Photophysics of octabutoxy phthalocyaninato-Ni(II) in toluene: Ultrafast experiments and DFT/TDDFT studies

Article abstract of DOI:10.1021/jp0457444

Reported herein is a combination of experimental and DFT/TDDFT theoretical investigations of the ground and excited states of 1,4,8,11,15,18,22,25- Octabutoxyphthalocyaninato-nickel(II), NiPc(BuO)₈, and the dynamics of its deactivation after excitation into the S₁(π,π*) state in toluene solution. According to X-ray crystallographic analysis NiPc(BuO)₈ has a highly saddled structure in the solid state. However, DFT studies suggest that in solution the complex is likely to flap from one D_2d-saddled conformation to the opposite one through a D _4h-planar structure. The spectral and kinetic changes for the complex in toluene are understood in terms of the 730 nm excitation light generating a primarily excited S₁ (π,π*) state that transforms initially into a vibrationally hot ³(d_z2,d_x2-y2 state. Cooling to the zeroth state is complete after ca. 8 ps. The cold (d,d) state converted to its daughter state, the ³LMCT (πd _x2-y2), which itself decays to the ground state with a lifetime of 640 ps. The proposed deactivation mechanism applies to the D_2d- saddled and the D_4h-planar structure as well. The results presented here for NiPc(BuO)₈ suggest that in nickel phthalocyanines the ^1,3LMCT (π,d_x2-y2) states may provide effective routes for radiationless deactivation of the ^1,3(π,π*) states.

Full text of DOI:10.1021/jp0457444

2078
J. Phys. Chem. A 2005, 109, 2078-2089
Photophysics of Octabutoxy Phthalocyaninato-Ni(II) in Toluene: Ultrafast Experiments and
DFT/TDDFT Studies
Tissa C. Gunaratne,† Alexey V. Gusev,† Xinzhan Peng,‡ Angela Rosa,*,§
Giampaolo Ricciardi,*,§ Evert Jan Baerends,| Corrado Rizzoli, Malcolm E. Kenney,*,‡ and
Michael A. J. Rodgers*,†
Contribution from The Center for Photochemical Sciences and Department of Chemistry,
Bowling Green State UniVersity, Bowling Green, Ohio 43403, The Department of Chemistry,
Case Western ReserVe UniVersity, CleVeland, Ohio 44106, Dipartimento di Chimica,
UniVersita` della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy, Afdeling Theoretische Chemie,
Vrije UniVersiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Dipartimento di
Chimica Generale ed Inorganica, UniVersita` di Parma, Viale delle Scienze, 43100 Parma, Italy
ReceiVed: September 19, 2004; In Final Form: December 10, 2004
Reported herein is a combination of experimental and DFT/TDDFT theoretical investigations of the ground
and excited states of 1,4,8,11,15,18,22,25-Octabutoxyphthalocyaninato-nickel(II), NiPc(BuO)8, and the
dynamics of its deactivation after excitation into the S1(π,π*) state in toluene solution. According to X-ray
crystallographic analysis NiPc(BuO)8 has a highly saddled structure in the solid state. However, DFT studies
suggest that in solution the complex is likely to flap from one D2d-saddled conformation to the opposite one
through a D4h-planar structure. The spectral and kinetic changes for the complex in toluene are understood in
terms of the 730 nm excitation light generating a primarily excited S1 (π,π*) state that transforms initially
3
2
2
2
into a vibrationally hot (dz ,dx -y ) state. Cooling to the zeroth state is complete after ca. 8 ps. The cold (d,d)
3
2
2
state converted to its daughter state, the LMCT (π,dx -y ), which itself decays to the ground state with a
lifetime of 640 ps. The proposed deactivation mechanism applies to the D2d-saddled and the D4h-planar structure
as well. The results presented here for NiPc(BuO)8 suggest that in nickel phthalocyanines the 1,3LMCT (π,dx -y
2
2
)
states may provide effective routes for radiationless deactivation of the 1,3(π,π*) states.
Introduction
knowledge of the excited-state dynamics is essential. However,
to the best of our knowledge, no studies on the fundamental
deactivation processes of nickel phthalocyanines have been
reported in the literature.
Metallophthalocyanines (MPcs) have been studied extensively
since the beginning of the century. These highly stable mac-
rocyclic π systems display interesting properties such as light
stability and efficient light absorption in the near-UV/visible/
near-IR region of the spectrum that make them potential
candidates for applications in high-tech fields, including mo-
lecular electronics, nonlinear optics, liquid crystals, and pho-
tovoltaic solar cells.1-3 Owing to the intense absorption in the
near-IR region, which coincides with the therapeutic window
where tissue absorption and scattering is minimal, metalloph-
thalocyanines have been extensively used as photodynamic
reagents for cancer therapy and other medical applications.4,5
Recently there has been increased interest in metallophthalo-
cyanines as photothermal sensitizers for tumor therapy purposes
due to the promising results obtained for Ni(II)5,9,14,18,23,-
27,32,36-octabutoxy-2,3-naphthalocyanine, NiNc(BuO)8.6 This
compound has been found to meet the conditions for the
photothermal process to be effective, viz., it shows efficient
absorption of the photon energy and rapid deactivation to
produce highly localized thermal effects that can subsequently
lead to cell death. To rationalize the factors governing the
efficiency of the photothermal behavior of NiNc(BuO)8, the
Previous studies on the photophysical properties of nickel
tetrapyrroles have mainly focused on nickel porphyrins (see ref
7 and references therein). Similar to NiNc(BuO)8, nickel
porphyrins in noncoordinating solvents show rapid radiationless
decay of the primarily excited (π,π*) state. Nickel(II) porphyrins
have a d8 metal electronic configuration, so in the absence of
2
axial ligands, the dz orbital is the highest filled metal orbital
2
2
and the dx -y orbital is empty. The lack of luminescence,
together with results of extended Hu¨ckel calculations, has led
to the proposal that the normally emissive 1,3(π,π*) excited states
of the porphyrin ring deactivate rapidly via low-energy
1,3
(dz ,dx -y ) states.8,9 Indeed, ultrafast absorption measurements
2
2
2
of common nickel porphyrins such as NiTPP and NiOEP have
revealed that a (d,d) state forms within <1 ps,10-12 followed
by deactivation to the ground state in 200-500 ps.13-19 The
excited state relaxation processes in such molecules are highly
complex since they display photoinduced transient nuclear
dynamics. For example, the transient optical bands of the (d,d)
excited state in noncoordinating media narrow and blue-shift
with wavelength-dependent kinetics on the 5-25 ps time
scale.10-12 This complex spectral evolution has been described
in terms of vibrational relaxation (cooling) of the hot metal
centered state.
* To whom correspondence should be addressed.
† Bowling Green State University.
‡ Case Western Reserve University.
§ Universita` della Basilicata.
The deactivation mechanism proposed for nominally planar
nickel porphyrins in noncoordinating solvents cannot be tout
| Vrije Universiteit.
Universita` di Parma.
10.1021/jp0457444 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/19/2005

