Singlet-singlet, triplet-triplet, and "optically-controlled" energy transfer in polychromophores. Preliminary models for a molecular scale shift register

Article abstract of DOI:10.1021/jp9821782

The photophysics of two trichromophoric molecules have been studied by a combination of absorption, fluorescence, phosphorescence, and laser flash photolysis. TRI-1 consists of phenanthrene and naphthalene terminal chromophores joined to a central biphenyl group by methyl ester bridges. Intramolecular singlet-singlet energy transfer (SSET) between the biphenyl and terminal chromophores occurs efficiently with k_SSET > 6 ×10¹⁰ s^-1. Longer range SSET from the naphthalene moiety to the phenanthrene group takes place with a lower rate, k ～ 2.5 × 10⁸ s^-1. In TRI-2, the biphenyl moiety is replaced by a benzophenone group. SSET here occurs from the phenanthrene chromophore to the central benzophenone chromophore although with a significantly lower rate than the biphenyl → phenanthrene rate in TRI-1. This is likely due to the change in configuration (ππ* to nπ*) of the excited singlet states involved in the energy transfer. Triplet-triplet (TTET) energy transfer between biphenyl and the terminal chromophores is not observed as a result of a low biphenyl triplet population. That both the phenanthrene and naphthalene triplets are observed following laser photolysis and have different lifetimes and different excitation wavelength concentration ratios indicates that there is no significant TTET between these two groups on the time scale at which the triplets are decaying. This is attributed to the large interchromophore distance (～ 2.5 A) as indicated by modeling studies. In TRI-2, TTET from benzophenone to the terminal chromophores is indicated by both phosphorescence and laser flash photolysis results. Two-laser flash photolysis of the phenanthrene triplet in TRI-1 results in the production of the naphthalene triplet by the following suggested route: (i) production of the upper excited triplet state of the phenanthrene group, (ii) energy transfer to the central biphenyl moiety, and (iii) further energy transfer to the naphthalene chromophore. The utility of this two-laser behavior as the basis for the operation of an optically coupled molecular scale shift register is discussed.

Full text of DOI:10.1021/jp9821782

J. Phys. Chem. A 1998, 102, 8679-8689
8679
Singlet-Singlet, Triplet-Triplet, and “Optically-Controlled” Energy Transfer in
Polychromophores. Preliminary Models for a Molecular Scale Shift Register
W. G. McGimpsey,* W. N. Samaniego, L. Chen, and F. Wang
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
ReceiVed: May 8, 1998; In Final Form: June 23, 1998
The photophysics of two trichromophoric molecules have been studied by a combination of absorption,
fluorescence, phosphorescence, and laser flash photolysis. TRI-1 consists of phenanthrene and naphthalene
terminal chromophores joined to a central biphenyl group by methyl ester bridges. Intramolecular singlet-
singlet energy transfer (SSET) between the biphenyl and terminal chromophores occurs efficiently with kSSET
> 6 × 1010 s-1. Longer range SSET from the naphthalene moiety to the phenanthrene group takes place
with a lower rate, k ∼ 2.5 × 108 s-1. In TRI-2, the biphenyl moiety is replaced by a benzophenone group.
SSET here occurs from the phenanthrene chromophore to the central benzophenone chromophore although
with a significantly lower rate than the biphenyl f phenanthrene rate in TRI-1. This is likely due to the
change in configuration (ππ* to nπ*) of the excited singlet states involved in the energy transfer. Triplet-
triplet (TTET) energy transfer between biphenyl and the terminal chromophores is not observed as a result
of a low biphenyl triplet population. That both the phenanthrene and naphthalene triplets are observed following
laser photolysis and have different lifetimes and different excitation wavelength concentration ratios indicates
that there is no significant TTET between these two groups on the time scale at which the triplets are decaying.
This is attributed to the large interchromophore distance (∼12.5 Å) as indicated by modeling studies. In
TRI-2, TTET from benzophenone to the terminal chromophores is indicated by both phosphorescence and
laser flash photolysis results. Two-laser flash photolysis of the phenanthrene triplet in TRI-1 results in the
production of the naphthalene triplet by the following suggested route: (i) production of the upper excited
triplet state of the phenanthrene group, (ii) energy transfer to the central biphenyl moiety, and (iii) further
energy transfer to the naphthalene chromophore. The utility of this two-laser behavior as the basis for the
operation of an optically coupled molecular scale shift register is discussed.
Introduction
More recently, larger molecules containing more than two
chromophores have been studied. Morrison has incorporated
Intramolecular charge and energy transfer in bichromophores
has been the subject of increasing interest in recent years.1-4
Particular attention has been paid to the kinetics of transfer
processes and how they are effected by molecular conformation,
the rigidity or flexibility of the connecting bridges, and
interchromophore distance. The observed relationships between
the efficiency of transfer and these parameters have been used
to suggest through-bond and through-solvent superexchange
transfer mechanisms. In general, the through-bond mechanism
seems to be favored for molecules in which the chromophores
are joined by rigid saturated hydrocarbon bridges such as those
synthesized and characterized by Closs,1 Morrison,2 Paddon-
Row and Verhoeven,3 Zimmerman,4 and ourselves.5 In these
bridges, orbital overlap facilitating a superexchange interaction
is provided by an “all-trans” arrangement of the σ bonds.
Bichromophores containing flexible bridges, on the other hand,
allow the chromophore-bridge-chromophore structure to
sample a large number of rotational conformations, only a small
number of which produce orbital overlap conducive to through-
bond transfer. In these molecules, the transfer process has
traditionally been regarded as a through-space interaction, with
the effect of the solvent being largely ignored. However,
recently Zimmt6 has used “C-clamp” bichromophores to show
the through-space mechanism is more accurately described as
a solvent-mediated superexchange interaction.
singlet and triplet energy donors and acceptors on a steroidal
backbone and has observed singlet-singlet and triplet-triplet
energy-transfer processes occurring with reasonably large rate
constants, via a through-bond mechanism.2d-2g Paddon Row
observed rapid charge separation between the terminal chro-
mophores in a trichromphore connected by fused norbornyl
spacers,3g and Lindsey constructed a 9 nm long polychro-
mophore from a series of tetraphenyl zinc porphyrin moieties
connected by ethyne linkages.7 The efficiency of singlet energy
transfer through this molecule was 75%. These studies build
upon recent literature that suggests large polychromophores
could be used as molecular wires and devices.8
We report here our initial study of two trichromophoric
molecules, TRI-1 and TRI-2, shown in Chart 1 along with
related model compounds used in this study. We have chosen
to study these molecules for several reasons including their
relative ease of synthesis, the well-characterized photophysics
of the individual chromophores, and the intriguing possibility
that large molecules such as these compounds may eventually
be used as molecular scale devices. As it turns out, these
compounds exhibit an unexpectedly rich photophysics that
includes singlet-singlet energy transfer between the central and
terminal chromophores as well as between the terminal chro-
mophores themselves. Triplet-triplet energy transfer from the
central chromophore to the terminal chromophores in TRI-2 is
10.1021/jp9821782 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/10/1998

8680 J. Phys. Chem. A, Vol. 102, No. 45, 1998
McGimpsey et al.
CHART 1
(CDC13): 33.68 (2C, CH2); 127.92 (4C, C3,3′,5,5′); 129.98 (4C,
C2,2′,6,6′); 137.51 (2C, C4,4′); 140.96 (2C; C1,1′).
