Balance between Triplet States in Photoexcited ... - ACS Publications

Jul 8, 2019 - The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved ele...
1 downloads 0 Views 883KB Size
Download PDF
Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4157−4163

pubs.acs.org/JPCL

Balance between Triplet States in Photoexcited Orthogonal BODIPY Dimers Yuri E. Kandrashkin,*,†,§ Zhijia Wang,‡,§ Andrei A. Sukhanov,†,§ Yuqi Hou,‡ Xue Zhang,‡ Ya Liu,‡ Violeta K. Voronkova,*,† and Jianzhang Zhao*,‡ †

Zavoisky Physical-Technical Institute FRC Kazan Scientific Center of RAS, Sibirsky Tract 10/7, Kazan 420029, Russia State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian 116024, P.R. China

Downloaded via UNIV OF SOUTHERN INDIANA on July 22, 2019 at 09:35:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electron spin polarization (ESP) of the triplet state of the dimers, accessed with spin−orbit charge-transfer ISC, is different from that of the monomer (spin−orbit coupling-induced ISC). The TREPR spectra show that the triplet state initially formed by charge recombination is localized on either of two subunits, with different preference and ESP patterns. On the basis of the relative orientation of the respective zero field splitting principal axes, the Tx state on one subunit and the Tz state on another subunit in the dimer are overpopulated. The balance between the two triplet states is confirmed by the temperature dependency of the population ratio. No quintet state was detected with TREPR down to 20 K; thus, the recently proposed singlet fission ISC mechanism is excluded.

B

molecules difficult. Normally, no chromophores with strong absorption of visible light were used in these dyads, and the triplet-state yields (ISC quantum yields) and the triplet-state lifetimes were rarely studied in detail. Therefore, designing small organic molecules with simple structures but showing strong visible light-absorption, efficient ISC, and long-lived triplet state is highly desired, but it is still challenging from the point of view of photochemistry. Using a shorter linker between the electron donor and acceptor may simplify the synthesis of the above-mentioned dyads showing RP ISC, but the short linker will increase the J values significantly; as a result, the energy gap between the 1CT and 3CT states of these compact electron donor−acceptor dyads will increase as well, and the RP ISC will be inhibited.16 It was known that the ISC of the compact electron donor− acceptor dyads will be enhanced, given the electron donor and acceptor subunits with their π-conjugation planes adopt orthogonal geometry (perpendicular geometry).18−22 This mechanism is the so-called spin−orbit charge-transfer ISC (SOCT-ISC),13,19,22 which has been known for decades.18,22 ISC is enhanced because of the conservation of the total angular momentum (compensation between the molecular orbit angular momentum and electron spin angular momentum). This mechanism is of particular interest for designing heavy atom-free triplet photosensitizers. Recently this area has

ODIPY derivatives (boron-dipyrromethene) as versatile fluorophores have attracted much attention because of their outstanding photophysical properties,1−4 for instance the strong absorbance of the visible light, photostability, and feasible derivatization chemistry.5 Applications of BODIPY dyes in various areas have been studied, e.g., in photoluminescence,6 molecular probes,7 photocatalysis,8 photodynamic therapy,2 and photon upconversion.9 Conventionally, the strengthening of the intersystem crossing (ISC) in BODIPY,8 and many other organic chromophores, is based on the heavy atom effect.10 However, this method suffers from the drawbacks of high cost, toxicity, poor photostability, and the shortened triplet-state lifetime by the heavy atom effect.11 Thus, designing heavy atom-free triplet photosensitizers is important from the point of view of both fundamental chemistry and applications.12 It is known that charge recombination (CR) of the charge transfer (CT) state can induce ISC in some electron donor− acceptor dyads.13−15 In these dyads the separation between the electron donor and acceptor is large; as such, the electron spin−spin exchange interaction (J) of the radical anion and the radical cation is much smaller than the Zeeman energy (typically 0.3 cm−1). As a result, the 1CT and the 3CT states have close energies, and the hyperfine interaction (HFI) between the magnetic nuclei and the electrons will drive the electron spin rephrasing/flip; thus, the ISC occurs via the radical pair ISC (RP ISC).16,17 However, these electron donor−acceptor dyads are generally not suitable as triplet photosensitizers, because the rigid, long linker between the electron donor and acceptor makes the synthesis of the © XXXX American Chemical Society

