Visible Absorption and Fluorescence Spectroscopy of

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Visible Absorption and Fluorescence Spectroscopy of Conformationally Constrained, Annulated BODIPY Dyes Noël Boens, Volker Leen, Wim Dehaen, Lina Wang, Koen Robeyns, Wenwu Qin, Xiaoliang Tang, David Beljonne, Claire Tonnelé, Jose Manuel Paredes, Maria Jose Ruedas-Rama, Angel Orte, Luis Crovetto, Eva M Talavera, and Jose M. Alvarez-Pez J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp305551w • Publication Date (Web): 30 Aug 2012 Downloaded from http://pubs.acs.org on September 3, 2012

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The Journal of Physical Chemistry

Visible Absorption and Fluorescence Spectroscopy of Conformationally Constrained, Annulated BODIPY Dyes Noël Boens,a, * Volker Leen,a Wim Dehaen,a Lina Wang,a Koen Robeyns,b Wenwu Qin,c Xiaoliang Tang,c David Beljonne,d Claire Tonnelé,d Jose M. Paredes,e Maria J. Ruedas-Rama,e Angel Orte,e Luis Crovetto,e Eva M. Talavera,e Jose M. AlvarezPeze a

Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200f – bus 02404, 3001 Leuven, Belgium. b

Institute of Condensed Matter and Nanoscience – Molecules, Solids and Reactivity (IMCN/MOST), Université catholique de Louvain, Bâtiment Lavoisier, place Louis Pasteur 1, bte 4, 1348 Louvain-la-Neuve, Belgium.

c

Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China.

d

Laboratory for Chemistry of Novel Materials, Université de Mons, Place du Parc 20, 7000 Mons, Belgium. e

Department of Physical Chemistry, Faculty of Pharmacy, University of Granada, Cartuja Campus, 18071 Granada, Spain.

*

Corresponding author: Tel.: +32-16-327497, Fax: +32-16-327990, E-mail: [email protected] 1 ACS Paragon Plus Environment


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Abstract Six conformationally restricted BODIPY dyes with fused carbocycles were synthesized to study the effect of conformational mobility on their visible electronic absorption and fluorescence properties. The symmetrically disubstituted compounds (2, 6) have bathochromically shifted absorption and fluorescence spectral maxima compared to those of the respective asymmetrically monosubstituted dyes (1, 5). Fusion of conjugation extending rings to the α,β-positions of the BODIPY core is an especially effective method for the construction of boron dipyrromethene dyes absorbing and emitting at longer wavelengths. The fluorescence quantum yields Φ of dyes 1–6 are high (0.7 ≤ Φ ≤ 1.0). The experimental results are backed up by quantum chemical calculations of the lowest electronic excitations in 1, 2, 5, 6, and corresponding dyes of related chemical structure but without conformational restriction. The effect of the molecular structure on the visible absorption and fluorescence emission properties of 1–6 has been examined as a function of solvent by means of the recent, generalized treatment of the solvent effect, proposed by Catalán (J. Phys. Chem. B 2009, 113, 5951–5960). Solvent polarizability is the primary factor responsible for the small solvent-dependent shifts of the visible absorption and fluorescence emission bands of these dyes. Keywords: fluorescent dye · conformational mobility · BODIPY · solvent effect · Catalán solvent scales · quantum chemical calculations

2 ACS Paragon Plus Environment

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Introduction The class of bright fluorophores derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (better known as BODIPY, boron dipyrromethene 1 , 2 ) came into existence by the serendipitous discovery by Treibs and Kreuzer in 1968. 3 Since the mid-1990s, the potential of these fluorescent dyes became fully evident as they found applications in many different fields, such as optical engineering (organic light-emitting diodes, dyesensitized solar cells), analytical chemistry, biological in vivo imaging and sensing and materials science. 4 , 5 The huge popularity of BODIPY derivatives is understandable considering their excellent characteristics, such as relatively high molar absorption coefficients ε(λ) and fluorescence quantum yields Φ (leading to high brightness6), narrow emission bandwidths with high peak intensities, robustness towards light and chemicals, resistance towards self-aggregation in solution, fluorescence lifetimes in the nanosecond range, and excitation/emission wavelengths in the visible spectral range. Moreover, their spectroscopic/photophysical properties can be fine-tuned by synthetically introducing suitable groups at the right positions of the BODIPY core. The search for BODIPY dyes that absorb and emit well within the red visible (vis) or even near-infrared (NIR: 700– 2000 nm) spectral range continues relentlessly. Indeed, stable dyes with sharp absorption and fluorescence emission bands in the far-red or NIR region of the spectrum, combined with high brightness, are a prime scientific and commercial target. In recent years, a large number of research papers have focused on the syntheses, functionalization reactions and spectroscopic/photophysical properties of boron dipyrromethene dyes.2 Extension of the π-conjugated system is a proven method to construct BODIPY dyes with bathochromically shifted spectra. Annulation, i.e., the



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building of a ring onto some starting molecule can be used to expand the conjugated ring system and to reduce the nonemissive decay via rotational relaxation and hence to increase fluorescence quantum yield. Fusing the BODIPY core to rigid π-conjugated carbocycles can lead to NIR emitting dyes. Burgess et al. described that symmetrical, aryl-substituted BODIPY dyes with relatively rigid conformations have longer absorption and fluorescence emission maxima as well as higher ε(λ) and Φ values than their unconstrained counterparts. 7 Similar findings have been reported independently by Kovtun et al.,8 Shen, Rurack and coworkers,9 and Lee’s group.10 BODIPY dyes fused with aromatic rings at the β,β-positions have been reported by Ono, Rurack and coworkers. 11 A Cu2+ selective chemosensor based on a BODIPY with fused aromatic groups at the α,β-positions has been described. 12 Several groups reported BODIPY derivatives fused to large aromatic ring building blocks (anthracene, 13 phenanthrene, 14 biphenyl,

15

perylene,

16

spirofluorene,

17

porphyrin

18

) with concomitant NIR

absorption/fluorescence emission. The vast majority of these reported, structurally rigid ring-fused BODIPYs are symmetrically substituted systems. However, a series of asymmetrical benzo-fused BODIPYs has been reported by Jiao and coworkers.19 Fusion of a benzene ring at the 1,2-positions of the BODIPY core leads to increased conjugation, as shown by a red shift of approximately 60 nm of the absorption and emission maxima compared to classical BODIPY dyes. Since most studies of structurally constrained BODIPY dyes reported so far have a limited spectroscopic viewpoint, we decided to perform a detailed spectroscopic and photophysical study of asymmetrical as well as symmetrical BODIPY derivatives with restricted bond rotations. In addition, quantum chemical calculations were performed to better understand the experimental findings.


