 excited state were calculated at the CAM-TD-B3LYP/cc-pVDZ

Figure 4. Electronic absorption spectra of neutral o-TsABDI (solid lines) and o-TsABDI anion (dotted lines) in mixed CH3CN−H2O solvents (acetonitrile, dark blue; 2:1 CH3CN−H2O, red; 1:2 CH3CN−H2O, lime green; water, purple).

of mixed CH3CN−H2O solvents, the electronic absorption of TsABDI is slightly blue-shifted from 364 to 347 nm while the electronic absorption of the o-TsABDI anion is significantly blue-shifted from 450 to 362 nm. This indicates that water stabilizes the ground state of the o-TsABDI anion through hydrogen bonding much better than the ground state of o-TsABDI. When the water percentage of mixed CH3CN−H2O solvents reaches 100%, the electronic absorption of o-TsABDI looks like that of the o-TsABDI anion. It is likely that the basicity of the o-TsABDI anion is strong enough to grab a proton of water through hydrogen bonding, making the electronic absorptions of both o-TsABDI and the o-TsABDI anion look alike. We used Shi’s method50 to calculate the ground-state N−H acid strength (pKa) of o-TsABDI in DMSO to be 12.7 in DMSO, of which the acid strength is not strong. Hence, the basicity of the o-TsABDI anion is strong enough to grab a proton of water through hydrogen bonding. Insights into Electronic Transition from Ground-State o-TsABDI. The lowest-energy electronic absorption (S0 → S1) of o-TsABDI was calculated at the CAM-TD-B3LYP/cc-pVDZ E


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Figure 5. Optimized structure (left), MO 97 (HOMO, middle), and MO 98 (LUMO, right) of ground-state o-TsABDI at CAM-TD-B3LYP/ccpVDZ//B3LYP/cc-pVDZ level. I, bond length of I-bond; P, bond length of P-bond; H, bond length of N−H bond; τ, torsional angle around I-bond; φ, torsional angle around P-bond.

consistent with the experimental result that the lowest-energy electronic absorption of o-TsABDI is not regularly shifted with solvent polarity. S1 Excited-State Molecular Structure. The S1 excitedstate structure of o-TsABDI was optimized at the CAM-TDB3LYP/cc-pVDZ level (Figure 6). During the optimization, we

Scheme 2. Proposed ESIPT-Involved Photophysics of o-TsABDI

Figure 6. Optimized S1 excited-state structure of zwitterionic o-TsABDI at CAM-TD-B3LYP/cc-pVDZ level. I, bond length of I-bond; P, bond length of P-bond; H, bond length of N−H bond; τ, torsional angle around I-bond; φ, torsional angle around P-bond.

level (Table 3). The charge of sulfonamide nitrogen of o-TsABDI is −0.33, and it turns into −0.27 right after excitation to its Table 3. Mulliken Charges of the Ground State of o-TsABDI (GS) and Its Franck−Condon S1 Excited State (F−C ES) in the Gas Phase at CAM-TD-B3LYP/cc-pVDZ Level specified atom, GS Mulliken charge

did not find any global minimum in the S1 excited-state PES of o-TsABDI, but the proton on the sulfonamide nitrogen of o-TsABDI gradually migrated to the imidazolinone nitrogen until the S1 excited state of the zwitterionic o-TsABDI was successfully optimized, indicating o-TsABDI undergoes ESIPT, which is consistent with the experimental result. For the optimized S1 excited-state structure of the zwitterionic o-TsABDI, the torsional angle φ (19.4°) around the P-bond is a little bit larger, the bond length of the I-bond (139.0 pm) is longer, and the bond length of the P-bond (143.9 pm) is shorter in comparison with the single crystal X-ray diffraction structure and the optimized structure (Figure 5) of the ground state of o-TsABDI.34,35 Even though the I-bond is a little bit longer, it still has double-bond character. Although the P-bond is a little bit shorter, it still has more single-bond character. This indicates that both the P-bond and the I-bond linking the aniline and imidazolinone rings in the S1 excited state of the zwitterionic o-TsABDI still stay in a good conjugated resonance even though the torsional angle φ (19.4°) around the P-bond is a little bit larger. S1 Excited-State Acidity. Based on the thermodynamics of the Forster cycle, an excited state is a stronger acid than its ground state if the absorption or emission spectrum of the conjugated base is characterized by a red shift relative to that of the conjugated acid.8 Thus, the S1 excited state of o-TsABDI is a stronger acid than its ground state. The Forster equation is pKa* = pKa − (hc/λ1 − hc/λ2)/2.3RT, where λ1 is 362 nm for the electronic absorption of o-TsABDI in DMSO and λ2 is 463 nm

