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Spectroscopy and Photochemistry; General Theory
Rapid Excited State Deactivation of BODIPY Derivatives by a Boron Bound Catechol Rachel K. Swedin, Yuriy V. Zatsikha, Andrew T. Healy, Natalia O. Didukh, Tanner S. Blesener, Mahtab Fathi-Rasekh, Tianyi Wang, Alex J. King, Viktor N. Nemykin, and David A Blank J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00751 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019
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The Journal of Physical Chemistry Letters
Rapid Excited State Deactivation of BODIPY Derivatives by a Boron Bound Catechol Rachel K. Swedin,† Yuriy V. Zatsikha,‡ Andrew T. Healy,† Natalia O. Didukh,‡ Tanner S. Blesener,‡ Mahtab Fathi-Rasekh,¶ Tianyi Wang,‡ Alex J. King,¶ Victor N. Nemykin,∗,‡ and David A. Blank∗,† †Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA ‡Department of Chemistry, University of Manitoba, MB R3T 2N2, Canada ¶Department of Chemistry and Biochemistry, University of Minnesota Duluth, 1039 University Drive, Duluth, MN 55812 E-mail:
[email protected];
[email protected] 1
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Abstract The excited state dynamics and energetics of a series of BODIPY derived chromophores bound to a catechol at the boron position were investigated with a combination of static and time resolved spectroscopy, electrochemistry, and density functional theory calculations. Compared to the difluoro-BODIPY derived parent compounds, addition of the catechol at the boron reduced the excited state lifetime by three orders of magnitude. Deactivation of the excited state proceeded through an intermediate charge transfer state accessed from the initial optically excited π ∗ state in less than one picosecond. Despite differences in structure of the BODIPY derivatives, and absorption maxima that spanned the visible portion of the spectrum, all compounds exhibited the same, rapid, excited state deactivation mechanism, suggesting generality of the observed dynamics within this class of compounds.
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Keywords BODIPY, BOPHY, catechol linker, photo-initiated electron transfer, organic light harvesting, excited state deactivation
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Modular construction of molecular systems designed to translate the energy of absorbed light into the potential energy of charge separation has received considerable attention. 1,2 Typical covalent constructs consist of three core components: a chromophore to absorb the light and also acts as an excited state oxidant or reductant, an electron donating or accepting partner, and the molecular bridge between them. Chromophores based on modification of a BODIPY core can provide strong light absorption, good stability under photolytic conditions, and broadly tunable absorption and redox properties. 3,4 Substitution chemistry replacing the fluorines on the boron with a catechol, as illustrated at the top of Fig. 1, offers a synthetically efficient route to subsequent attachment of a redox partner. There were reports of successful light driven charge separation from BODIPY derivatives to a fullerene electron acceptor attached via the catechol linker. 5–8 This was interesting given a prior report of complete quenching of the fluorescence in BODIPY derivatives by the catechol, suggesting efficient excited state deactivation of the BODIPY by the linker. 9,10 To investigate the general potential for this approach, and better understand the underlying dynamics, we recently prepared a series of BODIPY-catechol-fullerene triads. 11 While the parent BODIPY derivatives exhibited large fluorescence quantum yields, no fluorescence was observed from any of the triad assemblies. Despite efficient fluorescence quenching, no compelling evidence of charge transfer to the fullerene was observed. Lack of fluorescence in the BODIPY-catechol dyad precursors led to the hypothesis that the initial photo-excited state on BODIPY derived chromophores were rapidly quenched by the catechol linker. This letter provides strong evidence in support of that hypothesis, quantifies the rate of deactivation, and elucidates the mechanism. Difluoro (1x) and catechol substituted (2x) BODIPY derivatives were prepared, purified, and characterized as described in the SI. The structures and absorption spectra are presented in Figs. 1 and 2 respectively. The 1x compounds all exhibited strong, roughly mirrorimage fluorescence with Stokes shifts of 20–40 nm. After addition of the catechol, there was no measurable fluorescence observed from any of the 2x compounds, Figs. S18–S27,
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All catechol-containing compounds 2a – 2e were prepared by the reaction between parent products in all tested solvent systems are very close to those of unidentified minor (1 -2%) fluorescent impurities found in the reaction mixture. in the presence of AlCl 3 as catalysts. It should be noted that compounds 1a – 1d20-23 and catechol The Journal of Physical Chemistry Letters impurities found in the reaction mixture. Synthesis purification of the target compounds 2a, 2d, and 2e requires extra care as the R f values of the desired
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All catechol-containing 2a – 2e were are prepared by the reaction between parent products incompounds all tested solvent systems very close to those of unidentified minor (1 -2%) fluorescent compounds 1a – 1d20-23 and catechol thereaction presence of AlCl 3 as catalysts. It should be noted that impurities found ininthe mixture. OH
purification of the target compounds 2a, 2d, and 2e requires extra care as the R f values of thechromophore desired chromophore OH
N
N
N
AlCl3 / DCM
N
B products in all tested solvent systems are very close to those of unidentified minor (1 -2%) fluorescent B
F
impurities found in the reaction mixture.
