Tuning the optoelectronic properties of core-substituted naphthalene diimides by the selective conversion of imides to monothioimides

Article abstract of DOI:10.1039/c5ra05920h

Selective sulfur substitution of the distal carbonyls of a core-substituted naphthalene diimide was obtained when a combination of core and imide substituents were used. The substituents appear to inhibit thionation of the proximal carbonyl by steric hindrance. Each thionation caused a 50 nm bathochromic shift of the visible absorption band and an anodic shift of the reduction potentials. The dithionated compound has a λ_max in the near-IR at 733 nm and an optical gap of 1.59 eV, which is unusually low for this type of molecule. Thionation of carbonyls offers a useful avenue for tuning optoelectronic properties of NDI-based materials.

Full text of DOI:10.1039/c5ra05920h

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Tuning the optoelectronic properties of core-
substituted naphthalene diimides by the selective
conversion of imides to monothioimides†
Cite this: RSC Adv., 2015, 5, 46534
a
a
b
c
a
*
F. S. Etheridge, R. Fernando, J. A. Golen, A. L. Rheingold and G. Sauve
Selective sulfur substitution of the distal carbonyls of a core-substituted naphthalene diimide was obtained
when a combination of core and imide substituents were used. The substituents appear to inhibit thionation
of the proximal carbonyl by steric hindrance. Each thionation caused a 50 nm bathochromic shift of the
visible absorption band and an anodic shift of the reduction potentials. The dithionated compound has a
lmax in the near-IR at 733 nm and an optical gap of 1.59 eV, which is unusually low for this type of
molecule. Thionation of carbonyls offers a useful avenue for tuning optoelectronic properties of NDI-
based materials.
Received 2nd April 2015
Accepted 19th May 2015
DOI: 10.1039/c5ra05920h
www.rsc.org/advances
Thionation of NDI was reported in a patent by Facchetti and
coworkers.8,9 NDI with cyclohexyl imide substituents gave a
Introduction
mixture of cis and trans isomers of dithionated NDIs, whereas
NDI with 2-ethylhexyl imide substituents gave a mixture of
monothionated, cis and trans isomers of the dithionated NDI
and a small amount of trithionated NDI. Orzeszko and co-
workers investigated the thionation reaction of various cyclic
imides using the Lawesson's reagent.11 They reported that steric
hindrance near the carbonyl inhibits the replacement of the
oxygen atom by sulfur. It follows that selective synthesis of thi-
oimides should be possible by selecting the right substituents.
2,6-Dialkylamino core-substituted NDI derivatives (Fig. 1)
offer a promising substrate to further examine thionation
chemistry of NDI-based molecules. These molecules have
strong absorption in the visible range (610 nm), strong emission
(630 nm) and reversible reductions and oxidations. We have
previously synthesized 2,6-dialkylamino core substituted NDI
derivatives with various core alkylamino and imide substituents
and studied their structure-property studies in organic solar
cells.12,13 The alkylamino core substituents not only tune the
optoelectronic properties of NDI due to their electron donating
character, but they also offer a way to add alkyl chains to
Naphthalene diimides (NDI) have attracted attention due to
their synthetic versatility, tunable optical properties, high
electron affinity, p-acidic aromatic properties and good electron
transport properties.1–3 NDI is the smallest member of the ary-
lene diimide family, with a planar naphthalene aromatic core
and two imide groups. Colorful NDI dyes can be made via core
substitution with electron donating groups such as alkylamino
to create push–pull systems, or with conjugated moieties such
as thiophene to extend the conjugation system.1–3
The use of thionating reagents to replace the oxygen atom of
carbonyls with sulfur, thus changing them into their thio-
carbonyl derivatives, is common for biological applications.4
Thionation of carbonyls in aromatic molecules is known to
cause a bathochromic shi of the absorption spectrum. For
example, thionation of guanine results in a red-shi of the
absorption spectra by ꢀ40 nm.5 Thionation of carbonyls in
conjugated molecules for use as liquid crystals or organic
semiconductors have been reported, including for phthali-
mides,6 diketopyrrolopyrroles and thienopyrroledione,7 and
arylene diimides.8–10 While thiocarbonyl compounds tend to be
unstable, thioimide compounds are stable.6
aDepartment of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106,
USA. E-mail: genevieve.sauve@case.edu
bDepartment of Chemistry and Biochemistry, UMass Dartmouth, Dartmouth, MA
02747, USA
cDepartment of Chemistry and Biochemistry, University of California at San Diego, La
Jolla, CA 92093, USA
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, MALDI-TOF MS spectra, XRD of RF2-1S, and computational data.
