Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions

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Metal-Metal Bond Formations in [Au(CN) ] (n = 3, 4, 5) Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions Munetaka Iwamura, Kenshi Kimoto, Koichi Nozaki, Hikaru Kuramochi, Satoshi Takeuchi, and Tahei Tahara J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03139 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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The Journal of Physical Chemistry Letters

Metal-Metal Bond Formations in [Au(CN)2-]n (n = 3, 4, 5) Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions Munetaka Iwamura1*, Kenshi Kimoto1, Koichi Nozaki1, Hikaru Kuramochi2,3,4, Satoshi Takeuchi2,3† and Tahei Tahara2,3*

1Graduate

School of Science and Engineering, University of Toyama, 3190 Gofuku,

Toyama 930-8555, Japan

2Molecular

Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198,

Japan 3Ultrafast

Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP),

RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 4PRESTO,

Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama

332-0012, Japan

ACS Paragon Plus Environment

1

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AUTHOR INFORMATION

Corresponding Author [email protected], [email protected]

Present Addresses

†Graduate

School of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori,

Hyogo 678-1297, Japan

ABSTRACT. Large oligomers of [Au(CN)2]n including pentamer were favorably formed in an aqueous solution containing tetra-ethyl ammonium chloride (1.0 mol/dm3), and intense transient absorption in the visible region was recorded by a selective photoexcitation of the oligomers.

Distinct oscillations at ~40-100 cm-1 were clearly

observed in the temporal profile of the excited-state absorption signal, and the frequency-wavelength two-dimensional analysis of the oscillation clearly distinguishes the coherent nuclear motion of different oligomers. The observed nuclear motions were


2

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assigned to Au-Au stretch vibrations in trimer, tetramer and pentamer induced by the bond formation in the excited states. The transient absorption exhibits significant changes with the time constant of 3-20 ps, reflecting intersystem crossing and structural change.

TOC GRAPHICS

Complexes containing heavy metal such as gold or platinum form oligomers in solution due to the metallophilic interaction, which is weak attractive force working between heavy metal atoms.1-6 Metallophilic oligomers have been attracting interest in a wide range of fields in chemistry such as functional molecular assemblies, soft materials and light emitting/sensing devices.7-18 In particular, [Au(CN)2]n oligomers,


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one of the most typical metallophilic oligomer systems, have attracted much attention as wavelength-tunable emitters because of variation in the emission wavelength which arises from their various degrees of oligomerization.11 Furthermore, it was recently recognized that the [Au(CN)2]n oligomer is a system suitable for real-time observation of the intermolecular bond formation because Au-Au covalent bonds are formed in the excited state with photoexcitation.6,

19-21

Actually, we performed time-resolved

absorption and emission measurements of K[Au(CN)2] aqueous solutions, and reported on the excited-state dynamics of the dimer and trimer with their Au-Au stretching nuclear wavepacket motions observed in the transient absorption signals.6,

19

The

frequencies of the nuclear motions well accorded with the Au-Au stretch vibrations of the excited-state oligomers that were computed by DFT, and it was concluded that the Au-Au stretch motion is induced by rapid shortening of the Au-Au distance due to the photo-induced bond formation between the gold atoms. This direct observation of the excited-state nuclear motion demonstrated that the vibrational information in the time domain is very useful for characterizing metallophilic oligomers generated with photoirradiation.

Very recently, Wakabayashi et al. reported that the degree of


4

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oligomerization of [Au(CN)2]n is significantly enhanced with addition of tetra-ethyl ammonium chloride (Et4NCl), which efficiently stabilizes the oligomers by reducing electrostatic repulsions among [Au(CN)2].22

The addition of the salts significantly

expands the tunability of the emission wavelength of the [Au(CN)2]n oligomer with considerable enhancement of the emission quantum yield from < 1 % (without the salts) to > 40 %. Such highly efficient emission and wide tunability open a new possibility in photochemistry and application of a large size of molecular assemblies of metal complexes.

However, it is difficult to well characterize each oligomer because the

oligomers with various sizes and different structures coexist in the equilibrium in solution.11, 22

In this letter, we report on an ultrafast time-resolved absorption study of [Au(CN)2]n oligomers in an aqueous solution containing Et4NCl, which was added to generate large oligomers in the solution.22 Several distinct oscillations in the frequency range of 40-85 cm-1 were observed in the temporal profiles of the transient absorption. To separate and identify signals of various oligomers, we have performed the


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frequency-wavelength two-dimensional (2D) analysis of the oscillation, where the magnitude oscillation was plotted as a function of its frequency and monitored wavelength.23 The characteristics of coherent nuclear motion clarified in the analysis clearly distinguished the contributions from the trimer, tetramer and pentamer with their distinct Au-Au stretch motions in the excited state.

