Two-Photon Absorption Properties of New ... - ACS Publications

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J. Phys. Chem. C 2009, 113, 4706–4711

Two-Photon Absorption Properties of New Fluorene-Based Singlet Oxygen Photosensitizers Kevin D. Belfield,*,†,‡ Mykhailo V. Bondar,⊥ Florencio E. Hernandez,†,‡ Arte¨m E. Masunov,†,§,| Ivan A. Mikhailov,§,# Alma R. Morales,† Olga V. Przhonska,⊥ and Sheng Yao† Department of Chemistry, CREOL, The College of Optics and Photonics, Nanoscience Technology Center, and Department of Physics, UniVersity of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366, and Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki, 46, KieV-28, 03028, KieV, Ukraine ReceiVed: NoVember 22, 2008; ReVised Manuscript ReceiVed: January 16, 2009

The degenerate two-photon absorption (2PA) spectra of several fluorene-based photosensitizers (PS) in solution were obtained over a broad spectral range (460-880 nm) by open aperture Z-scan and two-photon fluorescence methods under either picosecond or femtosecond excitation, respectively. A maximum 2PA cross section of ca. 300 GM was observed for the photosensitizers containing a benzothiazole substituent in the fluorenyl 7-position. The electronic structures and 2PA properties of these PS were analyzed using a time-dependent density functional theory method, resulting in reasonably good agreement between experimental and theoretical data. 1. Introduction The synthesis and characterization of efficient singlet oxygen photosensitizers (PS) is a subject of significant scientific and technological interest in the field of photochemical and biomedical applications.1-5 It has been established that molecular singlet oxygen, 1O2, can be produced not only using linear, one-photon, excitation, but under nonlinear, two-photon, excitation of PSs as well.6-9 The combination of efficient singlet oxygen generation with the advantages of two-photon absorption (2PA)10-12 offers great potential for further development of photodynamic cancertherapy(PDT)methods13,14 andnumerousotherapplications.15,16 Therefore, nonlinear optical characterization of efficient singlet oxygen photosensitizers, including reliable determination of their 2PA spectra over a broad spectral range, is of great importance. 2PA spectra of phenylene-vinylene derivatives, porphyrins, and phenylene-ethynylene-based PSs with maximum 2PA cross sections of ca. 200-900 GM were reported.7,9,17 Typically, the fluorescence quantum yield of efficient singlet oxygen PS is relatively low,18-20 and their 2PA spectra can be obtained by the measurement of two-photon excited phosphorescence of 1O2 at 1270 nm relative to the standard PS molecule.21,22 The main problem in the relative 2PA measurements with the direct observation of two-photon excited 1O2 phosphorescence is to find a standard with a well-defined 2PA cross section and two-photon induced 1O2 quantum yield. Arnbjerg et al. proposed to use 2,5-dicyano-1,4-bis(2-(4-diphenyl-aminophenyl)vinyl)-benzene and 2,5-dibromo-1,4-bis(2-(4-diphenyl-aminophenyl)vinyl)-benzene as a standards for 2PA measurements of PS based on the comprehensive characterization of these molecules in a number of solvents by optical and laser-induced optoacoustic spectroscopy methods.15 In the case of measurable * To whom correspondence should be addressed. E-mail: belfield@ mail.ucf.edu. † Department of Chemistry. ‡ CREOL. § Nanoscience Technology Center. | Department of Physics. ⊥ National Academy of Sciences of Ukraine. # On leave from Petersburg Nuclear Physics Institute, Gatchina, St. Petersburg 188300, Russia.

