Modulation of charge transfer by: N -alkylation to control photoluminescence energy and quantum yield

Article abstract of DOI:10.1039/d0sc02460k

Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimental, e.g., mediating emission quenching. N-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. N-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine's two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an N-alkylated analogue of an 'unnatural' protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This N-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

Full text of DOI:10.1039/d0sc02460k

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DOI: 10.1039/D0SC02460K
Modulation of Charge Transfer by N-Alkylation to Control
Photoluminescence Energy and Quantum Yield
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
b
a
Andrew T. Turley, Andrew Danos, Antonio Prlj, Andrew P. Monkman, Basile F. E. Curchod,
a
b,c
Paul R. McGonigal, * and Marc K. Etherington *
DOI: 10.1039/x0xx00000x
Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g.,
facilitating thermally activated delayed fluorescence, or detrimental, e.g., mediating emission quenching. N-Alkylation is
shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine
chromophores. We demonstrate this concept using quinine as a model. N-Alkylation causes changes in its emission that
mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine’s two N sites is
performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize
an N-alkylated analog of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is
inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the
associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard.
This N-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.
Here, we utilize quinine (Qn) as a model system to
demonstrate a strategy for modifying CT states of organic
compounds. Recent literature has shown that some of the
Introduction
‘
Your “epipolic” dispersion has given me the clue to a most
cinchona alkaloids display CT and proton transfer, which in
extensive field of research, which has occupied me during the
last year when sunlight permitted’ wrote Sir George Stokes to
Sir John Herschel on April 6, 1852. This prediction of an
extensive field of research has held true through the continued
use of spectroscopy to study epipolic dispersion that, also
thanks to Stokes, we now know as fluorescence. In recent
years, fluorescence spectroscopy has been used to
characterize a wide range of different compounds and uncover
new functional phenomena such as thermally activated
2
1,22
turn dictate their photophysical properties.
These studies
have previously been performed using protonation equilibria
in aqueous media. Here, we use N-alkylation as an alternative
in order to permanently modify Qn, allowing investigation into
the structure–property relationships between lone-pair
availability, charge, emission energy and quantum yield. We
demonstrate that N-alkylation is a versatile method for
studying the effects of amine quaternization in aqueous and
organic solvent media, not requiring acidic conditions or being
subject to equilibria. The kinetic stabilities of the N-alkylated
salts also make it possible to quaternize structures at positions
other than their most basic N site. We identify one N site of Qn
that can be manipulated to ‘turn off’ CT state formation and
another whose modification tunes emission colour. The
compound produced by double N-alkylation displays enhanced
photoluminescence quantum yields (PLQYs) and solubilities
across a range of solvents compared to acidified quinine
1
2
–7
delayed fluorescence (TADF),
aggregation induced
emission8 and room temperature phosphorescence.
–10
11–13
TADF is of particular interest as it increases the efficiency of
organic light-emitting diodes (OLEDs) used in displays and
devices. The TADF process is contingent on the formation of
charge-transfer (CT) states, i.e., the spatial redistribution of
electron density in the excited state. Controlling and tuning CT
states is crucial to the development of high efficiency TADF
materials.14 This redistribution can result in reduced overlap
between the highest occupied molecular orbital (HOMO) and
2 4
sulphate (H Qn·SO ), which is a common PLQY standard for
characterizing blue emitters. Overall, this N-alkylation
approach represents a simple, robust pathway for tuning the
emission and functional properties of Qn and other tertiary
4
,15
lowest unoccupied molecular orbital (LUMO). CT states also
have a significant impact on photochemistry and are
commonplace in natural chromophores – for example, they
2
3–26
amines
to impart functional properties such as improved
1
6–18
19,20
have been invoked in photosynthesis
and DNA repair
PLQYs and, potentially, TADF emission.
mechanisms. CT states can contribute to enhancing light
emission in organic compounds through mechanisms such as
TADF, but their presence is not always desirable. Identification
of their behaviour with respect to the electronic energy levels
can guide material design.
