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A: Kinetics, Dynamics, Photochemistry, and Excited States 1

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S/S Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/E Photoisomerization and E/Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group? Jun-Jia Xu, Robert Sung, and Kuangsen Sung J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b02924 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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The Journal of Physical Chemistry

S1/S0 Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/E Photoisomerization and E/Z ThermoIsomerization by An Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group? Jun-Jia Xu, Robert Sung† and Kuangsen Sung* Department of Chemistry, National Cheng Kung University, Tainan, Taiwan

ACS Paragon Plus Environment

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ABSTRACT. Any method that can enhance fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/E photoisomerization through intramolecular steric hindrance or relatively high bond order of C=C double bond in a S1 excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/E photoisomerization and E/Z thermo-isomerization phenomena. We found that the S1 and S0 potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S1 PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relative high bond order of the I-bond in the S1 excited state of 2b that makes it have a higher barrier for the Z/E photoisomerization, a smaller Z/E photoisomerization quantum yield and a higher fluorescence quantum yield. In its S0 PES, it is not the reduced bond order of the I-bond in the S0 ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/Z thermo-isomerization and a much smaller E/Z thermo-isomerization rate constant.


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INTRODUCTION Light emitted from fluorophores has many applications such as organic light-emitting diodes (OLEDs),1 organic field-effect transistors (OFETs),2 fluorescent biological labels,3 sensors4 and so on. Any method that can enhance fluorescence of fluorophores is highly desirable. It is wellknown that fluorescence enhancement can be accomplished by restricted Z/E photoisomerization, and it is usually achieved in a viscous solvent,5 by encapsulation in host molecules,6 by aggregation-induced emission (AIE)7,8 or by extra intramolecular covalent/ionic bonding.9 Recently, fluorescence enhancement accomplished by restricted Z/E photoisomerization around C=N bond through intramolecular steric hindrance of a bulky perylene group was reported.10 However, fluorescence enhancement accomplished by restricted Z/E photoisomerization through intramolecular steric hindrance has rarely been studied and documented.11 Besides, to the best of our knowledge, fluorescence enhancement accomplished by restricted Z/E photoisomerization through relatively high bond order of C=C double bond in a first singlet (S1) excited state has not been reported.10 In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to explore restricted Z/E photoisomerization through possible intramolecular steric hindrance or/and relatively high bond order of the C=C double bond in their S1 excited states. GFP and its mutants have attracted interest as fluorescent biological labels in recent two decades.12,13 They are known for their non-destructive nature and high sensitivity and have become one of the most widely used methods to label and to track biomolecules.12,13 The wild-type GFP (wtGFP) has its chromophore, p-hydroxybenzylideneimidazolinone (p-HBDI), surrounded by the peptides.14-16 Once the wtGFP is stripped of the peptide environment, its chromophore, p-HBDI, significantly loses fluorescence because the S1 excited state of p-HBDI undergoes much faster non-radiative relaxation17 through Z/E photoisomerization around the I-bond in aprotic solvents.18


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In protic solvents, internal conversion through solvent-solute hydrogen bonding becomes another important pathway to relax the p-HBDI excited state.18 Fluorescence enhancement of the isolated GFP chromophore has been one of interesting topics.12,13 To enhance fluorescence of the GFP chromophore, several methods have been tried. For examples, fluorescence quantum yield of p-HBDI was significantly increased at the temperature as low as 77 K.19 In glycerol glass at 150 K, the dramatic increase in fluorescence quantum yield and excited-state lifetime of the GFP chromophore has been achieved by viscous drag.20 Long-chain O-alkyl synthetic analogues of the GFP chromophore demonstrated a bright fluorescent emission in the crystalline state because of “freezing” of planar conformation of the fluorophores.21 In this article, the GFP chromophore analogues 1a,b and 2a,b were chosen to address two issues that are related to fluorescence enhancement. (Fig. 1) The first issue is whether an out-ofplane benzyl group restricts Z/E photoisomerization or E/Z thermo-isomerization of the GFP chromophore analogues through intramolecular steric hindrance. (Fig. 2) The second issue is whether Z/E photoisomerization of the GFP chromophore analogues can be restricted by relatively high bond order of the C=C double bond in their S1 excited states, which might be achieved by an in-plane m-pyridinium group through a charge transfer. What we address on the two issues will provide new physical insights into the two ways to restrict Z/E photoisomerization, which may be further contributed to fluorescence enhancement.


