Photophysical study of new versatile multichromophoric diads and triads with BODIPY and polyphenylene groups

Article abstract of DOI:10.1021/jp8051592

The photophysical properties of multichromophoric dyes with borondipyrromethene (BODIPY) and poly-p-phenylene (di-p-phenylene and tri-p-phenylene) groups in the same molecule are studied in detail. The excitation of the polyphenylene moiety in the UV region leads to a strong visible fluorescent emission of the BODIPY chromophore, via intramolecular excitation energy transfer between both groups. Consequently, these multichromophoric dyes are characterized by a large virtual Stokes shift, with a high fluorescence capacity and an efficient laser emission. On the other hand, the photophysical properties of a related dichromophoric dye with a hydroxy end group at the di-p-phenylene moiety show an important decrease in the fluorescent emission due to a photoinduced electron transfer process in basic media. Therefore, its photophysical properties are sensitive to the environmental acidity/basicity and could be applied as a proton sensor.

Full text of DOI:10.1021/jp8051592

10816
J. Phys. Chem. A 2008, 112, 10816–10822
Photophysical Study of New Versatile Multichromophoric Diads and Triads with BODIPY
and Polyphenylene Groups
J. Ban˜uelos,† F. Lo´pez Arbeloa,*,† T. Arbeloa,† S. Salleres,† F. Amat-Guerri,‡ M. Liras,‡ and
I. Lo´pez Arbeloa†
Departamento Qu´ımica F´ısica, UniVersidad del Pa´ıs Vasco-EHU, Apartado 644, 48080-Bilbao, Spain, and
Instituto de Qu´ımica Orga´nica, CSIC, Juan de la CierVa 3, 28006 Madrid, Spain
ReceiVed: June 12, 2008; ReVised Manuscript ReceiVed: September 03, 2008
The photophysical properties of multichromophoric dyes with borondipyrromethene (BODIPY) and poly-p-
phenylene (di-p-phenylene and tri-p-phenylene) groups in the same molecule are studied in detail. The excitation
of the polyphenylene moiety in the UV region leads to a strong visible fluorescent emission of the BODIPY
chromophore, via intramolecular excitation energy transfer between both groups. Consequently, these
multichromophoric dyes are characterized by a large “virtual” Stokes shift, with a high fluorescence capacity
and an efficient laser emission. On the other hand, the photophysical properties of a related dichromophoric
dye with a hydroxy end group at the di-p-phenylene moiety show an important decrease in the fluorescent
emission due to a photoinduced electron transfer process in basic media. Therefore, its photophysical properties
are sensitive to the environmental acidity/basicity and could be applied as a proton sensor.
Introduction
SCHEME 1: Molecular Structures of the
Dichromophoric Dyes P2ArAc and P3ArAc and of the
Parent Chromophores p-Terphenyl and PM567
In the past few years, borondipyrromethene (BODIPY or
BDP) dyes with the core group 4,4-difluoro-1,3,5,7-tetramethyl-
4-bora-3a,4a-diaza-s-indacene have been largely used for many
applications in different fields because of their unique photo-
physical properties.1-4 As laser dyes, these compounds usually
present tunable bands over the spectral region from the visible
to the near IR, with high thermostability and high photostability.
Their absorption bands are characterized by high absorption
coefficients, while the emission bands show high fluorescence
quantum yields. This large emission ability, together with their
low triplet-triplet absorption, induces high lasing efficiencies.5,6
Indeed, BDP dyes are becoming the most used active media of
tunable dye lasers. Several attempts have been made to develop
sintonizable dye lasers in the solid state based on this dye family,
improving laser efficiencies and photostabilities.7,8
Apart from their applications in photonics, BDP dyes have
been widely used as fluorescence probes in biology and
biomedicine.9,10 They have been also applied in analytical
chemistry, as molecular sensors to recognize different analytes
(cations, anions, or molecules) by means of the fluorescent on/
off switch via a photoinduced electron transfer process11,12 and
in optoelectronics (antenna systems),13-15 in photovoltaic cells,
and in digital information storage devices,16 among others.
The photophysical properties of BDP dyes can be modulated
by the incorporation of appropriate substituents to the chro-
mophoric core.4,6,17 Following this idea, in the last years, the
photophysics of a wide variety of BDP analogues with different
substituents at central 8 (or meso) position, has been analyzed.6,18,19
Thus, we have incorporated poly-p-phenylene (di- or tri-p-
phenylene) units at this 8-position in the dye PM567, instead
of its methyl group, rendering the diads P2ArAc, P3ArAc (both
p-substituted with a terminal acetoxymethyl group), and P2ArOH
(p-substituted with an OH), as well as the triad P3ArP (Schemes
1 and 2).
