Two-photon induced isomerization through a cyaninic molecular antenna in azo compounds

Article abstract of DOI:10.1039/d0cc08346a

We present a new design for non-linear optically responsive molecules based on a modular scheme where a polymethinic antenna section with important two-photon absorption properties is bonded to an isomerizable actuator section composed of a stilbenyl-azopyrrole unit. Upon two photon excitation, energy migration from the antenna-localized second singlet excited state to the stilbenyl-azopyrrole section allows for efficient indirect excitation and phototransformation of this actuator.

Full text of DOI:10.1039/d0cc08346a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Two-photon induced isomerization through a
cyaninic molecular antenna in azo compounds†
Cite this: Chem. Commun., 2021,
7, 3123
5
a
a
a
Emmanuel Villatoro, Leonardo Munoz-Rugeles,
Jesus Duran-Hernandez,
´
´ ´
˜
a
a
a
Received 25th December 2020,
Accepted 25th January 2021
Bernardo Salcido-Santacruz, Nuria Esturau-Escofet,
Jose G. Lopez-Cortes,
´ ´
b
a
M. Carmen Ortega-Alfaro
and Jorge Peon
*
´
DOI: 10.1039/d0cc08346a
rsc.li/chemcomm
We present a new design for non-linear optically responsive molecules future applications. Also, from their extended conjugation, they
based on a modular scheme where a polymethinic antenna section have transition energies appropriate for indirect excitation
with important two-photon absorption properties is bonded to an through energy transfer from specific upper states in the
4
isomerizable actuator section composed of a stilbenyl-azopyrrole unit. antenna section (see below). In our design, the antenna is
Upon two photon excitation, energy migration from the antenna- the polymethinic IR780 cyanine dye. Its symmetry properties
localized second singlet excited state to the stilbenyl-azopyrrole make the S
section allows for efficient indirect excitation and phototransformation 460 GM units at 860 nm).
of this actuator. Scheme 1 outlines the relevant energy levels for our design
and proposed mechanism. The S polymethinic state (formed
0 2
to S transition strongly two-photon allowed (with
5
2
Reactive response to light is inspiring new molecular control by 2PA) requires a long enough lifetime for the exciton migra-
1
schemes with applications in photo-pharmacology, catalytic tion to occur before internal conversion to the S state within
1
2
process initiation, and the manipulation of molecular gears.
The initial step in these processes is an electronic excitation
this chromophore. The large energy gap between the S
2
and the
S
1
cyaninic states (of about 1.0 eV), results in a small vibronic
1
,2
which triggers a specific molecular transformation. For many coupling. Thus, for this kind of cyanines, the S
2
lifetime can be
5
of these applications, it is crucial to have high spatial control of in the time scale of picoseconds.
the photochemistry. Such control can be achieved if the initial
excitation occurs by a non-linear absorption event.
Scheme 2 displays an overview of the synthetic method.
Briefly, a Mirozoki–Heck cross-coupling between 1 and 2
In this contribution, a new bichromophoric design for non- affords the intended SAP actuator. To accomplish this cou-
6
linear optically switchable molecules is proposed. The main pling, it was crucial the use of complex 3 as catalytic precursor.
idea is to use two chromophores to isolate the two-photon A nucleophilic reaction couples SAP through an ether bond to
absorption (2PA) properties in one section (the antenna), and the IR780 cyanine rendering the expected Cy-SAP system. Detailed
the photochemical transformation in another segment (the synthesis and characterization for the SAP and Cy-SAP molecules
actuator) as seen in Scheme 1. The moieties are joined through are included in the ESI.†
an ether type functionality in a way that the two keep their
3
independent properties as localized chromophores. In this
design, upon 2PA at the antenna, the energy associated with
the excitation is used to efficiently drive changes in the actuator
region after rapid exciton migration.
The actuator in our design is a p-extended stilbenil-
azopyrrole (SAP). Azoheteroarenes have shown outstanding
properties as photoswitches, providing a versatile scaffold for
a
Instituto de Qu ´ı mica, Universidad Nacional Aut o´ noma de M ´e xico,
Ciudad de M ´e xico, Mexico. E-mail: jpeon@unam.mx
b
Instituto de Ciencias Nucleares, Universidad Nacional Aut o´ noma de M ´e xico,
Ciudad de M ´e xico, Mexico
Scheme 1 Molecular structure of the Cy-SAP dyad. Relative energy
diagram of the electronic levels for the polymethinic ‘‘two-photon
antenna’’ based on the IR780 cyanine, coupled to a stilbenyl-azopyrrolic
(SAP) actuator.
