Insights into the Efficient Intersystem Crossing of Bodipy-Anthracene

Article Cite This: J. Phys. Chem. C 2019, 123, 265−274

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Insights into the Efficient Intersystem Crossing of BodipyAnthracene Compact Dyads with Steady-State and Time-Resolved Optical/Magnetic Spectroscopies and Observation of the Delayed Fluorescence Zhijia Wang,†,⊥ Andrey A. Sukhanov,‡,⊥ Antonio Toffoletti,§ Farhan Sadiq,† Jianzhang Zhao,*,† Antonio Barbon,*,§ Violeta K. Voronkova,*,‡ and Bernhard Dick*,∥

J. Phys. Chem. C 2019.123:265-274. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/20/19. For personal use only.

†

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Ling-Gong Road, Dalian 116024, P. R. China ‡ Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Russian Academy of Sciences, Kazan 420029, Russia § Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via Marzolo 1, 35131 Padova, Italy ∥ Lehrstuhl für Physikalische Chemie, Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany S Supporting Information *

ABSTRACT: Spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) is of particular interest for preparation of heavy atom-free triplet photosensitizers. Up to now, examples for SOCT-ISC dyads are limited and electron donor/acceptor SOCT-ISC dyads showing strong visible-light harvesting are rare. Herein, we studied the photophysics of a series of Bodipy-anthracene (BDP-An) compact dyads, especially the triplet state electron spin dynamics with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electronic coupling matrix elements (VDA * ) between the 1CT (charge transfer) 1 state and LE (locally excited) state are in the range 773−1545 cm−1. For one dyad, we observed three triplet states simultaneously with TREPR, that is triplet states confined on the anthracene (3An) and the Bodipy (3BDP) moieties as well as a 3CT state. Based on the electron spin polarization of these three triplet states and the optical experiments, the SOCT-ISC mechanism is confirmed and the radical pair-ISC mechanism as the main ISC channel was excluded. Triplet−triplet annihilation-induced delayed fluorescence was observed for the dyads, which is rare for Bodipy fluorophores.

1. INTRODUCTION Triplet photosensitizers (PSs) are versatile compounds and have been widely used in photodynamic therapy,1−5 photoredox catalytic organic reactions, 6−9 photocatalytic H 2 production by photolysis of H2O,10−16 photovoltaics,17−19 and triplet−triplet annihilation upconversion (TTA-UC).20−25 The desired photophysical properties of triplet PSs include strong absorption of light, efficient intersystem crossing (ISC), and a long-lived triplet excited state.2 Efficient ISC is one of the pivotal properties for a triplet PS. Normally most triplet PSs rely on heavy atoms, which are toxic, expensive, and the triplet lifetime is actually shortened due to the heavy atom effect.2,26−29 Exciton coupling,30 spin converter,31 and radical pair ISC (RP-ISC)32,33 were also employed for the design of triplet PSs. However, the synthesis of these triplet PSs is usually difficult. For instance, two similar chromophores must be connected by a specific conformation or linkage to ensure the exciton coupling effect, whereas long, rigid connection linkers are required for RP-ISC. Radical-enhanced ISC34−36 and establishment of energy-matched S1/Tn states2,28 are also © 2018 American Chemical Society

reported approaches, but the molecular structure usually affects the ISC efficiency greatly, appearing to be more art than science. As such, it is highly desired to develop a general molecular designing strategy for heavy atom-free triplet PSs with predetermined ISC, which should be based on simple, concise synthetic routes. Concerning this aspect, the recently proposed spin orbit charge-transfer ISC (SOCT-ISC) is of particular interest.37−40 For this ISC, the electron donor and acceptor are connected, preferably in orthogonal geometry. Electron transfer between these two units is accompanied by a change in molecular orbital angular momentum. The total angular momentum of the molecular system is preserved given the spin of the electron also flips. The spin orbit coupling matrix element between the 1 CT and 3LE states is hence large, and ISC is greatly enhanced.38,39 In this case, no heavy atom effect is required, that is, this ISC does not require precious metals or other Received: November 7, 2018 Published: December 6, 2018 265

DOI: 10.1021/acs.jpcc.8b10835 J. Phys. Chem. C 2019, 123, 265−274

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The Journal of Physical Chemistry C