The dyes P2ArAc and P3ArAc are models of BDP dyes
bearing polymerizable end groups, such as the methacryloyloxy
group, a good strategy for the covalent linkage of BDP dyes to
polymeric chains of, for example, poly(methyl methacrylate),
in the design of new tunable dye lasers in the solid state.20 In
addition, the terminal OH group in P2ArOH gives rise to a
photophysical behavior sensitive to the OH-ionization and,
hence, to the acidity/basicity of the medium. Indeed, some
authors have proposed pH fluorescence probes based on the on/
off switch of the fluorescence emission of appropriate BDP
* To whom correspondence should be addressed. E-mail:
fernando.lopezarbeloa@ehu.es. Tel.: +34 94 601 59 71. Fax: +34 94 601
35 00. Address: Departamento de Qu´ımica F´ısica Universidad del Pa´ıs
Vasco-EHU Apartado 644 48080-Bilbao, Spain.
† Universidad del Pa´ıs Vasco-EHU.
‡ Instituto de Qu´ımica Orga´nica, CSIC.
10.1021/jp8051592 CCC: $40.75
2008 American Chemical Society
Published on Web 10/04/2008

Versatile Multichromophoric Diads and Triads
J. Phys. Chem. A, Vol. 112, No. 43, 2008 10817
SCHEME 2: Synthesis of the Dyes P3ArP and P2ArOHa
the extract was washed with water and dried. Vacuum elimina-
tion of the solvent yielded a residue that was separated into
two main components by column chromatography on silica gel,
with dichloromethane as eluent. Data of 8-(4′-hydroxy-p-
biphenylene)-4,4-difluoro-2,6-diethyl-1,3,5,7-tetramethyl-4-bora-
3a,4a-diaza-s-indacene (P2ArOH): purified by crystallization
from hexane at -78 °C; yield, 6.5 mg (27%), red oil. 1H NMR
(300 MHz, CDCl3, 25 °C): δ 0.98 (t, J ) 7.4 Hz, 6 H, 2 ×
CH3CH2), 1.36 (s, 6 H, CH3-C1, CH3-C7), 2.30 (q, J ) 7.4 Hz,
4 H, 2 × CH3CH2), 2.55 (s, 6 H, CH3-C3, CH3-C5), 6.0 (s, 1
H, OH), 6.94 (d, J ) 8.4 Hz, 2 H, 2 × H-Ar), 7.30 (d, J ) 8.0
Hz, 2 H, 2 × H-Ar), 7.57 (d, J ) 8.4 Hz, 2 H, 2 × H-Ar), 7.67
(d, J ) 8.0 Hz, 2 H, 2 × H-Ar) ppm. 13C NMR (100 MHz,
CDCl3, 25 °C): δ 10.8 (CH3), 11.4 (CH3), 13.6 (CH3CH2), 16.0
(CH3CH2), 115 (C4′), 127, 128.4, 128.8, 128.9 (CH-Ar), 130.7
(C-7a, C-8a), 132.7 (C-2, C-6), 134.9 (C-1′′), 138.3 (C-1, C-7),
139.7 (C-8), 140.1, 140.8 (C-1′′, C-4′′), 153.8 (C-3, C-5). MS
EI (70 eV) m/z (%): 472 [nominal mass, M+] (100), 457 (99).
IR (KBr) νmax: 3150, 1539, 1474, 1314, 1184, 976 cm-1. Data
of 4′,4′′′-di(4,4-difluoro-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-
3a,4a-diaza-s-indacen-8-yl)-p-terphenyl (P3ArP): purified by
crystallization from hexane at -78 °C, red crystals, mp 280
1
°C. Yield 13 mg (31%), H NMR (300 MHz, CDCl3, 25 °C):
δ 0.99 (t, J ) 7.4 Hz, 12 H, 4 × CH3CH2), 1.38 (s, 12 H, 2 ×
CH3-C1, 2 × CH3-C7), 2.32 (q, J ) 7.4 Hz, 8 H, 4 × CH2CH3),
2.55 (s, 12 H, 2 × CH3-C3, 2 × CH3-C5), 7.38 (m, 4 H, 4 ×
H-Ar-(b)), 7.80 (m, 8 H, 4 × H-Ar-(a), 4 × H-Ar-(c)) ppm.
