Organic Letters
Letter
Table 1. Photophysical Properties of 2,2′-Bibenzothiazoles bisbtz-I−VI in Comparison with Pertinent Dipolar 2-H-
Benzothiazoles and bpy Congener of bisbtz-VI in Toluene
dye
λabs [nm]
εabs [M−1 cm−1
]
λf [nm]
Φf
⟨τ⟩ [ns]
λTPA [nm]
δTPAΦf [GM]
δTPA [GM]
bisbtz-I
bisbtz-II
bisbtz-III
bisbtz-IV
bisbtz-V
bisbtz-VI
btz-IV
439
419
437
428
441
450
365
381
392
42 500
43 700
55 600
65 100
49 000
74 300
31 500
34 500
52 100
495
496
527
497
527
518
405
429
443
0.99
0.82
0.75
0.86
0.68
0.75
0.75
0.96
0.64
1.70
1.34
2.00
1.22
1.92
1.24
800
800
820
810
840
830
730
760
780
538
480
819
873
679
939
34
543
585
1092
1015
998
1252
46
btz-VI
bpy
62
135
64
210
less than twice that of the single-photon absorption λabs (S0 →
S1), implying a deeper S0 → S2 transition associated mainly with
the HOMO − 1 → LUMO excitation. Both of these MOs have
the same a symmetry, obeying thus a parity selection rule for
quadrupolar dyes (see also Figure S9 and Table S9 in SI).
TPA cross sections of resonant maxima increase within the
bisbtz series nonmonotonically along with the elongation of the
π-conjugation length, while the vinylene π-spacer is found to be
more efficient than the ethynylene linkage. Interestingly, the
introduction of peripheral methoxy groups to bisbtz-II led to an
approximately 2-fold TPA enhancement, while the same
modification in the longer π-system (bisbtz-IV → bisbtz-V)
affected TPA activity marginally.
photon and two-photon absorption properties in the visible and
near-IR region, respectively, along with the chelating N^N
ability, make triarylamine-functionalized 2,2′-bibenzothiazoles a
more attractive alternative to notorious donor-functionalized
2,2′-bipyridines. On the basis of the high TPA cross sections,
strong emission, and bidentate coordination, the title quad-
rupolar biheteroarenes provide a privileged platform paving the
way to a large scope of efficient, small- to medium-sized, NLO-
phores with diverse (dipolar, quadrupolar, octupolar) architec-
tures, properties of which can be easily tailored by modifying the
peripheral arylamine moieties and/or metal fragments.
ASSOCIATED CONTENT
■
It is noteworthy that the TPA maximum cross section of
bisbtz-VI is ca. 6 times larger than that of bipyridine-cored
congener bpy12,13 measured in toluene under identical TPEF
conditions (literature data for bpy are 219 GM at 690 nm and
425 GM at 780 nm in chloroform and DMF, respectively).12,13
Although the TPA performance of bisbtz-VI in chloroform is
somewhat lowered (δTPA = 1018 GM at 840 nm), it is still about
4.6 times higher than that of bpy, yet with an emission maximum
shifted to the orange spectral region (Figures S4 and S12 in SI).
The TPA superiority of the bisbtz scaffold can be ascribed
according to the three-state model22 to a notably larger
transition-dipole moment between the ground and the first
excited state (μ01 = 17.8 D), which is also reflected in the higher
molar extinction coefficient (cf. εabs for bisbtz-VI and bpy in
Table 1), larger first-excited-to-second-excited state transition
moment (μ12 = 13.4 D), and smaller detuning energy (E1 − E2/2
sı
* Supporting Information
The Supporting Information is available free of charge at
Experimental and computational details, synthetic
procedures, compound characterization, crystal structure
data for bisbtz-I, and computed TPA cross sections
FAIR data, including the primary NMR FID files, for
compounds 1−4, btz-I−VI, bisbtz-I−VI (ZIP)
Accession Codes
CCDC 2056840 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
= 1.32 eV) as compared to the bpy-cored congener with μ01
=
14.4 D, μ12 = 9.9 D, and E1 − E2/2 = 1.65 eV. This, in turn, allows
researchers to achieve higher TPA activity within the confined
chromophore space with a shorter π-bridge, that is beneficial in
the engineering of small-molecular probes for TPEF microscopy
or photosensitizers for PDT.
AUTHOR INFORMATION
■
Corresponding Author
In addition, our pilot studies on the complexation of bisbtz-I
with transition-metal carbonyl compounds, such as ReCl(CO)5,
confirmed its bidentate N^N coordination, while forming a
purple complex [ReCl(CO)3(bisbtz-I)] with low-energy ICT
absorption bands positioned at 536 and 562 nm (Figure S5 in
SI). Complexation via central azole N^N nitrogen atoms is
unambiguously manifested by a high-frequency 15N NMR azole
shift from −70 ppm in the free ligand (bisbtz-I) to −123 ppm in
the ReI complex, which is in excellent agreement with the 15N
NMR shifts computed for the considered species by means of
relativistic calculations (Table S11 in SI).23
Peter Hrobárik − Department of Inorganic Chemistry, Faculty
of Natural Sciences, Comenius University, SK-84215
Bratislava, Slovakia; Laboratory for Advanced Materials,
Comenius University Science Park, SK-84215 Bratislava,
Authors
́
Patrik Osusky − Department of Inorganic Chemistry, Faculty of
Natural Sciences, Comenius University, SK-84215 Bratislava,
Slovakia
To conclude, homocoupling of push−pull C−H heteroarenes
is demonstrated to be a simple and atom-economical strategy to
achieve large (∼20-fold) TPA enhancement. Outstanding one-
Jela Nociarová − Department of Inorganic Chemistry, Faculty of
Natural Sciences, Comenius University, SK-84215 Bratislava,
Slovakia
5515
Org. Lett. 2021, 23, 5512−5517