The role of symmetric functionalisation on photoisomerisation of a UV commercial chemical filter

Article abstract of DOI:10.1039/c8cp06536e

Photoisomerisation has been shown to be an efficient excited-state relaxation mechanism for a variety of nature-based and artificial-based molecular systems. Here we report on the excited-state relaxation dynamics and consequent photostability of a symmet

Full text of DOI:10.1039/c8cp06536e

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The role of symmetric functionalisation on
photoisomerisation of a UV commercial chemical
Cite this:DOI: 10.1039/c8cp06536e
filter†
a
a
ab
a
Jack M. Woolley, Jack S. Peters, Matthew A. P. Turner, Guy J. Clarkson,
a
a
Michael D. Horbury* and Vasilios G. Stavros
*
Photoisomerisation has been shown to be an efficient excited-state relaxation mechanism for a variety
of nature-based and artificial-based molecular systems. Here we report on the excited-state relaxation
dynamics and consequent photostability of a symmetrically functionalised cinnamate by transient
electronic absorption spectroscopy, along with complementary computational and steady-state spectro-
scopy methods. The findings are then discussed in comparison to 2-ethylhexyl-E-4-methoxycinnamate,
a structurally related ‘off the shelf’ chemical filter present in commercial sunscreens with a similar
absorption profile. The present study allows for a like-for-like comparison beween 2-ethylhexyl-E-4-
methoxycinnamate and the functionalised cinnamate, driven by the need to enhance solar protection
across both the UVA and UVB regions of the electromagnetic spectrum.
Received 19th October 2018,
Accepted 17th December 2018
DOI: 10.1039/c8cp06536e
rsc.li/pccp
follow a growing trend of chemical filters with harmful photo-
products, both to humans and the environment, triggered by
Introduction
2
4–27
Overexposure to ultraviolet radiation (UVR) is an increasing photoexcitation.
As such, there is mounting impetus to
1
–5
concern in today’s Society.
Chemical filters, found in com- develop ‘safer’ chemical filters for use in formulations, such as
mercial formulations, have been designed to allay such over- derivatives of commercial chemical filters.
6
7,8
exposure. Chemical filters based on cinnamates are currently
The present work takes our current knowledge on photo-
being used as a class of UVR absorbing molecules in commercial isomerisation in these cinnamate systems one step further.
formulations due to their strong absorption of UVR and potent It assesses the influence of symmetric functionalisation of
9
–12
antioxidant properties.
Investigations into their photo- cinnamates through the addition of a second methyl acrylate
chemistry has shown a photoisomerisation pathway as a main moiety onto the base cinnamate, methyl cinnamate (highlighted
1
3–17
route of deactivation, irrespective of solvent environment,
and starting geometric isomer (either E or Z);
in red, Fig. 1a). This gives rise to (potentially) three distinct geo-
1
4,18
0
0
although a metric isomers: dimethyl 3,3 -(1,4-phenylene)(2E,2 E)-diacrylate,
change in lifetimes is noted. One such cinnamate-derivative dimethyl 3,3 -(1,4-phenylene)(2Z,2 E)-diacrylate and dimethyl
0
0
contained in sunscreen formulations is the FDA and EU approved
6
,19
2
-ethylhexyl-E-4-methoxycinnamate (E-EHMC).
shown that, following UVR excitation to the first electronic excited
S1) state, E-EHMC undergoes ultrafast relaxation, repopulating
the electronic ground (S ) state through an energetically accessible
Studies have
(
0
20–23
S
1 0
conical intersection, with a percentage forming Z-EHMC.
/S
The formation of Z-EHMC is cause for concern as recent studies
24
suggest its adverse effects on human liver cells. These reports
a
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry,
CV4 7AL, UK. E-mail: v.stavros@warwick.ac.uk, m.horbury@warwick.ac.uk
b
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry,
CV4 7AL, UK
Fig. 1 (a) The three geometric isomers for DPD, with the methyl cinnamate
base unit highlighted in red. (b) Steady-state absorption spectra for
E,E-DPD: at B10 mM concentration in ethanol (blue line) and acetonitrile
(red line), and (owing to poorer solubility) o10 mM for E,E-DPD in cyclo-
hexane (black).