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2079
TABLE 1: Experimental Data for the X-ray Diffraction
Study on Crystalline NiPc(BuO)8
court translated to nickel phthalocyanines since the differences
in the macrocyclic framework result in significant changes in
the orbital level pattern.20-22 Worth mentioning in this context
are (i) the reduced G-a1u/G-eg* (G ) Gouterman) energy gap
due to the destabilization of the G-a1u induced by the benzo
complex
NiPc(BuO)8
formula
a, Å
b, Å
c, Å
R,°
C64H80N8NiO8
14.107(1)
14.121(1)
16.871(1)
86.52(1)
72.52(1)
73.30(1)
3069.2(4)
2
2
2
2
rings and (ii) the increased dz /dx -y energy gap due to the
2
2
upshift of the dx -y related to the contraction of the coordination
cavity. Thus, on going from nickel porphyrin to nickel phtha-
locyanine, the S1(π,π*) state moves to longer wavelengths and
â,°
γ,°
1,3
2
2
2
V, Å3
Z
the (dz ,dx -y ) ligand field excited states move to shorter
1,3
2
2
2
wavelengths to the effect that the (dz ,dx -y ) ligand field
excited states which are central intermediates in the deactivation
of photoexcited low-spin d8 nickel porphyrins, might not be
involved at all in the radiationless decay of nickel phthalocya-
nines. Instead, a relevant role in the deactivation of photoexcited
low-spin d8 nickel phthalocyanines might be played by LMCT
states. Indeed, due to the upshift of the G-a1u, the 1,3LMCT states
formula weight
space group
t, °C
1148.10
P 1h (n. 2)
25
0.71073
1.242
3.76
0.903-0.978
0.066
λ, Å
F
calc, g cm-3
µ, cm-1
transmission coefficient
Ra
2
2
involving excitation out of the G-a1u into the Ni-dx -y MOs
might be low enough in energy to provide effective routes for
deactivation of the normally emissive 1,3(π,π*) states.
It is the aim of this paper to provide the first experimental
and theoretical investigation of the static and dynamic photo-
physical properties of nickel phthalocyanines, using Ni(II)1,4,8,-
11,15,18,22,25-octabutoxy-phthalocyanine, NiPc(BuO)8, as a
case study.
The peripheral butoxy groups impart high solubility to this
molecule in common organic solvents and, most importantly,
induce a red-shift of the Q-band by ca. 70 nm relative to the
unsubstituted analogue, which makes NiPc(BuO)8 suitable for
use as photothermal sensitizer. The complex has been re-
synthesized and structurally characterized through X-ray crystal-
lography, and its excited-state spectral and dynamic behavior
in a noncoordinating solvent (toluene) has been studied by
ultrafast transient absorption spectroscopy and DFT/TDDFT
calculations.
wR2
GOF
0.172
1.09
6264
13767
7120
712
N-observedb
N-independentc
N-refinementd
variables
a Calculated on the observed reflections having I > 2σ(I). b N-
observed is the number of the independent reflections having I > 2σ(I).
c N-independent is the number of independent reflections. d N-refinement
is the number of reflection used in the refinement having I > 1.5σ(I).
Ultrafast Pump-Probe Measurements. The pump-probe
instrument for ultrafast transient absorption measurements has
been described previously.25 The recent improvements to
enhance signal-to-noise characteristics have been communicated
elsewhere.26 In the current experiments, the excitation wave-
lengths at 660 and 730 nm were generated with an optical
parametric amplifier (OPA 800, Spectra Physics), pumped with
800 nm light from an amplified, mode-locked Ti:Sapphire laser
(Hurricane, Spectra-Physics).
Experimental Section
Materials. NiPc(BuO)8. Under Ar, a mixture of H2Pc(BuO)8
(173 mg, 0.158 mmol), Ni(CH3CO2)2‚4H2O (1.51 g, 6.07
mmol), and dimethylformamide (30 mL) was refluxed for
1.5 h and evaporated to dryness by rotary evaporation (45 °C).
The solid was chromatographed twice (basic Al2O3 III, ethyl
acetate-toluene, 1:2), further chromatographed (Bio-Beads S-X1
(Bio-Rad Laboratories, Hercules, CA), toluene), dissolved
in CH2Cl2 (5 mL), recovered with CH3OH (15 mL), washed
(CH3OH), vacuum-dried (room temperature), and weighed
(87 mg, 48%). UV-vis (toluene) λmax, nm (log ꢀ): 732 (5.42).
NMR (CDCl3): δ 7.50 (s, 2,3,9,10,16,17,23,26-Ar H), 4.78 (t,
OR-1 CH2), 2.19 (m, OR-2 CH2), 1.64 (m, OR-3 CH2), 1.07 (t,
OR-4 CH3). MS-HR-FAB exact mass m/z: calcd for
C64H81N858NiO8 (M+H)+, 1147.5530; found, 1147.5529,
1147.5527. The compound is a green solid. It is soluble in
CH2Cl2 and toluene, and slightly soluble in hexanes.
X-ray Crystallography. Crystal data and details associated
with structure refinement are given in Table 1 and in the
Supporting Information. Data were collected on a Bruker AXS
100 CCD diffractometer using graphite-monochromatized Mo
KR radiation at 298 K. A total of 1206 frames were collected
with a ∆ω of 0.3 degrees and an exposition time of 20 s.
Solution and refinement were carried out using the programs
SIR9723 and SHELX93.24
Cyclic Voltammetry and Spectro-Electrochemistry Ex-
periments. Electrochemical oxidations and reductions (controlled-
potential coulometry) were carried out using a Bioanalytical
Systems Epsilon unit. A standard three electrode system was
used except in the case of nonaqueous solvents where a silver
wire was used as the reference electrode. Tetramethylammonium
hexafluorophosphate was the supporting electrolyte. A home-
made spectro-electrochemical cell was employed. This cell
consisted of a screw cap 0.5 mm path length rectangular quartz
cuvette attached into a Teflon beaker. Platinum gauze, 100 mesh,
woven from 0.07 mm diameter platinum wire was used as the
semitransparent working electrode. The electrode was placed
in the 0.5 mm spectrophotometric cell and connected to the
potentiostat output by a platinum wire. The potential applied
to the electrode produced oxidized or reduced species that
diffused away from the wire electrode to saturate the 0.5 mm
layer of solution inside the spectrophotometric cell. The
absorption spectra (HP 8453 UV-visible spectrophotometer)
of the sample were taken until the potential-induced spectral
evolution was complete. The spectro-electrochemical cell filled
with pure solvent was used as reference.
Computational Details and Theoretical Methods. All
calculations were performed with the ADF (Amsterdam Density
Functional) suite of programs, release 2004.01.27-29
The calculations made use of the local density approximation
(LDA) functional of Vosko-Wilk-Nusair (VWN),30 plus the
generalized gradient approximation (GGA), employing Becke’s31
UV-Visible Absorption Spectra. The ground-state absorp-
tion spectra were recorded for Ni(II)Pc(BuO)8 (2-3 µM) in
toluene solutions using a Varian Cary 50 Bio single beam
spectrophotometer.

