Steric effects in hula-twist photoisomerization. 1,4-Dimethyl- and 2,3-dimethyl-1,4-diphenylbutadienes

Article abstract of DOI:10.1021/ja052312d

1,4-Dimethyl-1,4-diphenylbutadiene was shown to be able to execute regiospecific HT-1 photoisomerization around a methyl group when irradiated in a low-temperature organic glass, albeit at reduced efficiency. 2,3-Dimethyl-1,4-diphenylbutadiene exhibited a different type of steric effect, causing the E,E-isomer to undergo regiospecific HT-1 photoisomerization. Copyright

Full text of DOI:10.1021/ja052312d

Published on Web 06/09/2005
Steric Effects in Hula-Twist Photoisomerization. 1,4-Dimethyl- and
2
,3-Dimethyl-1,4-diphenylbutadienes
‡
,‡
†
,†
Lan-ying Yang, Robert S. H. Liu,* Natalie L. Wendt, and Jin Liu*
Department of Chemistry, UniVersity of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822, and
Department of Chemistry, Murray State UniVersity, Murray, Kentucky 42071
Received April 9, 2005; E-mail: rliu@gold.chem.hawaii.edu; jin.liu@murraystate.edu.
Hula-twist (HT) is a general mechanism of photoisomerization.1
It is a volume-conserving process of which a single C-H unit moves
2
across the plane of an unsaturated molecule. In contrast, the tradi-
tional one-bond-flip (OBF) mechanism turns over one-half of the
molecule:
The question whether the process is limited to a small C-H unit
Figure 1. Room temperature irradiation of isomers of 2. (a) Absorption
spectra of (E,E)-2 recorded during its irradiation (>310 nm, Corning O-54
filter) in EPA glass at 200 K; t ) 0, 2, 4, 6, 8, 10, and 12 s. (b) Difference
spectra (t - t0) of (E,E)-2. Insert: absorption changes (320 nm) versus
time. (c) Absorption spectra of (Z,E)-2 recorded during its irradiation (>310
nm, Corning O-54 filter) in EPA glass at 200 K; t ) 0, 2, 5, 8, 11, 15, 20,
25, and 30 s. (d) Difference spectra of (Z,E)-2. Insert: absorption changes
has not been answered, although the absence of conformational equili-
3
bration of 2,3-dimethyl-1,3-butadiene in argon matrix (in contrast
to the parent diene) has been interpreted as steric inhibition due to
increased size at the HT reaction center.1b However, the work was
carried out at a much lower temperature of 10 K (argon matrix); fur-
thermore, the compound simply could have opted for the unhindered
HT-1 process, thus leading to no detectable chemical changes. The
observed photoisomerization of 1,4-dideutero-2,3-dimethylbuta-
diene1b reflects this possibility. We now report steric effects in HT
isomerization of substituted diphenylbutadienes, addressing specif-
ically the issue of possible HT around a C-Me center.
(333 nm) versus time.
Recently, we reported the HT photoisomerization of isomers of
4
1
,4-diphenyl-1,3-butadiene (DPB), 1, in EPA glass, demonstrating
regioselective HT at center-1 (absence of the central s-cis product
expected of HT-2). This selectivity suggested that isomers of 1,4-
dimethyl-DPB, 2, and 2,3-dimethyl-DPB, 3, will be ideal for
probing possible HT around a methyl group. Results of such an
investigation are described below.
Figure 2. Low temperature irradiation of (Z,E)-2. (a) Absorption spectra
of (Z,E)-2 recorded during its irradiation (>310 nm, Corning O-54 filter)
in EPA glass at 78 K; t ) 0, 60, 300, 540, 900, 1380, 1920, 2520, and
3120 s. (b) Difference spectra. Insert: log of relative absorbance at 355
nm versus time of irradiation.
at 77 K, was unreactive upon irradiation with light >310 nm. This
feature is consistent with 1,2-diarylethenes.4
,7
Irradiation of (Z,E)-2 at 77 K led to a gradual change of the UV
absorption spectrum from a broad band to one with a resolved red-
shifted absorption band (Figure 2a), suggesting formation of the
E,E-isomer. It was a clean one-to-one conversion, as reflected by
the sharp isosbestic point at 313.5 nm. The changes are more clearly
Irradiation of an EPA (ether:isopentane:ethanol ) 5:5:2) solution
of either (E,E)- or (Z,E)-2 (200 K) readily produced a photosta-
5
0
revealed in the difference spectra (t - t ) (Figure 2b). Upon thawing
the glass medium and recooling to 77 K, the spectrum remained
tionary state mixture (Figure 1a,c). The presence of a sharp isos-
bestic point (especially in the difference spectra, Figure 1b,d)
suggests that only the E,E- and the Z,E-isomers are present in the
unchanged, thus indicating that a thermally stable photoproduct (the
4
E,E-isomer) was formed. Hence, same as the parent DPB (1), the
6
mixture (same as the parent DPB). In EPA glass at 77 K, the E,E-
photoisomerization must have taken place exclusively by way of
HT-1, that is, HT around a C-CH unit. After deconvoluting the
3
isomer, exhibiting a fine structure in its UV absorption spectrum
†
product spectrum, we found that the irradiated sample contained
18% (E,E)-2 and 82% of residual (Z,E)-2.
Murray State University.
University of Hawaii.
‡
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J. AM. CHEM. SOC. 2005, 127, 9378-9379
10.1021/ja052312d CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
3
HT involving the C-CH group was, expectedly, much less effi-
cient than that of C-H. The irradiation time needed for the same con-
version of (Z,E)-2 was approximately 6 times longer than that of
the parent (Z,E)-1. However, it is difficult to determine such num-
