Crystallization induced green-yellow-orange emitters based on benzoylpyrenes

Article abstract of DOI:10.1039/c6ce00610h

We report the synthesis, X-ray structures and photophysical properties of a few benzoylpyrene (BP) derivatives. Steric hindrance due to incremental benzoyl groups causes a systematic reduction in the orbital overlap (π-π) between vicinal pyrene units affording green-yellow-orange solid-state emitters. Crystallization induced emission could arise from: i) electronic (dipolar/excitonic) interactions, ii) arrested bond rotations, and/or iii) lack of solvation in crystalline 1-4BP (Φ_Fl ～ 2-26%) when compared to that in solution (Φ_Fl ≤ 1%). Our earlier effort [Chem. Commun. 2014, 50, 8644] on progressive acylation, in contrast to benzoylation, results in a gradual increase in the π-π overlap between vicinal pyrenes.

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ARTICLE
Crystallization Induced Green- Yellow-Orange Emitters Based on
Benzoylpyrenes
Shinaj K. Rajagopal, V. Sivaranjana Reddy and Mahesh Hariharan*
Received 00th January 20xx,
Accepted 00th January 20xx
We report the synthesis, X-ray structure and photophysical properties of a few benzoylpyrene (BP) derivatives. Steric
hindrance due to incremental benzoyl groups causes a systematic reduction in the orbital overlap (π−π) between vicinal
pyrene units affording green-yellow-orange solid-state emitters. Crystallization induced emission could arise from i)
electronic (dipolar/excitionic) interactions; ii) arrested bond rotations and/or iii) lack of solvation in the crystalline
(ΦFl~2−26%) 1-4BP when compared to that in solution (ΦFl≤1%). Our effort [Chem. Commun. 2014, 50, 8644] on
progressive acylation, in contrast to benzoylation, results in gradual increase in the π−π overlap between vicinal pyrenes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
1. Introduction
of pyrene based system. In our earlier effort,13a progressive
Pyrene is considered as fruit fly of photochemists by virtue of
its fundamental and applied research interests.1 Since the
discovery of pyrene in 1837,2 this unique chromophore is
acylation of pyrene resulted in remarkable decrease in the
interplanar angle between two neighbouring units causing
significant red-shift in the solid-state fluorescence. Significant
dimension of benzoyl unit (129.4 Å3)17 as compared to pyrene
(237.5 Å3) could potentially perturb strong π−π interactions
well−exploited for diverse applications in biology and material
science owing to its inherent photochemical stability.
Crystalline pyrene based blue organic light emitting devices
(OLED) suffer from strong excimer contributions.3 Disruption
of nearest neighbour interactions in crystalline pyrene is
expected to enhance the performance of OLEDs.4 A variety of
chemical5 and physical approaches6 have been utilized to
regulate the pyrene-pyrene interactions. Physical methods to
tune these interactions7 involve mechanical grinding,8
(−
13.8 kcal/mol)18 between the nearest pyrene units (center-
to-center distance of 3.53 Å; Fig. S1; ESI†). In addiꢀon,
conjugation arising from incremental benzoyl units
progressively influences the HOMO-LUMO gap that could
result in the bathochromic shift in electronic absorption and
sublimation9
or
solvent-/co-crystallization.6a
Chemical
methods10 employ steric effects imparted by bulky groups that
can be conveniently functionalized through electrophilic
substitution reactions. Most of such attempts led to the
conversion of dimeric to monomeric pyrene, resulting in a shift
from “deep-blue” to “sky-blue” solid state emission.11 Yet the
synthesis and structure-property correlation of pyrene based
non-blue solid state emitters has received less attention.
Recently, we utilised interchromophoric interactions of
pyrene to understand the distribution of parallel vs.
antiparallel G-quadrulex conformations in aqueous and non-
aqueous media.12 Efforts to tune the interchromophoric
interactions in molecular crystals using π−π 13
,
C− ,
H•••π 14
C−
H•••O15 and C16 contacts for favourable
C−
H•••H
−
photophysical properties facilitated us to employ the
combination of steric and conjugation effects of benzoyl units
Scheme 1. Row I: Molecular structure of 1–4BP; Row II: close packing arrangement in
the crystal a) 1BP; b) 2BP; c) 2’BP; d) 2’’BP; e) 3BP and f) 4BP; Row III: photographic
image of the crystals in daylight (above) and under UV illumination (below).