specified atom, F−C ES

SO2N

SO2NH−N

SO2N

SO2NH−N

−0.33

−0.28

−0.27

−0.29

Franck−Condon S1 excited state. The decrease of the negative charge by −0.06 on the sulfonamide nitrogen of o-TsABDI right after excitation is quite small and reasonable because its LUMO has a smaller electron density on the sulfonamide nitrogen than its HOMO. The slight decrease of the negative charge on the sulfonamide nitrogen of o-TsABDI should not be interpreted as CT. The dipole moments of the ground state of o-TsABDI and its Franck−Condon S1 excited state were calculated at the CAM-TD-B3LYP/cc-pVDZ level (Table 4). The dipole Table 4. Dipole Moments μ (in D) of Ground State of o-TsABDI (GS) and Its Franck−Condon S1 Excited State (F−C ES) and Their Dipole Moment Difference Δμ (in D) in the Gas Phase at CAM-TD-B3LYP/cc-pVDZ Level GS dipole moment, μ Δμ

F−C ES

4.71

4.32 −0.39

moment of the ground state of o-TsABDI is very close to that of its Franck−Condon S1 excited state. This result is F


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The Journal of Physical Chemistry A for the electronic absorption of the o-TsABDI anion in DMSO. Hence, the ΔpKa* (=pKa* − pKa) of o-TsABDI in DMSO is −12.7. It is quite unusual that the pKa* of the o-TsABDI photoacid is significantly decreased by 12.7 in comparison with its ground-state pKa. Thus, the pKa* value of the S1 excited state of o-TsABDI in DMSO is 0, which is low enough to be able to protonate its amidine nitrogen,52 causing the ESIPT. Insights into Fluorescence Emission from S1 Excited State of Zwitterionic o-TsABDI. The fluorescence emission (S1 → S0) of the zwitterionic o-TsABDI was calculated at the CAM-TD-B3LYP/cc-pVDZ level. The predicted fluorescence emission energy of the zwitterionic o-TsABDI is 1.46 eV with an oscillator strength of 2 × 10−4, which is consistent with the experimental value of 2.07 eV (598 nm, ϕF = 4 × 10−5) (Table 5).

does not undergo ESIPT, is still much higher than that of the zwitterionic o-TsABDI. Hence, the intramolecular hydrogen bonding in the zwitterionic o-TsABDI is not the major factor for its low fluorescence quantum yield. We suggest that the 1 (n, π*) CT excited state of the zwitterionic o-TsABDI could be partially responsible for its low fluorescence quantum yield because the fluorescence involves low-probability π* → n charge transfer.53 It seems to us that the 1(n, π*) CT excited state of the zwitterionic o-TsABDI is not caused by its intramolecular hydrogen bonding only because the fluorescence state of o-ABDI, which has intramolecular hydrogen bonding, is not a 1(n, π*) CT excited state but a 1(π, π*) CT excited state.26−28 We suggest that the 1(n, π*) CT excited state of the zwitterionic o-TsABDI is highly related to ESIPT. It is the ESIPT plus the thermodynamically favorable n → π electron transfer that coproduces the 1(n, π*) CT excited state of the zwitterionic o-TsABDI (Scheme 2). The Mulliken charges of the S1 excited state of the zwitterionic o-TsABDI and its Franck−Condon ground state were calculated at the CAM-TD-B3LYP/cc-pVDZ level (Table 6).

