Dendrimers as photochemical reaction media. Photochemical behavior of unimolecular and bimolecular reactions in water-soluble dendrimers

Article abstract of DOI:10.1021/jo0503254

Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the monomer to 0-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing microenvironmental properties of the aqueous solutions were assessed by pyrene solubilization studies. Upon establishing the presence of nonpolar microenvironments within the dendritic structures, solubilizations of few organic substrates were conducted and their photochemical behaviors were assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether and photodimerization of acenaphthylene were conducted. These studies revealed that the product distribution and the "cage effect" were more distinct and efficient for the third generation dendrimer, than for the first and second generation dendrimers. The photochemical studies of carboxylic acid functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether) dendrimers.

Full text of DOI:10.1021/jo0503254

Dendrimers as Photochemical Reaction Media. Photochemical
Behavior of Unimolecular and Bimolecular Reactions in
Water-Soluble Dendrimers
Lakshmi S. Kaanumalle,† R. Ramesh,‡ V. S. N. Murthy Maddipatla,†
Jayaraj Nithyanandhan,‡ Narayanaswamy Jayaraman,*,‡ and V. Ramamurthy*,†
Department of Chemistry, University of Miami, Coral Gables, Florida 33124, and
Department of Organic Chemistry, Indian Institute of Science, Bangalore, 560118, India
jayaraman@orgchem.iisc.ernet.in; murthy1@miami.edu
Received February 21, 2005
Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities
at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the mono-
mer to O-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers
of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were
thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing micro-
environmental properties of the aqueous solutions were assessed by pyrene solubilization studies.
Upon establishing the presence of nonpolar microenvironments within the dendritic structures,
solubilizations of few organic substrates were conducted and their photochemical behaviors were
assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether
and photodimerization of acenaphthylene were conducted. These studies revealed that the product
distribution and the “cage effect” were more distinct and efficient for the third generation dendrimer,
than for the first and second generation dendrimers. The photochemical studies of carboxylic acid
functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether)
dendrimers.
Introduction
defined inner cavities or microenvironments.2 The pres-
ence of such inner cavities is more profound in the larger
generation dendrimers than the lower generation den-
drimers, due to spherical and globular shapes of the
larger generation dendrimers. A number of studies have
An outcome of the branches-upon-branches structural
features present in dendrimers1 is the evolution of
† University of Miami.
‡ Indian Institute of Science.
10.1021/jo0503254 CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/20/2005
5062
J. Org. Chem. 2005, 70, 5062-5069

Dendrimers as Photochemical Reaction Media
SCHEME 1a
been conducted to encapsulate different types of organic
molecules and metal ions inside the dendritic cavities.2
Solubilization of water-insoluble organic substrates in
aqueous solutions of dendrimers is an area of active inter-
est.3 The host ability of dendritic microenvironments
forms a newer strategy to encapsulate guest molecules,
and, in this respect, dendritic microenvironments might
be considered analogous to well-known hosts, such as
cyclodextrins4 and micelles.5,6 We have recently reported
the synthesis of a series of phloroglucinol-based dendri-
mers, in which the phloroglucinol moiety acts as the core
as well as the branch junctures and these branch junc-
tures are linked with the aid of pentamethylene linkers.7
Photochemical reactions of few water-insoluble substrat-
es solubilized in aqueous phloroglucinol-based dendritic
media showed the possibility of product selectivity in these
novel reaction media. In few cases, such selectivities were
comparable or better than in reaction media originating
from traditional micelles.8 In an effort to modify the
peripheries of these phloroglucinol-based dendrimers
with functional groups that would allow water solubility,
carboxylic acid terminated dendrimers were synthesized.
Upon synthesis, the encapsulation properties of these
carboxylic acid modified dendrimers with photochemi-
cally active water-insoluble substrates were performed,
in aqueous solutions. Synthesis and photochemical stud-
ies with the new carboxylic acid modified poly(alkyl aryl
ether) dendrimers are reported herein.
a Reagents and conditions: (a) K2CO3, 2-butanone, 18-C-6 (cat.),
8 h.
SCHEME 2a
a Reagents and conditions: (a) K2CO3, 18-C-6 (cat.), 2-butanone
or DMF, reflux, 12 h; (b) KOH, MeOH:THF:water (2:2:1), 15 h.
ized with a bromopentyloxy linker, so as to afford the
activated monomer 2 (Scheme 1). O-Alkylation of 1,3,5-
tris-(5-bromopentyloxy)benzene (3) with 1 led to the
isolation of G-1(CO2Me)6 (4), which upon hydrolysis
afforded G-1(CO2H)6 (5) (Scheme 2). The monomer 2 was
subjected to O-alkylation of hydroxy group terminated
first (G-1(OH)6) and second (G-2(OH)12) dendrimers,
containing 6 and 12 hydroxyl group,7 respectively. The
O-alkylation was performed in the presence of K2CO3 and
18-C-6; in DMF and the corresponding methyl esters, 6
and 8 were obtained in good yields, respectively. Upon
multiple O-alkylation of the preformed dendrimers, the
methyl ester protecting groups were removed under basic
conditions, so as to afford the second and third generation
carboxylic acid terminated dendrimers, possessing 12 (G-
2(CO2H)12) (7) and 24 (G-3(CO2H)24) (9) carboxylic acid
groups, respectively, at their peripheries (Scheme 2). The
methyl ester protected and free carboxylic acid containing
dendrimers were characterized by IR, NMR spectroscopies,
and elemental composition analysis, so as to establish
their structural homogeneities. The molecular structures
of the carboxylic acid terminated dendrimers 5, 7, and 9
are presented in Figure 1. Dendrimers 5, 7, and 9 were
freely soluble in alkaline aqueous solutions pH > 9.