2080 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Gunaratne et al.
Figure 1. ORTEP side view (30% probability ellipsoids) of complex
NiPc(BuO)8. Disorder affecting some butyl chains has been omitted
for clarity.
gradient approximation for exchange and Perdew’s32 for cor-
relation. The excitation energies were calculated using time-
dependent density functional theory (TDDFT). In the ADF
implementation,33,34 the solution of the TDDFT response
equations proceeds in an iterative fashion starting from the usual
ground state or zeroth-order Kohn-Sham (KS) equations.34 For
these, one needs an approximation to the usual static exchange-
correlation (xc) potential Vxc (r). After the ordinary KS equations
have been solved, the first-order density change has to be
calculated from an iterative solution to the first-order KS
equations. In these first-order equations, an approximation is
needed to the first functional derivative of the time-dependent
xc potential Vxc(r, t) with respect to the time-dependent density
F(r′, t′), the so-called xc kernel. For the xc kernel, we used the
adiabatic local density approximation (ALDA). In this ap-
proximation, the time dependence (or frequency dependence
referring to the Fourier transformed kernel) is neglected, and
one simply uses the differentiated static LDA expression. In
our case the VWN parametrization was used.30 For the
exchange-correlation potentials which appear in the zeroth-order
KS equations, we employed the same GGA as in the DFT
calculations.
To facilitate the calculations on the title compound, a model
compound, NiPc(MeO)8 (Me ) methyl), was chosen in which
the butyl substituents were replaced by methyl groups. Geometry
optimizations were performed for the ground and selected triplet
excited states of the model system. Open-shell DFT calculations
for the triplets were carried out using the unrestricted formalism.
The ADF TZ2P basis set, which is an un-contracted triple-ú
STO basis set with one 3d and one 4f polarization function for
C, N, and O atoms, one 2p and one 3d polarization function
for H, and a triple-ú 3d, 4s basis with one 4p and one 4f function
for Ni, was used. The cores (C, N, O, 1s; Ni, 1s-2p) were kept
frozen.
Figure 2. SCHAKAL view of the packing along the [1-1 0] direction.
The structural data reveal that the macrocycle assumes a
saddle conformation, with the indole rings tilted alternately up
and down, almost as rigid bodies. The dihedral angles, formed
by the mean planes through the indole rings and the phthalo-
cyanine mean plane are 15.4(1)°, 16.5(1)°, 16.0(1)°, and 16.1-
(1)° for the indole rings associated to the N(1), N(2), N(3), and
N(4) atoms, respectively. The opposite N(1), C(1)‚‚‚C(8), N(3),
C(17)‚‚‚C(24) and N(2), C(9)‚‚‚C(16), N(4), C25)‚‚‚C(32) form
dihedral angles of 31.5(1)° and 32.4(1)° respectively.
The average absolute perpendicular displacement from the
phthalocyanine mean plane of the pyrrolic N and Câ atoms,
|∆Np| and |∆Câ|, and the meta carbon atoms of the benzo rings,
|∆Cm|, are 0.109(4), 0.670(5), and 1.319(6) Å respectively.
The structural data also revealed a gentle sideway tilt
(ruffling) of the indole rings superimposed on the quite large
vertical tilt. The molecular parameters that define the degree of
ruffling, i.e., the average transannular dihedral angle (CRNpNpCR)op
and the average out-of-plane displacement of the meso-nitrogen
atoms from the phthalocyanine mean plane, |∆Nmeso|, amount
to only 2.9(6)° and 0.056(4) Å, respectively.
The N4 core shows a small but significant tetrahedral
distortion ranging from -0.110(4) Å for N(2) to 0.110(4) Å
for N(1), the metal being displaced by 0.002(1) Å from the least-
squares plane through it. The Ni-Np bond distances (mean value
1.878(4) Å) are not remarkably different from each other and a
little (but significantly) shorter than those reported in the
literature for Ni-phthalocyanine complexes (mean value 1.904-
(18) Å calculated over four entries from the Cambridge
Crystallographic Data File).37-40
The N-C and C-C bond distances are, as expected,
consistent with a complete delocalization within the macrocycle.
As for the Co-O bond distances, they are indicative of a not
negligible conjugation between the oxygen lone pairs and the
phthalocyanine π system. The extent of the conjugation,
however, depends significantly on the displacement of the Cbut
atoms from the mean plane through the benzene moiety of the
corresponding indole ring that ranges from 0.078(6) to 1.209-
(12) Å, the mean value being 0.34(12) Å.
The molecular orbitals were visualized with Molden3.635 and
the ADF output data were converted to Molden format using
the ADF2MOLDEN program.36
Results and Discussion
X-ray Structure of NiPc(BuO)8. The experimentally deter-
mined crystal structure of NiPc(BuO)8 is shown in Figure 1.
Selected bond distances, angles, and metrical parameters are
quoted in Table 1.
In the crystal packing, the molecules are disposed in layers
parallel to the [1-1 0] direction (Figure 2) in such a way that
each molecule arranges the butyl chains as guests of the cavities
of over- and under-imposed layers, acting in the same time as

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2081
Figure 3. Optical absorption spectrum of NiPc(BuO)8 in toluene.
Figure 5. Perspective view of the first 10 ps of the A(λ, t) surface
collected during an ultrafast pump-probe experiment with 10 µM NiPc-
(BuO)8 in toluene excited at 740 nm. The red arrows indicate the times
of the first two of the spectral cuts shown in Figure 6.
Figure 4. Progress over 25 min of spectro-electrochemical oxidation
of NiPc(BuO)8 (10µM) in CH2Cl2 at a constant potential of +500 mV.
Inset: a cyclic voltammetry scan of the same solution showing two
oxidation and two reduction waves.
a guest for the butyl chains of adjacent molecules. In this
arrangement some C-H‚‚‚O and CH-N contacts involving the
C atoms from the butyl chains could be interpreted in terms of
weak hydrogen interaction, the shortest one being C(42)B‚‚‚
O(4)′, 3.344(13) Å; H(424)‚‚‚O(4)′, 2.60 Å; C(42)B-H(424)‚
‚‚O(4)′, 133.8° C(42)B‚‚‚N(6)′, 3.580(14) Å; H(424)‚‚‚N(6)′,
2.76 Å; C(42)B-H(424)‚‚‚N(6)′, 142.7° C(58)‚‚‚O(8)′′, 3.546-
(7) Å; H(582)‚‚‚O(8)′′, 2.67 Å; C(58)-H(582)‚‚‚O(8)′′, 150.4°
(the prime and double primes refer to transformations of -x, 1
- y, 1 - z and 1 - x, -y, 1 - z, respectively).
Ground-State Spectrum. The ground state absorption spec-
trum recorded in toluene at room temperature is shown in Figure
3. In the energy window of interest for the present study, it
shows two clear vibrational progression peaks corresponding
to the Q(1,0) and Q(0,0) bands and a broad absorption
immediately to the blue of the Q(1,0) vibronic band. Comparison
of the visible absorption spectra of NiPc(BuO)8 and NiPc reveals
that the butoxy groups at the ortho positions cause a substantial
red shift (more than 70 nm) of the Q(0,0) band of NiPc(BuO)8
and introduce additional absorptions to the blue of the Q(1,0)
band.
Figure 6. Three spectral cuts at 1.2 ps (dotted), 7.5 ps (dashed), and
70 ps (solid) taken from the surfaces shown in Figure 5.
shows the results of a cyclic voltammetry sequence on com-
pound I in DCM. The first oxidation wave is at 650 mV. Similar
experiments with the free base H2Pc(BuO)8 showed similar
behavior. Moreover the reduction cycle showed that the singly
reduced species had significant absorption to the red side of
the ground state 0,0 band, as in the case of the oxidized species.
Ultrafast Transient Absorption Spectrometry. A solution
of NiPc(BuO)8 (ca 10 µM) in toluene was irradiated with ca.
100 fs excitation pulses at 660 or 730 nm, and the pump-probe
method was employed to record optical absorption spectra at a
series of delay times after the excitation pulse. The transient
absorption spectra and kinetics were found to be identical
irrespective of excitation wavelength. Figure 5 shows a perspec-
tive view of the first 10 ps of the dynamic surface of the A(λ,t)
acquired by the spectrometer. Figure 6 shows absorption-time
plots extracted from the surface at 1.0, 7.5, and 100 ps after
excitation at 730 nm.
Voltammetric and Spectro-Electrochemistry Experiments.
Figure 4 shows the results of spectro-electrochemical oxidation
of compound I (10 µM) in dichloromethane solution. The
potential was held constant at 500 mV and the spectra were
recorded at times in the range of 0-25 min. At the end of the
oxidation sequence, the neutral compound (λmax ) 730 nm) was
seen to be wholly converted into the oxidized form. The inset
The main features of the transient spectrum at early times as
seen in Figures 5 and 6 were a prompt negative absorption
mirroring the ground-state spectrum (Figure 3) with extrema at
660 and 730 nm, a positive absorption from 450 to 650 nm and