(4′-Bromomethyl-4-methylnaphthoate)biphenyl. Potassium
naphthoate was prepared by neutralization of 2-naphthoic acid
with KOH (1 N) in ethanol, using phenolphthalein as an
indicator. The solvent was evaporated, and the solid used
without further purification. The salt (0.1006 g, 0.48 mmol)
was suspended in 2.0 mL of HMPA in a boiling flask, and
enough water was added to dissolve the acid.10 Finally, a
solution of 4,4′-dibromomethylbiphenyl (0.2060 g, 0.61 mmol)
in 2.0 mL of HMPA was added drop by drop, over a 3 h period.
After the addition was completed, the mixture was stirred at 25
°C for 24 h. The reaction was quenched by addition of 5%
HCl (20 mL) producing a white precipitate. The suspension
was extracted with ethyl ether (2 × 30 mL). The organic phases
were combined, washed with 10% NaOH, H2O, and brine, and
then dried over magnesium sulfate. The solvent was eliminated
to give 0.17 g of a mixture of unreacted halide and mono- and
diester. The mixture was purified by flash chromatography
(CH2Cl2/hexane) to give 77 mg (39% yield) of the pure
bichromophore. Mp: 142-143 °C. 1H NMR (CDCl3): 4.52
(s, 2H); 5.51 (s, 2H); 7.62 (m, 7H); 7.80 (m, 7H); 8.66 (s, 1H).
13C NMR(CDC13): 33.77; 66.98; 125.72; 127.13; 127.79;
127.92; 127.97; 128.15; 128.65; 128.78; 129.25; 129.82; 130.00;
131.69; 137.412; 141.24; 167.08.
also observed. We also present preliminary evidence showing
that re-excitation of the triplet state of the terminal phenanthrene
chromophore in TRI-1 results in energy transfer to the naph-
thalene group. We discuss the relevance of this process to the
operation of molecular scale devices such as a shift register.
Potassium Phenanthrene-9-carboxylate. Phenanthrene-9-
carboxaldehyde (0.1605 g, 0.78 mmol) and silver nitrate (0.4885
g, 2.8 mmol) were suspended in a mixture of 6 N NaOH (1.0
mL), water (3.0 mL), and ethanol (3.0 mL).11 The suspension
was protected from light and refluxed for 18 h. The hot solution
was filtered, the solid was washed with NaOH, and the
combined liquids were made acidic with HCl. The suspension
formed was extracted with ethyl ether (2 × 30 mL), the organic
phases were combined and dried over Na2SO4, and the solvent
was evaporated. The product, phenanthrene-9-carboxylic acid
(0.1636 g, 0.74 mmol, 95% yield), was neutralized with
ethanolic KOH, as described for the naphthoate, and used
without further purification.
Experimental Section
Materials and Methods. All solvents in spectroscopic and
laser studies were Aldrich spectrophotometric grade and were
used as received. Phenanthrene, 9-phenanthraldehye, biphenyl,
4,4′-dimethylbiphenyl, benzophenone, 4,4′-dimethylbenzophe-
none, naphthalene, and 2-naphthoic acid were obtained from
Aldrich and recrystallized twice prior to use. All other reagents
were used as received from Aldrich.
Synthesis. Flash chromatography was performed using
“Baker” Silica Gel, 40 µm Flash Chromatography Packing.
Melting points given are uncorrected and were measured with
a capillary apparatus. 1H and 13C NMR spectra were obtained
using a Bruker Avance 400 Digital NMR.
[4′-(Phenanthrene-9-carboxylate) Methyl-4-methylnaphthoate]-
biphenyl (TRI-1). In a boiling flask, (4′-bromomethyl-4-
methylnaphthoate)biphenyl (0.080 g, 0.20 mmol) and potassium
phenanthrene-9-carboxylate (0.0547 g, 0.21 mmol) were sus-
pended in 4.0 mL of HMPA, and water was added drop by
drop until the solid salt dissolved. The mixture was stirred at
25 °C for 24 h, and the final solution was treated as described
for (4′-bromomethyl-4-methylnaphthoate)biphenyl. The crude
product was purified by flash chromatography (CH2Cl2/hexane),
yielding 44 mg (39% yield) of the diester. Mp: 138-139 °C.
1H NMR (CDCl3): 5.47 (s, 2H); 5.54 (s, 2H); 7.63 (m, 16H);
7.95 (m, 4H); 8.15 (d, 2H); 8.53 (s, 1H); 8.68 (m, 2H). 13C
NMR(CDCl3): 67.00; 67.08; 122.67; 123.08; 123.27; 125.72;
127.02; 127.11; 127.37; 127.46; 127.83; 127.87; 127.90; 128.63;
129.26; 129.34; 129.45; 129.81; 130.46; 131.08; 132.25; 135.74;
137.30; 137.62; 141.13; 167.08; 167.80.
4,4′-Dibromodimethylbiphenyl. N-Bromosuccinimide9 (1.0154
g, 5.64 mmol), 4,4′-dimethylbiphenyl (0.5173 g, 2.84 mmol),
and benzoyl peroxide (26 mg, 0.3 mmol) were placed in a
boiling flask fitted with a condenser and an anhydrous CaCl2
hood. The solid mixture was suspended in 15.0 mL of CCl4
and brought to a gentle reflux with stirring. The solution was
refluxed overnight in order for the starting material to be
completely consumed. The hot mixture was filtered through a
fritted glass funnel, and the white insoluble solid was washed
(2 × 5 mL) with hot CCl4. The final yellowish solution was
chilled in an ice-water bath, and the precipitate was filtered.
This procedure gave 0.37 g of the pure product (38% yield).
The mother liquor was reduced to a volume of 10 mL, and 5
mL of hexane was added in small portions. The precipitate
formed was redissolved by heating, and the solution was allowed
to reach room temperature. The crystals were filtered and
washed with a few drops of cold CCl4. This produced an
additional 0.31 g of pure product (32% yield, total yield, 70%).
