Stacking among the clips of the poly-aromatic rings of phenazine with hydroxy-aromatics and photophysical properties

Article abstract of DOI:10.1039/c9ra07602f

Clip-like arrangements of molecules in the cocrystals of phenazine with hydroxy-aromatics in their respective self-assemblies and photophysical properties were presented. Phenazine cocrystals with 1,2-dihydroxybenzene provided assembly with butterfly-like

Full text of DOI:10.1039/c9ra07602f

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Stacking among the clips of the poly-aromatic rings
of phenazine with hydroxy-aromatics and
photophysical properties†
Cite this: RSC Adv., 2019, 9, 33403
*
Rinki Brahma, Munendra Pal Singh and Jubaraj B. Baruah
Clip-like arrangements of molecules in the cocrystals of phenazine with hydroxy-aromatics in their
respective self-assemblies and photophysical properties were presented. Phenazine cocrystals with 1,2-
dihydroxybenzene provided assembly with butterfly-like arrangements. In these cocrystals, the
phenazine molecules occurred in parallel pairs having extensive p-stacking. The clip-like cocrystals with
1,3-dihydroxybenzene also exhibited parallel pairs of phenazine molecules that were parallel cofacial p-
stacked. The hydrated cocrystals of phenazine with 1,2,3-trihydroxybenzene had chains of parallel
cofacial phenazine rings having three distinguishable p-separation distances among the centroids of the
phenazine rings. Also, 2,7-dihydroxynaphthalene formed a clip-like cocrystal with phenazine, which
encapsulated an additional molecule of phenazine. This cocrystal also provided chain-like parallel
arrangements of the phenazine molecules. The emission and quantum yields of the cocrystals were
determined by the integrating sphere method, which indicated that only the cocrystal of phenazine with
2,7-dihydroxynaphthalene showed monomer-like emission of phenazine and the rest of the cocrystals
were in a quenched state. In the solution phase, quenching of the emission of hydroxynaphthalene was
observed when phenazine was added to an independent solution of 2,7-dihydroxynaphthalene or
another hydroxynaphthalene. However, when hydroxybenzenes were added to a solution of phenazine,
fluorescence enhancements of phenazine occurred due to photo-electron transfer.
Received 19th September 2019
Accepted 3rd October 2019
DOI: 10.1039/c9ra07602f
rsc.li/rsc-advances
aggregates where different packings through the C–H/p
interactions or p–p interactions are present show distinguish-
able features.36–43 The solvent molecules within lattices are of
Introduction
The new discoveries on different aspects on the consequences
of weak interactions have been fuelling the quest to modulate
optical properties by them in solid and solution.1–7 Changing
weak interaction schemes in conformation polymorphs has
been routinely demonstrated to change photophysical proper-
ties.8–11 The cocrystals and salts are among the non-covalently
linked compounds, where the proton transfer12–18 and orga-
nized structures19,20 change the orientations of uorophores by
interactions with partner molecules. The p-interactions have an
immense role in photophysical properties,21–23 ion-molecular
recognition,24–26 and catalytic reactions.27 These interactions
are common in poly-aromatic compounds to guide the mech-
anism of photoluminescence processes.28–30 Stacked arrange-
ments through p-interactions contribute to dimer- and excimer-
like emissions31–34 and Dexter quenching.35 Different types of
concern in discerning optical properties.44 Thus, the structures
of self-assemblies are of general concern in the elucidation of
photoluminescence properties. The dimer-like emissions45–47
48
¨
and Forster resonance transfer emission require designs to
hold the uorophores in close proximity in denite orienta-
tions. We chose to study the self-assemblies of phenazine with
different aromatic diols in anticipation that the position and
stoichiometry would guide the stacking pattern.49,50 For
example, a diol forming an adduct with an equimolar ratio may
form a linear polymeric structure or cyclic-oligomers; in
contrast, a 1 : 2 ratio combination would provide discrete units,
and an arrangement shown in Fig. 1a for such a composition
would violate the Etter's rule. Such adducts would provide
various stacking arrangements from parallel eclipsing to
parallel translated arrangements, as shown in Fig. 1a. In the
latter case, one will have to leave the favorable hydrogen
bonding sites of phenazine free; in fact, such an observation can
be possible as the exceptional adducts of poly-hydroxyaromatics
violating the Etter's rule have been described with other hosts.51
The p-stacking among the phenazine rings or dihydroxyar-
omatics would provide avenues to study the photoluminescence
properties in the solid state. This is anticipated, as uorophores
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781
039, Assam, India. E-mail: juba@iitg.ac.in
† Electronic supplementary information (ESI) available: Powder XRD, 1H-NMR in
solution, solid state emissions of the cocrystals and partner molecules, quantum
yield of different naphthols, life-time decay proles of the cocrystals are available.