1
13
†
Electronic supplementary information (ESI) available: H and C NMR spectra,
crystallographic data, computed structures, additional transient absorption, and
steady-state spectra, fitting residuals, fluorescence quantum yields and power
dependency measurements. See DOI: 10.1039/c8cp06536e
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0
0
3
,3 -(1,4-phenylene)(2Z,2 Z)-diacrylate, henceforth termed E,E- recompression grating. One of these beams (3.5 W) was used
DPD, Z,E-DPD and Z,Z-DPD respectively and all shown in to produce the pump and probe pulses. The pump beam (2.5 W)
Fig. 1a. Drawing inspiration from the recent gas-phase experi- seeds an optical parametric amplifier (Topas-Prime with UV
ments reported by Kinoshita et al. on the influence of hydroxy extension, Light Conversion) allowing variability in pump wave-
substitution on the photochemistry of methyl hydroxycinna- length. The wavelengths of excitation were: cyclohexane 315 nm,
2
8
mates, we study the photochemistry of the geometric isomer ethanol 318 nm, and acetonitrile 317 nm, corresponding to the
E,E-DPD and track its possible evolution into the remaining peak maxima of E,E-DPD for each solvent as shown in Fig. 1. The
geometric isomers (vide supra). In doing so we attempt to grasp pump beam was focussed beyond the sample holder, to give a
a handle, on the influence of multiple geometric structures on beam diameter of 500 mm at the sample. 5% of the remaining
dynamics, and how these may ultimately influence function.
1 W of fundamental 800 nm was further attenuated and irised
To facilitate these studies, we use a combination of synthesis, (o0.05 W), before being focussed into a vertically translated
spectroscopy and theory. E,E-DPD was synthesised following a 2 mm thick CaF window to generate a white light superconti-
2
2
9
literature procedure. We subsequently used femtosecond tran- nuum from 330 to 680 nm. The relative polarisation between the
sient electronic (UV/visible) absorption spectroscopy (TEAS) to pump and probe pulses was held at magic angle (54.71) to negate
probe (E,E-DPD) in three solvents of varying polarity and ability dynamics from ensemble molecular reorientation. Pump–probe
to hydrogen bond: cyclohexane, acetonitrile and ethanol. Com- time-delays (Dt) were achieved out to the maximum temporal
plementary time dependent density functional theory (TD-DFT) delay of 2.5 ns using a motorized optical delay line in the probe
was also carried out to elucidate the relative energies of the beam. Changes in optical density (DOD) were measured through
different geometric isomers and provide insight into their asso- the change in probe intensities using a fibre-coupled spectro-
ciated electronic structure, ultimately assisting in the assign- meter (Avantes, AvaSpec-ULS1650F). Collated spectra were sub-
3
1
ment of the relaxation dynamics of these systems.
sequently chirp corrected using the KOALA package. Samples
were prepared in 1 mM concentrations and delivered to
the interaction region through a demountable liquid cell
(Harrick Scientific Products Inc.) with a sample thickness of
Experimental
Synthesis
1
2
00 mm placed between two CaF windows. The sample was
flowed through the cell by a diaphragm pump (SIMDOS 02) at a
rate to ensure that fresh sample was interrogated with each
laser pulse.
