Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches

Article abstract of DOI:10.1021/jacs.0c06327

The concepts of excited-state aromaticity and antiaromaticity have in recent years with increasing frequency been invoked to rationalize the photochemistry of cyclic conjugated organic compounds, with the long-term goal of using these concepts to improve

Full text of DOI:10.1021/jacs.0c06327

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Article
Photoinduced Changes in Aromaticity Facilitate
Electrocyclization of Dithienylbenzene Switches
Baswanth Oruganti, Péter Pál Kalapos, VARADA BHARGAV, Gábor London, and Bo Durbeej
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c06327 • Publication Date (Web): 15 Jul 2020
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Photoinduced Changes in Aromaticity Facilitate Electrocyclization of
Dithienylbenzene Switches
,†
‡
§
,‡
,#
Baswanth Oruganti,* Péter Pál Kalapos, Varada Bhargav, Gábor London* and Bo Durbeej*
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†Department of Chemistry and Biomedical Sciences, Faculty of Health and Life Sciences, Linnaeus University, SE-
5041 Kalmar, Sweden
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‡MTA-TTK “Lendület” Functional Organic Materials Research Group, Institute of Organic Chemistry, Research
Centre for Natural Sciences, 1117 Budapest, Hungary
§Department of Chemistry, GITAM Institute of Science, GITAM (deemed to be University), Visakhapatnam-530045,
Andhra Pradesh, India
#Division of Theoretical Chemistry, IFM, Linköping University, SE-58183 Linköping, Sweden
–
––––––––––––––––––––––
Corresponding Authors
E-mail: baswanth.oruganti@lnu.se (B.O.)
*E-mail: london.gabor@ttk.hu (G.L.)
E-mail: bodur@ifm.liu.se (B.D.)
*
*
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ABSTRACT
The concepts of excited-state aromaticity and anti-aromaticity have in recent years with increasing
frequency been invoked to rationalize the photochemistry of cyclic conjugated organic
compounds, with the long-term goal of using these concepts to improve the reactivities of such
compounds toward different photochemical transformations. In this regard, it is of particular
interest to assess how the presence of a benzene motif affects photochemical reactivity, as benzene
is well known to completely change its aromatic character in its lowest excited states. Here, we
investigate how a benzene motif influences the photoinduced electrocyclization of
dithienylethenes, a major class of molecular switches. Specifically, we report on the synthesis of
a dithienylbenzene switch where the typical non-aromatic, ethene-like motif bridging the two
thienyl units is replaced by a benzene motif, and show that this compound undergoes
electrocyclization upon irradiation with UV-light. Furthermore, through a detailed quantum
chemical analysis, we demonstrate that the electrocyclization is driven jointly and synergistically
by the loss of aromaticity in this motif from the formation of a reactive, anti-aromatic excited state
during the initial photoexcitation, and by the subsequent relief of this anti-aromaticity as the
reaction progresses from the Franck-Condon region. Overall, we conclude that photoinduced
changes in aromaticity facilitate the electrocyclization of dithienylbenzene switches.
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1
. INTRODUCTION
The concept of aromaticity is widely used in organic chemistry to predict and rationalize structures
and reactivities of cyclic conjugated molecules residing in their electronic ground state. It was first
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applied to electronic excited states by Dewar1 and Zimmerman2
⁠ ⁠ in the 1960s to explain the
formation of four-membered rings in photochemical electrocyclization reactions. These authors
proposed that such reactions proceed through aromatic transition structures, which are also known
to mediate thermal electrocyclization,1–3 sigmatropic rearrangement,3,4 non-pericyclic ring-
closure3,5 and carbonyl-olefin metathesis reactions. Subsequently, Baird
molecular orbital theory calculations to formulate rules for aromaticity and anti-aromaticity in the
lowest triplet excited state (T ) of cyclic conjugated hydrocarbons. These rules, which are the
6
7
⁠
used semi-empirical
1
reverse of Huckel’s rules for the ground state, assert that 4n+2 and 4n π-electron systems are
8
respectively anti-aromatic and aromatic in T
1
. As argued by Aihara in 1978, it follows from
Baird’s rules that reactions of benzene that are energetically hindered in the ground state due to a
loss of aromaticity, are favored in T due to a loss of anti-aromaticity. However, unlike
consequences of Huckel’s rules, this implication of Baird’s rules was not put to immediate test.
Much later, in 2008,9,10 Karadakov
demonstrated using multi-configurational quantum
chemical calculations that Baird’s rules apply also to the lowest singlet excited state (S ) of
cyclobutadiene,9 benzene9 and cyclooctatetraene.10
This triggered a renewed interest in excited-
state aromaticity (ESA), both in terms of obtaining spectroscopic evidence
applying it to the design of novel synthetic protocols.15–17 Furthermore, this trend led to the
1
⁠
1
⁠
⁠
⁠
for the concept11–14 and
⁠
discovery of ways of tuning ESA through steric and electronic substituent effects,18
⁠
and to the
development of strategies for using ESA to modulate double-bond photoisomerization,1
9–22
⁠
proton-transfer,2
photodissociation30
In a recent review2
1,23–25
⁠
electrocyclization,
26
⁠
conformational-planarization,
27–29
and
⁠
reactions.