2082 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Gunaratne et al.
Figure 7. Representative time profile cuts at 630 nm (a), 700 nm (b), 740 nm (c and inset), and at 774 nm (d).
TABLE 2: Selected Bond Distances (Å) and Angles (deg) for
NiPc(BuO)8
a positive absorption on the red side of the ground-state
bleaching, that is formed with a delay. The prompt absorption
changes were complete within the instrument response time
(∼180 fs). The decay of the positive absorption from 450 to
650 nm was best described by biexponential decay kinetics
(Figure 7a) with a fast component having a lifetime of 2.3 (
0.4 ps, and slower decay component of 15 ( 1 ps.
Ni(1)-N(1)
1.868(4) N(4)-C(25)
1.883(4) N(4)-C(32)
1.875(4) N(5)-C(8)
1.886(4) N(5)-C(9)
1.378(5) N(6)-C(16)
1.385(6) N(6)-C(17)
1.370(6) N(7)-C(24)
1.384(5) N(7)-C(25)
1.371(5) N(8)-C(1)
1.377(6) N(8)-C(32)
1.362(6)
1.373(5)
1.325(5)
1.321(5)
1.311(6)
1.322(5)
1.315(6)
1.325(6)
1.327(6)
1.311(6)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(4)
N(1)-C(1)
N(1)-C(8)
N(2)-C(9)
The negative absorption in the 650-740 nm region exhibited
complex recovery kinetics. In the spectral region from 690 to
710 nm the negative absorption recovered to a zero level (Figure
7b) in a biexponential manner with lifetimes of 2.2 ( 0.2 ps
and 15.2 ( 0.6 ps. These lifetimes correspond closely to those
measured for the positive visible absorption (625 nm). Further
to the red side (>715 nm), the negative signal recovery
proceeded above the pre-pulse zero (Figure 7c) before eventually
decaying back to the zero level with a lifetime of 640 ps. At
770 nm and beyond (Figure 7d), the absorbance post-pulse grew
up in intensity from the zero level with a lifetime of 2.2 ps.
Scrutiny of the dynamic surface in Figure 5 (first 10 ps) shows
that the positive absorption band appearing at the red edge of
the ground-state bleaching shows absorption maxima that shift
to the blue as the amplitude increases in time. At 755 nm, the
absorption band grew in with a 2.5 ps formation lifetime and
decayed back to a zero baseline with a lifetime of 18.6 ( 0.3
ps. The three spectra displayed in Figure 6 summarize the results
of the kinetic changes described. These, together with the kinetic
profiles presented in Figure 7, clearly reveal that there are three
kinetically and spectrally distinct species generated between the
excitation event and the final relaxation back to the initial state.
Ground-State Molecular and Electronic Structure. The
remarkable distortions from planarity of NiPc(BuO)8 in its solid
state structure (Figure 1) poses the question whether they are
dictated by intrinsic electronic factors, such as the necessity to
minimize the steric hindrance between the lone pairs of the
facing oxygen atoms while preserving an efficient nickel-
N(2)-C(16)
N(3)-C(17)
N(3)-C(24)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(3)
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(3)
N(2)-Ni(1)-N(4)
N(3)-Ni(1)-N(4)
C(1)-N(1)-C(8)
90.1(2)
173.4(2)
90.3(2)
90.3(2)
173.3(2)
90.1(2)
106.5(4)
C(16)-N(6)-C(17) 120.1(4)
C(24)-N(7)-C(25) 120.0(5)
C(1)-N(8)-C(32)
N(1)-C(1)-N(8)
N(1)-C(8)-N(5)
N(2)-C(9)-N(5)
N(2)-C(16)-N(6)
N(3)-C(17)-N(6)
N(3)-C(24)-N(7)
N(4)-C(25)-N(7)
120.0(4)
126.5(5)
126.7(4)
126.3(4)
127.1(4)
126.0(4)
127.2(4)
126.6(4)
C(9)-N(2)-C(16) 107.5(4)
C(17)-N(3)-C(24) 106.8(4)
C(25)-N(4)-C(32) 107.3(4)
C(8)-N(5)-C(9)
120.2(4)
N(4)-C(32)-N(8) 127.2(4)
phthalocyanine interaction, and hence are retained in solution,
or they are merely dictated by packing requirements. To answer
this question that is of some relevance since we are attempting
to understand the photophysics of NiPc(BuO)8 in diluted
solutions, we performed unconstrained geometry optimization
of the model complex NiPc(MeO)8 taking as starting geometry
that of the crystal structure of NiPc(BuO)8. The optimization
converged to a minimum energy structure of D2d symmetry with
the carbon atoms of the methyl groups, Cmet, coplanar with the
indole rings. The same minimal energy geometry was found
whether the optimization was symmetry-constrained or not. The
most important geometrical parameters calculated for this
structure are gathered in Table 3 and compared to the experi-
mental values of the crystalline NiPc(BuO)8.
According to the data in the Table, there is a close agreement
between the geometrical parameters calculated for the D2d

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2083
TABLE 3: Selected Bond Lengths (Å), Bond Angles (deg),
and Metrical Parameters in Crystalline NiPc(BuO)8
Compared with the Corresponding Theoretical, Optimized
Values of the Model NiPc(MeO)8 Complex in the D4h and
D2d conformations
NiPc(MeO)8
D4h
D2d
expt.a
Ni-Np
CR-Np
CR-Câ
Câ-Câ
CR-Nb
Co-O
1.918
1.384
1.454
1.405
1.317
1.364
1.900
1.382
1.453
1.406
1.319
1.363
1.878(4)
1.376(6)
1.455(7)
1.397(7)
1.320(6)
1.361(6)
CRNpCR
CRNbCR
O‚‚‚O
107.1
121.8
3.500
107.5
121.2
3.793
0.111
0.0
0.243
0.597
0.917
1.232
0.919
0.0
107.0(4)
120.1(4)
3.91(2)
|∆Np|
|∆Nb|
|∆CR|
|∆Câ|
|∆Co|
|∆Cm|
|∆O|
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.109(4)
0.056(4)
0.268(5)
0.670(5)
1.015(6)
1.319(6)
1.043(4)
2.9(6)
Figure 8. Energy level scheme for NiPc and NiPc(MeO)8.
(CRNpNpCR)op
(CRNpNpCR)ad
12.5
16.2(4)
pleteness, it is deemed necessary to consider also the D4h
structure.
a X-ray data, this work.
The highest occupied and the lowest unoccupied ground-state
one electron levels calculated for NiPc(MeO)8 in the D2d-saddled
and D4h-planar conformations are shown in Figure 8 where, for
comparison purpose, the ground-state one-electron levels of
NiPc are also shown. In our choice of the axes, the z axis is
perpendicular to the plane of the meso-nitrogen atoms, the x
and y axes bisect the indole rings in the planar D4h structure
and point to the meso-nitrogen atoms in the saddled D2d
structure.
conformer and the experimental averaged values. Thus, apart
from the modest ruffling of the phthalocyanine skeleton and
the random displacements of the Cbut atoms from the indole
rings (most likely related to packing requirements) the salient
features of the solid-state structure of NiPc(BuO)8 are substan-
tially preserved in the D2d geometry of the model system.
We have also tried to determine the motion from one saddle
structure to the opposite saddled structure (with the other indole
rings up and down) through a planar D4h structure (the most
commonly adopted by monomeric metallophthalocyanines).
Surprisingly, we found that the D4h is only ca. 1 kcal/mol less
stable than the D2d structure. Table 3 lists the relevant geo-
metrical parameters calculated for this geometry, whence it is
apparent that the most conspicuous effects of flattening the
molecule consist of lengthening the Ni-Np distance by 0.018
Å, and a considerable shortening (0.29 Å) of the distance
between facing oxygen atoms. The increased steric repulsion
between the facing oxygen lone pairs and the less efficient
metal-macrocycle interactions would lead to the expectation
that the planar structure should be far less stable than the saddled
one. That this is not the case suggests that the energy cost of
the saddling (mostly ring strain) almost cancels the energy gain
coming from the relief of steric repulsion between the facing
oxygen lone pairs and the improved metal-macrocycle interac-
tion.
Since an extensive discussion of the electronic structure of
NiPc has been published,20-22 we recall here only the most
salient features.
Among the NiPc levels in Figure 8, one may recognize, apart
from the π G-2a1u and the π* G-7eg (G ) Gouterman) orbitals,
2
2
the metal 3d orbitals. In the virtual spectrum, there is the 3dx -y
(11b1g), which is pushed up by antibonding with the Np lone
2
2
pairs. Actually, the 11b1g is composed of 3dx -y and Np lone
pairs in almost equal amount. The highest occupied 3d levels
2
are the 3dz (13a1g), which is an almost pure metal orbital, and
the 3dπ (6eg). The 6eg is heavily (41%) mixed with the Np based
π orbitals of the phthalocyanine ring. Its bonding counterpart
is the 4eg orbital, lying 1.3 eV below. The level pattern of the
NiPc(MeO)8 conformers shows that introducing methoxy groups
into the Pc destabilizes all of the levels, but preferentially the
G-2a1u (4a1u and 17b1 in the D4h and D2d conformers, respec-
tively). The preferential destabilization of the HOMO fits in
with the antibonding, visible in the plot of this orbital displayed
in Figure 9, between the Pc-a1u, which has large amplitude on
the ortho-carbons of the benzo rings, and the oxygen lone pairs
of the methoxy groups whose contribution to this orbital
amounts to ∼15%.
We have verified by explicit frequency calculations that the
saddled structures are true minima (all positive frequencies) and
that the D4h structure is indeed a maximum, with one imaginary
frequency corresponding to the saddling motion (of b2u sym-
metry). Given the inherent limitation of these calculations (basis
set choice, functional used, numerical accuracy, neglect of
solvation effects, no information on entropic effects, including
those from solvent environment, truncation of the alkyl chains)
we cannot be specific about the precise height of the barrier
and the frequency of the flapping motion. The calculations,
however, clearly indicate that while in the solid state the
molecule is “frozen” in the saddled structure, in solution it can
flap from one saddled conformation through the planar “maxi-
mum” to the opposite saddled structure. Therefore, for com-
Owing to the upshift of the HOMO, the energy gap between
the occupied G-a1u and the unoccupied G-eg* is reduced in NiPc-
(MeO)8 with respect to NiPc.
The 2b1u is another π orbital of the macrocycle which is
strongly destabilized by interaction with the oxygen lone pairs.
In NiPc(MeO)8, the 2b1u derived orbital, the 4b1u in the planar
and the 15a2 in the saddled conformer, is upshifted with respect
to the NiPc-2b1u by 1.8 eV (in NiPc the 2b1u is too low in energy