bers for solid samples in high precision. Plans are underway to pre-
pare appropriately substituted analogues so as to be able to provide a
3
more manageable internal reactivity ratio between HT-CH and HT-H.
Figure 3. Photoisomerization of (E,E)-3. (a) Absorption spectra of (E,E)-3
recorded during its irradiation (>310 nm, Corning O-54 filter) in EPA glass
at liquid nitrogen temperature; t ) 0, 20, 40, 60, 80, 100, 120, 140, 160,
and 200 s. (b) Difference spectra. Insert: green solid line, irradiation for
200 s; red dotted line, after warming up to thaw the glass then recooling to
low temperature.
We have also examined photoisomerization of isomers of 2,3-
5
dimethyl-DPB (3). The close proximity of the vicinal dimethyl
groups presents a different type of steric effect on photochemical
properties of the diene. For example, the E,E-isomer does not exist
in the planar form, as suggested by the blue-shifted (λmax 313 nm)
8
UV absorption band at 77 K (Figure 3a). Calculations revealed
that (E,E)-3 is likely to exist in a twisted (38°) s-trans conformation
with a close-lying (1.5 kcal/mol) twisted s-cis conformation in
equilibrium. The photoreactivity also reflects the nonplanarity in
that the E,E-isomer is light-sensitive even at 77 K (Figure 3a). A
more blue-shifted photoproduct was formed. However, only after
warming to room temperature and recooling to 77 K, the UV
absorption became identical to that of (Z,E)-3. This photochemical
reactivity is unique among all DPBs.
Figure 4. Photoreaction of (Z,E)-3 at low temperature. Insert a, left top:
absorption spectra recorded during the earlier stage of its irradiation (>310
nm, Corning O-54 filter) in EPA glass at 78 K; t ) 4, 8, 15, 25, 40, 60,
and 120 s. (a) Difference spectra (t - t ) during the early stage. Insert b,
0
left top: absorption spectra recorded during the second stage of its irradiation
under the same condition; t ) 160, 200, 240, 280, 320, and 400 s. (b)
Difference spectra in the late stage. Insert, right bottom: green solid line,
after irradiation for 400 s; red dotted line, after warming up to thaw the
glass then recooling to low temperature.
That a conformationally unstable Z,E-isomer was formed sug-
gests that the reaction originates either from the s-trans form (HT-
2
) or from the s-cis form (HT-1). We see no easy way to distinguish
by way of the unprecedented HT-Me within the confined cavity of
amorphous organic glass. The nonplanar 2,3-dimethyl-DPB intro-
duced unexpected reactivity. Even the E,E-isomer undergoes
regiospecific HT-1 isomerization.9
between the two processes. However, we suspect that the former
process involving a hitherto unobserved HT-2 process (especially
around a methyl group) is unlikely, and the required s-cis form for
HT-1 has been shown to be accessible by calculations.
Acknowledgment. The work was supported by grants from
Hawaii NSF-EPSCoR (CHE-01-32250) and Kentucky NSF-EPS-
CoR (4-65752-03-397).
Supporting Information Available: NMR and UV data of isomers
of 2 and 3, and complete ref 8. This material is available free of charge
via the Internet at http://pubs.acs.org.
Irradiation of pure (Z,E)-3 led to a more complex change. There
was first a facile reaction (<120 s) as reflected in a red-shift and in-
crease in intensity of the UV spectra (insert of Figure 4a), followed
by a slow rate of (>160 s) reaction, giving eventually to a more blue-
shifted product that exhibited the same fine structures in the high-
energy region of the UV absorption spectrum (Figure 4b). The spec-
tra became identical to those of (E,E)-3 only after warming up to
room temperature and recooling to 77 K. The difference spectra of
the E,E- and Z,E-isomers revealed that the first stage of photoreac-
tion for the Z,E-isomer is a mirror image of the reaction for the E,E-
isomer. This suggests that the more twisted isomer (E,E)-3′ is the
only photoreactive conformer for the E,E-isomer, which after HT-1
produced (Z,E)-3′, and (Z,E)-3′ after HT-1 produced (E,E)-3′ with
an isosbestic point at 273 nm. Calculations revealed that (Z,E)-3′
likely exists as an s-cis conformer with a close-lying (0.2 kcal/mol)
and similarly twisted s-trans conformer, (Z,E)-3, in equilibrium,
and the second stage of photoreaction should be due to the HT-1 of
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(Z,E)-3 giving the stable (E,E)-3. The small spectral changes upon
warming and cooling are likely due to changes in the ratio of the
(
9) Note added after acceptance of this paper. A recent article (J. Am. Chem.
8
conformers that are believed (based on energy calculations) to exist
Soc. 2005, 127, 6938-6939) claimed the absence of HT in their study of
1-(2-naphthyl)-2-phenylethylene. Unfortunately, the authors overlooked
in a ratio close to 1:1 and 10:1 for (Z,E)-3 and (E,E)-3, respectively.
In summary, both 1,4- and 2,3-methyl substituents played signifi-
cant roles on the photoreactivity of DPBs. While regiospecific HT-1
photoisomerization is retained for 1,4-dimethyl-DPB, it proceeded
the simple fact of the presence of two different HT processes in their
compound, which in fact is not expected to provide distinguishable data
between HT and OBF processes.
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