DOI: 10.1002/chem.201303291

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& Dyes/Pigments

New 2,6-Distyryl-Substituted BODIPY Isomers: Synthesis, Photophysical Properties, and Theoretical Calculations** Lizhi Gai,[a, b] John Mack,[c] Hua Lu,*[a] Hiroko Yamada,[d] Daiki Kuzuhara,[d] Guoqiao Lai,[a] Zhifang Li,*[a] and Zhen Shen*[b]

Abstract: A 2,6-distyryl-substituted boradiazaindacene (BODIPY) dye and a new series of 2,6-p-dimethylaminostyrene isomers containing both a- and b-position styryl substituents were synthesized by reacting styrene and p-dimethylaminostyrene with an electron-rich diiodo-BODIPY. The dyes were characterized by X-ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6-distyryl-substituted BODIPYs, relative to analogous 3,5-distyryl- and 1,7-distyryl-substituted BODIPYs. Although the fluorescence of the compound with b-position styryl substituents on both pyrrole moieties and one with

Introduction Boradiazaindacenes (BODIPYs) are a well-known type of highly fluorescent dye with unusually intense absorption and emission bands, high photochemical stability, and low sensitivity to [a] L. Gai, Dr. H. Lu, Prof. G. Lai, Prof. Z. Li Key Laboratory of Organosilicon Chemistry and Material Technology, Ministry of Education Hangzhou Normal University, Hangzhou, 310012 (P.R. China) Fax: (+ 86) 571-2886-8529 E-mail: [email protected] [email protected] [b] L. Gai, Prof. Z. Shen State Key Laboratory of Coordination Chemistry Nanjing National Laboratory of Microstructures School of Chemistry and Chemical Engineering Nanjing University, Nanjing, 210093 (P.R. China) Fax: (+ 86) 25-8331-4502 E-mail: [email protected] [c] Dr. J. Mack Department of Chemistry, Rhodes University Grahamstown 6140 (South Africa) [d] Prof. H. Yamada, D. Kuzuhara Graduate School of Materials Science Nara Institute of Science and Technology Ikoma, 630-0192 (Japan) [**] BODIPY = 4-bora-3a,4a-diaza-s-indacene. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303291. Chem. Eur. J. 2014, 20, 1091 – 1102

both b- and a-position substituents was completely quenched, the compound with only a-position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6-p-dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non-emissive charge-separated S1 states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.

the chemical environment.[1] BODIPYs have been widely used in a variety of organic functional materials, such as labeling reagents, chemosensors and laser dyes, and in applications such as photodynamic therapy.[2] Recently, there has been an increasing focus on the synthesis, functionalization, and photophysical properties of BODIPYs. One area of particular interest has been the design of new dyes with absorption and emission bands shifted towards the near-infrared (NIR) region and into the optical window in biological tissue.[3] Despite the lack of a cyclic perimeter, BODIPYs possess many of the characteristics of heteroaromatic compounds, such as planarity and high chemical stability, due to the manner in which the BF2 moiety interacts with the p system. A redshift of the main absorption band into the NIR or red region can be achieved by preparing substituted BODIPY structures in moderate yield through transition-metal-catalyzed cross-coupling reactions,[4] Knoevenagel condensations reactions,[5] and nucleophilic substitution reactions.[6] The introduction of styryl group is one of the most efficient ways to achieve an extension p-conjugation system, and hence a significant redshift of the main spectral bands. For example, the introduction of styryl substituents at the 3,5-positions leads to a profound redshift of approximately 120 nm relative to the corresponding tetramethyl-BODIPY compound.[7] In contrast, the shift induced by styryls at the 1,7-positions is only 70 nm.[8] Since the substituent position clearly has a strong influence on the spectroscopic properties, a logical

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Full Paper next step would be to explore the effect of introducing styryl substituents at the 2,6-positions. In this study, 2,6-distyryl BODIPYs have been synthesized by using the analogous diiodo-BODIPY through Heck reactions.[9] Surprisingly, the use of p-dimethylaminostyrene results in the synthesis of three geometric isomers with markedly different photophysical properties. The dimethylaminostyryl group was selected for detailed study due to its strong electron-donor properties, since this is likely to result in an enhanced redshift of the spectral bands. Protonation could also lead to applications as sensors for pH.[5e, 18b] In this study, we report the synthesis and characterization of 2,6-distyryl-substituted BODIPY dyes and the use of DFT and time-dependent (TD)-DFT calculations to explore the structure–property correlations.

Results and Discussion

Scheme 1. Synthesis of 2,6-distyryl-substituted BODIPY derivatives.