2084 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Gunaratne et al.
Figure 10. Schematic definition of the computed excitation energies.
∆E is calculated as E(S0,GT) - E(S0,GS0).
When comparing the level scheme of the two NiPc(MeO)8
conformers, it is apparent that the most conspicuous effect of
2
2
the D4h f D2d geometry change is the upshift of the Ni-dx -y
level (22b2), which ends up slightly above the G-eg*(38e). The
2
2
destabilization of the 22b2 (dx -y ), which is a strongly σ
antibonding Ni-Np orbital (see the plot in Figure 9), is in line
with the shortening of the Ni-Np distance on going from the
planar to the saddled conformation (see Table 3). As will be
2
2
seen later, the relative position of the unoccupied dx -y metal
2
level with respect to the occupied metal levels, notably the dz
and the ligand G-a1u are crucial for the photophysical behavior
of NiPc(BuO)8.
Excited States and Ground-State Absorption Spectrum.
TDDFT calculations were employed to examine the lowest
triplet and singlet excited states of the two NiPc(MeO)8
conformers. The vertical absorption energies (Eva) calculated
for the whole set of triplet and singlet states up to 1.9 eV are
Figure 9. Contour plots of the 17b1 (HOMO), 15a2, 38e (LUMOs),
and 22b2 orbitals of NiPc(MeO)8 in the D2d geometry.
gathered in Table 4 together with the adiabatic energies (Eadia
calculated for selected excited states.
The vertical absorption energies Eva have been evaluated at
the ground-state optimized geometry. The adiabatic energies
have been obtained according to the expression
)
to enter the diagram of Figure 8). Because of its large amplitude
(∼50%) on the carbon atoms at the ortho positions of the benzo
rings, the 2b1u heavily (30%) mixes in antibonding fashion with
the oxygen lone pairs, as inferred by the plot of the 15a2
displayed in Figure 9.
As for the metal states, the introduction of the methoxy groups
does not modify the salient features of the nickel-phthalocya-
nine interactions, although their energy and composition sense
to some extent the change in the relative energy of the
macrocycle and the metal. For instance, going from NiPc to
NiPc(MeO)8, the 3dπ contribution to the π bonding/antibonding
pair is reversed. Most of the 3dπ character is found in the 9eg
(36e), its antibonding counterpart, the 10eg (37e) having only a
20% of Ni-3dπ character.
Eadia ) Eve + ∆E
Eve is the vertical emission energy, which is calculated at the
TDDFT level using the relaxed excited-state geometry. The ∆E
term accounts for the change in energy of the ground state upon
deformation to the relaxed geometry of the excited state (for a
schematic definition of the calculated energies see Figure 10).
In Table 5, the vertical absorption energies (Eva) and oscillator
strengths calculated for the optically allowed excited states
TABLE 4: Excitation Energies (eV) Computed for the Low-Lying Excited States of NiPc(MeO)8
D2d D4h
character character
state
composition (%)
Eva
Eadia
state
composition (%)
Eva
Eadia
21A2
23A2
13B1
23B2
23E
100 (37e f 38e)
100 (37e f 38e)
100 (37e f 38e)
100 (37e f 38e)
80 (37e f 22b2)
20 (36e f 22b2)
100 (37e f 38e)
98 (22a1 f 22b2)
π,π*
1.90
1.88
1.86
1.83
1.83
11A2 g
11A1u
13A1u
13A2 g
13B2g
100 (10eg f 11eg)
100 (4b1u f 15b1g)
100 (4b1u f 15b1g)
100 (10eg f 11eg)
100 (10eg f 11eg)
π,π*
1.90
1.90
1.90
1.88
1.86
π,π*
π,π*
π,π*
LMCT
LMCT
π,π*
2
2
LMCT/dπ,dx -y
π,π*
13A1
13B2
π,π*
1.80
1.69
13A1 g
13Eg
100 (10egf11eg)
77 (10eg f 15b1g)
22 (9eg f 15b1g)
90 (4a1u f 11eg)
100 (17a1gf15b1g)
100 (4a1u f 15b1g)
100 (4a1u f 15b1g)
90 (4a1u f11eg)
π,π*
LMCT/dπ,dx -y
1.82
1.66
2
2
2
2
2
dz ,dx -y
1.39
11E
91 (17b1 f 38e)
π,π*
1.60
11Eu
π,π*
1.60
1.58
1.19
1.17
1.16
13B1g
11B1u
13B1u
13Eu
dz ,dx -y
LMCT
LMCT
π,π*
1.08
2
2
2
11A2
13A2
13E
100 (17b1 f 22b2)
100 (17b1 f 22b2)
100 (17b1f38e)
LMCT
LMCT
π,π*
1.28
1.27
1.15
1.15a
0.88a
1.13
0.87
1.09 (13B2)
1.04 (13B3u)
a Computed at the optimized geometry of the corresponding triplet.