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O

P-bond I-bond O N



N

N

N

N N

1b

1a O

I N

O

I N

N

N N

N

2b

2a

Figure 1. Structures of the GFP chromophore analogues 1a,b and 2a,b I

*

?

N

I

N

O O N

N



N

N

Z/E-photoisomerization * I-bond I

O

fluorescence

N

N

O

I N

N N

N

2b Figure 2. Does an out-of-plane benzyl group restrict Z/E photoisomerization of GFP chromophore analogue 2b through intramolecular steric hindrance, causing fluorescence enhancement?


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EXTERIMENTAL SECTION General method to prepare 1a,b. Preparation of 1a,b is similar to that of the p-pyridinyl analogues of the GFP chromophore, which has been described in the literature.22,23 (Z)-2-Methyl-1-phenyl-4-(pyridin-3-ylmethylene)-1H-imidazol-5(4H)-one (1a). color : orange yellow; weight: 1.1 g; yield: 42%; 1HNMR (400 MHz, CDCl3) δ 2.29 (s, 3H), 7.16 (s, 1H), 7.37-7.55 (m, 6H), 8.60 (d, 1H, J = 4.8 Hz), 8.73 (d, 1H, J = 8.0 Hz), 9.12 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 16.5, 123.6, 127.2, 129.0, 129.7, 130.3, 133.1, 138.4, 139.9, 150.3, 152.8, 163.0, 169.3; HRMS [FAB, (M+H)+] m/z calcd for C16H14N3O 264.1136, found 264.1135. (Z)-1-Benzyl-2-methyl-4-(pyridin-3-ylmethylene)-1H-imidazol-5(4H)-one (1b) color : orange yellow; weight:1.2 g; yield: 43%; 1HNMR (400 MHz, CDCl3) δ 2.27 (s, 3H), 4.82 (s, 2H), 7.12 (s, 1H), 7.22-7.35 (m, 6H), 8.56 (d, 1H, J = 6.4 Hz), 8.69 (d, 1H, J = 8.0 Hz), 9.07 (s, 1H); 13C

NMR (100 MHz, CDCl3) δ 16.1, 43.9, 123.5, 123.6, 127.0, 128.0, 129.0, 130.3, 135.7, 138.4,

140.2, 150.2, 152.8, 163.7, 170.1; HRMS [FAB, (M+H)+] m/z calcd for C17H16N3O 278.1293, found 278.1296. General method to prepare 2a,b. Preparation of 2a,b is similar to that of the p-pyridinium analogues of the GFP chromophore, which has been described in the literature.23 (Z)-1-Methyl-4-((2-methyl-5-oxo-1-phenyl-1H-imidazol-4(5H)-ylidene) methyl) pyridinium iodide (2a). color : dark brown; weight: 0.5 g; yield 74%;1HNMR (400 MHz, D2O) δ 2.24 (s, 3H), 4.38 (s, 3H), 7.19 (s, 1H), 7.32 (d, 2H, J = 6.4 Hz), 7.51-7.56 (m, 3H), 8.03 (t, 1H, J = 7.8 Hz), 8.69 (d, 1H, J = 6.4 Hz), 8.95 (d, 1H, J = 8.0 Hz), 9.38 (s, 1H); 13C NMR (100 MHz, d6DMSO) δ 16.9, 48.9, 117.1, 128.0, 128.1, 129.5, 129.9, 133.2, 134.3, 143.0, 145.2, 145.6, 147.5, 167.7, 168.8; HRMS [FAB, M+] m/z calcd for C17H16N3O 278.1293, found 278.1288.