Synthesis The 2,6-distyryl-substituted BODIPYs were synthesized from an electron-rich 2,6-diiodo-BODIPY by a reaction with styrene in dimethylformamide and triethylamine. Palladium diacetate was used as a catalyst. The 2,6-distyryl-substituted BODIPY 1 was formed in 41 % yield. The structure of 1 was determined by 1 H NMR spectroscopy and HRMS. Two doublet signals with J = 16 Hz at d = 6.88 and 6.62 ppm were assigned to the protons on the ethene moiety. Similar signals were reported in the 1 H NMR spectra of the analogous BODIPYs with styryl group at the 3,5- or 1,7-positions.[7, 8] The use of p-dimethylaminostyrene provides scope for introducing electron-donating groups, which can be used to further fine-tune the redox and optical properties. When the synthesis was carried out, however, three new BODIPY isomers were isolated (Scheme 1); one with b-position styryl substituents on both pyrrole moieties (2 a), another with both b- and a-position substituents (2 b), and another with only a-position substituents (2 c). The same molecular ion peak is observed at m/z 614 (calcd for [M] + = 614 amu) in the MALDITOF mass spectra of 2 a–c. In the 1 H NMR spectra of 2 a (Figure 1), the proton signals of the styryl moiety and methyl groups are shifted to highfield compared with those of 1. This is probably related to shielding effects caused by the electron-donating properties of the dimethylamino groups. Although singlet peaks Figure 1. 1H NMR spectra of 2 a–c. Chem. Eur. J. 2014, 20, 1091 – 1102

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at d = 4.89 and 5.70 ppm are observed in the 1H NMR spectra of 2 b (Figure 1) due to the terminal olefin protons, split peaks are observed for the methyl protons of the amino groups between d = 2.95–2.98 ppm and of the main BODIPY moiety at around d = 1.21, 1.46, 2.40, and 2.72 ppm due to the unsymmetrical structure. The pair of peaks at highfield can be assigned to the a-position styryl substituent, since the corresponding signals for 2 c lie at d = 1.21 and 2.40 ppm (Figure 1). Our research confirms that geminal substitution (to form the a-position substituents) is strongly favored by electron-donat-

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Full Paper ing substituents. This can be rationalized based on a consideration of the different cationic transition states that can be formed after Pd insertion into the double bond (see Scheme S1 in the Supporting Information).[10] A single crystal of 2 b was obtained by the slow diffusion of hexane into a dichloromethane solution. As shown in Figure 2, the boron atom is coordinated in a tetrahedral geometry by

Table 1. Spectroscopic and photophysical properties of 1 and 2 a–c in hexane and CH2Cl2 at 298 K, and those reported previously for 3 and 4.

1 2a 2 a + H + [b] 2b 2 b + H + [b] 2c 2 c + H + [b] 3[8] 4[7a]

Solvent

labs [nm]

lem [nm]

Demabs [cm1]

F

tf [ns]

hexane CH2Cl2 hexane CH2Cl2 CH2Cl2 hexane CH2Cl2 CH2Cl2 hexane CH2Cl2 CH2Cl2 MeOH toluene toluene

575 575 603 616 578 564 570 550 529 529 524 572 582 629

628 633 n.d[a] n.d[a] 619 n.d[a] n.d[a] 597 559 548 545 588 603 641

1470 1590 – – 1150 – – 1430 1010 660 740 480 600 300

0.02 0.01 – – 0.01 – – 0.02 0.49 < 0.001 0.46 0.72 0.44 0.59

2.08 1.14 – – 2.46 – – 4.22 6.51 4.65 5.05 n.a[c] n.a[c] 4.6

[a] Not detected. [b] After the addition of 100 equivalents of TFA. [c] Not available.

the 570–620 nm region than are typically observed for the main BODIPY absorption band (Figure 4).[7c, 11] In CH2Cl2, a redshift of 41 nm is observed for the band center of the lowest-

Figure 2. Front (top) and side (bottom) ORTEP views of the molecular structures of 2 b with the thermal ellipsoids set at 50 % probability.

two nitrogen and two fluorine atoms and the meso (or 8)phenyl ring lies almost orthogonal to the indacene plane with torsion angles of 88.6 8 (Figure 2). The indacene moiety is highly planar with an average root-mean-square (rms) deviation of 0.0166 . The C9C10 and C30C31 bond lengths (1.335 and 1.316 , respectively) are consistent with the double-bond character of terminal and styryl olefins. Head-to-tail p–p stacking interactions are observed in the molecular packing diagrams with interplanar separations of approximately 3.9  (Figure 3). There is a significant dihedral angle of 37.2 8 between the b-position styryl substituent and the main BODIPY fluorophore, due to the steric interactions with the methyl substituents at the 1- and 3-positions. The UV/Vis absorption and emission spectra of 1 and 2 a– c were measured in hexane and CH2Cl2. The photophysical properties are summarized in Table 1. The absorption spectra of 1 and 2 a,b exhibit broader and more structureless bands in

Figure 3. View of the p–p stacking interactions of 2 b. Chem. Eur. J. 2014, 20, 1091 – 1102

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Figure 4. Absorption spectra of 1 and 2 a–c in dichloromethane. Inset: Photograph of 1 and 2 a–c in dichloromethane.

energy absorption band of 2 a relative to that of 1. Absorption bands are observed at 529 and 570 nm for 2 c and 2 b, respectively, when there is a-position substitution of the styryl unit (Scheme 1). This represents a blueshift of 87 and 46 nm relative to the absorption band of 2 a. This can be readily attributed to weaker conjugation between the a-position-substituted styryl moiety and the main BODIPY fluorophore. The absorption spectra of 1 and 2 c are largely unaffected by solvent polarity, as is normally the case with BODIPY chromophores.[12] In contrast, the band maxima of 2 a,b are blueshifted by 13 nm when the solvent is changed from hexane to CH2Cl2. An increase in the ground state dipole moment would be anticipated based on the presence of the dimethylamino group on a b-position styryl substituent. The emission band of 1 at 633 nm exhibits mirror symmetry with the absorption band. There is a significant Stokes shift of

