Deuterium Tautomerization in Transient Isomers

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Monitoring Hydrogen/Deuterium Tautomerization in Transient Isomers of Single Porphine by Highly Localized Plasmonic Field Zhen Xie, Sai Duan, Chuan-Kui Wang, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00398 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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The Journal of Physical Chemistry

Monitoring Hydrogen/Deuterium Tautomerization in Transient Isomers of Single Porphine by Highly Localized Plasmonic Field Zhen Xie,†,‡ Sai Duan,∗,‡,¶ Chuan-Kui Wang,† and Yi Luo‡,§ †Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, Jinan 250014, China. ‡Department of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, S-106 91 Stockholm, Sweden ¶Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai, Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Key Laboratory of Computational Physical Sciences, Department of Chemistry, Fudan University, Shanghai 200433, People’s Republic of China. §Hefei National Laboratory for Physical Science at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China. E-mail: [email protected]

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Abstract Inner proton transfer between two trans isomers (tautomerization) in porphyrins plays a crucial role in many biological systems as well as molecular nanotechnology. Although the stepwise mechanism of tautomerization is well accepted, the involved intermediate cis isomer has not been directly detected owing to its short lifetime and the extremely weak intensities of corresponding hydrogen vibrations. Here, taking a single porphine as the prototype, we theoretically demonstrate that Raman intensities of the hydrogen vibrations become accessible under the highly localized plasmonic field because of the symmetry breaking effect. In addition, with the ultrafast incident excitations, we find that Raman signals of cis porphine could be distinguished from the stable trans isomer, suggesting a general protocol for the direct characterization of transient isomers. Moreover, calculated results reveal that the position of inner hydrogen/deuterium can be unambiguously visualized from Raman images of the corresponding stretching modes, providing a unique optical means for the chemical monitoring of tautomerization in porphine and its derivatives.

Introduction Labeled as “pigments of life”, 1 porphyrins are found in nature and play major roles in vital biological processes, e.g. oxygen transportation, oxygen activation, and photosynthesis. 2 Among various porphyrins, metal-free porphyrins have two inner protons that could migrate at four interior nitrogen sites. This migration process is known as the NH tautomerization, 3 which is significantly important in photosynthesis 2,4 as well as the potential applications for single-molecule switch device. 5–7 To shed light on the mechanism of the NH tautomerization in metal-free porphyrins, porphine is a good model as the parent compound of porphyrins. 8–10 It is now generally accepted that the double proton transfer between two stable trans configurations of porphine occurs via a complicated stepwise transfer network involving an intermediate of cis-porphine isomer, 3,8,9,11 as shown in Figure 1a. However, the 2


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(a)

(b) νNH

νCH

Raman Shift / cm−1

Figure 1: (a) NH tautomerization of porphine. m, α and β represent the symmetrically unique carbon atoms, and the distances (in Å) of N-H and H/H at the interior are labeled. (b) Experimental (black line) and theoretical (red line) normal Raman spectra of trans porphine. The experimental spectrum was extracted from Ref. 16 and the theoretical spectrum was convoluted by the Lorentzian function with a full width at the half-maximum (fwhm) of 6 cm−1 . significant tunneling nature of proton makes the lifetime of the metastable cis-isomer too short to be directly detected in experiments, 3,8,9,11 which hinders the final verdict of stepwise mechanism in tautomerization of porphine. The rate of tautomerism in porphine has been measured using dynamic nuclear magnetic resonance (NMR) spectra of inner nitrogen atoms by Limbach’s group. 9,12 They observed a large deuterium isotope effect (DIE) on NH tautomerization upon substituting transferred hydrogen atoms by deuterium (D), highlighting the effects of proton tunneling. This seminal work makes porphine an attractive model for the understanding of the nuclear tunneling. 13,14 Nevertheless, owing to the sensitivity, a very large amount of molecules have to be investigated in the NMR experiments. 15 Moreover, the ultimate goal of direct monitoring for the NH tautomerization is beyond the scope of the NMR technique. To further understand the NH tautomerization process, low-temperature scanning tunneling microscopy (STM) images have succeed to characterize the inner hydrogen atoms of a single trans porphine molecule adsorbed on different metallic surfaces. 17–19 However, to the best of our knowledge, the transient intermediate cis-isomer has not been captured by

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STM yet, even with the assist of DIE that can significantly enhance the lifetime of cis. Furthermore, the DIE itself is difficult to be addressed in STM because STM measurement only detects the local density of states of adsorbates. Specifically, STM images cannot chemically identify the detailed tautomerizations in a single deuterated porphine (HD-porphine) molecule. Considering the request of rapid characterization of transient processes with rich chemical information, techniques such as surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) 20 and time-resolved sum frequency generation (TRSFG) 21,22 are favored. But they have restrictive spatial resolution owing to the optical diffraction limit. To further increase the spatial resolution at ultrafast time scale, improving the temporal resolution possible with tip-enhanced Raman spectroscopy (TERS) 23–26 has recently proven to be non-trivial. 27–31 The progress of tools and techniques that enable ultrafast TERS with single molecule sensitivity has been made. For instance, Van Duyne’s group demonstrated a reliable TERS signal of rhodamine 6G molecule using picosecond excitation. 28 Wickramasinghe et al. further reported ultrafast imaging applications for an azobenzene thiol molecule in stimulated TERS. 29 These pioneer works highlight the capabilities of ultrafast TERS experiments toward investigating molecular transient processes on picosecond and even femtosecond timescales. In the present work, we theoretically propose TERS that takes the advantage of the near-field feature of spatially confined plasmon (SCP) 32–34 for direct monitoring of the NH tautomerization in different isomers of porphine. It is well known that ordinary Raman spectroscopy cannot provide the location of transferred protons because of the limited spatial resolution. It should be further emphasized that although CH vibrations could be observed in the high frequency region, conventional Raman spectrum fails to obtain the direct NH vibrational information of porphine as depicted in Figure 1b, owing to the symmetry-forbidden transitions or occasionally weak intensities. 16 In this context, both issues are circumvented by the near-field feature of SCP in TERS.