13C NMR (75 MHz, CDCl3, 25 °C): δ 11.8 and 12.5 (2 × Ar-
CH3), 14.6 and 17.0 (2 × CH2CH3), 127.4 (C-3′,C-5′, C-3′′′,
C-5′′′), 127.5 (C-2′, C-6′, C-2′′′, C-6′′′), 128.9 (C-2′′, C-6′′, C-3′′,
C-5′′), 130.7 (C-7a, C-8a), 132.8 (C-2, C-6), 135 (C-4′, C-4′′′),
138.3 (C-1, C-7), 139.5 (C-8), 139.8 (C-1′′, C-1′′′), 140.7 (C-
1′′, C-4′′), 153.8 (C-3, C-5) ppm. MS ESI+ m/z: 815 [M - FH]+.
IR (KBr) νmax: 2956, 2920, 2869, 1541, 1476, 1321, 1192, 1074,
978 cm-1. UV-vis (EtOH) λmax (ε): 523 nm (32800 L mol-1
cm-1). UV-vis (cyclohexane) λmax (ε): 526 nm (56000 L mol-1
cm-1).
a Reagents and conditions: (a) (Ph3P)4Pd, THF-aq; 2 M Na2CO3 2:1,
reflux, 2 h.
chromophores in acid/basic media due to the activation of
quenching processes.21,22
The absorption spectrum of some described dichromophoric
8-(poly-p-phenylene) BDP dyes is the addition of the individual
absorption bands of each chromophore, suggesting the absence
of any intramolecular π-π-interaction between both entities in
the ground state.20 Quantum mechanical calculations indicated
that both chromophores are disposed nearly perpendicularly,
without overlapping their electronic clouds. Excitation at the
BDP main absorption band of these dichromophoric dyes leads
to the expected emission from the free BDP chromophore, while
UV excitation at the poly-p-phenylene absorption band induces
the same visible fluorescent emission due to the existence of a
nonradiative intramolecular excitation energy transfer (intra-
EET) from the poly-p-phenylene moiety to the BDP group.20
Some authors have used this intra-EET process for the design
of dichromophoric BDP as luminescence traps23,24 and as
excitation energy injectors for the development of light harvest-
ing arrays, antenna systems, or molecular wires.25-28
In the present work, the intramolecular excitation energy
transfer from di-p-phenylene and tri-p-phenylene groups linked
to a BDP chromophore analogue to that of the dye PM567
(Schemes 1 and 2) is studied. The potential application of the
dye P2ArOH, with a terminal phenol group, as acid/base
fluorescent sensor is also analyzed, because its fluorescence
emission strongly depends on the pH.
Sample Preparation. Solutions of the dyes (ca. 2 × 10-6
M) in different media were prepared by adding the correspond-
ing solvent to an adequate amount of a concentrated stock
solution (ca. 10-3 M) of the corresponding dye in acetone, after
vacuum evaporation of the solvent. All the solvents were of
spectroscopic grade (Merck and Aldrich) and were used without
further purification. Solutions of P2ArOH in ethanol were
acidified or alkalinized by adding adequate drops of 7 M HCl
in ethanol (prepared by bubbling HCl gas through ethanol up
to saturation) or 1 M NaOH in the same solvent, respectively.
A special electrode was employed for measuring pH values in
ethanol, and a correction factor was used to take into account
the effect of this solvent on the value of the proton concentration.
Spectroscopic Techniques. UV-vis absorption and fluo-
rescence spectra were recorded on a Cary 4E spectrophotometer
and on a SPEX Fluorolog 3-22 spectrofluorimeter, respectively,
with 1-cm quartz cuvettes. Fluorescence spectra were corrected
for the monochromator wavelength dependence and the pho-
tomultiplier sensibility. Fluorescence quantum yields were
determined using as references solutions of PM567 in methanol
(Φ ) 0.91)31 (for the BDP Vis emission) and of p-terphenyl in
cyclohexane (Φ ) 0.77)32 (for the poly-p-phenylene UV
emission).
