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inorganics
Article
The Silacyclobutene Ring: An Indicator of Triplet
State Baird-Aromaticity
Rabia Ayub 1,2, Kjell Jorner 1,2 ID and Henrik Ottosson 1,2,*
1Department of Chemistry—BMC, Uppsala University, Box 576, SE-751 23 Uppsala, Sweden;
rabia.ayub@kemi.uu.se (R.A.); kjell.jorner@kemi.uu.se (K.J.)
2Department of Chemistry-Ångström Laboratory Uppsala University, Box 523, SE-751 20 Uppsala, Sweden
*Correspondence: henrik.ottosson@kemi.uu.se; Tel.: +46-18-4717476
Received: 23 October 2017; Accepted: 11 December 2017; Published: 15 December 2017
Abstract:
Baird’s rule tells that the electron counts for aromaticity and antiaromaticity in the first
ππ
* triplet and singlet excited states (T
1
and S
1
) are opposite to those in the ground state (S
0
).
Our hypothesis is that a silacyclobutene (SCB) ring fused with a [4n]annulene will remain closed in
the T
1
state so as to retain T
1
aromaticity of the annulene while it will ring-open when fused to a
[4n+ 2]annulene in order to alleviate T
1
antiaromaticity. This feature should allow the SCB ring to
function as an indicator for triplet state aromaticity. Quantum chemical calculations of energy and
(anti)aromaticity changes along the reaction paths in the T
1
state support our hypothesis. The SCB
ring should indicate T
1
aromaticity of [4n]annulenes by being photoinert except when fused to
cyclobutadiene, where it ring-opens due to ring-strain relief.
Keywords: Baird’s rule; computational chemistry; excited state aromaticity; Photostability
1. Introduction
Baird showed in 1972 that the rules for aromaticity and antiaromaticity of annulenes are reversed
in the lowest
ππ
*triplet state (T
1
) when compared to Hückel’s rule for the electronic ground state
(S
0
) [
1
–
3
]. The rule has subsequently been confirmed by a series of quantum chemical calculations [
3
,
4
],
and it has also been shown that 4n
π
-electron species can have triplet multiplicity ground states
(T
0
). Interestingly, the T
0
state cyclopentadienyl cation and the isomeric vinylcyclopropenium cation
(a closed-shell singlet) are nearly isoenergetic [
5
,
6
], revealing that Baird-aromatic stabilization of triplet
state species can be significant [
3
,
7
]. It has also been shown through computations that Baird’s rule
can be extended to the lowest
ππ
*excited singlet states (S
1
) of cyclobutadiene (CBD), benzene, and
cyclooctatetraene (COT) [
8
–
13
]. Thus, [4n]annulenes display aromatic character in both their T
1
(or T
0
)
and S
1
states whereas [4n+ 2]annulenes display anti-aromaticity. With Hückel’s and Baird’s rules it
becomes clear that benzene has a dual character and can be labelled as a molecular “Dr. Jekyll and
Mr. Hyde” [
3
,
14
,
15
]. This is in line with the early conclusions by Baird as well as by Aihara [
1
,
16
],
and the excited state antiaromaticity explains the photoreactivity of many benzene derivatives [
14
].
On the other hand, CBD and COT are both aromatic in the T1state [17–19].
In the last few years, the excited state aromaticity and antiaromaticity concepts (abbr. ES(A)A)
have gained gradually more attention [
20
–
22
], even though the pioneering experimental studies were
presented by Wan and co-workers already in the 80s and 90s [
23
–
28
]. We earlier stressed that Baird’s
rule can be used as a qualitative back-of-an-envelope tool for the design of photochemically active
materials, as well as for the development of new photoreactions [
29
,
30
]. Indeed, a recent combined
experimental and computational study of a chiral thiopheno-fused COT compound by Itoh and
co-workers reveals that the aromatic stabilization energies in both the T
1
and S
1
states are extensive
(~21 kcal/mol) [
31
], in agreement with previous computational estimates of T
1
state (anti)aromatic
Inorganics 2017,5, 91; doi:10.3390/inorganics5040091 www.mdpi.com/journal/inorganics
Inorganics 2017,5, 91 2 of 16
(de)stabilization [
32
,
33
]. For experimental identification of excited state aromatic cycles vs. anti- and
nonaromatic ones, there is a need for suitable indicator moieties. Based on both computations and
experiments we recently reported that the cyclopropyl (cPr) group can differentiate T
1
and S
1
state
aromatic rings from those that are antiaromatic or nonaromatic in these states [
34
]. Yet, the cPr group
also has a drawback in that the products formed upon ring-opening are not easily identified as they
are complicated mixtures or polymeric material.
Herein, we discuss a computational study on the effects of T
1
state (anti)aromaticity of [4n]- and
[4n+ 2]annulenes on the photochemical ring-opening of a silacyclobutene (SCB) ring fused with an
annulene. The SCB ring is interesting because of its ring-strain and high chemical reactivity [
35
].
