Nonlinear Absorption in Phthalocyanines and Naphthalocyanines
- PDF / 302,131 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 87 Downloads / 182 Views
It is of considerable importance for the design of new materials to determine how the reverse saturable absorption is modified by changes in chemical structure. EXPERIMENTAL The experimental apparatus consists of a dye laser synchronously pumped by a modelocked, doubled Nd/Yag laser. The dye laser output was amplified to provide 1.2 psec pulses with energies up to 1 mW.Nonlinear transmission experiments were performed by directing the output of the laser through the sample and measuring the transmitted intensity as a function of incident intensity. Transient absorption measurements were made by splitting the laser into two beams. The first, or pump beam was used to excite the sample at variable optical delays. The second, or probe beam, was focussed into a water cell to provide a broad band continuum. This continuum was filtered, recollimated, and directed onto the sample. Both the sample beam and a reference beam were focussed onto the slits of a monochromator and the output imaged onto a CCD array. The change in optical density due to the formation of the excited state is calculated from the ratio of sample to reference signal after subtraction of the background signal. The polarization vector of the excitation beam was -54.7 degrees relative to that of the continuum to remove any effects due to orientational relaxation. RESULTS AND DISCUSSION The molecular structures of PbPc(CP) 4 and PbNc(a-PO)8 are shown in figure 1. H1 1 C5 0 0C 5 H1 N
H1 1 C5o
H3
H3
CN3
CH3
AN
N0-
C 5H1 1
I
N N3 1 C5 0
(C3
B
A
Figure 1 The molecular structure of A) PbNc(a-PO)8 and B) PbPc(CP) 4. The spectra of a 7.5x10 3 M solution of PbPc(CP)4 and a 1.03x10 2 M solution of PbNc(aPO) 8 in toluene are compared in Fig. 2. The absorption spectra are typical of a phthalocyanines. There is a strong lr-r" transition, the Q band, peaked near 720 nim in PbPc(CP)4 . This material has a relatively high transmission from about 430 nm to 620 rnm. In PbNc(a-PO)8 the Q-band is shifted to 910 nm, primarily because of the more delocalized 7r orbitals in the
662
2
0
01
0)0 900 700 Wavelength (nm)
500
1100
Figure 2 The spectra of a 7.5x10 3 M solution of PbPc(CP)4 (left) and a 1.03x10 2 M solution of PbNc(a-PO)8 in toluene. The laser excitation wavelengths are indicated. larger ring system. The region of relatively high transmission in this material is wider than that in PbPc(CP)4; it extends from -500 nm to -800 rim. The nonlinear transmission of solutions of PbPc(CP) 4, measured with 1.2 psec pulses
5.3 mM PbPc(CP)4 in toluene 590.5 nm
7.5 mM PbPc{CP)4 in toluene 675 nm
0.79
0.28
.026
• 0.78
i 0.24 0.76
0.22
0.75
A_
0
1
2 3 4 5 Intensity (GW/cm z)
6
0.2 0
7
1
2 3 4 2 Intensity (GW/cm )
5
6
Figure 3 Nonlinear transmission of PbPc(CP)4 at 590.5 rum and 675 rim. at 590.5 rm and 675 nim, are shown in Figure 3. Values of the nonlinear absorption coefficient, a 2 = 6.7 cm/GW at 590.5 nm and a 2 = -120 cm/GW at 675 mu were derived from the data in Fig. 2 using the equations given by Van Stryland et. al.! The data in Fig
Data Loading...