Pyrene as a Probe To Measure the Interior
Hydrophobicity. Upon establishing the synthesis and
water-soluble nature of the carboxylic acid terminated
dendrimers, solubilization studies of pyrene in dendrimer
solubilized aqueous solutions were conducted. Pyrene has
a solubility in the range of 10-7 M in aqueous solutions
and thus is practically insoluble in water. Thus, observing
pyrene solubilization in aqueous solutions containing the
host is an indirect reflection of the ability of the host to
include an organic molecule. The encapsulation of the
pyrene molecule can be followed by its fluorescence
emission bands, typically the ratio of the I1 and I3
emission bands. The I1/I3 ratio is also a good measure of
assessing the medium polarity.9 Table 1 provides the
Results and Discussion
Synthesis. Syntheses of carboxylic acid functionalized
dendrimers were performed by following protocols estab-
lished previously for the synthesis of phloroglucinol-based
poly(alkyl aryl ether) dendrimers. A divergent synthetic
route was utilized accordingly to synthesize the carboxy-
lic acid functionalized poly(alkyl aryl ether) dendrimers.
5-Bromopentyloxy methylisophthalate (2) was chosen to
afford the carboxylic acid functionalities in the dendrim-
ers. As the required monomer, diester 1 was functional-
(1) Many aspects of dendrimer chemistry are covered in: (a) Top.
Curr. Chem. 1998, 197. (b) Top. Curr. Chem. 2000, 210. (c) Top. Curr.
Chem. 2001, 212. (d) Top. Curr. Chem. 2002, 217.
(2) (a) Smith, D. K.; Diederich, F. Chem. Rev. 1904, 4, 1353. (b)
Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74. (c)
Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99,
1665. (d) Newkome, G. R.; He, E.; Moorefield, N. Chem. Rev. 1999, 99,
1689. (e) Inoue, K. Prog. Polym. Sci. 2000, 25, 453. (f) Gorman, C. B.;
Smith, J. C. Acc. Chem. Res. 2001, 34, 60. (g) Oosterom, G. E.; Reek,
J. N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Angew. Chem.,
Int. Ed. 2001, 40, 1828.
(3) For example: (a) Hawker, C. J.; Wooley, K. L.; Frechet, J. M. J.
J. Chem. Soc., Perkin Trans. 1 1993, 1287. (b) Baars, M. W. P. L.;
Froeling, P. E.; Meijer, E. W. Chem. Commun. 1997, 1959. (c)
Sideratou, Z.; Tsiourvas, D.; Paleos, C. M. Langmuir 2000, 16, 1766.
(d) Morgan, M. T.; Carnahan, M. A.; Immoos, C. E.; Ribeiro, A. A.;
Finkelstein, S.; Lee, S. J.; Grinstaff, M. W. J. Am. Chem. Soc. 2003,
125, 15485. (e) Meijer, E. W.; van Genderen, M. H. P. Nature 2003,
426, 128.
(4) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-
Verlag: New York, 1978.
(5) Moroi, Y. Micelles, Theoretical and Applied Aspects; Plenum:
New York, 1992.
(6) (a) Photochemistry in microheterogeneous systems; Kalyana-
sundaram, K., Ed.; Academic Press: New York, 1987. (b) Photochem-
istry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH:
New York, 1991.
(7) Nithyanandhan, J.; Jayaraman, N. J. Org. Chem. 2002, 67,
6282-6285.
(8) Kaanumalle, L. S.; Nithyanandhan, J.; Pattabiraman, M.; Ja-
yaraman, N.; Ramamurthy, V. J. Am. Chem. Soc. 2004, 126, 8999.
J. Org. Chem, Vol. 70, No. 13, 2005 5063

Kaanumalle et al.
FIGURE 1. Structures of the carboxylic acid terminated first (G-1), second (G-2), and third (G-3) generation dendrimers.
