(Reproduced from Reference 59 with permission of IOP Publishing Ltd.
online version available: https://doi.org/10.1088/1361-648X/aaa1ac )
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Keywords: ultrafast dynamics, molecular dyad, electron transfer, energy transfer, thin films of titania and alumina
1. Introduction
Energy and electron transfer processes in photoswitchable dyads in solution have been intensively studied and optimized in the past two decades for applications in material and life sciences [1,2]. However, reversible light modulation of intramolecular and interfacial electron and energy transfer processes within photoswitchable dyads on semiconductor surfaces remain to be a challenge. The DTE-BODIPY conjugates A and B, figure 1, reported herein contain a carboxylic acid anchoring group for attachment to the TiO2
surface. In the dyads different arrangements of the fluorescent BODIPY (boron-dipyrromethene) donor and the DTE (dithienylethene) acceptor were realized. These conjugates were designed to investigate the electron transfer at the TiO2 (titanium dioxide) surface in a two-state scenario, and the Al2O3 (aluminium oxide) surface (insulator) was chosen as a passive reference system (scheme 1).
Figure 1. Chemical structures and photochromism of the conjugates BODIPY543-DTE-COOH (A) and BODIPY585-DTE-BCP-COOH (B) in solution.
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ultrafast dynamics, molecular dyad, electron transfer, energy transfer, thin films o ultrafast dynamics, molecular dyad, electron transfer, energy transfer, thin films of
Energy and electron transfer processes in photoswitchable dyads in solution have been intensively studied Energy and electron transfer processes in photoswitchable dyads in solution have been intensively studied
for applications in material and life
for applications in material and life sciences [1,2].
and interfacial electron and energy
and interfacial electron and energy transfer processes remain to be a challengeto be a challenge. The DTE
reported herein contain a carboxylic acid anch
reported herein contain a carboxylic acid anchoring group for attachment tooring group for attachment to ifferent arrangements of
ifferent arrangements of the the fluorescent fluorescent BODIPYBODIPY were realized
were realized.. These conjugates were designed to investigate the These conjugates were designed to investigate the (titanium dioxide) surfacesurface in a twoin a two-state
surface (insulator) was chosen as a passive reference
surface (insulator) was chosen as a passive reference system (scheme
A cc epte
Figure 1.Chemical structures Chemical structures
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(figure 1). According to the point dipole approximation in Förster theory no energy transfer should occur in this dyad in solution, and coulombic interactions do not contribute to the electronic coupling. However, for such orthogonal transition moments an extended Förster model has to be applied including vibrational motions because of low wavenumber ground state vibrations, in addition to structural and electronic deformations induced by solvent fluctuations. These deformations break the orthogonal arrangement of the transition dipole moments, and are responsible for a noise-induced FRET (Förster resonance energy transfer) mechanism with a generally much weaker distance dependence (~1/R3). Besides the orthogonal arrangement, conjugate BODIPY543-DTE-COOH (A) necessitates a phenyl ring instead of the ethynyl unit directly attached to the BODIPY donor BODIPY543 (543 nm: emission maxima), to prevent complete planarity in the immediate surrounding [4]. In BODIPY543-DTE-COOH (A) the carboxylic acid anchoring group was directly attached to the DTE (figure 1).
Another option is the linear arrangement of an appropriate BODIPY donor and the DTE photoswitch.
However, when chromophores come in close proximity to each other, the dipole approximation for FRET becomes inadequate. Therefore, in donor-acceptor systems with parallel dipole moments a strong mixing of electronic and vibrational degrees of freedom is expected, and through-bond contributions to the energy transfer can be observed by modifying the donor-acceptor spacer in length and chemical character, e.g. the exchange interaction decays exponentially with donor-acceptor distance [5,6]. First attempts to realize a planar arrangement in a conjugate of BODIPY donor BODIPY585 (585 nm: emission maxima) containing the same DTE as in conjugate A, and a directly attached carboxylic acid anchoring group, proved to be in vain. The conjugate could not be switched between two states due to pronounced conjugation.
