A study of the spiropyran-merocyanine system using ion mobility-mass spectrometry: Experimental support for the cisoid conformation

Article abstract of DOI:10.1039/c3cc47697a

The spiropyran-merocyanine system was studied using ion mobility-mass spectrometry (IM-MS) and three major conformers were identified. Assignment of conformers is based on DFT-B3LYP energy minimized structures and collision cross-sections as light-induced changes in IM-MS. The three conformers were assigned to the spiropyran, cisoid and transoid structures.

Full text of DOI:10.1039/c3cc47697a

Featuring research from William Brittain’s
laboratory / Department of Chemistry & Biochemistry,
Texas State University, San Marcos, Texas, USA
As featured in:
A study of the spiropyran–merocyanine system using ion
mobility-mass spectrometry: experimental support for
the cisoid conformation
Spiropyran photoisomerization was studied using ion
a
mobility-mass spectrometry and three major conformers were
identified. Assignment of conformers is based on DFT-B3LYP
energy minimized structures and collision cross-sections
for data collected on light-induced changes in samples. The
three conformers were assigned to the spiropyran, a cisoid
merocyanine and a transoid merocyanine. This is the first
experimental evidence for a cisoid merocyanine intermediate in
spiropyran photoisomerization on a millisecond timescale.
See Xiaopeng Li,
William J. Brittain et al.,
Chem. Commun., 2014, 50, 3424.
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A study of the spiropyran–merocyanine system
using ion mobility-mass spectrometry:
experimental support for the cisoid
conformation†
Cite this: Chem. Commun., 2014,
50, 3424
Received 7th October 2013,
Accepted 18th November 2013
Robert A. Rogers, Allison R. Rodier, Jake A. Stanley, Nick A. Douglas, Xiaopeng Li*
and William J. Brittain*
DOI: 10.1039/c3cc47697a
www.rsc.org/chemcomm
The spiropyran–merocyanine system was studied using ion
For a thermally equilibrated system, there are eight possible
mobility-mass spectrometry (IM-MS) and three major conformers geometric isomers of MC. Studies have demonstrated both
were identified. Assignment of conformers is based on DFT-B3LYP zwitterionic4 and quinoidal5 character for the MC structure.
energy minimized structures and collision cross-sections as light- Isomers with the trans configuration about the b bond (TTT,
induced changes in IM-MS. The three conformers were assigned to TTC, CTT, and CTC) are strongly favoured based on quantum
the spiropyran, cisoid and transoid structures.
chemical calculations,6 time-resolved emission spectroscopy7
and NMR.7,8 NMR studies based on systems that stabilize the
The photoinduced and thermal isomerization of spiropyrans MC isomer through additional aromatic substituents8c–e indi-
(SP) have been extensively studied.1 The first step in the photo- cate that TTT (major isomer) and TTC are the predominate
chemical process is Cspiro–O bond cleavage to generate an excited isomeric forms in solution.7,8d,f However, these not-commonly
triplet or singlet state that decays within picoseconds and studied systems may not reflect the parent system behaviour.8c–e
produces a mixture of geometric isomers of merocyanine (MC) Quantum chemical calculations have shown that TTC is the
that differ in cis/trans (C or T) conformations about the a, b and most stable and TTT second most stable.6,8d,f,9 Several compu-
g bonds linking the indole and chromene subunits (Scheme 1). tational studies used a reduced atom set to facilitate computa-
For 10,30,30-trimethyl-6-nitrospiro[chromene-2,20-indoline] (6-nitro- tions making comparison to the parent system more difficult.
BIPS, SP-1), the photochemical reaction proceeds via a triplet
Here we used electrospray ionization (ESI) ion mobility-mass
mechanism followed by intersystem crossing to 3CCC mero- spectrometry (IM-MS)10 to examine both equilibrated and irradiated
cyanine and subsequent conversion to 3CTC and 3TTC. Theoretical samples of spiropyrans 1–3 in methanol (Scheme 1) to gain further
investigations suggest 3CCC* decays in picoseconds while the experimental insight into the photoisomerization.
