Structural control of the photodynamics of boron-dipyrrin complexes

Article abstract of DOI:10.1021/jp0525078

Boron-dipyrrin chromophpres containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-terf-butylphenyl) group show low fluorescence yields (～0.06) and short excited-singlet-state lifetimes (～500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (～0.9) and long excited-state lifetimes (～6 ns). The X-ray structures indicate that the phenyl or 4-terf-butylphenyl ring lies at an angle of ～60° with respect to the dipyrrin framework whereas the angle is ～80° for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.

Full text of DOI:10.1021/jp0525078

J. Phys. Chem. B 2005, 109, 20433-20443
20433
Structural Control of the Photodynamics of Boron-Dipyrrin Complexes
Hooi Ling Kee,† Christine Kirmaier,† Lianhe Yu,‡ Patchanita Thamyongkit,‡
W. Justin Youngblood,‡ Matthew E. Calder,‡ Lavoisier Ramos,§ Bruce C. Noll,
David F. Bocian,| W. Robert Scheidt, Robert R. Birge,§ Jonathan S. Lindsey,‡ and
Dewey Holten*,†
Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130-4889, Department of Chemistry,
North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, Departments of Chemistry and of
Molecular and Cell Biology, UniVersity of Connecticut, Storrs, Connecticut 06268, Department of Chemistry,
UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, UniVersity of California
RiVerside, RiVerside, California 92521-0403
ReceiVed: May 12, 2005; In Final Form: August 18, 2005
Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward
aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved
optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low
fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%)
by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an
o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes
(∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60° with
respect to the dipyrrin framework whereas the angle is ∼80° for mesityl or o-tolyl groups. The calculated
potential energy surface for the phenyl-substituted complex indicates that the excited state has a second,
lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin,
which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative
probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap
factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex
bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability
involving conformations at or near the initial Franck-Condon form of the excited state. These combined
results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class
of widely used dyes.
Introduction
CHART 1
Boron-dipyrrin dyes were first discovered by Treibs and
Kreuzer in 1968.1 Since then, boron-dipyrrin dyes have been
widely used as markers in life-sciences research.2 One limitation
to the even broader use of boron-dipyrrin dyes has been the
construction of the dipyrrin chromophore with use of â-substi-
tuted pyrrole units (Chart 1), which are available only via quite
lengthy synthetic procedures. A decade ago we discovered a
simple entre´e into 5-substituted dipyrromethanes by reaction
of an aldehyde with excess pyrrole at room temperature,3 and
have since refined this simple procedure.4,5 Dolphin showed that
such dipyrromethanes can be oxidized to the corresponding free
base dipyrrins upon treatment with DDQ or p-chloranil.6
Complexation of a free base dipyrrin with BF3‚O(Et)2 in the
presence of TEA yields the desired boron-dipyrrin complex.
We initially synthesized a boron-dipyrrin dye that incorpo-
rated an aryl group at the 5-position and a methyl group at each
of the pyrrolic R-positions (1, Chart 2).7,8 Such dyes were
incorporated in a variety of architectures, including a molecular
photonic wire,7,9 an optoelectronic gate,10 and a light-harvesting
array.11 To our surprise, 5-substituted boron-dipyrrin dyes were
only weakly fluorescent (Φf ) 0.058 for 1), in contrast to the
intense fluorescence of the analogous â-substituted dipyrrin-
based dyes first prepared by Treibs and widely used in the life
sciences. Time-resolved studies showed that the 5-aryl-
substituted dyes exhibited a rather short singlet-excited-state
lifetime of ∼500 ps, with an even faster ∼15 ps component to
the excited-state relaxation dynamics.11 Ab initio calculations
indicated that the two kinetic components are associated with
two energetically accessible conformers that differ in the rotation
* Address correspondence to this author. E.mail: holten@wustl.edu.
† Washington University.
‡ North Carolina State University.
§ University of Connecticut.
University of Notre Dame.
| University of California, Riverside.
10.1021/jp0525078 CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/08/2005

20434 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Kee et al.
CHART 2
CHART 4
CHART 3
Chart 4). The boron-dipyrrin dyes 2a-d are more readily
synthesized than the corresponding dyes (4, 5) bearing substit-
uents at the pyrrolic â-positions. We also synthesized and
studied five analogues bearing alkyl groups rather than fluorine
atoms on the boron (2a-Me2, 2a-Bu2, 2d-Me2, 2d-Bu2, 2d-BN).
The fully unsubstituted dye (6), although unknown, is a
benchmark for theoretical calculations.
Materials and Methods
Synthesis. Details of the synthesis and characterization of
N,N′-difluoroboryl-5-phenyldipyrrin (2a), N,N′-difluoroboryl-
5-(4-tert-butylphenyl)dipyrrin (2b), N,N′-difluoroboryl-5-ortho-
tolyldipyrrin (2c), N,N′-difluoroboryl-5-mesityldipyrrin (2d),
N,N′-(dimethylboryl)-5-phenyldipyrrin (2a-Me2), N,N′-(dibu-
tylboryl)-5-phenyldipyrrin (2a-Bu2), N,N′-(dimethylboryl)-5-
mesityldipyrrin (2d-Me2), N,N′-(dibutylboryl)-5-mesityldipyrrin
(2d-Bu2), N,N′-(9-borabicyclo[3.3.1]-non-9-yl)-5-mesityldipyrrin
(2d-BN), and 5-(o-tolyl)dipyrrin (8c) are given in the Supporting
Information. Compounds 5-(o-tolyl)dipyrromethane (7c),4 5-phen-
yldipyrrin (8a),6,12 5-(4-tert-butylphenyl)dipyrrin (8b),12 and
5-mesityldipyrrin (8d)12 were prepared according to literature
procedures. Compounds 4 (BDPY 3921) and 5 (BDPY 3922)
were purchased from Molecular Probes Inc.
Photophysical Characterization. Static absorption (Cary
100) and fluorescence (Spex Tau2) measurements were per-
formed as described previously.14 For emission studies, non-
deaerated samples with an absorbance e0.15 (typically 0.3-
0.8 µM) at λexc were employed. Emission quantum yields were
measured relative to fluorescein in 0.1 M NaOH (Φf ) 0.92)15
and were corrected for the solvent refractive index.
Fluorescence lifetimes were obtained on samples that had
concentrations of 0.5-10 µM and were deaerated by bubbling
with N2. Lifetimes were determined by the fluorescence
modulation (phase shift) technique, using a Spex Tau2 spec-
trofluorometer. Samples were excited at various wavelengths
and emission detected through appropriate colored glass filters.