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(Z)-1-Methyl-3-((2-methyl-5-oxo-1-benzyl-1H-imidazol-4(5H)-ylidene)

methyl)pyridi-

nium iodide (2b). color : dark brown; weight: 0.6 g; yield 85%; 1HNMR (400 MHz, D2O) δ 2.30 (s, 3H), 4.35 (s, 3H), 4.83 (s, 2H), 7.14 (s, 1H), 7.23-7.36 (m, 5H), 8.00 (t, 1H, J = 6.3 Hz), 8.66 (d, 1H, J = 6.0 Hz), 8.89 (d, 1H, J = 8.4 Hz), 9.31 (s, 1H); 13C NMR (100 MHz, d6-DMSO) δ 16.4, 43.7, 48.8, 117.3, 127.3, 128.0, 128.1, 129.3, 134.2, 136.6, 143.0, 145.1, 145.7, 147.5, 168.6, 169.6; HRMS [FAB, M+] m/z calcd for C18H18N3O 292.1449, found 292.1454. Z/E Photoisomerization and E/Z thermo-isomerization. The procedures for Z/E photoisomerization and E/Z thermo-isomerization is like that of the p-trimethylammonium analogues of the GFP chromophore, which has been described in the literature.22 For the measurement of Z/E photoisomerization quantum yields of the samples, the samples in CD3CN were irradiated with 350-nm UV light for 20 min in a photo-reactor at room temperature, followed by immediate measurement of the samples with a 1H NMR spectrometer to identify the amount of Z and E-isomers. The known Z/E photoisomerization quantum yield of p-HBDI in CH3CN is 0.48,18 and it was used a standard for the determination of Z/E photoisomerization quantum yields of the samples. Fluorescence quantum yield measurement. The fluorescence quantum yields of 1a,b and 2a,b were determined with the comparative method,24 which compare the wavelength-integrated intensity of the test samples with a standard at the same excitation wavelength by using the formula of Q = QR(I/IR)(ODR/OD)(n2/nR2), where Q is the fluorescence quantum yield, I is the integrated fluorescence intensity, n is the refractive index of the solvent, OD is the optical density in the range of 0.1~0.01 for the sake of accuracy and the subscript R refers to the reference fluorophore with known fluorescence quantum yield. The standard we used was quinine sulfate in 0.1 M H2SO4, of which the known fluorescence quantum yield is 0.577 at the excitation wavelength of 350 nm.25


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COMPUTATIONAL DETAILS All the calculations have been done by using Gaussian09 program.26 Geometry of a fluorescent state was optimized by the coulomb-attenuating method (CAM) / time-dependent (TD) / density functional theory (DFT) method27-34 with the correlation-consistent polarized valence double-zeta basis set of Dunning (cc-pVDZ)35 without any symmetry restriction. Geometry of a ground state was optimized by the DFT method with the cc-pVDZ basis set without any symmetry restriction. To determine the nature of the located stationary points, analytical vibration frequencies of the optimized geometries were calculated at the same level. The molecular orbitals were visualized by GaussView. Constrained geometry optimization at the CAM-TD-B3LYP/ccpVDZ level was used to calculate the S1 and S0 potential energy surfaces (PESs).36 The rotational barriers in the S1 and S0 PESs were calculated by the minimum energy path (MEP) computation.36,37 RESULTS AND DISCUSSION Like synthesis of the p-pyridinium analogues of the GFP chromophore, 1a,b and 2a,b were prepared similarly.23 (Scheme 1) The structure of 1a,b was confirmed to be Z-configuration by their single crystal X-ray diffraction structures. (Fig. 3 and the Supporting Information) No Econfiguration has been found for them based on their 1H and 13C NMR spectra. Even though the in-plane m-pyridinyl substituent of 1a or 1b is well conjugated with the imidazolinone moiety through the P-bond and the I-bond, leading to a planar structure, the N-phenyl substituent of 1a is twisted with the imidazolinone moiety with the dihedral angle C(8)-N(1)C(6)-C(1) = 126.3°. For 1b, one thing that deserves notice is that the phenyl ring of the Nbenzyl substituent of 1b is on the top of the molecular plane with the dihedral angle C(3)-N(1)-


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C(10)-C(11) = 98.5°. The edge of the phenyl ring of the N-benzyl substituent points towards the center of the imidazolinone moiety with the dihedral angle N(1)-C(10)-C(11)-C(12) = 14.9°, and this out-of-plane N-benzyl substituent in 1b or 2b may have potential to restrict Z/E photoisomerization or E/Z thermo-isomerization around the I-bond. (Fig. 2 and 3) Scheme 1. Synthesis of 1a,b and 2a,b O 1. Ac2O / NaOAc / N-acetylglycine