TABLE 1. Ratio of I1/I3 Bands of Pyrene in Acid Dendrimer and Water
phenolic
dendrimers
[pyrene] M
[host] M
I1/I3 ratio
acid dendrimers
I1/I3 ratio
1 × 10-4
1 × 10-4
1 × 10-4
1 × 10-4
2 × 10-4
2 × 10-4
2 × 10-4
G-3(OH)24
G-2(OH)12
G-1(OH)6
1.18
1.30
1.48
1.70
G-3(CO2H)24
G-2(CO2H)12
G-1(CO2H)6
1.23
1.47
1.60
1.70
water (pH > 9)
water (pH > 9)
ratios of I1 to I3 bands of pyrene upon its solubilization
in aqueous solutions containing the carboxylic acid
terminated dendrimers. A comparison of the I1/I3 ratio
across the carboxylic acid terminated dendrimers showed
an increasingly hydrophobic microenvironment being
encountered by the pyrene molecule as the dendrimer
5064 J. Org. Chem., Vol. 70, No. 13, 2005

Dendrimers as Photochemical Reaction Media
SCHEME 3
generation advanced. The peripheral hydrophilic region,
constituted with carboxylic acid functionality and the
relatively hydrophobic inner pentamethylene linker re-
gions, should afford an increasing nonpolar microenvi-
ronment at the interior of the dendritic structure. Results
of similar studies performed for the corresponding hy-
droxyl group terminated dendrimers,8 reported previ-
ously, are also included in Table 1. Comparison of I1/I3
values obtained from these dendrimers reveals that the
phenolic dendrimers are more hydrophobic than the
carboxylic acid dendrimers.
Photochemistry of Organic Guests Included
within the Dendrimer Core. Upon identifying the
solubilization properties of the dendrimers, their ability
to function as photochemical reaction media was studied,
particularly, with respect to the unimolecular and bimo-
lecular photochemical reactions. Unimolecular reactions
involving photoysis of 1-phenyl-3-p-tolyl-propan-2-one (4-
methyl dibenzyl ketone) and photolysis of benzoin ethyl
ether were undertaken. Bimolecular photoreaction using
the dendrimers as the media was concerned with the
photodimerization of acenaphthylene. Results of these
reactions are discussed individually below. In all of the
cases studied, an analysis of the results involving the
carboxylic acid terminated dendrimers is compared with
that of the hydroxyl group terminated dendrimers.
Photolysis of 1-Phenyl-3-p-tolyl-propan-2-one.
1-Phenyl-3-p-tolyl-propan-2-one (10) upon photolysis un-
dergoes R-cleavage leading to primary radical pairs (11
and 12) as shown in Scheme 3.10,11 In hexane solution,
diaryl ethane products 14 (AA), 15 (AB), and 16 (BB)
were formed, through the secondary radical (13) in a 1:2:1
ratio. The cage effect for this reaction was calculated
according to the equation:
TABLE 2. Photolysis of 4-Methyl Dibenzyl Ketone in
Water-Soluble Hosts
relative ratio
medium
host:guest
AA
BB
AB
cage effect
hexane
1.4
1.6
1.6
1.8
1.0
1.0
1.0
1.0
1.0
1.0
2.4
2.8
3.2
4.4
6.1
0.00
0.04
0.09
0.18
0.50
aq KOH
G-1(CO2H)6
G-2(CO2H)12
G-3(CO2H)24
6:1
2:1
1:1
a [10] ) 2.9 × 10-4 M; [G-1(CO2H)6] ) 1.8 × 10-3 M;
[G-2(CO2H)12] ) 6 × 10-4 M; [G-3(CO2H)24] ) 3 × 10-4 M. pH )
10-11.4. b Irradiation time G-1 ) 12 h, G-2 ) 20 h, G-3 ) 48 h to
achieve 30% conversion to photoproducts. c Cage effect in corre-
sponding phenolic dendrimers G-1, G-2, and G-3 are 0.11, 0.42,
and 0.77, respectively.
effect increased progressively as the generations ad-
vanced (Table 2). A cage effect of 0.5 was thus obtained
for the third generation dendrimer, and this value was
also higher, when compared to the dendrimer generations
1 and 2. Although cage effect in G-3 acid dendrimer is
greater as compared to isotropic solution, the cage effect
is still less when compared to the corresponding G-3
phenolic dendrimer (see footnote c in Table 2). Cage effect
results suggest that the G-3 phenolic dendrimer is more
confining (less open) than the G-3 acid dendrimer, the
results of which are consistent with the pyrene polarity
studies.
Photolysis of Benzoin Ethyl Ether. The second
guest that we have investigated to ascertain the confine-
ment provided by the acid dendrimers is benzoin ethyl
ether (17). This substrate upon photolysis can lead to
products arising from either cleavage (Norrish type I) or
hydrogen abstraction (Norrish type II).12 Type I products
formed from triplet radical pair 19 are pinacol ether (22),
benzil (23), and the rearrangement products (ortho
isomer (24) and para isomer (25)) (Scheme 4).13 Type II
products formed from radical pair 18 include deoxyben-
zoin (20) and oxetanol (21).
AB - (AA + BB)
(AB + AA + BB)
cage effect )
(1)
Analysis of the product distribution, arising from the
involvement of the dendrimers, revealed that the cage
In an organic solvent like hexane and in aqueous
solution (pH ) 7.0), type I products 22 and 23 were
formed in greater amounts (97% yield), while in basic
aqueous solution type II product deoxy benzoin (20) was
the major product (Table 3). Reasons for the enhanced
(9) (a) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977,
99, 2039-44. (b) Dong, D. C.; Winnik, M. A. Photochem. Photobiol.
1982, 35, 17-21.