Femtosecond time-resolved absorption spectra

Oligomers of [Au(CN)2] including the trimer, tetramer and larger oligomers are formed in [Au(CN)2] aqueous solution containing 1.0 mol/dm3 Et4NCl, as clearly seen in steady-state absorption (See SI, Figure S1) as well as time-resolved emission (Figure S2). Figure 1a-e show the time-resolved absorption data measured with excitation at 327 nm that photoexcites the oligomers larger than the dimer.6 Immediately after the photoexcitation, an intense excited-state absorption (ESA) peaked at around 630 nm is observed, and this ESA band shows a gradual rise in ~500 ps (Figure 1a,e). During this rising process, the band first exhibits a slight blue-shift in ~15 ps, and then shows a red shift till 500 ps. Temporal profiles of the transient absorption signal in the 450-720 nm


6

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region are well fit with multi-exponential functions with four time constants of 3 ps, 22 ps, 0.6 ns, and very long one (260 ns, fixed). The 3- and 22-ps dynamics appear as the rise of the ESA signal. (The fitting results and the decay-associated spectra are shown in Figure S3 and S4 in SI, respectively). The 3-ps time constant is close to the reported time constant of the bent-to-linear structural change of the excited-state trimer (~2 ps).6 (We note that there is a group that assigned this 3-ps dynamics to the Au-Au bond shortening process.21) The 22-ps time constant is essentially the same as the fluorescence lifetime of the pentamer (with larger oligomers) determined by the picosecond time-resolved emission measurement (24 ps, see SI). Thus, this component is attributed to the intersystem crossing (ISC) of the pentamer. The 0.6-ns dynamics corresponds to the red shift of the ESA band.

This red shift is assignable to the

formation of even larger oligomers by the collision of the excited-state oligomers with the ground-state monomer because it is close to the time constant of the diffusionlimited reaction calculated by Debye-Smoluchowski equation (~1 ns).6,

24

The

corresponding sub-nanosecond component was observed also as a red shift of the phosphorescence spectrum that was recorded in the picosecond time-resolved


7

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emission measurements (See Figure S2 in SI). The transient absorption in the late time region is attributable to the Tn T1 absorption whose lifetime was determined as 500 ns in an Ar purged solution (Figure S5), which agrees well with the phosphorescence lifetime (520 ns in Ar purged, 260 ns in air saturated solutions).22

The ISC time of the pentamer (22 ps) was found to be drastically longer than that of the trimer (0.5 ps).6

The singlet lifetime as long as 20 ps implies that such large

oligomers exist as the excited-state singlet in the first several ps time region while the trimer is quickly converted to the triplet within ~1 ps.


8

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Figure 1. (a) Femtosecond time-resolved absorption spectra of [Au(CN)2] in an aqueous solution containing Et4NCl ([Au] = 0.27 mol/dm3 [Et4NCl] = 1.0 mol/dm3, ex = 327 nm). (b) Two-dimensional plot of the time-resolved absorption data up to 1.4 ps. (c, d, e) Temporal profiles of the time-resolved absorption signal at the selected wavelengths displayed for three different delay time ranges.

Interestingly, the time-resolved absorption data show clear oscillatory features up to ~2 ps as shown in Figure 1b and c. Notably, the most prominent oscillatory feature


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exhibits the opposite phase at the shorter (e.g., 550 nm) and longer (e.g., 670 nm) wavelength sides of the ESA intensity maximum (Figure 1c). The apparent oscillation period of ~0.5 ps corresponds the frequency of ~60 cm-1, indicating that the oscillation is due to the Au-Au stretching motion.6, 16, 19 The lower frequency than the Au-Au stretching motion of the trimer (87 cm-1)6 suggests substantial contribution from the oligomers larger than the trimer.

Figure 2. 2D plot of the oscillatory components in transient absorption of K[Au(CN)2] in an aqueous solution containing 1.0 mol/dm3 Et4NCl. (a, b) 2D plot of the amplitude of


10

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oscillations for the delay time (vertical) and probed wavelength (horizontal). (c, d) 2D plot of the amplitude of the real part of Fourier transform of the oscillation for the frequency (vertical) and probed wavelength (horizontal). ([Au] = 0.27 mol/dm3, [Et4NCl] = 1.0 mol/dm3; ex = 327 nm for (a) and (c), ex = 345 nm for (b) and (d))

Frequency – wavelength 2D analysis of the oscillation of ESA

For analyzing the observed excited-state nuclear motion, we extracted the oscillatory components by subtracting slowly-varying population components (Figure 2a) and calculated their Fourier transform (Figure 2c). In this analysis, we focused on the dominant ESA band in the 500-700 nm region. Because the observed oscillations are cosine-like, we only discuss the real part of the Fourier transform hereafter, and its amplitude is plotted for the frequency (vertical axis) and the probed wavelength (horizontal axis) in a 2D manner (Figure 2c). This 2D plot clearly shows that two frequency components (60 and 85 cm-1) predominantly contribute to the oscillation of the ESA signal. Furthermore, each oscillation exhibits two lobes having the opposite sign, implying the vibrational phase flips at a certain wavelength. It is known that the oscillation due to coherent nuclear motion of a totally symmetric vibration flips its phase


11


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at the peak of the electronic transition,25 so that the node between the two lobes is considered the peak position of ESA of the oligomer that gives rise to the oscillation. Figure 2c shows that the lobes of the 85-cm-1 oscillation exhibit the node at ~560 nm, whereas the node for the 60-cm-1 oscillation appears at ~ 625 nm. This implies that these two oscillatory components are attributable to two different excited-state oligomers that give their ESA bands peaked at around 560 nm and 625 nm.