fluorescence intensity of the PS, their 2PA spectra can be obtained by a relative two-photon induced fluorescence (2PF) method, comprehensively described in refs 11 and 23. Extremely high 2PA cross sections of phorphyrin dimers PS (∼104 GM at the maximum wavelength) were obtained by this method in the spectral range of 820-890 nm.24,25 In this paper, we continue our investigation of efficient fluorene-based PSs, which exhibit high quantum yields of 1O2 generation (between 0.65 and 0.93) in order to better understand molecular parameters for the design of two-photon absorbing PSs.18,19 The degenerate 2PA spectra of these molecules were obtained over a broad spectral range by the well-known Z-scan technique26 using a modified fitting procedure27 and the 2PF method for derivatives with appreciable fluorescence.11 Quantumchemical calculations of the electronic structure, as well as linear and nonlinear absorption spectra, of fluorene-based PS were performed with a time-dependent density functional theory (TDDFT) approach.28 2. Experimental Section 2.1. Linear and Nonlinear Spectral Measurements. The chemical structures of the fluorene-based PS 1-4 are presented in Figure 1. Iodine atoms and nitro groups were incorporated into the structures to increase the rate of intersystem crossing to the triplet state for efficient singlet oxygen generation. Linear and nonlinear spectral properties of PS 1-4 were measured in a polar solvent (acetonitrile, ACN) and a nonpolar solvent (cyclohexane). All solvents were of spectroscopic grade and used without further purification. Compounds PS 1 and 2 exhibited low solubility in cyclohexane; hence, their nonlinear optical properties were investigated in ACN only when solutions of relatively high concentration were required. The linear, onephoton absorption (1PA) spectra were recorded with an Agilent 8453 UV-vis spectrophotometer in 0.01, 0.1, and 10 mm path length quartz cuvettes for the range of molecular concentrations C ∼ 10-2-10-6 M, respectively (i.e., higher concentrations in shorter path length cuvettes). No concentration dependence of the PS absorption spectra or values of extinction coefficients was observed. The corrected steady-state emission spectra were

10.1021/jp8102832 CCC: $40.75  2009 American Chemical Society Published on Web 02/23/2009

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Figure 1. Molecular structures of the photosensitizers PS 1-4.

Figure 2. Bond lengths of PS 5 and PS 6, optimized at the B3LYP/ pc-2 level of theory.

obtained in 10 mm spectrofluorometric quartz cuvettes for dilute solutions (C ∼ 10-6 M) with a PTI Quantamaster spectrofluorimeter in the photon-counting regime of the photomultiplier tube (PMT) detector. Fluorescence quantum yields of PS 1-4 were determined at room temperature relative to 9,10 diphenylanthracene in cyclohexane.29 The 2PA spectra of PS 1-4 were measured by two different methods. The first method was an open aperture Z-scan technique with a modified fitting procedure that takes into account excited-state absorption and stimulated emission processes.27 For these measurements a picosecond Nd:YAG laser (PL 2143 B Ekspla) coupled to an optical parametric generator (OPG 401/SH) was used.30 The excitation laser beam with pulse duration of ≈35 ps (fwhm), pulse energies of 0.05 µJ e EP e 5 µJ, and repetition rate of 10 Hz was tuned over the spectral range of 460-800 nm. 2PA spectra of PS 1-4 were measured in 1 mm quartz cuvettes at concentrations of 10-3 e C e 10-2 M. Special attention was paid for the quality of the Gaussian spatial distribution of the laser beam, which is important for the accuracy of 2PA measurements. The second method for the determination of 2PA spectra of the fluorene-based PSs was a relative 2PF technique,11 using rhodamine B in methanol and fluorescein in water (pH ) 11) as standards.23 A PTI Quantamaster spectrofluorimeter and femtosecond Clark-MXR CPA-2001 laser (output 775 nm) pumped an optical parametric generator/amplifier (TOPAS, Light Conversion), with pulse duration of ≈140 fs (fwhm), 1 kHz repetition rate, tuning range of 560-900 nm, and pulse energies of 0.05 µJ e EP e 0.15 µJ, were used. Compounds PS 1 and 3 have extremely low fluorescence quantum yields in ACN and cyclohexane (∼10-4)19 and could not be investigated by the 2PF method. The 2PA spectra of PS 2 and 4 (quantum yields ∼10-2) were measured in 10 mm path length fluorometric

Figure 3. Structures B1 (ref 36), B2 (ref 37), and B3 (ref 38). (The structure B2 from ref 37 contains fullerene attached to the methyl group next to the oxygen atom.)