DPD was prepared using a method adapted from the litera-
2
9
ture. Briefly, 5 drops of dimethylformamide (99.8% purity,
Sigma-Aldrich) were added to a mixture of 1 g p-phenylene-
diacrylic acid (97% purity, Sigma-Aldrich) and 10 ml thionyl Steady-state spectroscopy
chloride (97% purity, Sigma-Aldrich) and was refluxed over-
Steady-state UV/Vis absorption measurements were taken on
night. The mixture was allowed to cool and the excess of thionyl
chloride was removed at reduced pressure to give a yellow
powder. 50 ml of methanol (99% purity, Alfa-Aesar) was added
gradually to the powder, which started to reflux due to the
exothermic nature of the reaction. The mixture was refluxed for
a Cary 60 spectrophotometer with sample concentrations of
B10 mM. Steady-state irradiation (of varying time-delay) measure-
ments of B10 mM samples (ethanol and acetonitrile solvents,
410 mM in cyclohexane) were also taken using a Horiba
FluroLog-3, with an 8 nm slit width centred on the TEAS
excitation wavelength and irradiation power equal to that of
solar irradiance. Samples (each 10 mM, 410 mM in cyclo-
4
hours and then the solid was isolated via filtration. The
reaction mixture was analysed using thin layer chromatography
TLC) with dichloromethane (DCM, 99.8% purity, Sigma-
(
1
hexane) for H NMR analysis, were irradiated for 1 h using the
Aldrich) as an eluent, which showed that the starting material
had been consumed and that multiple new spots were visual-
ised under UV irradiation. The mixture was recrystallized using
a 2 : 1 solution of DCM and petrol (Sigma-Aldrich), to form a
light-yellow crystalline product. The resulting compound was
full available power of the Xenon lamp at the TEAS excitation
wavelength using an 8 nm slit width. Samples were then dried
of solvent and subsequently dissolved in deuterated chloroform
1
for H NMR analysis. Fluorescence measurements were per-
formed on the FluroLog-3, at excitation wavelengths matching
those for the TEAS measurements. A 2.5 nm slit width was
applied for both excitation and emission measurements.
A quartz cuvette with a 1 cm path length was used for UV/Vis,
1
13
analysed using H, C NMR along with X-ray crystallography
see ESI,† Fig. S1–S4).
(
Transient electronic absorption spectroscopy (TEAS)
1
fluorescence, steady-state irradiation and H NMR irradiation
TEAS measurements were executed within the Warwick Centre measurements.
for Ultrafast Spectroscopy (WCUS; www.go.warwick.ac.uk/WCUS),
Computational
the capabilities of which have been previously detailed and thus
3
0
only summarised here. 800 nm pulses (12 W, 1 kHz, 40 fs) were The structures of DPD were generated for all three isomers
3
2
generated by a commercially available Ti-Sapphire regenerative (E,E, E,Z, Z,Z), using visual molecular dynamics, with the
amplified laser system (Spectra-Physics, Dual Ascend Pumped molefacture plugin. Each of these structures underwent a
Spitfire Ace) seeded by a Mai Tai (Spectra-Physics). The beam geometry optimization within density functional theory using
3
3,34
was split into four fractions, each having an independent a cc-pVTZ basis set and PBE0 functional,
using the
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3
5
NWChem software. Following this, time-dependant density along with the first singlet vertical excitations of each isomer in
functional theory calculations were conducted on the optimized each solvent.
geometries of the E,E, E,Z and Z,Z forms. Again, the level of theory
Fig. 2 shows the transient absorption spectra (TAS) resulting
was cc-pVTZ/PBE0 and the NWChem software was employed, from photoexcitation of E,E-DPD, at the absorption maximum:
generating the vertical excitation energies along with the oscilla- 317 nm (3.91 eV) in acetonitrile and 318 nm (3.90 eV) in ethanol
tor strengths for each transition. For all calculations, a conductor- (cf. Fig. 1). We note the good agreement between the calculated
like screening model (COSMO) was used to replicate the effect of vertical and experimentally measured excitation energies for
36,37
the solvent.
The default COSMO implicit solvent model for E,E-DPD with differences being between 10–12 nm across all
each environment within NWChem was used, the descriptors of solvents. From the TAS presented as false colour heat maps in
38
which are based on the Minnesota Solvent Descriptor Database.