One could potentially use the SCB ring as a substituent on an annulene ring, and here it is already
known that 1,1-dimethyl-2-phenyl-1-silacyclobut-2-ene ring-opens photochemically [
36
], likely a route
for excited state antiaromaticity relief. However, when used as a substituent the opening of the SCB
ring will also be affected by conformational factors (Figure 1a). Instead, if fused it sits in the same
arrangement regardless of annulene, and thus, should be a more unbiased indicator (Figure 1b).
Figure 1. The silacyclobutene (SCB) ring as a substituent (a) and fused to an annulene ring (b).
We argue that the T
1
aromaticity of [4n]annulenoSCBs will hinder the SCB ring from opening
as this will lead to loss of T
1
aromaticity, while the T
1
antiaromaticity of [4n+ 2]annulenoSCBs
instead will enhance the rate for ring-opening. We base this argument on the Bell–Evans–Polanyi
principle that says that the activation energy will be proportional to the reaction energy for reactions
of the same type [
37
]. This will lead to a photoreactivity difference which can allow the SCB ring
to function as a T
1
aromaticity indicator. Noteworthily, the T
1
state potential energy surfaces (PESs)
for electrocyclization reactions of compounds with 4n
π
-electrons were earlier explored by Mauksch
and Tsogoeva [
38
], and Möbius aromatic transition states were identified in compounds with 8,
10 and 12
π
-electrons. Yet, the focus of our paper is not on the T
1
state electrocyclic ring-opening
of SCB but instead on the explicit effect of the T
1
state (anti)aromaticity of annulenes on the SCB
ring-opening when these annulenes are fused to the SCB ring. Earlier, we reported that the shapes
of T
1
state PESs for twists about the C=C double bonds (cf. T
1
state Z/E-isomerizations) of [4n]-
and [4n+ 2]annulenyl substituted olefins are connected to changes in T
1
state (anti)aromaticity of
(hetero)annulenyl substituents [
39
–
42
]. From this, one can infer that the T
1
state PES of also other
photoreactions will vary in dependence of a neighboring [4n+ 2]- or [4n]annulene ring. BenzoSCB
(
2a
, Figure 2) should photorearrange to o-silaxylylene (
2b
), containing a highly reactive Si=C double
bond [
43
], while cyclobuteno-(
1a
) and cyclooctatetraenoSCB (
3a
), based on our hypothesis, should be
resistant to photochemical ring-openings. Indeed, photochemical ring-opening of
2a
in the S
1
state to
the transient
2b
, trapped by alcohol solvents to yield isolable silylethers in 40–80% yield (Scheme 1),
was earlier reported by Kang and co-workers [
44
]. Additions of alcohols to silenes, particularly
naturally polarized ones (Si
δ+
=C
δ−
), proceed over very low activation barriers (a few kcal/mol) [
45
]
due to the high oxy- and electrophilicity of the sp
2
hybridized Si atom, making these reactions highly
suitable for rapid trapping of transient SCB ring-opened isomers. A potential benefit of the SCB
ring over the cyclopropyl group is the persistence of the products formed upon photochemical SCB
ring-opening followed by trapping [44,46].
Inorganics 2017,5, 91 3 of 16
Scheme 1.
The photochemical SCB ring-opening and subsequent trapping of ortho-silaxylylene reported
by Kang, K.T. et al. [44].
Our investigation is focused on the T
1
state ring-openings rather than the S
1
state processes as
the triplet states are more easily amenable to computations. We used different aromaticity indices
to examine the (anti)aromatic character of ring-closed and ring-opened isomeric structures. If T
1
aromaticity of a [4n]annulene hinders the SCB ring from opening, then the absence of this reaction upon
irradiation should indicate T
1
aromaticity. The SCB ring-opening could tentatively be connected to the
bond dissociation enthalpies (BDEs) because the C–C BDE in a strain-free compound (90.4 kcal/mol) is
slightly higher than the Si–C BDE (88.2 kcal/mol), which in turn is higher than the Si–Si (80.5 kcal/mol)
BDE [
47
,
48
]. Although the BDE difference between the strain-free C–C and Si–C bonds is small, strain
could be more important in the SCB ring than in the all-carbon cyclobutene ring (vide infra). Here it can
be noted that the cyclobutene and disilacyclobut-3-ene rings are less suitable than the SCB ring because
the former opens only rarely upon photolysis (e.g., benzocyclobutene does not undergo photochemical
ring-opening unless further derivatized) [
14
,
49
], while the latter is unstable and readily oxidized in
air to 1,3-disila-2-oxacyclopentenes [
50
]. It should also be noted that strained four-membered rings
with heteroatoms from Groups 15 and 16 are problematic in the context of excited state aromaticity
indicators as these heteroatoms provide lone-pair electrons that will interact electronically with the
π
-conjugated annulene. Additionally, the lowest excited states of compounds with such rings could be
of n
π
* rather than of
ππ
* character, leading to excited states for which Baird’s rule is not applicable.