1
⁠
outlining how ESA may improve photochemical reactivity, two broad
strategies were discussed, as illustrated in Scheme 1. The first involves utilizing a gain of anti-
aromaticity (or a loss of aromaticity) from the initial electronic excitation,23 and the second
involves exploiting a gain of aromaticity during the subsequent geometric relaxation from the
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vertically excited Franck-Condon (FC) point.20,22 Interestingly, however, a recent investigation
26
⁠
noted that photoinduced changes in aromaticity need not always influence photochemical
reactivity in a positive direction. Specifically, through transient absorption spectroscopy and time-
dependent density functional theory (TD-DFT) studies of dihydroazulene-vinylheptafulvene
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photoswitches operated through electrocyclization reactions,⁠ it was found that the loss of
aromaticity in a benzene motif from the initial electronic excitation hampers the photochemical
reactivity by increasing an already existing barrier along the excited-state potential energy surface
26
(PES). In this light, it is worthwhile to investigate whether a similar conclusion is applicable also
to other types of photoswitches utilizing electrocyclization reactions and featuring a benzene motif.
Scheme 1. (a) Facilitating an Excited-State Proton Transfer Reaction through a Gain of Anti-
2
3
Aromaticity in the Excited State. (b) Facilitating a Z/E Photoisomerization Reaction through a
Gain of Aromaticity in the Excited State.20,22
To this end, we here consider dithienylethene switches, which are one of the most
extensively studied class of electrocyclization-based photoswitches,31–39
⁠w ith applications in
photopharmacology,4
0,41
⁠
molecular electronics,34,38,42,43 and in the design of complex functional
materials.4
⁠
4–46
Specifically, we report on the synthesis and the experimental and computational
characterization of a dithienylbenzene switch where the typical non-aromatic, ethene-like motif31–
3
9
bridging the two thienyl units, is replaced by an aromatic benzene motif. The corresponding
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switch in its isomeric open (1a) and closed (1b) forms, and in a closed-oxidized form (1c), are
shown in Scheme 2.
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Scheme 2. Chemical Structures of the Open (1a), Closed (1b) and Closed-Oxidized (1c) Forms of
the Dithienylbenzene Switch Studied in this Work.
Photoswitches like 1a with an aromatic benzene π-linker have a potential key advantage
over those with a non-aromatic linker in that the energy stored in the ring-closed isomer 1b is
generally larger, because of the transformation of the benzene motif in 1a into a non-aromatic
cyclohexadiene motif in 1b. This makes these switches interesting candidates as molecular solar
thermal energy (MOST) storage systems.47–50 However, a recent study
⁠
51
⁠ on dithienylethene
switches with pyrazine-based aromatic π-linkers predicted the photocyclization of such switches
to be hindered by the energetic expense associated with the ensuing loss of ground-state
aromaticity. Although reached systematically through absorption spectroscopy and
crystallographic measurements, and with support from DFT computations,51
⁠t his prediction seems
to us somewhat simplistic. In fact, it relies essentially on the assumption that the ground-state
aromaticity of the π-linker remains unchanged in the excited state. However, following the
foregoing discussion of ESA based on Baird’s rules7
⁠
and their extensions,
⁠
this assumption is
not valid for a switch featuring a benzene π-linker. Furthermore, it is well known that the ground-
state aromaticity of the thienyl units does not impede the photocyclization of dithienylethene
switches.34,37,38 Therefore, it seems natural to expect the same to hold true for the ground-state
aromaticity of the π-linker. It should also be mentioned that Kitagawa and co-workers53
⁠ recently
reported spectroscopic evidence for photocyclization of a fluorinated derivative of 1a, in the form
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of a large absorption red-shift upon UV irradiation. As pointed out by these authors,53
⁠ the non-
detection of the ring-closed isomer in previous studies of such photoswitches is not necessarily an
indication that photocyclization does not occur, but is rather a consequence of the fact that
formation of the ring-closed isomer is immediately followed by fast thermal cycloreversion (that
restores the ground-state aromaticity of the π-linker).
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In this work, the potential problem with detecting the ring-closed isomer 1b is bypassed by
allowing, as shown in Scheme 2, for its subsequent oxidation to the more stable product 1c, which
is isolated and characterized by means of UV-Vis absorption and NMR spectroscopy. This
oxidation is facilitated by the presence of hydrogens instead of alkyl groups at the ring-closing
C11 and C13 positions. In particular, this work presents detailed computational evidence
supporting the idea that photoinduced changes in the aromaticity of the benzene motif of
dithienylbenzene switch 1a exerts a distinctly positive influence on the photocyclization reactivity
of this compound.
2
2
. RESULTS AND DISCUSSION
1
.1. Experimental UV-Vis Absorption and H NMR Spectra. The UV-Vis absorption spectra
of 1a shown in Figure 1 display high-energy bands and do not exhibit any solvent dependence,
which is further corroborated by spectra recorded in additional solvents given in Figure S1 of the
Supporting Information (SI). Upon irradiation of 1a in MeCN with UV-light of 254 nm, no
apparent color change was observed. However, as illustrated in Figure 2, new absorption bands
appeared in the near-UV region. Prolonged irradiation of the sample produced clear isosbestic
points at 229, 271 and 279 nm, indicating the formation of a single, well-defined product. This
photochemical transformation was found irreversible and, as can be inferred from Figures S2–S4
of the SI, quite insensitive to the nature of the solvent.