Results and Discussion
Qn fluoresces blue with a high PLQY in acidified water (55% in
0
2 4
.1 ᴍ aqueous H SO ), but is only weakly emissive at UV
2
7
wavelengths in basic solution. Recent work has shown that a
similar pH-controlled increase in the PLQY of
dehydroquinidine, an analogue of Qn, is linked to the
availability of the quinuclidine lone pair electrons. It has been
proposed that formation of a through-space CT state between
the quinuclidine N and the quinoline chromophore leads to
a.
Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK.
Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK.
Department of Mathematics, Physics and Electrical Engineering, Northumbria
b.
c.
2
1
University, Ellison Place, NE1 8ST, UK.
†
1
Electronic Supplementary Information (ESI) available: For ESI see DOI:
0.1039/x0xx00000x. CCDC 1985987 (MeQn·BF4) and 1985988 (Me Qn·2BF4)
2
contain the supplementary crystallographic data for this paper.
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Journal Name
quenching of emission. This CT loss pathway is suppressed
when the quinuclidine N is protonated, which increases the
PLQY. We reasoned that N-alkylation of Qn could modulate its
CT state in a similar manner and that investigation of
N-alkylated derivatives would allow us to delineate the
separate effects of quaternizing each of the two N sites (N1
and N2, Scheme 1) independently of one another,
circumventing the intrinsic limitations of using protonation for
this task.
View Article Online
DOI: 10.1039/D0SC02460K
We synthesized three salts of methylated Qn (Scheme 1).
Synthetic procedures and NMR spectroscopic characterisation
can be found in Scheme S1 and Figs. S1–S13 in the ESI†.
Selective N-methylation of the quinuclidine N (N1) is achieved
2
8
+
by treating Qn with MeI at room temperature, giving MeQn ,
2
+
whereas a reaction temperature of 100 °C leads to Me
N-methylation of both N1 and the quinoline N (N2).
2
Qn by
2
9,30
This
selectivity follows the known basicity trend of N1 and
2
1,31,32
N2.
Methylation at each site gives characteristic changes
1
in H NMR chemical shifts (Figs. S14 and S15†) and is
confirmed by X-ray diffraction (XRD) analysis of single crystals
(Scheme 1).‡ By employing an allyl protecting group at N1
(
Scheme S1†), however, it is also possible to prepare an
+
isomeric form of MeQn that would be inaccessible using a
thermodynamically controlled quaternization approach, such
as reversible protonation under acidic conditions. After
methylation at N2, the allyl protecting group at N1 is removed
+
to give the kinetically stable ion iMeQn . Each of the cations
Fig. 1 (a) Steady-state emission spectra of Qn dissolved in a series of solvents and
excited at 3.75 eV, showing the LE and CT character of its emission. (b) The absorption
were isolated with halide counterions before exchanging to
BF salts by metathesis with AgBF to avoid heavy-nucleus ions
4 4
4
(solid lines) and emission (dashed lines) spectra of Qn (purple), MeQn·BF (green),
with large spin-orbit couplings that might complicate our Me Qn·2BF (blue), and iMeQn·BF (orange) in MeCN (20 µᴍ). A significant shift in the
2
4
4
absorption spectra is associated with the methylation of N2. The emission spectrum of
iMeQn·BF in MeCN is omitted as partial protonation of N1 by adventitious water
affects the spectrum. Instead, the absorption and emission spectra (black) of a MeCN
solution of iMeQn·BF (20 μᴍ) with Et N (10 mᴍ) are displayed to show there is no
significant change in absorption with the addition of a base and that there is no
emission in the visible region without protonation of N1. The excitation energies (Eex
used were 4.13 eV for Qn and MeQn·BF or 3.54 eV for Me Qn·2BF and iMeQn·BF to
allow comparison of PLQYs with known standards (Figs. S18–S26 and Table S2†).
photophysical investigations by quenching the singlet
emission.33
4
4
3
)
4
2
4
4
The photophysical properties we have measured and modelled
3
4
for Qn and its salts are summarized in Table 1. We first
recorded (Fig. 1a) emission spectra of Qn dissolved in a series
of solvents with a range of polarities. Although the absolute
emission intensities from Qn in polar organic solvents are low
on account of its near-zero PLQY, we could identify distinct
emissions from a locally excited (LE) and a CT state.