P-bond I-bond O N

R

N

3. pyridine

1a : R=Ph 1b : R=CH2Ph

P-bond I-bond O

I CH3I

N

H 2. RNH2

N

N

N

R

N

2a : R=Ph 2b : R=CH2Ph

Figure 3. Single crystal X-ray diffraction structures of 1b with thermal ellipsoids at the 50% probability level


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Methylation of 1a and 1b with methyl iodide selectively occurred at the pyridine nitrogen, instead of the amidine nitrogen, and generated 2a and 2b, respectively. According to their 1H and 13C NMR spectra, they both exist only in Z or E-configuration. The single crystal X-ray diffraction structure of 2a in the Supporting Information confirms that it exists in Zconfiguration. For 2a, its in-plane positively-charged m-pyridinium substituent is still conjugated with the imidazolinone moiety through the P-bond and the I-bond, but the two moieties are a little twisted with the dihedral angle  of 18° around the P-bond. In addition to that, its N-phenyl substituent is still twisted with the imidazolinone moiety with the dihedral angle C(8)-N(1)-C(6)-C(5) = 102.4°. The ground-state structures of 1a,b and 2a,b were also optimized at the B3LYP/cc-pVDZ level, and they are very close to their respective single crystal X-ray diffraction structures. (Fig. 4 and the Supporting Information) We paid more attention to the bond length of the I-bond and the P-bond of these compounds. All the I-bonds have a C=C double bond character while all the P-bonds have a C-C single bond character. The bond length of I-bond in both 2a and 2b is around 0.002 Å longer than that in both 1a and 1b. However, the bond length of P-bond in both 2a and 2b is around 0.004 Å shorter than that in both 1a and 1b. A possible explanation for that is that the pyridinium group as an electron-withdrawing group in 2a and 2b pulls electrons through resonance from the imidazolinone moiety via the I-bond and P-bond, making their I-bond longer and P-bond shorter.


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Figure 4. Optimized ground-state structures of 1a,b and 2a,b at the B3LYP/cc-pVDZ level The S1 excited-state structures of 1a,b and 2a,b were optimized at the CAM-TDB3LYP/cc-pVDZ level. (Fig. 5) According to the electronic structures of their HOMO and LUMO, these S1 excited states involve a   * electron transition, which is consistent with their high experimental molar absorptivity () in Table 1. Hence, the bond order of the I-bond in these S1 excited states is smaller than that in their S0 ground states. We pay more attention to the electron density distribution of their HOMO and LUMO. The S1 excited states of 2a and 2b are involved in a charge transfer, but the S1 excited states of 1a and 1b are not. These are consistent with their dipole moments. Calculated dipole moments of the S1 excited states of 1a, 1b, 2a and 2b are 4.71, 5.25, 16.4 and 15.0 Debye, respectively.


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Figure 5. Optimized S1 excited-state structures (left), HOMO (middle) and LUMO (right) of 1a,b and 2a,b at the CAM-TD-B3LYP/cc-pVDZ level


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Table 1. Photophysical Properties of 1a, 1b, 2a and 2ba compound

solvent

A()

Z-1a

CH3CN

351nm (2.5 × 104) 453nm [(5.8± 0.7) × 10-3]

Z-1b

CH3CN

352nm (1.2 × 104) 454nm [(3.2± 0.7) ×10-3] 0.51±0.03

Z-2a

CH3CN

365nm (4.0 × 103) 508nm [(5.7± 1.0) × 10-4]

H2O

354nm (4.3 × 103) 510nm [(1.0± 0.2) × 10-3]

CH3CN

368nm (2.8 × 103) 456nm [(1.0± 0.1) × 10-2]

H2O

358nm (1.9 × 103) 454nm [(1.2± 0.2) × 10-2]

Z-2b

a.

F(f)

ZE 0.42±0.02

0.13±0.01

0.47±0.03

excitation wavelength: 350 nm; f: fluorescence quantum yield; ZE Z/E photoisomerization quantum yield One interesting thing that deserves notice is that the bond length of the I-bond in the S1

excited states of 2a and 2b is around 0.02 Å shorter than that in the S1 excited states of 1a and 1b. This is opposite to what happens to their ground states, where the bond length of the I-bond in the S0 ground states of 2a and 2b is around 0.002 Å longer than that in the S0 ground states of 1a and 1b. We suggest that the charge-transfer character for the S1 excited states of 2a and 2b may reduce the electron-transition probability directly from -orbital of the I-bond to the *-orbital of the I-bond, making the bond order of the I-bond in their S1 excited states higher. Next, we will show that it is the relatively high bond order of the I-bond in the S1 excited states of 2a and 2b that makes them have a higher barrier for the Z/E photoisomerization than 1a and 1b.