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Figure 1. UV-Vis absorption spectra of 1a in different solvents.
Figure 2. Irradiation of 1a in MeCN solution at 254 nm.
The gradual change of the initial UV-Vis absorption spectrum of 1a in Figure 2 upon
irradiation at 254 nm suggests that the high-energy UV-light induces a 6-electron
electrocyclization of this compound. The resulting ring-closed product 1b is expected to undergo
either of two reactions: thermal cycloreversion back to the parent species (1a), which is favored
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by the regeneration of the three aromatic rings, or oxidation by atmospheric O to yield a highly
2
stable, aromatic 18 π-electron compound (1c) and H
2
O. In order to gain structural insight into
1
which of these two reactions is more likely, the photochemical transformation was followed by H
NMR spectroscopy. The results are presented in Figure 3 and in Figures S6–S8 of the SI. As can
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be seen from Figures 3 and S6, upon irradiation of a solution of 1a in CD CN, a new set of peaks
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appears in the H NMR spectrum, upfield to the signals before irradiation, while the intensities of
the original signals are reduced. Gratifyingly, based on an analysis of the new peaks (see Figure
S7), it was found that they correspond to 1c. Furthermore, consistent with this interpretation, water
is the only observed by-product (see Figure S8). Although 1c has previously been prepared by
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alternative methods, its photochemical generation has not been reported before.
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Figure 3. Irradiation of 1a in CD
3
CN at 254 nm, followed by H NMR spectroscopy. (a) H NMR
1
spectrum before irradiation. (b) H NMR spectrum after 90 minutes.
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Besides synthesizing 1a and generating 1c photochemically, we also synthesized a tetra-
methylated derivative of 1a bearing methyl substituents instead of hydrogens both at the “outer”
C9 and C15 positions and the ring-closing C11 and C13 positions. For this compound, which has
not been prepared earlier and is denoted Me -1a in Scheme 3, no apparent change of its absorption
4
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spectrum (see Figure S5 of the SI) was observed upon irradiation with UV-light. This may indicate
that the ring-closed species, being relatively insusceptible to oxidation by the presence of the C11
and C13 methyl groups, undergoes a fast thermal cycloreversion that precludes its spectroscopic
53
detection. We will return to this issue in Section 2.2.
Scheme 3. Chemical Structures of Additional Dithienylbenzene Switches Studied in this Work.
2.2. Thermal Electrocyclization and Oxidation. The first task for the computational part of this
work is to clarify how probable or improbable the thermal electrocyclization of 1a is. Such an
assessment will provide a frame of reference for the subsequent modeling of the photoinduced
electrocyclization process. Before this assessment, however, it should be noted that previous
studies on diarylethene switches have observed two possible conformations for the open isomer: a
parallel conformation with the aryl rings in mirror symmetry, and an anti-parallel conformation
with the aryl rings in C
symmetry.33,34,37 Analogously, the open isomer 1a also exhibits parallel
2
and anti-parallel conformations, shown in Figure 4 and hereafter denoted 1a-p and 1a-ap,
respectively. As can be seen in Table 1, each of the four DFT methods employed for the
55
calculations, which were performed using an SMD continuum solvation model representation of
a MeCN solvent, yields free energies of 1a-p and 1a-ap that are very similar. Furthermore (see
Table S1 of the SI), the predicted free-energy barriers for the thermal interconversion between
-1
these conformations are small, only 11–25 kJ mol , which suggest very fast interconversion
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processes. Overall, these findings are consistent with previously reported NMR data on
dithienylethene switches indicating that the two conformations are equally populated under
ambient conditions.33,34,38 Moreover, these studies showed that the parallel conformation does not
undergo photocyclization.33,34 For this reason, only the 1a-ap conformation was considered in the
present modeling of the thermal electrocyclization and photocyclization reactions of 1a.
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Figure 4. Conformations of 1a, 1b and 1c considered in the calculations of this work.
Table 1. Calculated Relative Free Energies for the 1a/1b/1c System (kJ mol-1)a
methodb
1a-p
70.6
61.6
53.2
55.4
1a-ap
72.1
64.0
55.0
56.8
1b
1c+H
2
B3LYP
183.3
186.6
177.9
177.2
0
0
0
0
M06-2X
wB97X-D
CAM-B3LYP
aEnergies given relative to the total free energy of the 1c+H
system. Geometry
b
2
optimizations carried out using the cc-pVDZ basis set and singlepoint calculations
carried out using the cc-pVTZ basis set, employing in both cases an SMD
description of a MeCN solvent.
While the free-energy barriers for the 1a-p « 1a-ap interconversion are small, Table 2
shows that each of the four DFT methods predicts a barrier for the 1a-ap ® 1b thermal
-1
electrocyclization of such magnitude (183–197 kJ mol ) that the reaction cannot be induced
thermally under reasonable conditions. This is in accordance with the reaction being a Woodward-
Hoffmann forbidden reaction. It is also notable that the electrocyclization transition structure
(
henceforth denoted TS) calculated by each method has substantial diradical character, which is
another key characteristic of Woodward-Hoffman forbidden reactions56–59 and is revealed by the
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non-zero (~0.7) expectation value of the spin-squared operator and the associated spin densities
given in Table S2 of the SI.
