Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays

Article abstract of DOI:10.1039/d0sc03446k

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S0-S1 energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (?f) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ?f values that range from 10-25% in THF solvent, and between 28-36% in toluene solvent over the 700-900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission. This journal is

Full text of DOI:10.1039/d0sc03446k

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Driving high quantum yield NIR emission through
proquinoidal linkage motifs in conjugated
supermolecular arrays†
Cite this: DOI: 10.1039/d0sc03446k
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
*
Erin J. Viere,
Wei Qi, Ian N. Stanton, Peng Zhang
and Michael J. Therien
High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long
wavelength include the facts that radiative rate constants increase proportional to the cube of the
emission energy, while nonradiative rate constants increase in an approximately exponentially with
decreasing S0–S1 energy gaps (in accordance with the energy gap law). This work demonstrates how the
proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural
relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and
shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-
bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that
are integrated into the backbone of these compositions, elucidate new classes of impressive NIR
fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of
neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence
quantum yield (ff) measurements, acquired using a calibrated integrating-sphere-based measurement
system, demonstrate that these supermolecules display extraordinary ff values that range from 10–25%
in THF solvent, and between 28–36% in toluene solvent over the 700–900 nm window of the NIR.
These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to
drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence
emission.
Received 22nd June 2020
Accepted 14th July 2020
DOI: 10.1039/d0sc03446k
rsc.li/chemical-science
optical band gap is diminished: while it is oen the case that
a large S0 / S1 absorptive extinction coefficient is correlated
Introduction
with a correspondingly large S1 / S0 radiative rate constant
congruent with the Strickler–Berg relationship,28 most strongly
Low band gap p-conjugated molecules and oligomers are
central to function in electro-optic applications that span exci-
tonic solar cells,1–7 eld-effect transistors,8,9 optical power
limiting (OPL),10 dye-sensitized solar cells (DSSCs),11,12 photon-
upconversion (UC) technologies,13–17 long-wavelength light-
emitting diodes,18–20 nonlinear optics (NLO),21–24 and biological
imaging.25–27 Desirable low band gap materials for these appli-
cations typically feature singlet manifold transitions (S0 / S1,
S0 / Sn, and S1 / Sn) that possess large absorptive oscillator
strengths covering broad spectral domains that include the
near-infrared (NIR).
Driving high uorescence quantum yields is perhaps the
most demanding design challenge for low band gap chromo-
phores. Engineering augmented S0 / S1 transition oscillator
strength is not sufficient to realize correspondingly high S1 /
S0 uorescence quantum yields (ff values), particularly as the
Department of Chemistry, French Family Science Center, Duke University, 124 Science
Drive, Durham, North Carolina 27708-0346, USA. E-mail: michael.therien@duke.edu
Chart 1 Chemical structures for (A) benchmark PZnn, (B) BTD-PZnn-
BTD and (C) PZn2-(BTD-PZn2)n and PZn-(BTD-PZn)n fluorophores. R
¼ [2,6-bis(3,3-dimethyl-1-butyloxy)phenyl].