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emission properties. We herein report simple Friedel-Crafts 90.17% C, 4.61% H; found: 90.31% C, 4.56% H. HRMS (ESI) m/z
benzoylation of pyrene (Scheme 1, row I) that results in the calculated for C23H14O [M]+: 306.1045, found: 306.1037.
formation of green-yellow-orange luminescent crystals. In 2BP (yield= 9.4%) M.p. 164-167 °C. 1H NMR [500 MHz,
contrast to acylation of pyrene,13a progressive benzoylation of DMSO(d6), δ]: 8.50 (d, J = 7.7, 2H), 8.43 (d, J = 9.3 Hz, 2H), 8.32
pyrene resulted in significant decrease in the π-π interactions (d, J =9.2 Hz, 2H), 8.25 (t, J = 7.7 Hz, 1H), 8.13 (s, 1H), 7.88 (d, J
causing systematic blue-shift in the emitted light in the = 8.2, 4H), 7.71 (t, J = 7.5, 2H), 7.57 (t, J = 7.8, 4H). 13C NMR
crystalline state. Crystallization induced emission (CIE) [125 MHz, DMSO(d6), δ]: 201.94, 131.41, 131.21, 130.37,
possessing precise emission colour tunability in benzoylpyrene 129.87, 128.03, 127.43, 127.20, 124.39, 124.18, 123.19, 30.71.
derivatives indicates the possibility for novel light emitting IR (KBr, cm-1): 3053, 1656, 1585, 1516, 1446. Elemental
devices.
analysis: calcd. value for C30H18O2: 87.78% C, 4.42% H; found:
87.63% C, 4.61% H. HRMS (ESI) m/z calculated for C30H18O2
[M]+: 410.1307, found: 410.1305.
2’BP (yield= 9.6%) M.p 233-238 °C. 1H NMR [500 MHz,
DMSO(d6), δ]: 8.38 (d, J = 8 Hz, 2H), 8.25 (d, J = 9.5 Hz, 2H),
8.18 (d, J = 9.5 Hz, 2H), 8.10 (d, J = 7.5 Hz, 2H), 7.73 (d, J = 7.7
Hz, 4 H), 7.64 (t, J = 7.1, 2H), 7.49 (t, J =7.7 , 4H). 13C NMR [125
MHz, DMSO(d6), δ]: 202.20, 133.33, 132.31, 129.07, 128.20,
127.73, 126.39, 125.35, 123.73, 30.64. IR (KBr, cm-1): 3051,
1649, 1571, 1490, 1442. Elemental analysis: calcd. value for
C30H18O2: 87.78% C, 4.42% H; found: 87.71% C, 4.69% H. HRMS
(ESI) m/z calculated for C30H18O2 [M]+: 410.1307, found:
410.1301.
2’’BP (yield= 37.5%) M.p. 155-158 °C. 1H NMR [500 MHz,
DMSO(d6), δ]: 8.42 (d, J = 7.5 Hz, 2H), 8.35 (s, 2H), 8.11= (d, J =
3 Hz, 2H), 8.09 (s, 2H), 7.71 (d, J = 8.25, 4H), 7.61 (t, J = 8.5,
2H), 7.47 (t, J = 7.25, 4H). 13C NMR [125 MHz, DMSO(d6), δ]:
197.82, 138.27, 134.29, 134.25, 132.99, 130.63, 129.43,
129.36, 128.57, 127.54, 126.14, 125.97, 126.14, 124.22. IR
(KBr, cm-1) 3051, 1654, 1595, 1446, 1446. Elemental analysis:
calcd. value for C30H18O2: 87.78% C, 4.42% H; found: 87.68% C,
4.59% H. HRMS (ESI) m/z calculated for C30H18O2 [M]+:
410.1307, found: 410.1298.