measured on an LP980 laser flash photolysis spectrometer (Edinburgh Instruments, UK), recorded on a Tektronix TDS 3012B oscilloscope, and excited with an OpoletteTM 355II +UV nanosecond pulsed laser (OPOTEK, USA; typical pulse length: 7 ns. Pulse repetition: 20 Hz. Peak OPO energy: 4 mJ). The wavelength is tunable in the range 410−2200 nm. Lifetime values were obtained with the LP900 software by fitting exponential functions to the decay traces of the transient signals. The fit of bimolecular or mixed kinetic models (for TTA) was done with home-written software. All samples in flash photolysis experiments were deaerated with N2 for ca. 15 min before measurement, and the gas flow was kept during the measurement. 2.3. TREPR Spectroscopy. The magnetic resonance experiments were carried out in frozen solutions of the compounds with a mixture of dichloromethane and isopropanol = 1:1 as solvent. Samples were excited by the second harmonic of a Nd:YAG pulsed laser (Quantel Brilliant, λ = 532 nm, pulse length = 5 ns; E/pulse = ca. 10 mJ). The timeresolved continuous-wave (TR CW) EPR measurements were performed on an X-band EPR Elexsys E-580 spectrometer (Bruker) at 80 K. The TR CW EPR spectra were obtained by the summation of the data in different time windows after the laser pulse. The EPR spectra were simulated using the EasySpin package implemented in the Matlab programming language.43 2.4. Theoretical Computations. For the computation of the potential curves along the torsional coordinate, the Firefly QC package was used,44 which is partially based on the GAMESS (US) source code.45 For the computation of the spin density surfaces of the triplet state of the compounds, computation was performed with density functional theory (DFT) by using Gaussian 09W software.46

heavy atoms. Moreover, the electron donor/acceptor can have a simple structure, which makes the synthesis more feasible as compared to that of the conventional triplet PSs.38,39 It is also fundamentally important to control the fate of the charge recombination, for instance, formation of a triplet excited state instead of fast decay to the electronic ground state (S0 state). Bodipy-anthracene (BDP-An) dyads have been reported showing efficient ISC.3,41 Our group designed BDP-An dyads with different mutual orientations between the Bodipy and the anthracene moieties (BDP-AN-1−BDP-AN-4. Scheme 1), and Scheme 1. Molecular Structures of the Target Triplet PSs BDP-AN-1, BDP-AN-2, BDP-AN-3, and BDP-AN-4, and the Reference Compounds Used in This Study

we applied these dyads in TTA-UC.42 We found that although the dihedral angle between Bodipy and anthracene is nearly 90° for both BDP-AN-2 and BDP-AN-3, the ISC ability varies substantially. However, this phenomenon was not further studied.42 Moreover, the SOCT-ISC mechanism of the BDPAn dyads was actually not confirmed unambiguously in previous studies.3,41,42 Herein, we performed a detailed study with the aim of clarifying the ISC mechanism. By using the time-resolved electron paramagnetic resonance (TREPR) spectroscopy and based on the electron spin polarization (ESP) of the triplet state of the dyads, the SOCT-ISC mechanism was confirmed for the dyads, and the RP-ISC mechanism as the main ISC channel was excluded.38,39 For one dyad, three triplet states were observed with TREPR, that is the 3An, 3BDP, and 3CT states. The ESP of these three triplet states unveiled the details for SOCT-ISC. Additionally, TTAbased delayed fluorescence was observed for the dyads, which is rare for Bodipy fluorophores.

3. RESULTS AND DISCUSSION 3.1. Molecular Structure Designing Rationales. Based on the derivatization chemistry of Bodipy,47−51 two kinds of dyads were used. One kind has a connection at the meso position of the Bodipy chromophore (BDP-AN-1 and BDPAN-2). The 9-phenylanthranyl moiety in BDP-AN-1 is a stronger electron donor than the anthryl moiety in BDP-AN-2, thus we expect stronger electronic coupling in BDP-AN-1 than in BDP-AN-2. In the second pair of dyads (BDP-AN-3 and BDP-AN-4), anthracene is attached at the 2-position of Bodipy. The geometries are with an extra dimension of difference.42 It will be interesting to study the effect of the relative alignment of the chromophores in BDP-AN-1/BDPAN-2 and BDP-AN-3 on the ISC efficiency and the ESP phase pattern of the TREPR spectra of the triplet excited states.38,39 3.2. UV−Vis Absorption and Fluorescence Emission Spectra. The absorption spectra of the BDP-An dyads are presented in Figure 1a. For BDP-AN-1 and BDP-AN-2, the absorption profiles of the BDP-An dyads are the sum of the spectra of the components, indicating negligible electronic coupling in the electronic ground state.39,52 This postulate is confirmed by the lack of any charge-transfer (CT) absorption band in the UV−vis absorption spectra.52,53 This is reasonable because the geometry between the anthryl and the Bodipy unit is orthogonal (Figure 2), which inhibits π-conjugation between the anthryl and the Bodipy units. For BDP-AN-3 and BDPAN-4, the absorption band at 505 nm is slightly red-shifted and broader, indicating a stronger electronic coupling at the ground

2. EXPERIMENTAL SECTION 2.1. Analytical Measurements. All chemicals used in the synthesis are analytically pure and were used as received. Solvents were dried and distilled before used for synthesis. UV−vis absorption spectra were taken on an Agilent 8453 UV−vis spectrophotometer (Agilent, USA). Fluorescence spectra were recorded on an RF-5301 PC spectrofluorometer (Shimadzu, Japan). Luminescence lifetimes were measured on an OB920 fluorescence lifetime instrument (Edinburgh Instruments, U.K.). 2.2. Nanosecond Transient Absorption Spectra (Emission Mode). The nanosecond transient absorption spectra (for the TTA delayed fluorescence measurement) were 266