A peak in the UV region with emission energy, Eem, of 3.4 eV is
present in low polarity solvent (PhMe), consistent with
emission from an LE state. An additional peak is present in the
visible region between 2.2 and 2.6 eV in more polar media. As
the polarity is increased when moving from chlorinated
Scheme 1. N-Alkylation of Qn to N-methylquinium tetrafluoroborate, MeQn·BF
dimethylquinium bis(tetrafluoroborate), Me Qn·2BF , and iso-N-methylquinium tetra-
fluoroborate, iMeQn·BF . XRD structures of MeQn·BF and Me Qn·2BF are shown in
stick representation with N atoms as balls. Further XRD structures can be found in Figs.
S16 and S17 and Table S1.† Reagents and conditions: i) a. MeI, rt, 3 d, b. AgBF
4
, N,N’-
2
4
4
4
2
4
, MeCN, solvents to MeCN, the increasing relative intensity of this
, MeOH, rt, 10 min, second peak and its further bathochromic shift are indicative
4
6
0 °C, 10 min, 74% over 2 steps; ii) MeI, MeCN, 100 °C, 4 h, b. AgBF
9% over 2 steps; iii) a. allyl bromide, CH Cl , rt, 16 h, b. MeI, MeCN, 100 °C, 3 h, c.
(5 mol%), Me2SO, 40 °C, 16 h, d. diisopropylaminomethyl
polystyrene, MeOH, rt, 1 h, e. AgBF , MeOH, rt, 10 min, 79% over 5 steps.
4
7
2
2
35–37
of emission from a CT state.
The emission spectra are
3 4
barbituric acid, Pd(PPh )
normalized relative to the peak of the LE (which has reduced
intensity in polar solvents) in order to better demonstrate the
solvatochromic shift. The intensities of the spectra alone do
4
2
| J. Name., 2012, 00, 1-3
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not illustrate the absolute populations of the states, but the
changes in intensity
View Article Online
DOI: 10.1039/D0SC02460K
Table 1. Photophysical properties of Qn and its methylated derivatives.
a
d
Φ / %
E
em / eV
Calculated Eem / eV Presence
LE
Red-Shifted
Absorption and
b
of CTe
Compound
MeCNb
H
2
O
MeCN
2
H O
CT
Emissionf
No
No
Yes
Qn
0
5
63
0c
22
32
70
60
3.45 (2.30) 3.20
3.75
3.69
3.00
-
1.41
-
-
Yes
No
No
Yes
MeQn·BF
Me Qn·2BF
iMeQn·BF
4
3.40
2.75
-
3.24
2.75
2.75
2
4
4
1.01
Yes
a
(Φ = 60%)38 and those of Me
Qn·2BF and
2 4
PLQYs (Φ) for Qn and MeQn·BF
4
were measured with respect to a standard of 2-aminopyridine in 0.1 ᴍ aqueous H
2
SO
4
27 b
iMeQn·BF
4
were measured with respect to a standard of H
2
Qn·SO
4
in 0.1 ᴍ aqueous H
2
SO
4
(Φ = 55%).
Anhydrous MeCN was used throughout the spectroscopic
c
d
study. Anhydrous MeCN solution with 10 mᴍ Et
3
N used to suppress the formation of trace amounts of N1 protonated species resulting from adventitious water.
Excited-state energies in eV for protonated (rather than methylated) compounds calculated at the LR-TDDFT/ωB97X-D/6-31G* level of theory with state-specific
e
implicit solvation (MeCN). The presence or lack of an accessible CT state was determined by experimental observation (Fig. 1) and/or theoretical calculation (Tables
f
S5 and S6†). Relative to the absorption and emission of Qn.
can be rationalised. In apolar solvents there is a higher PLQY of 5% in MeCN, compared to the near-zero PLQY of
population of molecules emitting from the LE state, having a neutral Qn in MeCN. Quaternization at N1 prevents formation
high oscillator strength. In polar solvents an important of a CT excited state, removing one of the pathways for rapid
population of molecules is now emitting from the CT state, but nonradiative decay and, as a result, enhancing the PLQY.
the low oscillator strength results in a similar magnitude of Similarly, CT state formation is also suppressed for neutral H
2
O
emission relative to the LE state. Linear-response time- solutions of Qn, in which the major species at equilibrium is
3
1
dependent density functional theory (LR-TDDFT) calculations monoprotonated HQn+, giving a PLQY of 22%. The PLQY of
performed at the ωB97X-D/6-31G* level of theory with state- MeQn·BF in H O solution is 32%.