3BP (yield= 18.9%) M.p. 183-187 °C. 1H NMR [500 MHz,
DMSO(d6), δ]: 8.63 (d, J = 9.7 Hz, 1H), 8.54 (d, J = 9.3 Hz, 1H),
8.42 (d, J = 9.2 Hz, 1H), 8.35 (s, 2H), 8.30 (d, J = 7.8 Hz, 1H),
8.23 (s, 1H), 8.89 (m, 4H), 7.85 (d, J = 7.7 Hz, 2H), 7.75 (m, 3H),
7.63 (m, 6H). 13C NMR [125 MHz, DMSO(d6), δ]:197.18, 196.63,
196.52, 137.60, 137.48, 137.43, 135, 133.93, 133.90, 132.89,
132.65, 131.81, 130.42, 130.27, 130.26, 130.17, 129.77,
128.90, 128.88, 128.85, 128.06, 127.34, 127.22, 127.17,
126.44, 125.50, 124.50, 124.15, 123.64. IR (KBr, cm-1): 3055,
1654, 1593, 1568, 1446. Elemental analysis: calcd. value for
C37H22O3: 86.36% C, 4.31% H; found: 87.41% C, 4.45% H. HRMS
(ESI) m/z calculated for C37H22O3 [M]+: 514.1569, found:
516.1552.
4BP (yield= 1.9%) M.p. 283-285 °C. 1H NMR [500 MHz,
DMSO(d6), δ]: 8.33 (s, 4H), 8.20 (s, 2H), 7.81 (d, J = 9.25 Hz,
8H), 7.65 (t, J = 7.85 Hz, 4H), 7.50 (t, J = 7.85 Hz, 8H). 13C NMR
[125MHz, DMSO(d6), δ]:196.93, 137.82, 134.56, 134.39,
130.80, 130.19, 129.41, 127.75, 127.46, 124.62. IR (KBr, cm-1):
3055, 1654, 1593, 1568, 1446. Elemental analysis: calcd. value
for C44H26O4: 85.42% C, 4.24% H; found: 85.63% C, 4.31% H.
HRMS (ESI) m/z calculated for C44H26O4 [M]+: 618.1831 found:
618.1820.
2. Experimental
2.1 Materials and methods
Reactions were carried out in oven-dried glassware prior to
use and wherever necessary, were performed under dry
nitrogen in dried, anhydrous solvents using standard gastight
syringes, cannulae, and septa. Solvents were dried and distilled
by standard procedures. Flash column chromatography was
performed using silica gel of 200-400 mesh employing a
solvent polarity correlated with the TLC mobility observed for
the substance of interest. Yields refer to chromatographically
and spectroscopically homogenous substances. High
Resolution Mass Spectra (HRMS) were recorded on Agilent
6538 Ultra High Definition (UHD) Accurate-Mass Q-TOF-LC/MS
system using either atmospheric pressure chemical ionization
(APCI) or electrospray ionization (ESI) mode. H and 13C NMR
1
spectra were measured on a 500 MHz Bruker advanced DPX
spectrometer. Internal standard used for 1H and 13C NMR is
1,1,1,1-tetramethyl silane (TMS). All the elemental analyses
were performed on Elementar Vario MICRO Cube analyzer. All
values recorded in elemental analyses are given in
percentages. Reference standard used for elemental analysis is
4-aminobenzenesulphonic acid (sulphanilic acid).
2.2 Synthetic procedure
Preparation of benzoylpyrene (1-4BP) derivatives:19 Pyrene (10
g; 0.049 mols) dissolved in CS2 (yellow solution, 250 mL) was
maintained at ambient temperature and subsequently
anhydrous AlCl3 was added. Benzoyl chloride (27.80 g; 0.198
mols) was then slowly syringed to the suspension.
Subsequently, hydrogen chloride was liberated. After 3 hrs the
mixture was added slowly to a vigorously stirred mixture of ice
and concentrated HCl. The resulting suspension was filtered
and vacuum dried and purified through column
chromatography (silica gel) to give benzoylpyrene (1-4BP)
derivatives.
