Luminescent Quadrupolar Borazine Oligomers: Synthesis, Photophysics, and Two-Photon Absorption Properties

Article abstract of DOI:10.1002/chem.201502268

A set of monodisperse bent donor-acceptor-donor-type conjugated borazine oligomers, BnNn+1 (n=1-4), incorporating electron-rich triarylamine donor and electron-deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon-boron and tin-boron exchange reactions. The effect of chain elongation on the electrochemical, one- and two-photon properties and excited-state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl-centered acceptor sites results in emissions with high quantum yields (Φ_fl>0.5) in the range of 400-500nm. Solvatochromic effects lead to solvent shifts as large as ?70nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two-photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410GM (n=4), and broadband excited-state absorption (ESA) attributed to long-lived singlet-singlet and radical cation/anion absorption. The excited-state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6-31G) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.

Full text of DOI:10.1002/chem.201502268

DOI: 10.1002/chem.201502268
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&
Solvatochromism
Luminescent Quadrupolar Borazine Oligomers:
Synthesis, Photophysics, and Two-Photon Absorption Properties
Pangkuan Chen,[a] Ariel S. Marshall,[b] San-Hui Chi,[b] Xiaodong Yin,[a] Joseph W. Perry,*[b] and
Frieder Jäkle*[a]
Abstract: A set of monodisperse bent donor–acceptor–
donor-type conjugated borazine oligomers, BnNn+1 (n=1–
4), incorporating electron-rich triarylamine donor and elec-
tron-deficient triarylborane acceptor units has been pre-
pared through an iterative synthetic approach that takes ad-
vantage of highly selective silicon–boron and tin–boron ex-
change reactions. The effect of chain elongation on the elec-
trochemical, one- and two-photon properties and excited-
state photodynamics has been investigated. Strong intramo-
lecular charge transfer (ICT) from the arylamine donors to
boryl-centered acceptor sites results in emissions with high
quantum yields (Ffl >0.5) in the range of 400–500 nm. Sol-
vatochromic effects lead to solvent shifts as large as ~70 nm
for the shortest member (n=1) and gradually decrease with
chain elongation. The oligomers exhibit strong two-photon
absorption (2PA) in the visible spectral region with 2PA cross
sections as large as 1410 GM (n=4), and broadband excited-
state absorption (ESA) attributed to long-lived singlet–sin-
glet and radical cation/anion absorption. The excited-state
dynamics also show sensitivity to the solvent environment.
Electrochemical observations and DFT calculations (B3LYP/6-
31G*) reveal spatially separated HOMO and LUMO levels re-
sulting in highly fluorescent oligomers with strong ICT char-
acter. The BnNn+1 oligomers have been used to demon-
strate the detection of cyanide anions with association con-
stants of log K>7.
Introduction
Because of its empty p-orbital, three-coordinate boron is elec-
tron deficient and a strong electron acceptor. Furthermore, the
vacant p-orbital of boron allows for p-conjugation in organic
systems, giving rise to unique photophysical and electronic
properties.[3] For example, researchers have taken advantage of
the emissive properties of organoborane chromophores and
demonstrated their efficacy in OLEDs and bioimaging applica-
tions.[4] Moreover, the binding of anions, for example, fluoride
or cyanide, perturbs the p-p orbital overlap, allowing for orga-
noboranes to be used as fluorescent sensory materials.[5]
Considerable efforts have been devoted to developing ambi-
polar molecules, in which donor (D) and acceptor (A) groups
are linked by p-conjugated moieties. They are attractive for ap-
plications in the area of materials science, such as electronic
devices (organic light emitting diodes (OLEDs), field effect tran-
sistors and organic photovoltaic cells), as well as for use in bio-
imaging and photodynamic therapy (PDT).[1] Triarylamines have
been extensively exploited as p-type semiconductor materials
that enable hole transport, thanks to their inherent electron-
donating capability.[2] Among the wide range of acceptor ma-
terials studied, triarylboranes are of significant current interest.