Table 5. Calculated Fluorescence Emission Energies ΔE (in eV) and Oscillator Strengths f from the S1 Excited State of the Zwitterionic o-TsABDI in the Gas Phase at CAM-TDB3LYP/cc-pVDZ Level CAM-TD-B3LYP compound zwitterionic o-TsABDI

expt

electron-density redistribn

ΔE

f

ΔE

ϕF

S1 → S0

1.46a

2 × 10−4

2.07b

4 × 10−5

Table 6. Mulliken Charges of S1 Excited State of Zwitterionic o-TsABDI (S1 ES) and Its Franck−Condon Ground State (F−C GS) in the Gas Phase at CAM-TDB3LYP/cc-pVDZ Level

The excited state is described mostly by 0.69[MO 97 → MO 98]. Solvent for the fluorescence emission is cyclohexane.

a

b

specified atom, F−C GS

The optimized S1 excited state of the zwitterionic o-TsABDI from where fluorescence takes place is described mostly by an excited Slater determinant where an MO 97−MO 98 excitation is considered. Hence, it is practically possible to understand the electronic structure of the optimized S1 excited state of the zwitterionic o-TsABDI at the MO level.51 Most of the electron density of the MO 98 is located on the π* orbital of the 4-benzylideneimidazolone moiety, while most of the electron density of the MO 97 is located on the n orbital of sulfonamide nitrogen (Figure 7). Thus, the fluorescence emission from the

Mulliken charge

specified atom, S1 ES

SO2N

SO2NH−N

SO2N

SO2NH−N

−0.59

−0.01

−0.38

−0.06

Right after the S1 excited state of the zwitterionic o-TsABDI emits fluorescence and forms its Franck−Condon ground state, the Mulliken charge of its sulfonamide nitrogen turns more negative from −0.38 to −0.59, which is quite significant, while the Mulliken charge of its 4-benzylideneimidazolone moiety turns less negative. This result confirms that the fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI accompanies the CT from the π* orbital of the 4-benzylideneimidazolone moiety to the half-filled n orbital of the sulfonamide nitrogen (Scheme 2). The dipole moments of the S1 excited state of the zwitterionic o-TsABDI and its Franck−Condon ground state were calculated at the level of CAM-TD-B3LYP/cc-pVDZ, and they are 1.51 and 7.79 D, respectively (Table 7). The dipole moment Table 7. Dipole Moments (in D) of S1 Excited State of Zwitterionic o-TsABDI (S1 ES) and Its Franck−Condon Ground State (F−C GS) and Their Dipole Moment Difference Δμ (in D) in the Gas Phase at CAM-TD-B3LYP/ cc-pVDZ Level

Figure 7. MO 97 (left) and MO 98 (right) for S1 excited state of zwitterionic o-TsABDI at CAM-TD-B3LYP/cc-pVDZ level.

S1 excited state of the zwitterionic o-TsABDI to its Franck− Condon ground state is best described as a charge transfer out of the π* orbital of the 4-benzylideneimidazolone moiety into the half-filled n orbital of sulfonamide nitrogen (Scheme 2). As a result, the S1 excited state of the zwitterionic o-TsABDI is assigned to a 1(n, π*) CT excited state. Interestingly, the S1 excited state of o-DMABDI, which does not have intramolecular hydrogen bonding, is a 1(π, π*) CT excited state,28 of which the fluorescence quantum yield is much higher than that of the zwitterionic o-TsABDI.26 Although the intramolecular hydrogen bonding in the zwitterionic o-TsABDI may be partially responsible for its low fluorescence quantum yield, the fluorescence quantum yield of o-ABDI,26,27 which has intramolecular hydrogen bonding but

F−C GS dipole moment, μ Δμ

S1 ES

7.79

1.51 −6.28

difference Δμ between its S1 excited state and Franck−Condon ground state is −6.28 D. The magnitude of the dipole moment difference is high and comparable with those (10.8 and 10.9 D) of o-ABDI and o-DMABDI, which display intramolecularcharge-transfer (ICT) fluorescence.26−28 Hence, fluorescence from the S1 excited state of the zwitterionic o-TsABDI should involve ICT. The sign of the dipole moment difference is negative, and opposite that of o-ABDI or o-DMABDI.26−28 G


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Figure 8. S1 excited-state and S0 ground-state rigid PESs for ESIPT of o-TsABDI (left) and S1 excited-state and S0 ground-state relaxed PESs for ESIPT of o-SABDI (right) at CAM-TD-B3LYP/cc-pVDZ level.