Table 2. Calculated Free-Energy Barriers for the 1a-ap ® 1b Thermal
Electrocyclization and the 1b ® 1a-ap Thermal Cycloreversion of the Parent
1
1
1
1
1
1
1
1
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*
-1 a
Switch 1a and Its Methylated Derivatives Me
4
-1a, Me
2
-1a and Me -1a (kJ mol )
2
switch
method
electrocyclization
182.6
cycloreversion
70.5
1
1
a
a
B3LYP
M06-2X
197.0
76.0
1a
wB97X-D
CAM-B3LYP
wB97X-D
wB97X-D
wB97X-D
190.0
66.6
1a
190.3
68.8
Me
Me
Me
4
2
2
-1a
-1a
210.0
72.1
184.7
77.6
*
-1a
216.9
60.1
aAll calculations carried out using the aug-cc-pVTZ basis set and an SMD description
of a MeCN solvent.
As for the calculated relative free energies of 1a-ap and 1b, these predict that the overall
-1
electrocyclization process is endergonic by 111–123 kJ mol (see Table 1), which is readily
explained by the loss of aromaticity in the π-linker. Should the electrocyclization be inducible by
sunlight, this large endergonicity would be an attractive feature of systems of this kind for potential
MOST applications.47–50 Such applications would also require that the free-energy barriers of 67–
-1
7
6 kJ mol (see Table 2) that the calculations ascribe to the 1b ® 1a-ap thermal cycloreversion
can be increased, so as to ensure that the absorbed solar energy can be stored for a long time. In
order to briefly explore this possibility, cycloreversion barriers were also calculated for three
methylated derivatives of the parent switch (see Scheme 3), including the aforementioned Me
4
-1a
compound and two di-methylated compounds differing with respect to whether the methyls occur
at the “outer” C9 and C15 positions (Me
2
-1a) or the ring-closing C11 and C13 positions (Me -
2
*
1a ). Incorporated into Table 2, the results of these calculations suggest that the C9 and C15
-1
methyls increase the barrier by 11 kJ mol , whereas the C11 and C13 methyls decrease it by 7 kJ
-1
mol . Accordingly, in Me
4
-1a, the two effects add up to a slight increase of the barrier relative to
the parent switch. Interestingly, this indicates that the absence noted in Section 2.1 of a change in
the experimental absorption spectrum of Me
4
-1a upon irradiation with UV-light, is not related to
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the influence of the C11 and C13 methyls on the cycloreversion, but rather to their ability to
prevent oxidation of the ring-closed species.53
Turning thus to the oxidation of 1b by atmospheric O
2
, which yields 1c and H O, the
2
calculated free energies in Table S3 of the SI suggest that this reaction proceeds with an
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-1
astoundingly large exergonicity (DG
2
) of 400–420 kJ mol . Thereby, the experimental detection
of 1c described in Section 2.1 can be rationalized by the fact that the driving force for oxidation is
-1
much larger than the driving force for thermal cycloreversion (111–123 kJ mol ). Moreover, the
driving force for oxidation can be partitioned into two components: the first (DG ) being associated
with the formation of H O from H and O , and the second (DG - DG ) reflecting the gain in
3
2
2
2
2
3
aromaticity upon formation of the central six-membered ring that fuses the two thienyls in 1c (see
Scheme 2). As can be seen from Table S3, these two components amount to 217–233 and 177–
-1
1
87 kJ mol , respectively.
2
.3. Calculated UV-Vis Absorption Spectra. The second task for the computational part of this
work is to assess how well the TD-DFT methodology adopted for the modeling of the
photocyclization of 1a reproduces the experimental UV-Vis absorption spectrum of this compound
in MeCN solution. To this end, the absorption of 1a-p and 1a-ap was calculated with the same four
DFT methods employed for the modeling of the thermal processes discussed in Section 2.2. Again,
5
5
an SMD description of the MeCN solvent was used, in combination with the cc-pVTZ basis set.
For each species, vertical excitation energies from the ground state (S ) to the ten lowest singlet
0
excited states and the associated oscillator strengths were calculated to derive – through
convolution with Gaussian functions with the full width at half maximum set to 0.5 eV – the UV-
Vis absorption spectra shown in Figure 5a. In addition, “total” spectra of 1a-p and 1a-ap were
derived by assuming the individual contributions of 1a-p and 1a-ap to be quantifiable in terms of
their Boltzmann weights. The corresponding spectra are given in Figure 5b, which also includes
the experimental spectrum of 1a for comparison.
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Figure 5. (a) UV-Vis absorption spectra of 1a-p and 1a-ap in MeCN solution calculated with
different DFT methods. (b) Boltzmann-weighted UV-Vis absorption spectra of 1a-p and 1a-ap in
MeCN solution calculated with different DFT methods and compared with the experimental
spectrum of 1a.
As can be inferred from the excitation energies and oscillator strengths summarized in
Table S4 of the SI, the high-energy band in the spectra of both 1a-p and 1a-ap in Figure 5a
calculated with the M06-2X, ωB97X-D and CAM-B3LYP methods is dominated by the S
transition, which occurs at ~5.5 eV/~225 nm. Furthermore, the absorption shoulder at ~250 nm
predicted by these methods can be assigned to the S ® S ππ* transition, which is located at 4.84–
.09 eV/244–256 nm (at the B3LYP level, this transition results in a clearly identifiable lower-
energy band). From Figure S13 of the SI, it can be seen that the frontier π-molecular orbitals
involved in the S ® S transition are primarily localized on the benzene π-linker, with negligible
0
® S
3
0
1
5
0
1
contributions from the atoms of the two thienyls. This suggests that it is indeed this moiety, and
not the thienyls, whose aromaticity may influence the photocyclization of 1a.