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Note that boron dipyrromethene derivatives fused to heterocycles are not the topic of this paper. The reader interested in a detailed discussion of such BODIPY dyes is referred to ref 20. Herein, we describe six 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dyes (1–6, Chart 1) with restricted conformational flexibility. We investigated their spectroscopic and photophysical characteristics in a large number of solvents by UV–vis spectrophotometry and steady-state and time-resolved fluorescence spectroscopy. These experiments allowed us to determine the wavelengths of the spectral maxima [λabs(max), λex(max), λem(max)], the full width at half height of the maximum of the absorption (fwhmabs) and fluorescence emission (fwhmem) bands, the Stokes shifts [ ∆ ν = 1/λabs(max) – 1/λem(max)], the fluorescence quantum yields (Φ) and fluorescence lifetimes (τ). We used the recent, generalized treatment of the solvent effect based on a set of four empirical solvent scales (dipolarity, polarizability, acidity and basicity of the medium)21 to describe the solvent effect on the position of the spectral maxima ν abs = 1/λabs(max) and ν em = 1/λem(max) and the rate constants of fluorescence (kf) and nonradiative (knr) decay of 1–6. Optical properties of 1, 2, 5, and 6 as well as their unconstrained corresponding forms (Chart 3), were investigated by means of density functional theory (DFT)

and

time-dependent

density functional

theory (TD-DFT)

Additionally, the crystal structure of dye 2 has been determined.


calculations.


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N

B F2

N

N

B F2

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N

N Cl

2

1

B F2

N

3

7

8

1

6

Br

N

B F2

N

N

4

B F2

2

N 4 N B 5 F2 3

N

6

5

Chart 1. Structures of BODIPY dyes studied. The IUPAC numbering scheme for 6 is given as an example.

Results and discussion Synthesis The fluorescent dyes 1–6 (Chart 1) were prepared following literature procedures for the synthesis of symmetrical22, 23 and asymmetrical24, 25 BODIPY dyes. The synthetic route outlined in Scheme 1 can be utilized for the synthesis of both symmetrical and asymmetrical boron dipyrromethene dyes. Alternatively, symmetrical dyes can be prepared following Scheme 2, starting from suitably substituted pyrrole (I) and triethyl orthoacetate or acetyl chloride. Only dyes 1 and 5 are new. Kovtun et al. have described conformationally restricted 2.23 Compounds 3 and 4 have been reported previously by us.24, 25 Rigidified BODIPY derivative 6 has been described by Thiele and coworkers.22 R2

R

+

1

N H

I

POCl3

O H 3C

N H

CH 3

CH 3

R3

R

4

R2

NH R1

II

R3

HN

1. Et 3N

R2

N

2. BF 3 . Et2O

R4

III

Scheme 1. Synthesis of compounds 1–6.


R1

B F2

R3

N

1-6

R4

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R2 AcCl or MeC(OEt)3/H + R

1

N H

R2

∆ R1

I

R2

NH HN

Et 3N

R2

N

.

R1

BF3 Et2O

R1

IV

B F2

R2

N R1

2, 6

Scheme 2. Alternative synthesis of symmetrical BODIPY dyes 2 and 6.

Spectroscopic and photophysical properties The BODIPY dyes studied are strongly colored solids with a metallic luster that form intensely colored solutions with bright fluorescence when irradiated. The UV–vis absorption and fluorescence emission spectra of 1 dissolved in a selection of solvents are shown in Fig. 1. Compound 1 displays the typical absorption features of classic BODIPY dyes in all 20 solvents studied: that is, a narrow absorption band with the maximum λabs(max) positioned within a very narrow range (546–557 nm) and which is red-shifted with increasing solvent polarizability (from 546 nm in acetonitrile to 557 nm in chlorobenzene). This absorption band is assigned to the S1←S0 transition. (Notice that there is a narrow, optically transparent window around 425 nm.) An additional, considerably weaker, broad absorption band can be observed around 350 nm and is attributed to the S2←S0 transition.



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Fig. 1. (a) Normalized absorption spectra of 1 in a selection of solvents. (b) Corresponding normalized fluorescence emission spectra upon excitation at 515 nm.

Derivative 1 also shows the characteristic emission features of BODIPY: i.e., a narrow, slightly Stokes-shifted band of mirror image shape, which is red-shifted with increasing


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solvent polarizability [λem(max) moves from 560 nm in methanol and acetonitrile to 570 nm in chlorobenzene]. The fluorescence quantum yields Φ are in the 0.71–0.87 range. Table 1 compiles the spectroscopic and photophysical data of 1 as a function of solvent. The absorption and fluorescence emission spectra of BODIPY analogue 2 (Fig. S1, Electronic Supporting Information, ESI) are of similar shape as those of 1. The λabs(max) values are bathochromically shifted with increasing solvent polarizability (from 613 nm in methanol, acetonitrile, and acetone to 625 nm in chlorobenzene). The same trend is observed for λem(max), which moves from 625 nm in methanol, acetonitrile, and diethyl ether to 637 nm in chlorobenzene. Rigidization of two 3,5-phenyl groups, which is equivalent to the fusion of two dihydronaphthalene moieties to the BODIPY core of 2, gives rise to a red shift of the absorption and emission maxima of approximately 60–70 nm compared to 1, concurrently with a favorable increase of Φ (Table S1, ESI). The improved, more extended conjugation (and planarity) of the chromophore accounts for the observed bathochromic shifts of λabs(max) and λem(max). (Here as well, take note of optically transparent window centered around 450 nm, Fig. S1.) The quantum yields of 2 are higher than those of 1, with Φ values in the 0.91–1.00 range. Dyes 3 and 4, which are respectively 3-chloro and 2-bromo analogues of 1,24,25 also have similar absorption and emission spectra. The λabs(max) of 3 and 4 are slightly red-shifted (approximately 10 nm) in relation to those of 1 and are also located within a very narrow range: 554–566 nm for 3 and 555–568 nm for 4. The same is true for λem(max): similar small bathochromic red shift compared to 1: 568–577 nm for 3 and 574–584 nm for 4. The spectroscopic and photophysical data on 3 and 4 are compiled in Tables S2 and S3,