quartz cuvettes at concentrations of PS, C ∼ 5 × 10-5 M. For these measurements possible reabsorption of fluorescence emission was analyzed and taken into account.29 The quadratic dependence of the fluorescence intensity on excitation power was checked for all excitation wavelengths, indicating a pure 2PA process and negligible influence of saturation, excited-state absorption, and photobleaching effects. 2.2. Computational Methodology. All quantum-chemical calculations were performed using Gaussian 98 and Gaussian 03 suites of programs.31 To save computer time, aliphatic side chains in PS 1/PS 3 and PS 2/PS 4 were replaced with methyl groups. This is a reasonable approach since the substituents in the 9-position of the fluorene ring are not in conjugation with the aromatic system and exhibit no noticeable effect on the electronic distribution or photophysical properties of the chromophore system.19,32,33 Hence, the resulting structures PS 5 and PS 6 were studied (see Figure 2). The ground-state geometries were optimized with a DFT approach using the B3LYP exchange-correlation functional and pc-2 basis set,34 (cc-pVTZPP basis set and the effective core potential were used for the atom I in PS 5), whereas 1PA and 2PA spectra were calculated at the TD-B3LYP/6-31G (6-311G for iodine) level of theory. A choice of molecular geometry is critically important for electronic structure predictions. In particular, the bond length alternation parameter (BLA), defined as a difference between the lengths of single and double bonds, is known to be a determining factor for calculation of nonlinear optical properties.35 A minor increase in BLA can significantly destabilize excited states. In order to determine a best method to predict BLA for the molecules under study, benchmark calculations of fluorene derivatives B1, B2, and B3, for which single-crystal X-ray crystal structures were available (Figure 3), were con-

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Figure 4. Linear 1PA (1) and 2PA (2 and 3) spectra of PS 1 (a) and PS 2 (b) in ACN obtained by the open aperture Z-scan (2) and 2PF (3) methods.

Figure 5. Linear 1PA (1) and 2PA (2 and 3) spectra of PS 3 (a) and PS 4 (b) in ACN obtained by the open aperture Z-scan (2) and 2PF (3) methods.

ducted. The X-ray data were taken from refs 36, 37, and 38, correspondingly. The results are presented in Table 1. Three representative BLA parameters are reported: the difference a - b reflects the degree of conjugation between the donor and acceptor substituted phenyl rings across the biphenyl bridge (a quinoid character of the fluorene moiety), the difference e - b is indicative of hyperconjugation between the phenyl rings through the methylene bridge, and the difference d - c can serve as a measure of electron delocalization from the donor group to the adjacent phenyl ring. As one can see from the Table 1, the ab initio Hartree-Fock (HF) approach underestimates all delocalization effects: donor-acceptor conjugation across the biphenyl bridge (too large a - b), hyperconjugation (too small e - b), and donor-phenyl interaction (too small d - c). The basis set without polarization functions somewhat weakens HF errors by forcing delocalization. This error cancelation at the HF/6-31G level of theory was used for geometry optimization in the previous calculations.39,40 The AM1 method overestimates hyperconjugation (as much as a factor of 3 and 7 for e - b in B2 and B1, respectively), underestimates the donor-acceptor conjugation, but reproduces donor-phenyl interactions well. The B3LYP method, on the other hand, overdelocalizes all π-electrons with the smaller basis sets but systematically converges to the experimental values with increase of basis set size. The further details of this benchmark study will be published elsewhere. On the basis of these results, B3LYP/pc-2 was found to be the best method to reproduce the experimental geometry and was used for all geometry optimizations. The bond lengths are reported in Figure 2. Although the AM1 method provides the best performance/price ratio and was used for theoretical predictions earlier,32,41 it is not parametrized for iodine and cannot be used for the PS 5 molecule.

