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Exceptional Intersystem Crossing in Di(perylene bisimide)s: A Structural Platform toward Photosensitizers for Singlet Oxygen Generation Yishi Wu, Yonggang Zhen, Yuchao Ma, Renhui Zheng, Zhaohui Wang,* and Hongbing Fu* Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
ABSTRACT Photosensitized reactions of molecular oxygen have found far-reaching applications in various fields, and the development of new photosensitizer compounds is of crucial importance. We here describe a new class of triply linked bay-fused diperylene bisimides (DiPBIs) which exhibited several unique features, rendering them a new structural platform for the development of highly efficient and photostable photosensitizers. (i) The extended π-conjugation shifts its absorption into the body's therapeutic window. (ii) The nonplanarity of the distorted cores enhances the spin-orbit coupled intersystem crossing. (iii) The long-lasting highenergy T1 state facilitates singlet oxygen generation via energy-transfer reaction between T1 and ground-state oxygen. SECTION Energy Conversion and Storage
hotosensitized generation of singlet oxygen (1O2) has attracted a great deal of interest1 and has found farreaching applications in various fields, including organic fine synthesis,2 wastewater bleaching,3 and photodynamic therapy (PDT) for cancer treatment.4 The mechanism behind PDT involves excitation of photosensitizers (PSs) by a light that can pass through the body's therapeutic window (650900 nm), followed by intersystem crossing (ISC) from its lowest singlet excited state (S1) to the lowest triplet excited state (T1); then, energy transfer (EnT) from the PSs' T1 to ground-state oxygen (3O2) generates the reactive oxygen species, which causes oxidative damage of targets.5 Most of reported or commercially available PSs are predominantly based on various types of polypyrrole macrocycles, including the clinically used Photofrin.1,6 These PSs have limitations, such as low photostability and structural instability. For example, Photofrin is actually an undefined mixture of dimeric and oligomeric compounds derived from the acidic treatment of hematoporphyrin. In this regard, discovery of a nonporphyrin-based structure platform is crucial for rational design and development of novel PSs.7-10 Perylene-3,4:9,10-tetracarboxylic acid bisimides (PBIs) are one of the most intensively investigated chromophores and strongly absorb in the visible region at 525 nm.11 Coreexpanded terrylene and quaterrylene bisimides exhibit progressively red-shifted absorption from the red to near-infrared region.12 However, the high fluorescence quantum yields (ΦF) of rylene bisimides11,12 usually suggest negligible efficiency of ISC (ΦISC). The triplet-state population for PBI dyes can be achieved through intra- and/or intermolecular sensitization,13,14 covalent linkage of the heavy metal atom,15 and the multicomponent electron donor-acceptor framework facilitating a cascade of nonradiative processes.16 Akkaya and co-workers
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reported the first application of bay-substituted PBI derivatives in the sensitization of singlet oxygen toward PDT,17 indicating the potential sensitized utilization of PBI dyes. Recently, we have synthesized triply linked bay-region fused di(perylene bisimide)s (DiPBIs).18,19 Here, we disclose that several features unique for DiPBIs render them a new structural platform for the development of highly efficient and photostable PSs. (i) The extended π-conjugation of DiPBIs produces an absorption > 650 nm with extremely high absorption coefficients. (ii) Unlike planar rylene bisimides, the nonplanarity of the distorted cores of DiPBIs enhances the spin-orbital coupling, resulting in ΦISC > 0.9. (iii) The higher energy of the DiPBI's T1 state (1.1 eV) than that of 1O2 (0.98 eV) as well as its long lifetime makes the energytransfer reaction between them highly efficient. Indeed, the efficiencies of 1O2 generation (ΦΔ) are determined to be 0.67 and 0.59 for DiPBI-1 and -2, respectively. The DiPBI-2 compound was synthesized based on coppermediated coupling of tetrachloro-PBIs, while the BuchwaldHartwig coupling reaction between chlorine-functionalized DiPBI (S2) and ethyl 4-aminobenzoate afforded the monobay-functionalized DiPBI-1 (see Scheme 1).18,19 The UV-vis absorption and steady-state fluorescence spectra of DiPBI-1 and -2 in CH2Cl2 solutions are shown in Figure 1. Table 1 summarizes the absorption band maxima with the corresponding molar absorption coefficients and the luminescence peak wavelengths. Both DiPBI-1 and -2 have broad absorption with high absorption coefficients across almost the whole
Received Date: June 17, 2010 Accepted Date: August 2, 2010 Published on Web Date: August 06, 2010
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Figure 1. Steady-state absorption spectra (a) and fluorescence spectra (b) of DiPBI-1 (black line) and DiPBI-2 (red line) in CH2Cl2 solutions. Scheme 1. Synthetic Route to DiPBI-1 and -2
Figure 2. (a) Femtosecond time-resolved absorption spectra of DiPBI-1 in CH2Cl2 at delay times of 1 (black), 130 (red), 730 (green), and 3490 ps (blue). (b) Time-absorption profiles at 845 (triangle) and 965 nm (circle). The solid lines present the global fittings. (c) Time-absorption profiles at 970 nm recorded by nanosecond flash photolysis for DiPBI-1 in air-saturated (inset) and N2-saturated CH2Cl2 solutions at room temperature. (d) Near-infrared phosphorescence spectra of DiPBI-1 in N2-saturated (black) and airsaturated (red) CH2Cl2 at 77 K.