CCDC 1946193–1946196. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9ra07602f
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Fig. 1 (a) Some possible hydrogen-bonded assemblies of phenazine with aromatic diols and (b) structures of phenazine and different organic
aromatic diols used in this study.
such as anthracene p-stacked in different ways operate with the required wavelength in a Horiba Jobin Yvon Fluoromax-4
different emission mechanisms.52,53 With the above points in spectrouorometer by Petite integrating sphere method. The
mind, we studied the structures and photoluminescence prop- uorescence emission (Ec) and the scatter (Lc) for the sample
erties of the cocrystals of the diols and triols listed in Fig. 1b.
and blank (La and Ea) were recorded. From these spectral
measurements (sample and blank), the quantum yields were
calculated by using the equation 4 ¼ [(Ec ꢀ Ea)/(La ꢀ Lc)].
Experimental
General
Preparation and spectroscopic features of the cocrystals
Infrared spectra of the solid samples were recorded on a Perki- The cocrystals of 12DHB and 13DHB with phenazine were ob-
nElmer Spectrum-One FT-IR spectrophotometer by making KBr tained by slow evaporation of a solution of phenazine (360 mg, 2
pellets. Powder X-ray diffraction patterns were recorded using mmol) with the respective coformer (110 mg, 1 mmol) in
a Bruker powder X-ray diffractometer D2 phaser. 1H-NMR methanol (20 ml) carried out at room temperature. Aer about
spectra of complexes were recorded on a BRUKER Ascend-600 30 h, crystals appeared, which were decanted and collected for
MHz NMR spectrometer using TMS as the internal standard. characterization. Similarly, the cocrystals of 27DHN and
A PerkinElmer Lamda-750 spectrometer was used to record the 123THB with phenazine were obtained from crystallization by
solid-state UV-visible spectra by diffuse reectance. Fluores- slow evaporation at 27–30 ꢁC of the respective methanol (20 ml)
cence emissions were measured in a Horiba Jobin Yvon solution containing phenazine (450.5 mg, 2.5 mmol) with
Fluoromax-4 spectrouorometer by taking denite amounts of 27DHN (160.2 mg, 1 mmol) or 123THB (126.1 mg, 1 mmol). In
solutions or denite amounts of solid samples and exciting at these cases also, aer about 25–30 h, the solution was reduced
the required wavelength. Quantum yields were measured by to about 5 ml and the cocrystals were formed as exclusive
taking denite amounts of solids and solutions and exciting at crystalline products. We used only the pure crystals for study
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and did not use the bulk of the completely evaporated product with Mo Ka radiation (l ¼ 0.71073 A) by a Bruker Nonius
as it carries starting components. 2(Phen)$12DHB: isolated SMART APEX CCD diffractometer equipped with a graphite
yield of crystals: 62%. 1H-NMR (600 MHz, DMSO-d6): 8.82 (s, monochromator and an Apex CCD camera, and those for the
2H), 8.27 (m, J ¼ 6 Hz, 4H), 7.99 (m, J ¼ 6 Hz, 4H), 6.72 (t, J ¼ other two cocrystals were collected on an Oxford SuperNova