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2085
TABLE 5: Vertical Excitation Energies and Oscillator Strengths (f) Computed for the Optically Allowed Excited States of the
D4h and D2d NiPc(MeO)8 Conformers Contributing to the Q-Band Region Are Compared to the Experimental Data
D4h
D2d
exp.a
state
composition (%)
Eva (eV/nm)
f
state
composition (%)
Eva (eV/nm)
f
E (eV/nm)
11Eu
21Eu
90 (4a1u f 11eg)
93 (4b1u f 11eg)
1.60/775
2.07/599
0.492
0.391
11E
11B2
21E
91 (17b1 f 38e)
92 (37e f 38e)
92 (15a2 f 38e)
1.60/775
2.02/614
2.07/599
0.513
0.012
0.357
1.69/734 Q
sh
a Toluene solution spectrum of NiPc(BuO)8, this work.
TABLE 6: Optimized Geometrical Parameters (Å and deg)
for NiPc(MeO)8 in the D2d Ground State (1A1) and in the
Lowest Triplet Excited States
contributing to the Q-band region are compared to the experi-
mental data. Tables 4 and 5 also include the compositions of
the BP/ALDA solution vectors in terms of the major one-
electron MO transitions.
1A1
13B2
13A2
13B2
a
a
Ni-Np
CR-Np
CR-Câ
Câ-Câ
CR-Nb
Co-O
CRNpCR
CRNbCR
O‚‚‚O
1.900
1.382
1.453
1.406
1.319
1.363
107.5
121.2
3.793
1.972
1.376
1.460
1.413
1.328
1.363
109.5
123.6
3.844
1.957
1.374
1.465
1.410
1.327
1.362
108.8
123.1
3.917
1.900/1.900
1.386/1.384
1.464/1.440
1.403/1.416
1.308/1.338
1.360/1.361
107.8/107.6
120.5
3.916
0.093/0.114
0.003
0.257/0.294
0.684/0.725
1.069/1.107
1.452/1.481
1.068/1.109
0.0
We discuss first the TDDFT results for the optically allowed
excited states in order to provide an assignment of the ground-
state absorption spectrum of NiPc(BuO)8 in the Q-band region.
On the basis of the computed energies and oscillator strengths,
the intense Q(0,0) band with maximum at 734 nm is assigned
to the lowest optically allowed excited state, the 11E, computed
for the model system, NiPc(MeO)8, at 1.60 eV (775 nm), in
satisfactory agreement with the experiment. As inferred from
the composition of the excited states, the Q state is an almost
pure G-a1u f G-eg* (17b1 f 38e) state. The large destabilization
of the 17b1 induced by the methoxy groups causes the G-a1u/
G-eg* energy gap to be in NiPc(MeO)8 sensibly narrower than
in the parent NiPc, which fits in with the Q-band being in NiPc-
(BuO)8 red shifted relative to NiPc by ∼70 nm.
|∆Np|
|∆Nb|
|∆CR|
|∆Câ|
|∆Co|
|∆Cm|
|∆O|
0.111
0.0
0.069
0.0
0.045
0.0
0.243
0.597
0.917
1.232
0.919
0.0
0.205
0.552
0.901
1.242
0.908
0.0
0.225
0.641
1.024
1.401
1.028
0.0
(CRNpNpCR)op
(CRNpNpCR)ad
12.5
12.6
15.9
15.9
In the energy regime of the Q-band, we predict a second
intense excited state, the 31E. This state, computed at 2.07 eV
(599 nm), accounts for the broad absorption appearing in the
spectrum of NiPc(BuO)8 to the blue of the Q(1,0) band. This
assignment is fully consistent with this feature being related to
the presence of the butoxy groups at the periphery of the
phthalocyanine ring. Indeed, the 31E is largely derived from
promotion of one electron from the 15a2, a π orbital of the
macrocycle which has large amplitude on the oxygen lone pairs,
to the G-eg* derived 38e. A third, very weak, excited state, the
11B2, is predicted in the Q-band region. This state, computed
at 2.02 eV (614 nm), contributes to the broadening of the region
to the blue of the Q(1,0) band.
a The geometry optimization was performed under D2d symmetry
constraint.
electron transition, the 17b1 being a π orbital of the macrocycle
2
2
and the 22b2 a nearly 50/50 mixture of Ni-3dx -y and Np lone
pairs.
As inferred from the data in Table 4, the adiabatic energies
of the 13A2 and 11A2 (the Eadia value of the 11A2 was obtained
using the relaxed geometry of the corresponding triplet, so it
can be considered as upper limit to the actual value) are
significantly decreased by 0.13 and 0.14 eV relative to the value
of the corresponding vertical absorption energies. This fits in
with the relaxed geometry of the 13A2 showing significant
changes with respect to the ground-state geometry, as can be
seen in Table 6 which collects the optimized geometries of the
lowest-lying triplet excited states and, for the sake of compari-
son, the geometry of the ground state. The relaxed geometry of
the 13A2 is characterized by a considerable lengthening of the
Ni-Np distance and a sizable expansion of the macrocycle core.
These geometrical changes are directly related to occupation
of the 22b2, a strongly σ antibonding Ni-Np orbital (see plot
in Figure 9).
According to the TDDFT results in Table 5, the electronic
transitions computed for the D4h conformer are essentially
identical in energy, intensity, and character to those computed
for the D2d conformer. This indicates that the flapping motion
of the molecule from one saddled conformation through the
planar “maximum” to the opposite saddled structure, if any,
would not be detectable in the optical spectrum.
Besides the optically allowed excited states above-discussed,
a manifold of triplet and singlet symmetry forbidden excited
states are calculated in the energy regime of the Q-band (see
Table 4). These “dark” states are likely to play a role in the
observed radiationless decay of the primarily excited S1 (π,π*)
state and thus deserve detailed discussion. The TDDFT results
for the D2d structure indicate that three “dark” states lie vertically
below the S1 (π,π*) state. They are the 11A2, 13A2, and 13E,
which appear vertically at 1.28, 1.27, and 1.15 eV, respectively.
These are the most plausible candidates as deactivation channels
for the S1 (π,π*).
As for the 13E (π,π*) excited state, it is doubly degenerate
and is expected to Jahn-Teller (J-T) distort into a D2 or C2V
structure along the b1 or b2 active modes, respectively. To
estimate the vibrational relaxation shift of the 13E (π,π*) f
11A1(So) transition we performed geometry optimization of the
13E (π,π*) state. It turned out that the D2d symmetry is no longer
stable in this state and it is lowered to C2V. The resulting 13B2
electronic state, which originates from the HOMO-32a2 (17b1
in the D2d symmetry) to the 38b1 (the xz component of the D2d
38e) one-electron excitation, fails to show significant geo-
metrical relaxation, as can be inferred from the structural
parameters gathered in Table 6. In line with the minor
conformational relaxation, the adiabatic So f T1 (3B2) transition
energy is only 0.06 eV lower than the vertical absorption energy.
According to the composition of the BP/ALDA solution
3
vectors in Table 4, the 13E is the normally emissive (π,π*)
state, whereas the 11A2 and 13A2 are pure 17b1 f 22b2 LMCT
41-46
1
states.
the delocalized character of the MOs involved in the one-
The A2/3A2 splitting is very small, on account of