The SCB ring when used as a substituent can also influence the annulene through
π
-conjugation, yet,
when fused onto an annulene its C=C bond is joint with the annulene. Thus, the SCB ring could hold
a unique position as a tentative excited state aromaticity indicator unit.
Figure 2.
The annulenoSCB ring-openings and the postulated (anti)aromatic characters in the S
0
and
the T
1
states of
1a
,
2a
,
3a
,
1b
,
2b
, and
3b
, respectively, with A = aromatic, AA = antiaromatic, and NA
= non-aromatic.
Inorganics 2017,5, 91 4 of 16
2. Results and Discussion
We first discuss the changes in energies, geometries and (anti)aromaticities in the T
1
states
during ring-opening reactions of the three molecules (Figure 2) in which a SCB ring is fused with
either a [4n]annuleno-(cyclobuteno- and cyclooctatetraeno-) ring or a [4n+ 2]annuleno-(benzo-) ring.
We discuss reaction and activation energies, and subsequently analyze (anti)aromaticity changes
through the harmonic oscillator model of aromaticity (HOMA), nucleus independent chemical shift
(NICS) and isomerization stabilization energy (ISE) indices as well as anisotropy of the induced
current density (ACID) plots. Finally, openings of SCB rings fused with 5- and 7-membered annulenyl
cations and anions are discussed. We also explored the T
1
PES for SCB ring-opening when fused
with polycyclic systems (see Scheme S1), and for comparison the all-carbon analouges of
2
and
3
were analyzed.
2.1. Energy Changes
For the first three compounds (
1
–
3
), three different methods, two density functional theory
methods (B3LYP and OLYP), and one Coupled Cluster method (CCSD(T)) were tested to ensure that
the results do not vary extensively with method. Similar results were mostly obtained at the two DFT
(B3LYP and OLYP) and CCSD(T) levels, and therefore, only B3LYP energies are given for the remaining
compounds unless otherwise noted. Compound
2a
(
T1
) had a higher relative energy than that of its
ring-opened isomer,
2b
(
T1
), by 33.7–38.3 kcal/mol depending on the computational method (Figure 3).
Moreover, the activation energy for Si–C bond scission in the T
1
state was merely 9.0 kcal/mol,
38.3 kcal/mol lower than in the S
0
state at the B3LYP level. This suggests that an antiaromatic
destabilization of the benzene ring in the T
1
state affected
2a
(
T1
) making it highly unstable and prone
to cleave the Si–C bond.
Figure 3.
T
1
state potential free energy surface diagrams for ring-openings of 2,3-cyclobutadieno-1-SCB (
1
),
2,3-benzo-1-SCB (
2
) and 2,3-cyclooctatetraeno-1-SCB (
3
) at (U)B3LYP/6-311G(d,p) (normal print),
(U)OLYP/6-311G(d,p) (italics), and (U)CCSD(T)/6-311G(d,p)//(U)B3LYP/6-311G(d,p) (underlined)
levels. Values in parenthesis are activation energies. Values for the S
0
state in black and values for the
T1state in red.
Inorganics 2017,5, 91 5 of 16
Interestingly, the T
1
PES of
1
displayed a similar shape as that of
2
, since
1b
(
T1
) is of lower
energy than
1a
(
T1
) despite the fact that CBD is weakly T
1
-aromatic [
8
,
13
,
17
,
51
–
53
]. At the two DFT
levels,
1b
(
T1
) was even lower than
1a
(
S0
), likely due to ring-strain relief in
1b
(
T1
). Furthermore,
the activation energy in the T
1
state was lower than that in the S
0
state by 7.7 kcal/mol at B3LYP
level. An opposite behavior was observed for
3
in its T
1
state because
3b
(
T1
) is higher in energy
than
3a
(
T1
) by 11.8–26.1 kcal/mol. Moreover, the activation energies in the T
1
state are very high,
31.0–39.2 kcal/mol, revealing that the SCB ring will remain closed. Thus, for compounds
2
and
3
,
the energy changes upon ring-opening in the T
1
states fell in line with our hypothesis; ring-opening
releases energy in
2
in the T
1
state, while it requires energy in
3
. The question was, to what extent
these energy changes were linked to changes in (anti)aromaticity (vide infra).
The Si–C bond lengths in
1a
,
2a
, and
3a
are 1.95 Å, 1.92 Å, and 1.90 Å, respectively, which is
slightly longer than the normal Si–C bond length (1.87–1.89 Å) [
54
]. Thus, as one goes to gradually
larger annulene rings, the SCB ring gets successively less strained when evaluated based on Si–C
bond lengths.