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization of CySAP. Experimental and computational details. Time-resolved spectral
data for individual components. See DOI: 10.1039/d0cc08346a
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 3123–3126 | 3123

View Article Online
Communication
ChemComm
Scheme 2 Synthetic methodology. (i) 0.1% of 3, DMF, 160 1C, 2 h.
3
(ii) IR780, Et N, MeCN, r.t., 24 h.
The key design element of the Cy-SAP system relies on its Fig. 2 Calculated equilibrium geometry of Cy-SAP. The red area shows
modular structure. The chromophores keep their respective the biphotonic antenna and the blue section highlights the isomerizable
actuator. The arrows indicate the transition dipole moments where the
electronic transition patterns despite the covalent linkage. As
blue vector corresponds to the first transition localized in SAP, the purple
seen in Fig. 1, the Cy-SAP absorption spectrum replicates the
vector corresponds to the first transition localized in the antenna section,
merged spectrum formed by the simple sum of the SAP and
and the red vector corresponds to the second transition localized in the
IR780 spectra. From the match, it can be concluded that the antenna (S2 - S0).
SAP unit and the cyaninic section of Cy-SAP keep their excited
singlet manifold order and character. The latter is crucial for
our design as the cyanine keeps its symmetry and state ordering
TD-DFT calculations at the PBE0//M06 6-311++G(d,p) (PCM =
CH CN) level of theory were performed to estimate the energy
3
7
properties. These properties are essential for a large two-photon
transfer parameters. The relevant transition moments are
5
n
absorption cross-section into its S (n 41) states.
depicted in Fig. 2 and are shown within the optimized Cy-SAP
Polymethine chromophores have been shown to produce
structure. The IR780 antenna S
transition vector oriented along the short molecular axis in
the direction of the meso substituent. The SAP actuator has a
2
state bears an emission
5
a
significant emissions from their S
2
singlet. Fig. 1 shows the
state at 528 nm.
well-defined emission band from IR780’s S
2
This well-defined emission results from the remarkably slow
decay of this upper singlet state of several cyanines compared
to that of other common chromophores. As stated above, our
scheme is based on energy transfer coupling between the two
sections. Thus, it requires the presence of isoenergetic electro-
0 1
strong S - S transition dipole oriented along its main axis
and towards the antenna. These vectors are nearly collinear and
5
a
2
correspond to a high orientation factor, k , for energy transfer:
3.76 (for PBE0) and 3.73 (for the M06 functional).
Our estimation of the coupling between the two chromo-
8
nic transitions. Such energy matching will involve the S
2
state
¨
phores of Cy-SAP considers a Forster mechanism. The para-
meters include the spectral overlap shown in Fig. 1, the
orientation factor taken from our TD-DFT calculations, and
of the antenna with the first excited state of the acceptor (SAP).
The spectral overlap is shown in the inset of Fig. 1.
2
the experimental lifetime of the cyaninic S state in IR780.
Further details are found in the ESI.† The predicted antenna to
actuator energy transfer yield is between 75.0% (PBE0) and
74.8% (M06). Such large yield for transfer from the S2 state
localized at the antenna section to the S state localized at the
1
2
actuator comes from four main factors: (1) a favourable k
orientation factor, (2) a large oscillator strength for the acceptor
4
À1
À1
transition of 4 Â 10 M cm , (3) a long-lived upper singlet
state for the cyanine-localized S donor singlet state of 1.45 ps
see below), and (4) the proximity of the chromophores of
.92 nm (center to center).
The photochemical transformation of the actuator of Cy-SAP
2
(
0
was first studied by direct linear excitation of the SAP unit
within Cy-SAP and compared against the behaviour of the
isolated actuator SAP. The results are summarized in Table 1.
Upon irradiation, the Cy-SAP system rapidly evolves towards a
Fig. 1 Absorption spectrum of Cy-SAP (dashed red line) and, for
comparison, the sum of the absorption spectra of SAP and IR780 (dashed photo-stationary state (PSS). Fig. 3 shows the complete thermal
green line). We also include the absorption spectra of SAP (black line) and
IR780 (blue line). Inset: Absorption spectrum of Cy-SAP and emission
back isomerization. NMR spectroscopy of the PSS shows a clear
mixture of E and Z isomers as seen in the inset of Fig. 3.
A complete and clean thermal isomerization back to the E
spectrum from the second excited singlet state of the IR780 cyanine. All
spectra were taken in acetonitrile. The red area indicates the spectral overlap
isomer was observed within several minutes for both molecules.
