In situ Raman monitoring of [2+2] cycloaddition of pyridine substituted olefins induced by visible laser

Article abstract of DOI:10.1039/c4cc06808d

Visible laser induced [2+2] cycloaddition of solid-state pyridine substituted olefins into cyclobutane has been monitored by an in situ Raman technique. The laser power and wavelength can dramatically alter the reaction kinetics, as a prior melting process (heating from laser irradiation) is required for this [2+2] photoreaction. This journal is

Full text of DOI:10.1039/c4cc06808d

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. Chen, X. Han,
W. Jin, Y. Du and P. Xu, Chem. Commun., 2014, DOI: 10.1039/C4CC06808D.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

View Article Online
DOI: 10.1039/C4CC06808D
Page 1 of 3
ChemComm
Journal Name
RSCPublishing
COMMUNICATION
In-situ Raman monitoring of [2+2] cycloaddition of
pyridine substituted olefins induced by visible laser
Cite this: DOI: 10.1039/x0xx00000x
Dengtai Chen, Xijiang Han, Wen Jin, Yunchen Du and Ping Xu*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
distinguished, and melting point of the formed crystals can also
influence the reaction process. We believe this study may open up
new avenues for studying photo-induced chemical reactions.
Crystals of pyridine substituted olefins were synthesized through a
crystal engineering method, where silver salts were used to induce
the crystallization of the one end capped pyridine substituted olefins
(see Experimental Section in ESI for detail†).22 Here, crystals of
Visible laser induced [2+2] cycloaddition of solid-state
pyridine substituted olefins into cyclobutane has been
monitored by an in-situ Raman technique. The laser power
and wavelength can dramatically alter the reaction kinetics,
as a prior melting process (heating from laser irradiation) is
required for this [2+2] photoreaction.
[Ag2(4-VP)4][TFA]2 (1) (4-VP
=
4-vinylpyridine, TFA
=
Cyclobutanes constitute the backbone of many natural products
and pharmaceutically and biologically interesting molecules, which
can also act as reactive, intermediates for further ring-opening and
ring expansion reactions.1-3 [2+2] photodimerization of olefins into
cyclobutanes has been widely studied for its ease in controlling the
reactions and high conversion rates.4 Typically, UV light is applied
as the light source to initialize the [2+2] photoreactions, and solid-
state nuclear magnetic resonance (NMR) and X-ray diffraction
(XRD) techniques are used to characterize the reaction products.5-7
For instance, a recent study demonstrated a single crystals popping
phenomenon under UV light, which actually derives from a
photosalient effect triggered by a [2+2] cycloaddition reaction.8
Besides UV triggered reactions, laser induced chemical reactions
has attracted more and more attention,9-11 including coupling
trifluoroacetate) and [Ag2(PEA)4][TFA]2 (PEA = Ethyl-(pyridine-4-
yl)prop-2-enotate) (2) were prepared for the study of laser induced
[2+2] cycloaddition. For comparison, cocrystal of (BPE)2·(RES)2
(BPE = trans-1,2-bis(4-pyridyl)-ethylene, RES = resorcinol) (3) and
(BPEP)2·(RES)2 (BPEP = 1,3-bis(trans-4-pyridylethylenyl)pyridine)
(4) were also prepared for laser induced [2+2] cycloaddition, where
hydrogen bond formation leads to rigid supermolecular structure
with high melting point of these two crystals.
13
15
reactions,12,
photodimerization,14,
photochromic reactions.16
Moreover, in-situ Raman spectroscopy is appealing for studying
photoreactions in a way that the reactions can be induced by the laser
source, and simultaneously, the reaction processes (bond formation)
can be monitored by the variation of the Raman fingerprints.17-19 Our
previous works have also shown visible laser induced catalytic
reactions,20, 21 which utilizes the surface plasmon generated at metal
surface to initialize the chemical reactions. Herein, we demonstrate
an in-situ Raman monitoring of visible laser induced [2+2]
cycloaddition of solid-state pyridine substituted olefins (Scheme 1),
where crystals of pyridine substituted olefins are formed using
different silver complexes. Disappearance of C=C double bonds and
formation of cyclobutanes can be easily captured from the time-
dependent Raman spectra. Notably, laser power- and wavelength-
dependent reaction rates of this [2+2] photoreaction have been
Scheme
1 Schematic illustration of [2+2] cycloaddition of pyridine
substituted olefin crystal 1 and 2 induced by visible laser irradiation.
The as-prepared solid-state crystals of pyridine substituted olefins
were placed under laser irradiation on a Renishaw In Via confocal
Raman system, where in-situ Raman spectra were taken during the
reaction process. Here, Raman line mapping was carried out over the
crystal surface using a 633 nm laser at different powers (Fig. 1a). As
shown in Fig. 1b, Raman spectrum of crystal 1 is characterized by
the C=C double bond stretching at ~1633 cm-1, pyridine ring
stretching at ~1610 cm-1, CH2 deformation at ~1415 cm-1, CH in-
plane deformation at ~ 1204 cm-1, pyridine ring breathing at ~1020
cm-1, and CH in-plane bending at ~1065 cm-1.23 It has been found
that [2+2] cycloaddition can be realized immediately under 100% (3
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