Materials and Experimental Methods
Synthesis. The synthesis of the dichromophoric dyes P2ArAc
and P3ArAc has been previously reported.20 The news dyes
P2ArOH and P3ArP were synthesized by a Suzuki cross-
coupling reaction,29 as follows (Scheme 2): A mixture of the
8-(p-iodophenyl) BDP dye 130 (50 mg, 0.1 mmol), benzene-
1,4-diboronic acid (8.2 mg, 0.05 mmol), and tetrakis(triph-
enylphosphine)palladium(0) (11 mg, 0.01 mmol) in THF-aq; 2
M Na2CO3 2:1 v/v (25 mL) was refluxed for 3 h. The reaction
mixture was extracted at room temperature with CH2Cl2, and
Radiative decay curves were registered with the time-
correlated single-photon counting technique (Edinburgh Instru-
ments, model FL920, with picosecond time-resolution). Emis-
sion was monitored at the maximum emission wavelength after

10818 J. Phys. Chem. A, Vol. 112, No. 43, 2008
Ban˜uelos et al.
UV excitation leads to a weak fluorescence emission of the
corresponding poly-p-phenylene unit in the UV, and to the
typical strong visible fluorescent band of the BDP group (Figure
1B). The model dye PM567 does not show any important
fluorescent emission upon UV excitation. These results indicate
the existence of an intra-EET process from the donor poly-p-
phenylene to the acceptor BDP group. Indeed, the fluorescence
emission of the poly-p-phenylene chromophore is strongly
quenched (Φ < 0.02), favoring the visible emission from the
BDP chromophore. This emission is far away from the UV
excitation, leading to a very large “pseudo” Stokes shift (ca.
17700 cm-1). This is a very attractive feature for laser dyes
and fluorescent labels and probes, since scattering interferences
of the pumping/excitation light are neglectable at the emission
detection region.34
The absorption spectrum of the trichromophoric dye P3ArP
(Figure 1A, curve c) does not show any new absorption band,
and the shape resembles that of P3ArAc (Figure 1A, curve b).
These experimental data suggest the absence of intramolecular
interaction between the two BDP groups in P3ArP. This could
be due to the length and rigidity of the tri-p-phenylene group,
avoiding a possible overlapping between the electronic π-sys-
tems of the BDP moieties. However, intramolecular BDP-BDP
interactions have been reported by some authors when both
chromophoric groups are linked through appropriate flexible
chains.35 The intra-EET process between BDP and tri-p-
phenylene groups is very efficient, since almost no emission is
detected from the tri-p-phenylene chromophore under UV
excitation, and only the typical bright emission of the BDP group
is observed (Figure 1B, curve c). The efficient intra-EET process
is attributed to the presence of two acceptor units connected at
both sides of the donor tri-p-phenylene moiety.
Figure 1. Absorption (A) and fluorescence (B) (UV excitation) spectra
of diluted solutions of P2ArAc (a), P3ArAc (b), P3ArP (c), and PM567
(d) in cyclohexane. For a better comparison, the visible absorption band
of P3ArP is shown divided by two.
excitation at 410 nm by means of a diode laser (PicoQuant,
model LDH410) with 150 ps fwhm pulses, 10 MHz repetition
rate, and a power supply of 0.65 mW. The fluorescence lifetime
(τ) was obtained from the slope after the deconvolution of the
instrumental response, obtained by means of a Ludox scatter
solution. The goodness of the deconvolution was controlled with
the chi-square (ꢀ2) and Durbin-Watson (DW) parameters and
with the analysis of the residuals. The decay curves were also
registered at different excitation and emission wavelengths, in
this case by means of a single-photon counting instrument with
a nanosecond time resolution (Edinburgh Instruments model
ηF900), and exciting with a hydrogen flash-lamp with 1.5 ns
FWMH pulses and 40 kHz repetition rate.
Theoretical Methods. Quantum mechanical calculations were
performed with the Gaussian 03 software.33 Ground state
geometry was optimized by the B3LYP method using the double
valence 6-31G basis set. The absorption and fluorescence
Franck-Condon transition was predicted by the time dependent
(TD-B3LYP) method. The effect of ethanol in all the above
properties was simulated by means of the polarizable continuum
model (PCM).
The photophysical properties of P3ArP are shown in Table
1. In general, the spectral properties are similar to those of
P2ArAc and P3ArAc.20 The most remarkable feature is the high
molar absorption coefficient, which is nearly twice that of the
dyes with a single BDP group, indicating that the absorption
transition of each BDP chromophore is additive.36 Indeed,
quantum mechanical calculations reveal that the S0-S1 absorp-
tion band of P3ArP corresponds to promotion of an electron
from the HOMO to the LUMO + 1 state and from the HOMO
- 1 to the LUMO of the whole molecule. Looking to these
molecular orbitals (Figure 2), the HOMO and LUMO + 1 states
actually represent the HOMO and LUMO states of one of the
two BDP units, and the HOMO - 1 and LUMO states of P3ArP
correspond to the HOMO and LUMO states of the second BDP
unit. Therefore, the S0-S1 absorption band of P3ArP is the
electronic transition of two independent HOMO-LUMO BDP
transitions. This theoretical result corroborates experimental data
in the sense that there is not any delocalization of the excitation
energy between both BDP groups and that the absorption is
additive, about twice the transition probability in the model dye
PM567.