Therefore, herein we study in detail the conjugate BODIPY585-DTE-BCP-COOH (B) with a parallel arrangement of the BODIPY donor BODIPY585 (β-substituted fragment in conjugate B: lmax,abs = 541 nm, lmax,em = 585 nm for lexc = 480 nm (C2H4Cl2), see figure SI-4 up to SI-12) and a 4,4'-dimethyl-substituted DTE unit (figure 1). Additionally, the BCP unit besides a phenyl ring was incorporated between the anchoring group and the DTE, to effectively interrupt the conjugation between the donor and the anchoring group.
Linker units between BODIPY donor and DTE acceptor have to be carefully chosen. For instance, 2-methyl-substituted dithienylethene-bridged diporphyrins with short spacers, e.g. ethynyl or phenyl, do not undergo any photoreaction under light illumination, probably through intramolecular energy transfer quenching. However, longer 4-ethynylphenyl and di-4-phenyl-ethynylene spacers furnished 20 % and 75 % of the closed form DTE-c in the pss (photostationary state) [7], and an unsymmetrically substituted 5,5'-dicarboxylic acid-substituted PDI(perylene bisimide)-DTE dyad yielded 69 % of the closed form DTE-c in the pss [8]. Therefore, appropriate choice of the spacer units and of the anchoring group in DTE-BODIPY
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). According to the point dipole approximation in Förster theory no energy transfer should ). According to the point dipole approximation in Förster theory no energy transfer should occur in this dyad in solution, and coulombic interactions do not contribute to the electronic coupling. However, in this dyad in solution, and coulombic interactions do not contribute to the electronic coupling. However, for such orthogonal transition moments an extended Förster model has to be applied including vibrat for such orthogonal transition moments an extended Förster model has to be applied including vibrational
number ground state vibrations, in addition to structural and electronic number ground state vibrations, in addition to structural and electronic deformations induced by solvent fluctuations. These deformations break the orthogonal arrangement of deformations induced by solvent fluctuations. These deformations break the orthogonal arrangement of induced FRET (Först(Förster resonance energy er resonance energy mechanism with a generally much weaker distance dependence (~1/R1/R3). Besides the orthogonal ). Besides the orthogonal ) necessitates a phenyl ring instead of the ethynyl unit ) necessitates a phenyl ring instead of the ethynyl unit
emission
emission maxima)maxima), to prevent complete DTE-COOH COOH ((AA) the carboxylic acid anchoring ) the carboxylic acid anchoring
Another option is the linear arrangement of an appropriate BODIPY donor and the DTE photoswitch.
Another option is the linear arrangement of an appropriate BODIPY donor and the DTE photoswitch.
However, when chromophores come in close proximity to each other, the dipole approximation for FRET However, when chromophores come in close proximity to each other, the dipole approximation for FRET
acceptor systems wi
acceptor systems with parallel dipole moments a strong mixing th parallel dipole moments a strong mixing of electronic and vibrational degrees of freedom is expected, and through
of electronic and vibrational degrees of freedom is expected, and through energy transfer can be observed by modifying the donor
energy transfer can be observed by modifying the donor-acceptor spacer in length and chemical character, acceptor spacer in length and chemical character, exchange interaction decays exponentially with donor
exchange interaction decays exponentially with donor a planar arrangement in a conjugate of BODIPY donor a planar arrangement in a conjugate of BODIPY donor containing the same DTE as in conjugate
containing the same DTE as in conjugate AA, and a directly , and a directly
he conjugate could not be switched between two states due to pronounced conjugation.
he conjugate could not be switched between two states due to pronounced conjugation.
Therefore, herein we study in detail the conjugate Therefore, herein we study in detail the conjugate
of the BODIPY donor
of the BODIPY donor BODIPY585 BODIPY585 nm for lexc== 480480 nm (Cnm (C2H4Cl2), see
1). Additionally, the BCP unit besides a phenyl ring was incorporated between the anchoring 1). Additionally, the BCP unit besides a phenyl ring was incorporated between the anchoring group and the DTE, to effectively interrupt the conjugation between the donor and the anchoring group.
group and the DTE, to effectively interrupt the conjugation between the donor and the anchoring group.