† Electronic supplementary information (ESI) available: Synthetic details,
characterization data, additional spectroscopic and computational data. See
DOI: 10.1039/d0sc03446k
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absorbing NIR chromophores are not impressive emitters, due PZnn chromophores, and serve to partly counter-balance the
to the substantial S1 state non-radiative rate constants (knr impact of augmented Franck–Condon mediated internal
values) that ensue from the energy gap law.29
conversion rate constants (kic values) that typically accompany
With respect to high oscillator strength NIR absorbers, diminished S0–S1 energy gaps in the NIR. These impressive
meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays PZnn photophysical properties have been exploited in a wide
(Chart 1A)17,30–46 constitute a notable exception to these range of uorescence imaging applications.54–61
generalizations. These supermolecules – conjugated struc-
The photophysical properties of PZnn chromophores may
tures realized from strong coupling of simple chromophoric be extensively modulated via incorporation of proquinoidal
building blocks – manifest such strong electronic and exci- units into these emblematic conjugated arrays. For example,
tonic interactions between the constituent (porphinato)zinc proquinoidal spacer (Sp) moieties such as 4,7-diethynylbenzo
oscillators that entirely unique photophysical entities are [c][1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-
dened. PZnn structures, for example, possess uorescence PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]qui-
quantum yields comparable to the most impressive NIR laser noxaline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c0]
dyes in the 750–900 nm regime,41 and rival the highest re- bis([1,2,5]thiadiazole) (E-BBTD-E) that link bis[(porphinato)
ported for organic dyes in this spectral window;47–49 impor- zinc] units can be utilized to modulate the relative degrees of
tantly, PZnn emitters do not suffer from the commonly cited quinoidal character that characterize the ground and elec-
drawbacks of tricarbocyanine dyes that include poor photo- tronically excited singlet states in these highly conjugated
stability and uorescence quantum yields that drop precipi- supermolecules.62 Recent work highlights how electronic
tously with decreasing solvent polarity.25,50,51 PZnn modulation of proquinoidal conjugation motifs can also be
chromophores feature low energy Q-state derived p–p* exploited as a powerful means to modulate independently the
excited-states that are polarized exclusively along the long dynamics of excited-state relaxation pathways in these
molecular axis, and lowest energy transitions that gain in systems, enabling, for example, exceptional NIR absorbers
intensity and progressively red-shi with increasing numbers that possess long-lived electronically excited triplet states (sT1
of PZn units. Extended conjugation in these highly polarizable > ms) that are generated at unit quantum yield.63 In this
chromophores drives red-shiing of PZnn uorescence contribution, we (i) show how proquinoidal BTD building
spectra. Because triplet excitons are more spatially conned blocks can be utilized to minimize the extent of excited-state
than singlet excitons in these systems,31,34,35,39,52,53 an unusual structural relaxation relative to the ground-state conforma-
dependence of ff magnitude upon increasing emission tion in highly conjugated porphyrin oligomers, and (ii)
wavelength [lmax(S1 / S0)] is manifest that confounds that demonstrate that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD)
anticipated simply from the energy gap law.41 Because of this units that terminate PZnn chromophores (Chart 1B), and E-
disparity in lowest energy singlet (S1) and triplet (T1) state BTD-E spacers that are integrated into the backbone of these
delocalization, diminished S1–T1 intersystem crossing rate compositions (Chart 1C), enable elucidation of new classes of
constants (kisc values) track with increasing conjugation in impressive NIR uorophores.
Scheme 1 Synthetic routes for PZn-(BTD-PZn)n compounds.
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Scheme 2 Synthetic routes for BTD-PZnn-BTD compounds.
and terminal BTD units. These structures exploit [2,6-bis(3,3-
dimethyl-1-butyloxy)phenyl] groups as 10- and 20-meso-
porphyrin substituents, which facilitate solubility and
Results and discussion
Synthesis
1
straightforward assignment of H-NMR spectra.37,64 The nature
Schemes 1–3 outline the synthetic strategies for the fabrication
of BTD-PZnn-BTD, PZn-(BTD-PZn)n, and PZn2-(BTD-PZn2)n
chromophores. These structures were synthesized by palladium
(Pd)-mediated cross-coupling reactions involving appropriately
substituted (porphinato)zinc(II) (PZn) compounds, BTD spacers,
of the functionalized PZn, PZnn, and BTD moieties used in the
syntheses of these BTD-PZnn-BTD, PZn-(BTD-PZn)n, and PZn2-
(BTD-PZn2)n arrays varied with the degree of conjugation in the
precursor molecules. Synthetic details may be found in the ESI.†
Scheme 3 Synthetic routes for PZn2-(BTD-PZn2)n compounds.
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units were used to expand conjugation at the macrocycle meso-
carbon position;66 these designs led to PZn chromophores
characterized by enhanced transfer of B-to-x-polarized Q-state
oscillator strength, intensied Qx absorption bands, chromo-
phore structural rigidication, spectrally narrow uorescence
emission bands, small magnitude Stokes shis, and enhanced
radiative rate constant magnitudes relative to simple bench-
mark PZn complexes.