This indicates that o-ABDI or o-DMABDI has the ICT fluorescence state more polar than the ground state while the S1 excited state of the zwitterionic o-TsABDI is much less polar than that of its Franck−Condon ground state. This unusual result is consistent with the proposed charge-transfer mechanism (Scheme 2) and the experimental result that the fluorescence emission of the zwitterionic o-TsABDI is blue-shifted in a more polar solvent (Table 1 and Figure 1). Insights into ESIPT. Before the ESIPT, the dipole moment of the Franck−Condon S1 excited state of o-TsABDI is 4.32 D (Table 4). After the ESIPT, charge separation and the dipole moment of the S1 excited state of the zwitterionic o-TsABDI would be increased by the proton transfer because the Franck− Condon ground state of the zwitterionic o-TsABDI has a much higher dipole moment than the neutral ground state of o-TsABDI (Tables 4 and 7). However, the dipole moment of the S1 excited state of the zwitterionic o-TsABDI is 1.51 D, which is much lower than that of the Franck−Condon S1 excited state of o-TsABDI. In addition to that, we just elucidated that the Franck−Condon S1 excited state of o-TsABDI is a 1(π, π*) excited state while the S1 excited state of the zwitterionic o-TsABDI is a 1(n, π*) CT excited state (Figures 5 and 7 and Scheme 2). Hence, this means that the ESIPT from the Franck−Condon S1 excited state of o-TsABDI to the S1 excited state of the zwitterionic o-TsABDI must accompany an electron transfer, whose direction is exactly opposite that of the redistribution of electron density that occurs in the next CT fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI (Scheme 2). Hence, during the ESIPT of o-TsABDI, the accompanying electron transfer should move from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety, which is an exothermic and thermodynamically favorable process. This is consistent with what happens to the O−H photoacids, where the photoexcitation triggers an ICT from the oxygen atom to the aromatic ring system for the next proton transfer step.54 Potential Energy Surfaces (PESs) of ESIPT. The S1 excited-state and S0 ground-state rigid PESs for the ESIPT of o-TsABDI were calculated at the CAM-TD-B3LYP/cc-pVDZ level by changing the bond length of SO2N−H and the dihedral angle φ around the P-bond (Figure 8). In order to get the S1 excited-state and S0 ground-state relaxed PESs for the ESIPT and save computer time, o-TsABDI was represented by

o-SABDI, of which the ground-state and S1 excited-state structures calculated at the CAM-TD-B3LYP/cc-pVDZ level are shown in Figure 9. As shown in Figure 8, the S1 excited-state and S0 ground-state relaxed PESs for the ESIPT of o-SABDI along the bond length coordinate of SO2N−H were calculated at the CAM-TD-B3LYP/cc-pVDZ level. The relaxed and rigid PESs of the ESIPT look similar, but the major difference between them is the barrier of ESIPT (Figure 8). According to the relaxed PESs of the ESIPT, the barrier of the ESIPT from the Franck−Condon S1 excited state of o-SABDI to the S1 excited state of the zwitterionic o-SABDI is around 0.4 kcal/mol, which is quite small in comparison with the barriers of the ESIPT in other compounds such as 1-(1H-imidazol-2-yl)naphthalen-2-ol, 5.11 kcal/mol;55 2-(2′-hydroxyphenyl)imidazole, 9.3 kcal/mol;56 and 1-hydroxy-2-acetonaphthone. 6.26 kcal/mol.57 The barrier for its backward reaction is around 12.3 kcal/mol. This indicates that the ESIPT has a very low barrier and the S1 excited state of the zwitterionic o-SABDI is the major product of the ESIPT equilibrium. This is consistent with the experimental result that the fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI is the only fluorescence that can be seen after photoexcitation of o-TsABDI. We found that the ground state of the zwitterionic o-TsABDI is not stable because it is not a global minimum in its PES at the B3LYP/cc-pVDZ level. According to the PESs, the proton recombination of the zwitterionic o-TsABDI is a barrierless and exothermic process with ΔE = −39.2 kcal/mol. Hence, the proton recombination is an irreversible reaction. This is consistent with the experimental result. We further confirm that the proton recombination is a barrierless reaction from the viewpoint of acid-strength difference between the acid (HA) and the conjugated acid (HB+) of the base (B). According to the literature,58 a ΔpKa [=pKa(HB+) − pKa(HA)] value of around 5 is a turning point between activated and barrierless proton transfer reactions. The pKa(NH+) value for the acid group (the N-protonated imidazolone group) in the Franck− Condon ground state of the zwitterionic o-TsABDI is around 1.4 in water.52 The pKa(SO2NH) value for the conjugated acid of the base group (the sulfonamide anion) in the Franck− Condon ground state of the zwitterionic o-TsABDI is expected to be greater than 8.0 in water because it is 12.7 in DMSO. The ΔpKa = pKa(SO2NH) − pKa(NH+) > 6.6 in water, and that H