Linker units between BODIPY donor and DTE acceptor have to be Linker units between BODIPY donor and DTE acceptor have to be substituted dithienylethene
substituted dithienylethene
undergo any photoreaction under light illumination, probably through intramolecular energy transfer undergo any photoreaction under light illumination, probably through intramolecular energy transfer quenching. How
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conjugates will retain photoswitchability for reversible regulation of intramolecular energy transfer (mechanism B) and fluorescence (mechanism A), respectively, and additionally for the modulation of electron transfer at the TiO2 surface (mechanism C, scheme 1). These processes necessitate effective electronic communication between the two chromophores. In this respect an ethynyl unit between the BODIPY chromophore and the DTE photoswitch will be beneficial (conjugates A, B), and increase the communication by promoting a planar geometry leading to a red shift of the fluorescence wavelength, a negligible effect on extinction coefficient (e), a decrease of fluorescence quantum yield (ffl), and a negligible influence or a decrease of fluorescence lifetime [4,9]. On the other hand, additional phenyl rings incorporated between the chromophore and the BODIPY or the TiO2 anchoring group will help to electronically separate the anchoring group by phenyl ring torsion. Alternatively, an aliphatic linker can be incorporated, between the BODIPY and the anchoring group, e.g. a BCP (bicycle[1.1.1]pentane) unit, to achieve separation as in conjugate B (figure 1). Such an interruption of π-conjugation between donor and anchoring group will not affect the electron injection rate, but will effectively slow down recombination processes via back electron transfer [10,11].
2. Methods
Transient Absorption Spectroscopy
A detailed description of the self-built pump-probe setup is given elsewhere [12].Ultra-short laser pulses (150 fs) were provided by a laser-amplifier system (Clark, MXR-CPA 2001) operating at a repetition rate of 1 kHz at a central wavelength of 775 nm. For the probe pulses, a single filament white light was generated by focusing the laser fundamental in a CaF2 crystal of 5 mm thickness. The pump pulses at 520 nm were produced using a non-collinearly phase-matched optical parametric amplifier [13]. The pump pulse energy was adjusted to ~90 nJ. The experiments were performed under magic angle (54.7° pump-probe polarization angle difference) conditions on both states of the samples, the open state and the pss. The results of these measurements are displayed as absorption difference (DAbs(t)) between the absorption spectra of the excited sample at variable delay times and the ground state absorption. Therefore, this results in positive signals when the absorption in the excited sample is higher and negative ones when the absorption is lower. During the measurements, the samples are periodically moved transversally to the laser beams to minimize possible sample degradation caused by the pump light.
Steady State Fluorescence Spectroscopy
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conjugates will retain photoswitchability for reversible regulation of intramolecular energy transfer conjugates will retain photoswitchability for reversible regulation of intramolecular energy transfer for the modulation of for the modulation of ese processes
ese processes necessitatenecessitate effective effective ethynyl unit between the ethynyl unit between the jugates AA, , BB)), and increase the , and increase the communication by promoting a planar geometry leading to a red shift of the fluorescence wavelengththe fluorescence wavelength
, a decrease of fluorescence quantum yield , a decrease of fluorescence quantum yield (f
On the other
On the other handhand, additional phenyl rings , additional phenyl rings or the TiOTiO2 anchoring group will help to anchoring group will help to electronically separate the anchoring group by phenyl ring torsion. Alternatively, an aliphatic linker can be electronically separate the anchoring group by phenyl ring torsion. Alternatively, an aliphatic linker can be
and the anchoring group, e.g. e.g. a BCPa BCP ((bicycle[1.1.1]pentane)bicycle[1.1.1]pentane) Such an interruption of π
Such an interruption of π--conjugation between donor and anchoring group will not affect the electron injection rate, but will effectively slow down recombination anchoring group will not affect the electron injection rate, but will effectively slow down recombination
built pump
built pump-probe setup probe setup fs) were provided by a laser-amplifier system (Clark, MXRamplifier system (Clark, MXR kHz at a central wavelength of 775
kHz at a central wavelength of 775 nm. For the probe pulses, a single filament white light was generated nm. For the probe pulses, a single filament white light was generated by focusing the laser fundamental in a CaF
by focusing the laser fundamental in a CaF2crystal of 5
produced using a non-collinecollinearly phasearly phase--matched optical parametric amplifier
was adjusted to ~90 nJ. The experiments were performed under magic angle (54.7° pumpnJ. The experiments were performed under magic angle (54.7° pump polarization angle difference) conditions on both states of the samples,
polarization angle difference) conditions on both states of the samples, results of these measurements are displayed as absorption difference ( results of these measurements are displayed as absorption difference (
spectra of the excited sample at variable delay times and the ground state absorption.excited sample at variable delay times and the ground state absorption.