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Theoretical Methods The Raman scattering consists of the absorption and spontaneous emission processes. 35–37 When a molecular system is under a highly inhomogeneous electromagnetic field, i.e., the SCP field, the amplitude distribution of electric field should be taken into account for the absorption process. 32,38,39 As a result, the interaction Hamiltonian may be modified as 39 r 0 Hσ,ω

=ı

2πω Mµ ˆ σ gσ a ˆω e(−ıω+γ)t + H.c. V

(1)

Here, σ represents the Cartesian coordinate, ω is the frequency of incident light, V is the P ˆ = − i ri is the dipole operator, g repsystem volume, M is the enhancement factor, µ resents the amplitude distribution of the electric field for the SCP, a ˆω is the annihilation operator for the absorption, γ is a positive infinitesimal, and H.c. is the hermitian conjugate. The enhancement factor M depends on the dielectric function of the tip and substrate as well as the wavelength of the incident light, which should satisfy the specific normalization condition. 39 Thus, M is set to a constant for all vibrational modes in our calculations. In experiments, the observation of the far field scattering signals is always facilitated. Therefore, the corresponding interaction Hamiltonian for spontaneous scattering reads 39 r 0 Hρ,ω = −ı s

2πωs † † (ıωs +γ)t µ ˆρ a ˆ ωs e + H.c., V

(2)

where ρ again represents the Cartesian coordinate, ωs is the frequency of scattering light, and a ˆ†ωs is creation operator for the emission. Based on the above interaction Hamiltonians (Eq. 1 and Eq. 2) and the standard secondorder time-dependent perturbation theory, the differential scattering cross section of Raman process between initial state |ii and final state |f i under SCP field can be obtained as 40 ωωs3 Md2 FP dσf i = |αf i |2 , 4 dΩ c

5


(3)


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where Ω is the solid angle, Md and FP are the directional radiation pattern factor 41 and Purcell factor 42 owing to the nano-structure, respectively, c is the speed of light, and

αf i,ρσ = M

X hf |ˆ µρ |rihr|ˆ µσ gσ |ii r

ωri − ω − ıγ

hf |ˆ µσ gσ |rihr|ˆ µρ |ii + ωri + ωs − ıγ

(4)

is the general polarizability. Here, |ri is the intermediate state and ωri is the frequency difference between |ri and |ii. Once the Raman cross section is obtained, the scattering intensity would be readily known if the irradiance of the incident laser I0 is determined, 43 i.e. Is =

dσf i ωs I0 . dΩ ω

(5)

In practical calculations, in preresonance conditions, both Franck-Condon and HerzbergTeller terms come into play. 44 According to the state-to-state mapping relationship between the Albrecht’s theory and the perturbation theory, 45 for the polarizability associated with Raman processes, the prior summation of all vibrational states leads to 38,39

αρσ,k =

eff ∂αρσ hv f |Qk |v i i, ∂Qk

(6)

where eff αρσ

=M

X hΨg |ˆ µρ |Ψr ihΨr |ˆ µσ gσ |Ψg i r

∆Erg − ω

hΨg |ˆ µσ gσ |Ψr ihΨr |ˆ µρ |Ψg i + ∆Erg + ωs

.

(7)

Here, Qk is the vibrational normal mode that can be naturally selected out, 39 |v i i and |v f i are the initial and final vibrational states in the electronic ground state |Ψg i, |Ψr i is the electronic excited state, ∆Erg is the vertical excitation energy between |Ψg i and |Ψr i, and ıγ is neglected. Using Eq. 6, the induced dipole could be calculated by

µ0,k = Md

p

F P α k · E0 ,

(8)

where E0 is the electric field amplitude of the incident light and the elements of tensor αk

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are given by Eq. 6 The calculation of all excited states as well as their derivatives in Eq. 6 and Eq. 7 is the bottleneck in practical simulations. Further simplification is possible if we assume ω ≈ ωs . As a result, we have eff ∗ αρσ

=M

X hΨg |ˆ µσ gσ |Ψr ihΨr |ˆ µρ |Ψg i ∆Erg − ω

r

hΨg |ˆ µρ |Ψr ihΨr |ˆ µσ gσ |Ψg i + ∆Erg + ω

.

(9)

It should be noted that the operators µ ˆρ and µ ˆσ gσ are exchanged for the direct extraction of results calculated by the linear response function implemented in Gaussian 46 program (see the computational details in the Supporting Information (SI)). As a result, the complex conjunction emerges in Eq. 9 owing to the relationship of operator exchange in linear response function for general operators and/or complex wavefunctions. 47 In other words,

eff αρσ

∗

= −M hhˆ µσ gσ ; µ ˆρ iiω .