2086 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Gunaratne et al.
An important point to arise from the above TDDFT results
is that in NiPc(BuO)8 the 1,3LMCT states involving excitation
The lowest excited state, the potentially phosphorescent 13Eu
(π,π*), relaxes to a rectangular D2h structure, due to J-T
instability. The geometrical relaxation is not very large, thus
the vertical emission energy of the resulting 3B3u electronic state,
originating from the HOMO-8au (4a1u in the D4h symmetry) to
the 11b3g (the yz component of the D4h 11eg) one-electron
excitation, is only 0.12 eV lower than the vertical absorption
energy calculated for the unrelaxed 13Eu (π,π*). Therefore, the
minimum of the relaxed energy hypersurface of the 13B3u (π,π*)
is somewhat higher than the minima of the relaxed energy
hypersurfaces of the 1,3LMCT states which, on the contrary,
experience a larger energy relaxation (∼0.3 eV).
2
2
out of the G-a1u into the Ni-dx -y MO may provide effective
routes for deactivation not only for the 1(π,π*) (the 1,3A2 LMCT
states are found to lie vertically ∼0.3 eV below the normally
fluorescent S1 state) but also for the normally phosphorescent
3
3(π,π*). Indeed, the minima of the 1,3A2 LMCT and (π,π*)
relaxed energy hypersurfaces are found to lie at nearly the same
energy.
Coming now to the excited states lying vertically above the
11E (π,π*) state, most of them are of (π,π*) character and are
not expected to undergo significant conformational relaxation.
Therefore, it is unlikely their relaxed energy surfaces would
cross the S1 (π,π*) surface. On the contrary, this may happen
for the 13B2(d,d) and 23E(LMCT/d,d) excited states.
3
In summary, the (dz ,dx -y ) and the 1,3LMCT states appear
to be the main actors in the radiationless decay of the primarily
excited S1 (π,π*) state regardless the conformation assumed
by the molecule.
2
2
2
3
2
The 1 B2 is mainly described by the 22a1 f 22b2 (dz
f
1
2
2
d
x -y ) transition. Its corresponding singlet, the 4 B2, is calculated
Summation of the Theoretical and Experimental Results.
In the following section, the experimental results of the ultrafast
studies are examined in the light of the insights that have been
gained from the theoretical efforts. To simplify the arguments,
we concentrate on the theoretical results obtained for the stable
at a much higher energy (2.92 eV). Such a large singlet/triplet
splitting fits in with the high localization of the unpaired
electrons on the metal center. Similar to the 13A2 LMCT state
discussed above, this state undergoes large conformational
relaxation. From Table 6 where the 13B2 relaxed geometry is
reported, the lengthening of the Ni-Np distance and the
expansion of the macrocycle core following population of the
strongly Ni-Np antibonding 22b2 MO are even larger than in
the 13A2. In line with the remarkable conformational relaxation,
the adiabatic excitation energy of the 13B2 is predicted to be
0.3 eV lower than the vertical absorption energy. The geo-
metrical relaxation of the 13B2 suggests that its relaxed potential
energy hypersurface crosses the 11E (π,π*) surface thereby
opening a route for an efficient ISC mechanism.
D2d structure of the model complex, NiPc(MeO)8, and later point
out the effects of the flapping motion through a planar D4h
structure.
For NiPc(BuO)8 in toluene, the spectral data presented in
Figures 5 and 6 indicate that photoexcitation at 660 or 730 nm
(both wavelengths pump the S0 f S1 electronic transition)
generated an absorbing entity in the 450 nm to 650 nm region
that is fully formed during the instrument response function (ca
180 fs), this is also true for the negative absorbing component
at 730 nm. However, in the spectral region to the red side of
the ground-state bleach (>770 nm), no absorption was present
at the earliest times (Figure 6, dotted), but a positive signal
grows up over the succeeding few ps (Figure 6, dashed). The 1
ps (dotted) spectrum shown in Figure 6 has a broad signature
in the 450 nm upward region, superimposed on which is the
inverse of the ground-state UV-vis spectrum as displayed in
Figure 3. Thus, according to expectations, the excitation pulse
removes ground-state molecules as it generates the first observed
transient (FOT). Since the excitation induces the S0 f S1
transition, it is conceivable that the 1 ps spectrum (dots) in
3
Besides the (d,d) just discussed, another excited state may
become involved in the decay of the primarily excited S1 (π,π*)
state, the 23E. This state is predicted to lie vertically 0.24 eV
1
3
2
2
2
above the 1 E (π,π*). However, similar to the (dz ,dx -y ) and
13A2(LMCT), it is expected to undergo large geometrical
relaxation. The 23E excited state is indeed described by one-
2
2
electron transitions, the 37e f 22b2 (π f dx -y ), and the 36e
f 22b2 (dπ f dx -y ), which involve population of the strongly
Ni-Np antibonding 22b2 MO.
2
2
The TDDFT results for the flat geometry (D4h in the ground
1
Figure 6 is that of the Q state (via the S1 f Sn transition) of
state) proved to be similar to those for the D2d geometry. The
3
NiPc(BuO)8, or it may be that of some species generated from
1Q at times shorter than can be resolved here. The S1 f Sn
transition is of π-character, being localized within the orbitals
of the phthalocyanine ligand. However, TDDFT results indicate
that at least five states, viz. the 23E, 13B2, 11A2, 13A2, and 13E,
may provide effective routes for the radiationless decay of the
primarily excited S1(π,π*) state. Thus the FOT could be a state
2
2
2
)
major effect of flattening the molecule is to lower the (dz ,dx -y
and 1,3LMCT excited states, which is a consequence of the
downward shift of the Ni dx -y MO upon lengthening of the
2
2
Ni-Np bond distance (cf Table 3 and Figure 8). Of the states
that are the most plausible candidates for radiationless deactiva-
3
2
2
2
tion of the initially populated S1(π,π*) state, the (dz ,dx -y
)
(13B1g) is found to lie vertically at nearly the same energy as
3
with some amount of metal character, e.g. the 13B2 (dz ,dx -y
2
2
2
)
the 1(π,π*) (11Eu). So is the 1,3LMCT (11,3B1u) pair with respect
or the 11,3A2 LMCT. If the FOT were the 13B2 its difference
spectrum would be expected to resemble the simulated spectrum
shown as the full line spectrum in Figure 11. This simulation
was made by removing a fraction of the absorbance causing
the dotted spectrum and adding an equal amount of that caused
by the dot-dash one. These spectral entities have the same
extinction coefficients within the Lorentzian envelopes and have
peak wavelengths shifted by 20 nm.
to the (π,π*) (13Eu). The 13B1g and 11,3B1u states, similar to
3
the corresponding D2d 13B2 and 11,3A2, undergo large confor-
mational relaxation. The geometrical changes (not reported
here), which are mainly related to occupation of the strongly σ
antibonding Ni-Np orbital, the 15b1g, consist of a considerable
lengthening of the Ni-Np distance and a sizable expansion of
the macrocycle core, both effects being largest in the 13B1g
excited state. On account of the remarkable conformational
relaxation, the adiabatic excitation energy of the 13B1g is
decreased by 0.5 eV with respect to the value of the corre-
sponding vertical absorption energy. This implies that the
1
The conversion of the Q state into the 13B2 generates a
version of the complex that has a ground state π-system with a
metal center in a nonground-state electronic configuration. Since
the absorption spectrum in the Q-band region of the metalloph-
thalocyanine is dominated by the π f π* transition, and this
originates from a ground state electronic configuration in the
3
2
2
2
minimum of the relaxed energy hypersurfaces of the (dz ,dx -y
)
will be located well below the minimum of the 11Eu (π,π*)
hypersurface.

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2087
represented by Figure 11, the implication being that the product
at 8 ps is a state in which the π-system of the ligand has a
ground state electronic configuration and the excitation is
resident on the metal, viz. a (d,d) state. According to TDDFT
3
3
2
2
2
calculations, this (d,d) state can only be the (dz ,dx -y ) (1 B2).
That the positive and negative lobes in Figure 6 (dashed) are
not equal (as in Figure 11) indicates that the stoichiometry of
the conversion is not 1:1 in terms of absorbance change, either
due to differences in extinction coefficient or because the (d,d)
state precursor (the FOT) is being depleted through competing
routes that lead elsewhere (see below).
The negative absorption centered at 740 nm that was formed
within the rise time of the instrument, decayed in a biexponential
manner to a nonzero baseline (Figure 7b). Lifetimes of 2.1 (
0.3 and 22.0 ( 3 ps were extracted. The faster component has
a lifetime that is close to that of the early component of the
decay of the S1 state (2.6(0.3 ps) and the first thought is that
it represents a rapid radiationless conversion of S1 to S0.
However, this transition is π* f π in nature and such transitions
in metallophthalocyanines with p-block metal centers are known
to occur with lifetimes in the nanosecond regime.26 It is more
likely that this fast component of the negative signal recovery
is actually the growth phase of an entity that has a positive
absorption underlying that of the ground state of the complex.
Such an entity is the (d,d) state represented by the 8 ps spectrum
(dashed) in Figure 6.
Figure 11. Simulation of the difference absorption spectrum of a d,d
electronic state (see text).
ligand, modified by a nonground state d-electron configuration,
the true absorption spectrum of the state should closely resemble
that of the ground state of the complex with a small spectral
shift due to the unusual d-electron configuration, as simulated
in Figure 11. Spectral profiles such as this have been reported
The conclusion that the S1 (π,π*) state (dotted spectrum in
3
3
2
2
2
Figure 6) converts into the (dz ,dx -y ), 1 B2, with a lifetime of
ca. 2 ps requires an intersystem crossing process to occur, which
might a priori be thought to be inconsistent with such a short
lifetime. However, according to the TDDFT calculations, the
2
2
2
for Ni(II) porphyrin excited states where the dz ,dx -y character
was confirmed.10-19 The difference spectrum of the FOT derived
from NiPc(BuO)8 (Figure 6, dotted) bears no resemblance to
the simulated one and so the putative assignment to the 13B2 is
discarded. Thus, we are left with the possibility that the FOT is
either the π-localized S1 state or a state with charge-transfer
character (LMCT). The optical absorption spectra for such CT
states would be expected to show similarities to those of the
one electron oxidized forms of the π-system. However, the
spectro-electrochemical data presented in Figure 4 show sig-
nificant absorbance to the red side of the ground-state Q-band,
whereas the 1 ps spectrum shown in Figure 6 has no absorbance
at the red side of the ground-state region. These arguments lead
to the conclusion that the post-pulse spectrum is due to the 1Q
state formed directly from photon absorption. The decay of this
entity (at 625 nm) was nonexponential but could be adequately
fitted with a pair of exponentials. The major, faster component
had a lifetime of 2.6 ( 0.3 ps; the weak, slower component
had a lifetime of 15.1 ( 4 ps.
3
2
2
2
relaxed potential energy hypersurface of the (dz ,dx -y ) is likely
to cross the 11E (π,π*) surface opening a route for an efficient
ISC mechanism. The narrowing of the optical absorption
spectrum at the red side of the ground state bleaching absorption
(Figure 5) over the first 8 ps suggests that the produced (d,d)
state is vibrationally hot and is capable of losing some energy
to achieve its relaxed structure. The dashed spectrum shown in
Figure 6 is that of the vibrationally cold (d,d) state. This state
converts to a daughter state (Figure 6, full line) with a lifetime
of ca. 20 ps. The difference spectrum is reminiscent of the
absolute spectrum of the ground state, having a major peak at
740 nm and a minor one at 660 nm with underlying ground-
state bleaching. According to the TDDFT calculations the decay
product of the (d,d) state might be one of the two LMCT states,
one triplet and one singlet, or the 3(π,π*) state, all lying in the
S1-S0 gap. However, the spectral signature (Figure 6, full) bears
no resemblance to that of the 3(π,π*) spectra of similar
metallophthalocyanines, which typically show broad featureless
absorptions with maxima near 600 nm.26 Therefore, the full line
spectrum (Figure 6) arises from one of the near degenerate
LMCT states, probably the triplet sub-state, since the precursor
Having argued that the 1 ps spectrum is that of the S1 (π,π*)
state generated directly by photon absorption, it is now necessary
to assign the later transient spectra in Figure 6 (dashed and full
lines). At 770 nm and beyond (Figure 7d), a positive absorption
is seen to grow in from zero with a growth lifetime of 2.1 ps
and having a subsequent decay with a 21 ps lifetime. Interest-
ingly the peak wavelength of this entity became increasingly
blue-shifted as time elapsed (Figure 5, first 10 ps). Similar
behavior to this blue-shifting absorption peak has been observed
for other phthalocyanines earlier in these laboratories when it
was attributed to the cooling of a vibrationally hot daughter
state with d-character.25 At or about 8 ps, post excitation the
blue shift is complete and at that point the difference spectrum
is as the dashed line in Figure 6. Now the negative absorption
minimum at 730 nm with the positive absorption maximum at
755 nm show the typical negative and positive lobe pattern
3
has triplet multiplicity. An MO analysis of the LMCT state
reveals that the π/π* energy gap increases relative to the ground
3
state, so the LMCT is expected to have a visible absorption
spectrum similar to, but blue shifted in wavelength from, the
ground state spectrum. This fits in with the full line spectrum
of Figure 6 resembling the difference spectrum between two
metallophthalocyanines with only a very small spectral shift to
the blue. It is not inconceivable that the 13E (π,π*) is, in fact,
3
populated through the decay of the (d,d) state, but if so the
only nonradiative decay route for this state is to the LMCT state,
so the net result would be the same as if the 13E state did not
intervene. Finally, the LMCT state converts to the ground-state