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state. This conclusion is supported by the shallow potential energy surface (PES) (Figure 2). The fluorescence emission of the compounds was also studied (Figure 1b). For BDP-AN-1, BDP-AN-2, and BDPAN-3, emission bands at 515 nm were observed, which is attributed to the 1 LE state emission of the Bodipy chromophore. In addition, an emission band at 650 nm was observed, which is broad, structureless and solvent polaritydependent. These bands are typical charge transfer (CT) emission bands.3,39,56 For BDP-AN-4, the two bands merged to a broad emission band which is assigned to mixed 1LE/1CT emission. The CT emission band indicates that communication between the anthracene and the Bodipy chromophore becomes more significant in the electronically excited state as compared to the ground state. The conformation of the dyads in the excited state has most probably a slightly bended structure (butterfly conformation), thus the interaction between the two parts may increase.57 The photophysical parameters of the BDP-An dyads in different solvents are summarized in Table 1. The ISC ability of the BDP-An dyads is strongly dependent on the solvents. In general, BDP-AN-1/BDP-AN-2 show significantly higher ISC efficiency than BDP-AN-3/BDP-AN-4. This will be further explained in the next section. 3.3. Potential Energy Surface. In previous theoretical studies of SOCT-ISC in compact dyads, the analysis was restricted to the single optimized geometry of the electronic ground state.39 Herein, we present a more detailed study considering the PES along the torsional motion between the two chromophores (Figure 2). The PES of BDP-AN-1 is rather flat in the range of 80−100° for the dihedral angle, but it rises steeply upon rotation toward more coplanar geometries. This is due to repulsion by the

Figure 1. (a) Normalized UV−vis absorption spectra and (b) normalized fluorescence emission spectra (λex = 480 nm) of BDPAN-1, BDP-AN-2, BDP-AN-3, and BDP-AN-4. c = 1.0 × 10−5 M. In acetonitrile (ACN), 20 °C.

Figure 2. (a) PESs of the ground states of BDP-AN-1, BDP-AN-2, BDP-AN-3, and BDP-AN-4 as a function of the rotational dihedral angle between the two chromophores of the dyad. All other geometrical parameters were optimized. (b) Enlarged view of the PESs near the minima. The thermal energy at room temperature (kBT = 0.026 eV) is indicated by the dashed line. The potential energy curves were calculated at B3LYP/6-311G(d,p) level with Firefly Ver 8.

Table 1. Photophysical Properties of the Compounds solventsa BDP-AN-1

BDP-AN-2

BDP-AN-3

BDP-AN-4

BDP

n-hexane toluene DCM THF ACN n-hexane toluene DCM THF ACN n-hexane toluene DCM THF ACN n-hexane toluene DCM THF ACN ACN

λabsb

εc

504/372 508/373 506/373 505/373 503/373 504/366 508/366 506/366 505/366 502/366 513/367 516/367 513/367 513/367 510/367 517/364 520/364 517/364 517/364 513/364 503

9.4/1.8 9.0/1.7 8.6/1.7 8.8/1.7 8.0/1.7 10/1.5 9.9/1.5 9.7/1.5 9.6/1.5 8.7/1.5 8.5/1.0 8.0/1.0 7.8/1.0 8.0/1.0 7.5/1.0 9.3/1.2 8.7/1.2 8.9/1.2 8.8/1.2 8.4/1.2 9.2

τT/(μs)d

ΦΔe

ΦTf

g

g

g

317 85 118 68

0.10 0.95 0.43 0.84

0.06 0.90 0.43 0.92

g

g

g

345 82 129 78 123 127 118 129 137 130 102 116 144 125

0.04 0.82 0.31 0.86 0.09 0.20 0.24 0.16 0.11 0.09 0.11 0.13 0.07 0.05

0.03 0.80 0.30 0.96 0.16 0.31 0.20 0.20 0.16 0.14 0.17 0.21 0.13 0.06

g

g

g

a

ET(30) values of the solvents are n-hexane (31.0), toluene (33.9), THF (37.4), DCM (40.7) and ACN (45.6), respectively. bIn nm. cMolar absorption coefficient (104 M−1 cm−1). dTriplet excited state lifetimes. Measured by nanosecond transient absorption in deaerated solutions. e Singlet oxygen quantum yield, with IBDP as the standard (ΦΔ = 0.87 in DCM).54 fTriplet state quantum yield, with IBDP as the standard (ΦT = 0.88 in toluene).55 gNot applicable. 267