CCDC 1056142. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5ra05920h
Fig. 1 2,6-Dialkylamino core-substituted NDI.
46534 | RSC Adv., 2015, 5, 46534–46539
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Scheme 1 Synthesis of thionated RF2 derivatives using Lawesson's
reagent.
increase solubility in organic solvents. The hydrogen on the
secondary amine group also hydrogen bonds with the nearby
carbonyl, which could prevent thionation of the proximal
carbonyl, potentially offering selective thionation of the distal
carbonyls.
Fig. 2 1H NMR (400 MHz) spectra of RF2, RF2-1S and RF2-2S in
CDCl3.
Here, we show the selective thionation of N,N0-di((thiophene-2-
yl)methyl)-2,6-bis(N-2-ethylhexyl)-amino-1,4,5,8-naphthalenetetra-
carboxydiimide (RF2, Scheme 1) to the monothioimide deriva-
tives. This selective thionation allowed us to tune the optical
and electrochemical properties of RF2 systematically, and
obtain new NDI-based molecules that combine absorption in
the near-IR with high electron affinity.
(iii) due to the methylene protons of the imide substituents. The
RF2-1S spectrum shows a second set of proton signals shied
downeld, corresponding to the amine (i), core (ii) and
methylene (iii) protons, due to the loss of symmetry. This
unique NMR pattern suggests that we have only one isomer of
RF2-1S, meaning that one of the two carbonyls is preferentially
thionated. The RF2-2S spectrum shows only one set of proton
signals, which are shied downeld compared to those of RF2.
This indicates that RF2-2S is a symmetrical molecule, and that
we have only one isomer.
Results and discussion
Thionation of RF2 was performed using 4 equivalents of Law-
To determine which carbonyl was thionated, we looked at
esson's reagent in reuxing xylenes (Scheme 1). The main the gHMBC NMR of RF2 and RF2-2S (Fig. 3, and S7, S8†). For
products were RF2-1S and RF2-2S, with very small amounts of RF2 (Fig. 3a), the carbon signal from the distal carbonyl (v) at
trithionated RF2. This result held true even when higher 163 ppm strongly couples with the core H(II) at 8.1 ppm. The
concentrations of Lawesson's reagent or longer reaction times carbon from the proximal carbonyl (iv) at 166 ppm also weakly
were used. Due to difficulties in separating the trithionated RF2 couples with the core H(II), indicating strong long-range W-
from RF2-2S, we limited the reaction time to 2 h to avoid tri- coupling in these planar conjugated molecules. For RF2-2S
thionated products and ease the purication process. The (Fig. 3b), the carbon signal from the thiocarbonyl is shied
products were separated by column chromatography to give downeld to 191 ppm and more strongly couples with the core
RF2-2S as a dark green solid (30% isolated yield) and RF2-1S as a H(II) than the other carbon signal upeld at 163 ppm, consistent
light green solid (22% isolated yield). To obtain analytically pure with the distal carbonyl being thionated. To further conrm the
RF2-2S, an additional column was required.
location of the thiocarbonyls, we attempted to crystallize RF2,
It is worth noting that compounds RF2-1S and RF2-2S were RF2-1S and RF2-2S. We were able to obtain small single crystals
air and light stable as solids and solutions during the time scale of RF2-1S suitable for X-ray crystallography. The crystal struc-
of our synthesis, purication, and analysis. This is in contrast to ture (Fig. 4) indicates that the distal carbonyl is thionated.