Fig. 2a and b, two main features are clearly visible: a stimulated
Computational output files, for the discussed calculations are emission centred at 375 nm and a strong excited state absorp-
available at: DOI: 10.5281/zenodo.1305150.
tion centred at 525 nm. We add here that in Fig. 2b, we see
evidence of a doublet around 350 nm, due to the ground state
bleach and stimulated emission being spectrally offset in ethanol
Results
(cf. acetonitrile this is a single feature). This is also evidenced in
the evolutionary associated difference spectra (EADS) presented
in Fig. 2c and d (discussed below). The excited state absorption
and stimulated emission features return to baseline within 50 ps
leaving a small excited state absorption (not immediately appar-
ent, highlighted in Fig. 2e and f) centred around 410 nm, which
persists to the maximum temporal delay in our measurements
The dominant product of the synthesis was found to be
E,E-DPD, determined through X-ray crystallography, which
showed half the molecule in the asymmetric unit with an
inversion centre in the middle of the phenyl ring, and E stereo-
3
9 1
chemistry at each double bond.
H NMR spectra also con-
firms the E,E-isomer as the major reaction product (see ESI,†
Fig. S1). Computed structures predict that the lowest energy
structure is the E,E-isomer, showing good agreement with the
experimental structures (see ESI,† Fig. S5). Table 1 shows
the calculated energy difference between the three isomers,
(Dt = 2.5 ns). Fig. 2e and f also show the presence of a ground
state bleach centred at 340 nm. We discuss this, as well as the
aforementioned excited state absorption, below. We note that
further studies were carried out in cyclohexane and additional
excitation wavelengths, showing similar spectral features. This
data is presented in the ESI,† Fig. S7–S9.
Table 1 Energies of the three possible geometric isomers of DPD, relative to
the energy of E,E-DPD, along with first singlet (vertical) excitation energies,
in acetonitrile, ethanol and cyclohexane
To quantitatively assign lifetimes to the reported TAS
features, a global fit across all wavelengths (330–680 nm), using
ꢀ
ꢁ
t1
t2
t3
a sequential model A ƒ! B ƒ! C ƒ! D , was performed
Solvent
Isomer
E,E
E,Z
Z,Z
4
0,41
using the software package: Glotaran.
The TAS were fitted
Acetonitrile Ground state difference
relative to E,E-DPD (eV)
Vertical excitation
0
0.27
0.54
with a three-step sequential model, with three lifetimes
returned; these incorporate a convolution with a Gaussian to
model our instrument response (FWHM = 80 fs). The extracted
lifetimes are shown in Table 2, along with the associated errors.
Transient slices are also provided in the ESI,† (Fig. S10 and S11)
to illustrate the quality of the fit using this model. We note that
where the error returned was less than 50% of our instrument
response, we elected to quote an error of 40 fs (i.e. 50% of our
instrument response). The corresponding EADS are also shown
3.79, 327 3.80, 326 3.81, 325
0.27 0.55
3.76, 330 3.80, 326 3.81, 325
0.35 0.58
3.80, 326 3.81, 325 3.81, 325
energies (eV, nm)
Ethanol
Ground state difference
relative to E,E-DPD (eV)
Vertical excitation
0
energies (eV, nm)
Cyclohexane Ground state difference
relative to E,E-DPD (eV)
Vertical excitation
0
energies (eV, nm)
Fig. 2 False colour heat maps of E,E-DPD following photoexcitation at (a) 317 nm in acetonitrile and (b) 318 nm in ethanol. Evolutionary associated
difference spectra (EADS) for the TAS of E,E-DPD in (c) acetonitrile and (d) ethanol. TAS at Dt = 2.5 ns of E,E-DPD in (e) acetonitrile and (f) ethanol.
Note the intensity scale in (a) is cropped as a visual aid to the dynamical features beyond the first 200 fs. An expanded intensity scale of (a) is presented in
the ESI,† Fig. S6.