The corresponding data for SAP is included in Fig. S14 (ESI†).
between the FRET pair. Fig. S7 in the ESI,† confirms that this emission band
results from electronic excitation of the cyanine into its S (n 41) states.
n
3
124
|
Chem. Commun., 2021, 57, 3123–3126
This journal is © The Royal Society of Chemistry 2021

View Article Online
ChemComm
Communication
Table 1 Isomerization parameters for SAP and Cy-SAP
b
PSS [%]
a
a
À1
c
t
1/2 [min]
k [s
]
E
Z
FE-Z [%]
À4
À4
Cy-SAP
SAP
5.5
5.5
7.2 Â 10
6.8 Â 10
56
32
44
68
22 Æ 2
29 Æ 1
a
For the first-order
Method described by Calbo et al.
Z
-
E
back thermal isomerization.
Method proposed by B ¨o rjesson.
b
4a c
9
À5
A 10 M acetonitrile solution and a 485 nm continuous laser were used
to obtain the photostationary state.
Fig. 4 Evolution from the E-isomer to the Z isomer of Cy-SAP as a
function of irradiation time. The electronic excitation in these experiments
corresponds to two-photon absorption of 860 nm, 100 fs laser pulses. The
experimental details are included in the ESI.† The lower inset shows the
square dependence of the transformation as a function of pulse intensity.
demonstrate a clean two-photon isomerization. Experiments at
different excitation powers show no photodegradation at the
powers of the experiments (see Fig. S19, ESI†). Back-to-back
tests with the SAP molecule (without the antenna) showed no
evolution at the same conditions and corroborate the essential
role of the antenna section. Given the biphotonic nature of the
excitation, the PSS shows a smaller E–Z transformation for this
PSS in the overall volume of the cell (see Fig. S19, ESI†).
However, this is not a concern since these systems are meant
for applications where the effects are microscopically localized,
like in organelles or liposomes, where localized single molecule
Fig. 3 Thermal Z–E back-isomerization was observed after the formation
of a photo-stationary state in Cy-SAP solutions. Main: UV-Vis kinetics.
1
Inset: H NMR spectroscopy kinetics. The spectrum shown is a section of
the complete spectrum shown in Fig. S15 in the ESI.†
1d,2
photo-transformations are crucial
The PSS spectra of SAP and Cy-SAP were deconvoluted into their effects).
E and Z components in Fig. S16 (ESI†). The E/Z ratio in the PSS Femtosecond-resolved fluorescence measurements were
(rather than large bulk
of Cy-SAP is only slightly smaller than that in SAP. Furthermore, used to directly observe the mechanistic aspects of the antenna
Fig. S18 (ESI†) indicates that both systems are capable of several S2 excitation and energy migration in Cy-SAP. Biphotonic
switching cycles without any noticeable fatigue. Such photo- fluorescence up-conversion is ideal for these characterizations
conversion properties are significant and appropriate for several since transient emissions from higher states can be resolved in
1
kinds of applications.
the sub-ps scale. Fig. S23 (ESI†) displays the detection of the
The fact that the isomerization quantum yield in going from emission from the upper excited states of Cy-SAP. Experiments
SAP to Cy-SAP only shows a small drop is quite relevant. The detecting the emissions from the isolated antenna (IR780) and
indirect non-linear excitation of the stilbenyl-azopyrrole section actuator (SAP) are depicted in the ESI,† in Fig. S23.
is to occur by energy transfer from a higher antenna-localized
The emission intensities of Fig. S23 (ESI†) correspond to the
excited state. However, a cyaninic lower energy state (S , 780 nm energy region of the spontaneous emission from the cyaninic-
1
band) is present and could provide deactivation channel due to localized S2 state. These signals exhibit a single exponential
back energy transfer (actuator back to cyanine). The minor decay of 200 Æ 60 fs for the Cy-SAP system. These decays are
E - Z yield drop (from 0.29 to 0.22) implies that this not a much faster in comparison with those from the S emission for
2
predominant pathway. Therefore, this potential drawback does the IR780 (antenna-only) case, which is congruent with the
not impede the photochemistry in the actuator section.