4
2
specific implicit solvation (see ESI for full all computational Methylation of N2 has a markedly different effect on the
details and benchmarking) support the assignment of these LE photoluminescence properties. We first investigated this effect
and CT bands of Qn (Fig. 2). The CT state, in this case, is a by quaternizing both N1 and N2. The doubly methylated salt
result of a through-space charge transfer between the
quinuclidine and quinoline system.
The calculations predict (i) UV emission associated with an
electronic transition localized on the quinoline ring system
(π→π*) and (ii) a lower energy emission from a CT excited
state, which is associated with an electronic transition from a
nonbonding orbital located on N1 to an unoccupied π orbital
on the quinoline ring (n→π*). We ascribe Qn’s extremely low
PLQY (Table 1) to the formation of this through-space CT state.
Rapid nonradiative decay from the low-energy CT state,
possibly through the low-lying LE triplet state (Fig. S27†),
3
9
serves as a loss pathway that suppresses photoluminescence.
To further confirm this hypothesis and to ‘switch on’ the UV
emission of Qn, we investigated the effect of alkylating N1
selectively. Pleasingly, we observed (Fig. 1b) only a single peak
in the emission spectrum of MeQn·BF at 3.4 eV in MeCN,
4
indicative of emission from the π→π* LE state of the quinoline
ring system. Compared to Qn, the peak wavelengths observed
in the absorption and emission spectra of MeQn·BF
largely unchanged. However, MeQn·BF shows no evidence of
the lower energy emission from a CT state found for Qn.
Consequently, MeQn·BF emits (Table 1) with an increased
4
are
4
4
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Me Qn·2BF
Journal Name
2
4
exhibits (Fig. 1b) significant bathochromic shifts in variation in PLQY may be a result of modulating ion-pairing
View Article Online
DOI: 10.1039/D0SC02460K
its absorption and emission energies relative to Qn and
between the fluorescent cation and its counterion. Previous
investigations of cationic fluorophores have also shown
solvent-dependent PLQY and have suggested interactions with
the counterion can provide radiationless decay pathways, e.g.,
Fig. 2 Natural transition orbitals (NTOs) and energies characterizing the singlet emission
+
+
2+
of Qn from its LE and CT states, as well as MeQn , iMeQn and Me
the LR-TDDFT/ωB97X-D/6-31G* level of theory with state-specific implicit solvation.
Indicates that protonated structures were used as electronically similar (Table S7)
models for the methylated salts.
2
Qn , calculated at
4
0,41
through electron transfer and heavy atom effects.
Overall,
however, simple counterion exchange can be used to tune the
solubility profile and emission properties. Work is ongoing in
our laboratories to optimize these materials as PLQY standards
¶
MeQn·BF
TDDFT (Table 1) and algebraic diagrammatic construction with solvent.
ADC(2)) calculations (Tables S3 and S4†). Like MeQn·BF
emission from Me Qn·2BF (Fig. 2) comes exclusively from the
4
. These shifts are reproduced qualitatively by our LR- and provide physical insight into the empirical changes in PLQY
(
4
,
2
4
π→π* LE state. However, the transition is brought into the Conclusions
blue region (2.8 eV) as a result of quaternizing N2.