6 Hz, 2H), 6.60 (d, J ¼ 6 Hz, 2H). IR (KBr, cmꢀ1): 3446 (br), 1623 diffractometer. Data reductions and cell renement for the
(m), 1589 (w), 1514 (s), 1468 (m), 1364 (s), 1248 (s), 1192 (s), 1141 Oxford SuperNova diffractometer were performed by CrysA-
(w), 1094 (s), 1038 (m), 911 (m), 848 (s), 743 (s), 595 (m). lisPro soware and for Bruker Nonius SMART APEX CCD
2(Phen)$13DHB: isolated yield of crystals: 69%. 1H-NMR (600 diffractometer, they were performed using SAINT and XPREP
MHz, CDCl3): 8.27 (m, J ¼ 6 Hz, 8H), 7.86 (m, J ¼ 6 Hz, 8H), 7.09 sowares. Structures were solved by direct methods and were
(t, J ¼ 6 Hz, 1H), 6.42 (d, J ¼ 6 Hz, 2H), 5.74 (s, 1H), 3.49 (s, 1H). rened by full-matrix least-squares on F2 using the SHELXL-
IR (KBr, cmꢀ1): 3449 (br), 2882 (w), 1597 (s), 1515 (s), 1472 (s), 2014 soware. All non-hydrogen atoms were rened in aniso-
1214 (s), 1178 (s), 1143 (s), 906 (m), 840 (m), 745 (s), 694 (w). tropic approximation against F2 of all reections. Hydrogen
2.5(Phen)$27DHN: isolated yield of crystals: 71%. 1H-NMR (600 atoms were placed at their geometric positions by riding and
MHz, DMSO-d6): 9.67 (s, 2H), 8.23 (s, 4H), 7.95 (s, 4H), 7.58 (d, J rened in the isotropic approximation. The crystallographic
¼ 12 Hz, 2H), 6.87 (s, 2H), 6.82 (d, J ¼ 12 Hz, 2H). IR (KBr, cmꢀ1): parameters are listed in Table 1.
3453 (br), 3049 (m), 1621 (s), 1602 (w), 1531 (m), 1514 (s), 1463
(s), 1432 (m), 1379 (s), 1279 (m), 1239 (m), 1198 (s), 1158 (m),
Results and discussion
1142 (m), 1118 (m), 859 (s), 742 (s). 2.5(Phen)$123THB. 2H2O:
1
isolated yield of crystals: 58%. H-NMR (600 MHz, DMSO-d6):
Four cocrystals that were studied were the cocrystals of phena-
zine with 1,2-dihydroxybenzene (2 : 1); 1,3-dihydroxybenzene
8.27 (m, J ¼ 6 Hz, 4H), 7.86 (m, J ¼ 6 Hz, 4H), 6.63 (t, J ¼ 6 Hz,
1H), 6.49 (d, J ¼ 6 Hz, 2H). IR (KBr, cmꢀ1): 3476 (br), 1629 (m), (2 : 1), 2,7-dihydroxynaphthalene (2.5 : 1) and 1,2,3-trihydrox-
1532 (w), 1514 (s), 1469 (s), 1433 (s), 1365 (m), 1326 (m), 1248 ybenzene (2.5 : 1); all of them were prepared through solution
(m), 1204 (m), 1143 (m), 1116 (m), 1065 (m), 1011 (s), 907 (m), crystallization. The cocrystals were anhydrous except the one
823 (s), 745 (s).
formed with 1,2,3-trihydroxybenzene, which was a hydrate. Our
interest was the p-interacting system without the interference of
third foreign molecules. Thus, we chose three independent
anhydrous cocrystals having different projections of the
Crystal structure determination
Single-crystal X-ray diffraction data for cocrystals 2(Phen)$ hydroxyl groups in their respective rings and also dealt with the
12DHB and 2.5(Phen)$123THB$2H2O were collected at 296 K hydrated cocrystal of phenazine with 1,2,3-trihydroxybenzene.
Table 1 Crystallographic parameters of the cocrystals of phenazine
Parameters
2(Phen)$12DHB
30H22N4O2
1946193
470.51
ꢀ
P1
7.403(3)
17.584(6)
18.374(6)
76.445(11)
79.816(12)
79.751(12)
2265.1(14)
1.380
2(Phen)$13DHB
2.5(Phen)$123THB$2H2O
2.5(Phen)$27DHN
Formula
CCDC no.