The main advantage of the Cy-SAP system is its control Fig. S23 (ESI†) shows a bi-exponential decay with time con-
through non-linear excitation with NIR light. Focused laser stants of 220 fs and 1.45 ps due to S - S internal conversion,
proposed mechanism. Specifically, the IR780 emission in
2
1
pulses tuned to energies slightly below the first cyaninic where the sub-ps dynamics are attributed to fast relaxation
transition (860 nm) induce the corresponding isomerization. events after excitation. The lack of the 1.45 ps component in
Fig. 4 depicts the resulting spectral evolution after 2PA. The Cy-SAP indicates that system evolves into the energy-transfer
E - Z photoinduced response of the system is identical to that state before the population of the relaxed cyaninic S state is
2
observed for linear photoisomerization with visible light in established. The initial (near t = 0) emission spectrum of
Fig. 3. The clear isosbestic point at 365 nm shown in Fig. 4 Cy-SAP resembles that of the IR780 cyanine in Fig. S23 (ESI†)
and the complete thermal back-isomerization in Fig. S21 (ESI†) indicating the nature of the originally excited state. However,
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 3123–3126 | 3125

View Article Online
Communication
ChemComm
the Cy-SAP traces decay much faster and display the formation
Authors acknowledge CONACyT-M ´e xico grant Ciencia de
of a shoulder at shorter wavelengths corresponding to the Frontera 2019-51496, CONACyT-M ´e xico 285722, and PAPIIT/
emission from the SAP excited states shown in Fig. S23 (ESI†). DGAPA/UNAM IG200621 for financial support, DGTIC-UNAM
The faster signal loss in Fig. S23 (ESI†) is consistent with the project LANCAD-UNAM-DGTIC-210 for computer time.
presence of a new channel for the evolution of the cyanine-
localized state (polymethinic S
antenna-to-actuator energy migration channel, indicated as
2
), due to the presence of the
Conflicts of interest
‘‘energy transfer’’ in Scheme 1.
There are no conflicts to declare.
Kinetic modeling of the emission signals agrees with the
observed changes between IR780 Cy-SAP. The population modeling Notes and references
used follows the proposed mechanism and is described in the ESI.†
The model accounts for the changes in the up-conversion signals
considering a F o¨ rster type energy transfer rate constant from the
1
(a) W. A. Velema, W. Szymanski and B. L. Feringa, J. Am. Chem. Soc.,
2014, 136, 2178–2191; (b) K. H u¨ ll, J. Morstein and D. Trauner, Chem.
Rev., 2018, 118, 10710–10747; (c) I. Tochitsky, M. A. Kienzler,
E. Isacoff and R. H. Kramer, Chem. Rev., 2018, 118, 10748–10773;
S
2
state.
(
d) C. Matera, A. M. J. Gomila, N. Camarero, M. Libergoli, C. Soler
and P. Gorostiza, J. Am. Chem. Soc., 2018, 140, 15764–15773;
e) M. Wegener, M. J. Hansen, A. J. M. Driessen, W. Szymanski
Besides the sub-ps spectral evolutions, time-resolved emis-
sion polarization anisotropies (r(t)) from the up-conversion
signals are presented as part of the ESI.† The near time-zero
(
and B. L. Feringa, J. Am. Chem. Soc., 2017, 139, 17979–17986.
(a) R. Dorel and B. L. Feringa, Chem. Commun., 2019, 55, 6477–6486;
(b) F. M. Raymo and M. Tomasulo, Chem. – Eur. J., 2006, 12,
2
3
(r(t E 0)) anisotropies of IR780, SAP and Cy-SAP show clear
3
186–3193; (c) D. Roke, S. J. Wezenberg and B. L. Feringa, Proc.
differences. Here, Cy-SAP shows a significantly smaller r(0)
value than the IR780 and SAP solutions. This low anisotropy
is consistent with the presence of the additional S2 decay
channel in Cy-SAP from exciton migration (see ESI†).
It should be noticed that the emission decays and anisotropies
from Cy-SAP in the 475 to 600 nm region are due to: two-photon
Natl. Acad. Sci. U. S. A., 2018, 115, 9423–9431.
(a) J. Moreno, M. Gerecke, L. Grubert, S. A. Kovalenko and S. Hecht,
Angew. Chem., Int. Ed., 2016, 55, 1544–1547; (b) M. Izquierdo-Serra,
M. Gasc o´ n-Moya, J. J. Hirtz, S. Pittolo, K. E. Poskanzer, E` . Ferrer,
R. Alib ´e s, F. Busqu ´e , R. Yuste and J. Hernando, et al., J. Am. Chem.
Soc., 2014, 136, 8693–8701; (c) J. Croissant, A. Chaix, O. Mongin,
M. Wang, S. Cl ´e ment, L. Raehm, J.-O. Durand, V. Hugues,
M. Blanchard-Desce and M. Maynadier, et al., Small, 2014, 10,
excitation into a superposition of S
n
states, the ‘‘not-so-rapid’’
state of approximately
, and exciton migration to form the SAP localized
1
752–1755; (d) G. Cabr ´e , A. Garrido-Charles, M. Moreno,
internal conversion to the cyaninic S
1
M. Bosch, M. Porta-de-la-Riva, M. Krieg, M. Gasc ´o n-Moya,
N. Camarero, R. Gelabert and J. M. Lluch, et al., Nat. Commun.,
12
À1
kIC E 10
s
2
019, 10, 907.
excited states. Our femtosecond experiments show clear differ-
ences between the dynamics of the separate fragments (IR780 and
SAP) and those of the antenna-actuator system Cy-SAP, which are
consistent with the mechanism indicated in Scheme 1.
