P.T. Anusha

Dinaphthoporphycenes: synthesis and nonlinear optical studies.

Naphthobipyrrole-derived porphycenes are synthesized for the first time via McMurry coupling of the β-alkylated 2,9-diformylnaphthobipyrrole derivatives, which in turn were prepared easily from 2,3-naphthalene bishydrazine in four steps. Insertion of nickel into the porphycene core results in transformation of the rectangular N4-core into a square type geometry owing to the fusion of naphthalene moiety onto the bipyrrole entities. These porphycenes show large, intensity dependent three-photon absorption.

Ultrafast excited state dynamics and dispersion studies of nonlinear optical properties in dinaphthoporphycenes

Ultrafast excited state dynamics of dinaphthoporphycenes were investigated using femtosecond and picosecond degenerate pump-probe techniques at 600 nm and 800 nm, respectively. Femtosecond pump-probe data indicated photo-induced absorption at 600 nm resulting from two-photon/single photon excitation, whereas picosecond pump-probe data demonstrated photo-bleaching which was a consequence of three-photon absorption. The fastest lifetimes (100–120 fs) observed are attributed to the intramolecular vibrational relaxation, the slower ones (1–3 ps) to internal conversion, and the slowest components (7–10 ps) to non-radiative decay back to ground state. Z-scan studies in the 560–600 nm range were also carried out.

Ultrafast nonlinear optical properties and excited state dynamics of phthalocyanine thin films

We have investigated the nonlinear optical properties, optical limiting thresholds, and figures of merits for five different phthalocyanine thin films, achieved through doping in PMMA, using the Z-scan technique at 800 nm with 2 ps laser pulses. From the open-aperture Z-scan data we established that these molecules exhibit strong two photon absorption (2PA) with the nonlinear coefficients in the range of 15-200 cm/GW. We have also estimated the sign and magnitude of real part of third order nonlinearity through the closed aperture scans. Preliminary femtosecond pump-probe data suggests that the lifetimes of excited states are in the sub-100 ps regime for all the molecules in film form. Our studies provide tangible evidence that these phthalocyanines are potential candidates for multi-photon imaging and optical limiting applications.

Multiphoton Absorption Studies in Porphycenes Using Picosecond and Femtosecond Pulses

We present our results obtained from the Z‐scan measurements in porphycenes performed using ∼2 picosecond (ps) and ∼40 femtosecond (fs), 800 nm pulses with a repetition rate of 1 kHz delivered by amplified Ti:sapphire laser systems. Open aperture Z‐scan data demonstrated interesting multi‐photon absorption properties for these molecules. In the ps regime these compounds exhibited two‐photon absorption (2PA) at lower peak intensities whereas three‐photon absorption (3PA) was dominant mechanism observed at higher peak intensities. In the fs regime saturable absorption was the mechanism detected. Closed aperture data was collected to evaluate the nonlinear refractive indices of these molecules in both the time domains. The magnitude of n2 was ∼10−17 cm2/W in the fs regime and ∼10−15 cm2/W in the ps regime.

Ultrafast nonlinear optical studies of cyclo[4]naphthobipyrroles

Expanded porphyrins belong to the class of porphyrinoids, where the core of the porphyrin macrocycle is increased either by incorporating additional pyrrolic units, or by increasing the number of bridging carbon atoms from more than four, or a combination of both. The significance of these classes of compounds lies in their novel photophysical and nonlinear optical properties. Superior nonlinear optical coefficients are usually observed for aromatic expanded porphyrins with large number of π-electrons owing to their distinct structural features. In this regard, cyclo[8]pyrrole is unique, owing to its large planar 30-π core macrocyclic ring in its diprotonated state. Here, all the eight pyrrole units are directly linked to each other through their á-positions. Recently, we have synthesized, cyclo[4]naphthobipyrroles, a unique class of cyclo[8]pyrroles, where alternate pyrrole units are fused with naphthalene moieties. This adds more rigidity to the resultant cyclo[8]pyrrole while further extending the resultant π-conjugation. Herein, we present some of our results from the picosecond nonlinear optical studies of a â-octa-isopropyl-cyclo[4]naphthobipyrrole. The nonlinear optical coefficients were extracted from the Z-scan measurements. The values of two-photon cross-sections obtained for these molecules were in the range of 104 GM.

Femtosecond Pump Probe Spectroscopy of Novel Corroles

We present our experimental results on the femtosecond (fs) pump-probe dynamics of four novel Corroles (a) Tritolyl Corrole (TTC) (b) Germanium Tritolyl Corrole (Ge-TTC) (c) Phosporus Tritolyl Corrole (P-TTC) (d) Triphenyl Corrole (TPC). The photo-physical response of these Corroles was studied with ∼ 60 fs, 1 kHz laser pulses at a wavelength of 600 nm. The transient response of change in probe transmission with pump and without pump at 100 Hz modulation was acquired. Photo-induced absorption (change in transmission was negative) was observed for all Corroles. The recovery time for TTC was ∼ 60 ps. With the introduction of Ge the life time was estimated was ∼ 53 ps while for phosphorous substitution the life time observed was ∼ 286 ps. TPC had an excited life time of ∼ 35 ps.