Mol. wt
C
C15H11N2O1
1946194
235.26
C72H60N10O10
1946196
1225.30
ꢀ
P1
9.7943(10)
12.0978(12)
13.8839(14)
89.390(3)
78.850(3)
72.021(3)
1533.1(3)
1.327
C80H56N10O4
1946195
1221.34
P21/c
9.382(6)
18.747(12)
17.729(11)
90
97.799(7)
90
3089(3)
1.313
Space group
C2/c
˚
a/A
14.2921(9)
11.4224(7)
14.4820(10)
90
102.490(6)
90
2308.2(3)
1.354
0.087
˚
b/A
˚
c/A
a/deg
b/deg
g/deg
3
˚
V/A
Density/g cmꢀ3
Abs. coeff./mmꢀ1
F(000)
0.089
984
0.090
642
0.083
1276
984
Total no. of reections
Reections, I > 2s(I)
Max. q/degree
7999
4744
25.047
2036
1417
25.042
5429
3512
25.048
5457
2434
25.048
Ranges (h, k, l)
ꢀ8 # h # 8
ꢀ20 # k # 20
ꢀ21 # l # 21
99.9
7999/0/653
1.126
ꢀ11 # h # 16
ꢀ13 # k # 9
ꢀ17 # l # 16
100.0
2036/0/165
1.075
ꢀ11 # h # 11
ꢀ14 # k # 14
ꢀ16 # l # 16
99.9
5429/2/429
1.064
ꢀ11 # h # 11
ꢀ22 # k # 22
ꢀ21 # l # 21
99.6
5457/0/426
1.073
Complete to 2q (%)
Data/restraints/parameters
GooF (F2)
R indices [I > 2s(I)]
wR2 [I > 2s(I)]
0.0828
0.2413
0.0503
0.1201
0.0631
0.1966
0.0596
0.1247
R indices (all data)
0.1310
0.2413
0.0732
0.1380
0.0956
0.2352
0.1491
0.1662
wR2 (all data)
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The cocrystals were characterized by recording their H-NMR, among the cofacial parallel rings of the phenazine molecules
IR, and powder-XRD spectra and by determining the single- revealed that there were very weak p-interactions among such
crystal structures.
rings located on either side of 1,2-dihydroxybenzene. In the 1H-
The structures helped us delineate the stacking patterns NMR spectrum recorded in DMSO-d6, the aromatic protons
between the phenazines or between the diols or both; hence, appear at similar chemical shi positions to that of the parent
only the relevant portion of the structures relating these issues components but the chemical-shi positions of the O–H
was presented. The portions of packing showing the p–p protons are affected showing the existence of hydrogen bonds.
interactions of the cocrystals are shown in Fig. 2. In this 2 : 1 In the solid-state IR spectrum, the OH stretching appears as
cocrystal of phenazine with 1,2-dihydroxybenzene, the latter a broad peak at 3446 cmꢀ1. As the powder XRD pattern of the
part serves as a bridge to hold two phenazine molecules through bulk of crystals shows no other phase and the crystals obtained
O2–H/N7 and O1–H/N4 interactions (Fig. 2a) to form are an identical invariant of stoichiometry, it can be conrmed
a propeller-like structure. The cocrystal self-assembled through that in solution, the 2 : 1 cocrystal was exclusively formed from
˚
C3–H/N8 and C6–H/N3 interactions (dD/A ¼ 3.368(5) A, phenazine with 1,2-dihydroxybenzene in the methanol solution.
ꢁ
:D–H/A ¼ 140ꢁ and dD/A ¼ 3.316(5) A, :D–H/A ¼ 142 ,
Phenazine also formed a 2 : 1 cocrystal with 1,3-dihydrox-
˚
respectively), forming linear chain-like arrangements, as illus- ybenzene, which also has a similar feature in holding the two
trated in Fig. 2a. The prominent hydrogen bond parameters are molecules of phenazine by moderate hydrogen bonds (Fig. 2b).