Noteworthily, in the S
0
state, the reaction energy for ring-opening of
2a
(
S0
) was of opposite sign
to the reaction energy in the T
1
state. This reversal in endergonicity and exergonicity when going
from the S
0
to the T
1
state of
2
should be a consequence of Baird’s rule being the exact opposite to
Hückel’s rule. With regard to compounds
1
and
3
in the S
0
states, the ring-opening of
1a
(
S0
) was
exergonic, in line with relief of both S
0
antiaromaticity and ring-strain, while ring-opening in the
non-aromatic isomer
3a
(
S0
) was endergonic. These energies were a combination of factors; (i) changes
in (anti)aromaticity, (ii) ring strain release, and (iii) changes in the bonding character at the Si atom as it
goes from sp
3
to sp
2
hybridized. Formation of a Si=C double bond is an unfavorable process which is
not sufficiently compensated by relief of ring strain in the least ring strained of the compounds (
3a
(
S0
)).
Indeed, the ring opening of the all-carbon congener (
allC-3
(
S0
)), where a C=C double bond is formed
instead, was exergonic by 6.4 kcal/mol (see Figure S13).
For
2
and
allC-2
, the S
0
electrocyclic ring-opening transition states were conrotatory, as expected
for a thermal reaction with 4nelectrons. In T
1
, this was reversed to a disrotatory fashion. In contrast,
for
3
and
allC-3
, both the S
0
and T
1
transition states were conrotatory. The conrotatory mode in T
1
could be explained by the fact that the spin density is delocalized both over the eight-membered
and four-membered rings in the TS (Figures S44 and S45). This is consistent with a 10-electron
electrocyclic ring-opening with Möbius orbital topology which would be allowed in T
1
. Indeed,
the ACID plots for
3
and
allC-3
supported this interpretation, as the ring current went over all 10 atoms
(Figures S54 and S55). However, the difference of mechanism in T
1
with 10-electron conrotatory for
3
and 4-electron disrotatory for the other molecules was not sufficiently large to prevent application of
the Bell–Evans–Polanyi principle, as they all fell on the same correlation line (Figure S40, vide infra).
In addition to the SCB ring fused to aromatic and antiaromatic annulenes, we also analyzed it
when fused to the non-aromatic reference compounds cyclobutene, cyclohexene, and cyclohexadiene.
When going from
4
to
6
over
5
, the reaction energies in the T
1
state became gradually less
strongly exergonic, while the activation energies increased slightly. Compounds
4
and
5
both have
nonconjugated C=C double bonds, yet, the cycloalkene ring was larger in
5
, leading to less ring-strain
than in
4
, as well as a T
1
state SCB ring-opening reaction energy which was lower by 11.3 kcal/mol.
When going from
5
to
6
, the reaction energy further decreased by 12.1 kcal/mol, revealing that the
length of the conjugated path also had an impact. An indication that T
1
state antiaromaticity was
alleviated in
2a
(
T1
) was the fact that the energy released when going from
2a
(
T1
) to
2b
(
T1
) (Figure 3)
was larger than when going from
5a
(
T1
) to
5b
(
T1
) (Figure 4). Indeed, the ring-closed
2a
(
T1
) was at an
even higher energy than
5a
(
T1
) where the triplet biradical was confined to an essentially planar olefin
bond. This clarified that the benzene ring in the T
1
state was strongly destabilized. The destabilized
nature of T
1
state benzene became obvious when regarding compound
6,
where the ring-closed isomer
has a SCB moiety with its C=C double bond being part of a conjugated, yet, nonaromatic segment
because the T
1
energy of
2a
(
T1
), was substantially higher than that of
6a
(
T1
) (78.0 vs. 47.5 kcal/mol,
Inorganics 2017,5, 91 6 of 16
respectively). Moreover, the activation energy for SCB ring-opening of
2a
(
T1
) was 6.6 kcal/mol lower
than that of 6a(T1), which indicated an influence of T1antiaromaticity in 2a(T1).
Figure 4.
Relative reaction and activation free energies (kcal/mol) for the SCB ring-opening when
fused to non-aromatic rings. The energies for transition states are given in bold. Black energy levels
represent the S0state and red ones represent T1states at (U)B3LYP/6-311G(d,p) level.
2.2. Changes in T1State (Anti)aromaticity upon Ring-Opening
2.2.1. Harmonic Oscillator Model of Aromaticity (HOMA) Values
Bond length equalization is one indicator of aromaticity, and we chose the geometric HOMA
index as one of the indices used (Table 1). The large negative HOMA values in
1a
(
S0
),
1b
(
S0
) and
1b
(
T1
) corresponded to antiaromaticity, while the small HOMA value of
1a
(
T1
) suggested that this
structure is non-aromatic. Ring-opening of
1a
(
T1
) to
1b
(
T1
) led to an increase in antiaromaticity
(∆HOMA(T1) = −0.69).