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respectively (ESI). Fluorescence quantum yields are in the 0.87–0.97 range for 3 and 0.73–0.83 for 4. BODIPY derivatives 5 (with one cyclohexane ring fused at the 2,3-positions) and 6 (with two cyclohexane rings fused at the 2,3- and 5,6-positions) have blue shifted λabs(max) and λem(max) compared to 1–4, with those of 6 more red shifted than those of 5. The λabs(max) of 5 and 6 are also located within a 10 nm wavelength range: 508–518 nm for 5

and 534–543 nm for 6. The same holds for λem(max): 520–528 nm for 5 and 543–551 nm for 6. Φ values for 5 range from 0.83 to 0.93 and from 0.76 to 0.89 for 6. The spectroscopic and photophysical data on 5 and 6 are listed in Tables S4 and S5, respectively (ESI). The normalized absorption and fluorescence emission spectra of 5 and 6 in a selection of solvents are shown in Figs. S2 and S3 (ESI), respectively. The Stokes shifts ∆ ν are small for 1−6 with the following average values (± standard uncertainties) in the 20 solvents tested: 396 ± 35 cm−1 for 1, 277 ± 41 cm−1 for 2, 371 ± 46 cm−1 for 3, 511 ± 62 cm−1 for 4, 390 ± 53 cm−1 for 5, and 243 ± 50 cm−1 for 6 (Tables 1, S1–S5). By far the largest Stokes shift is found for the 2-bromo analogue 4, whereas the smallest Stokes shifts are found for the disubstituted dyes 2 and 6 with the least conformational mobility. The fluorescence excitation spectra of 1–6 match the absorption spectra in all cases and, moreover, λex(max) are within 3 nm equal to λabs(max). The observation that the absorption and emission band positions of 1−6 do not exhibit any distinct trend as a function of solvent polarity implies that emission occurs from the weakly dipolar, relaxed Franck-Condon excited state of the dyes.


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Table 1. Spectroscopic and photophysical data of 1 as a function of solvent. The solvents are numbered according to increasing refractive index n.

No

Solvent

λabs(max) / nm

λem(max) / nm

∆ν

/ cm-

fwhmabs / cm-1

fwhmem / cm-1

424 458 390 423 423 421 421 354 387 418 385 417 384 320 415 349 417 349 349 409

876 915 834 876 908 869 940 794 831 831 828 828 891 722 857 851 839 748 813 785

793 824 788 821 758 725 758 720 749 782 779 779 717 685 720 742 893 712 709 867

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

MeOH MeCN Diethyl ether Acetone Ethyl acetate 2-Propanol Butanenitrile Dibutyl ether 1-Butanol THF 1-Pentanol 1,4-Dioxane CH2Cl2 Cyclohexane 1-Octanol CHCl3 Cyclohexanone CCl4 Toluene Chlorobenzene

547 546 549 548 548 549 549 552 551 551 552 552 553 554 553 556 552 556 556 557

560 560 561 561 561 562 562 563 563 564 564 565 565 564 566 567 565 567 567 570

Φa

τb /ns

kf c / 108 s-1

knr c / 108 s-1

0.84±0.02 0.72±0.02 0.83±0.02 0.71±0.02 0.78±0.02 0.81±0.01 0.87±0.02 0.81±0.01 0.86±0.08 0.84±0.04 0.82±0.02 0.84±0.02 0.83±0.02 0.82±0.01 0.86±0.07 0.84±0.02 0.81±0.01 0.83±0.04 0.87±0.02 0.82±0.01

5.707 5.812 5.667 5.649 5.471 5.433 5.586 5.260 5.310 5.322 5.427 5.159 5.346 5.104 5.171 5.243 5.127 4.999 4.960 4.908

1.47±0.04 1.24±0.03 1.46±0.04 1.26±0.04 1.43±0.04 1.49±0.02 1.56±0.04 1.54±0.02 1.62±0.15 1.58±0.08 1.51±0.04 1.63±0.04 1.55±0.04 1.61±0.02 1.66±0.14 1.60±0.04 1.58±0.02 1.66±0.08 1.75±0.04 1.67±0.02

0.28±0.04 0.48±0.03 0.30±0.04 0.51±0.04 0.40±0.04 0.35±0.02 0.23±0.04 0.36±0.02 0.26±0.15 0.30±0.08 0.33±0.04 0.31±0.04 0.32±0.04 0.35±0.02 0.27±0.14 0.31±0.04 0.37±0.02 0.34±0.08 0.26±0.04 0.37±0.02

a

Determined with rhodamine 6G in water (Φr = 0.76) as reference. The quoted errors represent one standard uncertainty. b Excitation wavelength = 532 nm. The standard errors on the lifetimes τ are obtained from the diagonal elements of the covariance matrix available from the global analysis fit of decay traces recorded at three different emission wavelengths and are between 13 and 15 ps. c Calculated according to eq 2. The propagated errors are calculated using the standard uncertainty of Φ and the standard error of τ.

The molar absorption coefficients εmax of 1–4 at the absorption maximum λabs(max) have been determined exemplarily in methanol, ethyl acetate, and toluene and are compiled in Table 2. The oscillator strength f is directly related to the integral of the absorption band as follows:5

f =

4.32 × 10 −9 ∫ ε (ν ) d ν n

(1)

where n is the solvent index of refraction and ν is the wave number (in cm-1). The oscillator strength values f of 1–4 in methanol, ethyl acetate, and toluene are also listed in Table 2. Compound 2 with two dihydronaphthalene-fused rings has considerably



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higher εmax, f, and Φ values than dye 1 with one such ring. As a consequence, the brightness – defined as the product of the molar absorption coefficient ε(λ) at the excitation wavelength λ and the fluorescence quantum yield [ε(λ) × Φ]

6

– intensifies

considerably by restricting the conformational flexibility. The maximum values of the brightness in methanol, ethyl acetate, and toluene are given in Table 2. Table 2. λabs(max), εmax, maximum brightness, and f values for 1–4 in methanol, ethyl acetate, and toluene. The maximum brightness is given by εmax × Φ. λabs(max) are in nm, εmax and εmax × Φ in L mol–1 cm–1. Dye 1

2

3

4

Methanol

Ethyl acetate

Toluene

λabs(max)

547

548

556

εmax

48600

44700

69000

εmax × Φ

40800

34900

60000

f

0.337

0.403

0.437

λabs(max)

613

614

624

εmax

71600

54100

85900

εmax × Φ

65100

54100

85900

f

0.522

0.510

0.662

λabs(max)

555

556

565

εmax

58900

51900

46400

εmax × Φ

51200

47700

43100

f

0.460

0.471

0.422

λabs(max)

556

557

567

εmax

51100

32000

50700

εmax × Φ

38800

25600

41600

f

0.453

0.261

0.360

The absorption and emission spectral bandwidths of 1–6 are quite narrow (average fwhm values ± corresponding standard uncertainties for the 20 solvents used: fwhmabs = 842 ± 54 cm−1 for 1, 633 ± 43 cm−1 for 2, 810 ± 71 cm−1 for 3, 1070 ± 222 cm−1 for 4 , 1388 ± 249 cm−1 for 5, and 739 ± 66 cm−1 for 6; fwhmem = 766 ± 55 cm−1 for 1, 606 ± 56 cm−1