Received: June 16, 2019 Accepted: July 8, 2019 Published: July 8, 2019 4157

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters witnessed a substantial development. Highly efficient photosensitizers based on SOCT-ISC,20,21,23−26 detailed photophysical properties,27 maximizing ISC yields of organic photoredox catalysts by invoking a charge-transfer state,28 and application of these new triplet photosensitizers in triplet− triplet annihilation upconversion25,29 have been reported. Orthogonal BODIPY dimers are representative compact electron donor−acceptor dyads mentioned above, and they were reported to show efficient ISC.30,31 However, the ISC mechanism of these dimers is still an open question. Initially, the ISC in these dimers was assigned to happen via the doubly excited states.32 Later it was confirmed with femtosecond transient absorption spectroscopy that the ISC proceeds via charge separation and charge recombination processes, i.e. by SOCT-ISC.21,33 Recently, singlet fission was proposed to be responsible for the ISC in these orthogonal BODIPY dyads.34 Thus, the mechanism of the ISC from the excited singlet state of the BODIPY dimer still needs to be clarified. It should be pointed out that the femtosecond transient absorption spectra are not fully convincing to discriminate between the RP ISC, SOCT ISC, and even singlet fission, because the same CT state may be involved in all these mechanisms.35 Pulsed laser excited time-resolved paramagnetic resonance spectroscopy (TREPR) is helpful to study the ISC mechanism and the triplet-state properties.17,36−38 TREPR has been used to investigate the SOCT-ISC in several electron donor− acceptor dyads with orthogonal geometry.13,18,19 Some of us studied electron donor−acceptor dyads based on anthracene or BODIPY.24 For some dyads, we observed the two localized triplet states and the 3CT state simultaneously.26 However, the ISC mechanism of the orthogonal BODIPY dimers was not studied with TREPR. A remaining challenge for the orthogonal BODIPY dimers is clarifying the intramolecular triplet energy transfer and the localization of the triplet-state wave function. Because of the similarity of the optical spectral feature of the two subunits in the BODIPY dimers (Scheme 1), it is difficult to confirm the intramolecular triplet energy transfer with transient absorption spectra. TREPR spectroscopy might be helpful to solve these puzzles.39

Figure 1. Normalized UV−vis absorption spectra (solid lines) and the fluorescence emission spectra (dashed lines) of the orthogonal BODIPY dimers BB-1 (black lines) and BB-2 (red lines). c = 1.0 × 10−5 M in acetonitrile; 20 °C.

that it is not feasible to discriminate the different triplet states on the two subunits and to study the dynamics of the two triplet states localized on the subunits of the dimer because of the identical ground-state bleaching band and the excited-state absorption feature of the two subunits.33,34 The TREPR spectra of BB-1 and BB-2 recorded after the laser flash are shown in panels A and B of Figure 2,

Figure 2. TREPR spectra of (A) BB-1 and (B) BB-2 recorded at 0.4 μs after the laser flash, integrated over the 50 ns time window. Panels C and D are the two simulated ESP patterns used to fit the experimental data in panels A and B, respectively. The ESP patterns in panels C and D are simulated for the spin state determined by the density matrix (1) and spin-Hamiltonian (2). The blue curves correspond to px ≠ 0, py = pz = 0 and the red curves correspond to pz ≠ 0, px = py = 0. The nonzero values of the sublevels weights are given in panels C and D. The simulations in panels A and B are the sum of the two spectra shown in panels C and D, respectively. The spectra were measured in frozen DCM/toluene (1:1, v/v) at 80 K.

Scheme 1. Structures of the Orthogonal BODIPY Dimers (BB-1 and BB-2) Showing Efficient ISCa

a

The ZFS principal axes of the subunits of the dyads are labeled. In each dimer the left subunit (in red) is arbitrarily designated as the h unit (h: horizontal) and the right subunit (in blue) is designated as the v unit (v: vertical) for the sake of clarity in discussion. Note for both dimers, the y axis is perpendicular to the BODIPY planes for both the h- and v-subunits.

respectively. The electron spin polarization (ESP) pattern of BB-1 is AAEAEE (Figure 2A) (In ESP, A stands for enhanced absorption, E stands for emission), while BB-2 shows ESP of EAEAEA (Figure 2B). The ESP excluded the RP ISC mechanisms for these dimers, which should give ESP as AEEAAE or EAAEEA.16,39 The values of the zero field splitting (ZFS) tensor of the dimers are similar to the values of the photoexcited BODIPY monomer derivative (2-iodo-BODIPY, Table 1), but the latter has a completely different ESP pattern of EEEAAA.40 These results indicate that the triplet-state wave function of the dyads is highly confined on one subunit in the dimer.41 No quintet state was observed with TREPR; thus, the

To clarify the above questions concerning the orthogonal BODIPY dimers, herein we use TREPR spectroscopy to study the triplet state of the two representative BODIPY dimers BB1 and BB-2 (Scheme 1). The steady-state ultraviolet−visilbe (UV−vis) absorption spectra and the fluorescence emission spectra of the two dimers are presented in Figure 1. It is clear 4158