Table 1.
Harmonic oscillator model of aromaticity (HOMA) values at the (U)B3LYP/6-311G(d,p) level
of compounds 1,2, and 3.
Compounds S0T1
a b ∆HOMA a b ∆HOMA
1−4.04 −1.11 2.93 0.10 −0.59 −0.69
20.97 0.07 −0.90 −0.32 0.83 1.15
30.08 −0.21 −0.13 0.89 0.21 −0.68
Compound
2
, in comparison, was aromatic in structures
2a
(
S0
) and
2b
(
T1
) while it was
non-aromatic in structures
2a
(
T1
) and
2b
(
S0
). Since
2a
(
T1
) is non-aromatic, ring-opening to
2b
(
T1
)
was favored, as aromaticity was gained (
∆
HOMA(T
1
) = 1.15). Benzene in the T
1
state can adopt
several different conformers depending on the starting geometry; (i) it can have a quinoidal structure
with two unpaired electrons in the para-positions and two double bonds parallel to the C
2
-axis (
3Q
),
(ii) it can have an anti-quinoidal structure with two allyl radical segments (
3AQ
), or (iii) it can be
described as a combination of a pentadienyl and a methyl radical (
3PM
) [
55
] (Figure 5). Isomer
2a
(
T1
) is geometrically most similar to
3AQ
since it has two long CC bonds and two allylic segments
(see Figure S9).
The small HOMA values of
3a
(
S0
),
3b
(
S0
) and
3b
(
T1
) indicate that these are nonaromatic while
a high positive value of
3a
(
T1
) suggests aromatic character. Thus, ring-opening of
3
in the T
1
state
entails an unfavorable reduction in aromaticity as
∆
HOMA(T
1
) =
−
0.68. Taken together, the HOMA
Inorganics 2017,5, 91 7 of 16
values support our hypothesis that T
1
state aromaticity is lost in SCB ring-openings of
1a
and
3a
, while
the T1state antiaromaticity of 2a is alleviated in this reaction.
Figure 5.
The quinoid (
Q
), anti-quinoid (
AQ
), and pentadienyl-methyl (
PM
) conformers of
T1-state benzene.
2.2.2. Nucleus Independent Chemical Shift (NICS) Scans
NICS is a magnetic indicator of aromaticity. The chemical shifts of NICS probe are scanned
over a certain distance (0–5 Å) above the center of the molecular plane. The out-of-plane component
obtained is then plotted against the distance. The NICS scans of
1
–
3
in their T
1
states in ring-closed
and ring-opened isomers are shown in Figure 6, while those in the S
0
states are found in Figure S1.
With regard to
1a
(
T1
), the out-of-plane component in the NICS scan had a negative value (
−
14.4 ppm;
1.1 Å) suggesting that this structure is significantly aromatic. Conversely, a high positive value of
the out-of-plane component in
1b
(
T1
) shows that this structure had antiaromatic character. For
2
,
it was found instead to be
2a
(
T1
), as it had a high positive value for the out-of-plane component
(90.9 ppm; 0 Å) revealing that this structure was T
1
antiaromatic. This T
1
antiaromaticity changed back
to aromaticity when the SCB ring opened, because a value of
−
19.8 ppm at 1.1 Å was calculated for
2b
(
T1
). With regard to
3
, the NICS scan showed
3a
(
T1
) to be aromatic with a value of
−
30.4 ppm at
0.9 Å, yet,
3b
(
T1
) was non-aromatic. Because of the non-planarity of the COT of
3b
(
T1
) a small kink
was observed in its NICS scan, in contrast to that of
3a
(
T1
) (Figure 6and Figure S1). Discontinuities
in NICS-XY scans due to non-planarities were earlier observed by Schaffroth and co-workers for
tetraazaacenes [
56
]. Thus, based on NICS, we have support for our hypothesis that T
1
(anti)aromaticity
influences the reaction energies for the ring-openings of
1
,
2
, and
3
. This led to loss of T
1
aromaticity in
1
and
3
, whereas it leads to alleviation of T
1
antiaromaticity in
2
. This reversal when going from
1
to
2
,
and then to
3
was also viewed clearly in Figure 6, since the structures with negative (aromatic) NICS
values were successively 1a(T1), 2b(T1) and 3a(T1).
Figure 6. Cont.
Inorganics 2017,5, 91 8 of 16
Figure 6.
Nucleus independent chemical shifts (NICS) scans of (
a
)
1a
(
T1
) and
1b
(
T1
); (
b
)
2a
(
T1
) and
2b
(
T1
); as well as (
c
)
3a
(
T1
) and
3b
(
T1
) at the GIAO-(U)B3LYP/6-311+G(d,p)//(U)B3LYP/6-311G(d,p)
level. Only the out-of-plane components are displayed.