The adiabatic TD-DFT in the Runge-Gross formulation is currently the method of choice for a description of excited states for large molecular systems.42-44 An extension for the TD-DFT method based on coupled electronic oscillators formalism (CEO)45 was also developed for the calculations of 2PA profiles.39 It demonstrates a 2-4% deviation from experimental excitation energies for both 1PA and 2PA and close to 30% in 2PA cross sections when used with the B3LYP functional. The CEO code45 was used to calculate dynamic third-order polarizability, based on TD-DFT transition densities and CIS operators, printed out from the locally modified version of Gaussian 98 code. State-to-state transition dipole moments and permanent moments of the excited states were calculated using the a posteriori Tamm-Dancoff approximation recently introduced.46 3. Results and Discussion 3.1. Linear Spectral Properties. The linear 1PA spectra and other main photophysical parameters of PS 1-4 in ACN and cyclohexane are presented in Figures 4-6, (curves 1) and Table 2. All compounds exhibit a broad long-wavelength absorption band in the 300-400 nm spectral range, presumably corresponding to the S0 f S1 one-photon allowed electronic transition (S0 and S1 are the ground and first excited singlet electronic state of the PS, respectively). The spectral shape and maximum extinction coefficients of the absorption spectra remain nearly constant over the concentration range of 10-6 e C e 10-2 M. A comparison of the absorption spectra of the PS as a function of solvent polarity (Figures 5 and 6, curves 1) indicates the absence of any specific solvent-solute ground-state interactions. A small blue shift, narrowing of the main absorption band, and weak vibronic structure were observed for the PSs in cyclohexane. An extremely low fluorescence quantum yield

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TABLE 1: BLA Values in 10-3 Å for Molecules Depicted in Figures 2 and 3, Calculated at Different Levels of Theory as Compared with the X-ray Crystallography Dataa HF

B3LYP

BLA

molecule

AM1

pc-0

6-31G

pc-1

pc-2

pc-0

6-31G

pc-1

pc-2

X-ray

a-b

B1 B2 B3 PS5 PS6 B1 B2 B3 PS5 PS6 B1 B2 B3 PS5 PS6

71 74

73 79 80 77 76 11 13 11 11 11 22 8 7 -2 -1

76 81 83 81 63 11 13 12 13 17 21 9 10 1 6

78 82 83 82 82 6 9 7 9 9 20 10 11 1 2

81 85 86 86 85 6 9 7 9 9 20 11 12 0 1

55 62 63 60 59 16 18 15 16 16 25 13 11 0 1

58 64 66 65 63 17 18 17 17 17 26 16 16 5 6

61 64 65 65 64 13 13 11 13 13 24 16 16 5 5

63 66 67 67 66 13 14 12 13 13 22 17 17 4 5

61 69 70

e-b

d-c

a

74 42 43 42 21 15 3

6 16 18 23 18 26

BLA is the difference between lengths of the bonds, marked a, b, c, d, e in the Figures 2 and 3.

TABLE 2: Linear Photophysical Parameters of PS 1-4 in ACN and Cyclohexane at Room Temperature: Maximum max Absorption Wavelength, λabs , Extinction Coefficient, εmax, Fluorescence Quantum Yield, ΦFL, and One-Photon Singlet Oxygen Quantum Yield, Φ∆ molecule solvent max λabs , nm εmax × 10-3, M-1 · cm-1 ΦFL Φ∆a a

PS 1

PS 2

ACN 340 ( 1 21 ( 2 e10-4 0.65 ( 0.07

ACN 362 ( 1 36 ( 4 (6 ( 2) × 10-3 0.93 ( 0.1

PS 3 ACN 342 ( 1 23 ( 2 e1.3 × 10-4 0.74 ( 0.08

PS 4 cyclohexane 336 ( 1 20 ( 2 e3 × 10-5

ACN 364 ( 1 40 ( 4 (7 ( 2) × 10-3 0.92 ( 0.1

cyclohexane 361 ( 1 45 ( 5 e2 × 10-4

Ref 19.

Figure 6. Linear 1PA (1) and 2PA (2) spectra of PS 3 (a) and PS 4 (b) in cyclohexane obtained by the open aperture Z-scan method.