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The Journal of Physical Chemistry C c12 ≈ ε(v)LE /ε(v)ref

methyl groups in the Bodipy unit at the 1,7-positions and results in a barrier for rotation. This energy barrier is much larger than the thermal energy at room temperature, kBT = 0.026 eV (T = 300 K). As a result, the conformation of BDPAN-1 will be restricted to the orthogonal arrangement of both chromophores, which is beneficial for SOCT-ISC.38,39 Similar results are obtained for BDP-AN-2. This geometry is in agreement with the high ISC yield of these two dyads (Table 1). BDP-AN-3 and BDP-AN-4 show much lower triplet state quantum yields than BDP-AN-1 and BDP-AN-2 (Table 1). The study of the PES rationalizes this difference (Figure 2). The PES of BDP-AN-3 is shallower than that of BDP-AN-2 in the dihedral angle range of 60−120°. For BDP-AN-4, the PES is even more flat from 0 to 180°, which is due to the connection of the 2-position of the anthryl unit at the 2position of Bodipy. The significant deviation from the orthogonal geometry may contribute to the lower triplet quantum yield of BDP-AN-3 and BDP-AN-4, as compared to that of BDP-AN-1 and BDP-AN-2, which is in accordance with the SOCT-ISC mechanism. The single point optimized geometries of BDP-AN-2 and BDP-AN-3 are both nearly 90° but the ISC efficiencies are dramatically different (Table 1), which can be rationalized by taking a full view of the PES. Thus, our study indicated that investigation of the potential energy curve is useful to attain indepth understanding of the energy landscape of the compact electron donor/acceptor dyads as well as the factors that dictate the SOCT-ISC efficiency. The study of the PES also supports the SOCT-ISC mechanism. 3.4. Electron Transfer and Electronic Coupling between 1CT and 3LE. Fast electron transfer is essential for SOCT-ISC. The rate constant of electron transfer, ket, can be qualitatively analyzed according to the Marcus theory ket =

where ε(v)LE and ε(v)ref denote the maximum molar absorption coefficients of S0 → 1LE transition of the Bodipy derivative and unsubstituted Bodipy, in M−1 cm−1. Here, ε(v)ref is 9.2 × 104 M−1 cm−1 in ACN. For the compact dyads discussed here, rather strong coupling, VDA * (773−1545 cm−1. Table 2), is found. At a Table 2. Fluorescence Properties of 1LE → S0 Bands and 1 CT → S0 Bands of Compounds in ACN BDP-AN-1 BDP-AN-2 BDP-AN-3 BDP-AN-4

λFa

ΦFb

f

f

512 /641g 510f /634g 524f /627g 566f /630g

0.2 /0.7g 1.8f /0.5g 1.0f /2.0g 1.3f /2.4g

τF/(ns)c 1.5 (30%); 5.3 (70%)f/2.9g 5.0f/3.0g 2.1 (83%); 4.9 (17%)f/1.3g 1.3 (85%); 4.7 (15%)f/1.3g

krd

VDA *e

f

1256

0.48 /2.4g 3.6f /1.7g 3.8f /15.4g 7.2f /18.5g

773 1545 998

a In nm. bFluorescence quantum yields, in percent, with BDP as the standard (ΦF = 90% in toluene).55 cFluorescence lifetimes, c = 1.0 × 10−5 M. The values in parentheses indicate the individual population. d Radiative rate constant kr = ΦF/τF, in 106 s−1. eElectronic coupling matrix elements between the 1CT state and 1LE state, calculated with eq 2. In cm−1. fThe transition of 1LE → S0. gThe transition of 1CT→ S0, the peaks were separated by a fit by Gaussian lineshapes with Origin 5.0 software.

typical reorganization energy of 0.5 eV and at ambient * value of 1.7 cm−1 is sufficient for electron temperature, a VDA transfer to occur on a subnanosecond time scale.58 Thus, fast electron transfer is ensured, which is mandatory for efficient SOCT-ISC. 3.5. TREPR Spectroscopy: SOCT-ISC Mechanism and the Observation of the Coexistance of 3BDP, 3AN, and 3 CT States. TREPR has been used as a powerful tool to study the triplet excited states, either from a structural point of view (localization of the triplet state and geometry of the molecule at the T1 state) or for the determination of formation/ depletion of the triplet states (ESP and spin selectivity). It is worth mentioning, in particular, its capability to discriminate different ISC mechanisms, including RP-ISC, SOCT-ISC, or ISC via the normal heavy atom effect (SO-ISC).38,39,60−65 The ESP pattern of the TREPR spectra provide a special fingerprint for the different ISC mechanisms.38,39 The zero-field splitting (ZFS) parameters, |D| and |E|, instead, are useful to study the spatial confinement (or the localization) of the triplet state and the symmetry of the molecule in the T1 state, respectively; this information is hardly accessible with other optical spectroscopies, for instance time-resolved transient absorption spectroscopy.66 The experimental TREPR spectra of the triplet states of 2,6diiodoBodipy (IBDP, Scheme 1), BDP-AN-1, BDP-AN-2, BDP-AN-3, and BDP-AN-4 at 80 K are presented in Figure 3 together with their simulations (the fitting parameters are presented in Table 3). In general, all dyads show the half-field signal (ΔMS = 2) at 160−165 mT (Figure 3), which is the typical signal of a triplet excited state. Despite the different polarization of the spectra, we note that in all BDP-An dyads, the spectral features relative to the principal directions are positioned at the same fields (within experimental accuracy). They are related to the same triplet states with the D and E-values of −85.0 and 17.5 mT,

* )2 exp( −ΔG # /kBT ) 2π 3/2(V DA h(λkBT )1/2

(3)