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for 2, 715 ± 40 cm−1 for 3, 950 ± 44 cm−1 for 4, 1107 ± 54 cm−1 for 5, and 828 ± 32 cm−1 for 6). The disubstituted derivatives 2 and 6 have narrower absorption and emission bandwidths than their corresponding monosubstituted 1 and 5. These spectroscopic properties can be compared to those of analogous BODIPY dyes published in the literature. Introduction of two phenyl groups at the 3,5-positions (in 2u,8 Chart 3) produces bathochromic shifts of approximately 50 nm compared to 7 (Chart 2, marketed by Life Technologies26 as “BODIPY® 493/503”),1 which may serve as model for classic BODIPY dyes. Replacing the meso-methyl group in 7 by meso-phenyl (giving rise to 4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-4-bora-3a,4a-diaza-s-indacene) causes a red shift of approximately 5 nm in λabs(max) and λem(max).2a For symmetrical, constrained dye 2 (Chart 1), a clear batho- and hyperchromic effect in absorption (in CH2Cl2: εmax(621 nm) = 134000 M–1 cm–1 for 2 vs. εmax(544 nm) = 57000 M–1 cm–1 for 2u) is accompanied by a significant increase in the fluorescence quantum yield compared to its unbridged counterpart 2u (in CH2Cl2: Φ = 0.86 for 2 vs. 0.64 for 2u).8 Moreover, λem(max) of 2 shows a large red shift relative to dye 2u. These differences reflect both the

improved conjugation and the inhibited free rotation of the benzene molecular fragments due to the ethylene bridge linkers in 2 relative to 2u. Rigidization of two 3,5-phenyl groups, which is achieved by fusing two dihydronaphthalene moieties (with their 1,2positions) to the 2,3- and 5,6-positions of the BODIPY core of 2, leads to a bathochromic shift of approximately 80 nm relative to the unconstrained 3,5-diphenyl analogues (such as 2u), together with a valuable increase of Φ . Bridging has similar effects on BODIPY derivatives when the meso-methyl substituent in 2 and 2u is replaced by a polymethine moiety.8 Compound 2 (Chart 1) with a 8-methyl group has blue-shifted λabs(max) and



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λem(max) compared to 8a–b (Chart 2) with an aromatic meso-substituent.7 Dyes 8a–c

have lower fluorescence quantum yields than 2 because rotation of the aromatic mesosubstituent is a major nonradiative deactivation pathway of their singlet excited state. Derivatives 9 (Chart 2) with methoxy and methyl substituents on the benzene rings all have bathochromically shifted λem(max) and with the inexplicable exception of 9c also red-shifted λabs(max) in comparison to 2.10 BODIPY 6 (compound LD540 in the paper by Thiele et al.22) in which two aliphatic rings have been fused to the pyrrole fragments at the α,β-positions has red shifted λabs(max) and λem(max) in comparison to BODIPY 5 bearing one aliphatic ring or to dye 7 (Chart 2) with acyclic aliphatic substituents. Boron dipyrromethene dye 2 with two dihydronaphthalene units fused at the α,β-positions can also be compared to the corresponding β,β-ring fused systems 10a–c (Chart 2).9 Φ values for 10a–c are consistently lower than for 2. The spectral maxima λabs(max) and λem(max) for 10a–c are blue-shifted vs. those of 2. This indicates that there is a better πconjugation at the α-positions (3,5-positions) in 2 than at the 1,7-positions in 10a–c. The Stokes shifts ∆ ν of 2 (conformationally locked phenyl groups at the 3,5-positions) are smaller than those of 10b with a meso-methyl and 10c with a meso-phenyl (conformationally locked phenyl groups at the 1,7-positions). Introduction of two phenyl groups at the 1,7-positions (in 11, Chart 2) entails red shifts of approximately 40 nm relative to 7, a model for classic BODIPY dyes. 27 Comparison of dye 11 (two freely rotating phenyl groups at the 1,7-positions) and of its constrained analogues 10a–c shows that rigidization of two 1,7-phenyl groups, which is caused by the fusion of two dihydronaphthalene units to the 1,2- and 6,7-positions of the boron dipyrromethene core, results in further spectral shifts of approximately 20 nm.


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I

N

B F2

N

7 MeOH abs 493 nm, em 504 nm

N

X Y

R N

CHCl3

Y

Z

Z

Y CHCl3 9a: X=OMe, Y=Z=H, abs 634 nm, em 657 nm 9b: X=Y=H, Z=OMe, abs 646 nm, em 676 nm 9c: X=H, Y=Z=OMe, abs 590 nm, em 641 nm 9d: X=Z=Me, Y=H, abs 641 nm, em 668 nm

R

B F2

N

X

I

N

B F2

Y

8a: R=H, Y=H, abs 634 nm, em 647 nm, 0.38 8b: R=H, Y=OMe, abs 658 nm, em 673 nm, 0.13 8c: R=Me, Y=H, abs 619 nm, em 629 nm, 0.72

R

N

B F2

N

10a: R=H, abs 558-563 nm, em 566-569 nm, 0.50-0.55 10b: R=Me, abs 559-564 nm, em 598-601 nm, 0.36-0.52 10c: R=Ph, abs 565-584 nm, em 598-612 nm, 0.10-0.46 N

11:

abs

B F2

N

538-545 nm,

em

549-556 nm,

0.93-0.99

Chart 2. Structures of BODIPY dyes 7,2a 8,7 9,10 10, 9 and 1127 from the literature discussed here. λabs and λem are abbreviations for λabs(max) and λem(max), respectively.

Quantum chemical calculations To better understand the results of the spectroscopic measurements and to elucidate the differences between 1, 2, 5, and 6 and dyes of related chemical structure but without conformational constraints (1u, 2u, 5u, and 6u, respectively, Chart 3), quantum chemical calculations were performed.



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N

B F2

N

N

B F2

1u

N

2u CH2Cl2 544 nm, 0.64 abs

N

B F2

N

5u

N

B F2

N

6u

Chart 3. Structures of unbridged BODIPY dyes. Compound 2u has been described in ref 8, whereas the other derivatives have not yet been reported.

All calculations were performed with the Gaussian (G09) program package. 28 Density functional theory (DFT) was used with the CAM-B3LYP29 hybrid functional and 6-31G* basis set for geometry optimizations. From these equilibrium structures, UV–vis absorption and emission spectra were computed in the framework of time-dependent density functional theory (TD-DFT). To take the solvent effect into account (acetonitrile), the polarized continuum model (IEFPCM formalism 30 ) was adopted using linear response31 (LR, see Table S6 in ESI) or state-specific32,33 (SS) approaches available in G09. The computed electronic absorption spectra of 1 and 2 both present a low-lying ‘HOMO→LUMO’ transition band, at 430 nm and 487 nm, respectively (Table 3). Although these values are underestimated with respect to the experimental results (Fig. 2), the red shift measured upon disubstitution of the BODIPY core, namely when going from 1 to 2, is properly reproduced with a calculated value of ~57 nm (compared to a measured value of ~67 nm). The origin for the bathochromic shift can be better


