Light and Temperature Control of the Spin State of Bis(p-methoxyphenyl)carbene: A Magnetically Bistable Carbene

Article abstract of DOI:10.1021/jacs.5b11696

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon mat

Full text of DOI:10.1021/jacs.5b11696

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Article
Light and Temperature Control of the Spin State of Bis(p-
methoxyphenyl)carbene: a Magnetically Bistable Carbene
Paolo Costa, Thomas Lohmiller, Iris Trosien, Anton Savitsky, Wolfgang
Lubitz, Miguel Fernández-Oliva, Elsa Sanchez-Garcia, and Wolfram Sander
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11696 • Publication Date (Web): 14 Jan 2016
Downloaded from http://pubs.acs.org on January 14, 2016
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Light and Temperature Control of the Spin State of Bis(p-
methoxyphenyl)carbene: a Magnetically Bistable Carbene
Paolo Costa†, Thomas Lohmiller§†, Iris Trosien†, Anton Savitsky§*, Wolfgang Lubitz§, Miguel FernandezꢀOliva‡, Elsa Sanchezꢀ
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Garcia‡*, and Wolfram Sander†*
†Lehrstuhl für Organische Chemie II, RuhrꢀUniversität Bochum, 44780 Bochum, Germany
§MaxꢀPlanckꢀInstitut für Chemische Energiekonversion, 45470 Mülheim an der Ruhr, Germany
‡MaxꢀPlanckꢀInstitut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany
KEYWORDS carbene; matrix isolation; EPR; IR; spin state
ABSTRACT: Bis(pꢀmethoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest
energy singlet and triplet states. At 3 K both states coexist indefinitely under these conditions. The carbene is investigated in argon
matrices by IR, UVꢀvis, and Xꢀband EPR spectroscopy, and in MTHF glasses by Wꢀband EPR and Qꢀband ENDOR spectroscopy.
UV (365 nm) irradiation of the system results in formation of predominantly the triplet carbene, whereas visible (450 nm) light
shifts the photostationary equilibrium towards the singlet state. Upon annealing at higher temperatures (> 10 K), the triplet is conꢀ
verted to the singlet, however, cooling back to 3 K does not restore the triplet. Therefore, depending on matrix temperature and
irradiation conditions, matrices containing predominantly the triplet or the singlet carbene can be generated. Controlling the magꢀ
netic and chemical properties of carbenes by using light of different wavelengths might be of general interest for applications such
as information storage and radicalꢀinitiated polymerization processes.
splitting ꢀHST of 4 kcal/mol in isooctane,8 while for bis(pꢀ
methoxyphenyl)carbene 1b calculations predict that the lowest
INTRODUCTION
Carbenes are molecules bearing divalent carbon atoms, and
the reactivity of these species is highly variable, ranging from
the openꢀshell (triplet or singlet) reactivity expected for a 1,1ꢀ
diradical to the extreme philicities of closedꢀshell (singlet)
carbenes that resemble 1,1ꢀzwitterions. The stability of carꢀ
benes varies from fleeting intermediates with lifetimes in the
range of picoseconds to entirely stable compounds that can be
isolated at room temperature. The spinꢀdependent reactivity of
carbenes has been subject to intense research for several decꢀ
ades.1ꢀ3
lying singlet and triplet states should be almost degenerate
(Chart 1).
Chart 1. C-C-C bond angles at the carbene center and
singlet – triplet splittings of 1a and uu-1ba
Very simplified, the electronic structure of a carbene can be
described in a twoꢀorbital twoꢀelectron picture, where the
occupancy of the energetically closeꢀlying σ and π orbitals
determines their spin state and philicity.4 Thus, the spin state,
triplet (S = 1) or singlet (S = 0), depends strongly on the subꢀ
stituents at the carbene center: most alkylꢀ or arylꢀsubstituted
carbenes show a triplet ground state, whereas σꢀaccepting and
πꢀdonating substituents such as O, N or halogens stabilize the
singlet states.
A similar effect is observed for substitution in the para posiꢀ
tions of arylcarbenes. Song and Sheridan studied the influence
of methoxy substitution on the spin state of triplet pheꢀ
nyl(trifluormethyl)carbene and observed that para substitution
leads to a switching of the spin state to singlet, whereas meta
substitution does not change the spin state.5 While fluorenyliꢀ
dene
shows
a
triplet
ground
state,6
3,6ꢀ
dimethoxyfluorenylidene is a singlet carbene.7 Diphenylcarꢀ
aThree conformers of 1b were considered with respect to the
position of the methoxy group: downꢀdown (dd), upꢀdown (ud),
bene 1a exhibits a triplet ground state with a singletꢀtriplet
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and upꢀup (uu). Energies relative to uuꢀSꢀ1b and ꢀEST computed
at the CCSD(T)/ccꢀpVDZ//B3LYPꢀD3/def2ꢀTZVP level of theoꢀ
ry.