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Full Paper 1590 cm1, which is much larger than those typically observed for BODIPYs due to the rigid structure of the fluorophore.[13] Compounds 2 a,b do not exhibit any fluorescence, whereas only very weak emission is observed for 1. This suggests that there is a greater degree of conformational flexibility in the S1 excited state, because strong emission is observed for 3 and 4 Figure 5. Molecular structures of distyryl-substituted BODIPY dyes. (Figure 5).[14] In contrast, compound 2 c exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane (Table 1). In both of the solvents investigated the fluorescence decay profiles of 1 and 2 c could be described by a singleexponential fit on a nanosecond timescale. The low fluorescence quantum yields of styryl-substitution at the 2,6-position of the BODIPY core are reflected in the rates of nonradiative decay knr (knr = (1F)/t = 8.7  108 s1 in CH2Cl2), which increase around 10-fold relative to the corresponding 3,5-disubstituted BODIPY (knr = 0.89  108 s1 in toluene).[7b] The absorption spectra of 2 a– c exhibit interesting changes during titrations with trifluoroacetic acid (TFA; Figure 6). Several isosbestic points are observed. There is a hypsochromic shift of the maximum absorption band and a slight increase of the extinction coefficient. The observed blueshift is consistent with the elimination of the mesomeric interaction between the dimethylamino group and the p system. The main emission band gradually increases in intensity with an almost unchanged wavelength when the Figure 6. Absorption (left) and fluorescent (right) spectra of 2 a–c in CH2Cl2 by addition of TFA. lex is 540 (for 2 a), electron-donating properties of 530 (for 2 b), and 510 nm (for 2 c). dimethylamino substituents are eliminated upon protonation.[15] This is related primarily to a significant change in the fluorescence quantum yield (Table 1). the relatively low quantum yield values (F = 0.01 for 2 a and The TFA titration spectra demonstrate that 2 a–c could be used 0.02 for 2 b) when 100 equivalents of TFA are added. as “turn-on” fluorescence sensors for changes in pH despite Chem. Eur. J. 2014, 20, 1091 – 1102

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Full Paper Discussion Theoretical calculations have been used to gain greater insight into the electronic structures and to account for the observed photophysical properties. The B3LYP functional of the Gaussian 09 software package[16] was used with 6-31G(d) basis sets to optimize the structures of 1, 2 a–c, 3, 4, an unsubstituted meso-phenyl-BODIPY model complex (5), and a 3,5-distyrylsubstituted analogue of 2 a (6 a), without symmetry restraints. The B3LYP optimized geometries were also used to carry out a second set of TD-DFT calculations using the Coulomb-attenuated B3LYP functional (CAM-B3LYP) functional with 6-31G(d) basis sets, Tables 2 and 3, since the results of TD-DFT calcula-

Table 2. Observed and calculated wavelengths for the main absorption bands of 1, 2 a–c, 3, and 4 calculated by using the B3LYP and CAMB3LYPs functional with 6-31G(d) basis sets, both in the gas phase and dichloromethane using the polarizable continuum model (PCM).

Experimental B3LYP (PCM) B3LYP CAM-B3LYP (PCM) CAM-B3LYP

1

2a

2b

2c

3

4

575[a] 547 537 486 475

616[a] 669 626 514 499

564[a] 636 594 488 475

529[a] 588 541 453 441

582[b] 560 535 519 497

629[b] 590 556 555 525

[a] Dichloromethane. [b] Toluene.

tions are known to be problematic when significant chargetransfer character is involved.[17] The CAM-B3LYP functional includes a long-range correction of the exchange potential, which incorporates an increasing fraction of Hartree–Fock (HF) exchange as the interelectronic separation increases. If the systematic underestimation of the energies of the transitions is set aside, the general trend predicted for the wavelengths of the main spectral bands of the 2,6-p-dimethylaminostyryl-substituted compounds 2 a–c relative to 1, 3, and 4 in calculations with the CAM-B3LYP functional is in closer agreement with the experimental data (Table 2 and Figure 4) than those obtained with the B3LYP functional. The lowestenergy band of 2 a is predicted to lie at 499 nm (f = 1.13) and to arise almost exclusively from the HOMO!LUMO one-electron transition (Table 3). The HOMO and LUMO of 2 a are both destabilized relative to those of 1 due to the introduction of the electron-donating amino substituents (Figure 8). There is significant mixing between the HOMO of the BODIPY fluorophore and an MO introduced by the styryl substituents, which is the HOMO2 of 1 and 2 a–c (Figure 7). There is a significant redshift of the absorption bands of 2 a, since the destabilization of the HOMO is greater than that of the LUMO, which has very small MO coefficients on the styryl substituents (Figure 7 and 8). The a-position styryl substituents of 2 b,c are predicted to stabilize the HOMO (Figure 8), but to have little effect on the LUMO, thus increasing the HOMO– LUMO gap, resulting in a significant blueshift of the main absorption band relative to that of 2 a. The lowest-energy transitions of 1 and 2 b,c are also dominated by the HOMO!LUMO Chem. Eur. J. 2014, 20, 1091 – 1102

