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tion, and that of the flanking methyl groups at C1 and C7, on
their structural and photophysical properties. The meso-substi-
tuted 1,3,5,7-tetramethyl and 3,5-dimethyl BODIPY dyes, CF3-
T1 and -D1,[6] CH3-T2 and -D2,[6,8] COOMe-T3 and -D3,[9]
COOtBu-T4 and -D4,[9] CHO-T5[10] and -D5,[11] CN-T6[12] and Cl-
T7 and -D7[13] (Scheme 1), were prepared following slight
modifications of previously described procedures. CN-D6 was
synthesized in two steps from CHO-D5 by condensation with
hydroxylamine to yield the corresponding oxime, which was
then dehydrated with PPh3 and N-chlorosuccinimide (NCS)
(Scheme 2A). iPr-D8 and iPr-T8 were synthesized by the reac-
sulting in a decreased HOMO–LUMO gap (see Figures S61–S64
in the Supporting Information).
Particularly interesting is the observation that 3,5-dimethyl
BODIPYs substituted at the meso position with electron-with-
drawing carbonyl groups (COOR and CHO) exhibit absorption
and emission maxima that are redshifted more than those of
their 1,3,5,7-tetramethyl counterparts. For example, the respec-
tive absorption (labs.max) and emission (lem.max) maxima of
COOMe-D3 in CHCl3 are at 541 nm and 604 nm, which is far to
the red of COOMe-T3 (labs.max =516 nm, lem.max =543 nm). Simi-
lar trends exist in other 1,3,5,7-tetramethyl and 3,5-dimethyl
BODIPYs (COOtBu-T4 vs. COOtBu-D4; CHO-T5 vs. CHO-D5).
These differences are attributed to the flanking methyl groups
at C1 and C7 in the 1,3,5,7-tetramethyl-BODIPY series (T),
which force the electron-withdrawing groups at the meso posi-
tion to pivot out of p-conjugation, reducing their electronic in-
fluence. Indeed, whereas the calculated optimized geometry
for COOMe-D3 shows an ester carbonyl conjugated with the
BODIPY core (418 angle, Figure 2, right), that for COOMe-T3
presents an ester carbonyl orthogonal to the boradiazainda-
cene plane (908 angle, Figure 2, left). In a similar fashion, the
3,5-dimethyl BODIPY analogues, COOtBu-D4 and CHO-D5,
show much smaller torsion angles than their respective 1,3,5,7-
tetramethyl counterparts, COOtBu-T4 and CHO-T5 (Figur-
es S62–63). Electronic conjugation is optimal for CHO-D5,
which presents unusually redshifted absorption (600 nm) and
emission (632 nm) bands, and for which calculations show
a carbonyl group coplanar with the BODIPY plane. The oppo-
site behavior is seen for CF3-D1 (544/563 nm) and CF3-T1 (553/
622 nm). For dyes bearing this inductively electron-withdraw-
ing but not p-conjugated group, preferential destabilization of
Scheme 2. Synthesis of meso-substituted BODIPYs CN-D6 (A) and iPr-T8 and
iPr-D8 (B).
tion of the required pyrrole with iso-butyryl chloride in the
presence of Et3N followed by treatment with BF3·OEt2
(Scheme 2B). All the BODIPY derivatives are soluble in
common organic solvents, such as THF, CHCl3, ace-
tone, and CH3CN, but insoluble in water.
Table 1. Photophysical properties of the meso-substituted BODIPYs.[a]
[e]
[g]
[e,h]
Compound labs e (104)
[nm] [mÀ1 cmÀ1] [nm]
lem FF
tav
[ns] [sÀ1
kr[f]
knr
[sÀ1
DnSt tav
[cmÀ1] [ns]
]
]
Spectroscopic properties of the BODIPY dyes in so-
lution
CF3-T1
CF3-D1
CH3-T2
CH3-D2
COOMe-T3 516 10.3
COOMe-D3 541 5.6
COOtBu-T4 515 10.1
COOtBu-
D4
CHO-T5
553 5.1
544 6.4
499 11.6
508 11.2
622 0.008[c] 0.21 3.8107 4.7109
563 0.94[c] 6.51 1.4108 9.2106
2006 0.23
620
7.06
5.90
6.12
0.16
The absorption and fluorescence properties of the
prepared 1,3,5,7-tetramethyl and 3,5-dimethyl BODI-
PYs in CHCl3 solution are summarized in Table 1 and
Figure 1. The absorption and emission profiles, and
fluorescence quantum yields of the dyes in solution
are greatly affected by meso substituents as well as
methyl groups at C1 and C7. As previously observed
for CF3-T1,[6] introduction of electron-withdrawing
meso substituents (i.e., CF3-D1, COOMe-3, COOtBu-4,
CHO-5, CN-6) leads to bathochromic shifts with re-
spect to analogues with alkyl substituents at the
meso position (CH3-2 and iPr-8). The absorption and
emission maxima of meso-chloro-substituted BODIPYs
(Cl-7) are, however, comparable to those of the corre-
sponding meso-methyl-substituted BODIPYs (CH3-2).
As described earlier,[6] these observations are in quali-
tative agreement with DFT calculations, which indi-
cate that electron-withdrawing substituents at the
meso position preferentially stabilize the LUMO, re-
520 1.00[b] 5.22 1.9108 ꢀ1.9106 809
526 0.99[b] 5.30 1.9108 1.9106
543 0.005[b] 0.16 3.1107 6.2109
604 0.44[c] 7.26 6.1107 7.7107
544 0.007[b] 0.19 3.7107 5.2109
600 0.52[c] 7.34 7.1107 6.5107
674
964
1928 4.03
1035 0.20
2201 4.03
530 5.4
515 5.7
545 0.002[b] 1.43 1.4106 7.0108
1069 2.15,
0.3[i]
CHO-D5
CN-T6
CN-D6
Cl-T7
Cl-D7
iPr-T8
iPr-D8
600 3.4
561 7.1
568 6.9
504 9.5
516 9.1
508 9.0
509 8.4
632 0.34[d] 4.81 7.1107 1.4108
585 0.50[c] 6.28 8.0107 8.0107
586 0.60[c] 7.24 1.0108 6.8107
527 0.51[b] 3.99 1.3108 1.2108
535 0.84[b] 5.54 1.5108 2.9107
545 0.014[b] 0.26 5.4107 3.8109
528 0.90[b] 5.54 1.6108 1.8107
844
731
541
866
688
n.a.[j]
0.26
6.46
0.24
6.38
1336 0.63
707 5.46
[a] In CHCl3. [b] Quantum yields relative to fluorescein in 0.1 N NaOH (FF =0.95).
[c] Quantum yields relative to rhodamine 6G in ethanol (FF =0.95). [d] Quantum
yields relative to cresyl violet perchlorate in ethanol (FF =0.54). [e] The weighted
mean lifetime. [f] kr =FF/t. [g] knr =(1ÀFF)/t. [h] In 99:1 (v/v) water/CH3CN. [i] tav
=
2.15 ns for 540 nm and tav =0.3 ns for 650 nm. [j] Not available due to its reactivity
with water.[14]
Chem. Eur. J. 2015, 21, 17459 – 17465
17460
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