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters Table 1. ZFS Parameters (D and E) of the Triplet States and Population Rates of the Sublevels of the T1 Excited State of the BODIPY Dyads monomer BB-1b BB-2b

a

D (MHz)

E (MHz)

px

py

pz

−2595 −2350 −2280

−575 +470 +470

0.1 0.85 0.54

0.15 0 0

1.0 0.15 0.46

magneton; and g is the g-factor of the triplet state, which in the organic molecules is close to the g-factor of free electron. Both triplet states, Th and Tv, have similar spectral properties and are described by the spin-Hamiltonian (eq 2). We were not able to differentiate the ZFS parameters of the individual triplet states from the fitting of the TREPR spectra. This is not a surprise. In general, the ZFS parameters of BODIPY monomers, like other chromophores, are expected not to be rather sensitive to the structure of the substituents and connections; the spin density surface is mainly confined on the π-conjugation frameworks (see the electron spin density surface of the triplet state in the Supporting Information). The Zeeman interaction with the external magnetic field is much stronger than the ZFS interaction. Thus, the eigenstates of the Hamiltonian (eq 2) can be approximated by the highfield limit wave functions T±1,0. The TREPR spectrum records the resonance transitions between two spin sublevels as shown in Figure 3A−C. The initial nonequilibrium population of the

a

Recalculated from ref 39. bThe ESP of the dimers is assumed to be developed by the SOCT-ISC mechanism; therefore, py is set to zero for both triplets, Th and Tv, and px(h) = pz(v) = 0; the nonzero values px = px(v) and pz = pz(h) reflect the mutual weights of the populations of the triplet states Th and Tv.

singlet fission, recently proposed to be responsible for the ISC of the BODIPY dimers,34 is unlikely.35,42 ISC in the heavy atom-free BODIPY monomers is inefficient,43 while the observed signal from BB-1 and BB-2 is very intense. Thus, the initial ESP in these dimers is not generated by the regular ISC (SO ISC) of the unsubstituted subunits. Instead, the main channel populating the triplet state is proposed to be the SOCT-ISC mechanism.33 According to the SOCT-ISC mechanism, the CR of the CT state forms the well-isolated triplet state. The ESP developed by the SOCT-ISC mechanism is very sensitive to the mutual orientation of the subunits in the dyads.18,24,26 In general, the CR populates the spin state along the direction corresponding to the rotation axis of the orbital momentum.19 It has been shown that the π-conjugation planes of the subunits h and v in the two dimers (Scheme 1) are almost perpendicular to each other.33 For the given definition of the principal ZFS axes of the chromophore in the orthogonal dimers, the recombination to the triplet state of the h-subunit is governed by the rotation of the angular momentum along the z-axis in the h unit, while the recombination to the triplet state of the v-subunit requires the rotation along the x-axis of the v unit (Scheme 1).40 Therefore, the enhanced population of the triplet spin sublevel z is expected for the state Th localized on the h-subunit, and the enhanced population of the sublevel x is expected for state Tv localized on the v-subunit; that is, the triplet states formed by SOCT-ISC on the two subunits in the dimer BB-1 (or BB2) should have different ESP patterns. The initial density matrix of the triplet state generated by the ISC is determined by the principal axes of the spin−orbit interaction which typically are determined by the symmetry axes of the molecular structure. For mutually orthogonal geometry of the chromophores h and v (Scheme 1), the principal axes of the tensors of the spin−orbit interaction and ZFS are collinear, and the density matrix of the observable triplet state is described via the populations of the spin sublevels: ρ0 = px (1 − Sx 2) + py (1 − Sy 2) + pz (1 − Sz 2)

Figure 3. Polarization patterns for the triplet-state TREPR spectra derived for D < 0 and E > 0. The first row illustrates the resonance conditions for the canonical orientations of the external magnetic field along the principal molecular axes: B||x (panel A), B||y (panel B), and B||z (panel C). The stick spectra in the bottom row demonstrate the polarization patterns for different populations of the initial triplet state: px ≠ 0, py = pz = 0 (panel D); py ≠ 0, px = pz = 0 (panel E); and pz ≠ 0, px = py = 0 (panel F). The positive sign is set for the absorption (A) signal and negative sign corresponds to the emission (E) signal.