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one-electron transition but also contain a minor HOMO2! LUMO component, which arises from an MO associated primarily with the styryl substituents. When the systematic over-estimation of the energies of the spectral bands is taken into consideration, the HOMO2!LUMO one-electron transition is predicted to result in a second weaker absorption band with significant intensity close to the blue end of the absorption spectrum (Table 3), as is observed experimentally in the 350– 450 nm region (Figure 4). The MOs derived from the HOMO1 and HOMO2 of the parent unsubstituted BODIPY dye (compound 5) are not predicted to play a significant role in the visible region spectral bands of 1, 3, and 4. A marked destabilization of these MOs is predicted in the p systems of 2 a,b relative to the LUMO (Figure 8). This may account for the intensity observed in the 300–350 nm region of the absorption spectra of these compounds (Figure 4 and Table 3). Although very low fluorescence quantum yields are observed for the 2,6-distyryl-substituted BODIPY 1 (Table 1), the 1,7- and 3,5-distyryl-substituted BODIPYs 3 and 4 have high quantum yields of over 0.40. Both the HOMOs and LUMOs of 3 and 4 have significant MO coefficients on the styryl moieties (Figure 7), so there is only limited scope for charge-transfer character in the HOMO!LUMO transition. However, while the HOMO of 1 has a broadly similar set of nodal patterns to those of 3 and 4, the LUMO is centered almost exclusively on the BODIPY core, so there is significant charge transfer from the styryl groups to the main BODIPY fluorophore. This chargetransfer character is further enhanced in 2 a–c by the introduction of electron-donating amino groups. Although the rotation of styryl groups at the 2,6-positions of the BODIPY moiety of 2 a–c could result in fast internal conversion to the ground state, this would not account for the higher quantum yield observed for 2 c in non-polar solvents. In CH2Cl2, the emission intensity of 2 c increases markedly after the addition of 100 equivalents of TFA (F = 0.46) due to the protonation of the para substituents (Table 1). This suggests that charge transfer between the substituents and main BODIPY fluorophore plays a significant role. The fluorescence intensity can be further quenched in polar solvents as is observed with 2 c due to an enhancement of the rate of nonradiative decay through what are often referred to as intramolecular charge transfer (ICT) states.[5e, 18] These so-called ICT states are derived from the S1 state of the fluorophore based on a separation of charge on different portions of the structure upon electronic excitation. The rate of nonradiative decay is enhanced by the formation of conical intersections between this low-lying non-emissive S1 state and the ground state.[18, 19] In the context of 1 and 2 a–c, the quenching of the fluorescence may be further enhanced by the steric interactions between the styryl groups and the methyl substituents at the 1- and 3-positions (Figure 9 and Table 4), since it has been demonstrated that the charge-separated character of non-emissive S1 state is enhanced when there is a twisting of the bond linking the donor and acceptor moieties, similar to those predicted in the structure of 1 and 2 a (Figures 7 and 9, and Table 4).[19] Small Stokes shifts of around 600 and 300 cm1 are observed for the 1,7- and 3,5-distyryl-substituted BODIPYs 3 and 4 in tol-

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Figure 7. The frontier occupied and virtual MOs for an unsubstituted meso-phenyl-BODIPY model complex (5), 1, 2 a–2 c, 3, and 4 at the CAM-B3LYP/6-31G(d) level of theory using an isosurface value of 0.05 a.u.

uene, respectively, whereas the 2,6-distyryl-subsitituted BODIPY 1 has a much larger Stokes shift of 1590 cm1 (Table 1). The geometry of the potential energy surfaces involved in the relaxation of the S1 excited state after vertical excitation from the S0 state plays a vital role in determining the size of the Stokes shift.[14, 20] To rationalize the relative sizes of the Stokes shifts, the geometries of the ground and S1 excited states of 1, 2 a, 3, and 4 were optimized by using the Gaussian software packChem. Eur. J. 2014, 20, 1091 – 1102

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age.[16] The most prominent difference between the S0 and S1 geometries is the dihedral angle between the styryl and BODIPY moieties. The dihedral angles in the ground state for b-position styryl substituents are 44.3, 42.4, 37.9, 17.9, and 5.2 8 for 1, 2 a, 2 b, 3, and 4, respectively. In each case, the two moieties are predicted to have a significantly greater level of coplanarity in the S1 state. The dihedral angles are predicted to be much larger in the S1 states of 1, 2 a and 2 b, however, com-