results in positive positive signals when thsignals when th absorption is lower.
absorption is lower. During the measurements, the samples are periodically moved transversal laser beams to minimize possible sample degradation caused by the pump light.
laser beams to minimize possible sample degradation caused by the pump light.
Steady State Fluorescence Spectroscopy Steady State Fluorescence Spectroscopy
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Fluorescence spectra were measured on a Jasco FP-8500 spectrofluorometer, baselines were corrected and the wavelength-dependent instrument sensitivity was taken into account. Slit widths of 2.5 nm for excitation and emission were used. PMT voltage was set to 500 V.
Synthesis
The synthesis of the BODIPY-DTE-conjugates (scheme SI-1 and SI-2) is described in detail in the supporting information.
Sample Preparation for Spectroscopic Measurements
To obtain pure open isomer the samples were illuminated with visible light at l ≥ 650 nm. This could be done in case of dissolved molecules as well as for adsorbed ones. The closing reaction was induced by irradiation with UV light of a wavelength of ~320 nm and resulted in pss mixtures containing at least 40 % of closed isomer. This reaction was only performed in solution since it resulted in sample degradation in case of the adsorbed sample. Therefore the adsorbed samples were prepared in that way that a ready-made solution of pss mixture was used for the adsorption process (see SI). After the (transient) absorption measurements of the pss samples, illumination with visible light was used to create the open form which was subsequently measured. In case of TiO2 the visible light (pump of transient measurement and switching light) also causes degradation of the sample resulting in decreases of the dyads absorption band.
For this reason, to prevent accumulation of possible degradation products, illumination times were chosen as short as possible, only a low number of repetitions of transient measurements were made and no cw illumination was done during those measurements. Thus, reversible switching between pss and open state without degradation was only possible on Al2O3.
3. Results and Discussion
Steady State Spectroscopy
At first, the photochromism of the two BODIPY-DTE conjugates was investigated in solution (figure 1). The open isomers have a sharp S0-S1 band (BODIPY) at ~528 nm (meso-substitution in BODIPY543-DTE-COOH (A), e = 63.0 · 103 cm-1 M-1, see SI), or a broad S0-S1 band at ~540 nm (β-substitution in BODIPY585-DTE-BCP-COOH (B), e = 50.4 · 103 cm-1 M-1, see SI), both accompanied by a shoulder at higher energy around 500 nm due to the 0-1 vibronic transition (figure 2). A strong absorption band of the DTE unit is observed at 330 nm in the open form of both substitution patterns.