sidered. A very careful analysis of the IR, UVꢀvis, and EPR
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spectra allowed the authors to conclude that the singlet state Sꢀ
1c is stabilized by obtaining a perpendicular conformation
with the carbomethoxy group rotated out of plane, and that a
barrier between the two states is created by the pronounced
conformational change.20
Carbenes 1a and 1b are shortꢀlived at room temperature in
solution, but can be isolated at cryogenic temperatures in inert
gas matrices or in organic glasses.9,10 A carbene such as dipheꢀ
nylcarbene 1a is entirely stable in inert gas matrices, and its
bimolecular chemistry can be studied by doping these matrices
with small molecules such as O2,11,12 CO2,13 CO, H2O,14 or
CH3OH.2 Organic glasses are reactive matrices, and at temperꢀ
atures above 77 K carbene 1a decays rapidly, whereas below
50 K the carbene is stable even in solid alcohols. In 1966,
Trozzolo and Gibbons reported that photolysis of the dipheꢀ
nyldiazomethanes 2a or 2b in frozen 2ꢀmethyltetrahydrofuran
(MTHF) at 77 K results in the formation of the corresponding
carbenes in their triplet states Tꢀ1a and Tꢀ1b, respectively.15
The carbenes were characterized by EPR, UVꢀvis, and fluoꢀ
rescence spectra. The authors noted that the photolysis of 2b,
in contrast to that of 2a and several other derivatives studied,
proved to be rather anomalous. At low concentrations of 2b
the carbene Tꢀ1b could not be observed, and instead an EPRꢀ
silent intermediate x with a strong absorption at 390 nm was
found. The hypothesis that 1b has a singlet ground state and
intermediate x is indeed Sꢀ1b was ruled out by the observation
that at higher concentrations both the unknown compound x
and the triplet carbene Tꢀ1b are formed. From that, a triplet
ground state of 1b was inferred, and it was concluded that
“great caution should be exercised in assigning a given abꢀ
sorption to a methylene even if the ESR spectrum has been
obtained in different experiments under rather similar condiꢀ
tions.” Later, Humphreys and Arnold studied a series of paraꢀ
substituted diphenylcarbenes including Tꢀ1b by temperatureꢀ
dependent EPR spectroscopy, and from the linear CurieꢀWeiss
plot also concluded that Tꢀ1b has a triplet ground state, and
that Sꢀ1b is not populated to any significant extent.16,17
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Carbene 1c was also studied by ultrafast spectroscopy.21ꢀ23 It
was found that in cyclohexane as solvent the singlet carbene
Sꢀ1c is converted to Tꢀ1c with a lifetime of approximately 2
ns. In chloroform, acetonitrile, and Freonꢀ113, on the other
hand, singlet Sꢀ1c is the ground state, and no triplet is formed
at all. This indicates that in solid argon, where Tꢀ1c is the
ground state, singlet Sꢀ1c is stabilized in the ridged matrix
cage by restrictions for the large conformational change reꢀ
quired during the SꢀT interconversion.
So far, carbene 1c was the only carbene where both the triꢀ
plet (ground) state and an excited (metastable) singlet state
coexist for a period of hours at low temperatures. Recently, we
have shown that the singlet state of diphenylcarbene Sꢀ1a is
strongly stabilized by hydrogen bonding, and that the hydroꢀ
…
genꢀbonded Sꢀ1a HꢀOR coexists with Tꢀ1a in the same
matrix.2,14 In this case, Sꢀ1a HꢀOR is thermodynamically
…
…
more stable than Tꢀ1a HꢀOR, and Tꢀ1a is more stable than
…
Sꢀ1a. However, Sꢀ1a HꢀOR is only metastable, and with a
halfꢀlifetime of several hours it rearranges to the formal OꢀH
insertion product via quantum chemical tunneling.
We now report that, in contrast to previous findings,15ꢀ17
carbene 1b shows a singlet ground state Sꢀ1b in inert gas
matrices as well as in organic glasses. The excited triplet state
Tꢀ1b can be photochemically populated in high yields, and is
indefinitely stable at very low temperatures. Thus, both the
singlet and the triplet state of carbene 1b can be reversibly
generated, and the intersystem crossing rate between these
states is essentially zero.
When triplet ground state carbenes are generated by photolꢀ
ysis from singlet precursors such as diazo compounds 2, the
carbenes are initially formed in their singlet (excited) states
followed by rapid intersystem crossing (ISC) to the triplet
ground state.1 For diphenylcarbene 1a the ISC rate in CH3CN
was determined to be 311±22 psꢀ1 by picosecond fluorescence
spectroscopy,18 which was later confirmed by femtosecond
absorption spectroscopy.19 In nonpolar hydrocarbons as solꢀ
vents the ISC rates of 1a are roughly three times faster. Matrix
isolation of carbenes produces the carbenes in their thermodyꢀ
namically most stable electronic ground states, which can be
either singlet or triplet. The ISC generally is too fast to allow
for the detection of excited spin states under the conditions of
matrix isolation. The only exception was reported in 1999 by
RESULTS AND DISCUSSION
EPR and ENDOR Experiments. The diazo compound 2b
was photolyzed using visible light (510 or 530 nm) or UV
light (365 nm) in various matrices (Ar, Ne) and organic glassꢀ
es (MTHF) at cryogenic temperatures (3 – 10 K), and the
products were analyzed by EPR, IR, and UVꢀvis spectroscopy.
The EPR experiments demonstrate that photolysis of 2b proꢀ
duces Tꢀ1b with a characteristic triplet spectrum in all matriꢀ
ces, which is dominated by a large zeroꢀfield splitting (zfs)
due to the anisotropic dipolar interaction of the two electron
magnetic moments (see SI Figure S1A and refs. 24,25). The
Wꢀband (94 GHz) EPR spectrum in MTHF at 10 K is shown
in Figure 1A. The mS = ꢀ1 → mS = 0 transitions (denoted xꢀ1,yꢀ
1,zꢀ1 in the canonical orientations) contribute mostly to the
highꢀfield part of the EPR spectrum, which is of larger intensiꢀ
ty than the lowꢀfield part (mS = 0 → mS = +1 transitions, see SI
Figure S1). Thus, the sign of the zfs parameter D is positive,
which is characteristic for phenylꢀringꢀsubstituted carbenes.26
Bally,
McMahon
and
coworkers.20
2ꢀ
Naphthyl(carbomethoxy)carbene Tꢀ1c is a triplet ground state
carbene that on irradiation into its weak visible absorption (λ >
515 nm) is transformed into its metastable singlet state Sꢀ1c.