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Table 2 Lifetimes and associated errors (2s) extracted from the global identified the presence of an absorption feature at long time
sequential fitting of E,E-DPD and previously reported values E-EHMC, in
methanol and cyclohexane
delays, attributed to the formation of the corresponding geo-
metric isomer; in the present case, this would correspond to
Solvent
t
1
(fs)
t
2
(ps)
t
3
(ns) E,Z-DPD (or Z,Z-DPD). This may be the case here, evidenced
through the incomplete ground state bleach recovery of E,E-DPD
E,E-DPD
Acetonitrile
Ethanol
Cyclohexane
90 ꢀ 40
7.73 ꢀ 0.04
8.28 ꢀ 0.04
c2.5
c2.5
c2.5
(negative feature o350 nm; see Fig. 2e and f). However, the
positive feature extending 4400 nm points towards a long-lived
excited state, which would also restrict ground state bleach
recovery. We attribute this long-lived excited state to either an
400 ꢀ 40
480 ꢀ 40
10.34 ꢀ 0.15
a
E-EHMC
1
Methanol
Cyclohexane
260 ꢀ 90
1.10 ꢀ 0.30
1.80 ꢀ 1.20
c2.0
c2.0
np* or triplet state. We draw confidence that this is due to an
600 ꢀ 100
excited state absorption, given that neither E,Z-DPD or Z,Z-DPD
isomer is likely to absorb beyond 400 nm (see Fig. 4). Both these
features in the TAS, persist to Dt = 2.5 ns and are assigned a
a
Taken from ref. 20.
in Fig. 2c (acetonitrile) and d (ethanol). The associated residuals
for each TAS (difference between experimentally measured and
calculated TAS) are provided in the ESI,† (Fig. S12 and S13).
Discussion
From the reported lifetimes given in Table 2 and associated
EADS presented in Fig. 2c and d, we can now discuss the
reported TAS, with reference to the schematic shown in Fig. 3.
We note that we draw considerable insight from comparative
studies (with associated lifetimes; also shown in Table 2) of
20
1
1
E-EHMC. Photoexcitation to the S ( pp*) state leads to popula-
tion traversing out of the Franck–Condon region described by t1.
Notably t for acetonitrile is significantly shorter than that of
1
either ethanol or cyclohexane, as has previously been reported for
42,43
similar systems.
This also (likely) encompasses any geometry
rearrangement of solute and surrounding solvent. As with similar
systems, this geometry rearrangement is not thought to involve
1
6,44
a change in the electronic state.
2
t then describes the
evolution of excited-state population, as it moves towards an
S1/S0 conical intersection along the E - Z isomerisation
coordinate and then funnels through this conical intersection.
0
This latter step leads to effective repopulation of the ground (S )
7
,15,18,20,42,43
state. Previous reports of photoisomerisation,
have
Fig. 4 Steady-state UV/Vis absorption of 10 mM E,E-DPD in (a) acetonitrile
and (b) ethanol, following irradiation at solar fluences over the peak absorp-
tion (lmax = 317 nm (acetonitrile) and 318 nm (ethanol)). The colour bar
indicates length of irradiation in seconds. Selected region of (c) pre- (black)
1
Fig. 3 Schematic of relaxation for photoexcited E,E-DPD, showing the
progression of t and t . We add that t (not shown) would correspond to
population trapped in the E,Z-DPD and np* state.
and post-irradiation H NMR of E,E-DPD irradiated at 10 mM concentration in
1
2
3
acetonitrile and ethanol (red and blue respectively). (d) Peak assignment of
1
chemical shifts in E,E and E,Z isomers.
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7
,20,46
lifetime of t
further detail below.
3
. We discuss these features and the origin of t
3
in Fluorescence has also been reported for similar systems
and was again observed here with the quantum yield reported
Steady-state irradiation at solar irradiance intensity was as 0.039 ꢀ 0.002 in cyclohexane (see ESI,† Fig. S17), hence a
conducted to explore the long term photostability of E,E-DPD relatively minor pathway. Also, no phosphorescence was observed.