thionated diketopyrrolopyrroles and thienopyrroledione, which Based on this information, we surmise that RF2-2S is a trans
degraded within hours under ambient light and required isomer with the sulfurs at the distal positions, as drawn in
handling in the dark.7 This is consistent with thioimides being Scheme 1. This result is consistent with thionation occurring at
more stable than other types of thiocarbonyl compounds.6 the least sterically hindered carbonyls of the RF2 molecule. This
However, solutions of RF2-1S and RF2-2S exposed to ambient is reasonable considering that the most probable thionation
light visually degraded over several months, changing from mechanism utilizes an oxaphosphetane-like intermediate
clear green to cloudy yellow/blue with a yellow lm on the vial. found in Wittig mechanisms.11,14
Such changes were not observed if the vials were kept in the
To better understand which substituent guides the thiona-
dark. Long term quantitative stability tests were not performed. tion to the distal carbonyl, we analyzed the thionation products
1
Fig. 2 compares the H NMR spectrum of RF2, RF2-1S and for a series of NDI molecules with different core and imide
RF2-2S between 4.5 and 10.5 ppm. The spectrum of RF2 shows a substitution combinations (Scheme 2, Table 1). The reactions
triplet at 9.4 ppm (i) due to the amine protons, a singlet at 8.1 were performed under similar conditions and the product
ppm (ii) due to the NDI core protons and a singlet at 5.5 ppm mixtures were analyzed by MALDI-TOF MS. The NDI with no
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Fig. 4 ORTEP drawing with ellipsoids drawn at 50% level for RF2-1S.
For clarity only half of the atoms were labelled; second half of the
molecule is generated by symmetry. Sulfur S2 and oxygen O2 atoms
were found to occupy similar positions at 50% occupancy.
Scheme 2 Synthesis of NDI substrates with various core and imide
substituents. R ¼ 2-ethylhexyl (i) dibromoisocyanuric acid (DBI), sul-
phuric acid, 140 ꢁC, 16 h; yield: 33%; (ii) ammonium acetate, acetic
acid, 135 ꢁC, 1.25 h; yield: 84%; (iii) 2-ethylhexylamine, 140 ꢁC, 2 h, N2
atmosphere; yield: 60%; (iv) 2-ethylhexylamine, 150 ꢁC, 18 h, N2
atmosphere; yield: 88%; (v) 2-ethylhexylamine, acetic acid, 130 ꢁC, 1.5
h, N2 atmosphere; yield: 55%; (vi) 2-ethylhexylamine, 140 ꢁC, 2 h, N2
atmosphere; yield: 56%.
Fig. 3 (a) Zoomed in gHMBC of RF2; (b) zoomed in gHMBC of RF2-2S;
R ¼ 2-ethylhexyl.
Table 1 Thionation reaction products monitored by MALDI-TOF MSa
Reactant
Core
Imide
Major product
Minor products
core substituents, 1, gave a mixture of di-, with some mono- and
trithionated products. NMR of the product mixture showed that
several isomers were present (cis and trans), consistent with lack
of selectivity and similar to the literature for this molecule
(Fig. S12†).8 The NDI with alkylamino core substituents and no
imide substituents, 4, gave mainly tri and quad substitutions.
The high amounts of tri and quad substitutions suggest the loss
of selectivity because to get more than two substitutions, you
need to thionate proximal carbonyls. We therefore concluded
RF2
1
2
3
4
NHR
H
Br
NHR
NHR
TM
R
R
R
H
Di, mono
Di
Di
Mono, di
Tri, quad
Tri
Mono, tri
Mono
Starting
Di
a
NHR: 2-ethylhexylamino; R: 2-ethylhexyl; TM: thienylmethyl.
that a combination of imide and core substituents is required to products. These results are consistent with thionation selec-
obtain selectivity, most likely due to steric hindrance of the tivity. However, selectivity could not be conrmed by NMR.
substituents near the proximal carbonyl. Replacing the alkyla- Products from 2 had limited solubility in organic solvents and
mino core substituent with Br or replacing the imide therefore could not be separated and analyzed. The thionated
thienylmethyl substituent with 2-ethylhexyl also resulted in only products of 3 were soluble but could not be easily separated by
mono or dithionated products, with no tri or quad substituted column chromatography, in contrast to the thionated products
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of RF2, which could be separated by column chromatography,
pointing to a synthetic advantage of using the thienylmethyl
imide substituent.