In some cases the latter precipitate was purified by column
chromatography using hexane as the starting solvent (to
eliminate 4,4′-dimethylbiphenyl), and then a mixture of CH2-
Cl2/hexane. Mp: 145-146 °C. 1H NMR (CDCl3): 4.55 (s,
4H); 7.47 (d, 4H, 8.4 Hz); 7.56 (d, 4H, 8.4 Hz). 13C NMR
4,4′-(Bis(bromomethyl))benzophenone. N-Bromosuccinimide
(0.4967 g, 2.79 mmol), 4,4′-dimethylbenzophenone, (0.2931 g,
1.39 mmol) and benzoyl peroxide (5 mg) were placed in a
boiling flask fitted with a condenser and an anhydrous CaCl2
hood. The solid mixture was suspended in 15.0 mL of CCl4
and brought to a gentle reflux, with stirring. The solution was
refluxed overnight. Then the hot mixture was filtered through
a fritted glass funnel, and the white insoluble solid was washed
(2 × 5 mL) with hot CCl4. The volume was reduced in half,
and hexane was added, producing a precipitate. This solid was

Photophysics of Polychromophores
J. Phys. Chem. A, Vol. 102, No. 45, 1998 8681
TABLE 1: Ground-State Extinction Coefficients and Initial Excitation Distributions (ED Values) for TRI-1, TRI-2, and Model
Compounds
PM
NM
BPM
BZM
10 090
TRI-1
TRI-2
a
ꢀ226
ꢀ252
ꢀ260
ꢀ308
ꢀ355
24 500
47 800
34 700
9 600
400
59 900
3 600
4 500
900
5 100
45 100
103 300
107 500
91 500
42 200
155
84 100
11 300
610
25
10 010
<10
150
b
ED226
(1) for TRI-1
(2) for TRI-2
27%
29%
67%
60%
6%
11%
ED252
(1) for TRI-1
(2) for TRI-2
50%
4%
46%
ED260
(1) for TRI-1
(2) for TRI-2
ED308
43%
6%
51% (λmax
)
(1) for TRI-1
(2) for TRI-2
93%
90%
7%
8%
<0.5%
2%
ED355
(1) for TRI-1
(2) for TRI-2
73%
1%
26%
a Extinction coefficients have an estimated error of (0.5% b Excitation distributions have an estimated error (based on composite spectra) of
(0.3%
recrystallized once more (CCl4/hexane), producing 0.2251 g of
studies and transient absorption spectra, solutions were prepared
at concentrations sufficiently large to give absorbances in the
range 0.6-0.8 at the excitation wavelength. Unless otherwise
noted, the solutions, contained in a reservoir, were continuously
purged with a stream of nitrogen and were caused to flow
through a specially constructed quartz cell (7 mm × 7 mm) by
means of a peristaltic pump. This ensured that a fresh volume
of solution was exposed to each laser pulse, thereby avoiding
accumulation of any photoproducts. Samples were irradiated
with the pulses of a Lumonics EX 510 excimer laser (308 nm;
∼ 20 mJ/pulse; 8 ns pulse duration) or the frequency-tripled
output of a Continuum Nd:YAG laser (355 nm, ∼30 mJ/pulse,
5 ns). In two-laser studies, the 308 (or 355) nm pulse was
followed, after a period of 1-2 µs, by the pulse from a
flashlamp-pumped dye laser (output tuned to the particular T-T
absorption maximum; ∼120 mJ; 400 ns).
product (0.61 mmol, 44% yield). There was also product in
the mother liquour, which could be recovered by flash chro-
matography. Mp: 128-129 °C. 1H NMR (CDCl3): 4.54 (s,
4H); 7.51 (d, 4H, 8.3 Hz); 7.78 (d, 4H, 8.3 Hz). 13C NMR
(CDC13): 32.64 (2C, CH2); 129.50 (4C, C3,3′,5,5′); 130.94 (4C,
C2,2′,6,6′′); 137.59 (2C; C1,1′); 142.70 (2C, C4,4′); 195.68 (1C, Cd
O).
(4′-Bromomethyl-4-methylnaphthoate)benzophenone. The pro-
cedure described for (4′-bromomethyl-4-methylnaphthoate)-
biphenyl was followed. The monoester was purified by flash
chromatography, producing 0.2479 g of pure product (26%
yield). Mp: 83-84 °C. 1H NMR (CDCl3): 4.65 (s, 2H); 5.52
(s, 2H); 7.62 (m, 6H); 7.89 (m, 7H); 8.10 (d, 1H); 8.67 (s, 1H)
13C NMR (CDC13): 33.68; 66.72; 125.18; 125.97; 126.17;
126.77; 127.35; 127.56; 127.94; 128.33; 128.77; 129.65; 130.33;
130.61; 131.09; 136.94; 137.54; 138.36; 141.10; 141.78; 166.00.
[4′-(Phenanthrene-9-carboxylate) Methyl-4-methylnaphthoate]
Benzophenone (TRI-2). The procedure described for [4′-
(phenanthrene-9-carboxylate) methyl-4-methylnaphthoate] bi-
phenyl was followed. The asymmetric diester was purified by
flash chromatography, producing 0.0300 g of pure product
Calculations. To obtain minimum energy conformations of
the trichromophores, conformational space was explored using
ChemPlus 1.5 and the MM+ force field. The lowest energy
conformations were further minimized using AM1 and PM3
parameters in the Hyperchem semiempirical option. These
minimized conformations were used to obtain the spectroscopic
energies with ZINDO/S parameters.
1
(17.3% yield). Mp: 135-136 °C H NMR (CDCl3): 5.52 (s,
2H); 5.58 (s, 2H); 7.66 (m, 9H); 7.87 (m, 9H); 8.10 (d, 1H);
8.56 (s, 1H); 8.69 (m, 3H); 8.98 (d,1H). 13C NMR (CDC13):
66.06; 66.14; 122.67; 122.82; 125.19; 125.86; 126.50; 126.76;
127.02; 127.10; 127.53; 127.76; 127.79; 127.86; 128.30; 128.45;
129.38; 130.05; 130.43; 130.47; 132.47; 132.61; 132.78; 135.05;
135.54; 140.74; 166.47; 167.13; 195.76.
Absorption and Emission Spectroscopy. Ground-state
absorption spectra and extinction coefficients were obtained with
a Shimadzu 2100U absorption spectrometer. Fluorescence
emission spectra and quantum yields were measured in nitrogen-
and air-saturated acetonitrile and were found to be independent
of saturating gas. Spectra were recorded with a Perkin-Elmer
LS-50 spectrofluorimeter. Yields were measured using the
parent aromatic molecules as standards.12 Phosphorescence
spectra were recorded with the same instrument. Samples were
in 77 K 1:1 ethanol:methanol glasses.
Results and Discussion
Absorption and Fluorescence Spectroscopy and Singlet-
Singlet Energy Transfer (SSET). (i) Absorption and Fluo-
rescence Measurements. Table 1 gives ground-state extinction
coefficients for model compounds PM, NM, BPM, and BZM
as well as for both trichromophores TRI-1 and TRI-2 at various
wavelengths in the UV. The absorption spectra for the
trichromophores are virtually identical to composite spectra
constructed by adding proportional contributions from the model
compounds, indicating that there is little interaction in the
ground-state between the chromophores. Furthermore, ZIN-
DO/S calculations indicate that the highest six occupied
molecular orbitals and the lowest six unoccupied molecular
orbitals are localized on individual chromophores. Therefore,
it is likely that excitation of the localized ground state of one
of the chromophores initially will result in the production of
an excited state that is also localized on the same chromophore.
Laser Flash Photolysis. The laser flash photolysis system
has been described in detail elsewhere.5,13 Briefly, for kinetic

8682 J. Phys. Chem. A, Vol. 102, No. 45, 1998
McGimpsey et al.
TABLE 2: Fluorescence Maxima, Quantum Yields, Singlet Energies and Lifetimes, and Triplet Energies for the TRI-1, TRI-2,
and Model Compounds
PM
NM
BPM
321
BZM
TRI-1
TRI-2
λmax
380, 370(sh)
344, 358.5
not obsd
380, 367, 346 (sh)
374 (br), 345 (sh)
(λex ) 226 nm)
λmax
380.5, 369 (sh)
345, 358
321.5
not obsd
not obsd
381, 370 (sh) 345 (sh)
381, 370.5, 346 (sh)
380.5, 369.5 (sh) 345 (sh)
381.5, 371 (sh) 344 (sh)
380.5, 370 (sh), 345 (sh)
381.5, 370.5 (sh)
(λex ) 252 nm)
λmax
(λex ) 260 nm)
λmax
380.5, 369.5 (sh)
358.5, 345 (sh)
321.5
(λex ) 308 nm)
λmax
(λex ) 355 nm)
a
ΦFl
0.26
10.3(14)b
80.0
0.45
15.5(15)
84.0
0.32
τs
16.5(12)
100.1
65.8
∼0.02(12)
78.4
68.8
Esc (kcal/mol)
Etd (kcal/mol)
58.4
59.4
58.4
58.4
a From refs 14, 15. b Extrapolated to concentration used in these experiments from published self-quenching data. c From fluorescence and absorption
data. d From phosphorescence data. e Wavelengths quoted are (1 nm. sh: shoulder. br: broad.