upon exposure to UVR. The results for both acetonitrile and The possibility of the absorption feature being due to a radical
ethanol are shown in Fig. 4; steady-state irradiation studies for cation species, generated via a step-wise two-photon ionisation,
9,17,42
E,E-DPD in cyclohexane are shown in the ESI.† All absorption was considered (as seen for similar species),
spectra show a depletion of the initial main peak before due to the linear power dependence of the feature (see ESI,†
plateauing following 20 minutes (1200 seconds) of irradiation. Fig. S19). Taken together, we assign t to a combination of a long-
but dismissed
3
1
Accompanying this depletion is a small shift in the absorption lived np* state and presence of E,Z-DPD. We conclude our
to higher energy. The depletion of absorbance, is assigned to discussion of the long lived feature in the TAS by noting that a
the generation of a photoequilibrium between E,E-DPD and quantitative analysis of the conversion of E,E-DPD to E,Z-DPD is
E,Z-DPD; see Fig. 1 for associated structures. This depletion is not possible from the TAS given the convoluted nature of the
consistent with the calculated oscillator strengths (S
the calculated absorption maximum) between the two geometric Dt = 2.5 ns (from the presence of the np* state).
1 0
’ S , at features both at early times (from the stimulated emission) and at
1
isomers; e.g. 1.30 and 1.17 (arb. units) for E,E-DPD and E,Z-DPD
It is evident from the data thus far that E,E-DPD may possess
respectively in acetonitrile. To further prove the existence of this suitable properties for applications as a chemical filter. Notably, its
1
9,20
photoequilibrium, and to exclude formation of the Z,Z-isomer, absorption spectrum is similar to that of E-EHMC,
but with a
H NMR of the irradiated samples was performed. The samples slight red-shift (B10 nm), meaning it extends further into the UVA
were irradiated for 5 h using the full power of the Xenon lamp which is a sought-after quality for a chemical filter. Coupled to this
approximately 30 mW) at the absorption maximum (lmax = 317 is the rapid relaxation of the excited state through an accessible
and 318 nm, for acetonitrile and ethanol respectively), to obtain conical intersection, consistent with the previous literature of
1
(
1
7,42,43
sufficient conversion to the E,Z-DPD for H NMR identification; similar species.
Furthermore, the apparent degradation upon
the sample concentration was also increased to 10 mM to constant UVR is attributed to interconversion to the E,Z-DPD
1
facilitate this. The H NMR spectra in deuterated chloroform isomer. This finding is, once again, in good agreement with
2
0,23
for the region of interest for both pre- and post-irradiation are previous reports on E-EHMC versus Z-EHMC.
shown in Fig. 4c. Whilst there are similarities to E-EHMC, there are evident
1
From the analysis of the H NMR spectra, we can conclude differences. Firstly, the apparent interconversion between the
that E - Z isomerisation occurs, with peaks f and g showing a two isomers is longer for E,E-DPD in comparison with E-EHMC;
1
2
3 Hz splitting characteristic of the Z-isomer, along with the e.g. in cyclohexane, t B 10 ps for E,E-DPD versus B2 ps for
2
0
appearance of peak e indicating a change in environment of the E-EHMC (see Table 2). This indicates that photoisomerisation
terminating methyl. This is in contrast to peak b and d which is impinged by the different substituent in the para position. In
have a splitting of 16 Hz (see ESI,† Fig. S3), attributed to this case, the addition of a second methyl acrylate alters the
E-isomer. One would anticipate a progression towards Z,Z-DPD excited state dynamics by increasing the time for ground state
to be a stepwise multiple-photon process, with the first conver- repopulation (be it in E,Z-DPD (more likely) or E,E-DPD (less
sion to E,Z-DPD then subsequently conversion to Z,Z-DPD. From likely), Fig. 3). Whilst the underlying reason for this is not
the NMR we can deduce that no detectable presence of Z,Z-DPD known, this inevitably points to alterations to the excited state
is observed, confirmed through the splitting of the hydrogens on landscape along the photoisomerisation coordinate. We add
0
00
the aromatic ring (see signals c and c ) indicating unsymme- here that the increase in excited state lifetime could promote
trical substitution. competing pathways; the ns component (t ) of the E-EHMC can
3
To conclude our discussion, we shall return to the nature of entirely be attributed to occurrence of the Z-isomer whilst for
the long-lived features in the TAS (Fig. 2e and f) and the origin E,E-DPD, this appears to be a multicomponent, with the front
of t . It is evident from steady-state irradiation measurements runners being the E,Z-DPD isomer and a long-lived np* state.