View Article Online
DOI: 10.1039/C4CC06808D
ChemComm
Page 2 of 3
COMMUNICATION
Journal Name
mW) laser power, confirmed by a typical Raman spectrum of crystal S2 in ESI†). This indicates that in our case, [2+2] cycloaddition of
1 after laser irradiation (Fig. 1b). The disappearance of the C=C solid-state pyridine substituted olefins mainly arises from a heat
fingerprint and emergence of the peak at ~1145 and 844 cm-1 effect of the applied laser. To further confirm the above results,
corresponding to the cyclobutane skeletal C-C stretching mode and control experiment using UV irradiation was carried out (see Fig. S3
C(cyclobutane)-C(pyridine) stretching mode23 verify that olefins in ESI†), which shows similar spectroscopic variations relating to
have been completely converted into cyclobutane structures.
the vanished C=C bonds and newly formed cyclobutanes.
Fig. 2 Optical images of crystal 2 before (a) and after (b) laser irradiation; (c)
Schematic illustration of crystal 2 undergoing [2+2] cycloaddition induced by
laser irradiation; (d) The time-dependent Raman spectra showing the [2+2]
dimerization process of crystal 2 with 50% (1.57 mW) power of 633 nm laser.
Crystal 2, [Ag2(PEA)4][TFA]2, can also undergo visible laser
induced cycloaddition reaction, as shown in the time-depended
Raman spectra recorded on the solid crystal (Fig. 2). PEA was
synthesized and confirmed by 1H and 13C NMR spectra (see Fig. S4
and S5 in ESI†), and crystal information of 2 was determined
(CCDC number: 1025966). It can be seen from the optical images
Fig. 1 (a) Schematic illustrations of Raman line maps on crystal 1; (b) Raman
spectra recorded from the unreacted area (blue line) and reaction centre (red
line); (c) The collective diagram of Raman spectra line map with 100% (3
mW) laser power; (d) the collective diagram of Raman line map crossing the that the crystal surface will be melted by the laser irradiation. The
reacted area with 5% (0.15 mW) laser power. Laser: 633 nm.
laser induced cycloaddition can be manifested by the vanishing 1631
cm-1 peak (C=C double bond stretching), and the appearing peaks at
~1149 and 843 cm-1 corresponding to the skeletal C-C stretching of
cyclobutane and C(cyclobutane)-C(pyridine) stretching mode,
respectively.23 The split weak peaks at 1425 and 1379 cm-1 can be
assigned to the CH2 deformation cyclobutane. The disappearance of
carboxyethyl (-COOEt) at ~1701 and 1716 cm-1 (Fig. 2d) is caused
by disorder during cyclobutane formation.25 The slight peak shift for
the C=C and cyclobutane groups found for crystal 1 and 2 results
from the different substituting group at the end of the olefins.
Nevertheless, it has been verified that visible laser induced
cyclobutane formation does occur on these two solid state pyridine
substituted olefins. However, we have noticed that the exactly same
pyridine substituted olefins in liquid state cannot be converted into
cyclobutanes when identical laser irradiation is applied. In fact, [2+2]
photodimerization that can be completed in liquid phase normally
applies UV light or UV laser, and the presence of proton acid as
The laser induced reaction can also be confirmed by the fact that
the peak at 1415 cm-1 (CH2 deformation) was split into two peaks at
~1420 and 1384 cm-1.24 Through the Raman line map using 100% (3
mW) laser power in Fig. 1c, one can see that the crystal surface
exposed to the laser beam underwent rapid [2+2] cycloaddition
photoreactions, leading to complete conversion of olefins into
cyclobutanes. Notably, a laser power-dependent reaction feature was
witnessed (Fig. 1d). When a Raman line map with 5% (0.15 mW)
power was taken vertical to the previous one, such a power failed to
induce the cycloaddition reaction in a short time, but one can easily
find that photoreaction has been occurred in the irradiated part with
100% (3 mW) power (also see Fig. S1 in ESI†). However, a careful
time-dependent study reveals that such reaction is laser power-
dependent, and typically lower laser power consumes longer reaction
time to discover the occurrence of the cycloaddition reaction (see Fig.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