(∼10-4-10-5) was observed for PS 1 and 3 in ACN and PS 3 and 4 in cyclohexane (see Table 2). The nature of the lowefficiency fluorescence emission of PS 1-4 is described in ref 19. The fluorescence quantum yields of PS 2 and 4 in ACN

∼(6-7) × 10-3 were sufficient for 2PA measurements by the 2PF method. 3.2. Two-Photon Absorption Spectra of PS. The 2PA spectra of PS 1-4, obtained via the open aperture Z-scan and 2PF methods, are presented in Figures 4-6 (curves 2 and 3). The structures of PS 1 and PS 3 are very similar to each other, as well as PS 2 and PS 4, with the difference being the substituents at the fluorenyl 9-position ((CH2)2COOH and (CH2CH2O)2CH3, see Figure 1). It is reasonable to assume that these substituents do not play a substantial role in nonlinear optical properties.47 Hence, the 2PA spectra of the corresponding PSs should be similar in the same solvent. All four compounds are unsymmetrical molecules, exhibiting weak 2PA longwavelength bands (640-740 nm for PS 1 and 3 and 680-780 nm for PS 2 and 4), in the spectral range of the primary onephoton allowed electronic transition S0 f S1 (see Figures 4-6, curves 2 and 3). For centrosymmetrical molecules this transition is strictly forbidden by selection rules for 2PA.48,49 PS 2 and PS 4 in ACN and cyclohexane exhibit more intense shortwavelength 2PA bands (540-580 nm, Figures 4b-6b, curves 2), corresponding to S0 f Sn two-photon allowed electronic transitions (Sn is the higher excited singlet electronic state of PS). The maximum 2PA cross sections of PS 2 and PS 4 (∼300 GM) are 3-4 times larger than corresponding values for PS 1 and PS 3. This can be explained by the greater number of π-electrons in the molecular structures of PS 2 and 450 and increased values of the corresponding transition dipoles µ01 and µ1n (electronic transitions S0 f S1 and S1 f Sn, respectively). This trend is also observed for linear absorption (see the values of extinction coefficients, εmax, in Table 2). According to the simplified three-state model,11,51 the value of the 2PA cross 2 2 · µ1n and increases quadratisection depends on the product µ01 cally with an increase in transition dipoles. Nearly the same

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Belfield et al. PS upon electronic excitation. The short-wavelength maximum of the 2PA spectrum of PS 6 corresponds to the second resonance of the 1PA spectrum. This suggests a fairly complex nature of the electronic states of the PS that can simultaneously participate in one- and two-photon processes. The spectral position of the second 2PA resonance of PS 6 is close to the experimental ones, as seen in Figures 4b-6b (curves 2). In contrast to PS 6, for iodo-substituted PS 5 some deviation in the calculated 2PA efficiency was observed in the shortwavelength region. Lower values of 2PA cross sections ∼10-30 GM may be related to the restricted number of electronic excited states used in the CEO calculation for the iodine-substituted molecule. 4. Conclusions

Figure 7. Calculated 1PA (1) and 2PA (2) spectra of PS 5 (a) and PS 6 (b). Normalized experimental 1PA spectra (3) of PS 3 (a) and PS 4 (b) in cyclohexane.

2PA spectra were observed for the PSs in ACN and cyclohexane (Figures 5 and 6, curves 2), indicating only a weak influence of solvent polarity on the nonlinear absorption of PS 1-4. The values of the maximum 2PA cross sections of PS 1-4, obtained by two independent methods, were in good agreement with each other. The electronic, 1PA, and 2PA nature of the PSs was analyzed by quantum-chemical calculations described in the following section. 3.3. Results of Quantum-Chemical Calculations. The bond lengths in molecules PS 5 and PS 6, optimized at the B3LYP/ pc-2 theory level, are shown in Figure 2. The predicted 1PA and 2PA spectra of PS 5 and PS 6 are presented in Figure 7. Good agreement with the experimental data is evident in Figure 6 when comparing the spectra of PS 5 with PS 3 and PS 6 with PS 4. The 1PA absorption maxima, obtained with the TDB3LYP/6-31G method, are about 20 nm red-shifted relative to the experimental values. Their energies are probably close to the corresponding 0-0 transitions of the PS. Meanwhile, the maxima of the experimental spectra may correspond to excitations with changes of the vibration quantum numbers. The relative intensities of the 1PA spectra, as well as their spectral positions, are close to the experimental data for both PS 5 and PS 6. The 2PA spectra, simulated with the TD-B3LYP-CEO approach, including the absolute values of the cross sections, are also in relatively good agreement with experimental data. The first peak from the long-wavelength region of the 2PA spectra corresponds to the lowest energy 1PA transition, which is strictly two-photon forbidden for centrosymmetric compounds but can be observed for unsymmetrical polyene molecules.27 This transition is predominantly of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) nature, and its 2PA cross section is due to a relatively large change in the permanent dipole moment of the