In addition, the relatiVe energies predicted by ZINDO/S for the
HOMO-LUMO transitions in the trichromophores are consis-
tent with experimental absorption spectra of the models. As a
result, the extinction coefficients of the model compounds at
each wavelength can be used to estimate how the initial
excitation is partitioned between each chromophore in TRI-1
and TRI-2. These “excitation distributions” (ED), expressed
as percentages of the total excitation for each excitation
wavelength, are also shown in Table 1. For example, we
estimate that exposure of TRI-1 to λex ) 226 nm results in the
ED values 27%, 6%, and 67% for phenanthrene, biphenyl, and
naphthalene moieties, respectively. As is indicated by the ED
values in Table 1, the wavelengths chosen correspond to quite
different absorption conditions. Thus at 226 nm most of the
incident light is absorbed by the naphthalene chromophore, while
at 252 nm the phenanthrene and biphenyl chromophores have
roughly the same absorbance and at 308 nm most of the
excitation takes place in the phenanthrene chromophore. The
effect of using these different excitation wavelengths is reflected
in the results of fluorescence and phosphorescence emission
measurements which, in turn, allow a detailed description of
the photophysics of the trichromophores.
Table 2 shows the fluorescence maxima, quantum yields,
singlet energies, and lifetimes for the trichromophores and model
Figure 1. Fluorescence emission spectra of TRI-1 and the model
compounds. The quantum yields for the model compounds were
independent of excitation wavelength within experimental error
and agree closely with those published previously.12,14,15 Figure
1 shows the emission spectrum (λex ) 226) of TRI-1 as well as
the spectra of its associated model compounds, PM, NM, and
BPM. It is apparent from these spectra that the emission of
TRI-1 closely resembles that of PM, the phenanthrene model
compound. An additional feature of the TRI-1 emission is a
shoulder at 345 nm. The emission is quite similar at the other
two excitation wavelengths although there are small, but
important, differences. First, the magnitude of the 345 nm
shoulder decreases with increasing λex. Accompanying this
change is a shift of the 367 nm peak to 370 nm and a decrease
in the intensity of this peak relative to the other major peak at
380 nm. In effect, as λex increases, the spectrum of TRI-1
becomes more similar to that of PM. These changes can be
explained partially by the increase in the initial ED value for
the phenanthrene chromophore with increasing wavelength. The
decrease in the magnitude of the 345 nm shoulder, and the shift
in the 367 nm peak as well as its decrease in magnitude with
increasing λex also correlates roughly with the decrease in the
ED of the naphthalene group, indicating that there is a
contribution to the emission of TRI-1 from this chromophore.
compounds NM, PM, and BPM in N2-saturated acetonitrile (λex ) 226
nm).
Figure 2 shows the emission spectrum of TRI-1 (λex ) 226
nm) along with a composite spectrum constructed by summing
the emission spectra of all three chromophores in the proportion
indicated by their initial ED values for this wavelength (i.e.,
0.27(PM) + 0.06(BPM) + 0.67(NM)). The most striking
features of the TRI-1 spectrum when compared to the composite
are the absence of any emission associated with the biphenyl
moiety and the weak naphthalene emission. The lack of strong
naphthalene fluorescence is apparently at odds with the strong
absorption of this chromophore at 226 nm. While it could be
argued that the relatively small absorption expected for biphenyl
at 226 nm is responsible for the lack of biphenyl emission, the
same result was obtained at λex ) 306 nm, and at 252 nm where
roughly half of the absorption is due to the biphenyl group.
It is clear from Figure 2 that the contribution due to
naphthalene is considerably less than that expected from the
ED for this chromophore (67%). To determine the actual
contribution of the naphthalene emission to the TRI-1 spectrum,
composite spectra were created in which no biphenyl emission
was included and the relative contributions of naphthalene and

Photophysics of Polychromophores
J. Phys. Chem. A, Vol. 102, No. 45, 1998 8683
Figure 3. Energy diagram showing the intramolecular energy transfer
processes in TRI-1.
Figure 2. Fluorescence emission and composite spectra for TRI-1 in
acetonitrile. Composite spectra were calculated based on: (i) (filled
squares) the ED and emission spectra for the model compounds at λex
) 226 nm; (ii) (filled circles) an 80:20 PM:NM ratio.
(ii) Singlet-Singlet Energy Transfer (SSET). The results
described above for TRI-1 are consistent with the energy transfer
mechanism shown in Figure 3. The results support three
intramolecular singlet-singlet energy transfer pathways: bi-
phenyl f phenanthrene, biphenyl f naphthalene, and naph-
thalene f phenanthrene. Considering the singlet energies given
in Table 1, it is clear that energy transfer from the biphenyl
singlet to both the phenanthrene and naphthalene chromophores
is thermodynamically possible as is further energy transfer from
naphthalene to phenanthrene. That energy transfer occurs from
the biphenyl chromophore is most distinctly illustrated by the
lack of observed biphenyl emission, even at λex ) 252 nm where
the biphenyl group absorbs strongly. This indicates that there
is an efficient quenching pathway for the biphenyl singlet state.
To assign this quenching process as intramolecular energy
transfer it was first necessary to rule out the possibility of
photoinduced product formation. (Although CW UV irradiation
of BPM in MeCN does not lead to chemical conversion,
incorporation of this chromophore into TRI-1 could potentially
open up new chemical decay pathways). Product studies carried
out under steady-state lamp irradiation of TRI-1 failed to show
any significant conversion of starting material, indicating that
if there is a chemical deactivation route, it is inefficient.
Intermolecular processes, including energy transfer, could also
account for singlet quenching given sufficiently high concentra-
tions of TRI-1. However, the maximum concentration of TRI-1
used in any of the fluorescence experiments was less than 10
µM. Since the singlet lifetime of BPM has been reported as
∼16 ns12 (see Table 2), diffusion-controlled quenching could
account for a maximum reduction in biphenyl emission intensity
of only 0.16%, not the complete quenching observed. Of course,
using the singlet lifetime for BPM in this calculation assumes
that incorporation of the biphenyl group into the trichromophore
does not significantly alter the rate of the intrachromophore
photophysical decay pathways of the singlet state (i.e., those
decay routes available to the model compound, BPM, such as
ISC and internal conversion). While it is not possible to
determine with complete certainty the effect of the neighboring
chromophores on such processes, the lack of interaction between
chromophores in the ground state argues against a large effect.
phenanthrene were varied. Figure 2 also shows a composite
spectrum constructed by assuming a 80:20 ratio of emission
intensities, i.e., 0.80(PM) + 0.20(NM). This particular ratio
accurately reproduces the magnitude of the 345 nm shoulder.