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Like p-HBDI,18 1a, 1b, 2a and 2b also undergo Z/E photoisomerization. Hence, one of non-radiative relaxation pathways for 1a,b and 2a,b is supposed to go through a conical intersection at the dihedral angle  of around 90°, generating Z and E-isomers in a similar yield. Then, the quantum yield for generating Z and E-isomers through a conical intersection is around twice the Z/E photoisomerization quantum yield (ZE). As shown in Table 1, the Z/E photoisomerization quantum yields of 1a, 1b and 2b are close to 0.50, so most of their S1 excited states relax through the Z/E photoisomerization, leading to their low fluorescence quantum yields. This is similar to p-HBDI and several other GFP chromophore analogues such as p-ABDI and m-ABDI.18,38,39 However, the Z/E photoisomerization quantum yield of 2a is only 0.13, which is much smaller than that of 2b, but later we will show that both 2a and 2b have a similar barrier for the Z/E photoisomerization. Besides, the fluorescent quantum yield of 2a is as low as 5.7 × 10-4, which is consistent with its low calculated oscillator strength f of 0.06. (Table 1) All these results imply that 2a, other than 1a, 1b and 2b, relaxes through internal conversion at a much higher rate than its Z/E photoisomerization. We suggest that a loose-bolt effect40 might work on the S1 excited state of 2a because the N-phenyl group in its S1 excited state is almost coplanar with the imidazolinone moiety with a dihedral angle of around 23°. (Fig. 5) If we take a closer look at the HOMO and LUMO of the S1 excited state of 2a, its Nphenyl group is involved in a charge transfer, and that is the reason why the N-phenyl group is almost coplanar with the imidazolinone moiety. This unusual phenomenon can be further confirmed by its significantly red-shift fluorescent emission, which is around 50 nm more redshift than 2b. (Table 1) The unusual coplanarity between the N-phenyl group and the imidazolinone moiety in the S1 excited state of 2a does not occur in the S1 excited state of 1a and the ground states of 1a and 2a. That is why we do not see a similar loose-bolt effect in 1a.


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One thing that deserves notice is that the Z/E photoisomerization quantum yield of 2b is 4% smaller than that of 1b. Even though it is within an experimental error, it is consistent with the calculation result that the bond length of the I-bond in the S1 excited states of 2b is around 0.02 Å shorter than that in the S1 excited-state of 1b. To confirm this point again from another perspective, we will show that 2b has a higher barrier for the Z/E photoisomerization than 1b. Because 1a,b and 2a,b are all relaxed by Z/E photoisomerization through the I-bond torsion (), we explored their S1 and S0 potential energy surfaces (PESs) along the dihedral angle . As shown in Fig. 6, these PESs have a conical intersection or avoided crossing at  of around 90°. The barriers for the Z/E photoisomerization of 2a and 2b are around 10.9 and 7.8 kcal/mol, respectively, which are two or three times higher than those of 1a and 1b. These results echo another calculation result that bond length of the I-bond in the S1 excited states of 2a and 2b is around 0.02 Å shorter than that in the S1 excited-states of 1a and 1b. (Fig. 5) They are also consistent with the experimental results that the Z/E photoisomerization quantum yield of 2a is 29% smaller than that of 1a and the Z/E photoisomerization quantum yield of 2b is 4% smaller than that of 1b.