2.2.3. Anisotropy of the Induced Current Density (ACID) Plots
ACID is a magnetic indicator of aromaticity for visualizing ring-currents and electron
delocalization. The ACID plots (Figure 7) of compounds
1
,
2
, and
3
corroborated the results
of the NICS scans. Clockwise ring-currents for
1a
(
T1
),
2b
(
T1
) and
3a
(
T1
) indicated aromaticity,
while counter-clockwise ring-currents in
1b
(
T1
) and
2a
(
T1
) represented antiaromaticity. Yet, the
ring-currents in
1b
(
T1
) suggested this structure to be only weakly antiaromatic. The
3b
(
T1
) structure
was non-aromatic. Clearly,
2a
(
T1
) opened the SCB ring to alleviate T
1
antiaromaticity (a favorable
process), while 1a(T1) and 3a(T1) lost aromaticity upon ring-openings (unfavorable processes).
2.2.4. Isomerization Stabilization Energy (ISE) Values
ISE is an energetic index of aromaticity and it is based on the energy difference between the
calculated total energy of fully aromatic methyl isomer to that of the non-aromatic exocyclic methylene
isomer. We also utilized the isomerization stabilization energy (ISE) index of Schleyer [
32
] to estimate
either the aromaticity or antiaromaticity in ring-closed structures in the T
1
state. Here, we examined
only 1and 2, and only in their T1states. With regard to the smallest compounds, the 4- and 5-methyl
substituted
1a
(
T1
) derivatives showed negative ISE values (ISE
avg −
10.1 kcal/mol, Figure 8), indicative
of some T
1
aromatic stabilization. With regard to
2
, the methyl-substituted
2a
(
T1
) structures showed
positive ISE values from 10.1 to 12.6 kcal/mol, indicative of T
1
antiaromatic destabilization. Structure
2a
(
T1
) is highly destabilized, evident from the computed ISE values. On the other hand, the ISE values
Inorganics 2017,5, 91 9 of 16
reported for benzene in S
0
state is
−
33.2 kcal/mol [
57
]. Thus, the ISE values support our hypothesis
that 2a(T1) is destabilized to the same extent as 1a(T1) is stabilized.
Figure 7.
Anisotropy of the induced current density (ACID) plots at (U)B3LYP/6-311+G(d,p)//
(U)B3LYP/6-311G(d,p) level. Broken arrows in
1b
(
T1
) indicate weaker ring-currents. Aromatic = A,
antiaromatic = AA, weakly antiaromatic = WAA, and non-aromatic = NA.
Figure 8.
Isomerization stabilization energy (ISE) values (kcal/mol) of the 4- and 5-methyl
substituted
1a
(
T1
) derivatives, and the 5-, 6-, 7-, and 8-methyl substituted
2a
(
T1
) derivatives at the
UB3LYP/6-311G(d,p) level. ISEavg is the average ISE value for the various methyl substitutions.
Inorganics 2017,5, 91 10 of 16
2.3. Five- and Seven-Membered Carbocyclic Anions and Cations Fused to SCB
In order to explore if the findings on
1
–
3
can be extended to other [4n]- and [4n+ 2]annulenyl-SCBs,
we examined the potential energy surfaces of 5- and 7-membered annulenyl cations and anions fused
to SCB rings (
7
–
9
, Figure 9). This also allowed us to evaluate how the reaction energies depend on the
annulene size. It should be noted that we only investigated energy and geometry changes, and we
predicted
7
and
9
to resemble
1
and
3
, respectively, while
8
should resemble
2
. The SCB-fused Cp- was
excluded as its calculated T1state was of πσ* and not of ππ* character.
The
7a
(
S0
) structure was a transition state; being a 4
π
-electron species it is strongly singlet state
antiaromatic, it showed large CC bond length alternations (Figure S11), and it was unstable to SCB
ring-opening, leading to singlet state antiaromaticity alleviation. On the other hand,
7a
(
T1
) in the
T
1
state was a minimum on the T
1
PES; its geometry met the aromaticity criterion of bond length
equalization, and the spin density was uniformly distributed over the cyclopentadienyl fragment
(Figure S12). Interestingly, it was 2.9 kcal/mol lower in energy than
7a
(
S0
), similar to the parent
cyclopentadienyl cation which has a triplet ground state [
58
–
61
]. The ring-opening of
7a
(
T1
) to
7b
(
T1
)
was endergonic by 3.8 kcal/mol, opposite to the ring-opening of
1a
(
T1
) to
1b
(
T1
) which was exergonic
by 17.5 kcal/mol. The reason why
1a
(
T1
) does not behave similar to
7a
(
T1
) and
3a
(
T1
) could be
explained by the ring-strain in the CBD ring.