(1)

where VDA * is the electronic coupling matrix element between the 1CT state and the 1LE state, h is the Planck constant, kB is the Boltzmann constant, ΔG# is the activation energy for the electron transfer, and λ is the reorganization energy required to distort the product state to the equilibrium geometry of the reactant state. As the electron transfer rate constant, ket, depends on the VDA * values (eq 1),58 strong electronic coupling between the 1 CT state and the 1LE state (VDA * ) is necessary. The VDA * was estimated by eq 259 c A CT * (cm−1) ≈ 8049 × 2 × (E LE V DA ) − hcvabs c1 (2) where EALE is the 1LE state energy, and the value is taken as 2.46 eV for Bodipy derivatives; h [eV s] and c [cm s−1] are the Plank constant and the speed of light, respectively. vCT abs is the maximum of the S0 → 1CT transition band, in cm−1. However, we did not observe the CT absorption band, thus the vCT abs is estimated from the CT emission band. Equation 2 is calculated from a model that expands the true eigenstates of the system as a linear combination of the two states 1LE and 1CT with expansion coefficients, c1 and c2. The values of c1 and c2 = (1 − c12)1/2 can be estimated from the change of the S0 → 1LE absorption band intensity between the Bodipy derivative with unsubstituted Bodipy using eq 3 268


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mechanism (Table 3, the x, y, and z labeling is coherent with the assignment of the features reported in Figure 3). Interestingly, two different kinds of ESP patterns were observed for the TREPR of the BDP-An dyads (Figure 3). For BDP-AN-1 and BDP-AN-2, the ESP phase is (e, e, e, a, a, a) and the relative population rate is Px/Py/Pz = 0.00:0.47:1.00, whereas the ESP phase of the triplet state for BDP-AN-3 and BDP-AN-4 is (a, e, a, e, a, e), with the population of the sublevels of the triplet state having relative population rate of Px/Py/Pz = 0.10:1.00:0.00 (Table 3). The two types of polarizations are drastically different, and they are attributed to an overpopulation of the Tz state (Pz > Px, Py) for BDP-AN-1 and BDP-AN-2 (see Table 3) and to an overpopulation of the Ty state (Py > Px, Pz) for BDP-AN-3 and BDP-AN-4. We also note that although the polarization pattern in the spectra of BDP-AN-1 and BDP-AN-2 is similar to that of IBDP, the relative population rate of the Ty state is reinforced, thus making us deduce that there are differences for the ISC mechanism also in this case. A detailed examination shows that other features are clearly present in BDP-AN-2. Besides the major component of the 3 BDP state, a good simulation requires two additional species. The three contributions are displayed in Figure 4. It should be

Figure 3. Experimental TREPR spectra (black lines) of the compounds. Samples IBDP, BDP-AN-1, BDP-AN-3, and BDP-AN4 were photo-excited at 532 nm with pulse laser at energies of 10 mJ, and the repetition rate is 10 Hz. BDP-AN-2 was photoexcited at 532 nm with pulse energies of 20 mJ. Note for BDP-AN-2: three different triplet states were used in the simulation (see Table 3 for details). Solvent: dichloromethane and isopropanol = 1:1 (v/v). c = 1.0 × 10−3 M. Simulated spectra with parameters given in Table 3 are shown by red lines. The canonical orientations of each transition are indicated. T = 80 K, time window: 1−2.5 μs.

Table 3. ZFS Parameters (D and E) and Relative Population Rates Px,y,z of the Zero-Field Spin States of the IBDP, Anthracene, and BDP-An Dyadsa molecule IBDP anthracene BDP-AN-1 BDP-AN-2c

BDP-AN-3/4

D (mT)

E (mT)

Px

Py

Pz

−105.7 77.8b −85.0 −86.0 75.0 −40.0 −85.0

23.4 −9.0b 17.5 17.5 −15.0 8.5 17.5

0.00 0.70 0.00 0.00 1.00 0.18 0.10

0.15 1.00 0.47 0.47 0.20 1.00 1.00

1.00 0.00 1.00 1.00 0.00 0.00 0.00

Figure 4. TREPR spectra of the three components (normalized to the maximum of the intensity) used for the simulation of BDP-AN-2 reported in Figure 3. The spectra are the BDP-like triplet (red line), the anthracene-like triplet (black line), and the CT triplet states (blue line). See Table 3 and the main text for details.

a

Obtained from simulations of the triplet-state TREPR spectra of the indicated molecules in a dichloromethane/isopropanol = 1:1 matrix at 80 K. bThe sign of the D parameter is positive for anthracene. cThree different triplet states were used for the simulation; see Figure 4 and the main text for details.

pointed out that the individual species concentration is difficult to be derived from the simulations because the magnitude of the TREPR signal is more related to the magnitude of the ESP than to the species concentration. The two additional triplet states of BDP-AN-2 possess D values of 75.0 and −40.0 mT, respectively, and the E values are −15,0 and 8.5 mT, respectively. We supposed that the triplet with the larger D-(absolute) value is related to an anthracenelocalized triplet state. Indeed, we verified our hypothesis by acquiring the TREPR spectrum of a frozen anthracene solution (Supporting Information, Figure S4) and obtained the simulation, which provided the values of D = 77.8 mT and E = −9.0 mT (Table 3), rather close to the values that we found in BDP-AN-2 and to those reported by Wasielewski et al. (D = 77.81 mT and E = −8.81 mT).39 For BDP-AN-1, there is a difference between the simulation and the experimental TREPR curve, which could be attributed to the presence of the anthracene triplet, along with the BDP-localized triplet state as the main component. However, as the signal is very weak, we did not study its origin further.