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Figure 6. The S1 and S0 potential energy surfaces along the dihedral angle  of 1a,b and 2a,b at the CAM-TD-B3LYP/cc-pVDZ level The experimental fluorescent emissions and fluorescence quantum yields of 1a, 1b, 2a and 2b in acetonitrile are 453nm (f =5.8× 10-3), 454nm (f =3.2× 10-3), 508nm (f =5.7× 10-4), and 456nm (f =1.0× 10-2), respectively, which are consistent with their calculated fluorescent emissions and oscillator strengths, such as 3.15 eV (394 nm; f = 0.58) for 1a, 3.15 eV (394 nm; f = 0.54) for 1b, 2.11 eV (589 nm; f = 0.06) for 2a, and 2.82 eV (439 nm; f = 0.40) for 2b. (Table 1) The fluorescence quantum yields of 1a and 1b in acetonitrile are similar and they are around one order of magnitude higher than that (f = 2 ×10-4) of p-HBDI.12 We suggest that in contrast to 1a and 1b, p-HBDI excited state undergoes faster non-radiative relaxation through hydrogen bonding of phenol hydroxyl group with solvent.18,38,39 The fluorescence quantum yield of 2a in acetonitrile is 5.7×10-4, which is around one order of magnitude lower than that of 1a. (Table 1) This is consistent with its low calculated oscillator strength f (0.06). However, the Z/E photoisomerization quantum yield (0.13) of 2a is much smaller than that (0.42) of 1a, (Table 1) the barrier for the Z/E photoisomerization of 2a is


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around 7.5 kcal/mol higher than that of 1a, (Fig. 6) and the bond length of the I-bond in the S1 excited state of 2a is also around 0.02 Å shorter than that in the S1 excited-states of 1a (Fig. 5). A possible explanation for all the experimental and calculation results is that the S1 excited state of 2a should be relaxed mostly by much faster internal conversion through the loose-bolt effect caused by the almost coplanarity between the N-phenyl group and the imidazolinone moiety in its S1 excited state. We wonder if the higher fluorescence quantum yield (1.0×10-2) of 2b is ascribed to restricted Z/E photoisomerization through intramolecular steric hindrance between the out-ofplane benzyl group and the in-plane m-pyridinium group. At first, both 2a and 2b have the same bond length for the I-bond in their S1 excited states. (Fig. 5) According to the S1 PESs of 2a and 2b in Fig. 6, the barrier for the Z/E photoisomerization of 2a is around 3.1 kcal/mol higher than that of 2b and the Z/E photoisomerization quantum yield (0.13) of 2a is much smaller than that (0.47) of 2b, but 2a has a phenyl substituent that does not have any chance to cause intramolecular steric hindrance during the Z/E photoisomerization. As a result, there is no intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane mpyridinium groups in the S1 excited state of 2b during the Z/E photoisomerization. The higher fluorescence quantum yield of 2b is not ascribed to restricted Z/E photoisomerization through intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane mpyridinium group, either. According to the S1 PESs of 1a and 1b in Fig. 6, the barriers for the Z/E photoisomerization of 1a and 1b are similar. In Fig. 5, both 1a and 1b have the same bond length for the I-bond in their S1 excited states. As shown in Table 1, the Z/E photoisomerization quantum yield of 1a is even 9% smaller than that of 1b and the fluorescence quantum yield of 1a is also around twice


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higher than that of 1b. However, 1a has a phenyl substituent that does not have any chance to cause intramolecular steric hindrance during the Z/E photoisomerization. All these experimental and calculation results indicate that intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinyl group in the S1 excited state of 1b does not occur during its Z/E photoisomerization. This is like what happen to the S1 excited state of 2b. The fluorescence quantum yield of 2b in acetonitrile is as high as 1.0×10-2, which is around one order of magnitude higher than that of 1b. This is consistent with the experimental result that the Z/E photoisomerization quantum yield of 2b is around 4% smaller than that of 1b. These two experiment results echo the two calculated results: (1) the bond length of the I-bond in the S1 excited state of 2b is around 0.02 Å shorter than that in the S1 excited-states of 1b, (Fig. 5) and (2) the barrier for the Z/E photoisomerization of 2b is around 4.8 kcal/mol higher than that of 1b. (Fig. 6) Therefore, we suggest that the higher fluorescence quantum yield of 2b is ascribed to the restricted Z/E photoisomerization through relatively high bond order of the I-bond in its S1 excited state. (Fig. 7)

* relatively high bond order

O

I N

N N

O

I

h N

I

N



O N

N

N N

Figure 7. The restricted Z/E photoisomerization through relatively high bond order of the Ibond in the S1 excited state of 2b