It also reproduces the blue-shifted position (relative to PM) of
the 370 nm peak and the enhanced magnitude of the peak (again,
relative to PM). Small changes in this ratio (more than a 0.5%
change in the distribution of each chromophore) result in distinct
changes in spectral shape in the composite spectrum, and for
this reason we are confident that the ratio used is an accurate
reflection of the relative contributions of the phenanthrene and
naphthalene chromophores. This method was also used suc-
cessfully to reproduce the TRI-1 spectra obtained at λex ) 252
and 308 nm. For the latter wavelengths, the PM:NM ratios for
the matching composite spectra were 95:5 and 98:2, respectively.
Generally, the emission behavior observed for TRI-2 paral-
leled that of TRI-1 in that at all wavelengths the fluorescence
was similar to that of PM but also included a contribution due
to the naphthalene chromophore, especially at 226 nm where
naphthalene absorption is strongest. Construction of composite
spectra was hampered for this compound owing to the fact that,
unlike TRI-1, the emission intensities observed at all excitation
wavelengths were smaller than expected from the model
compound intensities and the ED values (vide infra). Therefore,
it was necessary to normalize the spectra of TRI-2 to the
composites in order to compare the band shapes. By following
this procedure it was possible to demonstrate that the contribu-
tion to the emission due to the naphthalene group was less than
expected from the ED. Thus, the PM:NM ratios were 75:25,
97:3, and 98:2 at 226, 260, and 306 nm, respectively. The extent
to which the emission intensity of TRI-2 decreased relative to
the model compounds was calculated by comparing the inte-
grated area for the TRI-2 spectrum with that of the PM spectrum
at 308 nm, i.e., at a wavelength where the majority of the
excitation is into the phenanthrene chromophore. The ratio of
the PM to TRI-2 areas was ∼7, indicating a reduction of about
7-fold in the phenanthrene emission in TRI-2.

8684 J. Phys. Chem. A, Vol. 102, No. 45, 1998
McGimpsey et al.
TABLE 3: Initial, Intermediate, and Final Excitation
At this point, we are unable to determine exactly how the
biphenyl singlet energy partitions between the naphthalene and
phenanthrene chromophores, although evidence given below
confirms that at least some of the energy is transferred to the
naphthalene group. Modeling the conformation of TRI-1 (see
Experimental Section for details) indicates that in the ground
state the biphenyl-naphthalene and biphenyl-phenanthrene
distances are similar (4.3 Å) as are the conformations of
naphthalene and phenanthrene relative to the biphenyl group.
Given previously reported results for bichromophores containing
naphthalene or phenanthrene groups linked with methyl ester
bridges in which singlet-singlet energy transfer was shown to
proceed by a Fo¨rster-type (and therefore “through-space”)
mechanism,15 it is reasonable to suggest that energy transfer
from biphenyl in TRI-1 follows both possible pathways and
that the relative efficiencies are roughly similar.
Distributions for TRI-1
initial
distribution after
energy transfer
from biphenyl
excitation
wavelength
(nm)
excitation
distribution
(ED)
final
distribution
(equal partitioning)
226
252
306
27:67:6
50:4:46
93:7:0
30:70:0
73:27:0
93:7:0
80:20:0
95:5:0
98:2:0
phenanthrene and naphthalene and the distribution of emitting
chromophores as indicated by the matching composite spectra.
The reduction in the naphthalene distribution from its value
following biphenyl energy transfer to that indicated by the
emission spectra reflects the actual efficiency of energy transfer
quenching of the naphthalene singlet state.
These fluorescence emission data also contain information
on the rates of biphenyl-naphthalene-phenanthrene singlet-
singlet energy transfer. In the case of energy transfer from
biphenyl, the lack of any observable biphenyl emission is an
indication that transfer is quite rapid. The sensitivity of our
fluorimeter is such that we are able to observe emission
intensities as low as 0.1% of the level that was recorded for the
biphenyl model compound. Given that the singlet lifetime of
the model is 16 ns (ks ) 6.25 × 107 s-1), we infer that the
lower limit for the energy transfer rate constant is 6 × 1010 s-1
(i.e., a smaller rate constant would lead to the observation of
biphenyl emission).
Evidence for energy transfer from the naphthalene moiety to
the phenanthrene group can be found in the spectra generated
at all three excitation wavelengths and a comparison of these
spectra with the composite spectra constructed as described
above. The spectrum at λex ) 226 nm provides the most direct
indication of naphthalene-phenanthrene energy transfer. Clearly,
the ratio of naphthalene to phenanthrene emission as indicated
from the composite spectrum is considerably smaller than that
expected from the initial ED. This indicates that some of the
naphthalene singlet states initially produced are subsequently
quenched. Since energy transfer to biphenyl is uphill energeti-
cally and there was no observed chemical conversion, this
quenching must be due to either inter- or intramolecular energy
transfer to the phenanthrene group. The reported singlet lifetime
of the naphthalene model compound, NM, is also ∼16 ns.15
Therefore, the same argument used against intermolecular
energy transfer for the biphenyl moiety can also be used here;
i.e., the extent of naphthalene singlet quenching is much greater
than the 0.16% quenching expected for intermolecular interac-
tion. That at least some of the naphthalene emission was not
quenched while that of biphenyl was completely quenched is
consistent with the larger interchromophore distance for naph-
thalene-phenanthrene and the attendant smaller rate constant
expected for energy transfer (vide infra). At λex ) 308 nm,
similar behavior is observed. The contribution to the emission
due to naphthalene as indicated by the composite spectrum is
2% whereas the ED is 8%. Thus, although when compared to
226 nm the naphthalene group absorbs considerably less, thereby
producing a smaller contribution to the emission, there is even
less contribution to the emission in the composite spectrum
(again, we found the spectral shape of the composite to be quite
sensitive to small changes in the emission ratio, and therefore
we are confident that this represents a real difference).
The extent of the reduction of naphthalene emission allows
an estimate of the naphthalene-phenanthrene energy transfer
rate constant. The data in Table 3 indicate that an average of
77% (admittedly with a large uncertainty) of the naphthalene
emission is quenched in TRI-1. Since the singlet lifetime of
the model NM is 16 ns, we estimate the naphthalene-
phenanthrene energy transfer rate constant to be ∼2.5 × 108
s-1, i.e., at least 50 times less than the biphenyl-phenanthrene
(or naphthalene) energy transfer rate constant. The smaller rate
constant is a reflection of the larger interchromophore distance
between phenanthrene and naphthalene (12.5 Å). These two
rate constants are consistent with an approximate R-6 distance
dependence consistent with a Fo¨rster type energy transfer
mechanism. We emphasize that these rate constants are
estimates and therefore this conclusion must be regarded as
speculative. However, as noted above, previously studied
bichromophores incorporating two NM molecules and variable
length spacers also showed an R-6 distance dependence.15
Figure 4 shows the energy transfer mechanisms proposed for
TRI-2. Here, unlike in TRI-1, the singlet state of the central
(benzophenone) group lies lower in energy than either of the
terminal chromophores, making it a more likely energy acceptor
than an energy donor. The fluorescence results give direct
evidence for phenanthrene f benzophenone energy transfer.
The noted 7-fold decrease in fluorescence intensity for TRI-2
compared to PM as measured at 308 nm indicates that a majority
of the phenanthrene excited states are quenched. Since there
is no observable photochemical products and as in the case of
TRI-1 the concentration used is too small to allow appreciable
intermolecular quenching, we attribute the quenching effect to
SSET from phenanthrene to benzophenone.