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appreciated by looking at the frontier orbitals involved in the transitions. As shown in Fig. S4, these are clearly delocalized over the phenyl rings in 2. The effect of conformational restriction on optical properties of such systems was also investigated by studying the corresponding unrestricted dyes, 1u and 2u, in which the phenyl group substituents are free to rotate (Chart 3). Table 3. Computed wavelengths/energies of the first absorption and emission vertical transitions with the SS approach and associated vertical transitions energy differences ∆Eabs/em for 1–6 and their corresponding unrestricted forms. The Stokes shifts are based on solvent relaxation, see text. absorption

emission

∆Eabs/em

Stokes shift

transition energy / eV

transition wavelength / nm

transition energy / eV

transition wavelength / nm

/ eV

/ cm-1

/ cm-1

1

2.8856

429.72

2.7040

458.58

0.1816

1465

169

1u

3.0323

408.93

2.8292

438.29

0.2031

1638

181

2

2.5472

486.81

2.3985

516.99

0.1487

1199

138

2u

2.8462

435.67

2.5898

478.80

0.2564

2068

128

5

3.1216

397.23

2.9672

417.90

0.1544

1245

209

5u

3.1451

394.26

2.9907

414.62

0.1544

1245

226

6

2.9359

422.36

2.8322

437.82

0.1037

836

151

6u

2.9732

417.06

2.8642

432.93

0.1090

879

155

As expected, the conformational restriction in 1 and 2 (compared to 1u and 2u) leads to a bathochromic shift of the absorption band by ~21 nm and ~51 nm, respectively (Table 3). These spectral changes arise from the extended delocalisation of the HOMO orbital over the ethylene bridge linker in 1 (Fig. 3) together with an increased planarity of the system. Constriction leads to a more rigid compound and partly prevents the rotation of the phenyl group at the 3-position: the value of the dihedral angle between the BODIPY core and the substituent changes from ~40° to ~23° when passing from 1u to 1.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 2. Experimental (solid lines) and theoretical (dashed lines) vertical transition energies for absorption and emission of 1, 2, 5, and 6.

Because the phenyl rings have some rotational freedom in the unrestricted conformation, we investigated the sensitivity of the calculated optical transitions to the dihedral angle between the BODIPY core and the substituent. In contrast to 1, which has a rather steep minimum at ~19°, 1u indeed presents a quite flat ground-state potential energy surface (with only ~1.4 kcal/mol between 15° and 37°, Fig. S5, ESI) and one can thus expect conformationally broadened absorption bands. Yet, the spectra – simulated using the optimized geometry and considering a Boltzmann distribution of conformers – are virtually superimposable, see Fig. S5 (ESI). Regarding the fluorescence emission process, the computed values of transition wavelengths are again underestimated. However, the general trends are well reproduced. Namely, the vertical transition from the relaxed excited-state geometry shows a large red shift of about 58 nm for 2 compared to 1, in line with the experimental data (bathochromic shift of 65 nm).


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As expected and measured in solution, we compute blue-shifted absorption and emission transitions for 5 and 6 (compared to 1 and 2), the former presenting the larger hypsochromically shifted band. The disubstitution of the BODIPY core (in 6) is responsible for a red-shift of the first absorption band by ~25 nm, in excellent agreement with the experimental findings (26 nm). This change is corroborated by the frontier orbitals of the two systems showing the larger delocalization for 6 (Fig. S4, ESI). The same trend is measured and computed for the emission band, with comparable values. Although no change in the absorption spectra is expected when passing from 5u to 5, it is most likely that the very small increase in transition wavelength (3 nm) predicted upon going from the unconstrained to the restricted form arises from enhanced hyperconjugation effects associated with the methyl groups in 5 (see Fig. 3, where small weights on the methyl substituents are circled). 1u

homo

1

lumo

homo

5u

lumo 5

homo lumo homo lumo Fig. 3. Graphical representation of the frontier orbitals of the monosubstituted compounds 1 and 5 and their corresponding unrestricted forms 1u and 5u.

Like for 5 vs. 5u, there is no significant spectral displacement upon conformational restriction of 6u. The computed absorption wavelengths for 6u and 6, ~417 nm and ~422



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nm respectively, present only a small red shift (~5 nm), expectedly slightly larger than for 5(u) as the BODIPY core is disubstituted. The same features are observed for the emission wavelengths. The oscillator strengths (f, Table S6, ESI) are in line with the description above. Larger values are obtained for the rigid structures compared to the non-locked compounds as a result of the improved conjugation. Similarly, the oscillator strengths are higher in the disubstituted compounds (2 and 6). Inner- and outer-sphere relaxations occur upon electronic transition from the ground state to the singlet excited state of the dye (in absorption) and vice-versa (in emission). The intramolecular relaxation is substantial, involves high-frequency vibrations and largely contributes to the energy difference between vertical excitations (see Table 3). It manifests itself as a vibrational progression, which is clearly seen (e.g., in the experimental spectra of 1 in Fig. 1) and has not been modelled further here. We have rather focused on the Stokes shift between the 0–0 energy transitions in absorption and emission that mostly originates from solvent relaxation. Thus, we have computed the vertical transition energies in absorption and emission successively based on the solvent reaction fields produced in the ground state and the lowest singlet excited state (and using the frozen ground- or excited-state optimized geometry). The theoretical values, although slightly underestimated, are in fair agreement with the measured Stokes shifts (Table 3). The relative trends are properly reproduced with the smallest computed Stokes shifts obtained for the most constrained compounds 2 and 6, in line with the measured values in solution.


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Finally, the ground and lowest excited state dipole, µg and µe, reported in Table 4, are found to be relatively small in all cases, supporting the experimental observations and conclusions of the solvatochromism study. Table 4. Computed ground and excited state dipoles for 1, 2, 5 and 6 and their corresponding unrestricted forms.

µg

µe

∆µgµe

1

6.2252

5.4952

0.7300

1u

5.7100

4.4717

1.2383

2

5.4617

4.1469

1.3148

2u

5.0044

3.3026

1.7018

5

6.0973

5.1431

0.9542

5u

6.0625

5.1244

0.9381

6

4.7756

3.8251

0.9505

6u

4.8397

3.9844

0.8553

To summarize, although the computed transition wavelengths for both absorption and emission processes are underestimated, the overall calculated trends support the experimental findings that increased rigidity of the systems as well as extended conjugation are accompanied by bathochromically shifted absorption and emission bands and reduced Stokes shifts.

Time-resolved fluorescence Fluorescence decay traces of 1–6 in different solvents were collected as a function of emission wavelength λem using the single-photon timing technique. 34 Simultaneous (global) curve-fitting of the time-resolved fluorescence histograms of 1–6 as a function of emission wavelength with τ linked over λem, confirmed that the decays are monoexponential [y(t) = α exp(–t/τ)] and do not depend on λem. Using the experimental



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Page 22 of 37

fluorescence quantum yield (Φ ) and single exponential lifetime (τ), the rate constants of radiative (kf) and nonradiative (knr) decay were calculated according to eq 2: kf = Φ f / τ

(2a)

knr = (1 – Φ f) / τ

(2b)

The results of the time-resolved fluorescence experiments of 1 and 2–6 are listed in Tables 1 and S1–S5, respectively.

