Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy.
We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)2(NPh2), NH2, OCH3, t-butyl, CH3, H, F, CF3, CN, benzothiazole, and NO2, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S0→ Sntransitions to higher energy states, above the lowest-energy S0→ S1transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ2∼ 300 GM (1 GM = 10-50cm4s), for R = NPh2. At transition energies overlapping with the lowest-energy S0→ S1transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ2< 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S0→ S1transition gradually increases with increasing EW strength, reaching values σ2= 700 GM for R = NO2, while in the S0→ Sntransition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S0→ S1transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S0→ S1transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S0→ Sntransition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.
Physics Department, Montana State University , Bozeman, Montana 59717, United States.|National Institute of Chemical Physics and Biophysics , 12618 Tallinn, Estonia.
Aleksander
A
Rebane
Physics Department, Montana State University , Bozeman, Montana 59717, United States.
Mikhail
M
Drobizhev
Physics Department, Montana State University , Bozeman, Montana 59717, United States.
Nikolay S
NS
Makarov
Physics Department, Montana State University , Bozeman, Montana 59717, United States.
Geoffrey
G
Wicks
Institute of Physical Chemistry, Polish Academy of Sciences , Warsaw, 01-224, Poland.
Pawel
P
Wnuk
Institute of Physical Chemistry, Polish Academy of Sciences , Warsaw, 01-224, Poland.
Yuriy
Y
Stepanenko
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.
Joy E
JE
Haley
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.|General Dynamics Information Technology, Dayton, Ohio 45431, United States.
Douglas M
DM
Krein
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.|SOCHE Student Research Program, Dayton, Ohio 45420, United States.
Jennifer L
JL
Fore
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.
Aaron R
AR
Burke
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.|Science Applications International Corporation, Dayton, Ohio 45431, United States.
Jonathan E
JE
Slagle
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.|Science Applications International Corporation, Dayton, Ohio 45431, United States.
Daniel G
DG
McLean
Materials and Manufacturing Directorate, Air Force Research Laboratory , Wright Patterson Air Force Base, Ohio 45433, United States.
Thomas M
TM
Cooper