1BP (yield= 5.1%) M.p. 102-105 °C. 1H NMR [500 MHz,
DMSO(d6), δ]: 8.40 (m, 4H), 8.43 (q, J = 9.66 Hz, 2H), 8.17 (m,
3H), 7.82 (d, J = 7.7 Hz, 2H), 7.73 (t, J = 7.2 Hz, 1H), 7.58 (d, J =
7.8 Hz, 2H). 13C NMR [125 MHz, DMSO(d6), δ]: 197.56, 138.0,
133.60, 132.82, 132.46, 130.70, 130.08, 129.07, 128.90,
128.83, 128.66, 127.25, 126.83, 126.68, 126.35, 126.04,
124.20, 124.10, 123.84, 123.55. IR (KBr, cm-1): 3037, 1651,
1595, 1506, 1446. Elemental analysis: calcd. value for C23H14O:
3. Results and discussion
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Adding stoichiometric quantity of benzoyl chloride to
a
the lifetime at the far red shifted emission. Time-resolved
solution of pyrene (P) and AlCl3 in carbon disulfide (CS2) at measurement at the longer emission wavelength exhibited a
ambient temperature rendered the benzoyl derivatives (1–
4BP) in low-moderate yields (Scheme S1; ESI†). Though Harvey
and co-workers19 reported 1BP, 2BP and 2’BP, the synthesis of
2’’BP, 3BP and 4BP are yet to be explored. Single crystal X-ray
analyses (SCXRD) of 1–4BP were not reported earlier. SCXRD
analyses were performed on 0.20 x 0.20 x 0.15 mm crystalline
samples obtained through slow evaporation of 1–4BP from a
varying composition of chloroform:acetone mixtures (Table S1;
ESI†). Benzoyl derivaꢀves 1BP (P-1), 2BP (P-1) and 3BP (P-1)
yield solvent free triclinic crystal system while 2’BP (
2”BP ( 2/ ) and 4BP ( 21/n) exhibit solvent free monoclinic
crystal system (Scheme 1, row III, Table S2; ESI†).
P21/n),
C
c
P
Frontier molecular orbital (FMO) analysis, cyclic
voltammetric, UV-Vis absorption and emission measurements
were employed to investigate the extent of perturbations in
pyrene imparted by incremental benzoyl groups. FMO analyses
of 1-4BP shows that the electron density of HOMO (Fig. 1a) is
distributed in pyrene units while electron density of LUMO is
mostly localized on pyrene chromophore with a moderate
extension to the carbonyl group(s). Low-lying excited
Fig. 1 a) Frontier molecular orbital (FMO) analysis of 1-4BP calculated from B3LYP/6-
311G**+ level of theory. Lower and upper plots represent the HOMOs and LUMOs
respectively; b) Cyclic voltammograms of 1-4BP in acetonitrile; c) Absorption spectra of
1-4BP; area filled spectrum (grey) represents the absorption spectrum of pyrene23 and
d) Nanosecond transient absorption spectrum of 4BP in chloroform.
electronic states mainly results from well-described π–π*
transitions with a minor contribution from n– * character. A
π
very short lifetime (<100 ps pulse width at λex=375 nm) similar
decrease in the HOMO-LUMO gap from 3.59 eV (1BP) to
3.18eV (4BP) is attributed to the increase in the effective
conjugation due to carbonyl groups on the pyrene unit (Table
S3, ESI†). Cyclic voltammetry (0.1 M nBu4NPF6 in acetonitrile)
of 1BP (Fig. 1b) exhibits oxidation peak at 1.47 V while
reduction peak appear at -1.50 V. Decrease in the HOMO-
LUMO gap of benzoylpyrenes (2.97 eV for 1BP; 2.61 eV for
4BP; Table S3; ESI†) in comparison to P (3.37 eV) is in
agreement with the FMO analysis.20
to that observed at the respective emission maxima. An
obvious shoulder peak at around 550 nm was observed in the
emission spectrum of 2’’BP. Wavelength dependent time-
resolved fluorescence and excitation spectra invalidates any
possibility of aggregates of 2’’BP in solution state under these
conditions.