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Figure 9. Optimized S0 structure of o-SABDI at B3LYP/cc-pVDZ level and optimized S1 excited-state structure of zwitterionic o-SABDI at CAMTD-B3LYP/cc-pVDZ level.

Figure 10. S1 excited-state (blue line) and S0 ground-state (red line) relaxed PESs of o-SABDI along dihedral angle coordinate τ (left) or φ (right) at CAM-TD-B3LYP/cc-pVDZ level.

makes the proton recombination of the zwitterionic o-TsABDI become a barrierless proton transfer reaction. Nonradiative Relaxation of S1 Excited States of o-TsABDI and Zwitterionic o-TsABDI. To understand why the fluorescence quantum yield of the zwitterionic o-TsABDI is so low, we explored possible nonradiative relaxation pathways from the S1 excited states of the zwitterionic o-SABDI and o-SABDI, which is the representative of o-TsABDI in order to save computer time. The relaxation pathways, where the chromophore is free from host constraints, are thought to proceed via the one-bond-flip mechanism.59 In the S1 excited-state and S0 ground-state relaxed PESs of o-SABDI that were generated by rotating around the P-bond, the S1−S0 energy difference is greater than 55 kcal/mol (Figure 10). According to the energy gap law,53 the internal conversion through rotation around the P-bond should not be fast. The S1 excited-state rotational barrier around the P-bond is around 9.2 kcal/mol. Part of the barrier could be due to overcoming the intramolecular hydrogen bonding. For hydrogen bonding energy of related compounds,

it was reported that each hydrogen bonding energy of C(O)NHOC is around 5−8 kcal/mol in polypeptides.60,61 The similar S1 excited-state rotational barrier around the P-bond is also found in the p-HBDI anion, of which the fluorescence state passes a 0−0.5 kcal/mol barrier to reach another 7 kcal/mol more stable S1 excited-state minimum, followed by overcoming a 12 kcal/mol barrier to reach its conical intersection.25 On the other hand, rotation around the I-bond of o-SABDI makes its S1 excited-state and S0 ground-state relaxed PESs come into contact at the dihedral angle τ of around 80° (Figure 10). According to the relaxed PESs in Figure 10, the S1 excited-state rotational barrier around the I-bond of o-SABDI is around 6.5 kcal/mol. Part of the barrier could be also due to overcoming the intramolecular hydrogen bonding. The similar S1 excited-state rotational barrier around the I-bond is also found in the p-HBDI anion, of which the fluorescence state passes a 2 kcal/mol barrier to reach another 9 kcal/mol more stable S1 excited-state minimum, followed by overcoming a 14 kcal/mol I


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The Journal of Physical Chemistry A barrier to reach its conical intersection.25 To save computer time, we used o-SABDI to represent o-TsABDI in optimizing its S1/S0 conical intersection structure at the CASSCF(6,6)/431G level by starting from the avoided-crossing structure in Figure 10. The CI(I) in Figure 11 is the S1/S0 conical intersection