The results obtained at 252 nm were somewhat different but
can still be explained in the context of naphthalene-phenan-
threne energy transfer. The absorption due to the naphthalene
group in TRI-1 at 252 nm is minor (4%) compared to that of
phenanthrene and biphenyl. The composite spectrum that most
closely reproduces the actual emission spectrum indicates a 5%
contribution due to naphthalene, i.e., an increase over the ED
rather than a decrease as observed at 226 and 308 nm. This
result would appear to be at odds with the energy transfer
interpretation described above. However, if decay of the
biphenyl singlet state partitions between naphthalene and
phenanthrene, the actual concentration of naphthalene singlet
states produced following 252 nm excitation will be considerably
greater than 4%. Table 3 shows the ED, the distribution
assuming equal partitioning of the biphenyl excitation to
In TRI-2, it is somewhat surprising that the phenanthrene
fluorescence is observed at all. Since the benzophenone-
phenanthrene separation in TRI-2 is approximately the same
as the biphenyl-phenanthrene distance in TRI-1, one could
expect comparable rates of SSET. As the biphenyl and
phenanthrene model compound singlet lifetimes are comparable,

Photophysics of Polychromophores
J. Phys. Chem. A, Vol. 102, No. 45, 1998 8685
Figure 4. Energy diagram showing the intramolecular energy transfer
processes in TRI-2.
this should result in no phenanthrene emission observed. There
are two possible explanations for this observation. First, the
proximity of the phenanthrene and benzophenone singlet
energies (∆Es ) 1.9 kcal/mol) could result in a singlet-singlet
equilibrium between the two chromophores. The viability of
this equilibrium will depend on the magnitude of the energy
transfer rate constants in both directions (which may or may
not be equal) and how these compare with the ISC rate constant
in the benzophenone chromophore (kISC ∼ 1011 s-1). Assuming
energy transfer rates that are comparable with that estimated in
TRI-1 (k > 6 × 1010 s-1) could result in an equilibrium and
the observed phenanthrene emission intensity. On the other
hand, ππ* f nπ* intramolecular SSET has been reported to
be considerably slower than ππ* f ππ* SSET for similar
distances of separation.16 If this holds in TRI-2, the SSET rate
constants would be too slow to support an equilibrium, and at
the same time the reduced efficiency of energy transfer could
account for the observation of phenanthrene emission. Pre-
liminary studies of the effect of temperature on the observed
TRI-2 emission intensity failed to show any effect on the
emission intensity, arguing against an equilibrium and in favor
of the second explanation. The rate constant for SSET from
phenanthrene to benzophenone can be estimated from the
observed fluorecence intensity and the PM singlet lifetime.14
Figure 5. Phosphorescence emission spectra for TRI-2 and its model
compounds obtained at 77 K in a N2-saturated 1:1 ethanol:methanol
glass. (For TRI-2, λex ) 260 nm.)
sample tubes were used) and the motion of the tube in the Dewar
contributes to fairly large variations in the effective path length.
In addition, solubility of the compounds vary in the low-
temperature glasses contributing both to light scattering and
differences in O.D. from sample to sample.
The phosphorescence spectra of TRI-2 and its model com-
pounds are shown in Figure 5. As in the case of TRI-1, it is
clear that the emission of TRI-2 is nearly identical to the spectra
of the PM and NM model compounds. There was also no
evidence for emission from the central benzophenone chro-
mophore although the benzophenone model exhibits very strong
phosphorescence under similar excitation conditions.
(ii) Triplet-Triplet Energy Transfer (TTET). The clearest
indication of TTET from the phosphorescence measurements
was found for TRI-2. In this spectrum the lack of phospho-
rescence emission from the benzophenone chromophore must
be due to TTET to either or both of the terminal chromophores.
From the triplet energies given in Table 2 and Figure 4 it is
apparent that transfer to either phenanthrene or naphthalene is
energetically feasible. The lack of benzophenone phosphores-
cence in TRI-2 is not surprising given that TTET over similar
distances in benzophenone-naphthalene bichromophores has
been shown to proceed efficiently with a large rate constant (k
∼ 1010 s-1).16,18 In the absence of quantitative emission intensity
or lifetime data it is not possible to estimate the rate constant
in TRI-2. However, from the phosphorescence lifetime for the
benzophenone model, BZM (τ ) 6 ms),12 a lower limit to the
rate constant can be assigned on the basis of an assumed
instrument sensitivity. Thus, kTTET > 1 × 105 s-1. As discussed
below, laser flash photolysis measurements indicate that this
rate constant is actually considerably larger. Owing to the
similarity of the phenanthrene and naphthalene emission spectra,
it was not possible to speculate on the occurrence of TTET
between these two chromophores. We are currently attempting
to synthesize a trichromophore consisting of a phenanthrene-
benzophenone-anthracene sequence. The triplet energy of
anthracene is considerably lower than that of phenanthrene and
Thus k ∼ 4.5 × 108 s-1
.
Phosphorescence Spectroscopy and Triplet-Triplet En-
ergy Transfer (TTET). (i) Phosphorescence Measurements.
Phosphorescence spectra of the trichromophores and their model
compounds measured in 77 K 1:1 ethanol:methanol glasses yield
the triplet energies shown in Table 2. The spectra of the model
compounds PM and NM were nearly identical indicating similar
triplet energies for the two chromophores. The major feature
of the spectra of TRI-1 obtained at a variety of excitation
wavelengths was an emission that closely resembled that of PM
and NM, although it was not possible to distinguish between
the two. There was no evidence for biphenyl phosphorescence
even at wavelengths where this chromophore absorbs strongly
(the BPM model phosphoresces strongly12). For several reasons,
composite spectra (analagous to the fluorescence composite
spectra) could not be used to determine the relative contributions
due to the phenanthrene and naphthalene chromophores. First,
the combination of the small sample tube (2 mm i.d. quartz

8686 J. Phys. Chem. A, Vol. 102, No. 45, 1998
McGimpsey et al.
Figure 6. Transient absorption spectra obtained for TRI-1 (a) and TRI-2 (b) in N2-saturated acetonitrile, 1 µs following 308 nm excimer laser
photolysis.
should afford the opportunity to distinguish between the possible
energy transfer pathways.
TTET in TRI-1 could not be confirmed from the phospho-
rescence spectra. Although there was no observed biphenyl
emission, this is likely caused by the inefficiency of singlet f
triplet ISC in the biphenyl chromophore relative to the efficient
SSET already discussed. A comparison of the rate constants
further supports this conclusion. The rate constant for ISC in
the DBM model compound as indicated from yield data12 is
roughly 3 orders of magnitude less than the value of kSSET
estimated above.
Laser Flash Photolysis and Triplet-Triplet Energy Trans-
fer (TTET). (i) Laser Flash Photolysis. Figure 6a shows the
transient absorption spectra obtained following 308 nm excimer
laser photolysis of TRI-1, PM, and NM in nitrogen-saturated
acetonitrile. The spectrum for BPM was not included since the
absorption due to the biphenyl chromophore at 308 nm was
negligible compared to the other two model compounds. The
spectrum was obtained ∼1 µs after the laser pulse.