(10) (a) Robbins, W. K.; Eastman, R. H. J. Am. Chem. Soc. 1970,
92, 6076-6077. (b) Engel, P. S. J. Am. Chem. Soc. 1970, 92, 6074-
6075.
(11) (a) Turro, N. J. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 609-
621. (b) Gould, I. R.; Turro, N. J.; Zimmt, M. B. In Advances in Physical
Organic chemistry; Gold, V., Bethell, D., Eds.; Academic Press: London,
1984; Vol. 20, pp 1-53. (c) Turro, N. J. Chem. Commun. 2002, 2279-
2292. (d) Turro, N. J. Acc. Chem. Res. 2000, 33, 637-646.
(12) Lewis, F. D.; Lauterbach, R. T.; Heine, H.-G.; Hartmann, W.;
Rudolph, H. J. Am. Chem. Soc. 1975, 97, 1519-1525.
(13) (a) Turro, N. J.; Gould, I. R.; Baretz, H. B. J. Phys. Chem. 1983,
87, 531-532. (b) Lunazzi, L.; Ingold, K. U.; Scaiano, J. C. J. Phys.
Chem. 1983, 87, 529-530.
J. Org. Chem, Vol. 70, No. 13, 2005 5065

Kaanumalle et al.
SCHEME 4
TABLE 3. Photolysis of Benzoin Ethyl Ether in
Water-Soluble Hostsa
SCHEME 5
medium
20
21
22
23
hexane
4
5
54
82
23
13
2
40
3d
4
water
10
70
80
88
95
21
KOHb
3
G-1(CO2H)6
G-2(CO2H)12
G-3(CO2H)24
3
4
5
5
1
2
2
c
G-3(OH)24
57
1
a G-1(CO2H)6 ) 2.5 × 10-3 M, G-2(CO2H)12 ) 8.4 × 10-4 M,
G-3(CO2H)24 ) 7.8 × 10-4 M, [benzoin ethyl ether] ) 4.16 × 10-4
M. b pH ≈ 11. c In addition to the tabulated products [20-23],
rearrangement products 24 and 25 were also formed in 8% and
14% yields, respectively. pH ≈ 10. d % yield of benzil is less because
benzoyl triplet radical reacts to give benzaldehyde in aqueous
solution. Benzaldehyde being volatile and soluble in water, % yield
was not reported.
ers. In continuation of this comparative studies, we have
carried out a bimolecular reaction, photodimerization of
acenaphthylene both in phenolic and carboxylic group
terminated dendrimers. Acenaphthylene (26) on photoly-
sis undergoes dimerization to form syn- and anti-dimers
(Scheme 5).14
Dimerization can occur from both singlet and triplet
states.9 Syn- and anti-dimer ratios vary with the nature
of the excited state, the concentration of acenaphthylene,
and the polarity of the solvent. In a highly concentrated
solution and from the S1 state (short-lived excited state),
acenaphthylene molecules being present in close proxim-
ity form the syn-dimer (27), whereas in dilute solution
and from the T1 state (long-lived), dimerization results
in both syn- and anti (28)-dimers. Experimental results
obtained with pyrene, 1-phenyl-3-p-tolyl-propan-2-one,
and benzoin ethyl ether in acid and phenolic dendrimers
made us conclude that acid dendrimers that are more
open and less hydrophobic would be more effective in
solubilizing organic guest molecules than phenolic den-
drimers. Solubility studies (Table 4) conducted with
acenaphthylene support our conclusion. Acenaphthylene
that is poorly soluble in water (<2.2 × 10-5 M) becomes
more soluble in the presence of phenolic and acid den-
drimers. As expected acenaphthylene is more soluble in
the presence of phenolic dendrimer (175 × 10-5 M, G-3)
than in the presence of acid dendrimer (92 × 10-5 M,
G-3). Such an enhanced local concentration would be
expected to lead to more efficient photodimerization.
Consistent with this, acenaphthylene photodimerization
was found to be faster within dendritic media as com-
yield of 20 under basic conditions are yet to be under-
stood. When benzoin ethyl ether encapsulated G-3 acid
dendrimer was irradiated, deoxy benzoin is formed in
95% yield, while cage escape type I products 22 and 23
were formed only in trace amounts (Table 3). Because
deoxybenzoin is formed in considerable amount in basic
aqueous solution even in the absence of dendrimer, now
the question arises whether the reaction is taking place
within the hydrophobic pockets of the dendrimer or in
basic aqueous solution. The absence of cage escape
products 22 and 23 suggests that the photoreaction is
probably taking place within the dendrimer. Despite the
increased yield of Type II products (with respect to Type
I) in both phenolic and carboxylic acid dendrimers,
differences in product distribution between them are
noteworthy. While in carboxylic acid dendrimers the
main Type II product was deoxybenzoin, in phenolic
dendrimer both oxetanol and deoxybenzoin were ob-
tained. Further, in phenolic dendrimer rearrangement
products 24 and 25 were obtained in an overall yield of
22% (G-3). These differences suggest the headgroup has
a role in dictating the interior character of the dendrim-
ers. Because of the complication by the basic medium,
clear-cut conclusions concerning the origin of Type II
products in carboxylic acid dendrimers could not be made.