triplet sublevels leads to the generation of the EPR signal of both signs A and E. The observed polarization pattern depends on the populations of the spin sublevels of the triplet state and the signs of the ZFS D and E parameters. In the case when the external field is aligned along the molecular frame axis x, the wave function of this state becomes T0 and two other wave functions are mixed equally to give T± states (Figure 3A). For the given signs of the ZFS parameters (D < 0 and E > 0), if the triplet state was initially populated along the molecular x-axis (px ≠ 0, py = pz = 0), the low-field transition will absorb the microwave while the high-field transition will emit (Figure 3A). If the triplet state initially populated along the molecular y or z axes, the positions of the absorption and emission transitions will swap (Figure 3A). The ESP patterns at other orientations of the external magnetic field can be observed similarly. The ESP pattern of randomly orientated triplet-state molecules can be found by combination of the transitions for the magnetic field aligned along three principal axes (Figure 3A−C). The AAEAEE type spectrum is expected for the initial population px along the molecular x axis (Figure 3D), AEAEAE for py (panel E), and EEEAAA for pz (panel F). BODIPY is a prolate molecule, and the parameter D is assumed to be negative.39

(1)

Here the spin operators are defined in the molecular frame of the observable chromophore. The energies of the triplet sublevels are determined by the spin-Hamiltonian that includes the Zeeman interaction with the external magnetic field and ZFS. H = μB g BS + D(Sz 2 − S2 /3) + E(Sx 2 − Sy 2)

(2)

The parameters D and E are the principal values of the ZFS tensor; B is the external magnetic field; μB is the Bohr 4159

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters The positive sign of E is chosen to correspond to the experimental data. Two states will be predominantly populated by the SOCT-ISC mechanism: x state in the v-subunit and z state in the h-subunit for the given geometry of the dimer (Scheme 1). The experimental spectra (Figure 2A,B) can be reproduced by a combination of the polarization patterns AAEAEE for px ≠ 0 (Figure 3D) and EEEAAA for pz ≠ 0 (Figure 3F) derived from (D < 0, E > 0). The ESP pattern for px ≠ 0 and E < 0 is equivalent to the ESP pattern for py ≠ 0 and E > 0; that is, it becomes the AEAEAE type. It is clear that the experimental TREPR spectra cannot be reproduced by a combination of two patterns, AEAEAE and EEEAAA. Thus, the sign of E should be positive. The spectra were simulated based on the initial density matrix (eq 1) and the spin-Hamiltonian (eq 2). The relative contributions of the initial populations of the Tz and Tx states (corresponding to the enhanched populations of the h-subunit and the v-subunit, respectively) are shown in Figure 2C and 2D. They relate to the ESP patterns shown in Figure 3D,F. The simulated ESP patterns fit well the experimental data (Figure 2A,B). Thus, the comparison of the experimental and simulated spectra gives unambiguously the positive sign E in the case of the SOCT-ISC mechanism of the ESP. The simulation also validates that the TREPR spectra of the dimers are composed of two different ESP patterns which can be referred to the triplet states, Tz (on h-subunit) and Tx (on vsubunit). The difference between the experimental and simulated spectra is due to the oversimplified model used for the simulations, which does not include the relaxation and spectral diffusion-like processes initiated by the intramolecular energy transfer (EnT) between two subunits. The simulations of the experimental TREPR curves show that the experimental spectra can be decomposed in two types of the ESP patterns, corresponding to the enhanced populations of the x- and zsublevels. Assuming the dominance of the SOCT ISC, we can refer these states to the different triplets, Tv and Th, correspondingly. Thus, the spectrum of the photoexcited dimer BB-1 can be interpreted as a sum of the spectrum of the Th state with the overpopulated z-sublevel and the spectrum of the Tv state with the overpopulated x-sublevel. Assuming that other sublevels are not populated, the relative weights of triplet states Th and Tv can be estimated as pz(h):px(v) = 0.15:0.85. Similarly, the relations between the weights of the two triplets in BB-2 is pz(h):px(v) = 0.46:0.54. Thus, although we cannot differentiate the spectral properties of the triplet states Th and Tv, assuming the dominant SOCT-ISC mechanism, we can separate the contributions of these states based on the ESP properties. The simulation parameters and the initial population rates are presented in Table 1. The populations of the initial spin state in dimers are different in comparison with the monomer. The analysis of the polarization patterns presented in Figure 3 shows that the population rates are py < px, pz, and in the following we set py = 0 assuming for the sake of simplicity that only the SOCT-ISC process populates the triplet state. More systematic analysis of the TREPR data including the processes of the spin−lattice relaxation (SLR) is required to obtain the weight of the population py relative to the px and pz. This analysis is outside the scope of this Letter and will be presented elsewhere. The excited states in chromophores are formed mostly by πtype orbitals distributed over the molecular plane. There are two processes, the direct SOCT-ISC and the intramolecular

triplet EnT between the h-subunit and the v-subunit of the dimers, which require the change of the orbital from the plane of one chromophore to another. Based on the geometry of the dimers (Scheme 1), these processes involve the rotations of orbital momentum around the direction of the bridge connecting two chromophores which is collinear with the zaxis of the h-subunit and with the x-axis of the v-subunit (Scheme 1). Therefore, the initial populations px and pz developed during the CR correspond to the populations Tx and Tz states on the v- and h-subunits, respectively. Later, these populations can be redistributed by the process of the intramolecular EnT. In the absence of the external magnetic field, during the EnT the Tz population of the h-subunit becomes the Tx population of the v-subunit, and vice versa.44 In total, both processes, the CR and the following EnT, will keep the dominant population of the triplet states along the rotation axis of the orbital momentum, as illustrated in Figure 4.