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Full Paper large Stokes shift observed for 1 and 2 a–c (Table 1), but also for why no quenching of the fluorescence of 3 and 4 has been reported (Table 1), since there is significantly less scope for conical intersections to form between a charge-separated S1 state and the ground state when the styryl and BODIPY moieties effectively form a single near planar p-conjugation system (Table 4). Although BODIPY dyes do not formally fit Hckel’s rule for aroFigure 8. The frontier MO energies and HOMO–LUMO gaps of an unsubstituted meso-phenyl-BODIPY model commaticity, their properties are plex (5), 1, 2 a–c, 3, 4 and a 3,5-disubstituted analogue of 2 a (6 a) calculated using the CAM-B3LYP functional with broadly similar to those of an ar6-31G(d) basis sets. Diamonds, circles, squares, and triangles are used to denote MOs that exhibit the same nodal patterns as the frontier p-MOs of the parent BODIPY fluorophore. omatic p-system, since the coordination of the boron atom holds the dipyrromethene ligand Table 3. Calculated electronic excitation energies (< 4.0 eV), oscillator strengths and the related wave functions in a rigidly planar conformation. for 5, 1, 2 a–c, 3, and 4 calculated by using the CAM-B3LYP functional with 6-31G(d) basis sets. The p-MOs associated with the indacene plane can be com[a] [b] [c] [d] E l f Wave function # pared to those of an aromatic [eV] [nm] C12H122 cyclic perimeter, which 5 S1 3.14 397 0.43 H!L (95 %), H1!L (4 %) has MOs arranged in a ML = 0,  3.97 314 0.09 H1!L (95 %), H!L (4 %) S2 1,  2,  3,  4,  5, 6 sequence 4.19 297 0.00 H3!L (87 %), H2!L (11 %) S3 4.30 290 0.29 H2!L (86 %), H3!L (11 %) S4 in ascending energy terms 2.62 475 1.10 H!L (95 %) 1 S1 (Figure 10). The angular nodal 3.47 359 0.05 H1!L (87 %), H6 (H3)!L (10 %) S2 patterns for the p-MOs of 3.55 351 0.22 H2!L (87 %), H5 (H1)!L (10 %) S3 a BODIPY core follow a similar 2.50 499 1.22 H!L (87 %), H2!L (8 %) 2a S1 3.18 392 0.04 H1!L (85 %), H4 (H3)!L (13 %) S2 sequence, but the introduction 3.44 362 0.31 H2!L (66 %), H3 (H1)!L (29 %) S3 of the BF2 moiety, the cross-links 2.62 475 1.00 H!L (82 %), H2!L (12 %) 2b S1 and pyrrole-nitrogen atoms re3.34 373 0.15 H1!L (46 %), H2!L (31 %), H4 (H3)!L (13 %), H!L (6 %) S2 sults in a marked lifting of the 3.54 352 0.10 H1!L (40 %), H2!L (38 %), H3 (H1)!L (10 %), H4 (H2)!L (10 %) S3 3.92 317 0.11 H4 (H3)!L (51 %), H3 (H1)!L (17 %), H2!L (8 %), H1!L (7 %), S4 degeneracies of the MO energies H8!L (6 %) due to the C2v symmetry. This re2.83 441 0.84 H!L (72 %), H2!L (25 %) 2c S1 sults in a HOMO and a LUMO 3.55 350 0.02 H1!L (89 %), H4 (H3)!L (9 %) S2 that are well-separated in energy 3.59 347 0.07 H2!L (60 %), H3 (H1)!L (24 %), H!L (15 %) S3 3.87 321 0.05 H3 (H1)!L (68 %), H2!L (11 %), H!L (12 %), H9!L (5 %) S4 terms from the other p-MOs. 4.08 305 0.03 H4 (H2)!L (74 %), H8!L (17 %), H1!L (8 %) S5 Theoretical calculations predict 2.51 497 1.01 H!L (98 %) 3 S1 that the lowest-lying S0 !S1 tranS2 3.04 409 0.00 H2!L (93 %) sition is associated almost 100 % 3.18 392 0.38 H1!L (94 %) S3 2.37 525 1.00 H!L (97 %) 4 S1 with the HOMO!LUMO transi3.75 332 0.04 H2 (H1) ! L (65 %), H3!L (30 %) S2 tion. The a2 MOs with a short3.80 327 0.50 H1!L (89 %) S3 axis nodal plane passing through 4.08 305 0.02 H6!L (82 %), H!L + 1 (7 %), H4 (H3)!L (5 %) S4 the boron atom are largely unaf4.11 303 1.31 H!L + 1 (84 %), H6!L (5 %) S5 fected in energy terms relative [a] Excited state. [b] Energy of excited state. [c] Oscillator strength. [d] MOs involved in the transitions. Oneto a C12N2H10 heteroaromatic electron transitions between MOs associated primarily with the BODIPY p-system are highlighted in italics and where necessary the corresponding MO of the parent meso-phenyl-BODIPY model compound 5 is provided in model compound, whereas the parentheses. Only one-electron transitions that provide a contribution of greater than 5 % are included. b2 MOs with nodal planes aligned with the long-axis are significantly stabilized due to stronger bonding interactions between the two pyrrole moietpared with those predicted for 3 and 4. This greater conformaies, larger MO coefficients on the electronegative nitrogen tional flexibility is due to the steric interactions between the atoms, and the cross-linking between atoms on the outer pestyryl substituents and the methyl groups (Figure 9 and rimeter due to the incorporation of the two pyrrole moieties. Scheme 1). This provides an rationalization not only for the Chem. Eur. J. 2014, 20, 1091 – 1102

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Figure 9. Optimized geometries of 3, 1, and 4 calculated with the 6-31G(d) basis set at the B3LYP level.