Modulation frequencies from 20 to 300 MHz were utilized and
both the fluorescence phase shift and modulation amplitude were
analyzed.
of the 5-aryl group and in distortions of the boron-dipyrrin
framework. The aryl-ring rotation and accompanying chro-
mophore distortions allow access to an excited-state conformer
with low radiative probability and facile nonradiative deactiva-
tion to the ground state, thereby limiting the fluorescence yields
of the dyes.11 The fluorescence properties of an unsubstituted
analogue of 1, which contained only the 5-phenyl group (2a),
also exhibited a low fluorescence quantum yield (Φf ) 0.053).8
More recently, we have prepared and characterized a series
of bis(dipyrrinato)zinc(II) complexes (Chart 3).12 The complex
with a 5-phenyl group (3a) exhibits a very short excited-state
lifetime (90 ps) and a low fluorescence quantum yield (0.006).
By contrast, the complex with a 5-mesityl group (3d) exhibits
a normal excited-state lifetime (∼3 ns) and much larger
fluorescence quantum yield (0.36).13 The photophysical behavior
of complex 3b, which contains a 4-tert-butylphenyl group but
lacks the 2,6-dimethyl groups, resembles complex 3a, indicating
that electronic effects are not the source of the altered excited-
state behavior. The profound increase in excited-state lifetime
is attributed to steric inhibition of rotation of the 5-mesityl group
by the mesityl o-methyl substituents. This striking result has
prompted us to revisit the boron-dipyrrin complexes with a
particular focus on the role of steric effects in controlling the
photodynamics of this class of dyes.
In this paper, we describe the synthesis and structural,
photophysical, and theoretical characterization of a series of
boron complexes of 5-substituted dipyrrin dyes. The substituents
include phenyl, 4-tert-butylphenyl, o-tolyl, and mesityl (2a-d,

Boron-Dipyrrin Complexes
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20435
SCHEME 1
Time-resolved absorption spectroscopy was performed as
described previously.13 Samples (∼10 µM) in 2 mm path length
cuvettes at room temperature were excited at 10 Hz with ∼130
fs, 5-30 µJ pulses at the appropriate wavelength and probed
with white-light probe pulses of comparable duration. Spectra
shown at <1 ps were constructed from spectra at closely spaced
time intervals to account for the time-dispersion of wavelengths
in the white-light probe pulse. Kinetic data were fit to functions
consisting of one to three exponentials (convoluted with an
instrument response) plus a constant asymptote.
Theory. Ground-state potential energy surfaces were gener-
ated by scanning the aryl dihedral angle and optimizing the
remaining degrees of freedom, using density functional methods
and redundant internal coordinates. We selected the B3LYP
hybrid functional of Becke, Lee, Yang, and Parr16,17 and used
a 6-31G(d) basis set. This combination provides a near optimum
combination of accuracy and computational efficiency.18 The
lowest excited triplet state surfaces were generated by using
identical procedures, and the triplet state geometry was then
used as the guess geometry for the excited singlet state
optimizations. The lowest singlet state geometries were calcu-
lated by using gradients generated via full single excitation
configuration interaction with inner shell orbitals (e.g., carbon
1s) frozen. The resulting energies were then adjusted for
correlation by using double CI as provided by a windowed [32-
(filled)+32(open)] symmetry-adapted cluster configuration in-
teraction (SACCI) calculation.19 The Opt(CIS)+∆E(CID/
SACCI) approximation can be justified on the basis of two
observations. First, as we will demonstrate below, the lowest
singlet state is an ionic state dominated by singly excited
configurations (>80%). Second, test calculations using SACCI
with single and double CI indicate that single CI optimization
generates a geometry that is within a few percent of that
generated with single and double CI in terms of redundant
internal coordinates when the aryl group dihedral angle is fixed.
Nevertheless, double CI plays an important role in the energy
correlation of the excited singlet state when the aryl ring
approaches planarity with respect to the boron-dipyrrin frame-
work.
SCHEME 2
Results and Discussion
Excited-state spectroscopic properties were calculated by
using MNDO-PSDCI20,21 and SACCI19 methods with partial
single and double CI assuming vacuum conditions. Test cal-
culations using full CISD on high-symmetry molecules indicated
that the states calculated with restricted basis set had converged
to within 0.1 eV of the full CISD values. The MNDO-PSDCI
and SACCI methods generate comparable energies and proper-
ties for the lowest four excited singlet states, but these methods
diverge for the higher energy transitions. We report here only
the results for the SACCI calculations, which we consider to
be more accurate and reliable. The SACCI calculations used
the Huzinage-Dunning D95 double-ú basis set, which has been
shown to be optimal for spectroscopic calculations.19
Synthesis. The synthesis of the boron-dipyrrin dyes requires
access to the corresponding dipyrrin, which is obtained in turn
from the dipyrromethane (7). The oxidative conversion of a
dipyrromethane to a dipyrrin can be performed with DDQ or
p-chloranil in THF.6,12 Dipyrrins 8a, 8b, and 8d have been
prepared previously.12 The synthesis of dipyrrin 8c is shown in
Scheme 1. Treatment of 5-(o-tolyl)dipyrromethane (7c)4 with
DDQ in THF afforded dipyrrin 8c in 84% yield.
Boron-dipyrrin complexes such as 2a were prepared previ-
ously from the dipyrromethane (without isolation of the dipyrrin)
via a two-step one-flask reaction.4,8,11 Here we have employed
excess TEA and BF3‚O(Et)2 to complete the reaction (Scheme
2). Thus, treatment of a free base dipyrrin 8 with TEA and
BF3‚O(Et)2 (10 molar equiv each) in CH2Cl2 at room temper-
ature for 30 min afforded the desired boron-dipyrrin com-
plex 2 in fair yield (19-68%). Each boron-dipyrrin complex
(2a-d) is less polar than the corresponding starting material
(8) or other byproducts and could be easily separated via flash
column chromatography. In this manner, boron-dipyrrin com-
plexes 2a-d were obtained.
The ab initio and density functional calculations were carried
out using Gaussian-03 Linux Revision B0522 with Linda as the
parallel resource software running under Red Hat Linux 9.0 (Red
Hat, Inc., Raleigh, NC). The hardware environment was a 16-
node (32 processors) Beowulf cluster manufactured by Western
Scientific (San Diego, CA). Each node is composed of two 1.266
MHz Intel Pentium III processors, 1 GB of DDR RAM, and a
single 34 GB SEAGATE hard drive for local data buffering.