Turning to the Boltzmann-weighted total spectra in Figure 5b, especially M06-2X, ωB97X-
D and CAM-B3LYP achieve good overall agreement with the experimental spectrum of 1a.
1
Furthermore, given that the H NMR analysis in Section 2.1 attributed the appearance in Figure 2
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of new near-UV absorption bands upon irradiation of 1a at 254 nm to the formation of 1c, it is also
gratifying that the lower-energy band in the calculated spectra of 1c (see Figure S14b of the SI) is
indeed red-shifted relative to those in the calculated spectra of 1a-p and 1a-ap (see Figure 5a).
Overall, then, we believe that the results in Figures 5 and S14b lend credence to the use of TD-
DFT for the modeling of the photocyclization of 1a, which we turn to next. Since 1a-ap, but not
1a-p, undergoes photocyclization,33,34 all calculations pertaining to compound 1a described
henceforth were carried out using strictly the 1a-ap species, which is hereafter referred to as simply
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1
a.
2.4. Photocyclization. The results from the modeling of the 1a ® 1b photocyclization in the S
1
state through static TD-DFT calculations with the ωB97X-D method are summarized in Figure 6,
which for comparison also includes results relevant for the symmetry-forbidden thermal
electrocyclization reaction. For example, as was already noted in Table 2, it can be seen that the
thermal reaction is prohibited by a very large barrier. Contrarily, following light absorption and
population of the vertically excited S
the C11–C13 (see Scheme 2) reaction coordinate proceeds in a completely barrierless fashion,
whereby the S and S states approach degeneracy (see Figure 6b) in the region of a presumed
/S conical intersection (CI) seam. The excited-state evolution toward this seam, which affords
a decay channel to the S state, is driven by a large decrease in S energy relative to the FC point
at C11–C13 = 3.29 Å. For example, at C11–C13 = 1.80 Å, the decrease in S energy amounts to
1.70 eV. In order to corroborate this result in light of potential problems in the treatment of static
1
FC point of 1a, Figure 6a shows that photocyclization along
1
0
S
1
0
0
1
1
6
0
correlation effects by TD-DFT, singlepoint calculations carried out with the complete active
space second-order perturbation theory (CASPT2) method61,62 yield a value of very similar
magnitude (1.74 eV, see Table S6 of the SI). Furthermore, based on the observation that 1b is
produced by a ωB97X-D S
0
geometry optimization started in the vicinity of the presumed S
1
/S
0
CI seam at C11–C13 = 1.80 Å, it also seems plausible that 1b is indeed the S
photocyclization of 1a.
0
photoproduct of the
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Figure 6. (a) Calculated S
geometry along the thermal S
a and S energies at the corresponding geometries.
1
photocyclization path of 1a and relative energies of 1a, 1b and the TS
0
electrocyclization path. (b). Calculated S photocyclization path of
1
1
0
It should be noted that even though the calculations summarized in Figure 6 suggest that
the 1a ® 1b photocyclization is a favorable reaction, the underlying procedure to perform S
1
geometry optimizations along a pre-defined C11–C13 reaction coordinate neither accounts for
possible competition from other reactions, nor provides an estimate of the time scale for the overall
photocyclization process. In order to address these issues, we also carried out non-adiabatic
molecular dynamics (NAMD) simulations63–67 with Tully’s fewest switches algorithm at the
level of B3LYP for the DFT and TD-DFT parts. These simulations, which describe the
simultaneous evolution of the electronic and nuclear degrees of freedom of the photoswitch, were
68
started from the S FC point of 1a and were run for maximally 200 fs with ten different initial
1
nuclear velocities generated randomly from a Maxwell-Boltzmann distribution at 300 K. The key
results are summarized in Figure 7.
As can be seen from Figure 7a, there is an apparent tendency for the excited-state dynamics
to allow the S
1
and S states to approach degeneracy, and to do so very quickly (within ~100 fs in
0
many trajectories). Furthermore, it is clear from the rapid decrease in C11–C13 distance observed
in all ten trajectories (see Figure 7b) that a key component of the dynamics is indeed ring-closing
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along the C11–C13 coordinate. In fact, within 200 fs, four out of ten trajectories oscillate around
a C11–C13 distance of 1.5 Å. Since all ten trajectories are evolving in the S state at that stage (see
0
Figure S15 of the SI), this means that 40% of the trajectories have completed the reaction and
formed the 1b photoproduct within 200 fs. Thus, the 1a ® 1b photocyclization appears to be very
fast and not much impeded by competing processes. Loosely, this 200-fs time scale is consistent
with findings reported in time-resolved spectroscopic studies of ring-closing in diarylethene
switches.69–72 Finally, on a technical note, complementary NAMD simulations reported in Figures
S16 and S17 of the SI performed with initial nuclear velocities generated at 500 K instead of 300
K, give results similar to those just described.