Extrapolating from our observations based on MeQn·BF
In summary, we have demonstrated an operationally simple
method to tune and elucidate the CT excited states of emissive
organic materials by N-alkylation. In the present investigation,
this approach has allowed us to systematically toggle on and
off the presence of a CT excited state, to change the emission
colour, and to improve the PLQY of Qn – a compound whose
4
and
Me Qn·2BF , we would expect that selective quaternization of
2
4
only N2—the less basic N site—would give rise to an ion that
retains the CT character of Qn on account of the available
quinuclidine lone pair electrons (N1), but whose emission and
absorption are red-shifted. The LR-TDDFT calculations fit with
this hypothesis, predicting (Fig. 2) formation of a new low-
energy CT state at lower electronic energy. Moreover, the LR-
TDDFT indicates that, upon relaxation of the excited state, the
CT state becomes the primary (singlet) ‘emissive’ state. The
selectivity and kinetic stability of the N-alkylation approach
allows us to test this hypothesis experimentally using
photophysical properties were first studied by Herschel and
42–44
Stokes over 150 years ago.
This approach can be applied
broadly to N-heterocycles, which are pervasive structural
motifs in many organic chromophores. Resulting insights into
their CT excited states will progress our understanding of
16–18,45
natural photoactive systems
performances of emissive
and improve the
2–7,39
and light-absorbing46–48 devices.
iMeQn·BF
As predicted, the absorption spectrum of iMeQn·BF
matches closely (Fig. 1b) the spectrum of Me
Photoluminescence measurements, which were carried out in
MeCN with 10 mᴍ triethylamine (Et N) to prevent protonation
4
.
The kinetic stabilities of the N-alkylated compounds open up
the possibility of quaternizing the chromophores selectively at
sites other than their most basic N site, achieving structures
that are inaccessible by protonation, while the choice of
counterion can serve as a handle to control physical
properties.
4
in MeCN
Qn·2BF
2
4
.
3
of N1 by adventitious water, are also consistent. Unlike the
other methylated derivatives, there is no detectable
4
photoluminescence from iMeQn·BF . Population of the
extremely low-energy CT excited state predicted by calculation Conflicts of interest
appears to lead solely to non-radiative decay. Overall,
There are no conflicts to declare.
therefore, selective alkylation of different sites can be used to
tune the photophysical behaviour of Qn, modulating
independent properties in an orthogonal manner. It is a useful
approach to control CT and emission energy. However, the
kinetic stabilities and modified solubility profiles of the Qn
salts also give them advantages over protonated analogues for
use as functional materials. For example, while a doubly
Acknowledgements
We thank The Royal Society for access to their archives to
retrieve the introductory quote, which was transcribed by Melody
Bishop. We thank Dr Dmitry Yufit for solving single crystal XRD
structures. ATT acknowledges an EPSRC DTG. AD, APM and
MKE acknowledge the HyperOLED Horizon 2020 project no.
2 4
protonated Qn salt, H Qn·SO , is used routinely as a
fluorescence standard for relative PLQY measurements in the
blue region,[ its use is limited to acidic aqueous solutions – it
is insoluble in common organic solvents and its
photoluminescence is quenched in the aggregated state. The
732013. MKE thanks the Hatfield Trust. This work made use of
10]
the facilities of the Hamilton HPC Service of Durham University.
2 4
physical properties of Me Qn·2BF , on the other hand, make it
appealing as a more versatile PLQY standard.
Our measurements show that it exhibits an enhanced PLQY of
Notes and references
‡
4 2 4
CCDC 1985987 (MeQn·BF ) and 1985988 (Me Qn·2BF )
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
7
0
0% in neutral H
.1 ᴍ aqueous H
2 2 4
O compared to the 55% PLQY of H Qn·SO in
2
7
2
SO
4
2 4
and, unlike H Qn·SO , it is soluble in Crystallographic Data Centre via www.ccdc.cam.ac.uk/
2 2 2
organic solvents such as MeCN, EtOH, Me CO, CH Cl , and
EtOAc, giving rise to useful PLQYs (Figs. S18–S26 and
Table S2†) of 63%, 43%, 54%, 48% and 13%, respectively. The
1
G. G. Stokes, Sir George Gabriel Stokes to Sir John Frederick
William Herschel, 6th April 1852., The Royal Society
4
| J. Name., 2012, 00, 1-3
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Chemical Science
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DOI: 10.1039/D0SC02460K
A versatile N-alkylation strategy controls the presence of charge-transfer excited states and the
emission colour of N-heterocyclic chromophores.