After several hours in the dark at 12 K or upon 450 nm illumiꢀ
nation, Sꢀ1c is converted back to Tꢀ1c almost quantitatively.
This system is highly complex, since numerous conformers of
the carbene and several rearranged products have to be conꢀ
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SpinꢀHamiltonianꢀbased simulations yield zfs parameters of
D = 0.4035 cmꢀ1 and E = ꢀ0.0201 cmꢀ1.
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We employed ENDOR to structurally characterize Tꢀ1b by
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means of its H /2H electronꢀnuclear hyperfine interactions.
They provide insight into the spin density distribution across
the molecule and hence access to its spatial conformations.
ENDOR is a very efficient method to observe nuclear spin
transitions directly in the radio frequency domain. Two standꢀ
ard ENDOR pulse sequences, Davies and Mims, are usually
used for nuclei with large (1H) and small (2H) hyperfine interꢀ
actions, respectively (see SI section 1 and ref. 27). The
ENDOR spectra for triplet species differ from those of S = ½
species in that the nuclear resonances are not symmetric
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around the nuclear Larmor frequency νn due to the orientation
selectivity resulting from the zfs (see ref. 28). Measured at the
field positions of the z edges (see Figure 1A), the ENDOR
spectra are most selective and hence simple. There, the differꢀ
ence ꢀνRF between the transition frequency and νn corresponds
directly to the hyperfine tensor component Azz (zꢀ1) or ꢀAzz
(z+1), the magnitude and sign of which can thus be read off
from the ꢀνRF axis.
The Qꢀband (34 GHz) 1H Davies ENDOR spectrum at z+1 of
Tꢀ1b in MTHF at 5 K is shown in Figure 1B, trace a. In addiꢀ
tion to the large central line from nuclear transitions within the
mS = 0 electronic spin manifold as well as weakly coupling
matrix protons, seven well resolved ENDOR lines are obꢀ
Figure 1. A) Wꢀband echoꢀdetected EPR spectrum of Tꢀ1b in
MTHF recorded at 10 K (black) and a simulation thereof (red).
The canonical magnetic field positions are marked. B) Qꢀband 1Hꢀ
Davies ENDOR spectra of a) Tꢀ1b in MTHF, b) Tꢀ1bꢀd4 (deuterꢀ
ated at positions 2, 2’, 3, 3’) and c) Tꢀ1bꢀd6 (methyl groups deuꢀ
terated) in tolueneꢀd8 measured at 5 K at the z+1 EPR spectral
position, as well as d) a simulation for the ud conformer of Tꢀ1b.
The numbers above trace a show the assignment of ENDOR lines
to corresponding Tꢀ1b protons as numbered in panel A. The
simulation employs the DFTꢀcalculated g and Aꢀtensors and the
Dꢀtensor, rescaled to match the experimental D, from udꢀTꢀ1b.
The spectra in A and B were recorded after 510 nm photolysis of
the corresponding (isotopically labeled) diazo precursors 2b at
10 K. For experimental settings see SI Figures S1 (A) and S2 (B).
served. Four of them have transition frequencies smaller than
1
the H Larmor frequency (ꢀ
ν
RF < 0), while the other three
have larger frequencies. In analogy to the spin density distriꢀ
bution and 1H hyperfine tensors in Tꢀ1a,29,30 the lines at
ꢀνRF = ꢀ5.5 MHz and ꢀ3.3 MHz can be assigned to the ring
protons 2, 2’, 3 and 3’ having positive hyperfine coupling
values (for proton numbering see Figure 1A). The ENDOR
lines at positive ꢀ
and correspond to large negative hyperfine couplings.29,30 The
remaining two lines at ꢀ RF ≈ ꢀ2.2 and ꢀ1.4 MHz must then
νRF stem from the ring protons 1, 1’, 4 and 4’
ν
originate from protons of the methoxy group. As further proof
of the assignment, Tꢀ1bꢀd4 and Tꢀ1bꢀd6, deuterated at the
positions 2, 2’, 3 and 3’ or at the methoxy groups, respectiveꢀ
ly, were investigated (Figure 1B, traces c, d). This selective
isotopic labelling allows complete assignment of the resoꢀ
Photolysis of 2b in solid argon at 5 K also resulted in the
formation of triplet carbene Tꢀ1b. The Xꢀband (9.5 GHz) EPR
spectrum (Figure 2, trace a) was simulated with zfs parameters
of D = 0.4096 cmꢀ1 and E = ꢀ0.0190 cmꢀ1 (SI Figure S4), close
to the values measured in MTHF. In the temperature range
between 5 K and 30 K a linear signal dependence on the inꢀ
verse temperature was observed, as expected for a ground state
triplet or nearly degenerate singlet and triplet states (Figure 2,
inset). However, if the matrix was cooled back to 5 K after
annealing for 10 min each at various higher temperatures, the
original signal intensity could not be recovered, indicating an
irreversible reaction of Tꢀ1b to an EPRꢀsilent singlet species.
After annealing for 10 min at 8 K the loss was 10%, at 15 K
33%, at 20 K 48%, and at 30 K 69% (SI Figure S5). Irradiaꢀ
1
2
nances in both the H Davies ENDOR and H Mims ENDOR
(SI Figure S2) spectra. The well resolved ENDOR lines of the
protons of the methyl groups clearly reveal that these groups
are nonꢀrotating at these temperatures (see also SI Figure S3).