Steady-state absorption and emission spectroscopy of BTD-
PZnn-BTD, PZn-(BTD-PZn)n, and PZn2-(BTD-PZn2)n
chromophores
Steady-state electronic absorption and uorescence spectra
recorded for reference PZnn chromophores (PZn2, PZn3, and
PZn5; Fig. 1A),17,30–46,54–56,58,59,61,63,65 as well as PZn-(BTD-PZn)n
[PZn-BTD-PZn,62 PZn-(BTD-PZn)2, and PZn-(BTD-PZn)4; Fig. 1B],
PZn2-(BTD-PZn2)n [PZn2-BTD-PZn2 and PZn2-(BTD-PZn2)2;
Fig. 1C], and BTD-PZnn-BTD [BTD-PZn-BTD,66 BTD-PZn2-BTD,
BTD-PZn3-BTD, and BTD-PZn5-BTD; Fig. 1D] arrays, are shown
in Fig. 1. The overall characteristics of the electronic absorption
spectra for these PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-
PZnn-BTD chromophores resemble those described previously
for dimeric and multimeric PZn compounds that feature a meso-
to-meso ethyne-linkage topology (PZnn arrays); these spectra
exhibit two distinct intense absorption manifolds that are
derived from the porphyrin B- (Soret, S0 / S2) and Q-band (S0
/ S1) transitions.30,32–34,37,38,40
Fluorescence emission metrics of BTD-PZnn-BTD, PZn-(BTD-
PZn)n, and PZn2-(BTD-PZn2)n chromophores
Long wavelength absorption maxima (lmax, S0 / S1), corre-
sponding extinction coefficient measurements, uorescence (S1
/ S0) emission maxima, full-width at uorescence half-
The observed perturbations from these benchmark PZnn
spectra trace their origin to the proquinoidal BTD units that are
connected to the porphyrin macrocycle meso carbons via ethynyl
moieties. Previous investigations demonstrate that the nature
of proquinoidal unit conjugation to the PZn macrocycle exerts
a pronounced impact on the magnitude of B- and Q-state mix-
ing;62,63,66 for example, for PZn–(proquinoidal Sp)–PZn chro-
mophores, the long axis-polarized Q state (Qx) absorption
maxima can be modulated from 689 to 1006 nm, depending
upon the extent of the quinoidal resonance contribution to the
electronically excited singlet state.62 A combination of semi-
empirical electronic structure calculations and electro-
chemical data underscore the cardinal role that PZn and pro-
quinoidal fragment orbital energy differences play in xing the
radical cation and anion state energy levels in these
structures.62,63
The principles that informed design of these PZn-(BTD-
PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD chromophores
stem from prior work that examined the photophysics of (por-
phinato)metal–(proquinoidal Sp)–(porphinato)metal super-
molecules.63 This work highlighted the central importance of
the magnitude of the energy separation between the (porphi-
nato)metal and proquinoidal Sp fragment frontier orbitals in
determining whether radiative, internal conversion, or inter-
system crossing decay channels dominated the relaxation
dynamics of the initially prepared electronically excited states of
these complexes. When the (porphinato)metal and proquinoi-
dal Sp fragment LUMO levels featured energy separations on
the order of a few tenths of an eV, as they are for the 5-ethynyl-
PZn, 4-ethynyl-BTD, and 4,7-diethynyl-BTD fragments of these
PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD struc-
tures, chromophores having multicongurational S1 states
characterized by a modest degree of charge transfer (CT) char-
acter are anticipated; such supermolecules were shown to
display large S1 / S0 radiative rate constants and substantial
uorescence quantum yields. Other factors that informed the
blueprint of the Fig. 1B–D supermolecules included insights
Fig. 1 Electronic absorption and emission spectra (figure panel insets)
recorded for: (A) benchmark PZnn chromophores (see ref. 30 and 41),
derived from monomeric PZn complexes in which ethynyl-BTD and (B) PZn-(BTD-PZn)n, (C) PZn2-(BTD-PZn2)n, and (D) BTD-PZnn-
BTD compositions. Experimental conditions: T ¼ 298 K, THF solvent.
