Photophysical studies on multichromophoric cyclotriphosphazenes. Trinuclear excimer formation in hexakis(2-naphthyloxy)cyclotriphosphazene

Article abstract of DOI:10.1016/j.tetlet.2005.03.006

Hexakis(2-naphthyloxy)cyclotriphosphazene showed an interesting emission behavior between its monomer and excimer forms, the latter nearly completely dominating in water. Encapsulation studies with β-cyclodextrin in water partially revived the monomer emission.

Full text of DOI:10.1016/j.tetlet.2005.03.006

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3089–3092
Photophysical studies on multichromophoric cyclotriphosphazenes.
Trinuclear excimer formation in
hexakis(2-naphthyloxy)cyclotriphosphazene
Nitin Chattopadhyay,* Basudeb Haldar, Arabinda Mallick and Saumitra Sengupta*
Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India
Received 14 December 2004; revised 19 February 2005; accepted 1 March 2005
Available online 16 March 2005
Abstract—Hexakis(2-naphthyloxy)cyclotriphosphazene showed an interesting emission behavior between its monomer and excimer
forms, the latter nearly completely dominating in water. Encapsulation studies with b-cyclodextrin in water partially revived the
monomer emission.
Ó 2005 Elsevier Ltd. All rights reserved.
With a view to understanding the mechanism and
dynamics of natural photosynthetic reactors,1 photo-
physical studies of multichromophoric scaffolds have
gained much current attention. These scaffolds are also
useful for harvesting diffused sunlight for potential use
in solar energy conversion and in the construction of
luminescent biological and trace metal sensors.2 Early
studies on multichromophoric systems were carried
out with polymers that were substituted at intervals with
chromophoric units.3 However, due to their inherent
flexibility and thus a number of different coiled confor-
mations, these polymers turned out to be poor mimics
of the natural photosynthetic reactors. In search of bet-
ter mimics, Jullien and co-workers recently studied a
number of naphthyl substituted b-cyclodextrins (b-
CDs) containing up to 14 chromophore units.4 The
structural rigidity of these systems held the naphthyl
chromophores in spatially well-defined positions and
in close proximity with each other. This led to efficient
energy hopping between the naphthyl units in these mole-
cules. Inspired by these results, we initiated a study on
new multichromophoric scaffolds as molecular mimics
of the natural photosynthetic antennae.
Our initial task was to seek a conformationally rigid
multitopic core that could be substituted with multiple
chromophoric units. Previous studies from these labora-
tories have shown that tetraphenylmethane, a centrally
rigid tetrahedral scaffold, can be effectively used in this
regard.5 However, tetraphenylmethane can accommo-
date only four chromophores. While searching for a
higher valent core structure, we were attracted to the
cyclotriphosphazenes (CTPs), which have hexavalent
inorganic cores. In this paper, we describe the unusual
excited state photophysical properties of hexakis(2-
naphthyloxy)cyclotriphosphazene 3 arising from highly
pre-organized trinuclear excimer formation.
Cyclotriphosphazene (CTP) is a well known hexatopic
core, which can be easily per-functionalized via substitu-
tion reactions on the corresponding hexachloride
(NPCl2)3, especially with heteroatomic nucleophiles.6
In symmetrically per-substituted CTP derivatives, the
inorganic ring is nearly planar, the phosphorus atoms
almost tetrahedral (sp3) and the nitrogen atoms ap-
proach an sp2 geometry.7 As a result, the geminal sub-
stituents on the phosphorus atoms are oriented in a
well defined spatial arrangement, one above and one
below the CTP ring. Moreover, in homogeneously per-
substituted CTPs, the three substituents on either side
of the CTP ring are also mutually equidistant. CTP rings
are photochemically inert and do not have any low en-
ergy absorption band of their own. Hence, they do not
interfere with the photophysical properties associated
with the attached chromophores. The ease with which
Keywords: Cyclotriphosphazene; 2-Naphthol; Excimer; b-Cyclo-
dextrin.
*
Corresponding authors. Tel.: +91 33 473 4266; fax: +91 33 414
6266; e-mail addresses: pcnitin@yahoo.com; jusaumitra@yahoo.
co.uk
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.006