excited state of o-TsABDI could not be found in its S1 excitedstate PES. After ESIPT, in addition to fluorescence from the S1 excited state of the zwitterionic o-TsABDI, we wondered if it also competitively relaxes through rotation around the I-bond or the P-bond. To save computer time, we used the zwitterionic o-SABDI to stand for the zwitterionic o-TsABDI. According to the S1 excited-state and S0 ground-state relaxed PESs of the zwitterionic o-SABDI that were prepared by rotating around its I-bond or P-bond, there are two avoided crossings that were found at the dihedral angle τ of around 70° or the dihedral angle φ of around −80° (Figure 12). The two avoided-crossing structures were used as starting structures to optimize its S1/S0 conical intersection structures at the CASSCF(6,6)/4-31G level. As shown in Figure 13, the two S1/S0 conical intersections CI(I″) and CI(P″) in the S1 excited-state and S0 groundstate relaxed PESs of the zwitterionic o-SABDI were found by rotating around its I-bond or P-bond. The S1 excited-state rotational barriers around the I-bond and the P-bond of the zwitterionic o-SABDI are around 18.9 and 2.7 kcal/mol, respectively, which are very close to those (19 and 2.8 kcal/mol) of the zwitterionic p-HBDI.63 For the S1/S0 conical intersection CI(I″), its dihedral angle τ is 81.8°, which is less than 90° and smaller than that of CI(I). Its I-bond and P-bond have more single-bond character. If the S1 excited state of the zwitterionic o-TsABDI relaxes radiationlessly through the S1/S0 conical intersection CI(I″) by rotating around the I-bond, the zwitterionic o-TsABDI would likely keep the same cis-configuration after photoexcitation, because the dihedral angle τ of its S1/S0 conical intersection CI(I″) is less than 90°. In the S1/S0 conical intersection CI(P″), the P-bond has a single-bond character with the dihedral angle φ of −149.7°, but the I-bond has a double-bond character with the dihedral angle τ of 17.9°. Hence, the S1/S0 conical intersection CI(P″) can be reached by rotating around the P-bond of the zwitterionic o-SABDI. The S1 excited-state rotational barrier to reach the S1/S0 conical intersection CI(P″) of the zwitterionic o-TsABDI is around 2.7 kcal/mol, which is much smaller than that (18.9 kcal/mol) of reaching the S1/S0 conical intersection CI(I″) by rotating around the I-bond. Hence, the former would be a faster relaxation pathway than the latter, besides the competitive fluorescence pathway that involves low-probability π* → n charge transfer (Scheme 3). This result is similar to the computational result for the zwitterionic o-HBDI, of which the fluorescence state mainly undergoes radiationless decay through the S1/S0 conical intersection by rotating around the P-bond.30 cis−trans Photoisomerization. After 20 min of irradiation with 350 nm UV light in a photoreactor at room temperature, around 36% of p-HBDI was converted from cis-configuration to trans-configuration. This is consistent with the literature.20,52 However, in the same condition, o-TsABDI, which exists in 100% cis-configuration, was not converted to its transconfiguration at all (Figure 14). This experimental result is consistent with the proposed relaxation pathways for the Franck−Condon S1 excited state of o-TsABDI, which were calculated at the levels of CAM-TD-B3LYP/cc-pVDZ and CASSCF(6,6)/4-31G (Scheme 3). After photoexcitation, the Franck−Condon S1 excited state of o-TsABDI prefers ESIPT, instead of fluorescence directly from its primitive structure or nonradiative relaxation through the S1/S0 conical intersection CI(I) by rotating around the I-bond. After ESIPT, the

Figure 11. Optimized structure of S1/S0 conical intersection CI(I) found in S1 excited-state and S0 ground-state relaxed PESs of o-SABDI by rotating around the I-bond at CASSCF(6,6)/4-31G level.

that we found in the S1 excited-state and S0 ground-state relaxed PESs of o-SABDI by rotating around the I-bond. In the CI(I), the dihedral angle τ is 101.6° with the single-bond character of the I bond while the dihedral angle φ is −2.6° with the double-bond character of the P bond. The structure of the S1/S0 conical intersection CI(I) is very similar to that of the RFP chromophore.62 Even though there is an S1/S0 conical intersection CI(I) in the S1 excited-state and S0 ground-state relaxed PESs of o-SABDI by rotating around the I-bond, the S1 excited-state rotational barrier to reach the S1/S0 conical intersection is estimated to be around 6.5 kcal/mol, which is much higher than that (ca. 0.4 kcal/mol) of its ESIPT (Figures 8 and 10). As a result, we suggest that most of the Franck−Condon S1 excited state of o-SABDI undergoes ESIPT to form the S1 excited state of the zwitterionic o-SABDI, and few decay radiationlessly through the S1/S0 conical intersection CI(I) by rotating around the I-bond (Scheme 3). Besides, after photoexcitation, o-TsABDI Scheme 3. Proposed Relaxation Pathways of o-TsABDI after Photoexcitation

does not fluorescence from its primitive structure because we did not see it experimentally and a global minimum for the S1 J


