Intersystem Crossing Involving Strongly Spin Exchange-Coupled

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Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor−bridge−Acceptor Molecules Michael T. Colvin, Annie Butler Ricks, Amy M. Scott, Dick T. Co, and Michael R. Wasielewski* Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor−bridge−acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9anthracenyl)-julolidine (DMJ−An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ+•−PI−• or DMJ+•−NI−• quantitatively in 1−4 (τCS ≤ 10 ps), and that charge recombination occurs with τCR = 268 and 158 ps for 1 and 3, respectively, and with τCR = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ+• and PI−• or NI−•, giving 2J > 600 mT for 1−3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of 3*An upon charge recombination occurs by spin−orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1−3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1−4 depend on the donor−acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

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INTRODUCTION The development of molecular systems for solar energy conversion requires the rational design and synthesis of systems that absorb light, separate charge, and either collect the charge at electrodes or store the charge by forming chemical bonds. This requires a fundamental understanding of how charge transfer depends on system energetics and reaction rates and has resulted in the examination of structure/function relationships in a wide variety of donor−bridge−acceptor (D−B−A) molecules.1 Because the vast majority of photodriven charge transfer reactions involve single electron transfers, paramagnetic radical ion pairs (RPs, i.e., electron−hole pairs) are generated, so that their spin dynamics can have an important influence on the overall mechanism. Whereas time-resolved optical spectroscopy can often be used to determine many aspects of these mechanisms,1 time-resolved electron paramagnetic resonance (TREPR) spectroscopy is a powerful tool to characterize the paramagnetic intermediates following photoexcitation as well as their spin dynamics.1,2 Increasing the yields and lifetimes of charge-separated intermediates for solar energy conversion requires understanding and eliminating energy wasting pathways, which frequently include triplet state formation.1 This requires a detailed knowledge of the competitive intersystem crossing © 2012 American Chemical Society

(ISC) mechanisms that accompany charge separation and recombination. There are relatively few reports on ISC spin dynamics in D−B−A systems in which the two spins in D+•− B−A−• are strongly spin exchange coupled. In contrast, there are many studies of weakly coupled systems because they have been designed specifically to mimic the characteristics of photogenerated RPs observed within photosynthetic reaction center proteins.1,3 However, numerous donor−acceptor combinations of relevance to organic photovoltaics, such as the donor polymer−fullerene acceptor blends used in bulk heterojunction organic solar cells, frequently have the donor and acceptor in close proximity, so that charge separation yields RPs in which the spin exchange interaction is large.4 Most TREPR measurements on bulk heterojunction donor−acceptor blends have focused on the weakly coupled charge-separated state produced by charge transport following the initial charge separation step,4,5 whereas little attention has been focused on the products of charge recombination from the strongly coupled initial charge-separated state. Received: December 28, 2011 Revised: January 29, 2012 Published: February 1, 2012 1923

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Figure 1. Polarization pattern for (a) SO-ISC with B∥Y overpopulated, (b) S−T0 mixing, and (c) S−T+1/T0 mixing. The blue circles represent the population differences of the three triplet sublevels relative to the least populated sublevel, the red arrows represent emissive transitions, and purple arrows represent absorptive transitions.

where D and E are the zero-field-splitting parameters and Sx,y,z are the components of the total spin angular momentum operator (S) for the triplet state. The effect of this term is to lift the degeneracy of the triplet manifold in the absence of an external magnetic field as a function of the molecular symmetry. The spin−orbit intersystem crossing mechanism (SO-ISC) can be differentiated from the RP-ISC mechanism by the electron spin polarization pattern of the six EPR transitions of 3*(D−B− A), i.e., the two transitions at each canonical (x, y, z) orientation.2a In SO-ISC, the three zero-field sublevels TX, TY, and TZ of 3*(D−B−A) are selectively populated, and their relative populations are carried over to the high field energy levels, T+1, T0, and T−1. For example, assuming selective population of the TY zero-field level and D > 0, Figure 1a shows that the six EPR transitions from low to high field give an (e, a, e, a, e, a) polarization pattern, where a = enhanced absorption and e = emission of microwave radiation. In contrast, RP-ISC acts directly on the high-field triplet sublevels of the RP via S−T0 mixing if the exchange interaction is small, or S−T+1 (S−T−1) mixing, if 2J > 0 (2J < 0) and is on the order of the EPR quantum (350 mT at 9.5 GHz, X-band). Spin polarization is preserved upon RP recombination, resulting in an (a, e, e, a, a, e) polarization pattern for S−T0 mixing8b (Figure 1b) or an (e, e, e, e, e, e) polarization pattern for S−T+1/T0 mixing9 (Figure 1c). We recently reported a covalent, fixed-distance D−B−A molecule that upon photoexcitation undergoes ultrafast charge separation to yield a RP in which 2J between the two radicals is sufficiently large to result