Photodimerization of Acenaphthylene. With uni-
molecular reactions, we have shown that phenolic den-
drimers are much more confining than the acid dendrim-
(14) (a) Cowan, D. O.; Drisko, L. E. J. Am. Chem. Soc. 1970, 92,
6281-6285. (b) Cowan, D. O.; Drisko, L. E. J. Am. Chem. Soc. 1970,
92, 6286-6291.
5066 J. Org. Chem., Vol. 70, No. 13, 2005

Dendrimers as Photochemical Reaction Media
TABLE 4. Solubility Studies of Acenaphthylene (26) in G2 Dendrimer
amount of
dendrimer
in 5 mL of soln
amount of
acenaphthylene
solubilized
concn of
acenaphthylene
solubilized
concn of
dendrimer
0.017 mg
0.70 mg
1.33 mg
0.57 mg
0.70 mg
0.22 × 10-4
9.2 × 10-4
17.5 × 10-4
7.5 × 10-4
9.2 × 10-4
M
M
M
M
M
G2(OH)12
8 × 10-4
3.8 × 10-4
9 × 10-4
M
G3(OH)24
M
M
M
G2(COOH)12
G3(COOH)24
4.1 × 10-4
TABLE 5. The Photodimerization of Acenaphthylene (26) in Water-Soluble Hosts
average
27:28
average
27:28
medium
host:guest
medium
host:guest
G-1(OH)24
G-2(OH)24
G-3(OH)24
2:1a
1:1a
1:2a
71:29
75:25
80:20
G-3(CO2H)24
G-3(CO2H)24
G-3(CO2H)24
9:1b
1:1b
1:1b
68:32
59:41
43:57
a G-1(OH)24 ) 14 × 10-4 M, G-2(OH)24 ) 8 × 10-4 M, G-3(OH)24 ) 3.8 × 10-4 M. b G-1(CO2H)6 ) 22.6 × 10-4 M, G-2(CO2H)12 ) 9.04
× 10-4 M G-3(CO2H)24 ) 4.11 × 10-4 M.
pared to that in an organic solvent. For example, for the
same amount of acenaphthylene and irradiation time, the
yield of dimer within phenolic dendrimer was 40-50%,
while in methanol it was less than 5%.
Experimental Section
5-Bromopentyloxy Methylisophthalate (2). A mixture
of 1 (5 g, 0.02 mmol), 1,5-dibromopentane (22 g, 0.09 mmol),
K2CO3 (4.0 g, 0.03 mmol), and 18-C-6 (cat.) in 2-butanone (60
mL) was refluxed for 8 h, and the solvents were removed in
vacuo. The resulting residue was dissolved in CH2Cl2, washed
with water, dried (Na2SO4), concentrated, and purified (SiO2,
PhMe/EtOAc ) 98.5/1.5) to afford 2, as a white solid (7 g, 82%).
Mp: 41-43 °C. TLC: Rf ) 0.54 (PhMe/EtOAc ) 96:4). 1H NMR
(300 MHz, CDCl3) δ: 1.54-1.64 (m, 2H), 1.74-1.94 (m, 4H),
3.40 (t, J ) 6.6 Hz, 2H), 3.88 (s, 6H), 4.00 (t, 2H, J ) 6.3 Hz),
7.67 (s, 2H), 8.20 (s, 1H). 13C NMR (75.5 MHz, CDCl3) δ: 24.6,
28.1, 32.3, 33.4, 52.3, 68.0, 119.6, 122.7, 131.6, 158.9, 166.0.
HR-MS m/z: 381.0314 ([M + Na]+). FT-IR (neat): 1730. Anal.
Calcd for C15H19O5Br: C, 50.17; H, 5.33. Found: C, 50.37; H,
5.42.
Photolysis of acenaphthylene (26) encapsulated within
phenolic dendrimer solutions under nitrogen purging
conditions yielded the syn- and anti-dimers in the ratio
2.5:1 (G-1), 3:1 (G-2), and 4:1 (G-3) (Table 5). In case of
acid dendrimers, upon direct irradiation syn- and anti-
dimers were formed in the ratio 2.1:1 (G-1), 1.4:1 (G-2),
and 0.75:1 (G-3). On comparing the ratios of the dimers
formed within acid and phenolic dendrimers, the percent
of anti dimer (the triplet product) is formed in greater
amount in acid dendrimers. This increase in triplet
product (anti dimer) can be explained with the solubili-
zation data provided in Table 4. G-3 acid dendrimer was
able to encapsulate on an average only one acenaphth-
ylene molecule per unit, whereas the phenolic G-3
dendrimer was able to encapsulate on an average two
acenaphthylene molecules per unit. Due to this decrease
in local concentration of acenaphthylene molecules within
acid dendrimer, we believe that the triplet derived
product anti dimer is formed in greater amounts.