2PA spectra of efficient singlet oxygen fluorene-based PS were obtained over a broad spectral range by two separate methods: open aperture picosecond Z-scan with modified fitting and 2PF technique with femtosecond excitation. The nature of the absorption bands and maximum 2PA efficiency of PS 1-4 were analyzed on the basis of TD-DFT quantum-chemical calculations. The best optimized geometries of the fluorenyl photosensitizers were obtained with a B3LYP/pc-2 approach, and good agreement was obtained between the experimental and calculated 2PA spectra of the PSs in cyclohexane. These fluorene-based photosensitizers (PS 1-4) have well-defined 2PA spectra, acceptable values of two-photon cross sections (∼300 GM), and high 1O2 quantum yields (∼0.65-0.93), all important parameters, indicative of their promising potential for one- and two-photon singlet oxygen generation in biomedical (PDT) applications, among others. Acknowledgment. We acknowledge the National Science Foundation (ECS-0524533, CHE-0832622, and CCF-0740344), the Florida Hospital Gala Endowed Program for Oncologic Research, and the University of Central Florida Presidential Initiative for Major Research Equipment for partial support of this work. We are thankful to Sergei Tretiak and Sergio Tafur for providing computer codes. References and Notes (1) DeRosa, M. C.; Crutchley, R. J. Coord. Chem. ReV. 2002, 233234, 351. (2) Song, B.; Wang, G.; Tan, M.; Yuan, J. New J. Chem. 2005, 29, 1431. (3) van der Haas, R. N. S.; Tang, S.-H.; Herna´ndez, B. A.; de Jong, R. L. P.; Erkelens, C.; Liu, Y.; Gast, P.; Smijs, T. G. M.; Schuitmaker, H. J.; Lugtenburg, J. Eur. J. Org. Chem. 2005, 3813. (4) Chavan, S. A.; Maes, W.; Gevers, L. E. M.; Wahlen, J.; Vankelecom, I. F. J.; Jacobs, P. A.; Dehaen, W.; De Vos, D. E. Chem. Eur. J. 2005, 11, 6754. (5) Nishiyama, N.; Stapert, H. R.; Zhang, G.-D.; Takasu, D.; Jiang, D.-L.; Nagano, T.; Aida, T.; Kataoka, K. Bioconjugate Chem. 2003, 14, 58. (6) Karotki, A.; Drobizhev, M.; Kruk, M.; Spangler, C.; Nickel, E.; Mamardashvili, N.; Rebane, A. J. Opt. Soc. Am. B 2003, 20, 321. (7) Frederiksen, P. K.; McIlroy, S. P.; Nielsen, C. B.; Nikolajsen, L.; Skovsen, E.; Jørgensen, M.; Mikkelsen, K. V.; Ogilby, P. R. J. Am. Chem. Soc. 2005, 127, 255. (8) Oar, M. A.; Dichtel, W. R.; Serin, J. M.; Fre´chet, J. M. J.; Rogers, J. E.; Slagle, J. E.; Fleitz, P. A.; Tan, L.-S.; Ohulchanskyy, T. Y.; Prasad, P. N. Chem. Mater. 2006, 18, 3682. (9) McIlroy, S. P.; Clo, E.; Nikolajsen, L.; Frederiksen, P. K.; Nielsen, C. B.; Mikkelsen, K. V.; Gothelf, K. V.; Ogilby, P. R. J. Org. Chem. 2005, 70, 1134. (10) Belfield, K. D.; Schafer, K. J.; Liu, Y.; Liu, J.; Ren, X.; Van Stryland, E. W. J. Phys. Org. Chem. 2000, 13, 837.

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