Results and Discussion
Intramolecular Energy Transfer. The absorption and
fluorescence spectra of the multichromophoric dyes P2ArAc,
P3ArAc, and P3ArP are shown in Figure 1. Similar results were
observed for P2ArOH, an analogue of P2ArAc with a p-OH
group instead the p-acetoxymethyl group. The absorption
spectrum of all these dyes consists of a UV band (not observed
in the dye PM567) and the expected visible absorption band of
the BDP group (Figure 1A). The UV absorption band is
attributed to the 8-(poly-p-phenylene) substituent and, conse-
quently, the absorption spectrum of these multichromophoric
dyes can be explained as the addition of the absorption bands
of the individual chromophores.
To study the dynamics of the intra-EET, time-resolved
fluorescence decay curves at different excitation and emission
wavelengths were registered. Figure 3 shows illustrative decay
curves for the specific case of P3ArAc. The fluorescent decays
of the free related chromophores p-terphenyl and PM567 are
well-described by a monoexponential analysis, with correspond-
ing lifetimes of around 1 and 6 ns. The fluorescent decay of
the BDP chromophore in P3ArAc, after excitation at 490 nm,
was also analyzed (λfl ) 535 nm) as a monoexponential decay,
although the obtained lifetime (4.8 ns) is lower than that of the
parent dye PM567 (6.1 ns).31 This decrease of the fluorescence

Versatile Multichromophoric Diads and Triads
J. Phys. Chem. A, Vol. 112, No. 43, 2008 10819
TABLE 1: Photophysical Properties of the Dye P3ArP under Visible Excitation in Six Representative Solventsa
dye
solvent
λab ((0.2 nm) εmax ((0.5 104 mol-1 l cm-1
)
λfl ((0.2 nm) Φ ((0.05) τ ((0.1 ns) kfl (108 s-1
)
knr (108 s-1
)
P3ArP
F3-ethanolb
methanol
ethanol
acetone
ethyl acetate
c-hexane
521.4
522.2
522.8
522.0
522.4
526.0
526.2
525.8
10.4
9.6
536.5
536.9
537.8
537.7
537.9
540.6
538.0
536.9
0.63
0.57
0.54
0.50
0.55
0.45
0.51
0.51
6.4
4.9
5.0
4.5
4.9
3.6
3.4
3.5
0.98
1.15
1.08
1.11
1.13
1.25
1.49
1.46
0.58
0.87
0.92
1.11
0.93
1.52
1.43
1.40
11.4
14.4
14.7
13.0
7.0
P2ArAc c-hexane
P3ArAc c-hexane
7.0
a Absorption (λab) and fluorescence (λfl) wavelengths, molar absorption coefficient (εmax), fluorescence quantum yield (Φ), and lifetime (τ),
radiative (kfl) and nonradiative (knr) rate constants. The corresponding data of P2ArAc and P3ArAc in cyclohexane (c-hexane) are included for
comparison.20 b F3-ethanol ) 2,2,2-trifluoroethanol
Figure 2. Contour maps of the molecular orbitals HOMO - 1, HOMO,
LUMO, and LUMO + 1 of P3ArP, calculated by the AM1 method.
Figure 4. Spectral overlap of the p-terphenyl emission band and the
PM567 absorption spectrum. Inset: transition dipole moment orientation
of both entities in the dichromophoric dye P3ArAc.
mophore (ca. 4.6 ns) affects the fluorescence decay of the tri-
p-phenylene moiety when recorded in the experimental condi-
tions in which no emission from the BDP group was observed:
λexc ) 275 nm and λfl ) 370 nm.
Intra-EET mechanisms have been explained by two main
theories: the dipole-dipole coupling, or long-range energy
transfer, described by Fo¨rster38,39 and the exchange interaction,
or short-range energy transfer, formulated by Dexter.40 In the
present case, the studied multichromophoric dyes do not
appropriately fulfill some of the requirements needed for the
Fo¨ster mechanism. For instance, Figure 4 shows that the spectral
overlap between the normalized fluorescence band of the donor
tri-p-phenylene group and the absorption spectrum of the
acceptor BDP chromophore (PM567) is nearly neglectable.