Table 4. Dihedral angles for optimized geometric structures of 1, 2 a, 2 b, 3, and 4. Dihedral angles[a] [8]

3

1

2a

2b

4

qGS qES

17.9 5.6

44.3 31.6

42.4 33.0

37.9 22.6

5.2 0.0

[a] qGS is the average angle in the ground state between the two styryl group and fluorophore core. qES is the average angle between the two styryl group and the fluorophore core in the excited state. The dihedral angles were calculated using the Mercury 2.3 program.[21]

From the standpoint of obtaining a redshift of the main absorption and emission spectral band, a structural modification must alter the energies of the two frontier p-MOs in a manner that narrows the HOMO–LUMO gaps. In the context of the cyclic perimeters associated with heteroaromatic cyclic polyenes such as porphyrins, Michl has demonstrated that the inductive effect of the substituent on the energies of the HOMO and LUMO will usually be similar, but the mesomeric effects can differ significantly.[22] Meso substituents have only a minor effect on the energy of the HOMO, since there is a nodal plane at this position, but changes to the para substituent on a phenyl substituent can have a significant impact on the energy of the LUMO, since there is a large MO coefficient on the meso-carbon. The effects of substituents at the 1,7-, 2,6and 3,5-positions of the BODIPY structure can be rationalized in a similar manner. There is a nodal plane near the 2,6-positions in the LUMO but not in the HOMO, while there are nodal planes near the 1,7-positions in the HOMO but not in the LUMO. Although there are no nodal planes, there are larger MO coefficients at the 3,5-positions in the HOMO than in the LUMO. In each case, therefore, there is scope for narrowing the HOMO–LUMO gap based on the mesomeric effects of substituents when there is an extension of the p-conjugation system. For example, substitution at the 3,5-positions results in a significant extension of the p-conjugation system, since there are significant MO coefficients across the entire structure Chem. Eur. J. 2014, 20, 1091 – 1102

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(Figure 7). This leads to a narrowing of the HOMO–LUMO gap due to a significant destabilization of the HOMO (Figure 8) and accounts for the marked redshift of the main BODIPY band of 4 to 629 nm. This band lies about 54 nm to the red of the corresponding bands in the spectra of 2,6- and 1,7-distyryl-substituted dyes, 1 and 3. It should be noted, however, that the reasons for the redshift of the main absorption band of 4 relative to that of 1 may also be related to the large dihedral angles between the styryl moieties and the BODIPY core in the structures of 1 and 2 a (Table 4), which limit the extent of conjugation across the entire molecule and lead to significant differences in the sizes of the Stokes shifts (Table 1). A relatively minor mesomeric effect would be predicted for the styryl substituent based on the published Hammett parameters (sm = 0.03, and sp = 0.07),[23] so only minor mesomeric effects are expected for 1, 3 and 4, in which there is no electron-donating para substituents. The large torsion angle in the structure of 1, therefore, probably does not have a significant effect on the wavelength of the main absorption band of 1 relative to those of 3 and 4. Similar trends in the wavelengths of the main spectral bands have been reported for phenylethynyl-substituted compounds in which the torsion angles are significantly smaller.[8, 24] The structure of 2 a is more problematic from this standpoint, however, since a larger mesomeric effect would be anticipated. B3LYP geometry optimizations and TDDFT calculations (Table 5) were carried out for a series model complexes comprised of analogues of 1, 2 a, 4, and 6 a with no methyl substituents, so that the effect of substitution position on the energies of the frontier p-MOs could be further assessed. As would be anticipated, substantial destabilizations of the HOMO energies are predicted relative to that of the parent meso-phenyl-BODIPY compound 5 (the Supporting Information, Figure S5), due to the introduction of styryl substituents at positions in which there are significant MO coefficients. When the B3LYP functional is used, the mesomeric effect associated with the styryl moieties is predicted to lead to a greater narrowing of the HOMO–LUMO gap of 2 a relative to 6 a, and hence a larger redshift in the wavelength predicted for the

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Figure 10. MO energies and angular nodal patterns of a C12H122 cyclic perimeter, BODIPY and a C12N2H10 model compound (consisting of two pyrrole ring moieties linked by two sp2 hybridized carbon atoms). It is noteworthy that the nodal patterns are broadly similar in each case and that the rigid planarity of the BODIPY p system due to coordination by the BF2 moiety results in sets of MO coefficients of comparable magnitude on the two pyrrole moieties at the 3,5-, 2,6- and 1,7-positions, due to the two-fold symmetry axis of the BODIPY chromophore.

main absorption band. This suggests that in the absence of the tilting of the styryl moiety due to steric effects (Table 4), the incorporation of para substituents at the 2,6-positions, which introduce strongly electron-donating or -withdrawing mesomeric interactions with the BODIPY p-system may prove to be a better strategy for fine-tuning the magnitude of the redshift of the main spectral bands over a wide range of wavelengths than substitution at the 3,5-positions. The presence of a nodal plane near these positions in the LUMO (Figure 11), means that mesomeric effects are insignificant where the Chem. Eur. J. 2014, 20, 1091 – 1102

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energy of the LUMO is concerned, resulting in an enhanced narrowing of the HOMO–LUMO gap on moving from 1 to 2 a relative to moving from 4 to 6 a, since there is a smaller destabilization of the LUMO. Although these trends are not present in calculations with the CAM-B3LYP functional (Table 5 and Figure S5 in the Supporting Information), it should be noted that the CAM-B3LYP functional calculations underestimate the wavelength range spanned by 2 a–c relative to the experimental values and those predicted by the B3LYP functional (Table 2), so they may underestimate the importance of the

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Full Paper thesis by the Heck reaction proved to be problematic. The most likely rationalization is that the CH bonds of the pyrrole rings also participate in the reaction resulting in a complex mixture of products. Efforts are underway to identify a protocol that would enable the isolation of significant yields of 2,6-distyryl-meso-aryl-BODIPYs.