The transient spectra obtained for PM and NM were sensitive
to air, similar to the T-T absorption spectra of the parent
aromatic compounds, and had absorption maxima similar to
those reported previously for NM and a variety of compounds
similar in structure to PM.17 Thus, they are assigned to the
Figure 7. Transient absorption spectra of TRI-1 in N2-saturated
triplet states. The observed low wavelength maximum for PM
(350 nm) does not necessarily coincide with an actual absorption
maximum. Rather, strong fluorescence (as described above)
in the 360-390 nm region may interfere with the T-T
absorption. The spectrum obtained for TRI-1 was also air
sensitive and, on the basis of the maxima, we conclude that it
is a combination of the phenanthrene and naphthalene chro-
mophore T-T absorptions. Supporting this conclusion is the
observation of similar but not identical decay kinetics at 490
and 430 nm, the wavelengths associated with the model
compound triplets, indicating that there are two transients
present. This fact is demonstrated more clearly in Figure 7.
which shows the transient spectra for TRI-1 at four different
times after the laser pulse. It is clear from this figure that the
490 nm phenanthrene and 425 nm naphthalene absorptions are
acetonitrile obtained 0.4 (filled circles), 0.8 (filled squares), 2.0 (open
triangles), and 5.0 µs (filled triangles) following 308 nm excimer laser
photolysis.
decaying at different rates. For further confirmation, a com-
posite spectrum was constructed in which the spectra of NM
and PM were combined in a ratio reflecting the ED. Larger
errors are associated with these laser measurements than with
the emission expermiments described above, and therefore, an
exact match between the composite and the measured spectra
was difficult to obtain. Nevertheless, there was a reasonable
correspondence between the composite and measured spectra
using the ED values. However, as we have discussed above,
incorporating the naphthalene chromophore into TRI-1 causes
a 4-fold decrease in the observed fluorescence for this moiety

Photophysics of Polychromophores
J. Phys. Chem. A, Vol. 102, No. 45, 1998 8687
as a result of energy transfer to the phenanthrene chromophore.
Since the fluorescence and ISC quantum yields in NM are
comparable,15,19 we also would expect a 4-fold decrease in triplet
yield, leading to considerably less observed naphthalene triplet
in the transient spectrum of TRI-1 than indicated from the
composite spectrum. Notwithstanding the greater uncertainty
in these laser measurements, such a decrease should be
discernible. Possible reasons for this discrepancy are offered
below.
CHART 2
Figure 6b shows the transient absorption spectrum obtained
following 308 nm photolysis of TRI-2 in nitrogen-saturated
acetonitrile. As with TRI-1, the absorption is air-sensitive and
is therefore assigned to triplet states. In this case the features
in the spectrum are clearly identifiable as phenanthrene and
naphthalene triplets with relatively more naphthalene triplet
present than in the case of TRI-1. There was no absorption
observed due to the benzophenone triplet. Excitation at 355
nm also failed to yield a benzophenone triplet absorption
although ∼30% of the excitation at 355 nm is absorbed by this
chromophore. A composite spectrum indicated approximately
35-40% contribution due to the naphthalene chromophore,
again considerably more than the ED values. This is also
discussed below.
ficiency in the phenanthrene group is attenuated to the same
extentsa 7-fold decreasesas the fluorescence emission (i.e.,
the quantum yield of ISC and fluorescence in PM are compa-
rable), equal partitioning of benzophenone triplet to phenan-
threne and naphthalene would yield a 55:45 phenanthrene triplet:
naphthalene triplet ratio, similar to the ratio obtained from the
composite spectrum.
In reaching the conclusion above we have chosen to discount
the potential of a TTET equilibrium between the phenanthrene
and naphthalene chromophores that could be established owing
to the similarity of their triplet energies. An equilibrium could
contribute to a ratio of triplets dissimilar to that expected from
the ED values. We rule out the possibility of an equilibrium
primarily on the basis of (i) the expected large distance between
the two groups and (ii) the results we have obtained for two
analogous trichromophores (TRI-3 and TRI-4) shown in Chart
2.
(i) TTET follows an exchange mechanism requiring either
good through-bond orbital overlap or small donor-acceptor
separations for efficient transfer. Modeling studies indicate that,
in the ground state, there is a 14 Å separation in TRI-2. In
previous studies, the TTET rate constants in bichromophores
with large interchromophore distances (14-15 Å) were 3 orders
of magnitude smaller (k < 102 s-1) than the triplet decay rate
constants measured for TRI-2. Admittedly in these systems the
linking groups were rigid and thereby ensured a large separation.
In TRI-2, the methyl ester links are flexible and will allow a
certain degree of conformational freedom to the chromophores.
However, with the intervening benzophenone group, a relatively
large separation is still assured even if significant geometry
changes occur upon excitation. Furthermore, the flexible ester
bridges should greatly reduce the efficiency of through-bond
transfer, ensuring that the transfer follows a through-space
(through-solvent) path that is more directly distance-dependent.
Additional evidence against an equilibrium is provided by the
transient spectrum obtained for 355 nm excitation, which shows
a smaller phenanthrene:naphthalene ratio than for 308 nm
photolysis. Equilibrium concentrations of the two chromophores
should be independent of excitation wavelength. The relative
increase in naphthalene triplet can be explained by a smaller
ED for phenanthrene at 355 nm and a larger benzophenone
distribution.
(ii) Triplet-Triplet Energy Transfer (TTET). Missing from
the transient spectrum of TRI-2 is the T-T absorption due to
the benzophenone chromophore, which, judging from the
spectrum of BZM, should be found in the 500-600 nm region
(λ max ∼ 545 nm). From the ED values at 308 and 355 nm and
from the emission studies performed at these wavelengths, it is
clear that significant population of the benzophenone excited
singlet (and therefore, also, triplet) state should occur. The
absence of this T-T absorption is the clearest transient evidence
for TTET in the trichromophores studied and reinforces the
results obtained from the phosphorescence measurements on
TRI-2. The inability to detect the benzophenone triplet even
at short time scales confirms the efficiency of this process in
solution and allows an estimate of a lower limit for the rate
constant of TTET, k > 5 × 108 s-1 (based on the time resolution
of the detection system and the laser pulse duration). Further
confirmation of TTET is given by the observation of the
phenanthrene and naphthalene T-T absorptions, even with 355
nm excitation where the naphthalene ground-state absorption
is negligible. TTET from benzophenone to the naphthalene and/
or phenanthrene chromophores is expected to be rapid on the
basis of previous studies of benzophenone-naphthalene bichro-
mophores incorporating a variety of linking groups.16,18
While both the phenanthrene and naphthalene triplet absorp-
tions were observed in the spectrum of TRI-2, they were not in
a proportion consistent with the ED values (35-40% naphtha-
lene triplet absorption observed for an initial excitation of 8%).
However, given the SSET processes that occur upon excitation,
this is not surprising. Following excitation, the majority of the
excitation energy should be transferred to the benzophenone
singlet state from where it efficiently forms the triplet (recall
the significantly attenuated phenanthrene and naphthalene
fluorescence emission intensities). Therefore, we suggest that
to a large extent the amount of phenanthrene and naphthalene
triplets found in the transient spectrum is a reflection of how
the triplet benzophenone partitions to each of the other chro-
mophores and actually depends little on which chromophore
was originally excited. In fact, the proportion of naphthalene
triplet determined from the composite spectrum indicates that
the benzophenone triplet partitions roughly equally between
naphthalene and phenanthrene. Assuming that the ISC ef-
(ii) TRI-3 and TRI-4 are similar to TRI-1 and TRI-2 with
the exception that the naphthalene group has been replaced by
an anthracene group. The triplet energies of anthracene and
phenanthrene are dramatically different removing the possibility
of TTET between them and ruling out an equilibrium. Since
the anthracene triplet lies lower in energy than the phenanthrene,
intramolecular energy transfer would result in quenching of the
phenanthrene triplet and production of the anthracene triplet.