With respect to the orientations of the methoxy groups, the
three conformers uu, ud and dd of Tꢀ1b were considered
(Chart 1). According to DFT calculations, these conformers
are almost degenerate in energy (SI Table S10). Their calcuꢀ
1
lated H Azz values (SI Table S1) show that the two different
methyl group orientations entail distinct hyperfine couplings
in the respective pꢀmethoxyphenyl moieties of the molecule.
ENDOR simulations employing the parameters from udꢀTꢀ1b
(Figure 1B, trace d) reproduce the experimental spectra reaꢀ
sonably well, basically identical to a 50 : 50 mixture of uuꢀTꢀ
1b and ddꢀTꢀ1b (not shown). The peak intensities can be best
reproduced by a 40 : 60 mixture of methoxy group up and
down conformations. Thus, considering their similar energies,
a mixture of all three conformers, which do not interconvert
under the lowꢀtemperature experimental conditions, is most
likely present in the organic glass.
tion at 5 K with λ = 365 nm resulted in a recovery of Tꢀ1b,
and this behavior was reproducible in repeated annealꢀ
ing/irradiation cycles. This indicates that annealing of the
matrix results in the formation of an EPRꢀsilent reservoir
compound of Tꢀ1b, from which the triplet carbene can be
restored by UV irradiation. Interestingly, the 365 nm irradiaꢀ
tion produces, in addition to the original EPR signals assigned
to Tꢀ1b, a second triplet with slightly different zfs parameters
(D = 0.4053 cmꢀ1 and E = ꢀ0.0173 cmꢀ1, SI Figure S6). Most
likely, this second triplet is due to a different conformer of Tꢀ
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1b formed upon 365 nm irradiation. A similar loss of intensity the photolysis of 2b. Annealing at 25 K for 10 min increases
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of the triplet signals was observed during annealing of MTHF
glasses. However, since MTHF is a reactive matrix, signal loss
could also arise from reactions with the matrix, which is exꢀ
cluded in argon matrices.
the singlet/triplet percent ratio to 75 : 25. Subsequent UV
irradiation (365 nm) for 10 min decreases the ratio to 60 : 40,
visible light illumination (450 nm) for 10 min increases it
again to 75 : 25. Prolonged 365 nm irradiation (several hours)
leads to Sꢀ1b : Tꢀ1b ratios larger than 20 : 80. Thus, dependꢀ
ing on the wavelength of irradiation, matrices containing prefꢀ
erentially either Sꢀ1b or Tꢀ1b can be produced, and at 3 K no
thermal interconversion between the singlet and triplet states is
observed. These observations are in complete agreement with
the EPR experiments described above, and the EPRꢀsilent
reservoir compound of Tꢀ1b is therefore identified as the
singlet state Sꢀ1b. There is some evidence from the EPR and
IR spectra that the 450 nm irradiation is conformerꢀselective
(in the EPR the minor triplet with D = 0.4053 cmꢀ1 and E
= ꢀ0.0173 cmꢀ1 is bleached preferentially, SI Figure S7), howꢀ
ever, the data do not allow us to determine which conformer is
photoconverted preferentially.
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Figure 2. Xꢀband CW EPR spectra of an argon matrix showing
the decrease of signal intensity of Tꢀ1b after warming from 5 K to
30 K and cooling back to 5 K. a) Argon matrix at 5 K showing the
triplet spectrum of Tꢀ1b (black line, zfs parameters D = 0.4096
cmꢀ1, E = ꢀ0.0190 cmꢀ1). b) After annealing at 30 K for 10 min and
cooling back to 5 K, 69% of the signal intensity is lost (red line).
Insert: EPR signal intensity plotted vs. inverse temperature.
IR Spectra. To shed light on the unexpected thermal behavꢀ
ior of Tꢀ1b in solid argon, the photolysis of matrixꢀisolated 2b
was investigated by IR spectroscopy. Visible light irradiation
(530 nm) of 2b matrixꢀisolated in argon at 3 K resulted in the
disappearance of its characteristic C=N=N stretching vibration
and formation of a complex spectrum (SI Figure S8, traces a,
b). Comparison of the spectrum with that of the three triplet
conformers calculated at the B3LYPꢀD3/def2ꢀTZVP level of
theory (SI Figure S8, traces cꢀe) reveals the formation of a
mixture of conformers plus additional compounds clearly
differing from Tꢀ1b. Annealing of the matrix at 25 K for sevꢀ
eral minutes results in a substantial change in the IR spectrum
(Figure 3, trace a): all bands assigned to the conformers of Tꢀ
1b decrease in intensity and those of the additional compounds
increase. By comparison with DFT calculations (Figure 3,
trace b) we assign these additional compounds to the singlet
state of the carbene, Sꢀ1b. At 3 K, the singlet and the triplet
state of carbene 1b coexist in the matrix, and there is no therꢀ
mal interconversion between these states on a timescale of
hours. However, the interconversion can be induced photoꢀ
chemically: 365 nm irradiation results in an increase of Tꢀ1b
(Figure 3, trace c), whereas 450 nm irradiation leads to an
increase of Sꢀ1b (SI Figure S10, trace d).
Figure 3. IR spectra showing the thermal interconversion of Tꢀ1b
to Sꢀ1b. a) Difference spectrum showing the interconversion of Tꢀ
1b to Sꢀ1b calculated at the B3LYPꢀD3/def2ꢀTZVP level of
theory (red line). Equal contributions of the three conformers dd,
ud, and uu were used to simulate the IR spectrum. b) Difference
spectrum at 3 K showing changes after 10 min annealing of an
argon matrix containing both Tꢀ1b and Sꢀ1b at 25 K (black line).