As stated above, low ΦFl for 1-4BP could result from an
efficient intersystem crossing (ISC) induced by strong mixing of
nearly-degenerate singlet and triplet states (Fig. 1d, Table S5,
ESI†). ISC efficiency of 1-4BP was investigated by employing
nanosecond time-resolved absorption spectroscopy (Fig. S3,
Table S5; ESI†). Upon excitaꢀon at 355 nm, 1BP exhibits
absorption maximum at 520 nm corresponding to triplet-
Steady state absorption spectra of 1-4BP in chloroform
exhibit two distinct bands: a band centered around 250-300
nm and another around 300-425 nm (Fig. 1c). Time-dependent
density functional theory (TDDFT)21 calculations suggest that
observed bands are a combination of several electronic
transiꢀons (Table S4; ESI†) with the longer wavelength band
due to π−π* transition. λmax of long-wavelength transition of
1BP in chloroform is red-shifted by 12 nm compared to P. 2-
4BP display remarkable red-shift in the long-wavelength,
which could be attributed to the increase in number of benzoyl
group(s). Upon excitation at 380 nm, 1BP in chloroform
exhibits emission band centered around 450 nm (Fig. S2a,
Table S5; ESI†). The emission maximum is ca. 50 nm red-
shifted in comparison to P under similar conditions. However,
further increase in the number of benzoyl groups, the emission
maximum remains unchanged (ca. 450 nm). Very low
fluorescence quantum yield was observed for 1-4BP
triplet transition of pyrene24 having a lifetime of 2.1
features for triplet-triplet absorption were also observed for 2-
4BP (Table S5; ESI†). Esꢀmaꢀon of triplet quantum yield (ΦT
of 1BP based on triplet–triplet energy-transfer to
-carotene20
ꢀs. Similar
)
β
shows a value of 48% (Fig. S4, Table S5; ESI†). We observed
that ΦT decreases with increase in number of benzoyl groups
(Table S5; ESI†). Time-gated emission measurements of 1-4BP
in ethanol at 77K show extremely weak phosphorescence
when excited at 380 nm (Fig. S2c, Table S5; ESI†). With
increase in number of benzoyl groups, systematic red-shift
from 510 to 634 nm in the phosphorescence emission
maximum was observed. In addition, non-radiative pathways
could also operate to cause significantly reduced fluorescence
quantum yield in 4BP in chloroform when compared to P.
Bond rotations from benzoyl functional groups can contribute
to energy dissipative pathways.
(
ΦFl ≤ 1.0%, Table S5; ESI†) in chloroform when compared to P
(Φ
Fl = 65%).22
Picosecond time-resolved fluorescence measurements of
1–4BP indicate very short lifetime (<100 ps pulse width at
ex=375 nm) when monitored at respective emission maxima
(Fig. S2b; ESI†). In order to rule out the possibility of
aggregates of 1-4BP in chloroform (0.1-1 M), we monitored
Having established the photophysical properties of 1-4BP
in solution, further efforts were made to correlate the optical
properties in the crystalline state. Detailed analyses of single
λ
ꢀ
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crystal X-ray structure decipher the role of terminal benzoyl the two nearest pyrene units having a transverse slip (along
groups in impeding the pyrene-pyrene nearest-neighbour the molecular short axis of the pyrene unit) of 0.69 Å and a
interactions in crystalline 1-4BP. Single crystal X-ray analysis of longitudinal slip (along the molecular long axis of the pyrene
1BP shows an interplanar distance (π−π) of 3.50 Å between
Fig. 2 a) Absorption; b) fluorescence spectra of crystalline 1-4BP derivatives; λexc =380 nm.; area filled spectrum (grey) represent the absorption and emission spectra of crystalline
pyrene25; c) Indicates τFl (major component; □); ΦFl
(●) of 1-4BP in the crystalline state; ΦFl (ꢀ) of 1-4BP in solution state and % CꢀꢀꢀC interaction (π−π; ∆) obtained from Hirshfeld
analysis17 (Table S7; ESI†) and d) Jablonski diagram depicꢀng energy levels in soluꢀon (monomeric) and aggregated (crystalline) state in 1BP.