Figure 4. Balance of the excited triplet states in BODIPY dimers. The energy of the triplet state of the h-subunit (Th) is slightly higer in comparison with the energy of the triplet state of v-subunit (Tv) (see text).

The EnT leads to several properties observable by the TREPR spectroscopy. First, the change of the reference frame (ZFS principal axes) from one subunit to another leads to the change of the ZFS interactions. This induces the change of the resonance conditions observed as spectral diffusion of the ERP signal.44 Second, the spin-Hamiltonians (eq 2) of the triplet states Tz and Tx on the two subunits do not commute except for the orientations of the external magnetic field along the principal axes of the ZFS. Therefore, the EnT induces the spinflip processes which manifest as the anisotropic SLR. In particular, the near-zero rate of the induced SLR is assumed to be the origin of the relatively high intensity of the EPR signal for the orientations of the magnetic field along the x-axis of dimer BB-2 (compare the experimental and simulated spectra at B = 281 mT and B = 409 mT, Figure 3B). The population of the triplet state localized v-subunit is more significant in both dyads, which might reflect more efficient initial formation of this triplet state and its lower energy level, which is supported by the spin density analysis.45 To study the balance of two triplet states, the TREPR spectra at different temperatures were recorded (Figure 5). The figure shows the normalized population pz obtained at different temperatures and delay times after the laser flash. To reduce the impact of EnT and the resulting SLR, the data were collected within short delay after the laser flash. The relative weights were obtained from the fitting of the experimental data by two polarization patterns shown in Figure 2C,D. The population py is set to zero. In the case of BB-1 (Figure 5A), the relative population pz has a value between 5% and 15% in the temperature range of 120−80 K. At 100 K, it significantly 4160

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters

Figure 5. Delay time and temperature dependency of the normalized population pz (py = 0, px + pz = 1) found from the fitting of the TREPR data of the dimers (A) BB-1 and (B) BB-2 with the ESP patterns.

molecules (where h represents the horizontal and v the vertical subunit of the dimer). The relative weight of the electron polarization pattern associated with population of the zsublevel is lower in comparison with the population of the xsublevel which corresponds to the higher energy of the hsubunit than the v-subunit. The change of the relative weights of these populations with the temperature and the delay time after the laser flash demonstrates the intramolecular triplet energy-transfer processes. The model presented in this work gives an adequate explanation of the observed properties of the triplet state in orthogonal BODIPY dimers. The positive sign of the parameter E is predicted based on the observed ESP pattern. No direct evidence for the existence of a quintet state was observed, which goes against the recently proposed singlet fission ISC mechanism of the orthogonal BODIPY dimers.

decreases within a microsecond interval. As discussed above, the observed polarization pattern associated with z-population corresponds to the excited triplet state localized on the hsubunit. Thus, it is expected that the energy of the h-subunit is higher than that of the v-subunit, and the barrier between these triplet states is ca. 100 K. To study the change of the spectral properties, we have also measured the dimers BB-1 at lower temperature (20 K, not shown). The shape of the TREPR spectrum at 20 K remains the same as at 80 K, although the signal intensity is much weaker. Thus, no indication of the SF was found for these dimers even at very low temperatures. We found the SLR in BB-2 is faster than that in BB-1, which can be associated with the more intense EnT between two triplets in BB-2. The triplet state with the lower energy is expected to have a larger population. Thus, on the basis of the mutual weights between pz(h) and px(v), the energy of the state Tv is expected to be lower in BB-1 in comparison with the energy of the state Th. For both dyads, there is also a clear tendency of the reduction of the relative weight of the Th states with increasing the temperature. Therefore, the energy of the state Th in BB-2 is expected to be also higher than the state Tv. It is noteworthy that the slow SLR indicates a slow intramolecular energy transfer at low temperature (supported by results in Figure 5); otherwise, the fast EnT will cause fast thermalization of the spins.44 The decreasing of the population px with prolonged delay time in Figure 5A indicates the intramolecular EnT process. It should be noted that the intramolecular EnT in these dimers is difficult to monitor by transient optical spectroscopies, such as femtosecond transient absorption spectroscopy, because the two subunits of the dimers exhibit quite similar optical spectral features (such as absorption).33,34 The TREPR data reveal the balance between two triplet states as illustrated in Figure 4. The populations of two triplets in the dyad BB-1 on two subunits are nonequal (Table 1). The relatively large population of the z-sublevel in BB-2 can be attributed to the presence of the electron-withdrawing nitrophenyl group attached in the h-subunit, which may reduce the LUMO energy level, thus resulting in lower triplet state levels (see the electron spin density surfaces in the Supporting Information). On the basis of the relatively slow kinetics (Figure 5), the initial two different triplet states, Tv and Th, have weights of 0.85:0.15 for BB-1 and 0.54:0.46 for BB-2, respectively (note the two triplet states are not in one single dimer molecule). In summary, the TREPR spectra of orthogonal BODIPY dimers validate the spin−orbit charge-transfer intersystem crossing mechanism and the coexistence of the triplet states localized either on h- or on v-subunits in the isolated dimer