The nodes are networked by using fast Ethernet (Ethernet Pro
100). The semiempirical MNDO-PSDCI and Gaussian-03
survey calculations were run on a dual processor 1.25 GHz
Macintosh G5.
A series of dialkylboron analogues were prepared in sim-
ilar fashion (Scheme 3). The reaction of dipyrrin 8a with bro-
modimethylborane or dibutylboron triflate afforded 2a-Me2 or
2a-Bu2, respectively. The reaction of 5-mesityldipyrrin (8d) with
bromodimethylborane, dibutylboron triflate, or 9-BBN triflate

20436 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Kee et al.
SCHEME 3
Figure 1. Basic architecture and atom numbering of a 5-aryl-substituted
dipyrrin. Planes encompassing a number of architectural features are
indicated by the colored lines. The dihedral angles between these planes
are given in Table 1.
TABLE 1: Dihedral Angles between Planes in
Boron-Dipyrrin Dyes
2a
2b
2d
2c
2d-Me2
some of the small deviations from coplanarity of the planes in
the dipyrrin framework may reflect the effects of crystal packing
forces. One example is the effect of replacement of the 5-phenyl
ring in 2a with the 4-tert-butylphenyl ring in 2b.
phenyla tert-butylphenyla mesityla o-tolyla mesityla
plane
Fb
Fb
Fb
Fb
Meb
1,2
1,3
2,3
1,4
2,4
3,4
1,5
2,5
3,5
4,5
1,6
2,6
3,6
4,6
5,6
1,7
2,7
3,7
4,7
5,7
6,7
0.0
1.9
2.9
7.0
0.7
1.7
0.9
2.5
5.7
7.1
One of the most pronounced effects of the 5-aryl substituent
lies in the dihedral angles between the 5-aryl ring (plane 7) and
the planes defining various dipyrrin elements. The angles
increase from ∼50° (2b, 4-tert-butylphenyl) and ∼60° (2a,
phenyl), when no internal hindrance for aryl rotation is present,
to ∼75° (2d, mesityl) and ∼85° (2c, o-tolyl), when this motion
is restricted. Steric hindrance also tends to lengthen the
C5-C10 bond between the dipyrrin and the aryl group, from
1.478(3) Å for 2b and 1.4813(11) Å for 2a to 1.4912(10) Å for
2d and 1.4944(17) Å for 2c. These ground-state structural effects
are a harbinger of the distortions involving the planarity of the
dipyrrin framework and rotation of the aryl ring in the excited
states of the complexes, as is indicated in the photophysical
data and theoretical calculations described below.
Photophysical Characterization. Absorption and Emission
Spectra. Figure 2 gives UV/vis room temperature electronic
ground-state absorption spectra (s) and S1 f S0 fluorescence
spectra (---) of a series of the boron-dipyrrin complexes in
cyclohexane (panels A-D). Generally similar spectra are
observed in toluene and acetonitrile, and in a solid polyvinyl
acetate (PVA) matrix (Supporting Information). Regardless of
the presence or nature of a 5-aryl ring or the boron substituents,
all of the spectra contain a S0 f S1 origin band (490-500 nm)
and vibronic components spanning ∼25 nm to higher energy.
A parallel situation exists in the S1 f S0 fluorescence spectra.
These vibronic contours are analyzed theoretically below. There
is a significant spacing (Stokes shift) of 500-900 cm-1 between
the absorption and emission maxima, indicating excited-state
conformational changes, solvent reorientation, or both. One of
the most notable changes that occurs in the electronic absorption
spectrum upon an increase in solvent polarity from cyclohexane
(or toluene) to acetonitrile is alteration of the absorption contour
in the region between 300 and 400 nm, which includes an
increase in intensity of one of the contributing transitions (cf.
Figure 2E).
1.9
6.4
1.0
2.9
8.9
1.9
1.0
2.7
2.9
8.4
1.9
3.9
3.1
2.8
8.8
2.6
7.7
3.4
5.4
15.3
0.3
0.7
1.6
0.7
0.3
0.7
3.2
1.0
1.1
5.7
1.5
5.5
1.6
2.5
7.4
1.5
2.2
0.1
2.9
8.2
0.7
2.7
0.9
0.7
4.3
0.7
1.7
0.8
0.3
1.4
1.5
4.9
1.1
2.8
8.4
1.5
3.7
2.5
2.7
8.1
0.0
2.0
0.5
0.9
4.3
60.8
60.8
62.1
62.1
61.4
61.4
50.0
50.2
56.6
49.6
51.6
51.7
75.4
75.3
75.9
77.9
76.1
71.6
84.8
84.8
82.3
87.6
84.9
84.9
84.4
84.7
77.3
92.6
84.6
84.9
a Substituent group at the 5-position. b Substituent groups on boron.
afforded 2d-Me2, 2d-Bu2, or 2d-BN, respectively. Each com-
pound was isolated by chromatography on silica and was stable
to routine handling.
Structural Studies. The results of X-ray diffraction studies
of five boron-dipyrrin compounds are described in the Sup-
porting Information. The principal effects of the 5-aryl group
and boron substituent can be seen in Table 1 and Figure 1, which
give the dihedral angles between various mean planes in the
architectures. In all five compounds, the boron-dipyrrin frame-
work, which encompasses the two pyrrole rings and the central
six-member ring containing the boron atom, is essentially planar.
For the four compounds with fluorines on the boron, the
deviation of the pyrrole planes with respect to each other and
the central ring is typically only a few degrees. Somewhat larger
(but still very modest) deviations are observed in 2d-Me2, in
which methyl groups replace fluorine atoms on boron. In this
case, the dipyrrin framework is slightly puckered, as indicated
by the ∼15° dihedral angle between the two pyrrole rings
(planes 3 and 4 in Figure 1 and Table 1). For all five compounds,
Polarized fluorescence and fluorescence-excitation spectra
were acquired for 2a in PVA at room temperature (Supporting

Boron-Dipyrrin Complexes
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20437
sponding yield via the formula (ki)-1 ) τ/Φi. For comparison,
Table 2 gives our prior results for toluene solutions of 2a and
1 (Chart 2),11 along with fluorescence yield and lifetime data
for the boron dipyrrins in PVA (and ethylene glycol). These
latter studies were undertaken to determine if motional restric-
tions imposed by the medium would influence the S1 photo-
physics at room temperature.