Discrete ambipolar molecules that combine organoboranes
with triarylamines are attractive, because of the simultaneous
n- and p-type behavior and the possibility for interesting intra-
molecular charge-transfer (ICT) processes.[6] These features
have also been exploited in nonlinear optical (NLO) materi-
als.[3,7] Notable studies by Lambert,[8] Müllen and Perry,[9]
Fang,[10] and Marder[11] have led to dipolar, quadrupolar, octu-
polar, and dendrimer structures that possess two-photon ab-
sorption (2PA) cross-section (d) values as large as 1300 GM.[10]
Polymeric systems of this type are comparatively less ex-
plored.[12] A high molecular weight linear polymeric system, in
which arylamine donors and arylborane acceptors alternate in
the polymer main chain has been introduced by Jäkle and co-
workers.[13] The investigation of such D/A conjugated polymers
poses important questions concerning structural conformation,
electronic delocalization, ICT characteristics, and NLO proper-
ties in organoborane polymeric systems. Further study of the
NLO properties of organoborane polymers may allow for new
design strategies toward the enhancement of 2PA in organic
[a] Dr. P. Chen,+ Dr. X. Yin, Prof. F. Jäkle
Department of Chemistry, Rutgers University Newark
Newark, NJ, 07102 (USA)
E-mail: fjaekle@rutgers.edu
[b] Dr. A. S. Marshall,+ Dr. S.-H. Chi, Prof. J. W. Perry
School of Chemistry and Biochemistry and Center for Organic Photonics
and Electronics, Georgia Institute of Technology
Atlanta, GA, 30332-0400 (USA)
E-mail: joe.perry@chemistry.gatech.edu
[+] These authors contributed equally.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502268. It contains synthetic details and
chemical characterization, cyclic voltammograms, time-resolved fluores-
cence decays of all oligomers dissolved in hexane, toluene, and DCM, two-
photon absorption spectra, computed orbital plots of HOMO-1 and
LUMO+1, solvatochromic steady- and excited-state spectra, solvatochro-
mic TA kinetic traces of oligomers, and 1PA and emission spectra of CNÀ ti-
tration experiments.
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fluorophores for 2PA-based fluorescence applications.[14] Addi-
tionally, ambipolar materials that display strong NLO behavior
have been used to probe carrier motion directly in organic
electronic devices,[15] and would be very useful for further im-
provement in device performance and design.
nally, the anion-induced optical response of the oligomers is
investigated by monitoring cyanide titration experiments of
the linear borazine dyes by 1PA and emission spectroscopy.
Comparative investigations of structurally well-defined oligo- Results and Discussion
fluorenes (OFn),[16] oligothiophenes (OTn),[17] oligoanilines
Synthesis and structural characterization of oligomers
(OAn),[18] and fluoreneborane oligomers (OBn)[19] in what since
has been dubbed “the oligomer approach”[20] have provided
important information on the photophysical attributes and
electronic structures of the corresponding conjugated organic
polymers.[21] Moreover, electrochemical studies on BODIPY-
based oligomers led to valuable insights into the substantial
interactions between the active building blocks.[22] However,
corresponding studies on well-defined oligomers with aryla-
mine donor and arylborane acceptor groups have not been
performed thus far. In this regard, we introduce here a new set
of monodisperse organoborane oligomers with alternating tri-
arylamine donor and triarylborane acceptor groups arranged
in a quasi-linear quadrupolar and bent dipolar geometry
(BnNn+1). The alternating electron-donor and -acceptor pair-
ing in these oligomers can be visualized as the linear counter-
part to a p-expanded borazine reported by Jäkle et al.,[23] as
such, the chromophores in this work have been coined as
“linear borazine oligomers”. We provide a comprehensive
study of the electrochemical, one-photon absorption (1PA),
2PA, and excited-state absorption (ESA) properties. Because
the molecules feature strong donor and acceptor groups, inter-
esting changes in the spectra and photophysics are expected
with increased conjugation length. We also examine the sol-
vent effects on the photophysics of the chromophores in order
to gain insights into the charge-transfer (CT) characteristics
and the role of conformational relaxation upon excitation. Fi-
Retrosynthetic analysis of the targeted ambipolar oligomers
(BnNn+1) suggests that chain extension to give larger oligo-
mers should be readily achieved by activation of the trimethyl-
silyl end-capped triarylamine building block with BBr3 as
a boron source in a Si/B exchange reaction, followed by selec-
tive Sn/B exchange using two equivalents of the bifunctional
reagent 1-SiSn (Scheme 1). Subsequent stabilization of the
boron center is accomplished by introducing bulky aryl groups
using the copper reagent TipCu (Tip=2,4,6-triisopropylphenyl)
in refluxing toluene. Standard isolation of the crude samples
followed by purification using preparative size-exclusion
column chromatography on Bio-beads and subsequent recrys-
tallization afforded the spectroscopically pure oligomers as
pale-yellow powdery solids in good yields. In the solid state,
the products are stable in air for several weeks. They are mod-
erately soluble in nonpolar aliphatic hydrocarbons, but well
soluble in chlorinated and aromatic solvents.
