## Transcribed Text

Absorption Spectra of Conjugated Dyes
Objective
To obtain absorption spectra of three related conjugated dycs and uso the
data model the system with the quantum mechanical particle ina box problem
Introduction:
basic problem in quantun mechanics is that of the "particle ina box In
thisproblem particle mass Isconstrainedto move one dimension along
flength The particle has constant otential thergy, and isconfined to
a
box because the potential chergy infinite beyond the linc.
According quantum mechanic the encrgy the particli must be quantized
meaning that can only have spedific values andis equal to
n2h2
(1)
where the quantized chergy, nis the quantum number (which isa positive
intege= and h Planck constant- These energies definc cheroy levels the
partide lithe particle moves from one energy leve another the change chergy,
AE s the difference energics between states with different values ofn other
words
AE
8mL²
8mL2
(2)
Ifthe energy leveloris defined as nen+ the cquation an be simplified to
AE
+1)
(3)
8ml2
The energy change, AE, between to states can be observed bectroscopically
Since wavelength the bsorbance maximum
Absort anco spectroscopy apply the particleina box model to systems which
meet the constraints of that model In other words
AE
(4)
This equation can thenbe solved for the wavelength
SmcL²
2
(5)
h(2n +1)
Inthis exteriment. the particle ina hox model will bo applied to serics of
conjugated (and coloFful) dycs- Conjugatedsystemshave serics alternating
single double bonds electron that are part This conjugates system
are
free move anywhere nlong conjugated system. Figure shows three
tugated dycs thatwillb studiedinthis Looking figuro you
can sce that conjugated system exists between the two nitrogen atorns dve
there are three pairso electrons the conjugated system (the non bonding pair
on the hitrogen andthe two bonding pairs between the two nitrogen atoms Dyc
has four pairs of electronsand dyc 3 has five pairs.
02.00
CH,
can
O
C,H,
C,H,
e
Ondo
C,H,
Figure
The
1.1"
2,2°
Toapply the particle in to these conjugate dyes voll will
measure
absorption
maxima
using
spectroscopy determing the length of the conjugated (L). then calculate a
bond leneth for the conjugated system- The absorbance maxima represent the
transitions between the highest occupied molecular bital (HOMO) tothe lowest
unoccupied molecular orbital(t LUMO). Because each energy level c hold
electrons can determine quantum number HQM06 taking the
totalnumber electrons the system and dividing by For
example, dyc has three pair electrons the conjugated system so the HOMO Is
n=3 Using the absorbance maxima and value of for each dye molecule. you will
determine valuco rashon bond length-
Theoretically, carbon carbon dlength should be equal that of
benzene (b=0) nm). This means that should be equal tob times the number of
bonds (b) between the two nitrogen atoms plus) factor accolnt for the fact
that electrons can spillover past the nitrogen atoms other words =bls tlext
Defining asb 2(p -1), where Isthe number oft electron pates, equation 5can
berewritten
(6)
h(2p+1
You willuse this equation to determine your theoretical carbon carbon bond
length.
Experimental:
Obtain 10+Maglicolls olutions of cach dve from Volle instructor. Add
0-30 dyc. add 2.70 mL wate* and mix well. (This will dilute the dyes
factor 10). Using lvv/Vis tometer obtain spectrum between 400
and 800 nm. Record the wavelength of maximum absorbance for cach dye (Ifthe
absorbance greater that 08 further dilute the dvc by factor of and recorda
new spectru
Safety Dyc molecules can be carcinogenic Wear gloves when working with
the dyes and dispose of any contan mated material including pipette tips and paper
towels used to clean up spills) as haza+dous waste
Data Analysis:
1.
Use equation 5to leterminça value of for cach of Voll dyc molecules
2.
Plot the number each molecule and the equation
determine value of both andlest from your data-
3
Prepare plot Using Igor Pro. fit qullation the plot (Yoll will
need to defing Vour own equation Igor Pro- You will be solving for lext the
resto the values includingla (=0 139 nm are known Watch your units
Discussion:
Comment the following
1. Why the benzene carbon carbon bond length used the theoretica value
model?
2. Compare the valucsofl obtained from item 2 in the Data Analysis the
bond length of benzene and discuss.
3 Compare the values of lest obtained from items and in the Data Analysis-
The parameter counts for the fact that the nitt ogen toms not the
hard walls particleina box, with infinite potential energy. Insteadt
electrons can "spillover' pasta length ofa boxcalculated lasa multiple of
carbon- carbon bond lengths. How does your value Fless compare with that of
Docs
the
value
make
sense
5
Agng increases absorption wavelength decrease but as L
increases wavelength should increase Which of these
two
effectsis dominant the system studied (and why?)
6-
From the absorption spectra the dycs comment on why the dyc solutions
are colors you observed
References
Halpern, AM. McBane C.C. Experimental Physical Chemistry Third Edition New York W.H Freeman
d Company 2006.

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