Bands pointing downwards, assigned to Tꢀ1b, are disappearing,
and bands pointing upwards, assigned to Sꢀ1b, are appearing. c)
Difference spectrum at 3 K showing changes after 10 min of
365 nm irradiation of the same matrix. Bands pointing downꢀ
wards, assigned to Sꢀ1b, are disappearing, and bands pointing
upwards, assigned to Tꢀ1b, are appearing.
UV-vis Spectra. The same type of experiments as described
above were conducted with UVꢀvis detection (Figure 4). In
argon at 10 K the diazo compound 2b shows two strong bands
at 280 and 230 nm and a very weak band at 535 nm (assigned
to the n→π* transition of 2b), which upon 530 nm photolysis
decrease. Simultaneously, a weak band at 483 nm and two
medium strong and broad bands at 384 nm and 316 nm appear
during the photolysis. Annealing at 25 K results in a decrease
of the bands at 483 nm and 316 nm, which are therefore asꢀ
signed to Tꢀ1b, and an increase of the 384 nm absorption,
assigned to Sꢀ1b (SI Figure S12B). Subsequent irradiation
with UV light (365 nm) results in a decrease of the Sꢀ1b abꢀ
sorption at 384 nm and an increase of the Tꢀ1b bands, whereas
visibleꢀlight (450 nm) irradiation results in an increase of the
A quantitative evaluation of the IR spectra (SI Table S8) reꢀ
veals that the 530 nm photolysis of precursor 2b results in a
percent ratio of Sꢀ1b : Tꢀ1b of approximately 50 : 50. This
ratio is independent of the irradiation time (several minutes to
hours) and of the extent of 2b converted to 1b (small fraction
or complete). This indicates that either (i) the 530 nm photolyꢀ
sis of 2b produces the singlet and triplet carbene in a single
step with a branching ratio of approximately 50 : 50, or (ii) Sꢀ
1b and Tꢀ1b are formed in a photostationary equilibrium,
where the photoequilibration is more efficient and faster than
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384 nm absorption of Sꢀ1b (SI Figure S13) and a decrease of point of these scans the singlet and triplet surfaces crossed
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the bands of Tꢀ1b. A quantitative analysis of the UVꢀvis specꢀ
tra obtained under various conditions (irradiation wavelengths,
matrix temperature, SI Table S9) revealed very similar ratios
of Sꢀ1b : Tꢀ1b as found in the corresponding IR experiments
(see above).
each other, indicating that changes of these internal coordiꢀ
nates are not related to ISC in this system.
Calculations of the energies of uuꢀSꢀ1b and uuꢀTꢀ1as a
function of the CꢀCꢀC bond angle at the carbene center (reꢀ
laxed scan) reveal a minimum for the singlet at 119° with the
triplet lying 9.3 kcal/mol higher at the same bond angle (Figꢀ
ure 5). The minimum of the triplet is found at a CꢀCꢀC bond
angle of 141°, and at this bond angle the singlet is higher in
energy by 7.8 kcal/mol. Thus, at the geometry of the singlet,
the singlet is more stable than the triplet, and vice versa. The
crossing was located at a CꢀCꢀC bond angle of 131°, 2.52
kcal/mol above uuꢀSꢀ1b, and 1.41 kcal/mol above uuꢀTꢀ1b.
These calculations indicate that an activation barrier of 1.4
kcal/mol has to be overcome to convert uuꢀTꢀ1b into the more
stable uuꢀSꢀ1b.
With only Tꢀ1b absorbing in the visible range of the specꢀ
trum (483 nm), visibleꢀlight irradiation drives the photostaꢀ
tionary equilibrium towards Sꢀ1b. On the other hand, irradiaꢀ
tion into the strong 384 nm band of Sꢀ1b shifts the equilibrium
towards Tꢀ1b. Thus, the absorption characteristics of the two
states of 1b straightforwardly rationalize the observed photoꢀ
chemical behavior.
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Figure 4. UVꢀvis spectra showing interconversion of 2b, Tꢀ1b
and Sꢀ1b by irradiation and annealing. a) UVꢀvis spectrum of 2b
isolated in argon matrix at 8 K (dotted line). b) UVꢀvis spectrum
obtained after several hours of 530 nm irradiation of 2b at 8 K
(blue line). c) UVꢀvis spectrum of the same matrix after 10 min
annealing to 25 K and cooled back to 8 K (black line). d) UVꢀvis
spectrum after subsequent 10 min irradiation with λ = 365 nm
(red line). The band of Sꢀ1b decreases while those of Tꢀ1b inꢀ
crease during irradiation (indicated by green arrows). The weak
band of Tꢀ1b in the visible region of the spectrum is shown in the
insert (intensity multiplied by 20).
Figure 5. Energy profiles of the scans performed on the CꢀCꢀC
carbene angles of the uu conformers in the triplet (green) and the
singlet (red) state. The scans were performed at the B3LYPꢀ
D3/def2ꢀTZVP level of theory and the energies were corrected
using singleꢀpoint CCSD(T)/ccꢀpVDZ calculations.
Reaction of 1b with Water. In previous publications we
have shown that the singlet state of diphenylcarbene Sꢀ1a is
strongly stabilized by hydrogen bonding, whereas the triplet
state Tꢀ1a is only weakly interacting.2,14 In the case of 1b, both
the singlet and the triplet carbene are present in the matrix
after photolysis of 2b. Annealing at temperatures above 20 K
is required to allow for the diffusion of matrixꢀisolated water.