The structures of the oligomers were verified by HR MALDI-
MS, which in all cases showed the molecular ion peaks
(Table S1, Supporting Information). Monodisperse GPC traces
(“=1.01) ascertained the high purity of the samples
(Scheme 1). The number average molecular weights (Mn)
match the corresponding calculated values very well, although
they are measured relative to narrow low molecular polystyr-
ene standards (Table S1, Supporting Information). NMR spec-
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Figure 1. Square wave voltammograms for oligomers BnNn+1. a) Reduction
in THF/0.1m Bu4NPF6; b) oxidation in DCM/0.1m Bu4NPF6; recorded vs. the
Fc+/Fc redox couple.
Scheme 1. Chain extension of oligomers BnNn+1 and corresponding GPC
traces.
are consistent with separate one-electron transfer reduction
processes, except that the redox waves partially overlap in the
voltammogram for B4N5 (Figure 1a).
troscopic characterization further confirmed the proposed
structures (Figures S1–S8, Supporting Information). The integral
ratio between the Tip protons of the repeating units at d=
6.96 ppm and the terminal phenylene protons adjacent to the
silyl groups at d=7.40 ppm increases with the elongation of
the conjugated chains. 2D NMR data were acquired for the
representative oligomer B1N2, and Nuclear Overhauser Effect
(NOE) correlations between the aromatic protons and the tBu,
SiMe3, and iPr substituents allow for unequivocal signal assign-
ments. The presence of broad 11B NMR signals at approximate-
ly d=70 ppm is consistent with boron in a tricoordinate envi-
ronment and the number of quadrupole-broadened, boron-
bound carbon atoms in the 13C NMR matches the expected
one. Sharp-singlet peaks at approximately d=À4 ppm in the
29Si NMR spectra correspond to the trimethylsilyl moieties at-
tached to the terminal aryl groups.
In contrast to the reduction waves that are generally well
separated, the oxidation profiles in dichloromethane (DCM) are
more complex.[24] Two reversible oxidation waves are detected
for the D–A–D species B1N2 at E1/2 =0.46 and 0.60 V and they
are assigned to successive oxidation of arylamine donor
groups. All the higher oligomers give rise to three redox waves
with different relative intensities. Based on the square wave
voltammograms in Figure 1b, we assign the first two redox
waves to 2e/1e, 3e/1e, and 3e/2e oxidation processes for
B2N3, B3N4, and B4N5, respectively, resulting in the corre-
sponding cation radical states (Figure S10, Supporting Informa-
tion).[8a,25] A third wave at higher potentials (>0.8 V) is attribut-
ed to further oxidation of the terminal arylamines to dication
states.[26] The imperfect reversibility of the oxidation processes
for the higher oligomers in the CV plots (Figure S9, Supporting
Information) could be due to electrode deposition of the re-
sulting less soluble, highly charged species, or possibly related
to follow-up reactions.[8a]
Electrochemical characterization
Electrochemical measurements allow for insight into the fron-
tier orbital energies of the linear borazine systems and provide
information on the stability of radical ions in solution. The re-
sults of cyclic voltammetry (CV) and square wave voltammetry
(SWV) experiments are summarized in Table S2, Supporting In-
formation. As shown in Figure 1, the first reduction wave for
B1N2 was observed at E1/2 =À2.57 V, and those for the higher
oligomers occur at increasingly less negative potentials. For
the higher oligomers, successive reduction of the individual
boron sites gave rise to multiple waves as a consequence of
electronic interactions between the resulting radical anions in
the conjugated chain.[19] The square-wave voltammetry plots
The electrochemical HOMO and LUMO levels were estimated
from the first square wave potentials for the oxidation and re-
duction processes. The corresponding values are listed in
Table S2 (Supporting Information). As illustrated in Figure 2,
the HOMO levels remain almost unchanged upon chain exten-
sion from B1N2 to B4N5, whereas the LUMO levels decrease
gradually from À2.23 to À2.33 eV, resulting in an overall de-
crease in the HOMO–LUMO gap.
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ies show only very small contri-
butions to the LUMO levels; they
also contribute very little to the
HOMO of B4N5, which is again
consistent with the notion that
the nECL is approached. The satu-
ration of effective conjugation is
further supported by linear ab-
sorption
spectra
discussed
below and is in accordance with
the previous findings for oligo-
mers OBn.[19]
Linear absorption and emission
spectroscopy
To further understand the
charge-transfer and photophysi-
cal characteristics of D–A–D
linear borazine oligomers, we
have investigated the relevant
1PA and emission properties of
all molecules considered in this
study. As shown in Figure 3a,
the 1PA spectra for all BnNn+
Figure 2. Computed FMO plots (B3LYP, 6-31G*, isovalue=0.02 au) and HOMO/LUMO energy levels determined for
BnNn+1 oligomers. The HOMO/LUMO values were estimated from square wave potentials and computational
methods and are shown in black and blue, respectively. The methods for determining the HOMO/LUMO levels are
discussed in the Supporting Information. The units of energy are eV.