Triplet formation in organic molecules commonly occurs by one of two mechanisms, spin−orbit intersystem crossing (SOISC) and radical-pair intersystem crossing (RP-ISC). In SOISC a change in spin angular momentum compensates for the change in orbital angular momentum that occurs when an electron is moved between orbitals of differing symmetry, so that the total system angular momentum is conserved.2a,6 RPISC requires rapid formation of a RP upon charge separation and occurs through quantum mechanical mixing of the singlet and triplet RP sublevels induced by differences in the g-factors and sums of hyperfine interactions within the two radical ions,7 followed by spin selective charge recombination to either the ground state or the local neutral triplet state of either the donor or the acceptor.2a,6 TREPR spectroscopy provides information directly on the RP and/or the triplet state formed by charge recombination, both of which have received significant attention.2b,7,8 TREPR spectroscopy can be used to identify the intersystem crossing mechanism by examining the spin polarization pattern of the microwave-induced spin sublevel transitions within the triplet state formed following charge recombination.2,9 The main features of the EPR spectrum of 3*(D−B−A) arise from zero-field splitting (ZFS), which is a result of the magnetic dipole−dipole interaction between the two unpaired electrons in the triplet state, with the Hamiltonian given as10 /dipolar = D(Sz 2 − S2 /3) + E(Sx 2 − Sy 2)

(1) 1924

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in preferential RP intersystem crossing to the highest energy RP eigenstate (T+1) at the 350 mT magnetic field characteristic of X-band EPR spectroscopy. This behavior is unprecedented in covalent D−B−A molecules and is evidenced by the timeresolved EPR (TREPR) spectrum at X-band of 3*D−B−A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e, e, e, e, e ,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the increased Zeeman splitting of triplet sublevels diminishes S−T+1 mixing, which results in a 3*D− B−A TREPR spectrum at the W-band (94 GHz) that displays the (a, e, e, a, a, e) polarization pattern characteristic of S−T0 mixing. Although there are numerous examples of SO-ISC observed in organic molecules observed by TREPR,2a,11 there are relatively few reports of SO-ISC directly from strong-coupled charge transfer states in organic molecules, termed spin−orbit charge transfer intersystem crossing (SOCT-ISC).12 Using TREPR spectroscopy, van Willigen et al.12c observed SOCTISC in systems having a series of arene electron donors attached to the 9-position of 10-methylacridinium. They found that the relative population of the zero field triplet sublevels, TX, TY, and TZ were sensitive to the orientation of the donor and acceptor π systems, so that the SOCT-ISC rate was enhanced when the donor and acceptor π systems were oriented approximately perpendicular to one another. In another example, Wasielewski et al.12d compared the spin polarization pattern of the zinc meso-tetraphenylporphyrin (ZnTPP) triplet state with that formed following electron transfer from 1*ZnTPP to a tetracyanonaphthoquinodimethane (TCNQ) acceptor, which is covalently attached to the porphyrin so that the π systems of ZnTPP and TCNQ are fixed at approximately a 60° orientation relative to one another. Following photoexcitation of ZnTPP at 10 K, TREPR spectroscopy shows that 3*ZnTPP forms exclusively by SOISC that populates the out-of-plane TZ sublevel, whereas in the ZnTPP-TCNQ molecule, 3*ZnTPP displays an electron spin polarization pattern indicative of SOCT-ISC to the in-plane TX and TY sublevels. Herein we report on the photophysics and spin dynamics of photogenerated RPs and the neutral triplet states resulting from charge recombination in a series of D−B−A molecules in which 2J is large compared to those of most D−B−A molecules reported in the literature. The donor is a 3,5-dimethyl-4-(9anthracenyl)-julolidine (DMJ−An) donor, connected directly or connected by a single phenyl bridge to either a pyromellitimide (PI), 1 and 2, respectively, or a naphthalene1,8:4,5-bis(dicarboximide) (NI) acceptor 3 and 4, respectively. Transient optical absorption spectroscopy yields direct evidence for ultrafast charge separation to produce DMJ+•− PI−• or DMJ+•−NI−• quantitatively, whereas magnetic field effects on the triplet yield of 3*An resulting from charge recombination reveal that the charge separated RP state is very strongly spin exchange-coupled. We observe 3*An formation by TREPR spectroscopy in 1−4, where the dominant intersystem crossing mechanism in 1−3 is SOCT-ISC, whereas competition between SOCT-ISC and RP-ISC occurs in 4, which has the smallest 2J value of 1−4. Although the triplet states in 1−3 are formed by SOCT-ISC, the sublevel populations differ among the three molecules, which indicates that the triplet sublevel populations depend on the geometry of the charge-separated state.