Moreover, quantum mechanical calculations predict that the
transition dipole moment of each chromophore is polarized along
the longer molecular axis, leading to a perpendicular disposition
of both dipoles in P3ArAc (Figure 4), for which the Fo¨ster
formalism avoids any energy transfer process. On the other hand,
the exchange interaction mechanism for energy transfer implies
overlap between the electronic clouds of the involved partners,
which is not the case for P3ArAc, where the electronic
π-systems of both moieties are disposed nearly perpendicular
(Figure 4), as discussed above.
An alternative mechanism for the intramolecular energy
transfer consists of the transfer of excitation energy from the
donor to the acceptor through the chemical bonds linking both
moieties.41-44 This mechanism does not require a spectral
overlap, but demands an orbital interaction, which can take place
over a large distance via a conjugated linking chain (superex-
change).45 Moreover, some authors have reported efficient intra-
EET in rigid systems in which the donor and the acceptor are
separated far away by rigid nonconjugated linkers.41-43
Quantum mechanical calculations suggest that the UV
absorption of p-terphenyl and the visible fluorescence band of
Figure 3. Fluorescence decay curves of P3ArAc in methanol exciting
at 275 nm and monitoring the emission at 370 (a) and 535 nm (b).
lifetime, previously observed in a related dye with the 8-(p-
acetoxymethyl)phenyl group (5.2 ns),37 has been assigned to
an enhancement in the nonradiative deactivation rate constant
due to the presence of the phenyl group.37
The fluorescence decay of the tri-p-phenylene moiety in
P3ArAc after UV excitation (λexc ) 275 nm, λfl ) 370 nm)
becomes biexponential (Figure 3, curve a), with a main
component (98%) with a lifetime lower than that of the free
p-terphenyl unit (<1 ns, below the time-resolution of the ns
SPC instrument), and a minor component (2%) with a longer
lifetime (4.6 ns). The fluorescent visible decay of the BDP
chromophore in P3ArAc after UV excitation in the tri-p-
phenylene group (λexc ) 275 nm, λfl ) 535 nm) was analyzed
with a grown-in component (with a very short lifetime, lower
than the time-resolution of our ns SPC) and a decay component
with a fluorescence lifetime of 4.75 ns (Figure 3, curve b),
suggesting that in this case the maximum population of the
fluorescent excited-state is not reached just after the excitation
pulse, but rather after the short delay time required for the
transference of the excitation energy from the tri-p-phenylene
group to the BDP chromophore. The intramolecular energy
transfer process decreases the lifetime of the tri-p-phenylene
group to a value lower than 0.8 ns (the time resolution of the
flash-lamp SPC instrument). The lifetime of the BDP chro-

10820 J. Phys. Chem. A, Vol. 112, No. 43, 2008
Ban˜uelos et al.
SCHEME 3: (Left) Intramolecular Energy Transfer Mechanism in the Dyes P2ArAc and P3ArAc; (right) Electronic
Density of p-Terphenyl and PM567 in the HOMO and LUMO States
TABLE 2: Photophysical Properties of PAr2OH in Diluted
Solution (2 × 10-6 M) of Several Solvents
these two dyes differ in their photophysical behavior in acid/
c-hexane
acetone
ethanol
methanol
basic media, because the behavior of P2ArOH strongly depends
on the ionization of its phenol group and, hence, on the OH-
concentration.