Quantum chemical calculations
1 oligomers in hexane are characterized by a dominant, low-
energy ICT absorption band at 390–430 nm, along with two
high-energy bands at ~300 and ~250 nm. The structureless,
low-energy ICT absorption band is assigned to a transition
from the triarylamine to the triarylborane groups.[9b,28] In agree-
ment with this assignment, time-dependent DFT calculations
reveal that for all oligomers the S0!S1 transition is primarily
due to HOMO!LUMO excitation (Table S4, Supporting Infor-
mation). As is common to many series of homologous conju-
gated molecules,[21] the extension of the BnNn+1 oligomers
results in a redshift of the lowest-energy absorption band, an
increase in molar absorptivity, emax, and an increase in energy
gap between the first and second 1PA transitions.
To further understand the electronic structure of these quasi-
linear borazine oligomers, DFT calculations were performed at
the B3LYP/6-31G* level of theory. To minimize the computa-
tional cost, iso-propyl and tert-butyl groups on the phenyl
rings were replaced with H atoms. The validity of this approach
was confirmed for B1N2 and B2N3, which produced overall
similar geometries and orbital distributions with and without
the substituents (Table S3, Figure S11, Supporting Informa-
tion).[27] The calculated orbital energies along with the comput-
ed orbital plots for all the BnNn+1 oligomers are depicted in
Figure 2 with corresponding values listed in Table S3 (Support-
ing Information).
As shown in Figure 2, the calculated HOMOs are localized on
the arylamine p-spacers with contributions from the nitrogen
p-orbitals and remain at the same energy (À4.93 eV) upon
chain extension. Conversely, the LUMOs are localized on the ar-
ylborane moieties with strong contributions from the empty p-
orbitals on boron and gradually decrease in energy from À1.52
to À1.74 eV for B1N2 and B4N5, respectively. This implies that
chain extension with an increasing number of D and A sites
The redshift of the 1PA wavelength maxima, labs(1), from
398 nm for B1N2 to 423 nm for B4N5 in hexane agrees with
the reduction of the HOMO/LUMO energy gap determined by
electrochemical and computational methods (Figure 2, Ta-
bles S2–S4, Supporting Information). The difference of optical
opt
gap
bandgap (DE ) is ~0.13 eV between B1N2 and B2N3 but only
~0.03 eV between B3N4 and B4N5. An exponential fit of the
absorption wavelength maxima versus the number of repeat
units in the oligomers gives an estimate for the extended con-
jugation length of nECL =4 (R2 =0.9999) using the criterion of
Dlꢀ1 for convergence;[21,29] extrapolation to n!1 predicts
stabilizes the LUMOs more than the HOMOs. Consequently, the
DFT
gap
calculated HOMO–LUMO energy gaps (DE ) narrow signifi-
cantly from 3.41 for B1N2 to 3.19 eV for B4N5. The trend of
energy reduction of the LUMOs along with the reduction in
an absorption maximum at l =430 nm for an infinite polymer
1
DFT
gap
DE is consistent with the observations from electrochemical
chain (Figure S13, Supporting Information). A slightly larger
measurements (see Table S2, Supporting Information). They
converge with an increasing number of repeat units, which
suggests that the effective conjugation length (nECL) is reached.
The computed orbital plots reveal important additional in-
formation about the degree of conjugation experienced within
the linear borazine oligomers. The terminal diarylamine moiet-
nECL =5 had been deduced for fluoreneborane oligomers
OBn,[19] while a corresponding data analysis has to our knowl-
edge not been performed for oligoanilines, OAn. As opposed
to the fluorene groups of OBn, the phenylene linkers between
the arylborane and arylamine groups in the borazine oligomers
allow for additional twisting throughout the oligomer unit.
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Two-photon absorption (2PA) spectroscopy
The 2PA properties of the BnNn+1 oligomers were studied by
two-photon excited fluorescence (2PEF) spectroscopy[31] and
2PEF excitation spectra are presented in Figure 4 with corre-
sponding parameters listed in Table 1. As is typical for quadru-
polar molecules, the 2PA maxima of the BnNn+1 oligomers
are blueshifted from the 1PA maximum. All of the oligomers
Table 1. Experimental two-photon spectroscopic parameters of BnNn+
1 oligomers in DCM determined by 2PEF methods.