Solvatochromism Solvent-dependent spectral shifts are often analyzed in terms of a single parameter (i.e., by using a global single-parameter scale or one describing only nonspecific solvent effects arising from the solvent acting as a dielectric continuum). However, empirical single-parameter solvent scales regularly appear to be inappropriate because that specific single parameter is too dependent on the particular probe used to construct the singleparameter scale concerned that it fails to predict the behavior of other solutes with considerably different properties from those of the probe.21 A multi-parameter approach is often preferable and has been applied successfully to various physicochemical properties.5 The solvent effect on the physicochemical observable y can be expressed by the multilinear equation 3a: y = y0 + a A + b B + c C + d D

(3a)

where y0 denotes the physicochemical property of interest in the gas phase; a, b, c and d are adjustable coefficients that reflect the sensitivity of the physicochemical property y in a given solvent to the various {A, B, C, D} solvent parameters.


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In the literature, various solvent scales have been reported to describe the solvent dependence of y (see ref 21 for an overview). The generalized treatment of the solvent effect based on a set of four empirical, mutually independent solvent scales – recently proposed by Catalán21 – is by far the most superior. In this method, the polarizability, dipolarity, acidity and basicity of a certain solvent are characterized by the parameters SP,35 SdP,21 SA36, 37 and SB38, respectively (eq 3b). The {SP, SdP, SA, SB} parameters for a large number of solvents can be found in ref 21. y = y0 + aSA SA + bSB SB + cSP SP + dSdP SdP

(3b)

The spectroscopic observables y analyzed in this paper are the absorption maxima ν abs = 1/λabs(max) and the emission maxima νem = 1/λem(max), all expressed in cm–1. The kinetic parameters y analyzed are the excited-state deactivation rate constants kf and knr

(

)

3 and the ratio k f n 2 ν em (n denotes the refractive index of the solvent used).

Table 5 compiles the estimated regression coefficients y0, aSA, bSB, cSP, dSdP and their respective standard errors, and the correlation coefficients (r) for the multi-linear regression analyses of the maxima of absorption ( ν abs ) and fluorescence emission ( ν em ) of 1–6 according to eq 3b for the solvents of Tables 1 and S1–S5. Use of the solvent parameter set {SA, SB, SP, SdP} gives high-quality to excellent fits to νabs using r as goodness-of-fit criterion (r = 0.968 for 1, 0.969 for 2, 0.953 for 3, 0.964 for 4, 0.935 for 5, and 0.920 for 6). To visualize the goodness-of-fit of νabs as a function of the Catalán solvent parameters {SA, SB, SP, SdP}, as an example we made a graph (Fig. S6, ESI) of the absorption maxima νabs of 1 calculated according to eq 3b using the estimated values of y0, aSA, bSB, cSP, dSdP versus the corresponding experimental νabs values. As expected,



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an excellent linear relation (r = 0.968) is found. Similar graphs for the other dyes (2–6) were obtained (data not shown). To put these results into perspective, the analysis of the νabs data of 1–6 according to the popular Kamlet-Taft solvent scales using the {α, β, π*} parameters (eq S1, ESI) showed poor fits, as assessed by the much lower values of r (0.629 for 1, 0.672 for 2, 0.690 for 3, 0.723 for 4, 0.691 for 5, and 0.656 for 6) and the large standard errors on the parameter (Table S7, ESI). Excellent fits were also found for the multilinear analyses of the ν em data of 1–6 according to eq 3b. Indeed, the r values found were 0.982 for 1, 0.975 for 2, 0.970 for 3, 0.962 for 4, 0.955 for 5, and 0.964 for 6. Figure S6 (ESI), which shows the linear relationship between the emission maxima ν em of 1 calculated according to eq 3b using the estimated values of y0, aSA, bSB, cSP, dSdP versus the corresponding experimental ν em values, illustrates the goodness-of fit of the analysis of the ν em data. An excellent linear relationship (r = 0.982) is found. Contrary to the superb results obtained using the Catalán solvent scales {SA, SB, SP, SdP}, the ν em data of 1–6 could not be described adequately by the Kamlet–Taft {α, β, π*} solvent parameters, as judged by the much lower r values and the large standard errors on the regression coefficients (Table S7, ESI).


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Table 5. Estimated coefficients (y0, aSA, bSB, cSP, dSdP; in cm-1 for

ν abs and νem , in 108 s–1 for kf; eq 3b)

and correlation coefficient (r) for the multiple linear regression analysis of the absorption ( ν abs ) and fluorescence emission maxima ( νem ) of 1–6 as a function of the Catalán solvent scales {SA, SB, SP, SdP} for the 20 solvents listed in Tables 1 and S1–S5.