The fluorescence quantum yield found here for phenyl-
substituted boron dipyrrin 2a in toluene (Φf ) 0.062) is in good
agreement with our earlier value (0.053).11 We also obtained
the same excited-state lifetime determined by fluorescence decay
(τ ) 0.45 ns). Very similar results are obtained for the 4-tert-
butylphenyl analogue 2b (Φf ) 0.069, τ ) 0.55 ns). In contrast,
the introduction of steric constraints on the aryl rings causes
bright emission and long excited-state lifetimes. In particular,
the o-tolyl-substituted complex 2c in toluene has Φf ) 0.93
and τ ) 5.8 ns, and the mesityl analogue 2d has Φf ) 0.93 and
τ ) 6.6 ns. Interestingly, relatively strong fluorescence was also
observed for 2d-Me2 (Φf ) 0.33 and τ ) 3.7 ns), which contains
methyl groups in place of fluorine atoms on boron. On the other
hand, the fluorescence quantum yield declined markedly upon
increasing steric bulk at the boron atom [2d-Bu2, Φf ) 0.057;
2d-BN, Φf ) 0.014]. Quite low fluorescence yields were ob-
served with the dialkylboron analogues of the phenyl-substituted
dipyrrin [2a-Me2, Φf ) 0.008; 2a-Bu2, Φf ) 0.011]. The
differences in fluorescence behavior may derive from motions
involving the alkyl groups; however, these compounds are
somewhat unstable under prolonged illumination and were not
studied in detail. In agreement with previous results,22-26 intense
fluorescence and long excited-state lifetimes are also obtained
for 4 and 5, which contain fluorine atoms on boron; hydrogen
or methyl groups (rather than an aryl substituent) at the
5-position along; and methyl groups on the pyrrole rings (Table
2). The fluorescence lifetimes and yields (in toluene) for all of
the boron-dipyrrin complexes listed in Table 2 give a radiative
Figure 2. Room temperature electronic absorption spectra (solid) and
fluorescence spectra (dashed) of boron-dipyrrin dyes. The peak
wavelengths are indicated. The second value indicated for the absorption
in the 300-400 nm region in panels A-D is the peak amplitude
compared to the maximum of the lowest energy transition in the 490-
510 nm region. The fluorescence spectra were acquired with use of an
excitation wavelength of 450-460 nm. The spectra in panel E are
vertically enhanced ∼3-fold from the other panels to emphasize the
solvent dependence of the spectra in the 300-400 nm region.
rate constant in the range kf ) (6-10 ns)-1
.
These findings indicate that the low emission yields and short
excited-state lifetimes found in aryl-substituted complexes
bearing no internal steric hindrance derive from a facile S1-
excited-state nonradiative decay channel that is restricted when
internal steric constraints are imposed (or in the absence of a
5-aryl ring). As is shown below from the transient absorption
data and theoretical results, these low emission yields and short
S1 excited-state lifetimes result from excited-state conforma-
tional changes that drive fast internal conversion to the ground
state. In addition to internal steric hindrance, these motions also
can be restricted by external forces. For example, Table 2 shows
that the fluorescence yield and lifetime essentially double for
2a when the medium is changed from toluene solution (Φf )
0.062, τ ) 0.45 ns) to the solid PVA matrix (0.1, 1.1 ns) at
room temperature. The same effect is observed for 2b in toluene
(0.069, 0.55 ns) versus PVA (0.1, 1.1 ns) and intermediate
results are observed in ethylene glycol, which has intermediate
viscosity (Table 2). By contrast with the sterically unhindered
2a and 2b, the respective mesityl and o-tolyl complexes 2d and
2c, which have internal steric hindrance to aryl rotation, show
virtually no change with medium viscosity, namely, fluorescence
yields of 0.89-0.93 and lifetimes of 5.8-6.9 ns in both toluene
and PVA. Parallel but even more dramatic effects on the
fluorescence properties of dipyrrins with and without internal
steric hindrance to 5-aryl rotation are found with temperature,
as is described in the following.
Information). The excitation spectra obtained with fluorescence
detection at 518, 550, or 650 nm give a constant polarization
ratio of ∼0.27 over the main absorption profile from 400 to
450 nm. These results are consistent with parallel transition
dipoles for these absorption and fluorescence features, which
can be understood because common S0 T S1 transitions are
involved. On the other hand, the polarization ratio for the weaker
absorption feature(s) near 350 nm drops below zero, indicating
the absorption transition dipole(s) is aligned more perpendicular
to that for the S1 f S0 fluorescence. Generally similar
polarization results have been obtained previously for 4.22
Fluorescence Yields and Lifetimes at Room Temperature. The
optical spectra described above indicate that the substituents
on the 5-aryl ring of the boron-dipyrrin dyes have only modest
impact on the absorption spectra and even less significant effects
on the S1 f S0 fluorescence profiles. Nonetheless, the nature
of the 5-aryl ring has dramatic effects on the fluorescence
quantum yield (Φf) and the lifetime of the S1 excited state (τ),
as shown in Table 2. These two quantities are related to the
fundamental rate constants for S1 f S0 decay via fluorescence
(kf), S1 f S0 internal conversion (kic), and S1 f T1 intersystem
crossing (kisc) via the formulas τ ) (kf + kic + kisc)-1 and Φf )
kf/(kf + kic + kisc) ) kfτ. Table 2 gives estimates for the yields
of the two nonradiative decay pathways (internal conversion
and intersystem crossing) obtained from the transient absorption
data described below. This table also gives estimates for the
inverse rate constants (i.e., time constants) for the three S1 decay
pathways obtained from the excited-state lifetime and corre-
Temperature Dependence of the Fluorescence Yield. The
effect of temperature on the fluorescence behavior of the phenyl-

20438 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Kee et al.
