Use glass cuvettes and place pure methanol in slot 1 in the spectrometer. Methanol is the solvent for all the samples. You can place a sample in all the other slots of the rotating sample holder. Slot 2 will then contain sample 1, etc.
Operation of the spectrometer: Click on Applications/Wavescan/Setup. Make the spectrometer scan over wavelength from 350 nm to 800 nm. Use slow scan. To start the measurements, select 'run'. You can print the spectra on the printer above the spectrometer (the printer is sitting on the shelf).
Dilute the four solutions tenfold in methanol (in the hood). Use pipettes and volumetric flasks (not beakers). Test to see if the absorbance is small enough to be measured - it goes off scale for the undiluted solutions. Dilute again tenfold until each solution has small enough absorbance to be measured. You will notice a shoulder on the absorption peak. This can be due to dimerization of the molecules in solution. To reduce the effect of dimerization, dilute the sample further twofold and then fourfold and record the peak position for all three solutions. Repeat this procedure for all four samples.
Make a table of the measured peak position for each dilutions of the four compounds that was measured. The table should contain an entry for the concentration of each solution in one column and the observed peak position for each solution in another column. Make a drawing of the molecular structure of each compound and then draw just the atoms and bonds in the conjugated pi-bond chain. How many pi-electrons are there in each case? What is a reasonable estimate of the lenght of the pi-bonded chain (there is a table of bond lengths in the text book by Silbey et al.)? Explain carefully your reasoning for your estimate of the length of the pi-bonded chain.
Predict the position of the absorption peak from the particle-in-a-box model using your estimate of the lenght of the pi-bonded chain as the box-length. Make a table showing the measured peak position for each compound (best estimate from the most dilute sample), the estimated length of the "box", the number of pi-electrons in the compound and then the predicted position of the absorption peak from the particle-in-a-box model. The last entry in the table should show how much the length of the "box" needs to be adjusted in order to predict peak position in complete agreement with the one you measured. How well does the particle-in-a-box model work for these compounds? Does it predict the right trend in the peak position as the conjugated chain becomes longer? Does the model improve as the length of the conjugated chain increases (should it?)?