listed in Table 1S.† The phenazine rings were p-stacked and the The self-assemblies were different due to the difference in the
centroid-to-centroid distance of the parallel phenazine rings position of the OH groups on the ring. The C–H bond located in
eclipsing each on either side of the 1,2-dihydroxybenzene ring between the two hydroxyl groups of the diol was involved in very
˚
was different. The distance of separation on one side was 3.760 weak bifurcated C–H/N interactions (dD/A ¼ 3.316(5) A and
A, whereas on the other side, it was 3.789 A. Generally, self- :D–H/A ¼ 142ꢁ). These subsidiary interactions helped hold
˚
˚
assemblies possessing adjacent aromatic rings in parallel the two phenazine rings parallel to each other in pairs with
˚
cofacial, parallel displaced or edge-to-face orientations get a distance of separation of 3.758 A; this distance permits very
involved in p-interactions.54 The centroid-to-centroid distance weak p–p interactions.55,56 The N1 atoms were not involved in
˚
of up to 3.5 A was suitable to have such interactions, whereas the hydrogen bonds as all good hydrogen bonding atoms did
˚
the distance within 3.7–3.8 A for parallel displaced and cofacial not participate in the hydrogen bonds in self-assembly. Hence,
parallel arrangements of rings was established for weak inter- the cocrystal is an exception not following the Etter's rule. This
actions.55 Hence, the observed centroid-to-centroid distances exception occurred due to the stacking orientations that
Fig. 2 Portions of the self-assembly of (a) 2(Phen)$12DHB, (b) 2(Phen)$13DHN.
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provided a tight-packed structure, so that the nitrogen atoms positions were 3.804 A and 10.358 A. Hence, out of these three
did not directly face each other or they were placed in orienta- phenazine molecules, one is far away to have p-interactions and
tions that were not in close vicinity or suitable to form it may be considered as an isolated phenazine molecule. The
a hydrogen bond with the donor. Such examples are oen other three cocrystals had relatively shorter centroid-to-centroid
observed in closed-packed structures having a high p-stacking distances to have cofacial parallel stacked p-interactions
environment.57–59
between the phenazine molecules than such interactions in the
The 2,7-dihydroxynaphthalene cocrystal had a 2.5 : 1 molar cocrystal of phenazine with 27DHN. The cocrystal of 1,2-dihy-
ratio. In this example, two hydroxy groups of each 2,7-dihy- droxybenzene did not have clips similar to the one observed in
droxynaphthalene could hold two independent phenazine the cocrystals of phenazine with 1,3-dihydroxybenzene or 2,7-
molecules through O1–H/N4 and O2–H/N2 hydrogen bonds dihydroxynaphthalene. Due to the outward projection of the
(Table S1†), forming a clip-like structure (Fig. 3a). A third O–H bonds of 1,2-dihydroxybenzene participating in hydrogen
molecule of phenazine was placed over one of the phenazine bonds with two phenazine molecules, the cocrystal of phena-
molecules directly hydrogen bonded to 2,7-dihydroxynaph- zine with 1,2-dihydroxybenzene exhibited a attened structure.
thalene. Thus, there were three molecules of phenazines that This provided a buttery-like structure. Phenazine has struc-
were arranged as stacked molecules together in a repeated tural similarity with acridine; the orderly structures of acridine
manner. The hydrogen-bonded clips were arranged in the are observed in salts having carboxylic acid. These assemblies
opposite direction. The encapsulated phenazine molecule was encapsulate polyaromatic guest molecules.59 However, we have
located in between two adjacent clips. This phenazine molecule a relatively simple cocrystal between 2,7-dihydroxynaphthalene
actually became conned within the self-assembly as it with phenazine in a 1 : 2.5 ratio, where an additional phenazine
provided a bridge between two independent clip-like structures molecule gets encapsulated. Polymorphs of host–guest
located at its two ends through C–H/p interactions. In other complexes formed by naphthalimide-functionalized carboxylic
words, the two rings of 2,7-dihydroxynaphthalene of the clips acids with quinoline encapsulating one of the components,
were involved in the C9–H/p(C38) and C2–H/p(C31–36) inter- namely, quinoline were reported earlier by us.60 Comparing
actions with this encapsulated phenazine molecule. In the these examples, the cocrystal of 2,7-dihydroxynaphthalene with
overall assembly, phenazine molecules were located at parallel phenazine is an exception to contradict the Etter's rule.61
positions on the top and bottom of the guest. The eclipsed
Finally, the self-assembly of the hydrated cocrystal of 1,2,3-
phenazine molecules at parallel positions exhibited very weak trihydroxybenzene (common name pyrogallol) comprises
facial p–p interactions on one side but on the other side, there R66(12) synthons. The synthon has two trihydroxybenzene
were no facial p–p interactions. The p-separations between molecules and four water molecules, forming a hydrogen-
phenazine and the hydroxynaphthalene neighbors in parallel bonded cyclic structure (Fig. 3b). This cocrystal comprises one
Fig. 3 Portions of the self-assembly of (a) 2.5(Phen)$27DHN; (b) 2.5(Phen)$123THB.2H2O showing the p-interactions among the phenazine
rings.