With regard to
9a
(
T1
), it was merely 4.2 kcal/mol higher in energy than
9a
(
S0
), and its geometry
indicated a completely delocalized cycloheptatrienyl anion. This delocalization of the triplet biradical
character was also confirmed through its spin density (Figure S12). The ring-opening of
9a
(
T1
) to
9b
(
T1
) was energetically unfavorable, yet, not equally unfavorable as the ring-opening of
3a
(
T1
) to
3b
(
T1
). Thus, when going to gradually larger annulenes the SCB ring-opening energies in the T
1
state
were
−
17.5 (
1
), 3.8 (
7
), 12.2 (
9
), and 22.2 (
3
) kcal/mol, respectively. i.e., only the most ring-strained
compound (
1
) displayed an exergonic reaction energy. The activation energies in the T
1
state also
increased gradually and they were 9.5 (
1
), 21.7 (
7
), 24.6 (
9
) and 31.6 (
3
) kcal/mol, respectively. Hence,
the SCB ring, when fused with [4n]annulenes will in general not open in the T
1
state, a feature
that stems from T
1
aromaticity. When an SCB ring is attached to an annulene ring, the absence of
a photochemical ring-opening should therefore indicate T
1
aromaticity. Only when ring-strain is high,
as in 1a(T1), will T1state ring-opening occur in such species.
Figure 9.
Free energy changes in 5- and 7-membered annulenyl cations and anions fused with SCB
rings upon ring-openings at (U)B3LYP/6-311G(d,p) level. Compound
7a
(
S0
) is not a minimum in
the S
0
state. The black energy levels represent S
0
and those of red indicate T
1
state and energies for
transition states are given in bold.
Inorganics 2017,5, 91 11 of 16
Compound
8
showed the opposite behavior to that found for
7
and
9
. Structure
8a
(
T1
) was highly
skewed and its ring-opening to 8b(T1) was exergonic by 17.6 kcal/mol. Also, the ring-opened 8b(T1)
isomer was planar and had a CC bond delocalized structure. Thus,
8
having a [4n+ 2]annulene
moiety, displayed similar characteristics as
2
. Yet, the smaller the ring, the higher the exergonicity
of the ring-opening, explained by relief of ring-strain in addition to the relief of T
1
antiaromaticity.
Taken together, the T
1
state ring-opening reactions were markedly uphill for compounds
3
,
7
, and
9
,
and downhill for
2
and
8
. Compounds
3
,
7
, and
9
showed T
1
aromaticity similar to [4n]annulenes,
while compounds 2 and 8 showed T1 antiaromaticity analogous to [4n+ 2]annulenes, suggesting that
loss of T1 aromaticity was observed in ring-openings of [4n]annulenes while T1 antiaromaticity of
[4n+ 2]annulenes is alleviated through such reactions. Finally, the T1 state activation energies for SCB
rings of the T1 aromatic compounds 3, 7, and 9 were higher than those of the non-aromatic reference
compounds
4
–
6
(Figure 10), allowing the SCB ring to function as a T
1
state aromaticity indicator.
For the (4n+ 2)
π
-electron annulenoSCBs, the activation energies were similar or lower than those of
the nonaromatic references. Overall, the height of the activation barriers in the T
1
state were, to a
significant extent, correlated with the reaction energies (R
2
= 0.762, Figure S40), in accordance with the
Bell–Evans–Polanyi principle. This correlation was even stronger for the S0state (R2= 0.872).
Figure 10.
Activation free energies (kcal/mol) of all compounds
1
–
9
at the (U)B3LYP/6-311G(d,p) level
in the S0(unfilled circles) and T1(filled circles) states.
2.4. Polycyclic Structural Units Fused to SCB
In order to test the generality of the hypothesis, the reaction and activation energies for the
ring-opening of an SCB ring when fused to polycyclic moieties (10 and 12
π
-electrons) were also
explored (Scheme S1). With 10
π
-electrons, naphthalene is T
1
antiaromatic, and we found that the SCB
ring-openings of all three isomers of naphtho-SCB (
10a
–
10c
) were exergonic, in line with our hypothesis.
However, the activation energy for
10a
(23.3 kcal/mol) is significantly higher than observed for the
cPr-naphthalenes previously studied (8–11 kcal/mol) [
34
], suggesting that the SCB ring will remain
closed when fused to naphthalene. With regard to biphenylene, a 12
π
-electron compound which is T
1
state Baird-aromatic, the SCB ring-opening energies of
11a
–
11c
were modestly exergonic. Moreover,
the activation barrier for SCB ring-opening was nearly the same as that of naphthalene (24.2 kcal/mol).
Yet, it should be noted that polycyclic systems are more complex than monocycles because the SCB
ring-opened products can adopt aromaticity in some of the rings leading to stabilization, a feature
already observed for the ring-openings of the corresponding cPr substituted systems. Clearly, the SCB
ring should remain closed for T
1
aromatic polycyclic compounds, however, it may also not open for
polycyclic T1antiaromatic species, leading to limitations of its usage.