λab ((0.2 nm)
εmax ((0.5 104
mol-1 l cm-1
λfl ((0.2 nm)
Φ ((0.05)
525.5
4.1
522.0
4.0
523.0
4.7
522.0
3.9
)
The absorption spectra of P2ArOH in ethanol remain nearly
unaltered in media with pH values in the range 0-13.1 (data
not shown). However, the fluorescence intensity shows a drastic
decrease in basic media (Figure 5). The fluorescence quantum
yield slight decreases from Φ ) 0.60 in very acid media ([H+]
) 1 M) to 0.56 at moderate basic media (pH ) 9.2). Further
pH increases imply a strong decrease in the fluorescence
quantum yield Φ, reaching a value of less than 0.07 at pH )
13.1 (Figure 5B). The proton association constant (Ka) value
538.0
0.53
3.6
1.46
1.30
440
535.5
0.59
4.5
1.32
0.92
480
536.5
0.60
5.1
1.18
0.79
495
535.5
0.65
4.8
1.35
0.72
490
τ ((0.1 ns)
kfl (108 s-1
)
knr (108 s-1
∆VSt (cm-1
)
)
the model dye PM567 are due to the S1-S0 transition between
the corresponding HOMO and LUMO states of the respective
chromophores. The corresponding contour maps for each dye
are depicted in the right side of Scheme 3. When these
chromophores are linked through the 8-position of the BDP, as
in P3ArAc, an important orbital interaction was observed
between both chromophores at the linking position in the LUMO
state. This observation supports the validity of the through-bond
mechanism for the intra-EET process from the excited-state of
the tri-p-phenylene group to the BDP chromophore (left side
of Scheme 3). Such through-bond mechanism has been also
proposed for the intra-EET in anthracene-BDP or porphyrin-
BDP systems,14,27 which mainly depends on the nature of both
π-systems.24 In any case, small contributions of the Fo¨ster long-
distance EET cannot be excluded because of the weak overlap-
ping between the fluorescence band of the tri-p-phenylene group
and the S0-S2 absorption band of the BDP chromophore, as
has been proposed for several 8-pyrenyl substituted BDPs.46,47
Indeed, the preexponential ratio for the fluorescence decay
curves of P3ArAc, analyzed at the visible BDP emission after
excitation at the UV tri-p-phenylene moiety, did not reach the
-1 value expected for a complete population of the fluorescent
excited-state from the locally excited-state of the tri-p-phenylene
group, suggesting a partial direct excitation of the BDP
chromophore in the UV excitation (i.e., via S0-S2 excitation
and consecutive ultrafast internal conversion to the fluorescent
S1 state).
Proton Sensor. The dye P2ArOH is also obtained in the
synthesis of P3ArP (Scheme 1). Table 2 summarizes its
photophysical properties under direct excitation of the BDP
group. The spectral properties of P2ArOH are very similar to
those of P2ArAc,20 and the former dye also shows the above
commented intra-EET process under UV excitation. However,
Figure 5. (A) Fluorescence spectra of diluted ethanol solutions of
P2ArOH at different pH values: (a) 0, (b) 0.9, (c) 2.8, (d) 5.2, (e) 6.6,
(f) 7.8, (g) 9.2, (h) 9.9, (i) 10.4, (j) 10.6, (k) 10.8, (l) 10.9, 9m) 11.0,
(n) 11.2, (o) 11.9, (p) 12.2, (q) 12.9, (r) 13.1. (B) Plot of the fluorescence
intensity at the maximum wavelength versus the solvent pH. Inset: linear
fit of eq 1 (see text).

Versatile Multichromophoric Diads and Triads
J. Phys. Chem. A, Vol. 112, No. 43, 2008 10821
SCHEME 4: Energy (in eV) of the Frontier Orbitals in Ethanol of PM567 and of Biphenyl-4-ol in Its Neutral -ol (acid
pHs) and Ionized -olate (basic pHs) Forms, calculated by the B3LYP Method
can be evaluated for the phenol OH group by the linear
relationship of Henderson-Hasselbach (eq 1):
BDP chromophore (right side of Scheme 4). Consequently, when
the BDP core is excited, an electron from the HOMO state of
the ionized phenol can be transferred (a thermodynamically
favored process) to the HOMO state of the BDP, filling the
semivacant HOMO orbital and avoiding the radiative transition
of the excited electron back to the ground state.
Ifl(ac) - Ifl(pH)
Ifl(pH) - Ifl(bas)
log
) pH + logKa
(1)
from which an association constant Ka ) 1.6 × 10-11 is deduced
(inset Figure 5B, correlation coefficient r ) 0.989). This value
is quite similar to that reported for 4-phenyl-phenol (Ka ) 1.4
× 10-11 in water/ethanol mixture48). The association constant
in the S1 excited state (Ka*), estimated from the fluorescence
wavenumber shift of the acid and basic forms, and the Fo¨rster
cycle is 2.5 × 10-11, a value similar to that obtained in the
ground state.
The decay curves of P2ArOH (data not shown) are well
analyzed as monoexponentials with a lifetime of ca. 5.05 ns in
the pH range 0-10.8. In more basic environments the fluores-
cent decay becomes biexponential, with a major component
(98%) with a lifetime of 4.8 ns, and a minor one (2%) with a
shorter lifetime (0.7 ns). Moreover, the fluorescence quenching
in basic media is reversible, since the bright emission of the
BDP group is recovered after acidification. These results suggest
that P2ArOH can be applied as a reversible fluorescent on/off
switch, since it is highly fluorescent in acid-neutral media and
nearly nonfluorescent in basic media.