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N. Chattopadhyay et al. / Tetrahedron Letters 46 (2005) 3089–3092
`
CTP rings can be per-functionalized (vis-a-vis cyclodex-
trins) together with the structural rigidity of the prod-
ucts make CTP an attractive core for the construction
of multichromophoric scaffolds.
A
number of hexakis(aryloxy)cyclotriphosphazenes
were prepared from hexachlorocyclotriphosphazene
and the corresponding phenol (NaH, THF, reflux).8
The absorption and emission spectra of those derived
from simple phenols (phenol, 4-tert-butylphenol) were
similar to those of the individual phenols indicating that
there were no interactions between the chromophore
units either in the ground state or in the excited state.
However, hexakis(2-naphthyloxy)cyclotriphosphazene
3, derived from 2-naphthol (Scheme 1), showed an unex-
pected emission behavior, entirely different from the
hexakis(phenoxy) and the hexakis(4-tert-butylphenoxy)
derivatives, which attracted our attention for detailed
study.
240
280
320
360
Wavelength (nm)
Figure 1. Absorption (solid line) and excitation (dotted line) spectra of
3 in CH2Cl2.
no ground state interactions between the chromophores
in 3. If at all present, they must be very weak with mag-
nitudes smaller than the optical line width.
The absorption spectrum of 3 recorded in CH2Cl2 is
shown in Figure 1 (solid line). The positions of absorp-
tion are similar to those of 2-naphthol.9 The absorption
maxima do not show any appreciable solvatochromic
shift in solvents of widely different polarities (n-heptane,
dioxane, CH2Cl2, CH3CN, MeOH, water). However, in
contrast to the vibrational structures observed for
2-naphthol,9 the lower energy bands of 3, especially
the E2 band (kmax 276 nm with shoulders at 272 and
279 nm), are somewhat broadened. This is obviously
caused by the presence of six 2-naphthol units in 3.
From the similarity of the absorption spectrum of 3 to
that of 2-naphthol, it may be inferred that there are
The emission spectra of 3 (kexc 280 nm) recorded in dif-
ferent solvents are shown in Figure 2. Two emission
bands were observed, one centred at kmax 340 (struc-
tured) and the other at 386 nm (broad and unstructured)
with red tailing up to 500 nm. In comparison, the emis-
sion spectrum of 2-naphthol (10ꢀ6 M in CH2Cl2, kexc
274 nm) showed structured bands consisting of a maxi-
mum at 347 nm with a shoulder at 340 nm. Thus the
appearance of the 386 nm band in 3 was somewhat
unexpected. Moreover, the emission spectrum of 3 was
found to be highly dependent on solvent polarity. Thus,
on increasing the solvent polarity (n-heptane!diox-
ane!CH3CN!MeOH!water), the intensity of the
386 nm band gradually increased at the expense of the
340 nm band. In a highly polar solvent such as water,
the 386 nm band was practically the sole peak. Based
on these observations, we assigned the 386 nm band in
3 as that arising from excimer formation whereas the
shoulder at 340 nm represented emission from the unas-
sociated form. The excitation spectrum of 3 monitored
OH
(NPCl2)3 +
1
2
NaH, THF, reflux
DOX
DCM
ACN
MeOH
Water
O
O
O
O
O
N
P
P
N
N
P
O
300
350
400
450
500
Wavelength (nm)
3 (65%)
Figure 2. Emission spectra of 3 (kexc 280 nm) in different solvents
(DOX = 1,4-dioxane, DCM = CH2Cl2, ACN = CH3CN).
Scheme 1.