2088 J. Phys. Chem. A, Vol. 109, No. 10, 2005
Gunaratne et al.
routes for radiationless deactivation of the 1,3(π,π*) states. The
(d,d) ligand field states, which are crucial intermediates in the
deactivation of photoexcited low-spin d8 nickel porphyrins, do
not necessarily play such a dominant role in the photophysics
of nickel phthalocyanines. In NiPcs, the S1(π,π*) state is red
3
2
2
2
shifted and the (dz ,dx -y ) ligand field state is blue shifted with
respect to the porphyrin analogues. Therefore, if the primarily
excited S1(π,π*) state lies at too long wavelengths, as in NiNc-
6
3
2
2
2
(BuO)8 where the Q-band lies at 850 nm, the (dz ,dx -y ) might
not be involved at all in the radiationless decay. Or, as found
here for NiPc(BuO)8, it intervenes only at the early stage of
the decay process.
Conclusions
The excited-state spectral and dynamic behavior of NiPc-
(BuO)8 in toluene were investigated by ultrafast transient
absorption spectroscopy and interpreted in the light of DFT/
TDDFT theoretical studies. NiPc(BuO)8 was shown to have a
highly saddled structure in the crystalline state. In solution the
theoretical results indicated that it can flap from one D2d-saddled
conformation to the opposite one through a D4h-planar structure.
Under pulsed photoexcitation at 735 nm, the most prominent
features in the transient spectrum of the complex in toluene were
positive absorption bands having λmax ) 625 and 760 nm, and
a negative absorption (bleaching) in the wavelength region of
the ground-state absorption maximum with λmin ) 735 nm, and
a weaker band with λmin ) 655 nm. In the case of the 760 nm
positive band, the absorption maximum shifted over the first 5
ps, or so. Taken together with the indications from the theoretical
studies, the spectral and kinetic changes for the complex in
toluene can be understood in terms of the 730 nm excitation
light generating a primarily excited S1 (π,π*) state that
Figure 12. Schematic diagram illustrating the key photoprocesses for
the photoexcited NiPc(BuO)8.
surface with a lifetime of 640 ps. In sum, the Q state (S1) of
NiPc(BuO)8 in toluene, generated by photoexcitation in the
Q-band, deactivates in a cascade of events, populating in turn
the 13B2 (d,d) and the 13A2 LMCT states, before finally
regaining the ground state surface. The just described sequence
of events is represented schematically in Figure 12, which has
been constructed using the data in Table 4.
As a final note in this attempt to assign spectral signatures
to states, the (d,d) state decayed exponentially with a lifetime
(21 ps) that compares favorably with the lifetime of the slower
component (22 ps) of the recovery of the negative absorption
centered at 740 nm. As discussed above, it could be that this
21 ps component of recovery of absorption at 740 nm is actually
the growth of positive absorption at this wavelength arising from
the LMCT state, or it could indeed be a true ground-state
repopulation process. If the latter is the case, then it is necessary
to conclude that the (d,d) state decays to the ground-state surface
in parallel with its decay into the LMCT state. Unlike in the
case of the FOT, it is not possible on the present evidence to
determine whether there is branching in the decay of the (d,d)
state, or whether it converts to the LMCT state with unit
efficiency.
3
2
2
2
transforms initially into a vibrationally hot (dz ,dx -y ) state.
Cooling to the zeroth state is complete after ca. 8 ps. The cold
3
2
2
(d,d) state converted to its daughter state, the LMCT (π,dx -y ),
which itself decays to the ground state with a lifetime of 640
3
3
ps. Population of the (π,π*) through the decay of the (d,d)
state cannot be ruled out, but if so the only nonradiative decay
route for this state is to the LMCT state, so the net result would
3
be the same as if the (π,π*) did not intervene. The excited-
state calculations indicate that the proposed deactivation mech-
anism applies to the D2d-saddled and the D4h-planar structure
as well.
The final question to be addressed is whether the possible
conformational excursion of the complex from one saddled
conformation through the planar “maximum” to the opposite
saddled structure might lead to a somewhat different interpreta-
tion of the ultrafast experiments. The TDDFT results for the
flat geometry (D4h in the ground state) show that the major effect
Acknowledgment. The experimental studies conducted at
the Ohio Laboratory for Kinetic Spectrometry (BGSU) were
supported in part by NIH Grant CA 91027 and by an
instrumentation grant from the Hayes Investment Foundation
(Ohio Board of Regents). A.R. and G.R. thank the Italian MIUR
(Ministero dell’ Istruzione, dell’ Universita` e della Ricerca) and
the Universita` della Basilicata, Italy for support (Grant
2003038084_002). T.C.G. expresses gratitude to the McMaster
Foundation (BGSU) for a pre-doctoral fellowship.
3
2
2
2
of flattening the molecule is to lower the (dz ,dx -y ) and
1,3LMCT excited states. In the D4h molecule, the (dz ,dx -y
)
3
2
2
2
(13B1g) is predicted to lie adiabatically below the 1(π,π*) (11Eu).
So is the 1,3LMCT (11,3B1u) pair with respect to the (π,π*)
3
(13Eu). This suggests that, in the case the molecule flattens, the
primarily excited S1 (π,π*) deactivates in a cascade of events,
populating in turn the (dz , dx -y ) and the LMCT states, before
finally regaining the ground-state surface.
3
3
2
2
2
Supporting Information Available: ORTEP top view of
the NiPc(BuO)8 complex, data collection and structure refine-
ment details, tables giving crystal data, atomic coordinates,
isotropic and anisotropic displacement parameters, and bond
length and angles. This material is available free of charge via
Internet at http://pub.acs.org. Full X-ray structural information
is available as an X-ray crystallographic file in CIF format. CIF
has been submitted to the CCDC with entry number CCDC
237068. This material is available free of charge via the Internet
at http://pubs.acs.org.
The studies presented here on NiPc(BuO)8 point to an
3
2
2
2
important role of the (dz ,dx -y ) state, suggesting a resemblance
of the radiationless deactivation mechanism of NiPcs to the one
in nickel porphyrins such as NiOEP and NiTPP.10-19 We should
however caution that there are significant differences between
these compounds. In NiPcs, due to the upshift of the G-a1u, the
1,3LMCT states involving excitations out of the G-a1u into the
2
2
Ni-dx -y MO are sufficiently low in energy to provide effective

Photophysics of NiPc(BuO)8
J. Phys. Chem. A, Vol. 109, No. 10, 2005 2089
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