1

2

3

4

5

6

y0

cSP

dSdP

aSA

bSB

r

ν abs

19003 ± 104

-1359 ± 133

165 ± 25

-134 ± 50

11 ± 33

0.968

ν em

18661 ± 63

-1342 ± 81

75 ± 15

-83 ± 30

-33 ± 20

0.982

kf

0.50 ± 0.23

1.57 ± 0.29

-0.23 ± 0.06

0.26 ± 0.11

0.08 ± 0.07

0.890

ν abs

16976 ± 97

-1263 ± 125

149 ± 24

-153 ± 47

44 ± 30

0.969

ν em

16761 ± 68

-1240 ± 88

46 ± 17

-70 ± 33

-17 ± 21

0.975

kf

0.59 ± 0.33

1.55 ± 0.42

-0.20 ± 0.08

-0.01 ± 0.16

0.23 ± 0.10

0.790

ν abs

18689 ± 140

-1342 ± 179

208 ± 34

-177 ± 67

42 ± 44

0.953

ν em

18235 ± 78

-1131 ± 99

99 ± 19

-118 ± 37

19 ± 24

0.970

kf

0.74 ± 0.34

1.23 ± 0.44

-0.13 ± 0.08

-0.20 ± 0.16

0.31 ± 0.11

0.728

ν abs

18609 ± 138

-1350 ± 177

267 ± 34

-196 ± 66

61 ± 43

0.964

ν em

18183 ± 95

-1301 ± 122

73 ± 23

-120 ± 46

20 ± 30

0.962

kf

0.92 ± 0.15

1.10 ± 0.19

-0.17 ± 0.04

0.10 ± 0.07

0.03 ± 0.05

0.917

ν abs

20130 ± 154

-1077 ± 197

246 ± 37

-181 ± 74

12 ± 48

0.935

ν em

19916 ± 94

-1193 ± 121

76 ± 23

-159 ± 45

10 ± 29

0.955

kf

0.67 ± 0.29

1.26 ± 0.38

-0.23 ± 0.07

0.12 ± 0.14

-0.02 ± 0.09

0.833

ν abs

19203 ± 145

-1043 ± 185

185 ± 35

-204 ± 69

5 ± 45

0.920

ν em

19154 ± 79

-1208 ± 102

30 ± 19

-89 ± 38

-1 ± 25

0.964

kf

0.35 ± 0.19

1.51 ± 0.24

-0.17 ± 0.05

0.20 ± 0.09

0.06 ± 0.06

0.907



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It is striking that the small solvent-dependent spectral shifts (all ν abs lie within 362 cm–1 for 1, 313 cm–1 for 2, 383 cm–1 for 3, 412 cm–1 for 4, 380 cm–1 for 5, and 310 cm–1 for 6; all ν em are located within 313 cm–1 for 1, 301 cm–1 for 2, 275 cm–1 for 3, 298 cm–1 for 4, 291 cm–1 for 5, and 267 cm–1 for 6) can be described so well by the new Catalán solvent scales. The advantage of this generalized treatment of the solvent effect is that it allows one to split up the relative contributions of dipolarity, polarizability, acidity, and basicity of the medium. In contrast, in the Kamlet–Taft approach the solvent parameter π* combines solvent (di)polarity and polarizability effects, and hence, this latter methodology can never be used to entangle solvent polarizability and (di)polarity effects. Now we use the Catalán methodology to unravel which solvent properties are primarily responsible for the observed spectral shifts. The very large (negative) estimated cSP values compared to the {aSA, bSB, dSdP} estimates in the analysis of νabs of 1–6 according to eq 3b and the relatively large standard errors on aSA, bSB and dSdP in comparison to those on cSP (Table 5) indicate that the small change of ν abs may reflect principally a minor change in polarizability of the environment of the chromophore. Further supporting evidence comes from the analyses of ν abs according to eq 3b with {SA, SB, SP}, {SB, SP, SdP} and {SA, SP, SdP} as independent variables. These regression analyses all gave high-quality fits (for 1 r = 0.871, 0.953, and 0.968, respectively; for 2 r = 0.883, 0.947, and 0.965, respectively; for 3 r = 0.825, 0.930, and 0.950, respectively; for 4 r = 0.793, 0.942 and 0.959, respectively; for 5 r = 0.714, 0.907, and 0.934, respectively; for 6 r = 0.750, 0.870, and 0.919, respectively). The common independent variable in all these analyses is SP (solvent polarizability). In this series of analyses, the lowest r–values (indicated in bold) were consistently obtained when solvent (di)polarity 26 ACS Paragon Plus Environment

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SdP was left out, indicating that solvent (di)polarity should not be neglected as influential factor. Conversely, the analyses of ν abs according to eq 3b, in which solvent polarizability (SP) was eliminated, gave the lowest r-values (0.709 for 1, 0.726 for 2, 0.752 for 3, 0.808 for 4, 0.789 for 5, and 0.721 for 6). To summarize, solvent polarizability is the most crucial solvent property affecting ν abs , although solvent polarity must not be disregarded. Further evidence that the position of ν abs of 1–6 is determined largely by solvent polarizability comes from the plots of ν abs versus f(n2) = (n2 – 1)/(2n2 + 1). Indeed, acceptable linear correlations are found between ν abs and f(n2) [r = 0.940 for 1 (Fig. 4), 0.936 for 2 (Fig. S7, ESI), 0.900 for 3, 0.887 for 4, 0.853 for 5, and 0.837 for 6]. The adequate linearity of the plots of ν abs of 1–6 versus f(n2) confirms that van der Waals and excitonic interactions with a polarizable solvent can rationalize to a large degree the solvent dependence of the excitation energy.39 If a large difference in permanent dipole moment would exist between the ground and excited state, the excitation energy would depend linearly on the Lippert solvent parameter ∆f = f(ε) – f(n2) = [(ε – 1)/(2ε + 1)] – [(n2 – 1)/(2n2 + 1)] rather than on f(n2) alone and hence no linear dependence on f(n2) would be observed anymore.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 4. Plots of the absorption maxima ( ν abs , squares) and fluorescence emission maxima ( ν em , circles)

for compound 1 versus f(n2) = (n2 – 1)/(2n2 + 1) as a function of solvent. The numbers refer to the solvents listed in Table 1. The straight lines represent the best linear fits to the data (r = 0.940 for ν abs and r = 0.969 for ν em ).

Now we examine which solvent characteristics account for the shifts of ν em . The relatively large (negative) cSP estimates and the comparatively large standard errors on aSA, bSB and dSdP in comparison to those on cSP (Table 5) point to solvent polarizability as

major factor affecting ν em . The analyses of ν em according to eq 3b with {SA, SB, SP}, {SB, SP, SdP} and {SA, SP, SdP} as independent variables (the common independent variable in all these analyses is SP) all gave high-quality fits (for 1 r = 0.952, 0.973, and 0.978, respectively; for 2 r = 0.962, 0.967, and 0.974, respectively; for 3 r = 0.914, 0.950, and 0.969, respectively; for 4 r = 0.936, 0.944 and 0.961, respectively; for 5 r = 0.920, 0.915, and 0.954, respectively; for 6 r = 0.958, 0.950, and 0.964, respectively).


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Conversely, the analyses of ν em according to eq 3b, in which SP was disregarded, gave much low r values (0.551 for 1, 0.530 for 2, 0.662 for 3, 0.602 for 4, 0.576 for 5, and 0.513 for 6). Hence, solvent polarizability is the most crucial solvent property influencing ν em . Supplementary evidence that the position of ν em is controlled largely by solvent

polarizability comes from the excellent linear plots of ν em versus f(n2) [r = 0.969 for 1 (Fig. 4), 0.978 for 2 (Fig. S7, ESI), 0.960 for 3, 0.949 for 4, 0.950 for 5, and 0.963 for 6]. The linearity of the plots for 1–6 of ν em versus f(n2) confirms that van der Waals and excitonic interactions with a polarizable solvent are primarily responsible for the experimentally observed solvent-dependent shifts.39 Now we consider the solvent dependence of the deactivation rate constants (kf and knr) of the singlet excited state of 1–6. Analysis of kf of 1–6 according to eq 3b gives only moderately acceptable fits, as assessed by the r values (r = 0.890 for 1, 0.790 for 2, 0.728 for 3, 0.917 for 4, 0.833 for 5, and 0.907 for 6, Table 3). That kf is sensitive mostly to solvent polarizability can be derived from the large cSP estimates (Table 5) and the separate analyses according to eq 3b with {SA, SB, SP}, {SB, SP, SdP}, {SA, SP, SdP} and {SA, SB, SdP} as independent variables. Indeed, the least satisfactory fits are obtained when solvent polarizability (SP) is omitted (r = 0.629 for 1, 0.530 for 2, 0.534 for 3, 0.696 for 4, 0.683 for 5, and 0.590 for 6). However, solvent (di)polarity cannot be ruled out as additional factor influencing the value of kf. This is further corroborated by the linear regression analyses of kf of 1–6 versus f(n2) = (n2 – 1)/(2n2 + 1). Indeed, rather poor linear correlations are found for kf as a function of f(n2) [r = 0.801 for 1, 0.734 for 2, 0.579 for 3, 0.896 for 4, 0.832 for 5, and 0.842 for 6]. The inadequate linearity of the plots of kf of 1–6 versus f(n2) confirms that polarizability is not the only factor