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Figure 12. S1 excited-state (blue line) and S0 ground-state (red line) relaxed PESs of zwitterionic o-SABDI along the dihedral angle coordinate τ (left) or φ (right) at CAM-TD-B3LYP/cc-pVDZ level.

cyclohexane are around 4 or 2 times more than that of acetonitrile or THF. The fluorescence quantum yield of the zwitterionic o-TsABDI in DMSO or cyclohexane is around 1.5 or 2 times more than that in acetonitrile or THF (Table 1). This might suggest that solvent viscosity restricts or slows down the relaxation rotation around the P-bond of the zwitterionic o-TsABDI, which is not a volume-conserving process, and that might enhance its fluorescence a little bit.

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CONCLUSION The ESIPT of o-TsABDI involves four processes. The first process is photoexcitation of o-TsABDI at λ = 366 nm in cyclohexane, which involves an excitation of an electron out of the MO 97 (HOMO) into the MO 98 (LUMO). The photoexcitation does not involve CT. The dipole moments of o-TsABDI and its Franck−Condon S1 excited state are 4.71 and 4.32 D, respectively. The second process is the ESIPT from the Franck−Condon S1 excited state of o-TsABDI to the S1 excited state of the zwitterionic o-TsABDI. This ESIPT accompanies the

Figure 13. Optimized structures of S1/S0 conical intersections CI(I″) and CI(P″) found in S1 excited-state and S0 ground-state relaxed PESs of the zwitterionic o-SABDI by rotating around the I-bond or the P-bond at CASSCF(6,6)/4-31G level.

zwitterionic o-TsABDI emits fluorescence with low-probability π* → n charge transfer, which is in competition with nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond. Solvent-Viscosity Influence on Fluorescence of Zwitterionic o-TsABDI. The viscosities of DMSO and

Figure 14. 1H NMR spectra of o-TsABDI and p-HBDI in CD3CN (a) before and (b) after 20 min of irradiation with 350 nm UV light. K



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thermodynamically favorable electron transfer from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety, reducing its dipole moment. Thus, the dipole moment of the S1 excited state of the zwitterionic o-TsABDI becomes as low as 1.51 D. The barrier of the ESIPT is also as low as around 0.4 kcal/mol, which is much smaller than the barrier (12.3 kcal/mol) of its backward reaction. As a result, when o-TsABDI is excited, the fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI is the only fluorescence that can be seen. The fluorescence emission that is directly from the S1 excited state of o-TsABDI cannot be seen. The third process is CT fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI to its Franck− Condon ground state at λ = 598 nm in cyclohexane. The fluorescence emission accompanies charge transfer from the MO 98 to the MO 97, which is best described as a charge transfer out of the π* orbital of the 4-benzylideneimidazolone moiety into the half-filled n orbital of sulfonamide nitrogen, increasing its dipole moment. Thus, its dipole moment becomes 7.79 D right after the CT fluorescence emission. Hence, the CT fluorescence emission from the S1 excited state of the zwitterionic o-TsABDI to its Franck−Condon ground state is blue-shifted in a more polar solvent. The fourth process is proton recombination, which is an irreversible and barrierless reaction. After photoexcitation, o-TsABDI follows ESIPT much faster than the relaxation through the S1/S0 conical intersection CI(I) by rotating around the I-bond. After ESIPT, the zwitterionic o-TsABDI emits fluorescence involving low-probability π* → n charge transfer, which is in competition with the nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond. The proposed relaxation mechanism and pathways can explain why the cis−trans photoisomerization experiment of o-TsABDI does not produce any of its trans-isomer.

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REFERENCES

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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.8b01799. 1 H and 13C NMR spectra and calculation data (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kuangsen Sung: 0000-0003-3920-6674 Present Address †

R.S.: Faculty of Family Medicine, Northern Ontario School of Medicine, ON, Canada. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council of Taiwan for financial support (NSC104-2119-M-006-013) and the National Center for High-performance Computing for computer facilities and time. L


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