Table 5. Predicted wavelengths in TD-DFT calculations with the B3LYP and CAM-B3LYP functionals for the main absorption bands of 1, 2 a–c, 4, meso-phenyl-BODIPY (5), and 1-planar, 2 a-planar, 4-planar, and 6 a-planar model compounds that have no methyl substituents but are otherwise identical to 1, 2 a, 4, and 6 a, both in the gas phase and dichloromethane using the polarizable continuum model (PCM).

Conclusion

l [nm]

5

1-planar

2 a-planar

4-planar

6 a-planar

B3LYP (PCM) B3LYP CAM-B3LYP (PCM) CAM-B3LYP

411 402 409 397

619 611 514 507

804 744 568 552

593 563 554 527

704 639 612 570

A new family of 2,6-distyryl-substituted BODIPY dyes has been synthesized and characterized by X-ray crystallography and 1 H NMR spectroscopy. The similar absorption band and redshifts of emission bands were found to be comparable to those observed in the spectra of the analogous 1,7-disubstituted compound 3, but significantly smaller than those observed for the main absorption and emission bands of the 3,5-disubstituted compound 4. The photophysical properties of the three new p-dimethylamino-substituted isomers 2 a–c have been analyzed by carrying out a series of theoretical calculations. Although the fluorescence quantum yields of 2 a,b are negligible, a strong solvent polarity dependence is observed in the quantum yields of 2 c, which has only a-position styryl substituents. Compound 2 c is, therefore, potentially suitable for use as a turn-on fluorescence sensor in this regard. There is strong pH dependence in the absorption and emission bands of all three isomers, which could also lead to applications as sensors. Theoretical calculations provide strong evidence that styryl rotation and the formation of non-emissive charge-separated S1 states play a pivotal role in shaping the weak fluorescence properties of 1 and 2 a,b. MO theory has been used as a conceptual framework to describe the electronic structures of the BODIPY core. The markedly differing effects of substitution at different positions on the BODIPY core can be readily accounted for through a comparison of the angular nodal patterns of the frontier p-MOs of the BODIPY core. Calculations with model compounds demonstrate that results derived from TD-DFT studies need to be treated with caution when the main absorption band has significant charge-transfer character due to the extension of the p system with styryl substituents.

Experimental Section Materials and instrumentations

Figure 11. Nodal patterns of the HOMO and LUMO of an unsubstituted BODIPY model compound at an isosurface of 0.07 a.u.

charge transfer character arising from the mesomeric effect associated with the p-dimethylamino substituents. The preparation of 2,6-distyryl compounds without methyl substituents was attempted to explore this question further, but their synChem. Eur. J. 2014, 20, 1091 – 1102

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All reagents were obtained from commercial suppliers and used without further purification unless otherwise indicated. All air- and moisture-sensitive reactions were carried out under a nitrogen atmosphere. Glassware was dried in an oven at 100 8C and cooled under a stream of inert gas before use. Dichloromethane and triethylamine were distilled over calcium hydride. Dry THF was distilled from sodium metal using benzophenone as an indicator under a nitrogen atmosphere. 1H NMR spectra were recorded on a Bruker DRX400 spectrometer and referenced to the residual proton signals of the solvent. HR-MS were recorded on a Bruker Daltonics microTOF-Q II spectrometer. Mass spectra were measured with a Bruker Daltonics AutoflexII TM MALDI-TOF spectrometer. Melting points was measured on a WRS-1 A Melting-Point Apparatus. The melting points of all dyes exceed 200 8C.

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X-ray structure determination