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determining the solvent dependence of kf. There is little variation between the average kf values of 1–6. Indeed, the average kf values for the 20 solvents in Tables 1 and S1–S5 are (1.5 ± 0.1) × 108 s–1 for 1, (1.7 ± 0.1) × 108 s–1 for 2, (1.7 ± 0.1) × 108 s–1 for 3, (1.6 ± 0.1) × 108 s–1 for 4, (1.4 ± 0.1) × 108 s–1 for 5, and (1.4 ± 0.1) × 108 s–1 for 6. Inadequate fits

are obtained for knr of 1–6 according to eq 3b (r = 0.549 for 1, 0.784 for 2, 0.738 for 3, 0.516 for 4, 0.566 for 5, and 0.389 for 6). The average values of knr for the 20 solvents in Tables 1 and S1–S5 are (0.34 ± 0.07) × 108 s–1 for 1, (0.05 ± 0.06) × 108 s–1 for 2, (0.14 ± 0.04) × 108 s–1 for 3, (0.43 ± 0.06) × 108 s–1 for 4, (0.18 ± 0.04) × 108 s–1 for 5, and (0.27 ± 0.05) × 108 s–1 for 6.

Because the radiative decay rate constant kf is proportional to the product of the squared transition dipole moment ψ G µˆ ψ E

2

(from the lowest exited state to the ground state),

(

)

3 3 n2, and ν em (eq 4),40 we calculated the ratio k f n 2 ν em of 1 for the solvents of Table 1

to check the solvent dependence of the transition dipole moment squared ψ G µˆ ψ E kf =

16 π 3 2 3 n ν em ψ G µˆ ψ E 3 ε0 h

2

2

. (4)

In eq 4, µˆ stands for the dipole moment operator, n is the refractive index of the medium, h is Planck’s constant, and ε0 denotes the permittivity of vacuum. There is no obvious

(

)

3 correlation between k f n 2 ν em of 1 and the {SA, SB, SP, SdP} solvent parameters, as

(

)

3 derived from the analyses of k f n 2 ν em according to eq 3b (r = 0.679). Similar results

were found for 2–6: (r = 0.539 for 2, 0.640 for 3, 0.655 for 4, 0.610 for 5, and 0.611 for

6). The absence of an unambiguous correlation demonstrates that the transition dipole


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moment ψ G µˆ ψ E is practically invariant in 20 solvents with widely varying solvent properties used in the study.

Crystallographic structure of 2 As shown in Figs. 5 and S10 (ESI), the boron center of 2 adopts a tetrahedral configuration with bond angles of ca. 109.5° and bond distances of 1.38 Å (B–F) and 1.56 Å (B–N). The BODIPY moiety is flat, in which the maximum deviation of nonhydrogen atoms is within the 0.005–0.055 Ǻ range (maximum deviation for N2, see Fig. S10, ESI). The angle between the two pyrrole moieties is 1.26°, which is near the average of 5.7° (range 0°–25.3°) found in the Cambridge Structural Database (CSD, updated to August 2011). The two fluorine atoms are equidistant above and under the mean plane of the BODIPY core, and the F-B-F plane is almost perpendicular (86.3°) to the plane of the ring system. The molecule is characterized by an internal (pseudo)-2-fold axis (through the C-CH3 bond), which gives the molecule a slight helicoidal appearance. This is due to the screw-boat conformation of the cyclohexa-1,3-diene moiety (Cremer and Pople parameters; θ = 113.5(2)°, φ = 87.6(3)°, amplitude Q = 0.470(2) )41. The unit cell is displayed in Fig. S11 (ESI).

Fig. 5. Stick representation of compound 2.



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Experimental The NMR spectra of the new compounds, the experimental procedures for the spectroscopic measurements and the crystal structure determinations can be found in the Electronic Supporting Information.

Conclusions The UV–vis absorption and fluorescence emission spectra of 1–6 have been determined as a function of solvent. These BODIPY derivatives show small Stokes shifts and narrow absorption and emission bands. The small solvent-dependent shifts of the absorption and emission bands of 1–6 are primarily determined by solvent polarizability. Restricting the conformational flexibility results in red shifted spectra compared to related unconstrained systems, in accordance with quantum chemical modeling. Extra increase of the conjugation causes further bathochromic shifts. Hence, fusion of conjugation extending rings to the 2,3- and 5,6-positions of BODIPY is an especially efficient method for the construction of BODIPY dyes absorbing and emitting at longer wavelengths.

Acknowledgments The ‘Instituut voor de aanmoediging van innovatie door Wetenschap en Technologie in Vlaanderen’ (IWT) is acknowledged for a fellowship to VL. WQ thanks the Chinese ‘Program for New Century Excellent Talents in University’ (NCET-09-0444), the ‘Fundamental Research Funds for the Central Universities’ (lzujbky-2011-22 and lzujbky-2012-k13), and the ‘Scientific Research Fund for Introducing Talented Persons to Lanzhou University’. This study was supported in part by the ‘Key Program of National Natural Science Foundation of China’ (20931003). The collaboration between Leuven and Mons is supported by the Science Policy Office of the Belgian Federal


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Government (BELSPO-IAP). Research in Mons is also supported by FNRS/FRFC and Région Wallonne (Programme d’excellence OPTI2-MAT). DB is a Research Director of the Fonds National de la Recherche Scientifique (FNRS, Belgium). This work was supported in part by grant CTQ2010-20507 from the ‘Ministerio Español de Ciencia e Innovación (co-financed by the Fondo Europeo de Desarrollo Regional, FEDER)’.

Supporting Information: Compound characterization data, experimental details on fluorescence quantum yield determination and time-resolved fluorescence, UV–vis absorption and fluorescence spectra of 2, 5, 6, spectroscopic and photophysical data of 2–

6 as a function of solvent, selection of fluorescence decay traces, solvatochromic analyses, results of quantum chemical calculations, crystal structure determination of BODIPY dye 2. This material is available free of charge via the Internet at http://pubs.acs.org.



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Graphical abstract

Detailed study of the optical and photophysical properties of conformationally constrained BODIPY dyes and the effect of solvent on those properties.


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