At these temperatures we also observe the ISC of Tꢀ1b to Sꢀ1b
in the absence of water, and therefore water will interact with
both Tꢀ1b and Sꢀ1b. We expect that Sꢀ1b strongly interacts
with water under formation of the hydrogen bonded complex
DFT Calculations. The singletꢀtriplet gap ꢀEST of carbene
1b mainly depends on the CꢀCꢀC bond angle at the carbene
center. However, the question remains how the conformational
flexibility of the methoxy groups influences ꢀEST. Therefore,
the Sꢀ1b and Tꢀ1b conformers dd, ud, and uu were computed
at the CCDS(T)/ccꢀpVDZ//B3LYPꢀD3/def2ꢀTZVP level of
theory. The results reveal that the orientation of the methoxy
groups has basically no influence on the relative stability of
the conformers in the singlet or triplet state (SI Table S10).
With respect to ꢀEST, Sꢀ1b is estimated to be 0.24ꢀ0.36
kcal/mol more stable than Tꢀ1b, considering all combinations
of conformers (SI Table S11). This is in qualitative agreement
with the experiments indicating that the singlet state is slightly
more stable than the triplet.
…
Sꢀ1b HOH, whereas Tꢀ1b only weakly interacts, but this
weakly bound complex rapidly interconverts to the Sꢀ
…
1b HOH complex (Scheme 1).
To assess the influence of water on the singletꢀtriplet gap,
several water complexes were computed considering not only
the carbene center as a potential Hꢀbond acceptor, but also
both of the oxygen atoms of the methoxy groups. Thus, in
addition to 1:1, we also considered 3:1 complexes involving
all the possible donor centers at once (SI Figures S18, S19).
When the oxygen atoms act as donors, the effect on the SꢀT
The influence of the rotations of the methoxy group (Cphenyl
ꢀ
O) and of the methyl group (CmethylꢀO) on ꢀEST was also invesꢀ
tigated (SI Figures S16, S17). Relaxed potential energy scans
show that the rotations of the methoxy and methyl groups have
small influence on the singletꢀtriplet gap (variations of 1
kcal/mol and 0.1 kcal/mol respectively). Furthermore, at no
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gap is relatively small (~1 kcal/mol) with the triplet complexes
being more stabilized than the complexes with the singlet.
When the interaction takes place through the carbene center
this situation changes. The singlet carbene is a stronger base
than the triplet, with the stabilization energy for the singlet
complexes being about 6 kcal/mol larger. This difference in
stabilization energies ensures that the singlet remains the
ground state upon interaction with a single water molecule,
actually increasing the gap to ꢀ5.9 kcal/mol (SI Table S12).
When the interaction with multiple water molecules is considꢀ
ered (3:1 complexes) the stabilization energy of the singlet
complexes is again higher than for the triplet complexes. This
results again in a widening of the singletꢀtriplet gap. The preꢀ
dicted value of the gap in the quaternary complex (ꢀ5.2
kcal/mol) is slightly smaller than for the 1:1 complexes (SI
Table S13). This small difference can be rationalized by the
effect of the hydrogen bonding with the methoxy groups,
which somewhat tends to stabilize triplet states over singlets.
Still, it is not enough to overcome the larger effect of the inꢀ
teraction of water at the carbene center that strongly stabilizes
the singlet.
…
1a HOH, involving a proton transfer from the water moleꢀ
cule to the carbene center followed by barrierꢀless recombinaꢀ
tion to yield the corresponding alcohol. This is in complete
1
2
3
4
5
6
7
8
…
agreement with the behavior of Sꢀ1a HOH, which at similar
rates rearranges to the corresponding alcohol (benzhydrol) via
quantum chemical tunneling.14 Given the calculated activation
barrier for the insertion of Sꢀ1b into water of 6.3 kcal/mol, we
assume that this reaction at 3 K also proceeds via tunneling.
The strikingly different kinetic behavior of Sꢀ1b and its water
complex Sꢀ1b HOH easily allows to discriminate these
species and assign the spectra even in spectral regions with
overlapping absorptions.
…
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Scheme 1. S-T interconversion and reaction of carbene 1b
with water.
365 nm, 3 K
450 nm, 3 K
H3CO
OCH3
H3CO
OCH3
S-1b
+ H2O
ꢀ
, 25 K
T-1b
Figure 6. IR spectra showing the formation and degradation of
the complex between Sꢀ1b and water. a) Mixture of T-1b and S-
1b in an argon matrix doped with 1 % of H2O at 3 K (black line).
b) Difference IR spectrum of the same matrix showing changes
after annealing for 10 min at 25 K (blue line). Bands pointing
downwards, assigned to Tꢀ1b, are disappearing, and bands pointꢀ
H
H
O
H
OH
QMT
…
H3CO
OCH3
H3CO
OCH3
ing upwards, assigned to the complex Sꢀ1b HOH, are appearing.
c) Difference IR spectrum of the same matrix showing changes
after 15 hours at 3 K (red line). Bands pointing downwards are
3
S-1b...H-OH
…
assigned to Sꢀ1b HOH, bands pointing upwards are assigned to
the OꢀH insertion product 3.
If a 1% H2Oꢀdoped argon matrix containing carbene 1b (S,
T mixture as described above) is warmed from 3 K to 25 K, all
…
bands of Tꢀ1b decrease in intensity and Sꢀ1b HOH is formed
(Figure 6). As expected, the IR spectrum of Sꢀ1b in the comꢀ
plex is only slightly disturbed upon complexation with water.
The largest shift of +7.7 cmꢀ1 is found for the strong band of Sꢀ
1b at 1339 cmꢀ1 (SI Figure S11), assigned to the unsymmetꢀ
rical CꢀCꢀC stretching vibration involving the carbene center.