B1N2
B2N3
B3N4
B4N5
(2)
labs (nm)[a]
732
280
730
700
730
1000
730
1410
dmax (GM)[a]
(2)
[a] Maximum of the two-photon fluorescence excitation spectrum (labs
)
and peak two-photon absorption cross section (dmax); 1 GM
10À50 cm4 s/photon-molecule.
ꢁ
1”
(2)
exhibit an unexpected constant labs at ~730 nm, along with
a weak 2PA peak at ~800 nm, which we attribute to a dipolar
2PA contribution to the 2PA spectrum; we note that the
BnNn+1 oligomers possess a dipole moment that is perpen-
dicular to the chain, which could be responsible for the weak
band at ~800 nm. The change in shape and position of the
2PA band is minimal as the oligomer is extended, resulting in
(2)
(1)
an increased separation of the labs and labs band positions.
The corresponding DFT calculated wavefunctions of the oligo-
mers (see Figure S12, Supporting Information) show significant
electronic localization. For example, in the case of B2N3, the
HOMOÀ1 and LUMO+1 orbitals are localized to the periphery
and the central region of the molecule, respectively, and the
longer oligomers showed a similar trend. It may be that the
rather strong localization of the wavefunctions plays a role in
the essentially fixed wavelength of the strong 2PA band at
~730 nm. The two-photon excited state at ~730 nm contains
prominent wavefunction contributions from the HOMOÀ1!
LUMO for B1N2. For B2N3 and B3N4, there are sizeable contri-
butions to the two-photon excited state from HOMO-1!
LUMO and HOMO!LUMO+1. The maximum 2PA cross sec-
tion (dmax) increases from 280 GM for B1N2 to significantly en-
hanced dmax as the number of monomer units increases, lead-
ing to dmax =1410 GM for B4N5.
As shown in Figure 5, a length dependence of emax and dmax
is observed as both parameters increase linearly with the addi-
tion of monomer units (n). This is consistent with the notion
that d is correlated to the distance over which a charge may
be transferred.[32] However, when considering the length-nor-
malized one- and two-photon absorptivity values (emax/n and
dmax/n) the onset of saturation of the linear and nonlinear ab-
sorption properties are observed at n>2. If we describe the
length dependence of emax and dmax with a simple power law
(see Figure S13c, Supporting Information), we find the expo-
nents to be 0.63 and 1.4, respectively, leading to the following
relations: emax /n0.63 and dmax /n1.4, in which n is the length of
the oligomer. The saturated values for emax and dmax are signifi-
Figure 3. a) Normalized 1PA and b) fluorescence spectra of organoborane
oligomers in hexane (excited at labs(1)).
Photoexcitation of the BnNn+1 oligomers in hexane result-
ed in blue emission with the wavelength of maximum fluores-
cence (lfl) ranging from 420–440 nm and large fluorescence
quantum yields (Ffl) of >0.65 listed in Table S5, Supporting In-
formation. Although somewhat narrow in width, the emission
bands lack significant vibronic structure, suggesting a primarily
planar excited state with a small amount of photoinduced CT
character from the triarylamine groups to the triarylborane
centers. A bathochromic shift in lfl was observed with in-
creased chain length (see Figure 3b). The extension of conju-
gation in the BnNn+1 oligomers has very little effect on Ffl,
whereas the fluorescence lifetimes (tfl) determined in hexane
gradually decrease from ~2.0 ns for B1N2 to ~1.7 ns for B4N5
(Figure S10, Supporting Information). The rate constants for
the radiative (kr) and nonradiative (knr) decays have been esti-
mated from Ffl and tfl[30] and are included in Table S5, Support-
ing Information. Both the kr and knr for the BnNn+1 oligomers
change modestly with length, which is consistent with the pro-
posed planarization of the oligomers in the S1 state, as sug-
gested above.
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Solvatochromic effects in linear
absorption and emission spec-
troscopy
To gain further insight into the
conformation and CT character
of linear borazine oligomers, the
1PA and emission spectra were
obtained in solvents with differ-
ent polarities. While the 1PA of
the BnNn+1 oligomers shows
very little change with solvent,
a strong positive solvatochrom-
ism is observed in the fluores-
cence of each oligomer along
with a significant broadening of
the emission band (Figure S14,
Supporting Information). The
largest changes are observed in
B1N2, in which lfl is redshifted
by ~70 nm comparing hexane to
propylene carbonate (PC). As
shown in Figure 6a,b, the mag-
nitude of the changes in the
emission spectrum with in-
creased solvent polarity is re-
Figure 4. 1PA, fluorescence, and 2PEF excitation (labeled as 2PA) spectra of BnNn+1 oligomers in dichlorome-
thane. The solid line indicates the one-photon absorption spectrum; the 2PEF excitation spectrum is represented
by blue solid squares; the red line is the one-photon fluorescence spectrum (excitation wavelengths are
l=390 nm).