18

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The lowest-energy electronic absorptions of 2a and 2b in acetonitrile are located at 365 ( = 4.0 ×103 M-1cm-1) and 368 (= 2.8 ×103 M-1cm-1), respectively, which are around 15-nm red-shifted in comparison with those of 1a and 1b. (Table 1) The same phenomenon was also found in the p-pyridinium analogues of the GFP chromophore.23 The red shift in electronic absorption is unlikely caused by the resonance stabilization because the resonance stabilization in 2a and 2b is not better than that in 1a and 1b. The structure of 2a in the Supporting Information has a twisted structure with the dihedral angle  of 18° around the P-bond. We suggest the 15-nm red shift in electronic absorption (i.e., the decrease in HOMO-LUMO energy gap) is likely caused by the electron-withdrawing m-pyridinium substituent of 2a or 2b, which decreases the energy of LUMO more than that of HOMO. This is consistent with the literature.41 On the other hand, water shifts the lowest-energy electronic absorptions of 2a and 2b to the shorter wavelength. We suggest that the blue shift in electronic absorption is caused by ground-state stabilization of 2a and 2b through hydrogen bonding with water. Like p-HBDI,18 1a, 1b, 2a and 2b also undergo E/Z thermo-isomerization right after Z/E photoisomerization. After 20 min of irradiation with 350-nm UV light in a photo-reactor at room temperature, part of 1a, 1b, 2a and 2b were converted to their respective E-isomers. (Fig. 8) These E-isomers are less stable than their respective Z-isomers and will be thermally isomerized back to their respective Z-isomers. This is consistent with our calculation results. The kinetics of E/Z thermo-isomerization of E-1a,b and E-2a,b in CD3CN was measured by monitoring growth of their Z-isomers or decay of their E-isomers with their 1H NMR spectroscopy.


19


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Figure 8. The 1H NMR spectra of Z-1b (left) and Z-2b (right) in CD3CN (a) before UV irradiation, (b) right after 20 min of irradiation with 350 nm UV light, and (c) 28 days after UV irradiation. As shown in Table 2, the E/Z thermo-isomerization rate constant of E-1b is three times smaller than that of E-1a even though they both have the same I-bond length in the ground state. This is consistent with the computation result that the E/Z thermo-isomerization barrier of E-1b is around 3.8 kcal/mole higher than that of E-1a. All these experimental and calculation results indicate that the restricted E/Z thermo-isomerization through intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane pyridinyl group in the S0 ground state of 1b does happen. On the other hand, as we just compared 1a with 1b for their fluorescence quantum yields, the barriers of the Z/E photoisomerization, the Z/E photoisomerization quantum yields, and the bond length of the I-bond in their S1 excited states, we suggested that intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinyl group in the S1 excited state of 1b does not occur during its Z/E photoisomerization. As a result, the restricted rotation the S1 and S0 PESs of 1b experience could be different because the routes they follow are dramatically different.


20

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Table 2. The E/Z Thermo-isomerization Rate Constants (k) of E-1a,b and E-2a,b at 25 °C compound

k (s-1) in CD3CN

E-1a

(9.70 ± 1.31) × 10-6

E-1b

(2.59 ± 0.63) × 10-6

E-2a

(8.67 ± 3.68) × 10-6

E-2b

(4.18 ± 1.37) × 10-7

Even though the bond length of the I-bond in the S0 ground state of 2b is around 0.002 Å longer than that in the S0 ground state of 1b, the E/Z thermo-isomerization rate constant of E-2b is still six times smaller than that of E-1b. (Table 2) This is consistent with the computation result that the E/Z thermo-isomerization barrier of E-2b is around 2.7 kcal/mole higher than that of E1b. All the experimental and computation results indicates that the reduced bond order of the Ibond in the S0 ground state of 2b should not cause the extra restricted E/Z thermo-isomerization, but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane mpyridinium group in the S0 PES of 2b leads to the extra restricted E/Z thermo-isomerization. In comparison with 1b, 2b is subjected to the extra intramolecular steric hindrance during the restricted E/Z thermo-isomerization because the m-pyridinium group of 2b is bulkier than the mpyridinyl group of 1b. This explanation can also be confirmed from another perspective that is shown as follows. It was reported that an electron-withdrawing group at the phenyl moiety of the GFP chromophore accelerates its E/Z thermo-isomerization rate.42 However, the E/Z thermoisomerization rate constant of E-2b, which has an electro-withdrawing m-pyridinium substituent, is much smaller than that of E-1b. A possible explanation for the unusual phenomenon is that