(10)

It is well known that the required linear response function in Eq. 10 can be calculated by solving the linear equations 48,49 (E − ωS) κ = µ, where





 A B  E= , B A

  S=

7

(11)

 Σ

∆  , −∆ −Σ


(12)


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with Apq Bpq Σpq

i i E D hh ˆ 0 , Q† 0 = 0 Qp , H q i i E D hh ˆ 0 , Qq 0 = 0 Qp , H

= 0 Qp , Q†q 0

(13)

∆pq = h0 |[Qp , Qq ]| 0i µρ,p = h0 |[Qp , µ ˆρ ]| 0i . Here, |0i is the reference state, Q†q and Qp are excitation and deexcitation operators, respectively, and Hˆ0 is the unperturbed molecular Hamiltonian. For different positions of the SCP field, the solvation of Eq. 11 is identical. As a result, Eq. 11 could be solved only once, leading to the efficient calculations of Raman images. Once κ is obtained from Eq. 11, the final expression of the effective polarizability reads 48

eff αρσ

∗

= M U† κ,

(14)

where Uσ,p = h0 |[Qp , µ ˆσ gσ ]| 0i .

(15)

It notes that the emission operator is not modified by the plasmonic field, leading to unsymmetrical response function (Eq. 10). One can always resume the symmetric response function by employing both plasmon modified absorption and emission operators. However, this would result in the emission of a localized plasmon, which violates the far field detection in experiments. 50 In this work, SCP is confined in the xy plane (parallel to the substrate), while the plasmonic decay along the z-axis (norm of the substrate) is neglected, resulting in a cylinder Gaussian distribution. Our tested calculations reveal that this is a good approximation for realistic cases because of the flat physisorption of porphine (see following discussions). Besides, the cylinder Gaussian distribution can automatically remove

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the gauge problem in conventional phenomenologically modified interaction Hamiltonian. 40 It is noted that the used Gaussian distribution as well as the separation of out-of-plane and in-plane confinements in current work is consistent with the actual simulations obtained by the hybrid atomistic electrodynamics-quantum mechanical method. 51 In addition, only the z-component of SCP field was considered owing to the metallic substrate with a thin spacer in our modeling (Figure 2a). The size of SCP is thus represented by the full width at half maximum (fwhm) of the amplitude distribution in xy. More computational details can be found in SI.

Results and Discussion TERS of Trans Porphine Table 1: Assignment of Calculated Vibrational Frequencies (in cm−1 ) of Trans Porphine in D2h Point Group. Cm , Cα and Cβ Are Labeled in Figure 1a. symmetry label s ag νNH ν2 ν11 ν3 ν4 ν12 ν6 ν15 as b3u νNH ν37b ν40b ν41b ν47b b2u ν40a

frequency 3476.8 1549.5 1497.0 1429.4 1396.5 1350.2 992.7 956.9 3434.8 1506.4 1402.8 1397.7 974.9 1406.1

assignmenta NHs Cβ Cβ sd srdip + Cβ Cβ s Cβ Cβ sp + Cα N, Cα Cβ sd Cm H b + srdip Cα Cβ si srdip srdid + Cα Cβ sd NHs srdop + Cβ Cβ sp Cβ H bd + srdop Cm H b + srdop srdop Cm H b + srdod

a

: In accordance of Li and Zgierski nomenclature. 52 Specifically, Xp and Xd are deformation X on protonated pyrrole ring and on deprotonated pyrrole ring, respectively; srd, in plane pyrrole ring deformations; b, bending; s, stretching; superscript “i” in-phase, “o” out-of-phase.

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(a) y z

(b)

10 9 8 7 6 5 4 3 2 1 3600

x 1

SCP size

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 3400 1600 s as νNH νNH

(c)

ν37b

1 A˚

1400

1200

1000

Raman Shift / cm−1 νmix ν6 ν47b

ν11

1 A˚

3600

3400 1600

1400

1200

1000

Raman Shift / cm−1

Figure 2: (a) Schematic illustration of proposed experimental setup. The real and virtual locations of inner hydrogen atoms are marked by the red and blue circles, respectively. Cyan, blue and white balls represent C, N and H atoms, respectively, and the coordinates are also illustrated. (b) Waterfall plot of calculated Raman spectra along SCP size with the plasmon focused on the real location of inner hydrogen atom. (c) Calculated Raman spectra under a 1 Å SCP focused on the real (red line) and virtual (blue line) locations of inner hydrogen atom in trans porphine. The real location represents the position of interior hydrogen in trans (red circle area in (a)). The virtual location is the position of phantom hydrogen that bonded to the free interior nitrogen with the same N-H bond length (blue circle area in (a)). All calculated Raman spectra were convoluted by the Lorentzian function with fwhm of 6 cm−1 . The assignments of vibrational modes and Raman bands of the stable trans porphine have been comprehensively investigated in both experimental and theoretical works, 16,52–54 which would be good references for TERS of porphine system. Thus, the detailed TERS of trans porphine was first calculated. To monitor the NH tautomerization is equivalent to detect the inner hydrogen position in real space. The first intuitive choice is focusing SCP that is generated between tip and substrate in TERS on the hydrogen atom in the observable trans porphine (marked by the red circle in Figure 2a). In practical simulations, the wavelength of the incident laser was set to 530 nm, which is commonly used in TERS experiments. 50 In addition, a thin spacer layer between the adsorbed porphine and the substrate was employed to further decouple the porphine-substrate interaction. As a result, the effects of the sub-

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strate (including spacer) on the electronic structure of porphine could be neglected in our simulations. We note that owing to the emerging TERS imaging technology, no TERS images with spacer have been reported yet. However, similar spacers that could be insulators, two-dimensional van der Waals materials, or other inert coatings have been widely used in TERS, 55 scanning probing microscopy, 56,57 as well as surface enhanced Raman spectroscopy experiments. 58 Moreover, recent experiments demonstrated that the ultra-confined plasmons are not affected by such thin spacers. 59,60 All simulated spectra in this case with different SCP sizes were depicted as a waterfall plot shown in Figure 2b and the assignments of all related vibrational modes in accordance of Li and Zgierski nomenclature 52 were listed in s , located at Table 1. It is surprisingly noted that the target NH symmetrical stretching (νNH as , 3435 cm−1 ) are still unobservable even when 3477 cm−1 ) and asymmetrical stretching (νNH