duced for the longer oligomers (i.e., B1N2 vs. B4N5), indicating
a reduction in overall dipole moment as the oligomer is ex-
tended (see also Figure S15, Supporting Information). The re-
sults of solvent-dependent absorption and emission studies of
the oligomers were analyzed using Lippert–Mataga plots in
Figure 6c, in which the Stokes shift (nA–nF) is plotted versus
the solvent polarity, expressed in terms of Df=f(e)-f(n2) with
e as the solvent permittivity and n the refractive index of the
solvent. The reduced solvatochromism, which is reflected in
smaller slopes (Figure 6d), and the narrowing of the emission
band as the oligomer is extended suggest that there is an in-
crease in planarity of the excited state.
In addition to spectral shifts, changes were also observed in
the radiative and nonradiative decay rates from the S1 state of
the linear borazine oligomers in solvents of varying polarity.
The fluorescence decay traces for hexane, toluene, and DCM
are displayed in Figures S16–S18, Supporting Information. As
shown in Table S5, Supporting Information, all oligomers reveal
a substantial increase of tfl in DCM, relative to hexane, the larg-
est of which is observed in B1N2 for which tfl increases two-
fold. A reduction of Ffl is also observed as the solvent polarity
is increased. Reductions in Ffl along with increases in tfl result
in a ~50% decrease in kr for all oligomers, when comparing
hexane to DCM solvents, with the exception of B1N2 which
shows a 70% reduction in kr. The nonradiative decay rate,
however, is unaffected by solvent change. The results gathered
from the steady-state and time-resolved fluorescence measure-
ments suggest that the radiative decay processes are less fa-
vored in polar solvent environments. This may be attributed to
conformational changes in the oligomers as a result of the sta-
bilization of the CT state by the polar solvent.
Figure 5. Length dependence of emax, emax/n, dmax, and dmax/n observed for
the BnNn+1 oligomers. The dotted lines are only meant to guide the eye.
cantly smaller than those reported for heteroaromatic oligo-
mers, such as OTn, for which Thienpont et al. demonstrates
a saturation of the linear polarizability (a) and the second hy-
perpolarizability (g) at n>7.[33] The conformational disorder
and significantly localized wavefunctions are likely responsible
for the relatively small nECL deduced for the BnNn+1 oligo-
mers.
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The representative TA spectra and kinetics recorded with B1N2
and B4N5 in hexane and DCM are depicted in Figure 7, with
additional spectral and kinetic results of all BnNn+1 oligomers
dissolved in hexane, toluene, and DCM displayed in the Sup-
porting Information (Figures S19 and S20).
All linear borazine TA spectra show bleaching in the 1PA
band region due to the depletion of ground-state electrons,
which is also referred to as ground-state bleach (GSB). Nega-
tive DOD signals to the red of the 1PA band coincide with
photoluminescence and were determined to be a result of
stimulated emission (SE). The evolution of the TA spectra in
this region shows a slight but continuous redshift (~7 nm) of
the SE band as the time delay increases, which can be inter-
preted as solvent relaxation from the nonplanar Franck–
Condon state to the more planar equilibrium region of the S1
state of the oligomer (see Figure S21, Supporting Information).
Strong, broadband ESA over a range of 1150 nm in the visi-
ble and NIR spectral region (ca. 450–1600 nm) is observed for
all BnNn+1 oligomers. The ESA band also overlaps strongly
with the GSB and SE of the chromophores. Comparing the ESA
of B1N2 to B4N5 in hexane (Figure 7a to b), the absorbance
maxima at 600 and 980 nm lose their structure due to the
broader emission spectrum as the oligomer is extended. Addi-
tionally, the NIR absorbance band maxima redshifts ~20 nm as
the oligomer is extended. B1N2 shows spectral shifts of the
ESA from 1000 to 900 nm over a few picoseconds whereas the
extended oligomer exhibits a constant band structure at
900 nm throughout the entire temporal range. Such spectral
shifts may be a result of the twisted excited-state structure of
B1N2, which undergoes planarization at a slower rate com-
pared to other oligomers.