Inorganics 2017,5, 91 12 of 16
3. Computational Methods
All calculations were performed with Gaussian 09 revision D.01 [
62
]. The structures were
optimized at the (U)B3LYP and (U)OLYP density functional theory levels [
63
–
66
], with the
6–311G(d,p) basis set [
67
,
68
]. Frequency calculations were carried out at the same level to confirm
stationary points with real frequencies. Single-point energy calculations were performed at the
(U)CCSD(T)/6-311G(d,p)//(U)B3LYP/6-311G(d,p) level and thermal corrections at the B3LYP level
were added to get the free energies. Structural, magnetic and energetic indices were used to assess
the extent of aromaticity [
69
]. The harmonic oscillator model of aromaticity (HOMA) [
70
] values were
calculated at the (U)B3LYP/6-311G(d,p) level. Positive values approaching 1.0 correspond to aromatic
compounds, negative values to antiaromatic compounds, and values close to zero indicate nonaromatic
compounds. Nucleus independent chemical shift (NICS) scans along an axis perpendicular (z-axis) to
the ring planes were generated with the Aroma package 1.0 [
71
–
73
], using the Gauge-Independent
Atomic Orbital (GIAO) method [
74
] at the GIAO-(U)B3LYP/6-311+G(d,p)//(U)B3LYP/6-311G(d,p)
level. Scans were performed starting at the centre of the annulene to 5.0 Å above the ring plane
with increments of 0.1 Å. For aromatic compounds, the out-of-plane components show relatively
deep minima. For non-aromatic compounds, the values close to the molecular plane are positive,
decreases asymptotically and approach zero as the distance is increased. The antiaromatic compounds
display high positive values for the out-of-plane components which go to zero with increasing distance.
The anisotropy of the induced current density (ACID) calculations [
75
,
76
] were used to analyze the
ring-currents with the CGST method [
77
] at (U)B3LYP/6-311+G(d,p) level with AICD 2.0.0 software
package. The ACID plots were generated using (NMR = CGST IOp(10/93 = 1) and ultrafine grid
(integral = grid = ultrafine). Clock-wise ring-currents indicates aromaticity and counter clock-wise
ring-currents indicates antiaromaticity. Isomerization stabilization energies (ISE) [
32
,
33
,
57
] were
calculated at the (U)B3LYP/6-311G(d,p) level.
4. Conclusions
We have shown that the ring-opening ability of the SCB ring in the T
1
state can be used to sense for
T
1
aromaticity of a [4n]annulene to which it is fused, as its ring-opening disrupts the T
1
aromaticity of
the [4n]annulene, an unfavorable (endergonic) process. Conversely, it should open regardless if fused
to a T
1
non-aromatic or T
1
antiaromatic ring. By usage of a variety of (anti)aromaticity indices, we link
the shapes of the T
1
PES to changes in T
1
(anti)aromaticity. Consequently, the SCB ring could be used
as a T
1
aromaticity probe, in contrast to the all-carbon cyclobutene or disilacyclobutene rings which
are either too photoresistent or too labile. Moreover, as the silacyclobutene ring when fused does not
π
-conjugate with the annulene, it has a benefit when compared to strained four-membered rings with
Group 15 and 16 elements. The SCB ring also has a benefit over the cPr group examined earlier by us in
the context of excited state aromaticity indicators [
34
] because the transient intermediate formed upon
SCB ring-opening, in contrast to the ring-opened cPr intermediate, is easily trapped by alcohols to
yield photostable silylethers. Yet, the SCB ring is likely of limited applicability in polycyclic systems as
it may remain closed regardless of whether the ring system is T
1
aromatic or T
1
antiaromatic. Still, our
study can be interesting from an applications perspective as it reveals situations when the SCB ring as
a part in a larger molecule could lead to photoinstability of compounds used for various applications
in organic electronics [78].
Supplementary Materials:
The following are available online at www.mdpi.com/2304-6740/5/4/91/s1, NICS
scans, ACID plots, geometries, spin densities, and Cartesian Coordinates.
Acknowledgments:
We thank the EXPERTS III (Erasmus Mundus Action II program) and the Swedish Research
Council (VR) for financial support. We also thank the SNIC and UPPMAX for generous allotment of computer time.
Author Contributions:
Henrik Ottosson conceived the project and Henrik Ottosson and Rabia Ayub designed
the project. Rabia Ayub performed most of the Quantum Chemical calculations. Kjell Jorner performed the
calculations of the transition states. Rabia Ayub, Kjell Jorner and Henrik Ottosson co-wrote the manuscript.
Inorganics 2017,5, 91 13 of 16
Conflicts of Interest: The authors declare no conflict of interest.
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