Article

EXPERIMENTAL SECTION

The synthesis and characterization of 1−3 are detailed in the Supporting Information, whereas the synthesis and characterization of 4 were reported previously.13 Optical Spectroscopy. Ground state absorption measurements were made on a Shimadzu (UV-1800) spectrophotometer. The optical density of all samples was maintained between 0.1 and 0.6 at 416 nm, for both femtosecond and nanosecond transient absorption spectroscopy. Femtosecond transient absorption measurements were made using the 416 nm, 130 fs output from an optical parametric amplifier using techniques described earlier.14 Samples for femtosecond transient absorption spectroscopy were placed in a 2 mm path length quartz cuvette and freeze−pump−thawed five times. The samples were irradiated with 1.0 μJ per pulse focused to a 200 μm spot. The total instrument response function (IRF) for the pump−probe experiments was 180 fs. Samples for nanosecond transient absorption spectroscopy were placed in a 10 mm path length quartz cuvette equipped with a vacuum adapter and subjected to five freeze−pump− thaw degassing cycles. The samples were photoexcited by a 7 ns, 2.5 mJ, 416 nm laser pulse using the frequency-tripled output of a Continuum Precision II 8000 Nd:YAG laser pumping a Continuum Panther OPO. The excitation pulse was collimated to a 5 mm diameter spot and matched to the diameter of the probe pulse generated using a xenon flashlamp (EG&G Electro-Optics FX-200). Kinetic traces were observed from 430 to 800 nm every 5 nm using a monochromator and photomultiplier tube with high voltage applied to only 4 dynodes (Hamamatsu R928) and recorded with a LeCroy Wavesurfer 42Xs oscilloscope interfaced to a customized Labview program (Labview v. 8.5.2). The total instrument response time is 7 ns and is determined primarily by the laser pulse duration. Analysis of the kinetic data was performed at multiple wavelengths using a Levenberg−Marquardt nonlinear least-squares fit to a general sum-of-exponentials function with convolution of a Gaussian function to account for the finite instrument response. For the magnetic field effect experiments, the sample cuvette was placed between the poles of a Walker Scientific HV-4W electromagnet powered by a Walker Magnion HS-735 power supply and the field strength was measured by a Lakeshore gaussmeter with a Hall effect probe. The electromagnet and gaussmeter were interfaced with Labview, allowing measurements and control of the magnetic field to ±1 × 10−5 T during the data acquisition. To maintain sample integrity during the experiment, a probe light shutter was used to block the sample from irradiation when transient absorption kinetics were not being collected. The triplet yield was monitored at 440 nm and kinetic traces were collected in increments of 0.3, 1.5, or 5.0 mT with zero field ΔA(B=0) collection after four or five steps. To compensate for possible sample degradation, zero field kinetics were collected during the experiment in four or five step increments and plotted and fit with polynomial or linear trend lines. These functions were used to calculate the relative RP yield or triplet yield as a function of applied field strength (B) and plotted as ΔA(B)/ΔA(B=0). The results presented are an average of three or more experiments conducted on separate days with freshly prepared samples in spectrophotometric or freshly distilled ACS-grade toluene. TREPR Spectroscopy. EPR measurements at X-band (9.5 GHz) were made using a Bruker Elexsys E580 EPR 1925