SCP size is comparable with that of porphine molecule (around 10 Å). Only four allowed ag (in D2h point group) vibrational modes 52 (ν2 , 1550 cm−1 ; ν3 , 1429 cm−1 ; ν6 , 993 cm−1 ; and ν15 , 957 cm−1 ) are readily observed in this case, which agrees well with the Raman spectrum under the uniformed electromagnetic field (Figure S1). Therefore, the confinement of a 10 Å SCP is not enough to break down the conventional spectral selection rule for porphine sysas s bands are attributed to the occasionally weak and νNH tem and thus, the vanishing of νNH

intensity and symmetry-forbidden transition, respectively, as discussed above. With the decrease of SCP size to around 8 Å, the ν37b (belongs to b3u ) band located at 1506 cm−1 emerges as shown in Figure 2b, indicating the breakdown of the spectral selection for conventional Raman scattering. 61 In fact, in this case, the point group of the whole system that includes field and molecule reduces to C2v . As a result, the forbidden b3u modes in D2h become Raman active (Table S1), which is consistent with our numerical calculations. It notes that further decreased SCP (5 Å) also enhances the previously weak ag modes, e.g. ν12 located at 1350 cm−1 . It is interesting to note that the dominant ν2 band with large SCP first decreases and then increases with deceasing of the SCP size. This result should be attributed to the cancellation of the Raman transition density (see following discussions),

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s as leading to almost zero intensity under 5 Å SCP. Unfortunately, the desired νNH and νNH

bands are still obscure even with 4 Å SCP that is smaller than the size of porphine. This result should be attributed to the expected highly localized νNH , which also highlights the difficulty of direct monitoring of the NH tautomerization in porphine. When the SCP size is smaller than 3 Å, we finally observe the gradual increase of the s νNH band with the increasing of the confinement as shown in Figure 2b. Furthermore, when as the SCP size is smaller than 2 Å, the νNH band emerges as well, indicating a more localized as characteristic of νNH . With the field confinement of 1 Å that is comparable to the N-H s as bond length (Figure 1a), both νNH and νNH bands have significant intensities (Figure 2b

and Figure 2c), which facilitates the experimental detection. Therefore, a highly localized plasmonic field is demanded for direct monitoring of the NH tautomerization. Meanwhile, in the low frequency range, a dominant band around 1400 cm−1 (labeled as νmix hereafter) that is composed of four near-degenerate vibration modes (ν40a , ν40b , ν41b , and ν4 , Table 1) emerges. We also note that there are other seven bands (ν2 , ν37b , ν11 , ν3 , ν12 , ν6 , ν47b ) having as comparable intensities with that of νNH . All these modes are potentially related to the NH

tautomerization. In contrast, the intensity of the previous observable vibrational band (ν15 ) under large spot has been significantly decreased under 1 Å SCP, predicting insignificant association with the NH tautomerization. To further check the related modes, we move the 1 Å SCP from the real location of hydrogen (red-marked circle in Figure 2a) to the virtual location (blue-marked circle). As expected, the intensities of two νNH modes and four potentially related modes (ν37b , ν11 , ν6 , and ν47b ) become negligible, as noted by gray areas in Figure 2c. On the other hand, the intensities of other four candidate modes (ν2 , ν3 , ν12 , and previously dominant νmix ) preserve, owing to the association with atom motions in the deprotonated pyrrole rings (Table 1). For instance, the maintenance of the νmix band is mainly contributed by ν40a that is related to the motion of nitrogen in the deprotonated rings (Figure S2). As a result, these modes fail to provide the practical identification of inner hydrogen location (also see Figure S3).

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(a)

(b) 1

s νNH (3477)

as νNH (3435)

ν6 (993)

ν47b (975)

s νNH

as νNH

ν6

ν47b

s νNH

as νNH

ν6

ν47b

0

(c) 1

0

Figure 3: (a) Calculated normal modes that related to the NH tautomerization for trans porphine with the frequencies in cm−1 . Calculated Raman images for the modes in (a) with 1 Å (top) and 3 Å (bottom) SCP under low (b) and high (c) temperatures. The inner NH bonds are highlighted by white lines. The black solid lines in Raman images represent the skeleton of porphine molecule.

Raman Imaging of Trans Porphine We then move to the TERS imaging 32,38,50,62 since it provides a direct way for monitoring of the NH tautomerization. Our test calculations show that ν2 image with a 1 Å SCP agrees well with the theoretical counterpart under the confined plasmonic field between a Au2057 icosahedron tip and Au(111) surface (Figure S4), 62 revealing that the SCP confinement of 1 Å is possible. It should be stressed that the highly confined plasmonic field requires a picocavity with a protrusion at the single atom scale in the junction. 62–64 Thus, a fabrication with the precise control of the position of adatom in nano-technology plays the key role for experimental implementation. According to the above spectral analysis, we first considered the Raman images under 1 Å SCP that is critical for the monitoring of the NH tautomer-