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01741.

■

Materials, TREPR experimental conditions, spin density surfaces, and coordinates of the optimized triplet-state geometries (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.E.K.). *E-mail: [email protected] (V.K.V.). *E-mail: [email protected] (J.Z.). ORCID

Zhijia Wang: 0000-0002-8309-0050 Yuqi Hou: 0000-0002-8175-6568 Jianzhang Zhao: 0000-0002-5405-6398 Author Contributions §

Y.E.K., Z.W., and A.A.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Y.E.K, A.A.S., and V.K.V. appreciate the support of the Russian Foundation for Basic Research (Project 19-53-53013). J.Z. thanks the NSFC (21673031, 21761142005, 21911530095, and 21421005) and the State Key Laboratory of Fine Chemicals (ZYTS201901) for financial support. 4161

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters

■

W.; et al. Generation of Triplet Excited States via Photoinduced Electron Transfer in meso-Anthra-BODIPY: Fluorogenic Response toward Singlet Oxygen in Solution and in Vitro. J. Am. Chem. Soc. 2017, 139, 6282−6285. (21) Zhang, X.-F.; Yang, X.; Xu, B. PET-Based bisBODIPY Photosensitizers for Highly Efficient Excited Triplet State and Singlet Oxygen Generation: Tuning Photosensitizing Ability by Dihedral Angles. Phys. Chem. Chem. Phys. 2017, 19, 24792−24804. (22) Okada, T.; Karaki, I.; Matsuzawa, E.; Mataga, N.; Sakata, Y.; Misumi, S. Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers. J. Phys. Chem. 1981, 85, 3957−3960. (23) Chen, K.; Yang, W.; Wang, Z.; Iagatti, A.; Bussotti, L.; Foggi, P.; Ji, W.; Zhao, J.; Di Donato, M. Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in TripletTriplet Annihilation Upconversion. J. Phys. Chem. A 2017, 121, 7550−7564. (24) Hou, Y.; Biskup, T.; Rein, S.; Wang, Z.; Bussotti, L.; Russo, N.; Foggi, P.; Zhao, J.; Di Donato, M.; Mazzone, G.; et al. Spin-Orbit Charge Recombination Intersystem Crossing in PhenothiazineAnthracene Compact Dyads: Effect of Molecular Conformation on Electronic Coupling, Electronic Transitions, and Electron Spin Polarizations of the Triplet States. J. Phys. Chem. C 2018, 122, 27850−27865. (25) Wang, Z.; Zhao, J. Bodipy-Anthracene Dyads as Triplet Photosensitizers: Effect of Chromophore Orientation on Triplet-State Formation Efficiency and Application in Triplet-Triplet Annihilation Upconversion. Org. Lett. 2017, 19, 4492−4495. (26) Wang, Z.; Sukhanov, A. A.; Toffoletti, A.; Sadiq, F.; Zhao, J.; Barbon, A.; Voronkova, V. K.; Dick, B. Insights into the Efficient Intersystem Crossing of Bodipy-Anthracene Compact Dyads with Steady-State and Time-Resolved Optical/Magnetic Spectroscopies and Observation of the Delayed Fluorescence. J. Phys. Chem. C 2019, 123, 265−274. (27) Buck, J. T.; Boudreau, A. M.; DeCarmine, A.; Wilson, R. W.; Hampsey, J.; Mani, T. Spin-Allowed Transitions Control the Formation of Triplet Excited States in Orthogonal Donor-Acceptor Dyads. Chem. 2019, 5, 138−155. (28) Sartor, S. M.; McCarthy, B. G.; Pearson, R. M.; Miyake, G. M.; Damrauer, N. H. Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts. J. Am. Chem. Soc. 2018, 140, 4778−4781. (29) Wang, Z.; Zhao, J.; Di Donato, M.; Mazzone, G. Increasing the Anti-Stokes Shift in TTA Upconversion with Photosensitizers Showing Red-Shifted Spin-Allowed Charge Transfer Absorption but a non-Compromised Triplet State Energy Level. Chem. Commun. 2019, 55, 1510−1513. (30) Cakmak, Y.; Kolemen, S.; Duman, S.; Dede, Y.; Dolen, Y.; Kilic, B.; Kostereli, Z.; Yildirim, L. T.; Dogan, A. L.; Guc, D.; et al. Designing Excited States: Theory-Guided Access to Efficient Photosensitizers for Photodynamic Action. Angew. Chem., Int. Ed. 2011, 50, 11937−11941. (31) Epelde-Elezcano, N.; Palao, E.; Manzano, H.; Prieto-Castaňeda, A.; Agarrabeitia, A. R.; Tabero, A.; Villanueva, A.; de la Moya, S.; López-Arbeloa, l.;́ Martínez-Martínez, V.; et al. Chem. - Eur. J. 2017, 23, 4837−4848. (32) Duman, S.; Cakmak, Y.; Kolemen, S.; Akkaya, E. U.; Dede, Y. Heavy Atom Free Singlet Oxygen Generation: Doubly Substituted Configurations Dominate S1 States of bis-BODIPYs. J. Org. Chem. 2012, 77, 4516−4527. (33) Liu, Y.; Zhao, J.; Iagatti, A.; Bussotti, L.; Foggi, P.; Castellucci, E.; Di Donato, M.; Han, K.-L. A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing. J. Phys. Chem. C 2018, 122, 2502−2511. (34) Montero, R.; Martínez-Martínez, V.; Longarte, A.; EpeldeElezcano, N.; Palao, E.; Lamas, I.; Manzano, H.; Agarrabeitia, A. R.; López Arbeloa, I.; Ortiz, M. J.; et al. Singlet Fission Mediated Photophysics of BODIPY Dimers. J. Phys. Chem. Lett. 2018, 9, 641− 646.