To clarify the origin of the two oscillatory components, we also performed timeresolved absorption measurements with photoexcitation at 345 nm that does not efficiently excite smaller oligomers, compared to the 327-nm photoexcitation. In the frequency-wavelength 2D plot obtained with this longer wavelength excitation at 345 nm, the two lobes with the node at ~560 nm disappear (Figure 2d), indicating that the 85-cm-1 oscillation is attributable to the oligomer smaller than the oligomer giving rise to the 60-cm-1 oscillation with the node at 625 nm.

In our previous transient absorption study performed with selective excitation of the trimer,6 a clear 87-cm-1 oscillation was observed at 510 and 600 nm with a node at ~560


12

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nm. This oscillation was assigned to the Au-Au stretch vibration of the excited-state triplet trimer with the help of DFT calculation that calculated the triplet Au-Au stretch vibration in the range of 83-96 cm-1 depending on the structure.6,

19

Therefore, the

~560-nm ESA oscillating at 85-cm-1 is safely assigned to the excited-state triplet trimer. Then, the 60-cm-1 oscillation is attributable to the larger oligomer, i.e., excited-state tetramer. Spin multiplicity of this excited-state tetramer is unclear at the moment but it is likely the excited-state singlet. This is because the lifetime of the excited-state singlet tetramer is expected to be longer than ~1 ps on the basis of the observed drastic slowdown of the ISC process with the increase of the oligomer size: 0.5 ps for the trimer6 and 22 ps for the pentamer which was determined by time-resolved emission (see SI).

In the picosecond time-resolved emission measurements, we found that the phosphorescence from the pentamer (probably with some contribution of larger oligomers)

appears

at

around

470

nm

and

that,

actually,

this

pentamer

phosphorescence is predominant in sub-ns time region (see SI, Figure S2). Therefore, the nuclear motion of the pentamer is also expected to appear in the 2D plot shown in


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Fig. 2. Actually, the oscillation having a frequency lower than 40 cm-1 is recognized in the 2D plots in Figure 2c and d, which is assignable to the excited-state pentamer. (Since the fluorescence lifetime of the pentamer was determined to be 24 ps (see SI), this excited-state pentamer is singlet.) Interestingly, the amplitudes of the oscillations due to the pentamer are small although the contribution in the emission spectra is predominant (see SI). A possible reason of this small contribution of the pentamer to the oscillation of the ESA is the large variations of the ground-state structure, such as bent and/or zig-zags. In other words, the excited-state structure of the larger oligomers can have a broad distribution immediately after photoexcitation, which likely suppresses the vibrational coherence because of the difference in their Au-Au stretch frequencies. In fact, the node of the oscillation around 40 cm-1 is not very clear and it looks tilted, suggesting that the large oligomers with various structures coexist and give rise to the ESA bands whose peak wavelengths are distributed. Table I summarizes the frequencies of the nuclear wavepacket motion of the excited-state oligomers and the peak positions of their ESA bands that were determined by the position of the node in the 2D plot.


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In this study, we have successfully identified the excited-state oligomers of different sizes using frequency-wavelength 2D analysis for the oscillation of ESA. The analysis revealed the existence of the multiple excited-state oligomers such as the trimer, tetramer and pentamer in the solution, and clearly distinguished their nuclear wavepacket motions. The result of this study demonstrates that this type of ultrafast time-resolved absorption measurements and relevant 2D analysis provide detailed information that enables solid assignments of the transient absorption and excited-state nuclear motion even when multiple excited-state species coexist. Such analysis is very powerful for elucidating the excited-state dynamics of not only metal complex oligomers but also a wide range of complex molecular systems including molecular assemblies.

Table 1. Assignments of the coherent Au-Au stretch motions to the excited-state [Au(CN)2-]n oligomers in solution and the peak wavelengths of the relevant excited-state absorption bands. Assignment

Frequency / cm-1

Peak WL / nm


15


Trimer

85

560

Tetramer

60

625

< 40

(~700)

Page 16 of 20

pentamer (with larger

oligomers)

ASSOCIATED CONTENT

Supporting information available

Following contents are given in Supporting Information. 1. Experimental. 2. Steadystate absorption spectrum: Concentration dependence and salt effect. 3. Time-resolved emission data. 4. Analysis of temporal profiles of transient absorption. 5. Nanosecond transient absorption data. 6. 2D plots of the imaginary part of the Fourier transform of the oscillation in the transient absorption. 7. The transient absorption data obtained with excitation at longer wavelength (345 nm).”).


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Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP15K05447 and JP18H04509 to M. I., JP16H04102 to S. T. and JP25104005 to T. T.

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Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions

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