Note: Solutions of the compounds shown in Figure 1 have already been prepared using methanol as the solvent. Because of the tendency to dimerize, two concentrations of each dye are provided. If the band shapes are the same for both solutions, it is likely that the dimer concentration is very small.
We will be using the Spectral InstrumentsTM model SI 440 spectrophotometer, which is equipped with a fiberoptic probe and a CCD detector for our absorption studies.
The procedure you will use is similar to that used in the pKa experiment, and is briefly repeated here (text from K Zientek).
1. Immerse the fiberoptic probe in methanol and stir gently with the probe to dislodge any bubbles. Click 'Blank' to acquire the blank spectrum to be used by the software for absorbance calculation. These data are used until the wavelength range is changed or until a new blank is acquired. Click 'Lock Blank' to prevent accidental repeat of the blank measurement. (If a new blank is needed later, first click 'Unlock Blank'.)
2. Remove the probe from the water and gently blot the probe with a bed of KimwipesTM to remove liquid remaining on the mirror. Gently dry the outside of the probe.
3. Acquire and save the spectrum for each solution as follows: Immerse the dry probe in the sample, and click 'Sample'. Then click the red 'Save' box on the lower right of the display, and save the spectrum on your floppy (A drive) using an appropriate filename. (The 'Save' box will return to background color after the spectrum has been stored.).
4.Immerse the probe in the deisred sample. Then click 'Sample'. Record the three absorbances from the table, and store the spectrum on the floppy. Click 'Save Setup' to keep the same wavelengths in the table for all spectra.
5. Repeat this procedure for each of the dye samples. Save each new spectrum to your floppy disk.
6. When you have finished with all the samples, click on the "Export Files" button located to the right of the Save button. A window will open called "File Dialog". Select the A: drive and select the folder containing the saved spectra. A list of all your saved files should appear. Using the shift key and the left-mouse button, select all the spectra files and click "OK." A new window will open which will give you the option of saving the files in either a text or excel spreadsheet format. Choose the one most applicable to you.
7. When you have completed all measurements and created your spreadsheet file(s), ask your TA if the spectrometer is to be shut down. If so, click 'File' and 'Close' in the upper left corner of the display. When the Windows descktop appears, click 'Start' and 'Shut Down'. Answer 'Yes' to the query. Turn off the spectrophotometer module, the water bath, and the power strip.
1. Using the longest lmax calculate DEexp for each dye.
2. Use equation 5 to calculate DEtheo for each compound. Compare the result to the experimental value.
3. For each set of dyes use an iterative method to calculate the best-fit a. Recalculate DEtheo for each dye, and compare the results to the experimental values.
Discuss the effect of chain length on the observed lmax, and relate this to the "particle-in-the-box". For each dye relate the visually observed color to the absorbed wavelengths.