In fact this is what occurs. However, the decay/growth kinetics

8688 J. Phys. Chem. A, Vol. 102, No. 45, 1998
McGimpsey et al.
are slow and are correlated with the concentration of the
trichromophore, indicating that the interaction here is intermo-
lecular, not intramolecular.
excitation wavelength and corresponds to the maximum of the
T-T absorption band of C. Since the upper triplet of C is higher
in energy than triplet A, re-excitation allows TTET to continue
into the next triad. Each time photons of the appropriate
wavelength are supplied, this “data” shift step operates resulting
in the controlled flow of triplet energy from one triad to the
next. By coupling each shift step with a “write” operation
(writing a “1” bit or a “0” bit), an entire data string can be
stored.
In the case of TRI-1, the efficiency of TTET from the
biphenyl chromophore could not be probed owing to the
negligible absorption of this chromophore at 308 nm. In any
case, the fluorescence results indicated that SSET probably
depopulates the biphenyl singlet before it can undergo ap-
preciable ISC. The poor correlation between the phenanthrene:
naphthalene triplet ratio (as indicated from the composite
spectrum) and the expected ratio given the SSET processes
discussed above is puzzling. Certainly, if a triplet-triplet
equilibrium exists, the amount of naphthalene triplet produced
could be larger. However, the interchromophore distance in
TRI-1 is 12 Å, nearly as large as in TRI-2, arguing against an
equilibrium. Furthermore, excitation at 355 nm resulted in a
transient absorption spectrum different from that at 308 nm.
Also the different lifetimes for the phenanthrene and naphthalene
triplets at 490 aand 430 nm, respectively, were clearly evident
from spectra taken at different times after the pulse.
Finally, reading the shift register can also be optically
controlled. At the opposite end of the chromophore chain from
L another unique chromophore (E) is attached. E has a triplet
energy lower than that of C and a unique T-T absorption
spectrum. A sequence of shift steps moves the “1” and “0”
bits along the chain from one location to the next until they
reach E. Shifting a “1” bit to E causes excitation to its triplet
state. Photoexcitation of triplet E with a photon of the
appropriate wavelength results in population of the upper triplet
state. Since E is also chosen for its large reverse ISC and
fluorescence yields, this re-excitation step can result in strong
fluorescence emission. This means that fluorescence from E
coupled with the excitation of triplet E indicates that a “1” bit
has been read. No emission indicates a “0” bit.
Preliminary Evidence for Optically Controlled Energy
Transfer. Polychromophoric molecules that have demonstrated
long-range charge and energy transfer capabilities have been
suggested as molecular scale electronic and photonic devices
such as wires, switches, diodes, shift registers, and charge
coupled devices.7,8 We have designed TRI-1 and TRI-2 so as
to demonstrate the feasibility of a molecular scale shift register
such as that described below.
Our previous work with rigidly linked bichromophores has
clearly demonstrated that intramolecular triplet energy transfer
from an upper triplet state to an energy acceptor can occur
efficiently.5 In effect, these systems represent the C-A
chromophore pair that connects the triads in the shift register
and are directly involved in the data shift operation described
above. Although upper state energy transfer was observed in
these systems, they were not ideal models for demonstrating
the efficiency of the proposed data shift operation because there
was no “B” chromophore available to accept the triplet energy
once it had been transferred to A. This means that back-transfer
from the triplet of A to the lowest triplet of C can occur without
any competition from a forward-transfer step. Indeed, this back-
transfer step was observed in the systems studied. Molecules
TRI-1 and TRI-2 are more realistic models of the triad
connection since their triplet energies have the same order as
the C-A-B chromophore sequence; i.e., the lowest triplet states
of the terminal chromophores lie lower in energy than that of
the central chromophore, but the upper triplets are higher in
energy. Also, an alternative destination for the energy of triplet
A is provided in chromophore C.
In our view, the operation of a device such as a molecular
scale shift register based on energy transfer will rely on the
ability to control the flow of excitation energy within the
molecule. Some of our recent work on polychromophores has
focused on demonstrating that the intramolecular flow of triplet
excitation energy can be controlled by the application of
additional photons.5 We visualize a molecular scale shift
register somewhat analogous to that described previously,8c but
one that is entirely optically coupled to the macroscopic regime
and operates on the basis of triplet energy transfer rather than
charge transfer. In such a device, a data bit is written to and
read from the shift register with photons, and photons are used
to shift the bit within the register. A single memory location
consists of three covalently linked chromophores (A-B-C
triads) having triplet energies that decrease sequentially, A >
B > C. Identical triads linked together covalently in serial
fashion form the shift register ((A-B-C)-(A-B-C)n-...). A
unique chromophore, L, is attached to the A chromophore at
the end of the chromophore chain, absorbs in a region of the
spectrum where the other chromophores are transparent, has a
large ISC yield, and has a triplet energy greater than that of A.
A “1” bit is written to the chromophore chain by (i) photoex-
citation of L, (ii) population of the triplet state of L, and (iii)
efficient TTET along the first triad to form the triplet of C.
(Writing a “0” bit is the “null” case and involves no excitation.)
Further TTET to the A chromophore of the next triad is uphill
energetically, and therefore, with the exception of decay
processes normally associated with triplet C alone, the excitation
energy (“1” bit) is stored at this location for a finite period of
time.
To demonstrate that triplet energy transfer can be optically
controlled, the following experiment was performed. As already
described, pulsed UV irradiation of TRI-1 results in production
of both the naphthalene and phenanthrene triplet states. The
triplet state of phenanthrene was re-excited approximately 1.5
µs after the initial pulse with the output of a flashlamp pumped
dye laser tuned to the phenanthrene T-T absorption band (488
nm). At this wavelength, the majority of the absorption is due
to the phenanthrene triplet with a minor component due to the
naphthalene triplet (T-T extinction coefficients at 490 nm
determined by benzophenone sensitization:12,20 NM, 1070 (
200; PM, 7100 ( 1400 M-1 cm-1). The results of this two-
laser excitation are shown in Figure 8. At 490 nm, the second
laser pulse causes permanent depletion of the phenanthrene
triplet absorption indicating that the upper triplets produced by
the second pulse do not repopulate the lowest phenanthrene
triplet. The fate of the upper triplets can be determined from
the two-laser kinetics observed at 425 nm, the maximum of the
absorption for the naphthalene triplet. Here, little or no
depletion was observed. The T-T extinction coefficients for
Shifting this triplet energy (“1” bit) into the next triad
(memory location) can be achieved by re-excitation of triplet
C to an upper triplet level having an energy greater than that of
triplet A. The re-excitation step can be achieved with a second
laser pulse firing a short time after the first pulse. The
wavelength for the re-excitation is different from the original

Photophysics of Polychromophores
J. Phys. Chem. A, Vol. 102, No. 45, 1998 8689
Acknowledgment. This work was supported in part by
National Science Foundation Grant CHE-9617830. The authors
thank the donors of the Petroleum Research Fund administered
by the American Chemical Society, for partial support of this
work. The authors also thank Professor S. J. Weininger for
helpful discussions.
References and Notes
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