G1-(COOMe)6 (4). A mixture of 37 (0.23 g, 0.39 mmol), 1
(0.31 g, 1.3 mmol), K2CO3 (0.18 g, 1.30 mmol), and 18-C-6 (cat.)
in 2-butanone (20 mL) was refluxed for 12 h, and the solvents
were removed in vacuo. The resulting residue was dissolved
in CH2Cl2, washed with water, dried (Na2SO4), concentrated,
and purified (SiO2, PhCH3/EtOAc ) 92/8) to afford 4, as a
gummy solid (0.36 g, 94%). TLC: Rf ) 0.54 (PhMe/EtOAc )
90:10). 1H NMR (300 MHz, CDCl3) δ: 1.66-1.68 (m, 6H), 1.81-
1.93 (m, 12H), 3.93-3.97 (m, 24H), 4.07 (t, J ) 6.3 Hz, 6H),
6.08 (s, 3H), 7.74 (s, 6H), 8.26 (s, 3H). 13C NMR (75.5 MHz,
CDCl3) δ: 22.6, 28.8, 28.9, 52.3, 67.6, 68.3, 93.7, 119.7, 122.8,
131.6, 159.0, 160.8, 166.1. EI-MS m/z: 962 ([M + H]+), 984
([M + Na]+), 1000 ([M + K]+). FT-IR (neat): 1729. Anal. Calcd
for C51H60O18: C, 63.74; H, 6.29. Found: C, 63.61; H, 6.39.
G1-(COOH)6 (5). 4 (0.15 g, 0.16 mmol) was dissolved in
MeOH:THF:H2O (10 mL) (2:2:1) and KOH (0.11 g, 1.92 mmol).
The solution was stirred for 15 h at room temperature; solvents
were removed partly and acidified with aqueous HCl (10%) to
pH 4. The white precipitate was filtered, washed with cold
water, and dried to afford 5, as a white solid (0.14 g, 95%).
Mp: 140-150 °C (dec). 1H NMR (300 MHz, DMSO-d6) δ: 1.56
(br, 6H), 1.75 (br, 12H), 3.90 (br, 6H), 4.07 (br, 6H), 6.03 (s,
3H), 7.62 (s, 6H), 8.04 (s, 3H). 13C NMR (75.5 MHz, CDCl3) δ:
22.1, 28.3, 28.4, 67.2, 67.9, 93.5, 118.9, 122.2, 132.4, 158.6,
160.4, 166.4. EI-MS m/z: 878 ([M + H]+), 900 ([M + Na]+),
916 ([M + K]+). FT-IR (KBr): 1697, 3440. Anal. Calcd for
C45H48O18: C, 61.63; H, 5.52. Found: C, 61.22; H, 5.93.
G2-(COOMe)12 (6). A mixture of G-1(OH)67 (60 mg, 0.085
mmol), 2 (0.27 g, 0.76 mmol), K2CO3 (0.11 g, 0.76 mmol), and
18-C-6 (cat.) in DMF (20 mL) was refluxed for 30 h, and the
solvents were removed in vacuo. The resulting residue was
dissolved in CH2Cl2, washed with water, dried (Na2SO4),
concentrated, and purified (SiO2, PhCH3/EtOAc ) 88/12) to
afford 6, as a gummy solid (0.17 g, 84%). TLC: Rf ) 0.38 (PhMe/
Summary
Over the years, photoreactions have been examined in
various media. These include organic crystals, host-
guest assemblies in solid state and solution, micelles and
liquid crystals, etc. Dendrimers add an additional dimen-
sion to these media. In this report, we have shown that
dendrimers can effectively solubilize organic compounds,
and also the dendritic cavities can be efficiently used for
conducting photochemical reactions in aqueous solutions.
The hydrophilic and hydrophobic balance available for
the dendrimers in aqueous solutions determines the
extent of solubilization of an organic substrate. Thus, a
higher generation dendrimer, offering a better hydro-
philic-hydrophobic balance along with defined inner
cavities, would be more preferable as hosts than the lower
generation dendrimers. Additional advantages of recov-
ery and reuse of the dendritic media should entail these
new media to be considered on par with other known
reaction media. Utility of these new media in photo-
chemical reactions is fully established in this report.
J. Org. Chem, Vol. 70, No. 13, 2005 5067

Kaanumalle et al.
EtOAc ) 85:15). 1H NMR (300 MHz, CDCl3) δ: 1.65-1.70 (m,
18H), 1.83-1.90 (m, 36H), 3.92 (br, 60H), 4.07 (t, J ) 6.3 Hz,
12H), 6.01 (s, 12H), 7.70 (s, 12H), 8.21 (s, 6H). 13C NMR (75.5
MHz, CDCl3) δ: 22.7, 28.8, 28.9, 29.0, 52.4, 67.6, 67.7, 68.3,
93.8, 119.8, 122.8, 131.7, 159.1, 160.8, 166.2. MALDI-TOF MS
m/z: 2400 ([M + Na]+), 2417 ([M + K]+). FT-IR (neat): 1730.80,
1719.23. Anal. Calcd for C129H156O42: C, 65.14; H, 6.61.
Found: C, 64.81; H, 6.91.
series II gas chromatograph fitted with SE-30 or HP-5 column.