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s as ization. It is immediately noted that, in this case, the Raman images of νNH and νNH can

fully reflect the positions of inner hydrogen atoms (Figure 3b), indicating a unique optical means for the direct monitoring of the NH tautomerization. We could further observe minor differences of the νNH images. Specifically, the two bright “circle” patterns are merged in s as νNH image, while a clear boundary separates the bright “circle” patterns in νNH one. This

result should be attributed to different phase distributions in both modes (Figure 3a and as Figure S5), which also makes more localized patterns in νNH image as consistent with above

spectral analysis. We noted that in recent experimental observations, a 2.6 Å spatial resolution is obtained for the TERS spectra of a metal-free porphyrin molecule on Cu(111) surface. 65 Thus, we also depicted the calculated images under a more relaxed 3 Å SCP in Figure 3b. The phase effects are more important on the images with a 3 Å SCP. The cens image as the consequence of the construction of two tral patterns are totally merged in νNH

patterns. In the meantime, because the size of SCP is larger than the inner H/H distance (Figure 1a), the destruction of the inner patterns results in a disappeared central pattern for as νNH . It notes that the destruction of patterns with the opposite phases is also responsible

for the zero intensity bands of ν2 , ν37b and ν11 in spectra under the specific SCP size in Figure 2b (Figure S6). Previous analysis of spectra (Figure 2c) speculates that ν6 and ν47b , i.e., the in-phase and out-of-phase protonated pyrrole ring deformations, respectively 16 (Table 1), may associate with the inner NH position. Calculated images with 1 Å SCP indeed have moderate “boomerang” patterns around the inner NH bonds, which should be attributed to the deformation-induced translational motions of N-H bonds (Figure 3a). As a result, the moderate patterns are assigned as indirect association with the NH position. Besides, ν37b and ν11 images also exhibit such kind of indirect association with NH position (Figure S7). On the other hand, νmix image has four bright patterns around all interior positions (Figure S8). All these results are consistent with previous analysis for spectra. When a 3 Å SCP is employed, the merging and repulsion of the central patterns for ν6 and ν47b are observed, respectively,

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which is again attributed to the phase effects (Figure S9). Our tested calculations show that the effect of field decay along z-axis on calculated Raman images is insignificant (Figure 3b, Figure S10, and Figure S4), which could be attributed to the planar configuration of physisorbed porphine. The flat physisorption also leads to forbidden emission along z-direction of the Qx (2.27 eV) and Qy (2.42 eV) states that near the incident light energy (2.34 eV). As a result, the contribution of resonance Raman effects becomes extremely weak (see Table S2 and related discussions in SI for details), which makes preresonance conditions satisfied. All above results are associated with the condition of low temperature (the lifetime of trans porphine exceeds 102 s below 134 K 9 ), where the position of inner hydrogen is fixed. Owing to the tunneling effect, the lifetime would be less than 10−2 s when the temperature is higher than 220 K as reported by experimental measurements. 9 In other words, under high temperature, the inner hydrogen atoms would rotate and the NH tautomerization occurs. The corresponding Raman images that calculated by the average of low-temperature counterparts obtained from all possible configurations under 1 Å SCP shown in Figure 3c conscientiously capture the fact of NH tautomerization, where four bright patterns display at the interior in both νNH images. Again, the superposition and the hollow of the patterns in as s images, respectively, should be attributed to the individual phase distribution. and νNH νNH

For ν6 and ν47b , the moderate four-fold patterns at the interior are observed as well, which shows the indirect association of the NH tautomerization as discussed before. Comparing the results with and without consideration of the NH tautomerization (the images with 1 Å SCP shown in Figure 3), we can conclude that Raman images provide an unprecedented method for the direct monitoring of the NH tautomerization in porphine. When a larger SCP size of 3 Å that is available in current experiments is employed, the circle-shaped central pattern s in νNH with tautomerization only has minor differences with the oval-shaped counterpart

without tautomerization (Figure 3b). In addition, the destructive phase effect leads to no as patterns at the interior for νNH . Thus, although, for indirectly associated ν6 and ν47b modes,

the moderate patterns with four-fold characters at the interior are different from the two-

15



fold interior patterns in their counterparts without tautomerization, the direct monitoring of the NH tautomerization with 3 Å SCP is still difficult. We note that the picocavity that has the single atom protrusion in the junction plays the essential role for the generation of highly confined field. 62–64 Generally, cryogenic temperatures could stabilize the picocavity. 63 A very recent experiment 66 further showed that the typical lifetime of pico-cavity based on gold can be around 2 s even at room temperature. In spite of that, it is still too short to a TERS image scan 50 (approximately around 60 s for porphine). Fortunately, the observation of tautomerization would be probable around 192 K (lifetime of trans porphine is less than 0.1 s). 9 Thus, if the pico-cavity could stay for 60 s under this relatively low temperature, the NH tautomerization would be observed by TERS images. Considering the fact that the moving tip may destroy the formed pico-cavity at the required temperature, we can expect longer lifetime of the pico-cavity with harder materials than gold because the fixation of adatom is the decisive role for the generation of the pico-cavity. Thus, a good fabrication method that can precisely control the position of the adatom is the challenge that should be overcame in the future development for the experimental observation of current predictions under high temperature. On the other hand, the applied bias is not prerequisite for TERS. Thus, we can neglect the effects of the bias on the stability of pico-cavity, which is also an advantage of TERS. In addition, to facilitate the detection of a single porphine molecule on surface, an estimated localized field enhancement of 103 would be needed (see calculated TERS cross sections for trans porphine in Table S3 and related discussions in SI for details).