Interestingly, B4N5 shows an ESA band at 1500 nm that is
absent in the TA of the shorter oligomer in hexane. This long-
wavelength (1550 nm) ESA band is again observed in the TA of
all BnNn+1 oligomers dissolved in DCM with significantly in-
creased absorbance (see Figure 7c, d). In addition, an ESA peak
~700 nm was observed in all BnNn+1 oligomers dissolved in
DCM, but not hexane or toluene. These additional ESA bands
are attributed to radical ion generation. This hypothesis is sup-
ported by chemical oxidation of B1N2 at room temperature
under a N2 atmosphere, using a Ag salt with a weakly coordi-
nating anion, Ag[(Al(OC(CF3)3)4][34] (see the Supporting Informa-
tion). Spectroscopic changes in the 1PA upon chemical oxida-
tion of B1N2 are shown in Figure 8. Upon the addition of one
equivalent of the Ag salt, an intense absorption band at
695 nm appeared, along with a broad 1PA band in the NIR
with a maximum at ~1550 nm. The 695 nm band is in a similar
range to those reported for monomeric triarylamine radical
cations,[26a] while absorptions at wavelengths >1200 nm have
been observed in larger conjugated arylamine oligomers[35]
and assigned to intervalence (IV) transitions between the po-
larons and redox centers. Additionally, the initial absorption
band of B1N2 at 400 nm decreased slightly and split into two
bands at 372 and 404 nm. All these observations can be as-
cribed to the formation of the mono-radical cation, B1N2·+.
Further oxidation of B1N2 by the addition of two equiva-
lents of the Ag salt resulted in the complete disappearance of
Figure 6. a,b) Fluorescence spectra for B1N2 and B4N5 in solvents of differ-
ent polarity (hexanes, toluene, CHCl3, DCM, PC=propylene carbonate; excit-
ed at labs(1)). c) Lippert–Mataga plots for different oligomers. d) Plot of Lip-
pert–Mataga slopes vs. the number of repeating units, n, in oligomers
BnNn+1 (values for toluene are omitted).
Solvent-dependent excited-state absorption (ESA) spectros-
copy and kinetics
To understand the photophysical changes in ESA due to the
length of the BnNn+1 oligomers and their degree of charge-
transfer character, solvent-dependent transient absorption (TA)
spectroscopy was performed following excitation at 415 nm.
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Figure 7. Femtosecond TA spectra of B1N2 and B4N5 in various solvents. a) B1N2 in hexane, b) B4N5 in hexane, c) B1N2 in DCM, d) B4N5 in DCM. The sam-
ples were prepared at concentrations of 10À5 m and excited at 415 nm at 500 nJ/pulse.
band would suggest an increase in concentration of the delo-
calized polarons.
Comparison of the solvent-dependent TA of the BnNn+
+
1 oligomers with the 1PA of B1N2 and B1N22+ shows that
C
the ESA bands observed are well described by the existence of
radical cations and higher oxidized species. The ESA band at
approximately 900 nm, which was not observed in the 1PA of
+
B1N2 or B1N22+, is attributed to the generation of boron-
C
centered radical anions by comparison with the reported value
of labs =872 nm for the electrochemically generated species
CÀ
Mes2BPh (Mes=2,4,6-trimethylphenyl).[36] In hexane, the IV
band at ~1550 nm is not observed for B1N2 but is seen in the
TA spectra of the longer oligomers, as there are more redox
sites in the larger conjugated systems (see Figure 7a, b). In
DCM, the overlap of SE with the ESA of the oligomers pre-
cludes the observation of the short wavelength polaron
bands; however, prominent ESA can be seen at ca. 700 and
900 nm (see Figure 7c, d). The IV band is present for all oligo-
mers in DCM, and appears to be the strongest in B1N2 and
B2N3, for which the ESA in this region saturated the detector.
This undoubtedly is due to the delocalization experienced in
the shorter oligomers, whereas the longer oligomers are more
twisted localizing the polaron and decreasing the IV charge
transfer experienced across the molecule.
Figure 8. 1PA spectra of B1N2 before and after preparative oxidation with
one or two equivalents of Ag[(Al(OC(CF3)3)4] in DCM under a N2 atmosphere.