A known amount of internal standard was added before
analysis for mass balance studies. For substrate 10, dodecane
was used as the internal standard, and for substrate 17,
benzophenone was used as the internal standard.
Characterization of Photoproducts. Peaks in the GC
traces were identified by co-injecting with authentic samples,
which were prepared by solution irradiation. In the case of
10, the photoproducts AA (14) and BB (16) were commercially
available. Photoproduct AB (15) was identified based on the
GC-MS fragmentation pattern.
G2-(COOH)12 (7). 6 (0.35 g, 0.148 mmol) was dissolved in
MeOH:THF:H2O (10 mL) (2:2:1) and KOH (0.19 g, 3.55 mmol).
The solution was stirred for 15 h at room temperature; solvents
were removed partly and acidified with aqueous HCl (10%)
solution to pH 4. The white precipitate was filtered, washed
with cold water, and dried to afford 7, as a white solid (0.27 g,
Mass spectral data: m/z (relative intensity), 196 (M+, 17),
105 (100), 91 (12), 77 (11).
Among the photoproducts from 17, benzaldehyde, 23, and
20 were identified by comparison with the commercially
available samples (Aldrich). Photoproducts 22, 24, 25, and 21
were isolated from solution irradiation and characterized by
1H NMR and GC-MS.
1
83%). Mp: 140-150 °C (dec). H NMR (300 MHz, DMSO-d6)
δ: 1.49-1.58 (br, 18H), 1.68-1.80 (br, 36H), 3.90 (t, 6.3 Hz,
24H), 4.00 (t, 6.3 Hz, 12H), 6.02 (s, 12H), 7.60 (s, 12H), 8.04
(s, 6H). 13C NMR (75.5 MHz, CDCl3) δ: 24.8, 31.0, 70.0, 70.64,
96.3, 121.5, 135.2, 161.5, 163.2, 169.1. FT-IR (KBr): 1702,
1697, 3440. Anal. Calcd for C117H132O42: C, 63.57; H, 6.01.
Found: C, 63.51; H, 6.33.
1H NMR of 22 (400 MHz, CDCl3): δ 0.94 (t, 6H), δ 2.83-
3.65 (m, 4H), δ 4.30 (s, 2H), δ 7.09-7.58 (m, 10H).
Mass spectral data: m/z (relative intensity): 165 (4), 152
G3-(COOMe)24 (8). A mixture of G-2(OH)127 (36 mg, 0.019
mmol), 2 (0.13 g, 0.35 mmol), K2CO3 (48 mg, 0.35 mmol), and
18-C-6 (cat.) in DMF (20 mL) was refluxed for 35 h, and the
solvents were removed in vacuo. The resulting residue was
dissolved in CH2Cl2, washed with water, dried (Na2SO4),
concentrated, and purified (SiO2, PhCH3/EtOAc ) 86/14) to
afford 8, as a gummy solid (58 mg, 58%). TLC Rf 0.44 (PhCH3/
EtOAc ) 83:17). 1H NMR (300 MHz, CDCl3) δ: 1.63-1.69 (m,
42H), 1.80-1.90 (m, 84H), 3.86-4.01 (br, 132H), 4.05 (t, J )
6.3 Hz, 24H), 6.06 (s, 30H), 7.73 (s, 24H), 8.25 (s, 12H). 13C
NMR (75.5 MHz, CDCl3) δ: 22.1, 28.3, 28.4, 67.2, 67.9, 93.5,
118.9, 122.2, 132.4, 158.6, 160.4, 166.4. FT-IR (neat): 1725.
Anal. Calcd for C285H348O90: C, 65.66; H, 6.73. Found: C, 65.22;
H, 7.21.
(2), 136 (9), 135 (100), 107 (66), 79 (48), 77 (32), 51 (10).
1H NMR of 24 (400 MHz, CDCl3): δ 1.2 (t, 3H), δ 3.51 (q,
2H), δ 4.57 (s, 2H), δ 7.40-7.80 (m, 9H).
Mass spectral data: m/z (relative intensity), 240 (M+, 18),
211 (18), 196 (34), 181 (5), 167 (49), 166 (100), 152 (10), 133
(21), 126 (14), 105 (77), 89 (24), 77 (50).
1H NMR of 25 (400 MHz, CDCl3): δ 1.0 (t, 3H), δ 3.38 (q,
2H), δ 4.58 (s, 2H), δ 7.30-7.80 (m, 9H).
Mass spectral data: m/z (relative intensity), 240 (M+, 9),
211 (59), 194 (28), 181 (3), 165 (25), 152 (12), 133 (100), 105
(28), 77 (49), 51 (17).
1H NMR of 21 (400 MHz, CDCl3): δ 1.4 (d, 3H), δ 5.2 (q,
1H), δ 5.95 (s, 1H), δ 7.20-7.80 (m, 10H).
G3-(COOH)24 (9). 8 (72 mg, 0.014 mmol) was dissolved in
MeOH:THF:H2O (10 mL) (2:2:1) and KOH (37 mg, 0.66 mmol).