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molecule of 1,2,3-trihydroxybenzene with 2.5 molecules of the distances are >3.5 A but the increments are in close range of
phenazine and two molecules of water. To form self-assembly, this stipulated distance to have p-interactions,52 there are weak
the two water molecules are linked together at the two sides p-interactions among the phenazine rings. In literature, it is
of two intervening 1,2,3-trihydroxybenzene molecules. The found that phenazine with 1,3,5-trihydroxybenzene (also known
centrally located O–H bond (that is at the 2-position of the 1,2,3- as phloroglucinol) forms different hydrate as well as anhydrous
trihydroxybenzene ring) is involved in the bifurcated hydrogen cocrystals; these four cocrystals each differing in stoichiometry
bonds with those water molecules. Each water molecule of this or the extent of hydration were characterized.49 In our case, we
assembly has one O–H bond free to serve as a hydrogen bond obtained only one form of the hydrated cocrystal of phenazine
donor. The four phenazine molecules form an O–H/N with pyrogallol from solution crystallization. This result
hydrogen bond with them and get anchored to remain parallel compared with the literature suggests that the positions and
to each other but perpendicular to the synthon. On the other orientations of the hydroxy groups on the benzene ring guide
hand, the two O–H groups located at the 1 and 3-positions of the the stoichiometry of the cocrystals.
6
ring of trihydroxybenzene are not a part of the R6 (12) synthon;
Different self-interacting assemblies among the phenazine
they project away from the synthon.62 These two hydroxyl molecules in the self-assemblies provided a scope to have
groups are hydrogen-bonded to the phenazine molecules a rational approach to study the effects of stacking on photo-
through the O–H/N bonds, whereꢁ the O3–H/N2 bond has luminescence properties. The phenazine molecule emits at
˚
dD/A ¼ 2.757(3) A, :D–H/A ¼ 163 and O1–H/N5 has dD/A 489 nm and the emission at this wavelength was not observed in
ꢁ
˚
¼ 2.787(3) A, :D–H/A ¼ 162 . Thus, there are parallel arrays any of the cocrystal. It was seen that the cocrystals of 1,2- and
of phenazine molecule chains along the c-crystallographic axis. 1,3-dihydroxybenzene were very weakly emitting and they were
These parallel cofacial rings are located on both sides of the like a quenched state. The cocrystal with 1,2,3-trihydrox-
cyclic synthon; they are arranged like an innite chain of ybenzene was also weakly uorescent, and it emitted at 415 nm,
parallel rings of phenazine molecules. The distances between which was 74 nm lower than the phenazine emission. The
the centroids of the phenazine rings of the ones that are cocrystal of 2,7-dihydroxynaphthalene showed very weak emis-
hydrogen-bonded to the O–H bonds of water molecules, the one sion at 484 nm, which was similar to that of phenazine (Fig. 4).
connected to O–H of trihydroxybenzene and the ones connected Among the cocrystals of the diols, this cocrystal was exceptional
to phenolic O–H at the two ends, and the ones from adjacent to have a phenazine molecule encapsulated between the p-
˚
˚
˚
synthons are 3.743 A, 3.759 A and 3.722 A, respectively. Since stacks. The encapsulated phenazine molecules in the assembly
were C–H/p bonded to the 2,7-dihydroxynaphthalene ring. All
the other structures had parallel stacks of phenazine rings, but
they showed negligible emission. The C–H/p interactions
helped the phenazine molecule to remain in isolation to show
a monomer-like emission of phenazine. The C–H/p interac-
tions facilitated enhancement in emission in the cocrystals of
naphthalene-aldoxime63 and in anthracene derivatives.52 The
emission and quantum yield in the solid state of each cocrystal
were determined, and they are listed in Table 2. The uores-
cence life-time of the cocrystals showed bi-exponential decay
proles. The cocrystals of 1,2- and 1,3-dihydroxybenzene and
1,2,3-trihydroxybenzene showed similar proles, with higher
amounts of the molecules decaying through shorter life-times
that ranged between 0.596 ns and 0.662 ns. Smaller fractions
exhibited life-times in the range of 2.010–3.066 ns. On the other
hand, the 2,7-dihydroxynaphthalene cocrystal showed a similar
trend with much shorter lifetimes. In this case, a fraction of
75.20% followed the relatively shortened life-time of 0.351 ns
Fig. 4 Fluorescence emission of the solid samples of (i) 2.5(Phen)$
27DHN (lex ¼ 335 nm, lem ¼ 484 nm); (ii) 27DHN (lex ¼ 335 nm, lem
¼
380 nm); (iii) Phen (lex ¼ 355 nm, lem ¼ 489 nm).