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9
0
Figure 7. Changes in S
1
and S energies (a) and C11–C13 bond length (b) during the ten NAMD
0
trajectories run for 1a.
2.5. Aromaticity Indices. Having obtained computational evidence that the 1a ® 1b
photocyclization is an ultrafast and efficient reaction, we now set out to investigate if this finding
can be rationalized in terms of photoinduced changes in aromaticity of the benzene π-linker. To
this end, aromaticity indices of four different types – geometric, electronic, magnetic and energetic
– were calculated for this moiety. Starting with the geometric harmonic oscillator model of
aromaticity (HOMA) index,73,74 which probes the deviation of carbon-carbon bond lengths from
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an ideal aromatic reference value for benzene, aromatic and non-aromatic systems usually exhibit
HOMA values close to 1 and 0, respectively.73–75 Presenting the HOMA values of 1a and 1b in
the S
along the S
geometries, Figure 8a unsurprisingly shows that 1a (0.96) and 1b (0.10) are typical S
and non-aromatic systems, respectively. However, more interestingly, as the photoexcited system
evolves from the S FC point of 1a, the HOMA values are much reduced (from 0.96 to 0.46–0.65),
0
state calculated from the corresponding ωB97X-D geometries, as well as the HOMA values
photocyclization path of 1a calculated based on the corresponding TD-ωB97X-D
aromatic
1
1
1
1
1
1
1
1
1
1
1
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8
9
0
0
1
which suggests that there is a loss of aromaticity in the S
1
state relative to the S state that might
0
contribute to the photocyclization reactivity of 1a.
Figure 8. Changes in HOMA (a) and SA values (b) for the benzene π-linker along the S
photocyclization path of 1a. Also shown are the HOMA and SA values for 1a, 1b and the TS
geometry along the thermal S electrocyclization path.
1
0
Continuing with the electronic Shannon aromaticity (SA) index,76,77 which is formulated
7
8
in the framework of Bader’s theory of atoms in molecules and measures the variation in electron
density at bond critical points, the analysis of these results in Figure 8b (obtained from S and S
0
1
electron densities calculated with ωB97X-D and TD-ωB97X-D, respectively) supports the same
conclusion as the analysis of the HOMA values in Figure 8a. Specifically, noting that aromatic
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systems typically show smaller SA values than their non-aromatic analogues because of their more
uniform distribution of electron density,76,77 the SA values are increased from 0.0001 for 1a in the
S
0
state to 0.0014–0.0021 along the S
1
photocyclization path.
Despite the agreement between the HOMA and SA values, it is important to point out that
0
1
2
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0
these indices do not distinguish between non-aromaticity and anti-aromaticity. Moreover, as can
be inferred from Figure 8, they are not able to probe changes in aromaticity during the
photoexcitation from the S
0
state of 1a to the S
1
FC point. In order to circumvent these limitations,
and S states of 1a was assessed magnetically
the aromaticity of the benzene π-linker in the S
0
1
through calculation of nucleus-independent chemical shift (NICS) indices79,80 by means of a NICS-
scan procedure.81,82 These indices probe ring currents induced by circulating π-electrons, and were
calculated from wavefunctions obtained with the complete active space self-consistent field
83
(
CASSCF) method and using gauge-including atomic orbitals.
The results of this investigation are summarized in Figure 9. We see that the S state of 1a
0
shows negative NICS values between -11 and -22 ppm, indicating diatropic ring currents or
79
aromaticity. At the S
1
FC point, on the other hand, the NICS values are positive (23–70 ppm),
suggesting paratropic ring currents or anti-aromaticity.79 In the CI region along the S
1
photocyclization path, in turn, the NICS values are close to zero, reflecting the absence of ring
currents or non-aromaticity.79 Accordingly, the overall picture that emerges from Figure 9
regarding changes in aromaticity during the course of the photocyclization of 1a, is that the π-
linker goes from being aromatic in the S
0
state to anti-aromatic at the S FC point, and then turns
1
non-aromatic in the CI region. Thereby, it appears that the photocyclization is driven by the loss
of aromaticity and the concurrent formation of a reactive, anti-aromatic excited state from the
initial photoexcitation, and by the subsequent relief of this anti-aromaticity as the reaction
progresses from the FC point. In other words, the reaction is facilitated by photoinduced changes
in aromaticity.
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Figure 9. NICSzz-scan values for the benzene π-linker in the S
These values are denoted 1a (S ) and 1a (S FC), respectively. Also shown are the NICSzz-scan
values for a structure in the CI region (at C11–C13 = 1.80 Å) along the S photocyclization path
of 1a, for the TS geometry along the thermal S electrocyclization path of 1a, and for the S state
of 1b. These values are denoted CI (S ), TS (S ) and 1b (S ), respectively.
0
state and at the S FC point of 1a.