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Table 1 Electronic absorption and emission data in THF solvent for BTD-PZnn-BTD, PZn-(BTD-PZn)n, and PZn2-(BTD-PZn2)n chromophores
relative to corresponding PZnn benchmarks
3g@lmax
lmax (S0 / S1)
(S0 / S1)a [nm] [Mꢀ1 cm ꢀ1
FWHM
f
lmax
(S1 / S0) [nm] [cmꢀ1
(S1 / S0) Stokes
krf
knr
shib (cmꢀ1
)
ffc,d
sF [ns] [ꢁ108 sꢀ1
]
[ꢁ108 sꢀ1
]
e
]
]
PZn-BTD-PZn
689 (1194)
745 (1178)
777 (1558)
674 (672)
765 (1243)
809 (1663)
847 (1675)
780 (1822)
64 600
741
784
811
687
787
846
888
822
857
711
806
883
1605
977
846
727
802
872
1008
926
1154
810
875
955
1018
543
490
281
365
510
587
688
586
324
580
551
0.25 (0.31) 1.6
0.22 (0.36) 1.1
0.16 (0.29) 0.8
0.13 (0.13) 1.3
0.23 (0.30) 1.4
0.16 (0.29) 0.8
0.10 (0.28) 0.6
0.14 (0.34) 0.6
0.12 (0.36) 0.5
0.14 (0.16) 1.09
0.19 (0.27) 1.13
0.09 (0.11) 0.45
1.6
2.0
2.0
1.0
1.6
2.0
1.7
2.3
2.4
1.3
1.7
2.0
4.7
7.1
11
6.7
5.5
11
15
14
18
PZn-(BTD-PZn)2
PZn-(BTD-PZn)4
BTD-PZn-BTD
BTD-PZn2-BTD
BTD-PZn3-BTD
BTD-PZn5-BTD
PZn2-BTD-PZn2
132 700
184 000
84 000
101 600
157 500
238 300
145 400
340 000
51 400
PZn2-(BTD-PZn2)2 816 (1582)
PZn2
PZn3
PZn5
695 (1082)
770 (1386)
842 (1562)
7.9
7.1
20
116 000
230 000
a
b
Numbers in parentheses are spectral breadths (full-widths at half-maximum, FWHM) of the respective transitions in units of cmꢀ1
. Stokes shis
values correspond to the difference in energy between the low energy (Qx) absorption (S0 / S1) and uorescence (S1 / S0) band maxima.
c
Fluorescence quantum yields (ff values) were determined using an integrating sphere-based absolute emission quantum yield measurement
d
e
system (see ESI). Values in parentheses represent those determined in toluene solvent. These values were determined using S0 / S2
excitation (483 nm). Fluorescence lifetimes were measured via time-correlated single-photon-counting using a picosecond uorescence lifetime
measurement system. f Excited-state relaxation constants were calculated based on the following equations: sS1 ¼ 1/(kr + knr), ff ¼ kr ꢁ sS1
.
maximum (FWHM), Stokes shis, uorescence lifetimes (sF), wavelengths that range up to 1100 nm (see ESI† for a complete
radiative rate constant (kr), non-radiative rate constant (knr), and system description). Note in this regard that minor differences
uorescence quantum yield (ff) data, are tabulated in Table 1 between the Table 1 ff data acquired for PZnn chromophores
for the PZnn, PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD- and those reported originally41 stem from the fact that these
PZnn-BTD supermolecules.
earlier ff values were determined via the reference method.
These uorescence quantum yield data correspond to abso- Table 2 displays comparative integrated oscillator strengths of
lute measurements, acquired using a calibrated integrating- the B- and Q-band spectral regions of BTD-PZnn-BTD, PZn-
sphere-based measurement system, which collects photons (BTD-PZn)n, and PZn2-(BTD-PZn2)n chromophores relative to
emitted from the sample following excitation through NIR corresponding PZnn benchmarks.