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8500 spectrofluorometer, baselines were corrected 8500 spectrofluorometer, baselines were corrected
taken into account. Slit widths of 2.5 taken into account. Slit widths of 2.5 nm for nm for
described in detaildetail in in the the supporting
To obtain pure open isomer the samples were illuminated with visible light at To obtain pure open isomer the samples were illuminated with visible light at ll≥650
done in case of dissolved molecules as well as for adsorbed ones. The closing reaction was induced by done in case of dissolved molecules as well as for adsorbed ones. The closing reaction was induced by
and resulted in pss mixtures containing a and resulted in pss mixtures containing a reaction was only performed in solution since it resulted
reaction was only performed in solution since it resulted adsorbed sample
adsorbed sampless were prepared were prepared
made solution of pss mixture was used for the adsorption process (see SI). After the (transient) absorption made solution of pss mixture was used for the adsorption process (see SI). After the (transient) absorption
illumination with visible light with visible light was used to create the open form measured. In case of
measured. In case of TiOTiO22 the visible light (pump of transient measurement and the visible light (pump of transient measurement and degradation of the sample
degradation of the sample resulting in decreases of the dyads absorption bandresulting in decreases of the dyads absorption band to prevent accumulation of possible degradation
to prevent accumulation of possible degradation only a low number of repetitions of only a low number of repetitions of illumination was done during those measurements
illumination was done during those measurements. Thus, reversible switching between pss and open state without degradation was only possibl
without degradation was only possible on Ale on Al2O3
3. Results and Discussion 3. Results and Discussion
Steady State Spectroscopy Steady State Spectroscopy
At first, the photochromism of the two BODIPY At first, the photochromism of the two BODIPY open isomers have a sharp S
open isomers have a sharp S (A), e== 63.063.0·10103cm-1
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Typically, red-shifted and broadened absorption bands, and reduced extinction coefficients at 528 nm (A) and 540 nm (B), are consistently expected with an increase in conjugation length. However, upon formation of the closed form by UV-light illumination the S0-S1 absorption bands in both series of compounds increased in intensity, and new maxima at ~350 nm (A) or 380 nm (B), and additionally around 600 nm (A and B) are formed (figure 2). The reverse ring-opening was investigated by irradiation at 607 nm (A) and 655 nm (B), see SI-2 and SI-5. The ratios of the two isomers in the photostationary state -(pss) at 326 nm (A) and 300 nm (B) were determined, and ratios of c/o = 83:17 (A, CHCl3) and c/o = 54:46 (B, C2H4Cl2) were calculated (for additional information, including data in CHCl3 for B (c/o = 40:60) , see SI figures SI-1 up to and including SI-12.
Figure 2. a) UV/Vis spectral changes of BODIPY543-DTE-COOH (A) upon irradiation with HBO 200 (326 nm filter) in CHCl3. b) UV/Vis spectral changes of BODIY585-DTE-BCP-COOH (B) upon irradiation with HBO 200 W (300 nm filter) in C2H4Cl2.
In case of the adsorbed molecules, steady state absorption spectra (see figure 3b for B, and figure 3a for A) show that the absorption bands are slightly broadened compared to the spectrum in solution (SI, figure SI-13) but no significant spectral shifts are observable. The difference in absorption in the blue part of the spectrum between the adsorption on TiO2 and Al2O3 is explained by the underlying scattered light, which could not be completely subtracted in the case of TiO2. By comparing the absorbance of the pss and the open isomer on TiO2 surfaces, a clear degradation of both dyads (A and B) is observable. Since this light induced degradation was only observed on TiO2 surfaces, it is the first hint for an electron transfer reaction. For the transient absorption measurements degradation results in decreasing difference signals, restricts the number of experimental repetitions and thus limits the signal to noise ratio.
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a) UV/Vis spectral changes of BODIPY543BODIPY543--DTEDTE-COOHCOOH(A) upon irradiation with HBO 200 (326 COOH(B) upon irradiation ) upon irradiation with HBO 200with HBO 200
In case of the adsorbed molecules, smolecules, steady state absorption spectrateady state absorption spectra ) show that the absorption bands are
) show that the absorption bands are slightly broadened compared to the spectrum in solution but no significant spectral shift
but no significant spectral shift of the spectrum between the adsorption on TiO of the spectrum between the adsorption on TiO which could not be
which could not be completecompletely ly subtracted subtracted
which could not be completecompletely ly subtracted subtracted