unit) of 0.93 Å. Observed transverse/longitudinal displacement the significant
complements the phenyl rings to adopt an end to face shift in the emission wavelength was observed when
(CH••• ~2.9 Å interaction with -electron cloud of the compared to crystalline 1-4BP. A marked increase in the
π-overlap (π−π; 39.91%) in crystalline P, a blue-
π
)
π
neighbouring pyrene unit. With increase in the number of separation (dπ π ~ 3.53 Å) between the molecular planes of
-
benzoyl groups in 2-4BP, a progressive rise in the steric vicinal pyrene units in crystalline P (Table S6; ESI†) when
contribution causes an increase in the transverse/longitudinal compared to the crystalline BP derivatives could cause the
displacement of the vicinal pyrene units in the crystal structure blue-shift in the emission spectrum. Observed differences in
(Fig. S5&S6, Table S6; ESI†). In 4BP, the bulky benzoyl group the emission maxima for crystalline 1-4BP could be
hampers the strong aggregation, inducing a transverse shift combined consequence of different i) degree of overlap
(5.65 Å), prohibiting any -contacts between neighbouring and ii) distance between the molecular planes of the adjacent
a
π-π
π
pyrene units. The longitudinal and transverse offset imparts pyrene units. However, contributions from non-nearest
different degree of orbital overlap between the pyrene units neighbour can also contribute to overall electronic coupling
that are separated at their van der Waals distances (ca. 3.35- that can influence the peak positions in emission spectra.
3.59 Å) in crystalline 1-4BP (Scheme 1, row II, Table S6; ESI†). Significant red-shift in the emission wavelength dependent
This intermolecular offset of vicinal pyrene units reduces the excitation spectra compared to the corresponding steady-state
π−π interaction that attributes to the observed decrease in the absorption spectra of crystalline 1-4BP suggests the possible
orbital overlap from 45.92% (1BP) to 0.95% (4BP) (see ESI†). ground state interaction between the vicinal pyrene units (Fig.
Interplanar C–H•••O contacts26 support the pyrene units for S9; ESI†).27 Substantially red-shifted dimer/excimer like
an extended interaction along all axis in the crystalline emission of 1BP could arise from significant orbital overlap
arrangement of 1-4BP (Fig. S7; ESI†).
Intrigued by the
(
π−π; 45.92%) between the nearest pyrene units. Nearly
sterically
controlled slipped out (π−π; 0.95%) pyrene-pyrene stack along the
longitudinal/transverse shift of intermolecular vicinal pyrene molecular long axis in 4BP results in a monomer-like emission
units, further efforts were made to explore the photophysical possessing vibronic features.
properties of crystalline 1-4BP. Diverse colour in the crystalline
Upon excitation at 375 nm, picosecond time-resolved
1-4BP ranging from pale yellow-yellow-orange red (Scheme 1, fluorescence measurements of crystalline 1BP shows tri-
row III) were observed due to intermolecular offset of vicinal exponential decay having the lifetime of 2.5 ns (82%), 8.5 ns
pyrene units in the crystalline state. Diffuse reflectance (12%) and 0.7 ns (6%), when monitored at 620 nm (Fig. S10,
absorption spectrum of crystalline 1-4BP exhibits a broad band Table S5; ESI†). Long fluorescence lifetime in crystalline 1BP
centered around 350-450 nm, with an additional absorption when compared to that in solution (<100 ps) could arise from
tail extending to 500 nm in 1BP (Fig. 2a, Table S5; ESI†). excimer/ground state aggregate of neighbouring pyrene units
Presence of benzoyl group(s) results a red-shift in the UV-Vis possessing different orbital overlap in the crystalline state.
absorption, for example 114 nm (1BP) and 60 nm (4BP), when Similarly, crystalline 2-4BP show tri-exponential fluorescence
compared to crystalline P. Upon excitation at 380 nm, emission profile with a remarkable decrease in the lifetime of
crystalline 1BP exhibits a broad emission band with the the corresponding major component (Fig. 2c, Table S5; ESI†). A
maximum centered at 620 nm red-shifted by 148 nm radiative decay rate constant (kr) of 1.56×108 s−1 and 0.59×108
compared to crystalline P. A systematic blue-shift in the s−1 in solution and crystalline state of 1BP, respectively, is
emission maximum was observed upon further increase in estimated (Table S5; ESI†). The faster rate of radiative decay in
number of benzoyl groups, when compared to crystalline 1BP solution vs. crystalline state was similarly observed in 2-3BP.
