Modulation of TwoPhoton Absorption of AsymmetricallySubstituted Phthalocyanines by the Position of I

1
Modulation of Two-Photon Absorption of
Asymmetrically-Substituted Phthalocyanines by the
Position of Inner Protons (Tautomerization) and
Structure of Linking GroupAleksander Rebanea,
Mikhail Drobizheva, Nikolay S. Makarova, Gema de
la Torreb and Tomás TorresbaPhysics Department,
Montana State University-Bozeman, USAb
Departamento de Quimica Organica, Facultad de
Ciencias, Universidad Autonoma de Madrid, Madrid,
Spain
Two-Photon Absorption Spectra (B-band Region)
Abstract We have recently started a
comprehensive study of two-photon absorption
(2PA) properties of phthalocyanines (Pcs) in near
IR region 1. These investigations can lead to
further development of several intriguing
applications of Pcs, including optical power
limiting, 3D memory, and deeper-penetrating
photodynamic therapy. Here we study 2PA
properties of two metal-free push-pull
nitrophenylethynyl- and nitrostyryl-substituted
Pcs. We have found that in both molecules there
are two unequivalent tautomeric forms (T1 and T2)
simultaneously present at room temperature in
comparable amounts. At low temperatures these
forms can be reversibly photo-transformed, which
is important property for re-writable 3D memory
application. By monitoring the fluorescence
signal at a particular wavelength, corresponding
to spectral maximum of each tautomer, we were
able to obtain 2PA spectra of T1 and T2
separately. Intrinsic (femtosecond) maximum 2PA
cross section, attained near 800 nm, vary from s2
1.5 x 103 to 4 x 103 GM (1GM 10-50 cm4 s),
depending on particular molecule and tautomer.
Such large s2 values can be due to either (1)
strong resonance enhancement in three-level
system (with Q-state playing a role of real
intermediate state) or (2) to a 2PA transition in
two-level system with large change of permanent
dipole moment (with ground and final levels
working as intermediate states), or even to both,
with possible quantum interference between the
corresponding pathways 2. While solvatochromic
shifts imply large changes of permanent dipole
moment, ranging from 15 to 30 D, the resonance
enhancement effect dominates in excitation region
from 800 to 900 nm in both molecules and
tautomers.
Extraction of Permanent Dipole Moments in the
Ground State
NSPc
NPEPc
Four equations (1)-(4) contain 5 unknown
parameters. To solve them, we add a fifth
equation, based on independent measurement of
ltm0gt in NSPc in chloroform G. Rojo, G. de la
Torre, J. Garcia-Ruiz, I. Ledoux, T. Torres, J.
Zyss, F. Agullo-Lopez, Chem. Phys., 245 (1999)
27, which writes Our additional measurements
of the ratio of T1 and T2 Q(0-0) - bands in
chloroform and octane shows that in chloroform,
n1 n2 0.5. Substituting these numbers in (5)
and solving Eqs. (1)-(5) with the assumption that
all dipole moments are either parallel or
anti-parallel, we find the following molecular
parameters for T1 and T2 The change of
permanent dipole moment is found from
solvatochromic shifts of CT B-bands in excitation
spectra of T1 and T2 through equation
We find Dm0B 27 D for T1 and 17 D for T2.
Substitution of these values in Eq. (TLS) gives
the dipole contribution to s2, see Table, last
column. For both tautomers it is several times
less than the total experimental s2.
These spectra are obtained in octane by
selectively monitoring fluorescence intensity
either at the T1 (718 nm) and T2 (698 nm)
fluorescence maxima. 2PA cross sections were
calculated, by taking into account real number
density of each tautomer in solution at room
temperature.
What Is The Role of Charge Transfer in 2PA
Strength?
Since the molecules are strongly
non-centrosymmetrical (carry large permanent
dipole moment), two-photon absorption should be
allowed to the same states as one-photon
absorption. If the low-frequency part of B-band
is dominated by charge-transfer state, the 2PA to
this state can acquire a particular strength if
the change of permanent dipole moment is large.
Question. Which mechanism dominates the 2PA
cross section in this region (1) A transition in
two-level system with a large change of permanent
dipole moment or (2) a transition in three-level
system with Q-band playing a role of intermediate
state? In the case (1), 2PA cross section is
given by In order to obtain the magnitude
of the permanent dipole moment change, we
performed the solvatochromic analysis of
excitation and fluorescence spectra of both T1
and T2 tautomers.
Chemical Structures
NSPc
Absorption, Excitation and Fluorescence in Octane

• Conclusions
• We found that at room temperature in solutions
  push-pull substituted phthalocyanines present in
  a dynamic equilibrium of two spectroscopically
  different tautomeric forms.
• 2. By using selective fluorescence detection we
  obtained 2PA spectra of T1 and T2 separately.
• 3. The absolute s2max values of both forms in the
  region of long-wavelength side of B-band is
  large, 103 GM.
• While large changes of permanent dipole moment
  contribute considerably to the resulting 2PA
  strength, the resonance enhancement effect
  dominates in both molecules and tautomers.
• Nitrophenylethynyl- and nitrostyryl-substituted
  Pcs show similar 2PA strength.

NPEPc
NSPc
Solvatochromic Measurements for NSPc in Q-bands
1. Stokes shift as a function of solvent polarity
Results
Temperature dependence of absorption spectrum
2. Q1(0-0) absorption band shift as a function of
solvent polarity
Results
Acknowledgments This work was supported by
AFOSR, and MBRCT.