TABLE 2: Photophysical Data for Boron Dipyrrins
substituents
compd
solvent
toluene
toluene
PVA
5
boron
F
pyrrole
H
τ (ns)
Φf
ΦIC
ΦISC
(kf)-1 (ns)
(kic)-1 (ns)
<0.6
(kisc)-1 (ns)
>5
2a
2aa
2a
phenyl
0.45
0.44
1.2
0.062
0.053
0.12
>0.95
<0.05
7.3
8.3
11
2b
2b
2b
2c
toluene
PVA
ethylene glycol
toluene
PVA
toluene
PVA
toluene
toluene
toluene
toluene
t-BuPh
F
H
0.55
1.1
0.68
5.8
6.1
6.6
6.8
3.7
6.1
5.6
0.069
0.10
0.045
0.93
0.90
0.93
0.89
0.33
0.92
0.93
>0.95
<0.05
8.0
10
11
<0.6
>5
o-tolyl
F
F
H
H
<0.07
<0.07
<0.05
<0.05
6.2
6.8
7.1
7.8
11
6.6
6.0
9.0
>70
>70
>100
>100
2c
2d
2d
2d-Me2
4
mesityl
mesityl
H
Me
b
Me
F
F
H
Me
Me
c
<0.08
<0.07
>0.95
<0.05
<0.05
<0.05
>70
>70
<0.6
>100
>100
>5
5
1a
F
0.52
0.058
a From ref 11. b TMS-terminated phenylethyne. c Methyl groups only at the 1,9-positions (adjacent to pyrrole nitrogens).
ambient temperature. Hence the apparent activation energy may
represent in part the quantum-level dependence of tunneling
through a torsional barrier, and not an actual barrier crossing.
Regardless of the precise physical significance of the apparent
activation energy, the key finding is that the fluorescence
yield of the phenyl-substituted boron dipyrrin lacking inter-
nal steric constraint (2a) increases significantly when thermal
energy is reduced. The value at 78 K (Φf ∼ 0.35) has climbed
to 30-50% of that of the mesityl-substituted analogue, which
contains internal hindrance to aryl-ring rotation (2d). These
observations are another indication that excited-state conforma-
tion excursions are a key process in driving the observed
fluorescent behavior of aryl-substituted boron-dipyrrin dyes.
Transient Absorption Spectra and Kinetics. Figures 4 and 5
give representative transient absorption spectra and kinetic data
for 2a, 2b, and 2d in toluene at room temperature, employing
∼130 fs excitation flashes at ∼485 nm. Additional data are given
in the Supporting Information. Comparison with the static
absorption (solid) and fluorescence (dashed) spectra for 2a in
Figure 4B indicates that the transient absorption difference
spectra in Figure 4A are comprised of bleaching of the S0 f
S1 ground-state absorption band at ∼505 nm plus a tail to longer
wavelengths representing S1 f S0 stimulated emission (i.e., gain
in the white-light probe pulse in the fluorescence region). The
spectra have decayed completely to the ∆A ) 0 baseline by 2
ns, indicating complete deactivation to the ground state and
insignificant formation of a longer lived metastable state such
as the triplet (T1) excited state. Given the very small fluorescence
yield (∼0.06, Table 2), these combined data indicate that the
decay of photoexcited 2a in toluene occurs predominantly by
S1 f S0 nonradiative internal conversion.
The kinetic traces in Figure 4C show a major (>70%)
component with a time constant of ∼400 ps to the decay of
both ground-state bleaching (505 nm) and stimulated emis-
sion (545 nm); this value agrees well with the S1 lifetime of
0.45 ns observed via fluorescence decay (Table 2). The decay
of ground-state-absorption bleaching also shows a component
with a time constant of ∼1 ps along with a contribution from
a third phase having a time constant of ∼10 ps. The ∼10 ps
phase also contributes in the stimulated-emission region. Thus,
these two fast phases represent a combination of rapid deactiva-
tion to the ground state (in addition to the dominant 400 ps
phase) and conformational evolution on the S1 surface with
consequent spectral changes. These results corroborate the
biphasic kinetics observed previously for 2a and 1 in toluene,11
and reveal at least one additional contribution to the excited-
state dynamics.
Figure 3. Temperature dependence of the fluorescence for 2a (A) and
2d (B) in PVA. The samples were excited at 455 or 450 nm,
respectively.
substituted complex 2a was examined in parallel with the
mesityl-substituted analogue 2d. Solid PVA was used as the
medium in both cases because (1) PVA is transparent across
the temperature range studied (295 to 78 K), (2) discontinuities
at the freezing point of a solution sample are avoided, and (3)
changes in viscosity with temperature are small compared to a
solution sample, affording a more clear effect of temperature
(thermal energy). Figure 3 shows the results. The integrated
fluorescence of unhindered 2a in PVA increases about 6-fold
when the temperature is lowered from 295 to 78 K. In contrast,
the emission from hindered 2d appears to increase slightly with
the same reduction in temperature. [There is little (<20%)
temperature dependence of the absorbance at the 450 or 455
nm excitation wavelength in either sample.]
An apparent activation energy on the order of 1000 cm-1 is
obtained from an Arrhenius analysis. It must be born in mind
that this activated phenomenon may not simply reflect the
energy for photoexcited 2a to surmount a barrier for phenyl
rotation. Instead, this is likely to represent a rather low-energy
torsional motion (<50 cm-1), coupled with nonplanar distortion
of the dipyrrin framework. These low-energy motions are no
doubt populated to very high quantum levels, especially at

Boron-Dipyrrin Complexes
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20439
the value (0.55 ns) obtained by fluorescence decay (Table 2).
In contrast, a much longer deactivation time is observed for
the mesityl-substituted 2d (Figure 5B). For this complex, the
ground-state recovery is only about one-half complete by 3.5
ns. When the decay profile observed over the ∼4 ns limit of
this spectrometer is fit to an exponential decay function
containing ∆A ) 0 at long-time asymptote (either fixed or in a
free fit), a time constant of ∼6 ns is obtained. This value is in
good agreement with the S1 lifetime of 6.6 ns observed by
fluorescence decay. These combined results indicate that the
S1 decay for 2d leads predominantly to repopulation of the
ground state with little triplet formation. This result is expected
since the fluorescence yield of this compound is 0.93, which
returns the excited molecules to the ground state via this
emissive route.
Thus, the transient absorption data are fully consistent with
the results of the static spectroscopy and fluorescence lifetime
measurements and indicate that the S1 excited state of sterically
hindered boron dipyrrins 2d and 2c decay predominantly
(g90%) by fluorescence emission whereas the unhindered
analogues 2a and 2b deactivate predominantly (g90%) by
nonradiative internal conversion to the ground state.