Table 2 Comparative integrated oscillator strengths of the B- and Q-band spectral regions of BTD-PZnn-BTD, PZn-(BTD-PZn)n, and PZn2-
(BTD-PZn2)n chromophores relative to corresponding PZnn benchmarksa
Oscillator strength B-band regionb
Oscillator strength Q-band regionc
Total oscillator strength
PZn-BTD-PZn
2.11
4.65
5.98
2.24
3.54
5.23
7.64
6.04
10.49
2.134
3.240
5.986
0.72
1.03
1.58
0.37
0.89
1.53
2.10
1.30
2.84
0.303
0.716
1.622
Ð
3.52
5.67
7.57
2.63
4.42
6.76
9.74
7.35
13.32
2.438
3.956
7.608
PZn-(BTD-PZn)2
PZn-(BTD-PZn)4
BTD-PZn-BTD
BTD-PZn2-BTD
BTD-PZn3-BTD
BTD-PZn5-BTD
PZn2-BTD-PZn2
PZn2-(BTD-PZn2)2
d
PZn2
PZn3
PZn5
d
d
a
Integrated oscillator strengths (f) were calculated based on the following expression: f ¼ 4:3 ꢁ 10ꢀ9 3dV, where 3 is the experimental extinction
coefficient, and n is the energy (in wave numbers) of the absorption. Values noted derive from electronic absorption spectra recorded in THF solvent.
b
Oscillator strengths calculated over the following wavelength domains: PZn-BTD-PZn (ꢂ360 to 600 nm); PZn-(BTD-PZn)2 (ꢂ360 to 600 nm); PZn-
(BTD-PZn)4 (ꢂ360 to 605 nm); BTD-PZn-BTD (ꢂ360 to 560 nm); BTD-PZn2-BTD (ꢂ360 to 560 nm); BTD-PZn3-BTD (ꢂ360 to 560 nm); BTD-PZn5-BTD
c
(ꢂ360 to 560 nm); PZn2-BTD-PZn2 (ꢂ360 to 610 nm); PZn2-(BTD-PZn2)2 (ꢂ360 to 610 nm). Oscillator strengths calculated over the following
wavelength domains: PZn-BTD-PZn (ꢂ600 to 760 nm); PZn-(BTD-PZn)2 (ꢂ600 to 820 nm); PZn-(BTD-PZn)4 (ꢂ600 to 900 nm); BTD-PZn-BTD
(ꢂ560 to 720 nm); BTD-PZn2-BTD (ꢂ560 to 850 nm); BTD-PZn3-BTD (ꢂ560 to 910 nm); BTD-PZn5-BTD (ꢂ560 to 1050 nm); PZn2-BTD-PZn2
(ꢂ610 to 860 nm); PZn2-(BTD-PZn2)2 (ꢂ610 to 920 nm). d Ref. 41.
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Table 1 highlights that E-BTD units that terminate PZnn respectively at 822 and 857 nm in THF solvent, with corre-
chromophores (Fig. 1A) give rise to BTD-PZnn-BTD super- sponding quantum yields of 14 and 12%. As demonstrated for
molecules (Fig. 1D) in which lmax(S1 / S0) for BTD-PZn2-BTD, both PZn-(BTD-PZn)n and BTD-PZnn-BTD chromophores, these
BTD-PZn3-BTD, and BTD-PZn5-BTD redshi by 1358, 587, and uorescence quantum yields are dramatically enhanced in
64 cmꢀ1, respectively, relative to their PZn2, PZn3, and PZn5 toluene solvent [ff(PZn2-BTD-PZn2) ¼ 0.34; ff(PZn2-(BTD-
benchmarks. Note that as a function of the number of PZn units PZn2)2) ¼ 0.36]. These data acquired for PZn2-(BTD-PZn2)n
in these arrays, BTD-PZnn-BTD and PZnn supermolecules designs suggest additional approaches to realize related
display similarly narrow uorescence FWHM values as well as frameworks that make possible high quantum yield NIR emis-
modest Stokes shis; these energy differences between the low sion that include electronic modulation of the proquinoidal
energy (Qx) absorption (S0 / S1) and uorescence (S1 / S0) units that both terminate supermolecules that utilize PZnn
band maxima of these chromophores range from 325– building blocks, as well as those that are integrated into the
585 cmꢀ1. A key spectroscopic ramication of terminal ethynyl- conjugated backbones of these compositions.