3
3
lifetime of the CTC and TTC species is milliseconds.2 Evidence
IM-MS provides information on the molecules’ shapes and
for the CTC form of 6-nitro-BIPS comes from laser-desorption/ sizes based on collision cross-section (CCS), in addition to mass
electron diffraction and excited state dynamics.3
and compositional information. Our interpretation of the
IM-MS experimental data argues for presence of a long-lived
(>milliseconds) CCX (refers to two of the four MC isomers with
cis configuration at the central bond) isomer of MC. The life-
time of this proposed species is considerably longer than the
sub-nanosecond lifetimes reported previously.3
SP-1 is commercially available and SP-2 and SP-3 were pre-
pared according to literature.11 Each sample was measured under
three conditions: (1) dark equilibration for B24 hours, (2) UV
irradiation (300–400 nm), and (3) visible irradiation (>400 nm).
The irradiation was performed ex situ for two minutes per fresh
sample in a quartz cuvette and then transferred and injected via
syringe immediately (o1 min) after treatment. SP degradation
was minimized by using fresh samples from equilibrated stock
solutions of each compound.
Scheme 1 General structures of SPs 1–3 and their corresponding MCs.
Department of Chemistry and Biochemistry, Texas State University,
601 University Dr., San Marcos, TX 78666, USA. E-mail: wb20@txstate.edu
† Electronic supplementary information (ESI) available: DFT-B3LYP optimized
geometries, isomer populations, experimental details, conformer drift times and
mass spectra. See DOI: 10.1039/c3cc47697a
Buncel and co-workers12 reported that acid catalysis of SP-1
produces the O-protonated merocyanine (M-OH+). Kinetic analysis
3424 | Chem. Commun., 2014, 50, 3424--3426
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also supported SP protonation at the nitrogen to form SP-NH+
but this species did not produce MC. Ball and co-workers8a used
low-temperature NMR and theory to demonstrate O-protonation
of the MC as the dominant process in the presence of acid.
Cartesian coordinate input files for CCS calculations were
determined from DFT-B3LYP optimized geometries† using
the 6-31++G(d,p) basis set. Starting DFT geometries were based
on the ground state structures of Sheng and co-workers6b with
inclusion of protonation for the MC and SP structures and then
optimized to give final calculated structures. These structures
remained essentially the same as those determined by Sheng
and co-workers, except for the TCC isomer which assumed a
conformation better described as the CCT isomer after optimi-
zation. Sheng and co-workers did not include the indole methyl
groups and suggested that these methyl groups will alter the
preferred pathway from an intermediate TCC structure (no methyl
groups) to the CCC structure. Optimized geometries used for CCS
determination are given in the ESI.†
Fig. 2 Example of IM-MS data with identical isotopic patterns for singly
charged SP-1 ions, corresponding to isomeric structures. Similar plots for
SP-2 and SP-3 can be found in the ESI.†
Fig. 1 displays a summary of the IM-MS data. General
observations from the IM-MS data include: (1) all SP com-
pounds studied display three peaks corresponding to three
isomeric monomers, in order of increasing drift times (tD),
(2) the percent relative intensity of each peak is dependent on
sample treatment, and (3) the difference in drift time between
peaks 2 and 3 is 1.4–2.5 times larger than the difference
between peaks 1 and 2.
Fig. 2 shows the IM-MS spectra of SP-1 at equilibrium and
after irradiation at visible or UV wavelengths. Three singly
charged monomer peaks corresponding to SP-1 were identified
based on identical isotopic patterns, which clearly showed the
existence of discrete isomers instead of a doubly charged dimer
Table 1 MOBCAL collision cross-sections (Å2) of SP’s 1–3 using projec-
tion approximation (PA) and trajectory (TJ) method based on DFT
optimized structures
SP-1
SP-2
SP-3
Isomer
PA
TJ
PA
TJ
PA
TJ
SP
113.30
114.63
112.76
119.38
119.71
119.46
119.56
113.27
117.45
113.57
118.55
118.29
117.83
117.97
120.66
122.57
121.50
128.24
128.63
128.33
128.36
122.69
124.13
122.35
127.28
128.27
126.56
127.48
115.54
116.82
114.86
121.54
121.96
121.58
121.82
111.74
114.48
112.78
115.85
116.37
115.38
116.07
CCC
CCT
TTC
CTC
TTT
CTT
or triply charged trimer. Upon irradiation by visible or UV light, and trajectory (TJ) methods.13a,b It is important to note that the
the abundance ratio of the isomers changed significantly for CCSs obtained from modelling were used only qualitatively to
SP-1. The same measurements were performed for SP-2 and SP-3 determine the relative conformer size for correlation with drift
and similar figures are included with the ESI.†
times. MOBCAL uses helium as a buffer gas in CCS determi-
The CCS values, given in Table 1, for each derivative were nation via the TJ method while our experimental data was
calculated by MOBCAL using the projection approximation (PA) collected with N2 buffer gas. This should not significantly affect
our analysis because relative size increases are nearly uniform
and the size ordering remains qualitatively the same.13c
Specific isomers observed in Fig. 1 were assigned by com-
parison of the CCSs to experimental drift times (isomer popu-
lation data is provided as ESI†). We assume all isomers have
similar ionization efficiency. This comparison also assumes a
linear relationship between CCS and tD. We have omitted TCT
in our analysis based on the literature6b and omission of the
TCC isomer was based on our DFT optimizations. Thus, the
remaining 7 isomers (6 MC isomers and SP) fall into two
groups: the SP-CCX isomer group and the XTX isomer group.