Table 2 Absorbance and emission of solid samples of phenazine, 27DHN and cocrystals of phenazine
lab (nm)
lex (nm)
lem (nm)
Quantum yield (FF)
Life-time in ns (% fraction)
2(Phen)$12DHB
2(Phen)$13DHB
2.5(Phen)$123THB$2H2O
2.5(Phen)$27DHN
Phenazine
425
422
420
418
417
336
365
365
365
335
335
335
438
434, 464
415
484
489
0.0058
0.0085
0.0181
0.0105
0.0200
0.0582
0.662 (80.149); 3.066 (19.851)
0.624 (78.274); 2.840 (21.726)
0.596 (81.239); 2.010 xed (18.761)
0.351 (75.195); 1.197 (24.805)
0.075 xed (64.386); 0.988 (35.614)
6.379 (100.00)
2,7-Dihydroxynaphthalene
380
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and the fraction of 24.80% followed a shorter life-time path with dihydroxynaphthalene had a relatively higher quantum yield in
1.197 ns. This showed that the amounts of stable exciplexes acetonitrile (4 ¼ 0.3752) than that in a solution of phenazine (4
formed in the rst three cases were relatively high. Hence, ¼ 0.1050) in acetonitrile. Hence, the quenching of the emission
quenching occurred, whereas in the case of 2,7-dihydrox- of 2,7-dihydroxynaphthalene was clearly seen. We checked the
ynaphthalene, monomer-like emission was observed. The changes with other naphthalene derivatives such as 2-naphthol,
crystal structure of a as well as b-phenazine was devoid of 1,6-dihydroxynaphthalene and 1,5-dihydroxynaphthalene by
˚
˚
effective p-stacking; the p-separations were 3.54 A and 3.45 A, adding phenazine to each solution. The Stern–Volmer plot of
respectively.63,64
each case is shown in ESI (Fig. 22S†). These also showed
A solution study enabled us to differentiate photophysical quenching of the uorescence emission on interactions with
properties measured from solid samples. An acetonitrile solu- phenazine in solution. Among these naphthols, the lowest
tion of phenazine emits at 478 nm upon excitation at 420 nm. quenching was seen with 1,6-dihydroxynaphthalene, whereas
The independent titration with 1,2- or 1,3-dihydroxybenzene as the other three naphthols showed comparable quenching. It is
well as with 1,2,3-trihydroxybenzene causes enhancement in clear that due to different structures, the ability of quenching of
this emission. The enhancements in the emission intensity the emission of dihydroxynaphthalenes by phenazine varied
occurred without a signicant shi in the emission wavelength. with the isomers of aromatic diols.
A
representative case of 1,2-dihydroxybenzene causing
From the emissive nature of the cocrystal of 2,7-dihydrox-
enhancement in the emission of the phenazine solution is ynaphthalene with phenazine in the solid state and from the
shown in Fig. 5a. The enhancements in solutions are due to the observations on the quenching of the emission of 2,7-dihy-
formation of hydrogen bonds by aromatic diols with phenazine droxynaphthalene in a solution on the interaction of phenazine,
facilitating photo-electron transfer.28
it was inferred that the emission processes in solids and solu-
Such enhancements in the uorescent aromatic heterocycles tions were distinguishable. In the solid state, different clip-like
by active acidic hydrogen are well known.28 Furthermore, the and chain-like structures were possible, as illustrated in Fig. 6.