Since the water molecule is attached to the carbene center, this
CONCLUSION
The thermal and photochemical interconversion between the
singlet and the triplet state of carbene 1b has been investigated
by EPR, IR, and UVꢀvis spectroscopy in solid argon, by EPR
and ENDOR spectroscopy in MTHF, and by theoretical methꢀ
ods. These experiments clearly demonstrate that:
vibration is affected most. The other vibrations are shifted
ꢀ1
…
between 0 and 6 cm . Whether Sꢀ1b HOH is formed by the
(i) The singlet and triplet states of carbene 1b coexist in arꢀ
gon at 3 K, and there is no intersystem crossing (ISC) within
the timescale of our experiments (hours).
direct interaction of water with Sꢀ1b or via ISC of the weakly
…
bound Tꢀ1b HOH, or by both mechanisms, cannot be deterꢀ
mined from our experiments.
(ii) At 3 K in argon, photostationary equilibria between Sꢀ
1b and Tꢀ1b are established by irradiation either into the
strong UV absorption of Sꢀ1b or into the weak visible absorpꢀ
tion of Tꢀ1b. Thus, visible light irradiation (450 nm) shifts the
equilibrium towards Sꢀ1b, and UV irradiation (365 nm) toꢀ
wards Tꢀ1b.
…
The most striking difference between Sꢀ1b and Sꢀ1b HOH
is that the singlet carbene Sꢀ1b is thermally stable as long as it
is matrixꢀisolated, whereas its water complex is only metastaꢀ
ble even at 3 K and slowly rearranges with a rate of 7±3 x 10ꢀ6
sꢀ1 to alcohol 3 (Scheme 1, SI Table S5), the formal OH inserꢀ
tion product. The corresponding reaction pathways were comꢀ
puted for the complexes of the three Sꢀ1b conformers with
water (SI Table S14, Figure S20). The predicted transition
states were very similar to those previously reported for Sꢀ
(iii) Annealing at slightly higher temperatures (10 – 25 K in
argon) results in a conversion to Sꢀ1b. From that we conclude
that Sꢀ1b is thermodynamically more stable than Tꢀ1b and that
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these states are separated by an activation barrier that prevents
interconversion at 3 K.
Bruker ELEXSYS E580 spectrometer equipped with a homeꢀ
built TE011 microwave cavity35, a cryogenꢀfree closedꢀcycle
cryostat (Cryogenic Limited) with a Cryomech liquid He
compressor and an ENI 3200L radio frequency (RF) amplifier.
For further details about pulseꢀsequences, spectral simulations
and DFT calculations of EPR parameters see SI.
1
2
3
4
5
6
7
8
In MTHF the triplet disappears completely above 50 K and
can be restored by UV irradiation at 5 K. However, since
MTHF is reactive towards carbenes and Tꢀ1b might be formed
by photolysis from remaining precursor 2b, these experiments
are less conclusive than the experiments in argon. Trozzolo
and Gibbons investigated the photochemistry of 2b in MTHF
at 77 K already in 1966 and found, besides Tꢀ1b, an unknown
singlet compound with a strong absorption maximum at 390
nm which is close to the band at 384 nm we assign to Sꢀ1b.
Although these authors explicitly ruled out the presence of Sꢀ
1b in their experiments, it is now clear that both Sꢀ1b and Tꢀ
1b were formed in these early experiments.
Matrix isolation technique. Matrix isolation experiments
were performed by standard techniques. For cooling the specꢀ
troscopic windows to cryogenic temperatures (3 K or higher) a
Sumitomo Heavy industries twoꢀstaged closedꢀcycle helium
cryostats (cooling power 1 W at 4 K) was used. The cryostat
was evacuated using a diffusion oil pump for IR experiments,
while a turbomolecular pump was used for Xꢀband CW EPR
experiments. Matrices were generated by coꢀdeposition of 2b
with a large excess of argon (Messer Griesheim, 99.99%) with
a flow rate of approximately 1.80 sccm on the top of spectroꢀ
scopic windows cooled to the lowest temperatures possible (3
K for IR, 8 K for UVꢀvis, and 5 K for EPR). 2b was sublimed
at 80°C and coꢀdeposited on the spectroscopic window with a
large excess of argon. The temperature of the windows was
controlled by an Oxford ITC4 temperature controller. The
experiments in 1% waterꢀdoped argon matrices were carried
out as described earlier. Ultraꢀpure water used in these experꢀ
iments was degassed by several freezeꢀthaw cycles (for details
see SI).
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An obvious question is: why is the ISC in carbene 1b exꢀ
tremely slow and why can both states coexist in solid argon?
Singletꢀtriplet bistability has been predicted31,32 and observed
previously for diradicals such as Nꢀsubstitutedꢀ3,4ꢀ
dimethylenepyrrole biradicals33 or bis(aminoxyl)diradicals.34
The only carbene which shows a similar behavior is 1c.20
However, this carbene has a triplet ground state and the singlet
state is kinetically unstable and slowly decays back to the
triplet state in the dark. In 1c the singlet exhibits a very differꢀ
ent geometry with the carbomethoxy group rotated out of
plane, and thus the SꢀT interconversion requires a major geoꢀ
metrical change, which slows done the ISC in rigid matrices.