Compound 1: 2,6-diiodo-BODIPY (57.6 mg, 0.10 mmol) was dissolved in DMF (10 mL), and triethylamine (0.4 mmol, 56 mL), styrene (0.30 mmol, 30 mL), and [Pd(OAc)2] (10 mmol, 2 mg) were then added. The mixture was stirred under a nitrogen atmosphere at 65 8C until the reaction was complete. The reaction mixture was poured into dichloromethane (50 mL) and washed with water. The organic layer was dried over MgSO4, filtered, and evaporated to dryness. The crude product was purified by silica-gel flash column chromatography (60 % Hexane/CH2Cl2) and recrystallized from CH2Cl2/hexane to provide 1 as red crystals (21.6 mg, 41 %). 1H NMR (400 MHz, CDCl3): d = 7.54–7.52 (m, 3 H) 7.44 (d, J = 7.6 Hz, 4 H), 7.36–7.32 (m, 6 H), 7.24–7.23 (m, 2 H), 6.88 (d, J = 16.4 Hz, 2 H), 6.65 (d, J = 16.8 Hz, 2 H), 2.73 (s, 6 H), 1.47 ppm (s, 6 H); UV/Vis (CH2Cl2): lmax (e) = 575 nm (31 000 m1 cm1); MALDI-TOF: m/z calcd: 528.3 [C35H31BF2N2] + ; found: 527.9 [M] + , 508.8 [MF] + ; HRMS: m/z calcd for C35H31BF2N2 : 528.2542; found: 528.2542 [M] + C. Compounds 2 a–c: Compounds 2 a, 2 b, and 2 c were obtained by following a procedure similar to that of 1. Compound 2 a was obtained in 9 % yield as light-blue crystals. 1H NMR (400 MHz, CDCl3): d = 7.52–7.49 (m, 3 H), 7.34–7.32 (m, 6 H), 6.81–6.72 (m, 4 H), 6.67 (d, J = 16.8 Hz, 2 H), 6.56 (d, J = 16.4 Hz, 2 H), 2.98 (s, 12 H), 2.71 (s, 6 H), 1.45 ppm (s, 6 H); UV/Vis (CH2Cl2): lmax (e) = 616 nm (58 900 m1 cm1). Compound 2 b was obtained as purple crystals in 31 % yield. 1H NMR (400 MHz, CDCl3): d = 7.48–7.47 (m, 3 H), 7.35– 7.33 (m, 4 H), 7.21–7.18 (m, 2 H), 6.70–6.55 (m, 6 H), 5.69 (s, 1 H), 4.89 (s, 1 H), 2.98 (s, 6 H), 2.95 (s, 6 H), 2.72 (s, 3 H), 2.40 (s, 3 H), 1.46 (s, 3 H), 1.21 ppm (s, 3 H); UV/Vis (CH2Cl2): lmax (e) = 570 nm (54 000 m1 cm1). 2 c was obtained as red crystals in 14 % yield. 1 H NMR (400 MHz, CDCl3): d = 7.45–7.44 (m, 3 H), 7.35–7.34 (m, 2 H), 7.20–7.19 (m, 4 H), 6.64 (br s, 4 H), 5.71 (d, J = 0.8 Hz, 2 H), 4.89 (d, J = 0.8 Hz, 2 H), 2.96 (s, 12 H), 2.40 (s, 6 H), 1.22 ppm (s, 6 H); UV/Vis (CH2Cl2): lmax (e) = 529 nm (49 500 m1 cm1); MALDI-TOF: m/z calcd: 614 [C39H41BF2N4] + ; found: 614 [M] + , 595 [MF] + ; HRMS: m/z calcd for C39H41BF2N4 : 615.3465 [M+H] + ; found 614.3402 [M] + C (for 2 a), 615.3418 [M+H] + (for 2 b), 637.3235 [M+Na] + , 615.3404 [M+H] + , and 595.3361 [MF] + (for 2 c).

The X-ray diffraction data were collected on a Bruker Smart Apex CCD diffractometer with graphite monochromated MoKa radiation (l = 0.71073 ) using the w-2q scan mode. The structure was solved by direct methods and refined on F2 by full-matrix leastsquares methods using the SHELX-2000 program.[26] All calculations and molecular graphics were carried out using the SHELX-2000 and Diamond program packages. Compound 2 b: C39H41BF2N4 ; a purple block-like crystal with approximate 0.13  0.14  0.15 mm3 dimensions was selected for measurement. Space group P21/n, a = 10.4511(11) , b = 15.7350(16) , c = 20.686(2) , a = 90 8, b = 91.512(2) 8, g = 90 8, V = 3400.6(6) 3, Z = 4, F(000) = 1304.0, 1 = 1.200 Mg m3, R1 = 0.0504, wR2 = 0.1808, GOF = 1.038, residual electron density between 0.264 and 0.274 e 3. CCDC-951445 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Computational details Geometry optimization calculations were carried out on the sun cluster at the Centre for High Performance Computing in Cape Town, South Africa by using the B3LYP functional of the Gaussian 09 software package[16] with 6–31G(d) basis sets. The B3LYP and CAM-B3LYP functionals were used with 6–31G(d) basis sets during TD-DFT calculations of the B3LYP geometries. The calculations were made both in the gas phase and in dichloromethane using the polarizable continuum model (PCM). CIS calculations were carried out to optimize the geometry of the S1 states.

Acknowledgements We are thankful to the NSFC (nos. 21101049 and 21021062) for their financial support. Keywords: density functional calculations · dyes/pigments · fluorescence · NMR spectroscopy · photophysics · synthetic methods

Spectroscopic measurements UV/Visible absorption spectra were recorded on a Shimadzu UV2550 spectrophotometer, whereas fluorescence spectra were measured on a Hitachi F-2700 FL spectrophotometer equipped with a xenon arc lamp as the light source. The fluorescence lifetimes of the sample were measured with a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. Samples for absorption and emission measurements were contained in 1  1 cm quartz cuvettes. For all measurements, the temperature was kept constant at (298  2) K. Dilute solutions with absorbance of less than 0.05 at the excitation wavelength were used for the measurement of fluorescence quantum yields. Rhodamine 6G was used as the standard (FF = 0.88 in ethanol).[25] The quantum yield (F) was calculated by using Equation (1):

Fsample ¼ Fstd

    Isample nsample 2 Astd Istd Asample nstd

ð1Þ

in which the “sample” and “std” subscripts denote the sample and standard, respectively, I is the integrated emission intensity, A stands for the absorbance, and n is refractive index. Chem. Eur. J. 2014, 20, 1091 – 1102

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