We assign peaks 1 and 2, with smaller tD, to the SP-CCX
group. The small difference in theoretical CCS prohibits specific
assignment to SP vs. CCX. We assign peak 3 to the XTX isomer
group due to its larger CCS. Changes in peak intensity with
irradiation support our assignments. For SP-1 and -2, the
intensity of peaks 1 and 2 decrease with UV irradiation and
increase with subsequent visible exposure in contrast to SP-3
Fig. 1 Experimental IM-MS data. Vis, UV, and EQ denotes visible irradiation
(>400 nm), UV irradiation (300–400 nm) or equilibrated sample.
where little change is observed. These observations are consistent
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with literature values for the rate of thermal ring-closure (k) from (AI-0045) for support. X L gratefully acknowledges the sup-
MC to SP where: k(SP-3) E 50 ꢀ k(SP-1) E 2–6 ꢀ k(SP-2).2,11b,14 port from the Research Enhancement Program of Texas State
If we use experimental studies in solution as a guide, we speculate University.
that the identity of the XTX isomer is TTC.6b
The assignment of CCX to an intermediate with a lifetime of
milliseconds is controversial. Based on transient absorption
Notes and references
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studies, CCX species have lifetimes well below milliseconds on
the photochemical reaction pathway which would make detec-
tion difficult under our experimental conditions. However, we
note that our observations reflect a system which after initial
irradiation, is equilibrating thermally up to the moment of
transfer into the gas phase. Our conclusion that we have
observed CCX relies on the qualitative comparison of CCS
values to experimental drift times because direct quantitative
correlation between CCS values and drift times in travelling
wave ion mobility is not yet fully understood. Recent experi-
mental studies have argued that biomolecule structure is pre-
served during the ESI process and the structures largely reflect
that in solution.15 This suggests that dynamic behaviour is
temporarily ‘‘frozen’’ when the molecules are transferred from
solution (thermodynamically controlled) to gas phase (kinetically
controlled). We argue that the solution structure of the SP–MC
system is preserved during our experiments and our results
provide a unique perspective on this classic photochromic
system. Photoisomerization of SP in the gas phase has been
studied; ionization potentials were combined with theory but
their conclusions did not contradict theoretical results.16 This
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Mechanistic elucidation of the elementary steps in SP–MC
isomerization has been largely limited to transient spectroscopy
and model systems. The combination of ESI with IM-MS pro-
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important photochromic system.
WJB thanks the National Science Foundation (PREM Center
for Interfaces, DMR-1205670) and the ACS Petroleum Research
Fund (51997UR7) for financial support of this research. The 16 L. Poisson, K. D. Raffael, B. Soep, J. Mestdagh and G. Buntinx, J. Am.
Chem. Soc., 2006, 128, 3169.
17 G. Cottone, R. Noto and G. La Manna, Molecules, 2008, 13, 1246.
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authors also acknowledge support of the National Science
Foundation for X-ray and NMR instrumentation (CRIF:
MU-0946998 and MRI-0821254) and the Welch Foundation 19 R. Ganesan and F. Remacle, Theor. Chem. Acc., 2012, 131, 1255.
3426 | Chem. Commun., 2014, 50, 3424--3426
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