In contrast, in 1b the main geometric difference is the CꢀCꢀC
bond angle at the carbene center, which increases from 119° in
Sꢀ1b to 141° in Tꢀ1b. This is typical for all carbenes, where
the (closedꢀshell) singlets show narrower bond angles (typicalꢀ
ly 100° – 110°) and the triplets wider angles (120° – 150°),
and therefore, in this respect carbene 1b does not differ from
any other carbene. The only difference is that in 1b the singlet
and triplet states are essentially degenerate. At both the geomꢀ
etry of Sꢀ1b and Tꢀ1b the vertical excitation energies to the
excited triplet and singlet states, respectively, are large, and
the DFT calculations suggest that a thermal barrier of 1.4
kcal/mol has to be overcome for the ISC to occur. While the
calculated values might be a rough estimation only, the qualiꢀ
tative predictions are in excellent agreement with the experiꢀ
mental observations: Sꢀ1b is predicted to be slightly more
stable than Tꢀ1b, and a small thermal activation barrier preꢀ
vents the interconversion at extremely low temperatures.
Computational details. All gas phase calculations were
performed using the B3LYP density functional36ꢀ38 with D3
dispersion corrections. The def2ꢀTZVP basis set39 was used in
the calculations. All gas phase computations were carried out
with the Gaussian 09 program.40 CCSD(T) single point calcuꢀ
lations were performed using the ccꢀpVDZ basis set.41 The
MOLPRO program42 was used for these calculations.
Since it is a wellꢀknown fact that B3LYP overestimates the
SꢀT gap of carbenes43 and the carbene 1b is expected to have a
nearꢀzero SꢀT splitting, a CCSD(T) correction was added to
the computed values. Energy evaluations at the CCSD(T)/ccꢀ
pVDZ//B3LYPꢀD3/def2ꢀTZVP level were performed for the
carbenes and the 1:1 carbeneꢀwater complexes. The average
difference between the DFT and CCSD(T) energies (separateꢀ
ly evaluated for singlet and triplet states) was then used to
scale the DFT energies(Equation (1)):
ꢀꢁ. = ꢀꢂꢃꢄ + ꢅ
(1)
Ec is the corrected energy, EDFT is the zeroꢀpoint energy
(ZPE)ꢀcorrected energy computed at the DFT level and δ is the
CCSD(T) correction.
In summary, it could be demonstrated that for carbene 1b
the lowestꢀenergy singlet and triplet states coexist. We predict
that this bistability of states of different spin multiplicity reꢀ
sults from the nearꢀdegeneracy of these states, and therefore
should be a general phenomenon in similar systems. In a biꢀ
stable carbene the magnetic properties of the system can be
switched by irradiation with light of different color, and thus
this property might be of general interest for applications such
as information storage.
The relaxed scans were carried out by fixing the torsion coꢀ
ordinates in 10° or 15° intervals while reꢀoptimizing the rest of
the internal coordinates in each step. For further details see SI.
For Materials, synthesis of the diazoprecursors and preparaꢀ
tion of samples for frozenꢀsolution EPR see SI.
EXPERIMENTAL PART
Pulse EPR spectroscopy. Wꢀband (≈ 94 GHz) pulse EPR
measurements were carried out at 10 K using a Bruker
ASSOCIATED CONTENT
Supporting Information. Extended experimental and computaꢀ
tional details, supplementary figures and tables and Cartesian
coordinates are available free of charge via the Internet at
http://pubs.acs.org.
ELEXSYS E680 spectrometer employing
a homebuilt
ENDOR microwave cavity, which comprised a solenoid of
Teflonꢀcoated silver wire integrated into a commercial Wꢀ
band ENDOR probe head (Bruker). Qꢀband (≈ 34 GHz) pulse
EPR and ENDOR experiments were performed at 5 K using a
AUTHOR INFORMATION
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(21)
Wang, J. L.; Likhotvorik, I.; Platz, M. S. J.
Corresponding Author
1
2
3
Am. Chem. Soc. 1999, 121, 2883.
wolfram.sander@rub.de
esanchez@kofo.mpg.de
anton.savitsky@cec.mpg.de
(22)
Wang, Y. H.; Yuzawa, T.; Hamaguchi, H. O.;
Toscano, J. P. J. Am. Chem. Soc. 1999, 121, 2875.
(23)
Zhang, Y.; Kubicki, J.; Wang, J.; Platz, M. S.
4
5
6
7
8
9
Author Contributions
J. Phys. Chem. A 2008, 112, 11093.
(24)
Carrington, A.; McLachlan, A. D.
The manuscript was written through contributions of all authors.
Introduction to magnetic resonance; Harper & Row: New York,
1967.
Funding Sources
(25)
Gerson, F.; Huber, W. Electron Spin
This work was supported by the Cluster of Excellence RESOLV
(EXC 1069) funded by the Deutsche Forschungsgemeinschaft
(DFG).
Resonance Spectroscopy of Organic Radicals; WileyꢀVCH:
Weinheim, 2003.
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2006, 110, 12267.
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Sinnecker, S.; Neese, F. J. Phys. Chem. A
ACKNOWLEDGMENT
Schweiger, A.; Jeschke, G. Principles of pulse
This work was supported by the Cluster of Excellence RESOLV
(EXC 1069) funded by the Deutsche Forschungsgemeinschaft
(DFG). E. SꢀG. also acknowledges a Liebig Stipend of the Fonds
der Chemischen Industrie and funding by the SFB 1093 of the
DFG.
electron paramagnetic resonance; Oxford University Press:
Oxford, 2001.
(28)
Kurreck, H.; Kirste, B.; Lubitz, W. Electron
Nuclear Double Resonance Spectroscopy of Radicals in Solution:
Application to Organic and Biological Chemistry; Wiley: New
York, 1988.
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SYNOPSIS TOC
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Bis(pꢀmethoxyphenyl)carbene can be isolated in lowꢀtemperature matrices in both its singlet or its triplet state, and both
states coexist without interconversion.
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