BTD conjugation to these PZnn frameworks is highlighted in the
data chronicled in Table 2, which contrasts the integrated
oscillator strengths of the B- and Q-band regions of the elec-
tronic absorption spectra of these chromophores. Note that the
integrated Q-state oscillator strengths of these BTD-PZnn-BTD
chromophores are augmented by more than 50% relative their
corresponding PZnn benchmark. These factors, coupled with
Computed electronic structures of BTD-PZnn-BTD, PZn-(BTD-
PZn)n, and PZn2-(BTD-PZn2)n chromophores
The natures of the low energy S1 states of BTD-PZnn-BTD, PZn-
(BTD-PZn)n, and PZn2-(BTD-PZn2)n chromophores were probed
through frontier orbital (FO) population and transition matrix
a diminished nonradiative rate constant, play important roles
in driving the substantial, long-wavelength emission quantum
yield manifest by BTD-PZn5-BTD (ff ¼ 0.28) relative to that for
the parent PZn5 chromophore (ff ¼ 0.11) in low dielectric
toluene solvent (Table 1).
PZn-(BTD-PZn)n chromophores (Fig. 1B) contrast the elec-
tronic spectral properties of BTD-PZnn-BTD supermolecules;
PZn-BTD-PZn, PZn-(BTD-PZn)2, and PZn-(BTD-PZn)4 display Qx
state absorption maxima that are blue-shied 126, 436, and
994 cmꢀ1 with respect to the lmax(S0 / S1) transitions of their
respective PZn2, PZn3, and PZn5 benchmarks (Table 1, Fig. 1);
this effect derives from the fact that 4,7-diethynylbenzo[c][1,2,5]
thiadiazole provides diminished PZn–PZn electronic coupling
relative to the ethynyl linker. While PZn-BTD-PZn displays an
augmented Stokes shi (1018 cmꢀ1) relative to PZn2 (324 cmꢀ1
)
due to the greater cumulenic character in its relaxed electroni-
cally excited S1 state,62 the Stokes shis manifest for PZn-(BTD-
PZn)2, PZn-(BTD-PZn)4, PZn3, and PZn5 are similar in magni-
tude (ꢂ560 cmꢀ1), congruent with more modest structural
differences between the relaxed S0 and S1 states for these
supermolecules (vide infra).
PZn-BTD-PZn, PZn-(BTD-PZn)2, and PZn-(BTD-PZn)4 display
uorescence emission maxima centered at 741, 784, and
811 nm, respectively, and substantial uorescence quantum
yields in THF solvent [ff(PZn-BTD-PZn) ¼ 0.25; ff(PZn-(BTD-
PZn)2) ¼ 0.22; ff(PZn-(BTD-PZn)4) ¼ 0.16]. Similar to that
observed for BTD-PZnn-BTD chromophores, uorescence
quantum yields are signicantly enhanced in nonpolar solvent;
note in this regard that ff(PZn-(BTD-PZn)4) is amplied to 29%
in toluene (Table 1).
Fig. 1C highlights the absorptive and emissive spectral
properties that ensue when PZn2 chromophores are linked by
4,7-diethynylbenzo[c][1,2,5]thiadiazole units. These PZn2-(BTD-
PZn2)n chromophores display dramatic Qx absorption band
intensication with increasing conjugation (Tables 1 and 2);
note in this regard that PZn2-(BTD-PZn2)n is an exceptional
Fig. 2 Calculated frontier molecular orbitals, energy levels, and rela-
tive one-electron configurations that contribute to the lowest energy
(Qx) transition for the PZn3 benchmark. Calculations were performed
at the M11/6-311g(d) theory level.
long-wavelength absorber (3(816 nm) ¼ 340 000 Mꢀ1 cmꢀ1
;
Table 1). PZn2-BTD-PZn2 and PZn2-(BTD-PZn2)2 emit
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eigenvector analyses derived using TD-DFT methods. Fig. 2–4 a key role in the solvent-dielectric dependent uorescence
show FO diagrams for BTD-PZn3-BTDand PZn-(BTD-PZn)2, quantum yields evidenced in THF and toluene solvent.