respectively. The parameters indicate the same spatial distribution of the triplet wavefunction in all dyads. By comparison with the ZFS values of IBDP, we assign the triplet states observed for all BDP-An dyads to a BDP-confined triplet state (the smaller D parameters with respect to IBDP may result from a slightly more delocalized T1 state). This conclusion agrees with the nanosecond transient absorption studies of the dyads.42 The different polarization of the TREPR spectra, as pointed out at the beginning of this section, is attributed to the presence of different population channels, namely different spin-selectivity in the triplet formation. Following photoexcitation of IBDP at 80 K in the solid matrix by a pulsed ns laser, the TREPR was observed with an absorption/emission (a/e) ESP of (e, e, e, a, a, a) pattern, which is assumed to be obtained from a population of the triplet zero-field states Tx, Ty, and Tz with relative rates of Px/ Py/Pz = 0.00:0.15:1.00 because of the regular SO-ISC 269


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The Journal of Physical Chemistry C The species with the smaller (absolute) D-value (D = −40 mT) for BDP-AN-2 is attributed to a 3CT state with one electron localized on the BDP moiety and another on the anthracene moiety, thus, under the rough point-dipole approximation, the value of D is negative, we obtained a distance between the two electrons of the 3CT state of 4.2 Å, which is compatible with this electronic configuration (the center-to-center distance of the BDP and the anthracene moieties in BDP-AN-2, based on DFT geometry optimization, is 4.4 Å). The observation of two triplet states in the same molecule (for BDP-AN-2 and probably for BDP-AN-1) with TREPR is quite unusual,32 and likely it is related to the perpendicularity of the BDP and An moieties in BDP-AN-2 (and BDP-AN-1), thus keeping the overlap between the two π-systems close to zero. For BDP-AN-3 and BDP-AN-4, there is no such case because of the less rigid geometry (Figure 2), and in fact the intermediate species was not observed. In the spectra of IBDP, BDP-AN-1, and BDP-AN-2, beside the presence of the triplet states reported in the Table 3, a small contribution in pure absorption (centered at around 350 mT and a width of ca. 22 mT) has been added. Likely this is associated with the formation of clusters (the concentration of the solutions used for the TREPR study was rather high, 1 mM). As the relative population rates Px,y,z of BDP-An dyads differ from those of IBDP and anthracene, and all experiments were done with excitation of the heavy atom-free Bodipy moiety, the ISC mechanism is unlikely dominated by the general SO-ISC, for instance, by the ISC ability of the anthracene moiety. It should be noted that although the 3CT state was observed for BDP-AN-2, we believe the 1CT → 3CT (RP-ISC) should be inefficient and have little contribution to the overall ISC because of the absence of the RP-ISC ESP phase feature, that is, (a, e, e, a, a, e) or (e, a, a, e, e, a), for the triplet state TREPR spectrum of the BDP-An dyads. This is reasonable because in such a compact donor−acceptor dyad, the electronic coupling is expected to be strong (Section 3.4.) as compared to the ZFS effect. Thus, the relatively large electron exchange values J (|2J| ≫ gβB, the J value was calculated to be large according to the DFT calculation, Table S3) make any mixing of the 1CT/3CT states by the hyperfine coupling interaction not significant.39 Moreover, it was reported that the SOCT-ISC is fast (about 100 ps),3 whereas RP-ISC takes a few nanoseconds. Thus, we conclude that the RP-ISC should be inefficient in BDP-AN-2. Determination of the J value with magnetic spectroscopies is impossible in this case because a very high magnetic field will be required to satisfy the condition B0 = 2J required to observe any magnetic field effect on the ISC efficiency. Under this condition, the ES1 = ET+ when J > 0, or ES1 = ET− when J < 0.67,68 The higher ISC efficiency for BDP-An dyads with orthogonal geometry and steep PES, the solvent polarity dependency of ISC, and the polarization patterns of the TREPR spectra of the dyads are consistent with the presence of an SOCT-ISC mechanism. The photophysical processes of the dyads are presented in Scheme 2 with BDP-AN-2 as an exemplar. Excitation of the Bodipy moiety will lead to ultrafast charge separation, followed by charge recombination. During SOCT-ISC, triplet states for the BDP-An dyads, with the triplet first localized either on the Bodipy (3BDP) or on the anthracene (3An) moieties, can in principle be formed. Although the three triplet excited states were observed simultaneously for BDP-AN-2 with the TREPR,