Theoretical Analysis. Nature of the Lowest Excited Singlet
States and Assignment of the Optical Transitions. The energies
of the excited states of the boron-dipyrrin dyes were calculated
by using the symmetry-adapted cluster configuration interaction
(SACCI) method. A comparison of the SACCI excited singlet
state manifolds with the absorption spectra of 4 and 2a is shown
in Figure 6. In general, SACCI with single and double
configuration interaction reproduces the absorption spectra of
these compounds. The results support the notion that the lowest
lying excited singlet state is a strongly allowed B2 or B2-like
state. As shown in the state correlation diagram (Figure 7), the
ordering of the lowest three excited singlet states (B2, 1A1, 2B2)
is not sensitive to substitution and aryl rotation. However, the
position of the 2A1 excited state is very sensitive to rotation of
the aryl ring. When this ring is phenyl and is rotated to the
minimum energy position (Φ ) 50°), the 2A1 state drops
dramatically (∼10 kK) in energy as shown in the insert in the
bottom panel of Figure 6.
Figure 4. Time-resolved absorption difference spectra for 2a in toluene
at room temperature, using ∼130 fs excitation flashes at 483 nm (A);
the time (in picoseconds) for each spectrum is indicated. Static
absorption (solid) and fluorescence (dashed) spectra for this compound
(B). Representative kinetic traces and dual-exponential fits at 505
(ground-state absorption bleaching decay) and 545 nm (stimulated
emission decay) are shown in panel C.
We conclude that the strong absorption band at 30 kK (∼330
nm) is due mainly to the 2A1 transition and that the less intense
2B2 transition is associated with the smaller band observed at
26 kK (∼380 nm). Spectral fitting indicates that the lower of
these two bands has an oscillator strength roughly half that of
the higher energy feature, in reasonable agreement with the
calculated SACCI-CISD oscillator strengths for the 2A1 (fcalc
) 0.37) and 2B2 (fcalc ) 0.18) states. [For reference, the strongly
allowed lowest energy B2 state has a calculated oscillator
strength of 0.44.] This assessment of the contribution of the
2A1 state, along with 1A1 and 2B2 states, to the absorption in
the 300-400 nm region is consistent with several observations.
First, the absorption contour in this region for a given solvent
is dependent on the nature (or presence) of the 5-aryl ring
(Figure 2A-E). Since different dihedral angles are expected
for the different types of complexes (e.g., hindered versus
unhindered) on the basis of the ground-state structural studies
(Table 1 and Figure 1), the contribution of the 2A1 to absorption
in the 300-400 nm region will differ. Second, the calculations
indicate that the 2A1 state has a somewhat larger dipole moment
than the ground state (the B2 states have a slightly smaller dipole
moment than the ground state). Thus, an increase in solvent
polarity will tend to cause the aryl ring to rotate to enhance the
excited-state dipole moment and thereby increase solvent
Figure 5. Time-resolved absorption and kinetic data (at 505 nm) for
2b (A) and 2d (B). Other conditions are as in Figure 4.
Similar results to those found for the phenyl-substituted
complex 2a are also observed for the 4-tert-butylphenyl
analogue 2b (Figure 5A). In particular, deactivation to the
ground state is complete by 2 ns, and occurs predominantly
with a time constant of 500 ps that is in good agreement with

20440 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Kee et al.
Figure 6. Comparison of the absorption spectra of 4 (top two spectra) and 2a (lower spectrum) with the calculated singlet state transition energies
for 6, 4, and 2a with the phenyl group rotated orthogonal to the boron-dipyrrin plane. The ground-state geometries were optimized with B3LYP/
6-31G(d) methods and the spectroscopic properties were calculated with SACCI methods with single and double CI (see text). The heights of the
bands are proportional to the oscillator strengths of the bands. Forbidden or very weak bands are assigned an oscillator strength of 0.04 so that the
band is visible.
with the solvent polarity effects on the absorption spectra (Figure
2E and Supporting Information). Third, calculations indicate
that the A1 transitions are polarized perpendicular to the B2
transitions (see Table 2 described below). Thus, a contribution
of the 2A2 (and weaker 1A2) transition to the absorption contour
in the 300-400 nm region (and not simply just the 2B2
transition) is consistent with the finding that the polarization in
this region tends toward perpendicularity to the S0 T S1(1B2)
absorption and fluorescence bands.
In summary, the calculations and experimental results are
consistent with a lowest lying B2 state for all the compounds
investigated, and the fact that the lowest three excited singlet
states are A1 or B2 symmetry. Interestingly, the separation
between the lowest and the second excited singlet states is
relatively large, regardless of substitution or aryl geometry.
Figure 7. Comparison of the excited-state level ordering in 4, 6,
2a-Bu2, and 2a on the basis of SACCI methods including single and
Configurational Characteristics of the Lowest Excited Singlet
States. The configurational properties of the lowest lying B2
excited singlet state in the phenyl-substituted dye 2a are
presented in Figure 8, on the basis of SACCI calculations using
full single and double CI. Note that while Figures 6 and 7
display calculated excited-state transition energies using vertical
bars (Figure 6) or horizontal lines (Figure 7), the horizontal
lines in Figure 8 show the Hartree-Fock energies of the
molecular orbitals. The symmetry of the closed-shell ground
state of a molecule with C2V symmetry is A1, and the π
molecular orbitals of the boron-dipyrrin framework have a2
or b1 symmetry. For all the molecules investigated here, the
double CI (see text). The oscillator strengths for low-lying strongly
allowed states are shown above or below the horizontal band represent-
ing the transition energy. The calculations for 2a were carried out for
two geometries of the phenyl ring: orthogonal (Φ ) 90°) and parallel
(Φ ) 0°) to the boron-dipyrrin plane, for which the symmetry labels
are approximate. The calculation for 2a-Bu2 was carried out for an
orthogonal phenyl group (Φ ) 90°). Other details are as in Figure 6.
Note that a2 states are symmetry forbidden (f ) 0.0) and that all of the
low-lying b1 states are very weak (f < 0.01).
stabilization. According to the calculations, this will also increase
the oscillator strength and contribution of the 2A1 state to the
near-UV absorption contour. These predictions are consistent

Boron-Dipyrrin Complexes
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20441
shows the transition with the phenyl group rotated into the
dipyrrin plane. The symmetry labels for the latter case are
approximate. Single arrows represent single excitations and
double arrows represent double excitations, with the percentage
contribution for each excitation shown within the yellow circles.
The sum of the percentages is shown to the right of the set, and
in both cases, more than 90% of the state configurational
character is represented by the seven configurations. More
interestingly, the rotation of the phenyl group has only minor
impact on the configurational distribution, and the lowest B2
singlet state is described well as a simple HOMO to LUMO
transition. As shown in the top two rows of panels in Figure 8,
the HOMO is localized in the boron-dipyrrin framework
irrespective of phenyl-group geometry. In contrast, the LUMO
is localized in the boron-dipyrrin framework when the phenyl
group is orthogonal, but this orbital is extensively delocalized
into the phenyl ring upon rotation of the latter into the plane of
the dipyrrin system. This delocalization of electron density is
responsible for the decrease in the LUMO energy and the
corresponding decrease in the transition energy upon aryl ring
rotation (Figure 7). As is shown in Figure 8, the LUMO is the
single most important orbital in defining the configurational
characteristics of the lowest lying, strongly allowed B2 state
because the LUMO participates in over 90% of the configura-
tional description.