TERS and Imaging of Cis Porphine Dynamic NMR measurement estimated extremely short lifetime of cis porphine (around 10−7 s even at 5 K), 9 making it impossible to be detected within the STM time-resolution (about 1 ms). 67 Previous studies demonstrated that time resolution of optical methods is much superior, 31,68 which leads to the possibility of the detection for TERS singles of transient cis-isomer. Unlike the trans isomer (Figure 2a), the accurate virtual position for inner 16


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(a)

(b)

(c)

s νNH 0 (3255)

as νNH 0 (3222)

ν47b0 (966)

s νNH 0

as νNH 0

ν47b0 +ν60

(d) ν s NH0

as νNH 0

ν47b0 +ν60

ν60 (958)

1

0

1

0

Figure 4: (a) Simulated TERS under a 1 Å SCP located at the position of inner hydrogen atom (marked by red circle) in cis porphine. (b) Vibration modes that related to NH tautomerization with the frequencies in cm−1 . The correspondingly calculated Raman images under 1 Å SCP (top) and 3 Å SCP (bottom) without (c) and with (d) considerations of NH tautomerization. The inner NH bonds are highlighted by white lines. hydrogen in cis is equivocal due to the movement of pyrrole ring skeleton with NH tautomerization. Therefore, the center of SCP field is naturally focused on the real location of inner hydrogen atom in cis configuration (noted by the red circle in Figure 4a). Here the 1 Å SCP field is employed owing to the importance of highly confined SCP on monitoring NH tautomerization, which has been highlighted above. For cis spectrum in Figure 4a, our 17



theoretical calculations reveal that more observable vibration modes appear comparing to the counterpart of trans case in Figure 2c. This result is attributed to the lower Cs symmetry of cis system with SCP included than trans system (C2v , as discussed above). The significant difference between cis and trans Raman spectra with 1 Å SCP indicates that they can be unambiguously detected from TERS. We should stress that under large light spots, the similarity of Raman spectra between trans and cis porphines (Figure S11) makes them indistinguishable. Thus, the highly confined SCP filed is prerequisite for detecting s as cis porphine. We further note that the unobservable NH vibrational modes (νNH 0 and ν NH0 ,

Figure 4b) under large spots (Figure S11) display significant intensities under 1 Å SCP (Figure 4a), which again highlights the importance of localized field for the direct NH detection even in cis. Besides, the protonated pyrrole ring deformation vibration modes of cis (ν47b0 and ν60 around 960 cm−1 ) that analogize to ν47b and ν6 in trans also give intense signals. Correspondingly, the Raman images of these modes give the inner hydrogen positions s as of cis porphine in real space, as seen in Figure 4c. Specifically, for νNH 0 and ν NH0 that

directly associated with NH tautomerization, the 1 Å SCP results exhibit two localized Raman patterns reflecting the ortho-position of hydrogens, which clearly differs from the para-position of hydrogens in trans Raman patterns (Figure 3b). It notes that comparing to their counterparts in trans, there are two more moderate patterns near the nitrogen s as atoms of deprotonated pyrrole rings for both νNH 0 and ν NH0 in cis. For a 3 Å SCP, both

modes provide the indirect information of ortho-position for hydrogens, where the bright patterns are distributed in the ortho protonated pyrrole rings. It should be stressed that s as the characteristically merged and separated Raman patterns of νNH 0 and ν NH0 , respectively,

come from the phase distributions, which agrees with that of their counterparts in trans. For ν47b0 and ν60 , the mixed signals are expected in the experimental measurements owing to the slight frequency difference (8 cm−1 ). Thus, the Raman image labeled as ν47b0 + ν60 that constituted by ν47b0 and ν60 images (Figure S12) are given in Figure 4c. It finds that both 1 Å and 3 Å SCP contribute to the moderate patterns near the ortho hydrogen atoms

18


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for this mixed signal, and thus monitor the NH positions in cis. We want to stress that it is extremely harsh to stabilize the hydrogen positions in transient cis porphine even during an ultrafast imaging scanning, where the tip needs to be moved. Therefore, the Raman images of cis porphine with stabilized hydrogens in Figure 4c incline to be a theoretical blueprint. s In contrast, the frequency shift (for instance, the νNH of trans with 3477 cm−1 dramatically s reduces to 3255 cm−1 for νNH 0 of cis) is more facilitated for the detection of cis. Although the

resolution of the state-of-the-art TERS experiment with ultrafast imaging (6 nm) 29 is much poor than that of theoretical requirement (1 Å and 3 Å), it is expected that the cooperation of picocavity and ultrafast TERS would finally make the required SCP field accessible. The simulated Raman images with ultrafast proton transfer in cis porphine that are more s promising to be observed, are given in Figure 4d. It finds that under 1 Å SCP, νNH 0 and as νNH 0 Raman images with NH tautomerization show the internally four-fold patterns, which

are similar to their counterparts of trans (Figure 3c) excerpt more diffused internal patterns. This result is attributed to the participation of moderate patterns near the nitrogen atoms s as of deprotonated pyrrole rings for νNH 0 and ν NH0 in cis (Figure 4c). In addition, the mixed

ν47b0 + ν60 in cis displays moderate four-fold patterns at the interior that resembles ν6 of as as trans. When a 3 Å SCP plays a role, νNH 0 and the mixed ν47b0 + ν60 are similar to νNH and ν6

of trans, respectively. The similarities in Raman images show the difficult in distinguishing NH tautomerization between trans and cis. Nevertheless, as discussed from spectra, the frequency difference between the counterparts of vibration modes in trans and cis already s gives the distinction. Furthermore, it notes that νNH 0 gives obscure pattern at the interior s that significantly differs from the internally bright patter of its counterpart νNH in trans

(Figure 3c), which could be expected to distinguish NH tautomerization in trans.