21


Page 22 of 30

restricted rotation around the I-bond of 2b through the extra intramolecular steric hindrance between the out-of-plane benzyl substituent and the in-plane m-pyridinium group slows down the E/Z thermo-isomerization. (Fig. 9) On the other hand, as we just compared 1b with 2b for their fluorescence quantum yields, the barriers of the Z/E photoisomerization, Z/E photoisomerization quantum yields, and the bond length of the I-bond in their S1 excited states, we suggested that the higher fluorescence quantum yield of 2b is ascribed to the restricted Z/E photoisomerization through relatively high bond order of the I-bond in its S1 excited state, and the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinyl group in the S1 excited state of 2b does not occur during its Z/E photoisomerization. As a result, the restricted rotation the S1 PES of 2b experience is accomplished by relatively high bond order of the I-bond in the S1 excited state of 2b, but the restricted rotation the S0 PES of 2b experience is caused by the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane mpyridinyl group. I I

N

N O

O N

N

O

I N N



N N

N

Figure 9. The restricted E/Z thermo-isomerization through intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group in the S0 ground state of 2b


22

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CONCLUSIONS The I-bond length in 2a and 2b in the ground state is around 0.002 Å longer than that in 1a and 1b in the ground state because the pyridinium group as an electron-withdrawing group in 2a and 2b pulls -electrons through resonance from the imidazolinone moiety via the I-bond and P-bond, making their I-bond longer and P-bond shorter. On the other hand, the I-bond length in the S1 excited states of 2a and 2b is around 0.02 Å shorter than that in the S1 excitedstates of 1a and 1b because the charge-transfer character for the S1 excited states of 2a and 2b reduces the electron-transition probability from -orbital of the I-bond to the *-orbital of the I-bond, making the bond order of the I-bond in the S1 excited states of 2a and 2b higher. We found that the S1 and S0 PESs of these GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In the S1 PES of 2b, it is not the intramolecular steric hindrance between the out-ofplane benzyl group and the in-plane m-pyridinium group but the relative high bond order of the I-bond in the S1 excited state of 2b that makes it have a higher barrier for the Z/E photoisomerization than 1b. Some of the supporting results for that include (1) the Z/E photoisomerization quantum yield of 2b is 4% smaller than that of 1b, (2) the fluorescence quantum yield of 2b is around one order of magnitude higher than that of 1b, (3) the I-bond length in the S1 excited state of 2b is around 0.02 Å shorter than that in the S1 excited-states of 1b, and (4) the barrier for the Z/E photoisomerization of 2b is around 4.8 kcal/mol higher than that of 1b. In the S0 PES of 2b, it is not the reduced bond order of the I-bond in the S0 ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-


23


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plane m-pyridinium group that makes it have an extra higher barrier for E/Z thermoisomerization than 1b. Some of the supporting results for that include (1) the E/Z thermoisomerization rate constant of E-1b is three times smaller than that of E-1a even though they both have the same I-bond length in the ground state, (2) the E/Z thermo-isomerization barrier of E-1b is around 3.8 kcal/mole higher than that of E-1a, (3) the E/Z thermo-isomerization rate constant of E-2b is six times smaller than that of E-1b even though the bond length of the Ibond in the S0 ground state of 2b is around 0.002 Å longer than that in the S0 ground state of 1b, and (4) the E/Z thermo-isomerization barrier of E-2b is around 2.7 kcal/mole higher than that of E-1b. ASSOCIATED CONTENT Supporting Information 1H-

and

13C-NMR

Spectra, data of single crystal X-ray diffraction structures, electronic

absorption and fluorescent emission spectra, and calculation data. This material is available free of charge via http://pubs.acs.org. AUTHOR INFORMATION † Dr. Robert Sung’s current address: Faculty of Family Medicine, Northern Ontario School of Medicine, Ontario, Canada. E-mail: [email protected] Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors.


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TOC Graphic *

relatively high bond order

Z/Ephotoisomerization

O

I N

N

I

N

O

N

N N

E/Z-thermo-isomerization

h

I N O

I

O

N

N N

N



N


30

Z The

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