The solution was stirred for 15 h at room temperature; solvents
were removed partly and acidified with HCl (10%) solution to
pH 4. The white precipitate was filtered, washed with cold
water, and dried to afford 9 as a white solid (63 mg, 95%).
Mp: 140-150 °C (dec). 1H NMR (300 MHz, DMSO-d6) δ:
1.51-1.68 (br, 126H), 3.86 (br, 60H), 4.04 (br, 24H), 6.01 (s,
30H), 7.60 (s, 24H), 8.04 (s, 12H). 13C NMR (75.5 MHz, CDCl3)
δ: 22.2, 28.3, 28.4, 67.3, 68.0, 93.6, 119.0, 122.2, 132.6, 158.8,
160.5, 166.5. FT-IR (KBr): 1726.94, 1721.16, 3437.49. Anal.
Calcd for C261H300O90: C, 64.03; H, 6.20. Found: C, 64.06; H,
6.71.
Inclusion of Reactants within Dendrimers and Pho-
tolysis. The procedures adopted for all substrates were
similar, and one of them is described below. To a stirred
solution of a known amount of dendrimer in 5 mL of aqueous
NaOH (G3(CO2H)24) ) 3 × 10-4 M) was added substrate 10
(2.9 × 10-4 M). Following stirring for 12 h, the solution was
filtered through a Whatmann filter paper (medium porosity)
to remove any floating particles. Filtrate was purged with
nitrogen for 30 min and then irradiated in a Pyrex tube with
450 W medium-pressure Hg lamp. In case of substrate 10,
sample in aqueous G1 solution was irradiated for 12 h, samples
in aqueous G2 solution for 28 h, and sample in aqueous G3
solution for 48 h to obtain 30% conversion. Absorption by the
dendrimer was responsible for the low conversion. Two hours
of irradiation resulted in ∼30% conversion in the case of 17.
Substrate 26 was irradiated with a 380 nm filter for direct
irradiation studies and with a 510 nm filter for triplet
sensitization studies.
Mass spectral data: m/z (relative intensity), 196 (17), 167
(28), 152 (9), 134 (85), 133 (62), 118 (12), 105 (100), 91 (15), 77
(42).
Photoproducts 27 and 28 were isolated from solution ir-
1
radiation and characterized by H NMR and GC-MS.
1H NMR of 27 (400 MHz, CDCl3): δ 4.81 (s, 4H), 6.98-6.99
(d, 4H), 7.10-7.24 (m, 8H).
1H NMR of 28 (400 MHz, CDCl3): δ 4.07 (s, 4H), 7.49-7.51
(d, 4H), 7.56-7.60 (m, 4H), 7.71-7.73 (d, 4H).
The following conditions were used for GC analysis of the
photoproducts. The temperatures of the injection and the
detection ports were maintained at 225 and 250 °C.
Substrate 10, column SE-30; temperature program, initial
temp - 100 °C; initial time - 1 min; rate - 5 °C/min; final
temp - 270 °C; final time - 10 min; retention times: AA -
14.3 min; AB - 17 min; BB - 19.3 min; 10 - 22.3 min; 13 -
24.3 min.
Substrate 17, column SE-30; temperature program, initial
temp - 70 °C; initial time - 1 min; rate - 5 °C/min; final temp
- 175 °C; final time - 30 min; retention times: 20 - 25.5
min; 22 - 26.1 min; 23 - 28.6 min; 17 - 30.0 min; 25 - 32.3
min; 21 - 34.3 min; 24 - 43.4 min.
Substrate 26, column SE-30; temperature program, initial
temp - 70 °C; initial time - 1 min; rate - 10 °C/min; final
temp - 270 °C; final time - 20 min; retention times: 26 -
12.3 min; 27 - 27 min; 28 - 33 min.
Fluorescence Measurements. Fluorescence spectra were
recorded at room temperature on an Edinburgh FS900CDT
steady-state fluorimeter. The concentration of pyrene aqueous
solution used in the fluorescence measurement is 1 × 10-5 M.
Emission spectrum was recorded by exciting at 335 nm.
Excitation spectrum was recorded at 385 nm emission wave-
length. To the aqueous solution of pyrene was added dendrimer
solution such that the concentration of dendrimer was 2 × 10-4
M in solution. The solution was stirred for 3 h prior to
recording the emission/excitation spectrum.
Extraction of Photoproducts and Reactants from
Dendrimer Aqueous Solution. After photolysis, the basic
aqueous solution was acidified with 10% dilute HCl. Reactants
and products were extracted from aqueous solution using ethyl
acetate and acetonitrile (7:3) solvent mixture, dried over
anhydrous Na2SO4, concentrated, and analyzed on a HP-5890
5068 J. Org. Chem., Vol. 70, No. 13, 2005

Dendrimers as Photochemical Reaction Media
Acknowledgment. V.R. thanks the National Sci-
ence Foundation for support of the research (CHE-
9904187 and CHE-0213042). N.J. acknowledges finan-
cial support from Council of Scientific and Industrial
Research (CSIR) and Department of Science and Tech-
nology, India. J.N. thanks CSIR, India, for a research
fellowship.
JO0503254
J. Org. Chem, Vol. 70, No. 13, 2005 5069