TERS and Imaging of Deuterated Trans Porphine Identification of the tautomerization in HD-porphine is the most challenging case. By focusing a 1 Å SCP either on the inner hydrogen (red circle) or on the deuterium (blue circle), the 19



Page 20 of 35

(a) D

(b)

(c) 1

D νNH (3456)

D νND (2544)

ν6D (988)

D ν47b (966)

D νNH

D νND

ν6D

D ν47b

D νNH

D νND

ν6D

D ν47b

0

(d) 1

0

Figure 5: (a) Simulated TERS under a 1 Å SCP located at the positions of inner hydrogen atom (marked by red circle) and inner deuterium atom (marked by blue circle) in trans HDporphine. The substituted deuterium atom is represented by a red ball. (b) Vibration modes that related to NH/ND tautomerization with the frequencies in cm−1 . The correspondingly calculated Raman images with 1 Å SCP (c) and 3 Å SCP (d) under low (top) and high (bottom) temperatures. The inner NH and ND bonds are highlighted by white and red lines, respectively. calculated Raman spectra for trans HD-porphine are depicted in Figure 5a. The modes that related to the differences between hydrogen and deuterium are straightforwardly selected D out. As we expect, when the SCP is focused on hydrogen, the N-H stretching mode (νNH )

at 3456 cm−1 is the only observed band in the high frequency region. If the SCP is focused D on deuterium, the N-D stretching (νND ) band at 2544 cm−1 exclusively emerges. This re-

sult provides a direct spectral characterization of trans HD-porphine. It should stress that

20


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D D under large light spots, both νNH and νND have negligible intensities and the trans of HD-

porphine and HH-porphine are indistinguishable (Figure S11). Thus, the highly localized SCP together with the frequency fingerprint signatures of Raman spectroscopy are responsible for the direct characterization of HD-porphine. In the low frequency region, we find that three bands under 1000 cm−1 are different. Detailed analysis indicates that the two bands with higher frequencies are corresponding to ν6 (at 988 cm−1 , labeled as ν6D ) and ν47b (at D 966 cm−1 , labeled as ν47b ) in trans HH-porphine, while, the band with the lowest frequency

(at 954 cm−1 ) consists of two near-degenerate modes (Figure S13) without correspondence in trans HH-porphine. All other bands are independent of the SCP’s positions, revealing their irrelevance of the deuterium substitution. To further gain the insight of differences of tautomerization in trans of HH-porphine and HD-porphine, the Raman images of selected modes in spectral analysis are calculated D (Figure 5c). The sole bright pattern in νNH image with 1 Å SCP exactly reflects the position

of inner hydrogen atom. This result clearly demonstrates the normal mode redistribution D after the deuterium replacement. We also note that for νND , except a moderate central

pattern associated with the deuterium, there are more patterns related to the correspondingly deuterated pyrrole ring, which should be attributed to the fact that heavier deuterium has more effects on the atomic movements of skeleton. As a result, Raman images can truly reflect the differences between inner hydrogen and deuterium, which suggests a reliable method for monitoring the tautomerization in HD-porphine. It is interestingly noted that, in spite of D few frequency red-shift (less than 10 cm−1 ), ν6D and ν47b images are significantly different D from their counterparts in trans HH-porphine. Specifically, ν6D and ν47b images associate with

protonated and deuterated pyrrole, respectively, and are similar to half part of ν6 image. The D pattern of ν47b image is obscurely observed in ν47b one. This result should be attributed to

the redistribution of the normal modes in HD-porphine (Figure 5b), although the deuterium is much lighter than the weight of elements in pyrrole ring. The ability of reflection for the subtle changes in vibration modes suggests that Raman imaging is superior for monitoring of

21



the tautomerization in porphyrins and would readily have applications for other interesting systems. It should be stressed that the DIE dramatically slows down the tautomerization, 9,12 which avoids the harsh low temperature condition for the stabilization of the inner H/D atoms. As a consequence, the lifetime of trans HD-porphine exceeds 102 s when the temperature is below 174 K. Hence, the required temperature for suppression of tautomerization is 40 K above that of HH-porphine, which significantly facilitates the experiments. Moreover, the deuteration brings four-level conductance switch based on the tautomerization, which improves the twolevel conductance switch of trans HH-porphine. With the increase of temperature above 263 K, the lifetime of trans HD-porphine would be shorter than 10−2 s. Thus, proton transfer occurs repetitiously in the timescale of a typical TERS experiment measurement. 50 D D under 1 Å SCP (Figure 5c) are , ν6D and ν47b In this case, calculated Raman images for νNH

similar to the counterparts of trans HH-porphine (Figure 3c). Nevertheless, the frequency D red-shift of νNH (21 cm−1 ) itself could provide the distinction of HD-porphine and HHD porphine. For νND , the moderate patterns associated with interior and peripheral pyrrole as , which rings are different from the bright central and obscure peripheral patterns from νNH

could visually discriminate tautomerization in trans of HD-porphine and HH-porphine. The Raman images of trans HD-porphine with 3 Å SCP are further calculated (FigD ure 5d). Under low temperature, i.e., without tautomerization, νNH image has a sole moder-

ate pattern associated with inner hydrogen atom and a bright pattern related to protonated D D pyrrole ring. Whereas, the central pattern disappears in νND image. Thus, only νNH mode

could be used for the direct monitoring of the hydrogen position in trans HD-porphine with the relaxed SCP even under the low temperature. On the other hand, the indirectly associD ated patterns in ν6D and ν47b images could be useful for speculation of the positions of both

inner hydrogen and deuterium. Under high temperature, the Raman images from different modes in trans HD-porphine exhibit almost identical patterns to their counterparts in trans HH-porphine (Figure 3c). In spite of this, we should emphasize that the frequency shifts of

22


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(a)

(b)

(c) 1

D νNH 0

(3238)

D νND 0

D νsrd 0

(2391)

D ν47b 0

(1367)

D νNH 0

D νND 0

D νsrd 0

D ν47b 0

D νNH 0

D νND 0

D νsrd 0

D ν47b 0

(973)

0

(d) 1

0

Figure 6: (a) Simulated TERS under a 1 Å SCP located at the positions of inner hydrogen atom (marked by red circle) and inner deuterium atom (marked by blue circle) in cis HDporphine. The substituted deuterium atom is represented by a red ball. (b) Vibration modes that related to NH/ND tautomerization with the frequencies in cm−1 . The correspondingly calculated Raman images under 1 Å SCP (c) and 3 Å SCP (d) without (top) and with (bottom) considerations of NH tautomerization. The inner NH and ND bonds are highlighted by white and red lines, respectively. those modes can distinguish trans of HD-porphine and HH-porphine, which again highlights the vibrationally resolved feature of Raman imaging.