Scheme 2. Diagram Demonstrating the Photophysical Processes in the BDP-An Dyads on Photoexcitationa,b

a

Exemplified with BDP-AN-2. bThe energy level of the 1CT state (1[BDP−•-An+•]) in DCM and ACN was determined according to the CT emission band, the energy level of 1CT state in toluene (TOL) was estimated with the Weller equation. The relative population of the triplet state is represented by the number of the red balls.

the 3BDP state should be dominant due to its lower energy. In the nanosecond transient absorption spectra of the dyads, however, no distinct 3An state and 3CT state were observed for BDP-AN-2.42 We propose that the weak signal and minor amount of the 3An and the 3CT state may be covered by the strong 3BDP state signal. These results show that TREPR is a powerful method for study of the triplet excited state and the ISC mechanism. 3.6. Observation of Delayed Fluorescence. Previously Senge,3 Zhang,41 and our group42 have unambiguously confirmed with nanosecond transient absorption spectroscopy that the triplet excited states of the dyads are populated on photoexcitation. Here, we report the observation of delayed fluorescence of the dyads (Figure 5) by using the nanosecond transient absorption spectrometer in the emission mode. Delayed fluorescence is useful for luminescence bioimaging.69

Figure 5. Delayed fluorescence of BDP-AN-2 on nanosecond pulsed laser excitation (λex = 508 nm. 1.8 mJ/pulse. Pulse duration: 5 ns). (a) Nanosecond transient emission spectra of BDP-AN-2. (b) Decay traces at 520 nm. λex = 508 nm, in deaerated dichloromethane. 20 °C.

With BDP-AN-2, long-lived luminescence was observed on ns pulsed laser excitation (1.8 mJ per pulse. Pulse duration: 5 ns, Figure 5a). The emission spectra profile are the same as the steady-state fluorescence. Interestingly, the fluorescence lifetime was determined as 31.5 μs. This lifetime is 5700 times longer than the prompt fluorescence lifetime of BDP-AN-2 (5.5 ns),42 but it is in the same range as that of the triplet state of the compound (82 μs). Similar results were observed for 270


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The Journal of Physical Chemistry C BDP-AN-1, BDP-AN-3, and BDP-AN-4 (Supporting Information, Figures S5−S7). We attribute the long-lived luminescence to delayed fluorescence. Two alternative mechanisms can lead to delayed fluorescence, namely TTA (the P-type delayed fluorescence)70 or T1 → S1 reverse ISC (the thermally activated delayed fluorescence, also called E-type delayed fluorescence).71−73 We assign the delayed fluorescence observed here to P-type fluorescence: E-type fluorescence should decay with the lifetime of the triplet state (T1 and S1 are in thermal equilibrium), P-type fluorescence is proportional to the square of the triplet population and, hence, decays with approximately half of the triplet lifetime. In addition, P-type fluorescence is more sensitive to the exciting laser power than the E-type fluorescence (there is a square relationship between fluorescence intensity and excitation laser power for P-type fluorescence, whereas a linear relationship for E-type). In our case, with a ps pulsed laser (ca. 5 × 10−7 mJ per pulse. Pulse duration: 120 ps) used in the fluorescence lifetimes measured with TCSPC, no delayed fluorescence was observed, but it is readily observed with excitation by the more intense ns pulsed laser (1.8 mJ per pulse. Pulse duration: 5 ns). These results indicate that the delayed luminescence we observed with BDPAN-2 is P-type delayed fluorescence, that is, delayed fluorescence via the TTA mechanism. The intensity of P-type fluorescence is proportional to the square of the concentration cT of the triplet states. The latter decays by spontaneous decay with rate constant, k1, and by TTA with rate constant, k2. The corresponding differential eq 4 dc T = −k1c T − k 2c T 2 dt

Table 4. Fitting Results of the Delayed Fluorescence of the BDP-An Dyads by Using Eq 6a BDP-AN-1 A1 τ1 τ2 A2 τ0 a

(4)

ÄÅ ÉÑ−2 ÅÅ i t − t y i ÑÑ yz τ j z j Å 0 2 zz·jj1 + zz − 1ÑÑÑ ·ÅÅÅexpjjj z j z ÑÑ ÅÅ j τ1 z j τ1 z{ ÑÑÖ {k ÅÇ k ij t − t0 zy zz + A 2 ·expjjj− zz j τ 0 k {

10−2 102 10 10−1 10−2

BDP-AN-2 1.92 1.55 3.15 8.40 4.24

× × × × ×

10−1 106 10 10−2 10−1

BDP-AN-3 1.03 8.68 3.90 2.59 1.33

× × × × ×

10−2 106 10 10−1 10−1

BDP-AN-4 9.82 1.22 5.59 2.84 1.56

× × × × ×

10−3 106 10 10−1 10−1

The value of τ1, τ2, and τ0 are in μs.