the initial absorption band, the appearance of additional 1PA
bands at 499 and 793 nm, and an increase in amplitude and
slight blueshift of the low energy band in the NIR. Again, the
1PA bands in the visible spectral region coincide with those
seen in larger conjugated arylamine systems,[35b] for which the
band at ~500 nm could be due to double oxidation[26a] at one
of the nitrogen atoms. The increase in absorbance of the IV
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Anion binding behavior
Conclusion
Finally, the response of the oligomers to cyanide anions was
investigated by monitoring titration experiments by UV/Vis
and fluorescence spectroscopy. Free tricoordinate borane ac-
ceptor sites allow for effective charge transfer from the N
donor as discussed above. However, complexation with fluo-
ride or cyanide results in a highly electron-rich, negatively
charged borate, thereby disrupting the charge transfer from N
to B.[23] Stepwise addition of [Bu4N]CN to a solution of B1N2 in
toluene led to a gradual attenuation of both the absorption
and emission bands (Figure 9a). Addition of about one equiva-
lent CNÀ resulted in complete emission quenching. In contrast,
treatment of the higher oligomers B2N3, B3N4, and B4N5
with one equivalent CNÀ led to a distinct long wavelength tail-
ing of the emission band (Figure 9b, see also Figures S22–S26,
Supporting Information). We attribute the longer wavelength
emission to charge transfer from the highly electron-rich
borate moiety to the electron-deficient remaining tricoordinate
borane acceptor moiety. For example, the D–A–D–A–D species
B2N3 is initially converted to the negatively charged borate
D–D*–D–A–D upon cyanide complexation. In the presence of
a larger excess of cyanide the visible absorption and emission
features completely disappear, as expected upon generation of
the fully complexed D–D*–D–D*–D species. The binding con-
stants for these processes are very high (log K>7) and no evi-
dence of cooperative binding effects was found, that is, the in-
dividual borane moieties act as independent Lewis acid sites.
The latter is in contrast to systems in which borane moieties
are directly linked together by aromatic units.[37]
We have explored the electrochemical, photophysical, 1PA,
and 2PA characteristics of a family of linear organoborane olig-
omers that feature a quadrupolar or bent-dipolar structure.
The 1PA and emission spectra indicate significant extension of
conjugation as evidenced by bathochromic shifts upon chain
elongation. However, rapid saturation is observed for the
longer oligomers, indicating that there is a large degree of tor-
sion in the ground-state conformation of the BnNn+1 oligo-
mers. These experimental findings are supported by DFT calcu-
lations, which for the longer oligomers reveal localization of
the frontier orbital wavefunctions on the central portions or in
the periphery of the molecules. In contrast to the twisted
ground-state geometry, the lowest excited state appears to
adopt a much more planar conformation, especially for the
longer oligomers. These oligomers exhibit large two-photon
cross sections and a 2PA band that extends from the visible to
the near-IR region. Broadband singlet–singlet, radical cation,
and radical anion ESA from the oligomers was also observed.
Solvatochromic measurements revealed that the optical
properties of the linear borazine dyes are highly sensitive to
changes in solvent, especially in the emission properties of the
dyes. Most notably, the shorter oligomers possess strong ICT
character, resulting in a substantial redshift in the emission
band going from nonpolar to polar medium. The solvatochro-
mic emission effects diminish with increasing oligomer chain
length, causing an unexpected hypsochromic shift of the maxi-
mum emission wavelength with extension of conjugation in
polar solvents. Additional solvation-induced changes are ob-
served in the excited-state pho-
tophysics of the oligomers. The
BnNn+1 oligomers also showed
sensitivity to cyanide anions
with large changes observed in
the 1PA and emission character-
istics of the molecules upon
complexation to boron.
The sizable two-photon ab-
sorptivity of these organoborane
chromophores, along with their
unique optical and material
properties, would suggest that
this family of oligomers offer po-
tential utility for several NLO ap-
plications. For example, the large
d values coupled with high Ffl
indicates the BnNn+1 oligomers
are of interest for two-photon
sensing applications. In addition,
because of their anion binding
behavior, there is potential for
developing NLO molecular sen-
sors with enhanced sensitivity.
The overlap of the 2PA bands
with the ESA and radical cation
and anion absorption bands of
Figure 9. 1PA and fluorescence spectra corresponding to the titration of a) B1N2 and b) B2N3 with [Bu4N]CN in
toluene solution.
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power limiting applications by two-photon processes. Finally,
given the fluorescence and redox properties of the BnNn+
1 oligomers, they may also be of utility in 3D fluorescence mi-
croscopy and 3D microfabrication applications.
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Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grants No. CHE-1362460 and CHE-
1112195 (FJ), the U.S. Air Force Office of Scientific Research, Bi-
oPAINTS MURI Program (Grant No. FA9550-09-0669), the NSF
PREM (Grant No. DMR-0934212) and the DARPA ZOE Program
(Grant No. W31P4Q-09-1-0012) (J.W.P.). We thank J. Chen for
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Keywords: borane
·
conjugation
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donor–acceptor
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fluorescence · two-photon absorption
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Received: June 10, 2015
Published online on October 30, 2015
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