(Scheme 1, row III and Fig. 2b&S8, Table S5; ESI†). In spite of Observed decrease in the rate of radiative decay in crystalline
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1-3BP when compared to solution state indicates the
possibility of H-like aggregates and/or excimers of pyrene.
Unusual formation of fluorescent H-aggregates were observed
earlier in merocyanine dyes by Würthner and coworkers.28
Notably, small π−π contact of vicinal pyrene units in 4BP
imparts a monomer-like behavior in the crystalline state (Fig.
2c, Table S6; ESI†), as reported earlier.29 The twisted nature of
the benzoyl group(s) with respect to the plane of pyrene
chromophore drastically diminishes the aggregation of vicinal
pyrene units. Observed similar rate constant in crystalline state
and solution state of 4BP confirms the monomer-like behavior
in the crystalline state (kr ~ 0.59 × 108 s−1; Table S6; ESI†).
Acknowledgements
M. H. acknowledges Kerala State Council for Science,
Technology and Environment (KSCSTE) for the support of this
work, 007/KSYSA-RG/2014/KSCSTE. Authors thank Alex P.
Andrews, IISER-TVM for single crystal X-ray structure analyses.
Notes and references
(1) Figueira-Duarte, T. M.; Müllen, K. Chem. Rev. 2011, 111
,
7260.
(2) Laurent, A. Ann. Chim. Phys 1837, 66, 136.
(3) Chercka, D.; Yoo, S.-J.; Baumgarten, M.; Kim, J.-J.; Mullen,
K. J. Mater. Chem. C 2014, , 9083.
Unravelling the processes responsible for crystallization/
aggregation induced emission is vital for the design of new
light emitting materials. Recent preliminary theoretical
investigations have offered insights on the characteristics of
induced CIE in a variety of organic/inorganic chromophores.30
2
(4) Figueira-Duarte, T. M.; Del Rosso, P. G.; Trattnig, R.; Sax,
S.; List, E. J. W.; Müllen, K. Adv. Mater. 2010, 22, 990.
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Elsegood, M. R. J.; Yamato, T. Org. Lett. 2013, 15, 1318(b)
Moorthy, J. N.; Natarajan, P.; Venkatakrishnan, P.; Huang,
CIE could arise as
(dipolar/excitonic)
a
consequence of i) electronic
interactions28,31
ii) arrested
vibrational/rotational motions;32 and/or iii) reduced solvation33
in crystalline state. To investigate the extent of vibrationally
promoted ISC in monomer vs. dimer states, low-lying excited
electronic states of 1BP and 4BP were calculated employing
TDDFT method (Fig. S11; ESI†). The energy difference between
S1 and T1 states in monomeric and dimeric 1BP found to be
1.18 and 0.94 eV, respectively. Notable lowering of T2 energy
state was also observed from monomeric (2.95 eV) vs. dimeric
(2.13 eV) 1BP. Similar observations were made in monomeric
vs. dimeric 4BP. Non-significant differences in energy gap
between the close-lying singlet and triplet energy levels of
dimeric vs. monomeric 1BP and 4BP excludes the contribution
of vibrational processes in CIE. Restriction of intramolecular
rotations offered to the pendent benzoyl groups in the
crystalline (dimeric) state of 1-4BP may facilitate enhancement
in the fluorescence, consistent with the earlier reports.34
Hence, CIE in 1-4BP could arise due to i) fluorescent H-like
excitonic interactions; ii) arrested rotational motion of benzoyl
group(s) and/or iii) lack of solvation in the crystalline state (Fig.
2d).
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4. Conclusion
In summary, the extent of orbital overlap (π−π) between
vicinal pyrene units in crystalline 1-4BP were regulated by
exploiting the steric hindrance offered via successive
benzoylation. Progressive increment in the steric hindrance
causes an increase in the transverse/longitudinal displacement
of the vicinal pyrene units. As a result, a 48-fold reduction in
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the π-stacking between pyrene units was observed in 4BP with
respect to 1BP. Moderating the molecular stacking along with
CIE through arrested intramolecular rotations of pendent
benzoyl groups may facilitates green-yellow-orange solid-state
emitters. Modulating the emission properties of organic
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π-overlap
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high performance photo-functional materials and devices.
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