N. Chattopadhyay et al. / Tetrahedron Letters 46 (2005) 3089–3092
3091
at 386 nm (Fig. 1, dotted line) was found to be similar to
(i)
its absorption spectrum. Such correlation was also
found in other solvents, irrespective of the solvent polar-
ity. Since the excitation spectra of 3 were insensitive to
the solvent polarity and therefore, independent of the
relative proportions of the 386 nm and the 340 nm
bands, the former band is more likely due to excimer
formation than any ground state interactions.
(iv)
(i)
(iv)
Notably, the emission spectrum of a concentrated
solution of 2-naphthol (10ꢀ2 M in CH2Cl2), where inter-
molecular excimer formation is favored, showed a
maximum at 352 nm with a shoulder at 376 nm. While
the appearance of this 376 nm peak is due to excimer
formation and closely matches the position of the exci-
mer peak of 3 (kmax 386 nm), excimer formation with
2-naphthol occurred only in very concentrated solu-
tions. In contrast, excimer emission from 3 was obtained
in dilute solutions (10ꢀ6 M) suggesting that the excimer
formation in 3 is intramolecular, rather than intermolec-
ular, in nature. The positive solvatochromic shift ob-
served in 3 together with the large difference in its
excimer and monomer emissions (3500 cmꢀ1) point to-
wards intramolecular excimer formation. Semi-empiri-
cal AM1 calculations (Hyperchem 5.01) on 3 showed
that the central inorganic ring was almost planar with
a near perpendicular O–P–O bond angle.10 The six
naphthalene units are highly pre-organized, three on
either side of the central CTP ring. The three naphthal-
ene rings on either side are oriented in a propeller type
arrangement. Perhaps, due to the rigid geometry of the
central CTP ring,8c which orients the chromophores
(three on either side of the CTP ring) in close proximity,
strong p-stacking interactions can operate between the
naphthyl rings on either side of the CTP ring favoring
intramolecular excimer formation. The average interpla-
nar distance between the naphthyl rings on either side of
300
350
400
Wavelength (nm)
450
500
Figure 3. Emission spectra of 3 in water (2 lM) in the presence of
b-cyclodextrin (i: 0 mM, ii: 3.9 mM, iii: 7.8 mM, iv: 11.7 mM).
Isoemissive point at k 351 nm.
in 3, which completely dominated in water, is somewhat
inhibited since some of the naphthyl chromophores are
trapped inside the b-CD cavity and hence, cannot
participate in intramolecular charge transfer. An iso-
emissive point was obtained at 351 nm indicating a 1:1
complexation between the naphthyl rings of 3 and b-
CD. This is in agreement with the reported complex-
ation behavior of 2-naphthol and b-CD.11b Addition of
a-CD, however, had no effect on the emission spectrum
of 3 in water. The different effects of a- and b-CD can be
rationalized by comparing the cavity sizes of the two
types of cyclodextrins and the molecular dimension of
the naphthyl units. Semi-emipirical AM1 calculations
show that the naphthyl units in 3 have a diameter of
˚
ꢁ7.2 A. The size of the cavities for a- and b-CD are
˚
4.5 and 7.8 A, respectively. Thus, a-CD, with a smaller
cavity size cannot encapsulate the naphthyl units of 3
and hence, does not have any effect on its excimer emis-
sion. On the other hand, the cavity size of b-CD matches
favorably with the molecular diameter of the naphthyl
units leading to strong binding and consequently, partial
inhibition of the excimer emission. However, even in the
presence of b-CD, the excimer emission of 3 was only
partially quenched. This may be due to the fact that
all the naphthyl rings in 3 were not encapsulated, even
with a large excess of b-CD, due to steric hindrance.
The energy minimized structure of 310 clearly indicates
that encapsulation of all six naphthyl rings with large
CD rings would lead to severe steric hindrance.
˚
the CTP plane is ꢁ4.5 A, which is well within the geo-
metric limits for intramolecular exchange interaction
to take place. Therefore, trinuclear excimer formation
between the three naphthyl units on either side of the
CTP plane is expected to be a facile process. In compari-
son, hexaphenoxycyclotriphosphazene showed only
monomer emission (kmax 290 nm) with no excimer for-
mation. Perhaps, due to their smaller size, the phenol
rings in the latter do not reach the proximal distance
for excimer formation. Since energy trapping by pre-
formed excimers is an irreversible process and excimer
emission dominates in 3, we conclude that energy migra-
tion between the naphthyl chromophores should be very
fast. However, at this stage, we cannot comment on the
possibility of interfacial energy transfer across the CTP
ring in 3.
In conclusion, we have shown that cyclotriphosphazene
can be used as a rigid hexavalent scaffold for construct-
ing multichromophoric modules having unusual and
interesting photophysical properties. When a relatively
large chromophore is used, as in 3, the structural rigidity
of the CTP ring leads to a preferred conformation in
which three chromophores reside on either side of the
CTP plane forcing them into close proximity. Such a
pre-organization leads to facile intramolecular charge
transfer in the excited state and a near complete tri-
nuclear excimer formation. Taking advantage of this
conformational bias, we are currently preparing other
hexachromophoric CTP derivatives for light harvesting
studies.
Cyclodextrins (CD) can encapsulate various organic
chromophores in their hydrophobic cavity and have
been widely used as probes to investigate excimer form-
ation in aqueous solutions.11 Addition of b-CD to an
aqueous solution of 3 led to a gradual reappearance of
the monomer emission band at 340 nm along with a
concomitant decrease in the intensity of the excimer
band (Fig. 3). Reappearance of the monomer emission
upon b-CD addition indicated that excimer formation

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M. N.; Canciell, J.; Gratton, G.; Jullien, L.; Lehn, J. M.;
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Financial support from CSIR, Govt. of India is grate-
fully acknowledged. B.H. thanks CSIR for a research
fellowship. We thank Professor V. Snieckus for helpful
suggestions.
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