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the electronic coupling between DMJ−An and −PI or −NI is not strong enough to result in a new charge transfer absorption. A Jablonski diagram for all relevant states and photophysical processes is shown in Figure 3.

spectrometer outfitted with a variable Q dielectric resonator (ER-4118X-MD5-W1). Samples of 1−4 were prepared as toluene solutions (∼10−4 M), loaded into quartz tubes (4 mm o.d. × 2 mm i.d.), subjected to five freeze−pump−thaw degassing cycles on a vacuum line (10−4 mBar), and sealed by using a hydrogen torch. The EPR samples were stored in a freezer in the dark, when not being used. TREPR measurements were made using continuous wave (CW) microwaves with direct detection. The temperature was controlled by an Oxford Instruments CF935 continuous flow cryostat using liquid N2. Samples of 1−4 were photoexcited at 416 nm (1.2 mJ/pulse, 7 ns, 10 Hz) using the frequency tripled output from either a Quanta-Ray DCR-2 Nd:YAG laser pumping a Raman shifter filled with hydrogen gas at 400 psi or a Spectra Physics Quanta-Ray Pro 350 Nd:YAG laser pumping a Spectra Physics Basi-scan OPO. Following photoexcitation, kinetic traces of the transient magnetization were accumulated under CW microwave irradiation (2−20 mW). The field modulation was disabled to achieve a Q/πν ≈ 30 ns instrument response function (IRF), where Q is the quality factor of the resonator and ν is the resonant frequency, whereas microwave signals in emission (e) and/or enhanced absorption (a) were detected in both the real and the imaginary channels (quadrature detection). Sweeping the magnetic field gave 2D spectra versus both time and magnetic field. For each kinetic trace, the signal acquired prior to the laser pulse was subtracted from the data. Kinetic traces recorded at magnetic field values off-resonance were considered background signals, whose average was subtracted from all kinetic traces. The spectra were subsequently phased into a Lorentzian part and a dispersive part, and the former, also known as the imaginary magnetic susceptibility χ″, is presented. The triplet spectra were simulated with a home-written MATLAB15 program using published models.16

Figure 3. Energies and photophysical pathways for 1−4, where n = 0, 1 and A = PI or NI.

Transient Absorption Spectroscopy. Photoexcitation of 1 and 2 in toluene at 416 nm results in formation of DMJ+•− An−Phn−PI−• (n = 0, 1) as indicated by the appearance of a strong 720 nm PI−• absorption band17b (Figures 4 and S1,

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RESULTS Steady State Absorption Spectroscopy. The excited state and redox properties of DMJ−An, −PI, and −NI have been described previously.17 The steady state absorption spectra of 1−4 (Figure 2) show the presence of PI and NI at

Figure 2. Steady state UV−vis absorption of 1−4 in toluene.

Figure 4. Femtosecond transient absorption of 1 and 3 in toluene at times indicated following a 130 fs, 416 nm laser pulse.

wavelengths 0 predicts that 2J > 0.6 T for 3, whereas we have shown earlier that analogous systems with PI as the acceptor are approximately an order of magnitude more strongly coupled than those with NI,9,13 both

Figure 6. Schematic of radical ion pair energy levels as a function of magnetic field (2J > 0).

When the Zeeman splitting of the triplet radical pair sublevels equals 2J, there is an increase in RP-ISC efficiency. This increase translates into a maximum in triplet RP production and therefore a maximum in the neutral triplet yield upon charge recombination, so that 2J can be measured directly by 1927

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Figure 7. TREPR of 1−4 in toluene at 85 K, 150 ns after a 7 ns 416 nm laser pulse.