along with those for the PZn3 benchmark, and highlight the
For PZn3 (Fig. 2), seven single excitation congurations
prominent one-electron congurations that contribute to their describe the transition matrix eigenvector; in contrast, nine and
respective lowest energy Qx transitions; related data for the six single excitation congurations describe respectively the
PZn2-(BTD-PZn2)2 chromophore is presented in the ESI.†
transition matrix eigenvectors for BTD-PZn3-BTD and PZn-
The natures and energy separations between the PZn3, BTD- (BTD-PZn)2. For PZn3, the transition eigenvector is dominated
PZn3-BTD, and PZn-(BTD-PZn)2 FOs underscore the x-polarized by congurations (representing an ꢂ82% weighting in the CI
nature of the lowest energy excited state for these super- expansion) that do not redistribute to a signicant degree
molecules, and the globally delocalized character of their electron density. For BTD-PZn3-BTD and PZn-(BTD-PZn)2, the
respective S1 excited states. The low-lying excited states of these relative importance of such single excitation congurations that
supermolecules are described by extensive conguration inter- do not redistribute signicantly electron density drops respec-
action (CI). The S0 / S1 transitions of PZn3, BTD-PZn3-BTD, tively to 74 and 22%, highlighting the increased importance of
and PZn-(BTD-PZn)2 have large contributions (ꢂ55%) from the single-excitation congurations that redistribute electron
one electron HOMO / LUMO (H / L) conguration, and density in the CI expansions that describe the S0 / S1 transi-
highlight the importance of quinoidal resonance contributions tion eigenvectors for these supermolecules. While the bandgap
to this low-lying excited state; this resonance contribution plays represented by DEHOMO–LUMO varies to a minor degree and
spans ꢂ4.5 to 4.8 eV, note that the FO bandwidth in these
Fig. 2–4 uorophores varies signicantly: DHꢀ5–L+5 for PZn3 is
9.77 eV but only 7.76 eV for PZn-(BTD-PZn)2, a decrease of
2.01 eV.
Fig. 3 Calculated frontier molecular orbitals, energy levels, and rela-
tive one-electron configurations that contribute to the lowest energy
(Qx) transition for BTD-PZn3-BTD. Calculations were performed at the
M11/6-311g(d) theory level.
Fig. 4 Calculated frontier molecular orbitals, energy levels, and rela-
tive one-electron configurations that contribute to the lowest energy
(Qx) transition for PZn-(BTD-PZn)2. Calculations were performed at
the M11/6-311g(d) theory level.
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Note that as supermolecular conjugation evolves from PZn3 thus contrast the behavior of classic tricarbocyanine NIR dyes,
to BTD-PZn3-BTD and PZn-(BTD-PZn)2, an increased degen- displaying uorescence quantum yields ranging from 28–36%
eracy of energy eigenvalues is manifest in the Fig. 2–4 FOs. This in toluene solvent. Because these BTD-PZnn-BTD, PZn-(BTD-
increased density of states (DOS) evident in BTD-PZn3-BTD and PZn)n, and PZn2-(BTD-PZn2)n supermolecules display extraor-
PZn-(BTD-PZn)2 derives from the mixing of BTD and PZn dinarily large NIR uorescence quantum yields in hydrophobic
frontier orbitals. Congruent with Fermi's Golden Rule, transi- solvent, these designs underscore new opportunities to evolve
tion probability correlates with increased DOS. This larger DOS NIR-emissive nano- and mesoscale vesicles for uorescence
near the HOMO and LUMO levels serve to increase the multi- imaging applications in which the emissive irradiance exceeds
reference nature of the S1 state wave function; S1 congura- the impressive metrics already established for such structures
tions that involve these delocalized and mixed frontier orbitals that membrane-disperse PZnn uorophores.54–61,65
will thus exhibit enhanced S1 state wave function spatial delo-
calization. This computational trend is in line with the experi-
mentally determined transition moments, wherein Qx-
transition oscillator strengths for BTD-PZn3-BTD and PZn-(BTD-
Experimental section
Synthesis and characterization
PZn)2 exceed that of PZn3. Note also that this dispersion of FO
energies, which decreases progressively from PZn3 to BTD-PZn3-
BTD to PZn-(BTD-PZn)2 (Fig. 2–4), is correlated with increased p
conjugation which is reected in the computed electronic
delocalization range function for these uorophores (ESI†); due
to the nature of these orbitals and their diminished energy gaps
within the lled and empty regimes of the FO manifold,
a greater weighted fraction of single excitation congurations
having charge resonance character is manifest, congruent with
the augmented Qx absorption oscillator strengths observed for
BTD-PZnn-BTD and PZn-(BTD-PZn)n relative to PZnn (Fig. 1,
Tables 1 and 2).