F
O
1x
O
2x
Figure 1: Molecular structures and labels.
1
indicating a lack of any residual 1x parent compound in the samples and a significant 1 change in the excited state dynamics. Pump-probe spectroscopy was used to 1measure the
time constants associated with the excited state dynamics. The pump excitation was set at a wavelength near the absorption maximum for each compound. 1Complete experimental details, the transient change in optical density, ∆OD(λ, t), at all wavelengths probed, and fits as a function of time for a number of selected probe1 wavelengths are presented in Figs. S56–S80 for all 1x and 2x compounds in Fig. 1. All transient spectra for the 1x compounds consisted of a ground state bleach (GSB, ∆OD < 0), stimulated emission (SE, ∆OD < 0), and excited state absorption (ESA, ∆OD > 0). The transient spectra appeared within the time resolution of the experiments, ∼50 fs, did not change shape during the full range of time delays probed, and were well fitted by a first-order decay of the amplitude at all probe wavelengths. The fitted time constants are listed in Table 1 and assigned to the initial optically excited state relaxing directly back to the ground state. Pump-probe spectra for the 2x compounds differed significantly from the 1x precursors. The shape of the spectra changed within the first picosecond, and the transient spectra recovered to the baseline, ie the original ground state, with time constants that were ca. 1000 times smaller than the analogous 1x compounds. Pump-probe data for 2a is presented in Fig. 3. A GSB and SE appeared within the time resolution of the experiment. The
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1a 2a 1b 2b 1c 2c 1d 2d 2e
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Figure 2: Absorption spectra in toluene for the structures in Fig. 1. Table 1: First order time constants τπ∗ →gs (ns) τπ∗ →CT (fs) τCT→gs (ps) 1a 3.1 ± 0.1 2a 100 ± 6 2.9 ± 0.2 1b 1.9 ± 0.1 2b 150 ± 20 4.3 ± 0.9 2c < 500 1.8 ± 0.1 1c 1.8 ± 0.1 < 500 15.1 ± 0.5 1d 1.8 ± 0.2 2d 2e < 500 14.0 ± 1.0 π ∗ is the optically excited state, CT is the charge transfer state, and gs is the electronic ground state. negative SE around 580 nm evolved into a positive ESA within the first 100 fs, followed by subsequent return to the baseline. The data probed at 580 nm was fitted as two sequential first-order events with time constants of 100 ± 6 fs and 2.9 ± 0.2 ps, Fig. 3(b). The GSB around 520 nm recovered to the base line on a comparable time scale to the loss of the intermediate ESA, with a time constant of 3.5 ± 0.3 ps. The data demonstrated very rapid deactivation of the photo-excited state via an intermediate that is accessed on a sub-ps time scale. Similar excited state dynamics were observed for all 2x compounds. Time constants for the two sequential deactivation events are listed in Table 1. The time constants for 2b were determined in the same manner as 2a. Evidence for loss of initial SE and creation of new ESA features in the first picosecond was present for all 2x compounds, however, the transition from initial photo-excited state to intermediate was not as well resolved at any single point within transient spectra for 2c, 2d, and 2e. An upper limit of 500 fs for the time 5
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