Fig. 5 compares the solution absorption spectra for RF2, RF2-
1S and RF2-2S and the optical properties are summarized in
Table 2. Each thionation caused a 50 nm shi of the lmax
,
consistent with other thionated arylene diimides.8–10 A small
increase in molar absorptivity is also observed with thionation.
The characteristic absorption of the core naphthalene unit at
about 380 nm undergoes changes in energy and structure, with
the appearance of a peak at 406 nm for RF2-1S and at 461 nm for
RF2-2S. The new compounds had strong absorption in the red
to near-IR region of the spectra, with lmax of 683 nm and 733 nm
for RF2-1S and RF2-2S, respectively. The optical gaps of the
three molecules were estimated at 1.87 eV, 1.71 eV and 1.59 eV
for RF2, RF2-1S and RF2-2S, respectively (Table 2). Emission
could not be detected from RF2-1S and RF2-2S. The absence of
uorescence could be due to efficient intersystem crossing to
the triplet state, which has been observed for thionated perylene
diimides.10 To fully understand the cause, transient absorption
spectroscopy and quantum chemical calculations are required
and are beyond the scope of this paper.15,16
Fig. 6 Cyclic voltammetry of RF2 (solid-blue), RF2-1S (dashed-black)
and RF2-2S (dotted-green) in DCM.
RF2-2S were not. Table 2 summarizes the electrochemical
results. The rst reduction potential shis anodically from
ꢂ1.39 V for RF2 to ꢂ1.17 V and ꢂ1.03 V for RF2-1S, and RF2-2S,
respectively. The second reduction potential also shis anodi-
cally from ꢂ1.76 V for RF2 to ꢂ1.52 V and ꢂ1.34 V for RF2-S1
and RF2-S2, respectively. Thionation had a much smaller effect
on the oxidation potentials, with a similar small cathodic shi
from RF2 to RF2-1S or to RF2-2S. Thionation therefore stabilizes
the anions and improves the electron accepting properties of
NDI but does not signicantly affect cations or electron
donating properties of NDI. DFT calculations (see ESI†) show
that the LUMO energy levels decrease with thionation, while the
HOMO energy levels only slightly decrease with thionation,
consistent with the electrochemistry results and the observed
red-shi in optical absorption.
Fig. 6 compares the cyclic voltammograms of the three
compounds in solution, with Fc/Fc+ as the internal standard. All
compounds have two reversible reductions and two oxidations.
The oxidations of RF2 were reversible while those of RF2-1S and
Conclusions
In conclusion we have demonstrated selective thionation of
distal carbonyls by using a combination of core and imide
substituents. The thienylmethyl imide substituents allowed us
to separate the monothionated and dithionated products by
column chromatography. NMR and X-ray conrmed that only
the distal carbonyls were thionated, yielding only the mono-
thioimide isomer. Experiments with different core and imide
substituents suggest that the substituents inhibit thionation of
the proximal carbonyl by steric hindrance. Each thionation
caused a 50 nm bathochromic shi of the visible absorption
Fig. 5 Molar absorptivity plots of RF2 (solid-blue), RF2-1S (dashed-
black) and RF2-2S (dotted-green) in chloroform.
Table 2 Optical and electrochemical properties
lmax (nm) (3 (Mꢂ1 cmꢂ1))
lonset (nm)
DEopt (eV)
E1/2,oxa (V)
E1/2,reda (V)
RF2
RF2-1S
RF2-2S
628 (22.4)
683 (23.0)
733 (28.3)
663
725
780
1.87
1.71
1.59
0.61, 1.11
0.66b
ꢂ1.39, ꢂ1.76
ꢂ1.17, ꢂ1.52
ꢂ1.03, ꢂ1.34
0.67,b 1.04b
a
V vs. Fc/Fc+. b Ep,a values.
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