1
0
1
1
0
0
1
0
0
In order to quantify in energetic terms the loss of aromaticity during photoexcitation and
the loss of anti-aromaticity during the excited-state evolution, two novel aromaticity indices were
introduced and calculated for a methylated (at the benzene motif) derivative of 1a and its non-
aromatic exocyclic methylene isomer. Denoted 2 and 2-ref, respectively, these compounds are
shown in Scheme 3. The indices in question, which are reminiscent of the isomerization
84
stabilization energy (ISE) aromaticity index, quantify the difference ΔVEE = VEE(2) – VEE(2-
ref) in the vertical S
FCRE(2) – FCRE(2-ref) in the S
energy is the decrease in S energy from the FC point to the minimum on the S
0
® S
1
excitation energies of 2 and 2-ref, and the difference ΔFCRE =
FC relaxation energies of 2 and 2-ref (the S FC relaxation
PES). Thereby, if
1
1
1
1
there is a distinct loss of aromaticity during the photoexcitation of 1a and similarly a pronounced
loss of anti-aromaticity during its subsequent excited-state evolution, one expects these indices to
be positive and large. Thus, it is encouraging to note from calculations presented in Table S7 of
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the SI that this is indeed the case, with ΔVEE and ΔFCRE amounting to 1.11 and 0.71 eV,
respectively. This finding is consistent with a previous study on cyclopentadienes and siloles
demonstrating that aromatic compounds have relatively larger vertical excitation energies than
their non-aromatic or anti-aromatic analogues.85
0
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Finally, we note that it is of interest for future studies to explore how functionalization of
the benzene motif with different electronic substituents might influence the photoinduced changes
in aromaticity. In this regard, the attachment of strongly electron-withdrawing groups to benzene
8
has previously been found to reverse the character of its T
1
state from anti-aromatic to aromatic.1
⁠
As a brief investigation of this issue, ΔVEE and ΔFCRE values were also calculated for a
derivative of 2 (denoted 3 in Scheme 3) featuring an electron-withdrawing nitro group at the
benzene motif. Interestingly, the ΔVEE and ΔFCRE values of 0.82 and 0.36 eV for this compound
given in Table S7 are noticeably smaller than those of 1.11 and 0.71 eV calculated for 2. Hence,
the nitro group appears to inhibit the photoinduced changes in aromaticity.
3
. CONCLUSIONS
1
We have synthesized dithienylbenzene 1a and reported H NMR evidence that this compound
undergoes electrocyclization upon irradiation with UV-light of 254 nm. Furthermore, through a
detailed computational analysis, we have found that the electrocyclization process is facilitated by
photoinduced changes in the aromaticity of the benzene motif of 1a. More specifically, there is a
loss of aromaticity during the initial photoexcitation that produces a reactive, anti-aromatic excited
state (NICS values change from distinctly negative to distinctly positive), and subsequently a relief
of this anti-aromaticity as the photoexcited system evolves from the FC point toward the CI seam
(
NICS values change from distinctly positive to ~zero). Through NAMD simulations, this
evolution is predicted to be very fast (~100 fs) and to be followed by excited-state decay and
complete electrocyclization within 200 fs in many (40%) of the simulated trajectories. Altogether,
the results of this work show that the concept of ESA holds substantial promise as a tool for
designing potent electrocyclization-based photoswitches within the framework of
dithienylethenes. In future research, we plan to investigate whether this concept is fruitfully
applicable also to the design of photoswitches based on other reactions and frameworks.
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. EXPERIMENTAL AND COMPUTATIONAL DETAILS
.1. Synthesis. Dithienylbenzene 1a was synthesized in a single step via a two-fold Suzuki-
Miyaura cross-coupling of 1,2-dibromobenzene and 3-thienylboronic acid in excellent yield (see
Scheme 4). Full details are given in the SI.
1
1
1
1
1
1
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Scheme 4. Synthesis of 1a.
1
4
4
.2. UV-Vis Absorption and H NMR Spectroscopy. Full details are given in the SI.
.3. Quantum Chemical Calculations. Thermal and photochemical reactions were modeled
using DFT and TD-DFT,86–94 respectively. As for the former reactions, S
corresponding to 1a, 1b, 1c and the electrocyclization transition structure (TS) connecting 1a and
b were located by performing geometry optimizations using the B3LYP,95,96 M06-2X,97,98
stationary points
0
1
9
9
100
ωB97X-D and CAM-B3LYP hybrid density functionals in combination with the cc-pVDZ
basis set (in some cases also with the larger aug-cc-pVDZ basis set). For each structure optimized
at a specific level of theory, frequency calculations were carried out at that level to obtain Gibbs
free energies at room temperature, and to ensure that the structure in question is either a minimum
with real vibrational frequencies only (1a, 1b and 1c), or a first-order saddle point with one
imaginary vibrational frequency (TS). In order to refine the electronic contribution to the free
energies, the geometries obtained with a specific functional were subjected to singlepoint
calculations with that functional and the larger triple-ξ cc-pVTZ basis set. All calculations were
55
performed using an SMD continuum solvation model representation of a MeCN solvent.
As for the TD-DFT modeling of the 1a ® 1b photocyclization, in turn, this was done
within the Tamm-Dancoff approximation (TDA)101–104 to TD-DFT using exclusively the ωB97X-
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D functional. This choice was made in part because ωB97X-D is a range-separated hybrid
functional, which has proven a useful feature for the description of photoisomerization
reactions,105,106 and in part because ωB97X-D includes empirical 1/R atom-atom dispersion
corrections99,107,108 suitable for capturing weak intermolecular interactions between the two thienyl
6
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units. Starting from the ωB97X-D/cc-pVDZ S
0
geometry of 1a, whose C11–C13 distance is 3.29
FC point
Å, the S photocyclization path was computed by first locating the vertically excited S
1
1
through a singlepoint calculation with the cc-pVTZ basis set and the SMD (MeCN) model.