ÄÅ ÉÑ−1 ÅÅ i t − t0 yz t − t0 ÑÑÑ Å zz ÑÑ + A 2 ·expjjjj− y(t ) = A1·ÅÅÅ1 + zz Ñ j ÅÅÇ τ2 ÑÑÖ τ 0 { k

k 2 = 1/c0τ2

(7)

(8)

Table 5. Estimated Initial Triplet Concentration and Calculated Results of Bimolecular Rate Constant of TTA (5) c0/M k2/M−1 s−1

Based on this model, the decay curves of delayed fluorescence were fitted by using the following fit-function (eq 6). ij τ yz y(t ) = A1·jjj 2 zzz jτ z k 1{

× × × × ×

However, using this fitting function yields simulations that significantly deviate from the data. Hence, the result is sensitive to the triplet lifetime. Thus, we tested what happens when we fix the triplet lifetime at smaller values. We did this with the data set BDP-AN-2, which has the best S/N ratio. The result is shown in Supporting Information (Table S1). The result is interesting: decreasing and fixing the triplet lifetime by a factor of 10, 100, and even 1000 has almost no effect on the fitted value of τ2 (less than 5%). Thus, we can conclude that although the triplet lifetimes found in the fits of the delayed fluorescence are not reliable, the fitted values for τ2 are reliable. From the result of the nanosecond transient absorption spectra, the initial triplet concentration (c0) is estimated (for details, please refer to Table S2), then the bimolecular rate constant of TTA can be calculated based on eq 8. The results are presented in Table 5.

has the solution (eq 5) c0k1 c T(t ) = exp(k1t ) ·(c0k 2 + k1) − c0k 2

2.00 4.99 3.37 2.35 9.95

BDP-AN-1

BDP-AN-2

BDP-AN-3

BDP-AN-4

7.5 × 10−6 3.9 × 109

8.3 × 10−6 3.8 × 109

3.0 × 10−6 8.1 × 109

1.9 × 10−6 9.4 × 109

To the best of our knowledge, delayed fluorescence of Bodipy, one of the most frequently studied fluorophores, was rarely reported. In our work, the delayed fluorescence of Bodipy itself was observed with nanosecond transient absorption spectrometer in the emission mode and fitted well. Our work is important as delayed fluorescence may become useful in time-resolved (time-gated) luminescence imaging studies.

2

(6)

where the first part with amplitude A1 is the delayed fluorescence, modeled by the square of the triplet concentration. The second part with amplitude A2 accounts for the contribution of prompt fluorescence; τ1 is the unimolecular lifetime of the triplet; τ2 = 1/c0k2, which is the reciprocal of the initial concentration of triplets times the bimolecular rate constant of TTA; τ 0 is the lifetime of spontaneous fluorescence; and t0 is the time of excitation, t > t0. The results of the fitting are presented in Table 4. When all four parameters (A1, A2, τ1, and τ2) are optimized, the triplet lifetime τ1 reaches very large values for BDP-AN-2, BDP-AN3, and BDP-AN-4. This looks as if the unimolecular decay contributes very little to the total delayed fluorescence. This would mean that TTA is the main process responsible for the triplet decay. In this limit, the fitting function becomes eq 7

4. CONCLUSIONS In summary, the photophysical properties of the BDP-An dyads were fully studied, especially to confirm the SOCT-ISC mechanism with TREPR spectroscopy. The electronic coupling between the 1CT and 1LE states are quantified with the matrix elements, V*DA, in the range of 770−1545 cm−1, indicating that these compact electron donor (anthryl)/ acceptor(Bodipy) dyads are within the strong coupling regime. Three different triplet states, that is, the 3BDP state, 3An state and 3CT state were simultaneously observed for BDP-AN-2 with TREPR. Based on the ESP of the three triplet states, we propose that 1CT → 3CT is not the dominant ISC channel; the SOCT-ISC mechanism as the main ISC channel was 271


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confirmed. Effect of the mutual orientation of the electron donor/acceptor on the ESP, that is, the spin selectively for population of the three substates of the triplet state, was observed with TREPR. TTA-based delayed fluorescence was observed for the Bodipy fluorophore, which has potential in application in bioimaging. These studies present new in-depth understanding of the charge recombination for producing triplet states in electron donor/acceptor dyads and also supply the details for the SOCT-ISC mechanism. These studies will also be useful for artificial photosynthesis, photovoltaics, photoredox catalytic synthetic organic reactions, TTA-UC, and fundamental photochemistry concerning photo-induced charge separation and charge recombination.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10835.

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UV-vis spectra and emission spectra, time-resolved emission spectra and fluorescence lifetime, TREPR spectrum of photoexcited anthracene, delayed fluorescence, and DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (J.Z.). [email protected] (A.B.). [email protected] (V.K.V.). [email protected] (B.D.).

ORCID

Zhijia Wang: 0000-0002-8309-0050 Jianzhang Zhao: 0000-0002-5405-6398 Bernhard Dick: 0000-0002-9693-5243 Author Contributions ⊥

Z.W. and A.A.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.Z. thanks the NSFC (21473020, 21673031, 2160021, 21761142005, 21273028, 21576043, and 21421005), the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, and DUT2016TB12), and University of Padua, Italy (Visiting Scientist) for support. V.K.V. thanks the Russian Foundation for Basic Research in the part of TREPR experiment (16-03-00586) for financial support. B.D thanks Dalian University of Technology for the Haitian Professorship support.

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