TERS and Imaging of Deuterated Cis Porphine Finally, we take the cis HD-porphine as the last example in the present work. The deuteration significantly enhances the lifetime of cis isomer of porphine (from 10−7 s lifetime in the 23



pure cis HH-porphine to 10−5 s in HD-porphine at 5 K), 9 making it more possible to be directly detected in experiments. 69 It noted that under large light spots, cis HD-porphine has D D unobservable νNH 0 and ν ND0 modes and it’s Raman spectra can not be identified from non-

deuterated trans and cis HH-porphine isomers as well as the deuterated trans HD-porphine tautomer (Figure S11). On the other hand, when the highly localized SCP is introduced, the reduced symmetry of the whole cis HD-porphine system (Cs ) leads to more observable vibration modes in Figure 6a, which makes it significantly different from both trans HHporphine and HD-porphine. Specifically, if a 1 Å SCP is focused on the hydrogen, only D −1 D νNH displays intense signal in the high frequency range. In contrast, νND 0 at 3238 cm 0

at 2391 cm−1 exclusively performs observable intensity when the SCP center is moved to deuterium. This result provides spectral characterization of cis HD-porphine that is clearly D distinguishable from cis HH-porphine. Besides, the spectra selection shows that modes νsrd 0

at 1367 cm−1 identified as protonated pyrrole ring deformations (without correspondence in D −1 cis HH-porphine) and ν47b (with correspondence of ν47b0 in cis HH-porphine) in 0 at 973 cm

Figure 6b have intense signals when SCP is located on deuterium while perform suppressed intensities after moving SCP to hydrogen position. These modes reflect the existence of deuteration in cis HD-porphine. The Raman images with 1 Å SCP for the selected modes in cis HD-porphine are shown in D D Figure 6c. We find that νNH 0 and ν ND0 images are similar to the half part of their counterparts as s (νNH 0 and ν NH0 ) in cis HH-porphine (Figure 4c) except the slightly clearer patterns associated D with deuterated pyrrole ring in νND 0 image. This result comes from the more obvious impact D of heavier deuterium on atomic skeleton movement, as mentioned above. Besides, both νsrd 0 D and ν47b 0 give observable Raman patterns around the deuterium positions, in contrast to D the invisible patterns near the inner hydrogen atom. Consequently, νNH 0 mode exclusively D D D associates with the inner hydrogen atom while νND 0, ν srd0 and ν47b0 modes monitor the in-

ner deuterium atom. When the rapid tautomerization is considered for cis HD-porphine, all these selected modes display internal patterns with four-fold characters, reflecting the

24


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D D proton transfer. In this context, the Raman images of νNH 0 and ν 47b0 are similar to that of s D νNH 0 and the mixed ν47b0 + ν60 in cis HH-porphine (Figure 4d), respectively. For ν ND0 , the

slightly clearer patterns near the nitrogen atoms caused by heavier deuterium in cis HDporphine contribute the four-fold patterns around the corresponding four nitrogen atoms, as which visually differs the tautomerization from its counterpart νNH 0 in cis HH-porphine. We

want to stress that the deuterium substitution in cis HD-porphine makes an eight-state superposition with NH tautomerization, which is responsible for the eight bright patterns in D νsrd 0 (also see Figure S14). When the SCP size is increased to 3 Å, the Raman images of

these modes in Figure 6d do not give observable patterns associated with the inner hydrogen/deuterium, proposing a difficulty in direct visualization of hydrogen/deuterium positions and thus the tautomerization of cis HD-porphine, which again highlights the necessity of the highly confined plasmonic field in monitoring proton transfer.

Conclusions In summary, we propose that Raman spectroscopy with highly localized plasmonic field could be a unique optical means for the direct monitoring of tautomerization in HH-porphyrin and HD-porphine, where the monitoring of the latter one is unavailable by other techniques. Calculated Raman images show that subtle variations in vibrational modes could be captured, which provides a straightforward method for the investigation of molecular vibrations. Furthermore, the cis isomer of porphine that has not been directly detected in experiments are expected to be visualized, which can be effectively distinguished from trans. Our study suggests that the emerging Raman imaging technique can give chemical identification at the single molecular scale and thus has extensive applications in physics, chemistry, material science, and biology.

25



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Supporting Information Available Methodology and computational details of the reported results and supporting calculated data.

Acknowledgement This work was supported by the Ministry of Science and Technology of China (2017YFA0303500), the National Natural Science Foundation of China (21633007, 21790350 and 11874242), the Chinese Academy of Science (2016HSC-IU003), and Swedish Research Council (VR). The Swedish National Infrastructure for Computing (SNIC) was acknowledged for computer time. CKW thanks Taishan Scholar Project of Shandong Province, China.

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Graphical TOC Entry Raman images of

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νNH


Deuterium Tautomerization in Transient Isomers

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