The same strongly allowed B2 character of the lowest excited
state is found when the fluorine atoms of 2a are replaced by
ethyl groups, which were substituted for n-butyl groups of
2a-Bu2 to make the calculation more efficient. As is shown in
Figure 7 and described in the Supporting Information, the
calculations also indicate that the alkyl groups should prefer-
entially stabilize the A1 states, thereby reversing the ordering
of the second and third excited states (1A1 with 2B1), with
consequences on the absorption contour between 300 and 400
nm (see Figure 2). The calculations showed no differences in
the potential energy surfaces for the ground or lowest excited
state compared to 2a.
Figure 8. The primary configurational properties of the lowest lying
B2 singlet states of 2a for two geometries of the phenyl ring: orthogonal
to the boron-dipyrrin plane (Φ ) 90°, left) and parallel to the dipyrrin
plane (Φ ) 0°, right). The lowest excited singlet state is highly ionic
in character and is well described as a simple HOMO (a2) f LUMO
(b1) one-electron excitation. The nature of the highest occupied
molecular orbital (HOMO, a2 or a2-like symmetry) is relatively invariant
in both character and energy to phenyl rotation. In contrast, the lowest
unoccupied molecular orbital (LUMO, b1 or b1-like symmetry) extends
into the phenyl group upon rotation and is highly stabilized by the
increased extent of the π-electron system when the phenyl group is
parallel to the boron-dipyrrin plane (Φ ) 0°). Stabilization of the
LUMO is the dominant electronic driving force for phenyl group
relaxation toward planarity in the S1 excited singlet state.
Franck-Condon ActiVity in the Optical Spectra. The strongly
allowed low-energy absorption bands in these compounds are
unusually sharp, exhibiting full-widths at half-maxima of less
than 1200 cm-1 (Figures 2 and 6). This sharpness is observed
regardless of the substituents at the 5-position or on the pyrrole
rings. Comparisons of the calculated ground- and excited-state
geometries provide a perspective on this observation. Simply
stated, the ground- and lowest-excited B2 states have virtually
identical geometries in the Franck-Condon region. To a first
approximation, only A1 or A1-like vibrational modes are
Franck-Condon active and these modes are confined largely
to the ring system. The promoting modes in absorption and
fluorescence of 4 involve in-plane breathing vibrations of the
ring atoms coupled to a N-B-N angular mode (see illustrations
in the Supporting Information). These modes are relatively
insensitive to whether there is hydrogen or an aryl ring at the
5-position. The possible exceptions to this rule involve the
phenyl and 4-tert-butylphenyl complexes 2a and 2b, respec-
tively, which have no internal hindrance to aryl rotation. As
described below, 2a (and by comparison 2b) has a lowest excited
singlet-state geometry that is significantly different than the
ground-state geometry (Figure 9A,B). Nevertheless, this sig-
nificant change in geometry generates only a minor increase in
the optical bandwidths (Figure 6). There are two reasons for
this observation. First, the LUMO does not expand into the aryl
group significantly until the ring approaches within 30° of the
planar geometry, an orientation that is never explored in the
TABLE 3: Dominant Configurations of Boron-Dipyrrin
States
state
transition polarization
configurations
A1
A2
B1
B2
z
b1 r b1
b1 r b2
b1 r a1; a2 r b2
b1 r a2
forbidden
x
y
lowest unoccupied molecular orbital (LUMO) has b1 symmetry
and is notably isolated in energy from the other unoccupied
orbitals (Figure 8). Thus, the dominant configuration for most
of the low-lying excited states involves excitation from one of
the filled orbitals into the b1 LUMO. The situation is illustrated
in Table 3, which provides a symmetry analysis of the transition
polarization (x and y are in plane) and the lowest energy
configurations on the basis of the boron-dipyrrin orbitals. (The
A2 state is rigorously forbidden.) Since the highest occupied
molecular orbital (HOMO) has a2 symmetry, inspection of Table
3, along with the orbital energies in Figure 8, provides a clear
picture of why the lowest singlet state is an allowed B2 state
and why this state is well separated in energy from the second
excited singlet state.
We now explore the nature of the lowest B2 excited singlet
state in 2a in more detail. The calculation at the lower left of
Figure 8 shows the composition of the transition with the
5-phenyl group rotated orthogonal to the plane of the boron-
dipyrrin framework, while the calculation at the lower right

20442 J. Phys. Chem. B, Vol. 109, No. 43, 2005
Kee et al.
Figure 10. The ground and first excited singlet state potential energy
surfaces of 2d as a function of mesityl group rotation. Details are as in
Figure 9.
process. In reality, an ambient temperature system will explore
many different conformations during the aryl rotation so our
decision to plot only the lowest energy conformation is
appropriate.
Excited-State Dynamics for 2a. The most interesting potential
energy surfaces are calculated for 2a and are shown in Figure
9C. This molecule is calculated to have a barrier to phenyl
rotation in the ground state of approximately 50 kJ/mol; the
lowest energy conformation is one in which the phenyl group
is twisted to have an angle of about 55° (125°), namely 35°
from perpendicularity. This result is in good agreement with
the average dihedral angle of ∼60° determined from the X-ray
structure (Table 1). The phenyl group in the lowest excited triplet
state has a barrier to rotation of 25 kJ/mol, roughly half that
observed in the ground state, and displays a local minimum in
the planar geometry (Figure 9). In contrast, the lowest excited
singlet state displays a potential surface dramatically different,
with a minimum geometry at a dihedral angle of 180° (Figure
9A,B). The origin of this potential-surface minimum is inti-
mately associated with the delocalization of the LUMO as
discussed above and as shown in Figure 8. We have discussed
the basic driving forces behind this phenomenon in a previous
paper.11 However, our previous theoretical treatment used
semiempirical methods with limited double CI to explore the
excited-state surface. The higher level theory used in this study
provides a higher correlated singlet excited-state surface, and
reveals that the S1 surface provides virtually no barrier to rotation
of the phenyl group toward planarity in 2a (compare Figure 9
with Figures 7 and 8 of ref 11). The new calculations provide
a clear perspective on why 2a (and 2b) has a negligible
fluorescence quantum yield. The barrier-less or nearly barrier-
less S1 surface provides a rapid transfer of the excited-state
population to the delocalized puckered conformation. In this
form, the phenyl group has rotated to a dihedral angle of 180°,
where there is a new minimum in the excited-state surface
accompanied by enhanced electron delocalization. The S1
excited state in this conformation has efficient coupling to the
ground state via nonradiative processes. If any emission were
to occur, it would be in the infrared.