View Article Online
DOI: 10.1039/C4CC06808D
Page 3 of 3
Journal Name
ChemComm
COMMUNICATION
27
template is also required.26,
With visible laser source, some
3. H. Kashida, T. Doi, T. Sakakibara, T. Hayashi and H. Asanuma, J.
Am. Chem. Soc., 2013, 135, 7960-7966.
successful examples have been reported in liquid/ solid state.14 So, it
comes to conclusion that silver or proton ion as template is essential
for the cycloaddition reaction in solid/ liquid.
4. G. M. J. Schmidt, Pure Appl Chem, 1971, 27, 647.
5. J. Hallmann, W. Morgenroth, C. Paulmann, J. Davaasambuu, Q.
In addition to the laser power, applied laser wavelength can also
greatly alter the reaction kinetics of this [2+2] cycloaddition. Using a
similar power (1.18 mW) of 532 nm laser, v.s. 1.65 mW of 622 nm
laser, it turns out that the reaction rate of the [2+2] dimerization
(crystal 1 and 2) is much faster (see Fig. S6 in ESI†). Moreover,
carbonization of the formed crystals can be seen under an even
higher power of 532 nm laser (6.35 mW), as we can clearly
distinguish the D and G bands of carbon materials (see Fig. S7 in
ESI†). It needs to be pointed out that reaction under 532 nm laser
would be too fast to capture the spectroscopic evolution for this [2+2]
photoreaction, and thus here 633 nm laser is typically applied. We
believe this reaction rate variation under lasers of different
wavelengths results from a more efficient and rapid heating effect on
the crystals with shorter wavelength lasers.
Kong, M. Wulff and S. Techert, J. Am. Chem. Soc., 2009, 131
,
15018-15025.
6. M. Khan, G. Brunklaus, V. Enkelmann and H.-W. Spiess, J. Am.
Chem. Soc., 2008, 130, 1741-1748.
7. K. Honda, F. Nakanishi and N. Feeder, J. Am. Chem. Soc., 1999, 121
,
8246-8250.
8. R. Medishetty, A. Husain, Z. Z. Bai, T. Runcevski, R. E. Dinnebier, P.
Naumov and J. J. Vittal, Angew Chem Int Edit, 2014, 53, 5907-5911.
9. R. Bini, Accounts Chem Res, 2004, 37, 95-101.
10. M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012,
335, 1326-1330.
11. L. Schade, S. Franzka, M. Mathieu, M. M. Biener, J. Biener and N.
Hartmann, Langmuir, 2014, 30, 7190-7197.
It is important to note that when cocrystals 3 and 4 were used as
pyridine substituted olefins to study the laser induced cycloaddition,
no changes in the Raman spectra can be distinguished even with 100%
(3 mW) power of 633 nm laser (see Fig. S8 and S9 in ESI†). From
the optical images, one can see that cocrystals 3 and 4 cannot be
melted under laser irradiation. Therefore, we think the melting
process is necessary to trigger the cycloaddition reaction. The
hydrogen bonds formed in the rigid supermolecular cocrystal
structures give melting points of 225 and 211 oC for crystals 3 and 4.
While the crystals formed with silver salts have much lower melting
12. E. M. van Schrojenstein Lantman, T. Deckert-Gaudig, A. J. G. Mank,
V. Deckert and B. M. Weckhuysen, Nat Nano, 2012, 7, 583-586.
13. B. Dong, Y. R. Fang, X. W. Chen, H. X. Xu and M. T. Sun,
Langmuir, 2011, 27, 10677-10682.
14. N. K. Nath, K. Manoj, A. S. Gaz and P. Naumov, Chemistry, 2013,
19, 8094-8099.
15. R. W. Taylor, R. J. Coulston, F. Biedermann, S. Mahajan, J. J.
Baumberg and O. A. Scherman, Nano Lett, 2013, 13, 5985-5990.
16. K. Mori, Y. Ishibashi, H. Matsuda, S. Ito, Y. Nagasawa, H.