emission properties change the p-stacked aromatic molecules The structures having prominent p-stacking showed Dexter
with slight changes in the orientations of the rings.65 However, quenching in the solid state. The only exception was the coc-
in the case of 2,7-dihydroxynaphthalene interacting with rystal of 2,7-dihydroxynaphthalene with phenazine. This coc-
phenazine, no notable increase was observed. This showed that rystal exhibited two sets of phenazine molecules in different
the PET mechanism was not effective in this case. The synergic environments of p–p interactions. One set was involved in
effects of aromatic stacking interactions and intrinsic acidity cofacial parallel p–p interactions and the other set was encap-
causing protonation helped decide the emission process.53,66 sulated. The encapsulated set was devoid of cofacial parallel p–
Hence, a reverse uorescence titration of the 2,7-dihydrox- p interactions but was involved in the edge-to-face C–H/p
ynaphthalene solution by adding phenazine was carried out. interactions. It was earlier suggested that the contribution of
The acetonitrile solution of 2,7-dihydroxynaphthalene emitted different p-interactions changes with the stacking patterns.52 In
at 344 nm upon excitation at 280 nm. This emission peak was the same article, it was shown that the variations of cations in
completely quenched upon the addition of a solution of phen- different salts and the edge-to-face C–H/p interactions help in
azine. The Stern–Volmer plot for the changes is shown in exciplex emission to inuence the intensity and position of
Fig. 12S.† It shows a non-linear plot, suggesting no change in emission. We observed a higher quantum yield in the particular
the ground state during the emission. As the concentration of case of a cocrystal of 2,7-dihydroxynaphthalene with phenazine
phenazine increased, Dexter quenching was observed. Also, 2,7- over that of the parent phenazine in the solid state. In the
Fig. 5 (a) Fluorescence titration (excitation at 420 nm) of phenazine (10ꢀ3 M in acetonitrile) by adding 1,2-dihydroxybenzene (10 mL aliquot of
10ꢀ3 M in acetonitrile). (b) Fluorescence titration (excitation at 280 nm) of 2,7-dihydroxynaphthalene (10ꢀ3 M in acetonitrile) by adding phenazine
(10 mL aliquot of 10ꢀ3 M in acetonitrile).
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Fig. 6 Different types of arrangements that contribute as repeating units in the self-assemblies of the four cocrystals. The central portion of the
figure is on parallel and non-parallel stacks depicted by using a floral part of a plant.
cocrystal of phenazine with 2,7-dihydroxynaphthalene, the solid state. Dexter quenching was prominent in the solid state,
emission of 2,7-dihydroxynaphthalene at 380 nm was not whereas competition between photo-electron transfer emis-
observed, but it weakly emitted at 484 nm, which is a close sions with Dexter quenching was observed in solutions. The
emission wavelength to that of phenazine. This cocrystal emission properties in solutions exhibited wide differences
comprised strong and weak uorescent partners. The emission from the emissions of the solid samples. In solution emissions,
of this cocrystal at 484 nm showed that the inuence of the the ON process was observed upon the addition of hydrox-
partner molecule was prominent on the strongly emitting u- ybenzenes to phenazine due to photo-electron transfer, whereas
orophoric component, namely, on 27DHN, whose emission was the OFF emission of hydroxynaphthalenes was observed when
not observed. The extent of the p–p interactions between uo- phenazine was added to the corresponding solution of
rophores decreases the excited-state non-radiative relaxations, hydroxynaphthalene due to Dexter quenching.
which causes partial quenching.2 This cocrystal did not show
the emission features of the strongly emitting parent compo-
nent, but it showed the emission of the weakly emitting
component phenazine. This was an exceptional case as it has
entrapped phenazine molecules held to 27DHN by C–H/p
Conflicts of interest
The authors declare that there is no conict of interest.
interactions; hence, emission occurred from the entrapped
phenazine molecules. In solutions, molecules were not in xed
geometry; hence, the weak interactions were averaged out to
Acknowledgements
show complete quenching of 2,7-hydroxynaphthalene by
The authors thank the Ministry of Human Resources, New Delhi
phenazine.
(India) for the nancial support through grant (No. F. No. 5-1/
In conclusion, this study elucidated four examples of
2014-TS.VII).
different parallel cofacial p-stacked arrangements of phenazine
molecules in the cocrystals of phenazine with dihydroxyar-
omatics. The centroid-to-centroid distances among the parallel
phenazine molecules were inuenced by the directional nature
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