transfer from DMJ+• to An−• via their HOMOs to produce DMJ−3*An. The PI and NI π systems in 1 and 3, respectively, are each perpendicular to that of An and parallel to that of DMJ, so that charge recombination in DMJ+•−An−PI−• and DMJ+•−An− NI−• will formally require two electron and/or hole transfers between mutually perpendicular π systems to produce 3*An and necessitates consideration of the orbital geometry changes of both charge transfers. The electronic coupling matrix element for charge recombination via the superexchange mechanism is VDA = VDB·VBA/ΔEDB, where VDB and VBA are the donor-bridge and bridge-acceptor matrix elements, respectively, and ΔEDB is the energy gap between the initial donor state and the bridge virtual state.25 Thus, given that ΔE ≅ 0.7 eV for DMJ+•−An−PI−• → DMJ−An+•−PI−• and for DMJ+•−An−NI−• → DMJ−An+•−NI−•, whereas ΔE ≅ 1.2 eV for DMJ+•−An−PI−• → DMJ+•−An−•−PI and ΔE ≅ 1.5 eV for DMJ+•−An−NI−• → DMJ+•−An−•−NI, the energy gaps favor hole transfer. Thus, virtual hole transfer from DMJ+• to An should once again result in a strong spin angular momentum component in the y-direction as was observed earlier for DMJ−An (Figure 8). However, the additional formal hole transfer from An+• to PI−• or NI−• takes the hole from the y-directed orbital on An+• and places it in an x-directed orbital on PI−• or NI−• resulting in net spin angular momentum in the z-direction to produce 3*PI or 3*NI, which then energy transfers rapidly to An faster than we can observe by TREPR, preserving the spin polarization of the high field T+1, T0, and T−1 sublevels.26 The same mechanism can account for 3*An formation in 2 and 4; however, the presence of the phenyl spacer between An and PI or NI results in dihedral angles

Table 1. Zero-Field Splitting Parameters (D and E), Relative Triplet Sublevel Population Rates (AX,Y,Z) of the Zero-Field Spin States, and the Fraction of the Triplet Population Formed from the SO Mechanism (SO) Obtained from Triplet State TREPR Spectral Simulations of 1-4 in Toluene at 85 K compound

D (mT)

E (mT)

AX

AY

AZ

SO

1 2 3 4

72.7 68.9 72.9 77.0

−6.8 −6.4 −7.7 −8.5

0.30 0.62 0.32 0.3

0.20 0.39 0.41 0.06

0.80 0.67 0.67 0.1

1.0 1.0 1.0 0.77

of which are consistent with the absence of a resonance in 1 and 2. TREPR Spectroscopy. The spin polarization patterns of the triplet states formed following charge recombination in 1−4 are different from those observed previously in DMJ−An−Phn−NI systems where n > 1,17a DMJ−An alone,12a and for An itself.24 We observe that the triplet line shapes for 1 and 3 are similar and their polarization patterns are identical (a, a, a, e, e, e), which indicates that the B∥Z sublevel is overpopulated.2a,6 The polarization pattern of 2 is different from those of 1 and 3 (a, e, a, e, a, e) and is reflected in the triplet sublevel populations, which show that the B∥X and B∥Z sublevels are overpopulated.2a,6 In our previous report on DMJ−An,12a we argued that the triplet sublevels populated by SOCT-ISC following charge recombination of DMJ+•−An−• should follow the right-hand rule for angular momentum. Because the π systems of DMJ and An are sterically constrained to a perpendicular geometry, the observed dominance of B∥Y sublevel population in DMJ−3*An is consistent with hole 1928

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very large and should be considered when designing molecules for solar energy conversion.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details including the synthesis, transient absorption spectra and kinetics, and magnetic field effect data for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 8. Representative geometries and spin configurations of 1 and 3.

* Email: [email protected]. Notes

between the An/PI and An/NI π systems that are 0.6 T for 1−3 and the EPR quantum at X-band is 350 mT, the energy gaps between the S and T+1, T0, and T−1 sublevels are large, which rules out significant mixing between S and T0 and should also inhibit mixing between S and T+1, thus eliminating RP-ISC as a viable pathway for 2. Charge recombination in 4 is apparently slow enough, and 2J is small enough for RP-ISC to begin to compete with SOCT-ISC.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Office of Basic Energy Science, U.S. Department of Energy under Grant no. DE-FG02-99ER14999. M.T.C. thanks the Link Foundation for a fellowship.

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REFERENCES

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CONCLUSIONS We show that neutral triplet formation by charge recombination from a strongly coupled charge separated state occurs by a SOCT-ISC mechanism exclusively when 2J is larger than the EPR quantum at the X-band and occurs by a combination of spin−orbit charge transfer and radical pair mechanisms when 2J is comparable to the EPR quantum at X-band. In addition, the triplet sublevels populated by SOCT-ISC are shown to depend on the geometry of the charge separated state. These results show that additional pathways for triplet formation in D−B−A molecules become active when the spin exchange interaction is 1929

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