There are several different properties that molecules can possess to display color. In this experiment conjugated polymethine dyes and cadmium selenide (CdSe) nanoparticles are excited to their lowest excited energy state are investigated by ultraviolet-visible (UV-Vis) Spectroscopy. The family of 1,1’-diethyl-4,4’ dyes are compared with varying length of conjugated carbon chains of 7, 9, and 11 carbons long for cyanine, carbocyanine, and dicarbocyanine at , of 590, 706, and 812 nm respectively. The wavelength increased with length of the conjugated carbon chain due to the resonating electronic transition along the 𝜋 bonds which is related to the particle in a box model. The free electron model was used to calculate theoretical at 579, 706, and 832 nm for cyanine, carbocyanine, and dicarbocyanine, which had a 1.87, 0.06, and 2.47% difference to observed . The 1,1’-diethyl-2,2’-carbocyanine iodide dye was also investigated with a carbon chain of 5 and a of 604 nm, which had a longer wavelength than 1,1’-diethyl-4,4’-cyanine iodide although it had a shorter carbon chain. The CdSe nanoparticles are represented by the quantum dot model which are crystal semiconductors that confine excitons to three spatial directions. CdSe nanoparticles embedded in glass plates were heated to 700°C between 0.5 and 4 hours causing crystal size to vary. Crystal size was determined from the corrected absorption spectra by fitting the curves to a parabola using Origin and changing wavelength to energy (eV). The average crystal radii for each time was found at and calculated to be 2.027 ±0.0005, 2.661 ± 0.0008, 3.196 ± 0.0004, and 3.349 ± 0.0004 nm for 0.5, 1, 2, and 4 hours, respectively. Particle size increased with prolonged heating but slowed after 2 hours.
Varying amounts of dye (0.001-0.008 mmol) were added to a 100 mL flask and then diluted with methanol. The resulting solutions were further diluted by adding 1 mL solution to a 10 mL flask and diluting to the line with methanol with a final concentration that ranged from 1×10-5 to 8×10-5 M. UV-Vis spectra for each dye were obtained using a Jasco V-530 spectrophotometer scanning in the 450 to 800 nm range.
Table 1. The conjugated dyes studied in this experiment with their structure, molecular weight, CAS and amount used to make each solution.
|Reagent||Structure||Molecular Weight (g mol-1)||CAS||Dye Content||Amount (g)|
Figure 1. Color of dye solutions when diluted to 100 mL. Left to Right: 1,1’-Diethyl-4,4’-cyanine iodide, 1,1’-Diethyl-4,4’-carbocyanine iodide, 1,1’-Diethyl-4,4’-dicarbocyanine iodide, 1,1’-Diethyl-2,2’-carbocyanine iodide
CdSe quantum dots embedded in glass slides were obtained that had been heated to 700°C for varying amounts of time (0.5 to 4 hours). UV-vis spectra were obtained using a Jasco V-530 spectrophotometer scanning in the 400 to 700 nm range. OriginLab® was used to correct and analyze spectra.
Figure 2. CdSe glass slides heated to 700°C for 0.5, 1, 2, and 4 hours.
Table 2. The number of carbon atoms in each conjugated polymethine chain with the observed and theoretical ,𝜆–𝑚𝑎𝑥. and the percent difference.
|Conjugated Dye||Number of Carbon Atoms, P||Observed(nm)||Theoretical (nm)||Percent Difference|
Table 3. The maximum absorbance wavelength observed for CdSe embedded in glass slides heated to 700°C with calculated maximum photon energy (eV) and average particle radius (nm).
|Heating Time (Hours)||(nm)||Maximum Photon Energy (eV)||Particle Radius (nm)|
|0.5||506.7 ± 0.1||2.4472 ± 0.0005||2.027 ± 0.0005|
|1||562.2 ± 0.1||2.2056 ± 0.0004||2.661 ± 0.0008|
|2||590.6 ± 0.1||2.0996 ± 0.0004||3.196 ± 0.0004|
|4||596.3 ± 0.2||2.0795 ± 0.0004||3.349 ± 0.0004|
UV-visible spectroscopy was used to observe quantum properties of dyes and quantum dots. The emission energy of the conjugated dyes and the quantum dots was observed to be lower than the excitation energy observed by the color. The loss of energy occurred due to vibrational motion. The dyes emit color in the visible region due to the conjugated polymethine chain explained by the particle in a one dimensional box model. From Figure 3 and Table 2 it is seen that less energy is needed to excite an electron to the lowest excited state as conjugation increased with of 590, 706, and 812 nm for cyanine (P=7), carbocyanine (P=9), and dicarbocyanine (P=11), respectively. The was accurately predicted by the free electron model with theoretical of 579, 706, and 832 nm which had a 1.87, 0.06, and 2.47% difference to observed . for cyanine, carbocyanine, and dicarbocyanine. The dye 1,1’-Diethyl-2,2’-carbocyanine iodide had the shortest conjugation chain of P=5 but had 2,2’ bonding of the conjugated chain which stabilized the molecule and resulted in lower excitation energy needed.
The CdSe nanoparticles are characterized by the quantum dot model which restricts movement in three directions. Absorption spectra has a Gaussian shape due to the Gaussian particle distribution shown in Figure 4. The represents the average particle size after an amount of time heated to 700°C. Less energy was needed to reach the lowest excited energy state as the particle size grew. Longer heating resulted in larger particle size, Figure 5, which resulted in absorption peaks to be shifted to lower energy, Figure 4. The radii were 2.027 ± 0.0005, 2.661 ± 0.0008, 3.196 ± 0.0004, and 3.349 ± 0.0004 nm for 0.5, 1, 2, and 4 hours, respectively. Studying dyes and quantum dots has important applications as intermediates in medicine, such as aspirin and producing light-emitting diodes.
 M. Halpern and G.C. McBane, Experimental Physical Chemistry, 3rd ed., W.H. Freedman and Company, New York, 2006. Experiment 37.
 Atkins, P., J. De Paula “Physical Chemistry”, 9th ed., W. H. Freeman, New York (2010)
 Filin, A.; Absorption Spectra of Nano-Particles, Handout, Powerpoint.
 Garland, Carl W., Nibler, Joseph W., Shoemaker, David P. (SGN). Experiments in Physical Chemistry. 8th ed., pp. 393-398, McGraw-Hill Companies, Inc., New York (2009).
Figure 6A. Uncorrected absorption spectra of CdSe quantum dots.
P is the number of carbons and 𝛼 is a constant dependent on the set of dyes, in this experiment 𝛼=0 so that we can compared simple bond-orbital models with the free electron model.
Converting wavelength to energy,