3
that the geometric isomer E,Z-DPD has a weaker absorbance Additionally, compared to E-EHMC, the time taken to reach a
o350 nm than the E,E-DPD (cf. Fig. 4a and b, 0 and 7200 seconds photoequilibrium is increased to 20 minutes (cf. 2 minutes for
2
3
irradiation). Given the TAS represents a difference spectrum E-EHMC). We finally note that no detectable spectroscopic
(post-irradiated sample compared to pre-irradiated sample), evidence for Z,Z-DPD was observed in any experiments, leading
it is unsurprising that we see a negative feature in the TAS (along to the hypothesis that a photoequilibrium is only established
with an incomplete, and overlapping, ground state bleach recovery) between E,E-DPD and E,Z-DPD.
1
and is further evidenced, via H NMR data, of isomerisation from
E,E-DPD - E,Z-DPD. However, there is no absorption of either
isomer at wavelengths greater than 350 nm, pointing to a different
Conclusions
origin of the positive feature in the TAS 4400 nm. This relatively
1
small excited state absorption is likely due to either a np* or a This study highlights the effect of chemical substitution on an
1
triplet state. Several reports have suggested the presence of np* established photoisomerisation pathway. In this instance, we
8,28,45
states playing a role within relaxation of similar systems.
compare E,E-DPD with the commercial chemical filter E-EHMC.
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Following photoexcitation in solvents of varying polarity, the
excited state population in E,E-DPD evolves out of the Franck–
Condon region within 500 fs, along with geometry rearrangement
of the solute and surrounding solvent, enabling access to an
energetically accessible S /S conical intersection. As the popula-
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1
0
tion transfers to the electronic ground state, the first difference
between E,E-DPD and E-EHMC is in the relaxation times; B10 ps
for E,E-DPD and B2 ps E-EHMC (both in cyclohexane). Ultimately
this change is assigned to effects of the additional methyl acrylate
on the excited state landscape. The second difference relates to
the assignment of t . Here t describes the presence of the
3
3
corresponding E,Z-DPD isomer, along with a small percentage
1
of the excited state population trapped in an np* (or triplet)
state, which persists beyond the time-window of the present
measurements (ons). This ns component is absent in E-EHMC.
Furthermore no evidence for Z,Z-DPD was observed, leading to
the conclusion that its formation is unfavoured and the photo-
equilibrium is generated between E,E-DPD, and E,Z-DPD.
As a final comparison, the steady-state absorption spectra of
E,E-DPD (in solvents of varying polarity) are red-shifted relative to
the absorption spectra of E-EHMC, allowing for stronger absorp- 11 J. C. Murray, J. A. Burch, R. D. Streilein, M. A. Iannacchione,
tion within the UV-A region of the spectrum. This is a key quality,
which is expected for new chemical filters, given the growing
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potential blue-print into how a current chemical filter can be 13 J. Luo, Y. Liu, S. Yang, A. L. Flourat, F. Allais and K. Han,
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Conflicts of interest
3
0683–30694.
There are no conflicts to declare.
1
5 M. D. Horbury, W.-D. Quan, A. L. Flourat, F. Allais and
V. G. Stavros, Phys. Chem. Chem. Phys., 2017, 19, 21127–21131.
Acknowledgements
16 A. Espagne, D. H. Paik, P. Changenet-Barret, M. M. Martin
and A. H. Zewail, ChemPhysChem, 2006, 7, 1717–1726.
The authors thank Dr Michael Staniforth at the Warwick Centre
for Ultrafast Spectroscopy (WCUS; www.go.warwick.ac.uk/WCUS),
for experimental aid. The authors would also like to thank
Dr Nicholas Hine for valuable discussions regarding the theory.
J. M. W. is grateful to EPSRC and Newport Spectra-Physics Ltd
for a joint studentship. M. A. P. T. thanks EPSRC for a doctoral
studentship through the EPSRC Centre for Doctoral Training in
Molecular Analytical Science, grant number EP/L015307/1.
M. D. H. thanks the Leverhulme Trust for postdoctoral funding.
V. G. S. is grateful to the EPSRC for an equipment grant
1
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