REFERENCES

(1) Loudet, A.; Burgess, K. BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties. Chem. Rev. 2007, 107, 4891− 4932. (2) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. BODIPY Dyes in Photodynamic Therapy. Chem. Soc. Rev. 2013, 42, 77−88. (3) Frath, D.; Massue, J.; Ulrich, G.; Ziessel, R. Luminescent Materials: Locking π-Conjugated and Heterocyclic Ligands with Boron(III). Angew. Chem., Int. Ed. 2014, 53, 2290−2310. (4) Turan, I. S.; Cakmak, F. P.; Yildirim, D. C.; Cetin-Atalay, R.; Akkaya, E. U. Near-IR Absorbing BODIPY Derivatives as Glutathione-Activated Photosensitizers for Selective Photodynamic Action. Chem. - Eur. J. 2014, 20, 16088−16092. (5) Lu, H.; Mack, J.; Yang, Y.; Shen, Z. Structural Modification Strategies for the Rational Design of Red/NIR Region BODIPYs. Chem. Soc. Rev. 2014, 43, 4778−4823. (6) Kowada, T.; Maeda, H.; Kikuchi, K. BODIPY-Based Probes for the Fluorescence Imaging of Biomolecules in Living Cells. Chem. Soc. Rev. 2015, 44, 4953−4972. (7) Zhu, H.; Fan, J.; Wang, J.; Mu, H.; Peng, X. An “Enhanced PET”-Based Fluorescent Probe with Ultrasensitivity for Imaging Basal and Elesclomol-Induced HClO in Cancer Cells. J. Am. Chem. Soc. 2014, 136, 12820−12823. (8) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. The Triplet Excited State of Bodipy: Formation, Modulation and Application. Chem. Soc. Rev. 2015, 44, 8904−8939. (9) Ozdemir, T.; Bila, J. L.; Sozmen, F.; Yildirim, L. T.; Akkaya, E. U. Orthogonal Bodipy Trimers as Photosensitizers for Photodynamic Action. Org. Lett. 2016, 18, 4821−4823. (10) Geist, F.; Jackel, A.; Winter, R. F. Ligand Based Dual Fluorescence and Phosphorescence Emission from BODIPY Platinum Complexes and Its Application to Ratiometric Singlet Oxygen Detection. Inorg. Chem. 2015, 54, 10946−10957. (11) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction; University Science Books: Sausalito, CA, 2009. (12) Zhao, J.; Wu, W.; Sun, J.; Guo, S. Triplet Photosensitizers: From Molecular Design to Applications. Chem. Soc. Rev. 2013, 42, 5323−5351. (13) Colvin, M. T.; Ricks, A. B.; Scott, A. M.; Co, D. T.; Wasielewski, M. R. Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor-bridge-Acceptor Molecules. J. Phys. Chem. A 2012, 116, 1923−1930. (14) Harriman, A.; Mallon, L. J.; Ulrich, G.; Ziessel, R. Rapid Intersystem Crossing in Closely-Spaced but Orthogonal Molecular Dyads. ChemPhysChem 2007, 8, 1207−1214. (15) Bandi, V.; Gobeze, H. B.; Lakshmi, V.; Ravikanth, M.; D’Souza, F. Vectorial Charge Separation and Selective Triplet-State Formation during Charge Recombination in a Pyrrolyl-Bridged BODIPYFullerene Dyad. J. Phys. Chem. C 2015, 119, 8095−8102. (16) Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. Time-Resolved EPR Studies of Photogenerated Radical Ion Pairs Separated by p-Phenylene Oligomers and of Triplet States Resulting from Charge Recombination. J. Phys. Chem. B 2006, 110, 25163−25173. (17) Salikhov, K. M.; Molin, I. N.; Buchachenko, A. L. Spin Polarization and Magnetic Effects in Radical Reactions; Akadémiai Kiadó: Budapest, Hungary, 1984. (18) Van Willigen, H.; Jones, G.; Farahat, M. S. Time-Resolved EPR Study of Photoexcited Triplet-State Formation in Electron-DonorSubstituted Acridinium Ions. J. Phys. Chem. 1996, 100, 3312−3316. (19) Dance, Z. E. X.; Mickley, S. M.; Wilson, T. M.; Ricks, A. B.; Scott, A. M.; Ratner, M. A.; Wasielewski, M. R. Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine-Anthracene Molecules with Perpendicular π Systems. J. Phys. Chem. A 2008, 112, 4194−4201. (20) Filatov, M. A.; Karuthedath, S.; Polestshuk, P. M.; Savoie, H.; Flanagan, K. J.; Sy, C.; Sitte, E.; Telitchko, M.; Laquai, F.; Boyle, R. 4162