For a deeper interpretation of the influence of the acidity on
the photophysics of P2ArOH, the energy levels of the frontier
HOMO and LUMO orbitals of BDP (PM567) and 4′-methyl-
biphenyl-4-ol in its neutral -ol and ionized -olate forms, moieties
are illustrated in Scheme 4. Unfortunately to our knowledge,
the ionization potential and electronic affinity of the compounds
have not been published. For this reason, the energy values
included in Scheme 4 are obtained from quantum mechanical
calculations in which the effect of ethanol as solvent is taken
into account. The validity of these quantum mechanical data
have been previously proven.8,9
Therefore, quantum mechanical calculations suggest that the
loss of the fluorescent emission of P2ArOH in basic media could
be assigned to a photoinduced electron transfer (PET) process
from the HOMO state of the ionized phenol group to the
semivacant HOMO state of excited BDP group in the same
molecule. Unfortunately the oxidation potential of the donor
moiety 4′-methylbiphenyl-4-olate has not been reported in the
literature and, consequently, the Rehn-Weller equation cannot
be applied to calculate the free Gibbs energy change of the PET
process. The PET process is experimentally confirmed by
fluorescence decay curves (data not shown). The PET process
should be very rapid, probably in a few picoseconds time-scale,
much faster than the time resolution of our ps SPC instrument
(around 30 ps), and the corresponding rate constant cannot be
evaluated in the present paper. Moreover, quantum mechanical
data included in Scheme 4 confirm the viability of PET from
the HOMO level of the biphenyl-4-olate moiety to the semi-
vacant HOMO state of PM567 after excitation removing any
emission from the excited-state of PM567. The validity of the
quantum mechanical data is supported by the fact that the
reported reduction potential of PM567 in acetonitrile (-1.29
eV49) is very similar to that calculated from the quantum
mechanics methods used in the present paper (-1.35 eV).
Summarizing, P2ArOH is a versatile fluorescent dye that
shows a large Stokes shift under UV excitation at the di-p-
phenylene group, and the bright fluorescent emission generated
from the BDP chromophore comes from an efficient intra-EET
process. This dye can also be used as a proton sensor due to
the reversible dependence of its fluorescence emission with the
acidity/basicity of the medium.
In acid media (neutral -ol form), the HOMO and LUMO
orbitals of 4′-methylbiphenyl-4-ol are placed at lower and higher
energies, respectively, with respect to the corresponding HOMO
and LUMO states of the BDP chromophore (left side of Scheme
4). Such a disposition of the energy levels suggests that the
presence of diphenyl-4-ol does not affect the fluorescent
emission of the BDP chromophore. However, in highly basic
media the OH group is ionized and the energy of the HOMO
and LUMO frontier orbitals of the resulting -olate form of
biphenyl-4-ol partner increases in such a way that the HOMO
orbital is placed between the HOMO and LUMO levels of the
Conclusions
To find new photophysical processes and to extend the
potential applications of BDP laser dyes, an appropriate
methodology is to incorporate different chromophores to their
structures, for example at the central 8-position, to reach
multichromophoric systems. Poly-p-phenylene-BDP dyes herein
studied present an intramolecular energy transfer process from
the donor polyphenylene unit (excitation in the UV) to the
acceptor BDP group (emission in the visible), mainly via a

10822 J. Phys. Chem. A, Vol. 112, No. 43, 2008
Ban˜uelos et al.
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shift which reduces the effects of reabsorption and reemission
phenomena. Besides, these dyes convert the UV light into yellow
luminescence and, hence, can be used as antenna systems in
photovoltaic solar cells or, simply, to achieve light of different
wavelengths.
The dichromophoric dye P2ArOH is even a more versatile
system since, apart from its large “pseudo” Stokes shift, the
presence of the terminal OH-phenyl group induces fluorescence
properties sensitive to the acid/basic characteristics of the
surrounding environment. In basic media, the phenol OH group
is ionized and, after excitation of the BDP moiety, a photoin-
duced electron transfer process is activated, quenching the BDP
fluorescent emission. Thus, P2ArOH dye behaves as a proton
sensor with a reversible on/off switch of its visible fluorescence.
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Acknowledgment. The Spanish Ministerio de Educacio´n y
Ciencia (MEC) is thanked for financial support (project
MAT2007-65778-C02-02). S.S. thanks the University of the
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