The synthetic procedures and corresponding characterization
data of all new compounds, complete with the reaction
schemes, are given in the ESI.†
Instrumentation
Electronic absorption spectra were recorded on a Shimadzu UV-
1700 spectrophotometer. Steady-state emission spectra were
recorded on a FLS920 spectrometer that utilized a xenon lamp
(Xe900) as the excitation light source and an extended red
sensitive PMT (Hamamatsu R2658P side window photo-
multiplier, spectral range: 200–1010 nm) for detection. Emis-
sion spectra were corrected using a calibration curve supplied
with the instrument.
Conclusions
Fluorescence lifetime measurements
We describe a design strategy for (porphinato)zinc-based
supermolecules that possess large NIR uorescence quantum
yields. These PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-
PZnn-BTD uorophores feature either 4-ethynylbenzo[c][1,2,5]
thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-
bridged (porphinato)zinc (PZnn) arrays, or 4,7-diethynylbenzo
[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into
the backbone of these compositions. PZn-(BTD-PZn)n, PZn2-
(BTD-PZn2)n, and BTD-PZnn-BTD chromophores are character-
ized by enhanced transfer of B-to-x-polarized Q-state oscillator
strength relative to their corresponding PZnn benchmarks,
intensied Qx absorption bands, supermolecule structural
rigidication, spectrally narrow uorescence emission bands,
small magnitude Stokes shis, and large radiative rate constant
magnitudes. TD-DFT calculations point to the importance of
a greater weighted fraction of single excitation congurations
having charge resonance character that describe the S0 / S1
transition matrix eigenvector in PZn-(BTD-PZn)n, PZn2-(BTD-
PZn2)n, and BTD-PZnn-BTD emitters relative to PZnn oscillators
in driving these spectroscopic and dynamical properties.
Collectively, these systems dene an unusual family of intensely
absorbing vis-NIR absorbers that display strikingly high uo-
rescence quantum yields (ff values) over the 700–900 nm
Time-resolved emission spectra were recorded using a Hama-
matsu C4780 picosecond uorescence lifetime measurement
system. This system employs a Hamamatsu Streakscope C4334
as its photon-counting detector; a Hamamatsu C4792-01
synchronous delay generator electronically generated all time
delays. The excitation light source chosen was a Hamamatsu
405 nm diode laser. Fluorescence lifetimes were acquired in
single-photon-counting mode using Hamamatsu HPD-TA so-
ware and analyzed using the Hamamatsu tting module.
Quantum yield measurements
A Hamamatsu C9920-03 Absolute Quantum Yield Measurement
System was employed to make the quantum yield measure-
ments. Excitation initiates from a Xe-lamp, where the wave-
length is selected by a monochromator, and passed through
a 1 mm optical excitation ber. The inside of the sphere is
coated with Spectralon (Labsphere, Inc.) that has at least 99%
reectance over the 350–1650 nm spectral window. Added detail
regarding these measurements is provided in the ESI.†
Time-dependent density functional theory calculations
regime of the NIR. These THF ff values for PZn-(BTD-PZn)n, All electronic structure calculations were performed upon
PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD supermolecules, model compounds in which aliphatic chains were truncated to
perhaps without peer in a solvent of this dielectric strength over methyl groups (ESI†). Structure optimization and linear
this spectral window, range from 10–25%; notably these ff response calculations were performed with density functional
values are dramatically amplied in hydrophobic media and theory (DFT) using Gaussian 16, revision C.01.67 The M11 68
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No competing nancial interests have been declared.
Acknowledgements
This work was funded by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences,
of the U.S. Department of Energy through Grant DE-SC0001517.
The authors are grateful for support made available through the
Extreme Science and Engineering Discovery Environment
(XSEDE) Comet at the San Diego Supercomputer Center
through allocation CHE190106, which is supported by the
National Science Foundation through Grant ACI-1548562. E.
J. V. acknowledges the National Science Foundation for
a Graduate Research Fellowship.
29 R. Englman and J. Jortner, Mol. Phys., 1970, 18, 145–164.
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