The fluorescence character of DiPBIs indicates an unusual photophysical processes. Moreover, these emission lifetimes are insensitive to solvent polarity and remain the same in polar CH2Cl2 or nonpolar toluene (Table S1, Supporting Information). This suggests that the nonradiative decay of DiPBI's S1 state is not related to the intramolecular electrontransfer process.20 To gain further insight into the excited-state dynamics of DiPBIs, femtosecond time-resolved absorption measurements were performed. Figure 2a depicts the transient spectra for DiPBI-1 in CH2Cl2 solution upon excitation at 615 nm. An intense ground-state bleaching signal appears promptly in the region of 650-700 nm (designated as band I). Simultaneously, a positive broad signal covering 750-900 nm (band II), attributed to the Sn r S1 transition, is observed. Remarkably, accompanied with the decay of band II, another positive band peaked at 965 nm (band III) appears gradually. A pseudoisosbestic point at around 925 nm is observed, as indicated by the square in Figure 2a. Moreover, a significant groundstate bleaching signal remains even after complete decay of the singlet excited state at the later time delay of 3.49 ns. The generated 965 nm feature as well as the partially decayed bleaching indicates a long-lived transient species correlated with DiPBI. A global curve-fitting procedure is applied to the kinetic traces at 845 (triangle) and 965 nm (circle), respectively, yielding one decay component (τ(S1,decay) = 610 ( 20 ps) for the former and one rise component (τ(T1,rise) = 650 ( 20 ps) for the latter (Figure 2b). The similar rise and decay time constants revealed a close correlation between S1 decay and band III formation. Therefore, the positive absorption band at 965 nm in Figure 2a is ascribed to the Tn r T1 transition, as a result of the T1 r S1 ISC process. Accordingly, the efficiency of ISC (ΦISC) is estimated to be ΦISC = [1/τ(T1,rise)]/[1/τ(S1,decay)] = 0.94 for DiPBI-1.
Table 1. Photophysical Parameters of DiPBIs in CH2Cl2 Solutions sample
λabs/nm (log ε)
DiPBI-1
414 (4.86) 614 (4.64)
DiPBI-2
408 (4.88)
671 (4.89)
λF/nm (τF/ns, ΦF) 683 (0.61, 0.03) 665
λP/nm (τT/μs)
ΦISC
ΦΔ
1130
0.94
0.67
0.91
0.59
(56) 1120
601 (4.49) 655 (4.79)
(0.71, 0.07)
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visible region. The lowest absorption transition of DiPBI-2 at 655 nm, correlated with the S1 r S0 transition, is hypsochromatically shifted compared to that of DiPBI-1 at 671 nm because of the electron-withdrawing inductive effect of chlorine atoms. A similar trend can be found in the fluorescence spectra with the emission maxima of DiPBI-2 and -1 at 665 and 683 nm, respectively. However, in sharp contrast with common planar rylene bisimides, DiPBIs are only weakly emissive, with ΦF =0.03 and 0.07 for 1 and 2, respectively. Meanwhile, the fluorescence of DiPBI-1 and -2 decays monoexponentially, giving rise to a much shorter lifetime (τF =0.61 ( 0.01 ns for DiPBI-1 and 0.71 ( 0.01 ns for -2) compared with other rylene bisimides (Figure S1, in Supporting Information).11,12
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Figure 4. Approximate energy level diagram and photophysical processes of DiPBI-1 in CH2Cl2 solution. The inset shows the optimized configuration of DiPBI-1.