Nakagawa, K. Uchida, S. Yokojima, S. Nakamura, M. Irie and H.
Miyasaka, J. Am. Chem. Soc., 2011, 133, 2621-2625.
o
o
points, 64.5 C for crystal 1 and 66 C for crystal 2. Besides the
topochemical considerations (one end capped with H/COOEt can
give the double bonds more freedom to move/ dimerize28), the
melting point can be regarded as a criterion to evaluate the
photoreactivity of pyridine substituted olefins under laser irradiation.
In conclusion, visible laser induced [2+2] photodimerization of
solid state pyridine substituted olefins has been studied. The
formation of cyclobutane and disappearance of the vinyl group can
be easily monitored from the in situ Raman spectra. It has been
found that the laser induced cycloaddition reaction is laser power-
and wavelength-dependent, and the topochemistry and melting point
of the crystals can determine the reactivity of such reactions.
Crystals formed by TFA cation complexation can undergo [2+2]
cycloaddition reactions, while those formed by hydrogen bonds with
higher melting point cannot. Compared to UV induced [2+2]
photodimerization reactions, this laser induced reaction process on a
Raman platform offers a new avenue to study photochemical
reactions due to the possibility of simultaneously inducing and
monitoring the reactions.
17. J. J. McMahon, T. P. Dougherty, D. J. Riley, G. T. Babcock and R. L.
Carter, Surf Sci, 1985, 158, 381-392.
18. J. J. McMahon, T. J. Gergel, D. M. Otterson, C. R. McMahon and R.
M. Kabbani, Surf Sci, 1999, 440, 357-374.
19. R. R. Frontiera, A. I. Henry, N. L. Gruenke and R. P. Van Duyne, J.
Phys. Chem. Lett., 2011,
20. P. Xu, L. L. Kang, N. H. Mack, K. S. Schanze, X. J. Han and H. L.
Wang, Sci Rep-Uk, 2013, , 2997.
2, 1199-1203.
3
21. L. L. Kang, P. Xu, B. Zhang, H. H. Tsai, X. J. Han and H. L. Wang,
Chem Commun, 2013, 49, 3389-3391.
22. Q. L. Chu, D. C. Swenson and L. R. MacGillivray, Angew. Chem.-Int.
Edit., 2005, 44, 3569-3572.
23. K. Tashiro, K. Matsushima and M. Kobayashi, J. Phys. Chem., 1990,
94, 3197-3204.
We acknowledge the financial supports from NSFC (No.
21471039, 21203045, 21101041), Fundamental Research Funds for
the Central Universities (HIT.BRETIII. 201223), and China
Postdoctoral Science Foundation (2014M560253).
24. R. L. Garrell and K. D. Beer, Langmuir, 1989, 5, 452-458.
25. C. M. Johnson, E. Tyrode, S. Baldelli, M. W. Rutland and C. Leygraf,
J. Phys. Chem. B, 2004, 109, 321-328.
Notes and references
Department of Chemistry, Harbin Institute of Technology, Harbin 150001,
26. M. Pattabiraman, A. Natarajan, R. Kaliappan, J. T. Mague and V.
Ramamurthy, Chem. Commun., 2005, 4542-4544.
China. Email: pxu@hit.edu.cn
27. J. Liese and N. A. Hampp, J. Photochem. Photobiol. A-Chem., 2010,
209, 128-134.
† Electronic Supplementary Information (ESI) available: Experimental,
Fig. S1-S9, CIF for Crystal
2
and
4
. See DOI: 10.1039/c000000x/
28. E. Elacqua, P. Kaushik, R. H. Groeneman, J. C. Sumrak, D. K. Bucar
and L. R. MacGillivray, Angew Chem Int Ed, 2012, 51, 1037-1041.
1. S. Yoriko, in CRC Handbookof Organic Photochemistry and
Photobiology, Volumes 1 & 2, Second Edition, CRC Press, 2003.
2. E. Kumarasamy, R. Raghunathan, S. Jockusch, A. Ugrinov and J.
Sivaguru, J. Am. Chem. Soc., 2014, 136, 8729-8737.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3