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163

Letter

The Journal of Physical Chemistry Letters (35) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Phelan, B. T.; Krzyaniak, M. D.; Reddy, S. R.; Coto, P. B.; Horwitz, N. E.; Young, R. M.; White, F. J.; et al. Unified Model for Singlet Fission within a non-Conjugated Covalent Pentacene Dimer. Nat. Commun. 2017, 8, 15171. (36) Hirota, N.; Yamauchi, S. Short-Lived Excited Triplet States Studied by Time-Resolved EPR Spectroscopy. J. Photochem. Photobiol., C 2003, 4, 109−124. (37) Weber, S. Transient EPR. eMagRes. 2017, 6, 255−270. (38) Ogiwara, T.; Wakikawa, Y.; Ikoma, T. Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules. J. Phys. Chem. A 2015, 119, 3415−3418. (39) Richert, S.; Tait, C. E.; Timmel, C. R. Delocalisation of Photoexcited Triplet States Probed by Transient EPR and Hyperfine Spectroscopy. J. Magn. Reson. 2017, 280, 103−116. (40) Toffoletti, A.; Wang, Z.; Zhao, J.; Tommasini, M.; Barbon, A. Precise Determination of the Orientation of the Transition Dipole Moment in a Bodipy Derivative by Analysis of the Magnetophotoselection Effect. Phys. Chem. Chem. Phys. 2018, 20, 20497− 20503. (41) Richert, S.; Bullard, G.; Rawson, J.; Angiolillo, P. J.; Therien, M. J.; Timmel, C. R. On the Importance of Electronic Symmetry for Triplet State Delocalization. J. Am. Chem. Soc. 2017, 139, 5301−5304. (42) Nagashima, H.; Kawaoka, S.; Akimoto, S.; Tachikawa, T.; Matsui, Y.; Ikeda, H.; Kobori, Y. Singlet-Fission-Born Quintet State: Sublevel Selections and Trapping by Multiexciton Thermodynamics. J. Phys. Chem. Lett. 2018, 9, 5855−5861. (43) Benniston, A. C.; Copley, G. Lighting the Way Ahead with Boron Dipyrromethene (Bodipy) Dyes. Phys. Chem. Chem. Phys. 2009, 11, 4124−4131. (44) Akiyama, K.; Tero-Kubota, S.; Ikoma, T.; Ikegami, Y. Spin Polarization Conservation during Intramolecular Triplet-Triplet Energy Transfer Studied by Time-Resolved EPR Spectroscopy. J. Am. Chem. Soc. 1994, 116, 5324−5327. (45) Wu, W.; Cui, X.; Zhao, J. Hetero Bodipy-Dimers as Heavy Atom-Free Triplet Photosensitizers Showing a Long-Lived Triplet Excited State for Triplet-Triplet Annihilation Upconversion. Chem. Commun. 2013, 49, 9009−9011.

4163

DOI: 10.1021/acs.jpclett.9b01741 J. Phys. Chem. Lett. 2019, 10, 4157−4163