Subsequently, additional points along the path were obtained by performing a series of constrained
1
S geometry optimizations (again with the cc-pVDZ basis set) in which the C11–C13 distance was
varied between 3.20 and 1.80 Å in steps of 0.10 Å. In each optimization, all other geometric
degrees of freedom than the C11–C13 distance were allowed to relax. For computational
expedience, the optimizations were done in the gas phase, but were followed by singlepoint
calculations (again with the cc-pVTZ basis set) utilizing the SMD (MeCN) model. Finally, as a
complement to the TD-DFT modeling, singlepoint calculations along the S photocyclization path
1
were also carried out with the CASPT2 method,61,62 as further detailed in the SI.
4.4. NAMD Simulations. The NAMD simulations were done with the implementation of Tully’s
6
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64,109
fewest switches algorithm in TURBOMOLE
DFT and TD-DFT parts. The reason for using B3LYP, and not ωB97X-D, is that the latter method
is not implemented in TURBOMOLE. The simulations were started from the S FC point of 1a
at the B3LYP/cc-pVDZ level of theory for the
1
and were run for maximally 200 fs with two sets of ten different initial nuclear velocities generated
randomly from a Maxwell-Boltzmann distribution at 300 and 500 K, respectively. The exact
criteria used to enforce a hop between the S
1
and S states in the simulations are given in the SI.
0
4
.5. Aromaticity Indices and Software Used. Full details on the calculations of HOMA,73,74
SA76,77 and NICS79–82 indices for the benzene π-linker are given in the SI. Briefly, HOMA values
were calculated based on S and S geometries optimized at the ωB97X-D/cc-pVDZ and TD-
ωB97X-D/cc-pVDZ levels of theory, respectively. SA values, in turn, were derived from S and
electron densities computed at the ωB97X-D/cc-pVTZ and TD-ωB97X-D/cc-pVTZ levels of
theory, respectively. NICSZZ values,80 finally, were calculated following a NICS-scan
0
1
0
S
1
procedure81,82 using S
and S
wavefunctions obtained at the CASSCF/cc-pVDZ level of theory.
0
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More specifically, NICS values were calculated along the axis passing through the geometric
center of the π-linker at distances above the center ranging from 1.50 to 2.50 Å in steps of 0.10 Å.
This procedure is designed to avoid the undesirable arbitrariness associated with NICS-index
calculations that only consider one single point in space.81,82,110
1
1
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The calculations were done with the Gaussian 16,111 OpenMolcas 18.09,112 TURBOMOLE
7.4,109,113 Dalton2016.2114,115 and Multiwfn116 programs. Gaussian 16 was used for DFT and TD-
DFT calculations. OpenMolcas 18.09 was used for CASSCF and CASPT2 calculations.
TURBOMOLE 7.4 was used for NAMD simulations. Dalton2016.2 was used for NICS-index
calculations. Multiwfn was used for HOMA- and SA-index calculations.
ASSOCIATED CONTENT
Supporting Information
Description of general experimental procedures; description of the synthesis of compounds 1a
(
Scheme S1) and Me -1a (Scheme S2); complementary experimental UV-Vis absorption spectra
4
1
(Figures S1–S5); H NMR characterization of the photochemical ring-closing and oxidation of
compound 1a (Scheme S3 and Figures S6–S8); NMR spectra of the reported compounds (Figures
S9–S12); complementary computational details; complementary computational results (Figures
S13–S17 and Tables S1–S7); description of multimedia file; and Cartesian coordinates for
optimized S
0
and S geometries (PDF).
1
Multimedia file with an animation of a representative trajectory from the NAMD simulations
describing a successful 1a ® 1b photocyclization event (MP4).
AUTHOR INFORMATION
Corresponding Authors
Baswanth Oruganti – Department of Chemistry and Biomedical Sciences, Faculty of Health and
Life Sciences, Linnaeus University, SE-45041 Kalmar, Sweden
Email: baswanth.oruganti@lnu.se
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Gábor London – MTA-TTK “Lendület” Functional Organic Materials Research Group, Institute
of Organic Chemistry, Research Centre for Natural Sciences, 1117 Budapest, Hungary
Email: london.gabor@ttk.hu
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Bo Durbeej – Division of Theoretical Chemistry, IFM, Linköping University, SE-58183
Linköping, Sweden
E-mail: bodur@ifm.liu.se
Authors
Péter Pál Kalapos – MTA-TTK “Lendület” Functional Organic Materials Research Group,
Institute of Organic Chemistry, Research Centre for Natural Sciences, 1117 Budapest, Hungary
Varada Bhargav – Department of Chemistry, GITAM Institute of Science, GITAM (deemed to
be University), Visakhapatnam-530045, Andhra Pradesh, India
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
B.D. acknowledges financial support by the Olle Engkvist Foundation (grants 184-568 and 204-
0183) and the Swedish Research Council (grant 2019-03664), and grants of computing time at the
National Supercomputer Centre (NSC) in Linköping, Sweden. G.L. acknowledges financial
support by the Hungarian Academy of Sciences through the Lendület Program
(
LENDULET_2018_355) and the National Research, Development and Innovation Office,
Hungary (NKFIH Grant FK 123760).
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