Fluorescent Lifetime of the Singlet State and Fluorescent Quantum Yield of Zinc Phthalocyanines

Hua-Kuang Lee

University of Minnesota

Methods of Experimental Physics Spring 2013

Abstract

The alkoxide substituents effects on the quantum yield on the two different zinc phthalocyanines, zinc 2,3,9,10,16,17,23,24-octakis (octyloxy)-29H,31H-phthalocyanine (ZnPcOR1) and zinc 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (ZnPcOR2) are compared to the regular zinc phthalocyanine (ZnPc). The first excitation wavelength for ZnPcOR2 is red shifted 67 nm compare to ZnPc, where as ZnPcOR1 only has a red shift value of 6 nm. The fluorescent quantum yield (QYf) for ZnPcOR2 has the value of 18.6±2.1 %, which is 50.6±5.9 % less than the QYf for ZnPc. The unexpected 50.6±5.9 % drop in QYf in ZnPcOR2 is due to the alkoxide substituents in ZnPcOR2 have steric interaction with one and another that causes the deformation of the planar structure of phthalocyanines, which lowers the energy gap between the ground state and the first excited state.

Introduction

The typical efficiency of 3rd generation solar cell is around 10%, and they are dominate by organic and dye-sensitized cells. Zinc phthalocyanine is an example of dye-sensitized solar cells. The excitation properties of these materials can be altered by adding substituents on the basic material. Fluorescent quantum yield, the ratio of photon emission over photon absorption, is depends on the electron density of the molecules. Because substituents have different electron properties, therefore by changing in the electron donating ability of the substituents on phthalocyanines will result different quantum yield. Alkoxide, oxygen containing carbon chain, is electron donating group that donates the electron density into phthalocyanine. Hypotheses :Longer the alkoxide chain yield higher fluorescent quantum yield. Goal :Measure and study how does the substituents affects the raditative and nonradiative rate on ZnPc

Theory

Fluorescent quantum yield is defined by the ratio of absorbance and integrated area of fluorescent spectrum of the standard and sample with known QYf; Phe_p=(A_s*F_p*Phe_s)/(A_p*F_s), where s and p stands for standards and sample. The relationship between the time constant of depopulation of the excited state and the radiative and nonradiative rate (K_r and K_nr respectively) is equal to; T=1/(K_r+K_nr ) T can be determined by fitting the data with exponential decay formula. Once the QYf and the lifetime of the depopulation of the excited state are know, the radiative rate can be determined as represented; K_r=Phe_p/T

Experimental Setup

Absorption spectrum was obtained with Olis 14 UV-Vis absorption spectrometer. Emission spectrum was acquired using double monochrometer spectrometer. Depopulation of the excited state figure was measured with photon counter.

Figure 1: UV-Vis spectrometer

Figure 2: Photon counter

Figure 3: Fluorometer

Results

Figure 4 shows the structure of these materials.

Figure 4: The structures of the zinc phthalocyanines.

It is found that the absorption of the first excited wavelength of ZnPcOR2 is red shifted by 67 nm compare to regular ZnPc, and its fluorescent quantum yield decreased by 50.6 +/-5.9%. ZnPc has planar conformation, and because the position of the substituents in ZnPcOR2 causes the substituents to interact with each other and deform the planar conformation. The deformation of the planar conformation in porphyrin lowers the energy gap between the first excited state and the ground state, therefore the red shift of the first excitation wavelength. It is also found that the florescent quantum yield is lowered by about 60% in these distorted porphyrins. QYf of ZnPcOR1 is lower than the QYf of ZnPc by about 3.1%, there for the substituents has little effects on QYf, this is the opposite what we expected. It was found that the increase in the carbon chain length of the alkoxide substituents increases the phosphorescent quantum yield (QYp) and lowers the QYf compares on ZnPc.5 Knr is consist of the rate of intersystem crossing, S1 to first triplet state, and nonradtiative emission, but it is not possible to resolve the two in this experiment. It is possible that the increasing in the Knr in ZnPcOR1 may suggest the increase rate of intersystem crossing, which leads to the higher QYp in the article.

Figure 5: The emission spectrum of the three zinc phthalocyanines.

Figure 6: The absorption spectrum of the three zinc phthalocyanines.

Table 1: Experimental results.

Conclusion

ZnPcOR1 was originally expected to have higher QYf than ZnPc, because it has longer carbon chain in the electron donating substituents. We proposed that the increase of Knr in ZnPcOR1 has increase the rate of intersystem crossing that yields higher QYp as observed by other experiment with ZnPc with different length in alkoxide substituents.10 It is clear there is interaction between the substituents in ZnPcOR2 causes the distortion of the planar phthalocyanine conformation caused the major red shift of the first excitation state and lowered the QYf. If one wants to farther investigate the excitation properties of zinc phthalocyanines, it is recommended to have the substituents on the same position to separate out the positioning effects. A different direction would be study the excitation properties of the phthalocyaninies by varying the metal centers.

Acknowledgements

Thanks for Professor David Blank (in the chemistry department), his group, and professor Pryke for their indispensable wisdom throughout the semester.