4
(a) J. Calbo, C. E. Weston, A. J. P. White, H. S. Rzepa, J. Contreras-
Garc ´ı a and M. J. Fuchter, J. Am. Chem. Soc., 2017, 139, 1261–1274;
(b) S. Crespi, N. A. Simeth and B. K o¨ nig, Nat. Rev. Chem., 2019, 3,
1
33–146; (c) C. E. Weston, R. D. Richardson, P. R. Haycock,
A. J. P. White and M. J. Fuchter, J. Am. Chem. Soc., 2014, 136,
1878–11881.
In summary, the absorbance changes due to the E–Z iso-
merization as a function of the 860 nm excitation intensity in
Fig. 4 demonstrate the biphotonic nature of this transforma-
tion. Crucial back-to-back tests with SAP show no isomerization
evidence at these powers for the actuator-only solutions or
IR780 – SAP mixtures. Such experiments demonstrate that the
CySAP non-linear isomerization response requires the presence
of the antenna section. The cyaninic moiety provides the 2PA
properties in this modular design where actuator excitation
occurs indirectly. The Cy-SAP molecule represents a proof of
concept for this general design where the antenna section
provides the actuator with significant two-photon properties
1
5 (a) C. A. Guarin, J. P. Villabona-Monsalve, R. L o´ pez-Arteaga and
J. Peon, J. Phys. Chem. B, 2013, 117, 7352–7362; (b) J. Fu,
L. A. Padilha, D. J. Hagan, E. W. Van Stryland, O. V. Przhonska,
M. V. Bondar, Y. L. Slominsky and A. D. Kachkovski, J. Opt. Soc. Am.
B, 2007, 24, 67; (c) R. S. Lepkowicz, O. V. Przhonska, J. M. Hales,
J. Fu, D. J. Hagan, E. W. Van Stryland, M. V. Bondar, Y. L. Slominsky
and A. D. Kachkovski, Chem. Phys., 2004, 305, 259–270;
(
d) J. Rodr ´ı guez-Romero, C. A. Guarin, A. Arroyo-Pieck,
L. Guti ´e rrez-Arzaluz, R. L o´ pez-Arteaga, F. Cort ´e s-Guzm ´a n,
P. Navarro and J. Peon, ChemPhotoChem, 2017, 1, 397–407.
(a) L. Mu n˜ oz-Rugeles, D. Gallardo-Rosas, J. Dur ´a n-Hern ´a ndez,
R. L o´ pez-Arteaga, R. A. Toscano, N. Esturau-Escofet, J. G. L ´o pez-
Cort ´e s, J. Pe o´ n and M. C. Ortega-Alfaro, ChemPhotoChem, 2020, 4,
6
1
44–154; (b) F. Hochberger-Roa, S. Cort ´e s-Mendoza, D. Gallardo-
Rosas, R. A. Toscano, M. C. Ortega-Alfaro and J. G. L o´ pez-Cort ´e s,
Adv. Synth. Catal., 2019, 361, 4055–4064.
using a near-resonant step-like transition (cyaninic S at 1.5 eV)
1
to augment the 2PA properties of NIR light. This modular
design and the resulting non-linear properties can have appli-
cations in areas where highly localized E–Z photoisomerization
processes are important, including the de-stabilization of lipo-
somes for content-release, and the control of cis–trans isomeriza-
tion related to macromolecular control, or photopharmacological
7 M. J. Frisch, et al., Gaussian 09, Revision E.01, Gaussian, Inc.,
Wallingford CT, 2016.
8
M. Taniguchi, H. Du and J. S. Lindsey, Photochem. Photobiol., 2018,
4, 277–289.
9
9 K. Stranius and K. B o¨ rjesson, Sci. Rep., 2017, 7, 1–9.
10 (a) D. Liu, S. Wang, S. Xu and H. Liu, Langmuir, 2017, 33, 1004–1012;
(
(
b) M. J. Fuchter, J. Med. Chem., 2020, 63, 11436–11447;
c) J. Morstein, M. Awale, J. L. Reymond and D. Trauner, ACS Cent.
10
schemes.
Sci., 2019, 5, 60.
3
126
|
Chem. Commun., 2021, 57, 3123–3126
This journal is © The Royal Society of Chemistry 2021