spectra of DiPBIs in CH2Cl2 at 77 K (Figures 2d and 3d). In the N2-saturated solution (black curve in Figure 2d), a strong band peaked at 1130 nm (1.10 eV) is observed clearly and assigned to the phosphorescence of DiPBI-1 (1120 nm for DiPBI-2). This band is quenched in an air-saturated solution (red curve in Figure 2d), whereas a new spectral feature at 1270 nm (0.98 eV), characteristic of 1O2 phosphorescence, appears as a result of the energy transfer from T1 to 3O2. The aforementioned results of spectra and kinetics analysis prove clearly a highly efficient ISC process and the consequential singlet oxygen generation for DiPBI compounds upon direct photoexcitation. Figure 4 proposes the energetic diagram for related photophysical processes. The top inset is the energy-optimized structure of DiPBI-1 calculated by using density functional theory (DFT)at the B3LYP/3-21G(d) level. It can be seen that the aromatic cores of DiPBIs exhibit serious out-of-plane distortion due to the steric repulsion imparted by the bay substitution (Figure S2, Supporting Information).21,22 The nonplanarity of this distorted X-shaped configuration for DiPBIs leads to the breakdown of the σ-π orbital separation and thus enhances the spin-orbital coupling necessary for a highly efficient ISC (black wavy arrow).23,24 Furthermore, the higher energy of DiPBI's T1 state than that of 1O2 as well as its long lifetime makes energy transfer between them highly efficient (red wavy arrow). Comparison between 1 and 2 suggests that substitutions on the bay positions of DiPBIs do not influence the photophysics significantly. We also measured the actual efficiency of 1O2 generation (ΦΔ) by using 9,10-diphenylanthracene (DPA) as a chemical trap and methylene blue (MB) as a standard (ΦΔ = 0.57 in CH2Cl2).25 Values of ΦΔ = 0.67 and 0.59 were obtained for DiPBI-1 and -2, respectively (Figure S3, Supporting Information). Both of them show considerable singlet oxygen generation upon 658 nm light irradiation. Moreover, when MB was photobleached over 5%, no detectable degradation of DiPBIs was observed. This verifies the excellent photostability of DiPBIs, probably due to their resistance against photo-oxidation as a result of their high electron-deficient π-framework.11,14 In conclusion, the features of DiPBIs, including strong absorption (>650 nm) within the body's therapeutic window, fast T1 r S1 ISC, and highly efficient energy-transfer reaction from its T1 to 3O2, render them a new structural platform for the development of effective and photostable PSs. The rich reactions available on either bay or imide positions makes the
Figure 3. (a) Femtosecond time-resolved absorption spectra of DiPBI-2 in CH2Cl2 at delay times of 1 (black), 131 (red), 731 (green), and 3490 ps (blue). (b) Time-absorption profiles at 710 (triangle) and 926 nm (circle). The solid lines present the global fittings. (c) Time-absorption profiles at 930 nm recorded by nanosecond flash photolysis for DiPBI-2 in air-saturated (inset) and N2-saturated CH2Cl2 solutions at room temperature. (d) Near-infrared phosphorescence spectra of 2 in N2-saturated (black) and air-saturated (red) CH2Cl2 at 77 K.
In the case of DiPBI-2, it exhibits similar transient spectral patterns in which the Tn r T1 transition shifted hypsochromatically from 970 to 926 nm (Figure 3a). Corresponding global fitting on 710 and 926 nm traces also resulted in one decay component of 880 ( 50 ps and one rise component of 970 ( 40 ps. Similarly, ΦISC = 0.91 was obtained for DiPBI-2. The evaluated rate constants of intersystem crossing in these DiPBI compounds are on the same order with those of earlier studies on the perylenebisimide-perylenemonoimide dyad.16 These results account well for the extremely low fluorescence quantum yield, proving the generation of the triplet state through a highly efficient ISC pathway from the lowest 1DiPBI*. The S0 r S1 internal conversion was a minor competitive process because the sum of the quantum yields for fluorescence and ISC is close to unity in both cases. In order to trace the time evolution of the generated 3 DiPBI*, nanosecond flash photolysis was performed at room temperature. Figure 2c reveals the time profile at 970 nm of DiPBI-1 in an air-saturated solution on the microsecond time scale. The Tn r T1 absorption decays single-exponentially with a time constant (τEnT) of 0.52 μs. A much slower lifetime of 56 μs is obtained for DiPBI-1 in a nitrogen-saturated solution. For DiPBI-2, the 930 nm kinetics was also monitored and resulted in τEnT = 0.51 μs for the air-saturated solution and τT = 35 μs for the nitrogen-saturated one (Figure 3c). Apparently, the dissolved molecular oxygen quenched the 3 DiPBI* efficiently because of energy transfer from DiPBI-1's T1 to ground state 3O2. Taking into account that DiPBI possesses a high electron-deficient structure (E1/2(DiPBI/DiPBI-•) = -0.46 eV vs Fc/Fcþ),18 the possibility of the electron-transfer reaction from 3DiPBI to 3O2 can be ruled out. Therefore, the quantum efficiency for energy transfer is estimated to be ΦEnT =1 - τEnT/τT =99%. To determine the energy level of DiPBI's T1 state, we also measured the phosphorescence
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chemical modification of DiPBIs highly versatile and designable. We also noted that the remarkable singlet oxygen generation for the studied DiPBI compounds may open the way to other PS systems with similar distorted π-conjugated structures. We now work on the water-soluble ones decorated with functional groups that recognize specific proteins as an approach toward a novel PDT agent.
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SUPPORTING INFORMATION AVAILABLE Experimental
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details and other related results. This material is available free of charge via the Internet at http://pubs.acs.org. (15)
AUTHOR INFORMATION Corresponding Author:
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*To whom correspondence should be addressed. Tel and Fax: þ86-1062522039. E-mail:
[email protected] (H.F.); wangzhaohui@ iccas.ac.cn (Z.W.).
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ACKNOWLEDGMENT We thank Prof. J. P. Zhang for his assistance in nanosecond time-resolved measurements. This work was supported by National Natural Science Foundation of China (Nos. 20903108, 20873163), National Science Fund for Distinguished Young Scholars (No. 20925309), the Chinese Academy of Sciences (“100 Talents” program), and the National Research Fund for Fundamental Key Project 973 (2006CB806200, 2006CB932101).
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