Figure 9. The ground, first excited triplet state, and first excited singlet
state potential energy surfaces of 2a as a function of phenyl group
rotation are shown in C. As the phenyl group rotates into the plane of
the boron-dipyrrin framework, repulsions between the hydrogen atoms
on both groups force a puckering as shown in panels A and B. This
distortion requires that we define the angle of rotation with reference
to the rotation of the yellow and red planes. In practice, the dihedral
angle is calculated in terms of the bond centroids υ, ν, λ, and σ as
shown in panel A. The alignment shown in panel B represents a dihedral
angle of 0° (which because of symmetry is identical with 180° or 360°).
The ground and triplet state surfaces were calculated with B3LYP/
6-31G(d) methods. The excited singlet state surface was calculated by
using full single CI minimization with a correlative correction calculated
by using SACCI doubles (see text).
ground state at ambient temperature because the LUMO is
populated only in the excited state. Thus, the relaxed excited-
state geometry is outside of the Franck-Condon active region.
Second, the vibration that is responsible for dihedral relaxation
of the phenyl group is a low-frequency torsional mode that has
poor vibrational overlap with the ground-state modes regardless
of geometry. The result is a Franck-Condon distribution that
is dominated by in-plane A1-like modes of the dipyrrin
framework in all of the compounds investigated here. This
observation provides insight into the fluorescence quantum
yields in those compounds (2d, 2c) wherein aryl rotation is
restricted (Table 2).
Ground- and Excited-State Potential Energy Surfaces. Rota-
tion of the phenyl or aryl ring relative to the boron-dipyrrin
plane is the key conformational degree of freedom that
determines the photophysical properties of these molecules.
However, as the aryl group rotates, interactions between the
o-aryl groups and the hydrogen atoms of the dipyrrin framework
forces the latter to pucker, as shown in Figure 9A,B. To describe
the rotation of the aryl group with internal consistency, we use
two bond centroids on the boron-dipyrrin ring (µ and ν, Figure
9A) and two bond centroids on the aryl group (λ and σ) to define
the dihedral angle. The potential-energy surfaces for rotation
of the phenyl (2a, Figure 9C), mesityl (2d, Figure 10), and
o-tolyl (2c, Supporting Information) groups are plotted with
reference to this torsional angle. Furthermore, only the lowest
energy conformation as a function of this angle is plotted. In
computer models, the ring tends to pucker in an asymmetric
fashion as the aryl group approaches coplanarity with the
dipyrrin framework. The asymmetry depends on the initial
conformation and is an artificial component of the minimization
We note with interest that the triplet excited state does not
share a minimum energy delocalized puckered conformation.
The factors underlying this observation are described in the
Supporting Information. The time-resolved absorption data
described above indicate that internal conversion of the lowest
excited singlet state is so facile as to circumvent any appreciable
intersystem crossing to produce the triplet excited state.
Excited-State Dynamics for 2d. The ground and first excited
singlet state surfaces for 2d are shown in Figure 10. The o-

Boron-Dipyrrin Complexes
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20443
K. J. Org. Chem. 2000, 65, 2900-2906. (d) Burghart, A.; Thoresen, L. H.;
Chen, J.; Burgess, K.; Bergstro¨m, F.; Johansson, L. B.-A° . Chem. Commun.
2000, 2203-2204.
and o′-methyl groups cause increased repulsion between the
mesityl group and the boron-dipyrrin framework to such an
extent that the lowest energy ground state conformation is one
in which the mesityl ring lies essentially orthogonal to the
dipyrrin framework. As one might anticipate, the delocalized
puckered state in which the mesityl ring rotates approximately
coplanar with the dipyrrin is not the lowest energy excited
singlet state conformation (as it was for 2a), but lies 60 kJ/mol
higher in energy than the orthogonal or nearly orthogonal
conformations. We conclude that excitation into the excited
singlet manifold of 2d generates only a modest change in the
mesityl group orientation, and that fluorescence from the relaxed
S1 state should be efficient. This prediction is in excellent
agreement with the findings from static and time-resolved optical
spectroscopy.
Excited-State Dynamics for 2c. The ground and first excited
singlet state surfaces for 2c are shown in the Supporting
Information. The 2c surface is qualitatively identical with that
observed for 2d but has some interesting features in terms of
local minima that are unlikely to have an observable impact on
the excited-state dynamics. These conclusions are in keeping
with the finding that the photophysical behavior of o-tolyl-
substituted 2c is virtually identical with that of 2d. Thus, the
presence of only one o-methyl group on the aryl ring provides
sufficient hindrance to internal rotation to afford the substantial
change in excited-state dynamics and emission properties
compared to the unhindered analogues.
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A family of 5-aryl-substituted boron-dipyrrin dyes has been
synthesized and then characterized by X-ray diffraction, pho-
tophysical studies, and theory. The results demonstrate the
dominant role of aryl-ring rotation in governing the excited-
state dynamics and fluorescence properties of these dyes. The
results should facilitate the further use of this class of dyes and
related systems as optical probes in the life sciences and other
applications.
Acknowledgment. This research was supported by grants
from the Division of Chemical Sciences, Office of Basic Energy
Sciences, Office of Energy Research, U.S. Department of Energy
(J.S.L., D.F.B., and D.H.) and the National Institutes of Health
(GM-38401 to W.R.S and GM-34548 to R.R.B). Mass spectra
were obtained at the Mass Spectrometry Laboratory for Bio-
technology at North Carolina State University. Partial funding
for the NCSU Facility was obtained from the North Carolina
Biotechnology Center and the NSF.
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Supporting Information Available: Experimental details
for the synthesis and characterization of new compounds and
theoretical analysis of the excited-state surfaces and Franck-
Condon-active modes for selected compounds, static absorption
and emission spectra, time-resolved absorption and emission
spectra, and ORTEP diagrams of the structures; crystallographic
data (CIF files). This material is available free of charge via
the Internet at http://pubs.acs.org.
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