Phosphorescence ErythrosinPMMA

7/30/2019 Phosphorescence ErythrosinPMMA

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Fluorescence and Phosphorescence of Erythrosin

Purpose

The fluorescence, delayed fluorescence and

phosphorescence of erythrosin immobilized in

polymethylmethacrylate (PMMA) will be observed. The dye

is immobilized to promote phosphorescence. Lifetimes of

delayed fluorescence and phosphorescence will be

measured. Temperature dependence of the intensities of

delayed fluorescence and phosphorescence will give the

energy gap between the lowest triplet state and the first

excited singlet state of erythrosin. Quantum-chemical

calculations will give the singlet-singlet absorption energy and wavelength.

Introduction

Fluorescence and phosphorescence are both examples of luminescence.1 Erythrosin is a highly

colored molecule that absorbs light near 500 nm and emits longer wavelengths. Fluorescence is

fast, occurring on the order of nanoseconds. Phosphorescence occurs more slowly, in about a

millisecond. Phosphorescence is usually observed at low temperatures, but certain conditions

favor observation of room temperature phosphorescence (RTP) of a dye:2

elimination of solvent molecules from the immediate vicinity of the dye

a rigid matrix surrounding the dye molecule

For this experiment erythrosin is dissolved in polymethylmethacrylate (PMMA).

RTP of erythrosin is accompanied by "delayed fluorescence,"2,3 which is normal fluorescence

except that the fluorescent state is populated by transfer of molecules from the phosphorescent

state. That transfer is temperature dependent. Its activation energy Ea is approximately the energy

gap between the triplet and excited singlet states. Delayed fluorescence and room temperature

phosphorescence spectra of erythrosin B have been published for silica gel (reference 4, Figure 1),

for aqueous solutions (reference 5, Figure 2), for various plastics (reference 6), and for sucrose

films (reference 7, Figure 1). Wavelengths for erythrosin in PMMA have also been published. 5

erythrosinPMMA.odt 1

Figure 1: erythrosin

http://www.d.umn.edu/~psiders/courses/chem4644/labinstructions/erythrosinPMMA.pdf


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The diagram shows absorption from the singlet ground state S0 to excited vibrational levels of the

first excited singlet state, S1. The initial absorption is

followed by rapid relaxation to the ground vibrational

level of S0. Ordinary fluorescence corresponds to the

emission of a photon as the system returns rapidly

from S1 to S0. Some molecules, rather than

fluorescing, make a transition from S1 to the excited

triplet state T1. That transfer is "intersystem crossing."

T1 is initially formed in an excited vibration level that

rapidly relaxes. Phosphorescence occurs as T1 returns

to S0, a slow process because an electron spin flip is

required. Some of the molecules in T1 acquire enough

vibrational energy to back-intersystem-cross to S1

rather than phosphorescing. From S1 they then fluoresce. This fluorescence occurs long after the

initial fluorescence is finished so it is called "delayed fluorescence."

Ideally, delayed fluorescence occurs at the same wavelength as fluorescence but with the same

lifetime as room temperature phosphorescence. Also, delayed fluorescence intensity depends on

temperature because an activation barrier slows transitions from T1 to S1.

Kinetic scheme

Fluorescence is much faster than DF and RTP so it is regarded as instantaneous. The initial

concentration of molecules in T1 is established before fluorescence is complete, so at t=0. Let

[T1]0 be the initial concentration of molecules in T1. For simplicity, neglect nonradiative decay and

quenching of T1 and S1.

d[T1]/dt = -kP[T1] kTS [T1] (1)

where kTS is the rate constant for the back-isc T1 to S1 transfer, a first-order process. The first-

order rate constant for phosphorescence is kP. Solving for [T1],

[T1] = [T1]0 e

-(kP+kTS)t

= [T1]0 e-t /RTP

(2)

where RTP is the lifetime for room temperature phosphorescence. RTP will be measured directly in

this experiment. In terms of the present kinetic scheme, RTP=1/(kP+kTS). The net rate of formation

of the excited singlet is


Figure 2: Jablonski diagram


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d[S1]/dt = kTS[T1] kF[S1] (3)

where kF is the rate constant for fluorescence; kF= 1/ F is large so d[S1]/dt0 at all times.

Therefore,

[S1] (kTS/kF) [T1] (4)

Let iDF denote the intensity of delayed fluorescence and iRTP the intensity of room temperature

phosphorescence.

iDF = kF[S1] = kTS[T1] (5)

iRTP = kP [T1] (6)

The concentration of triplet cancels from the ratio of DF to phosphorescence intensities.

iDF/iRTP = kTS / kP (7)

The triplet-to-singlet back intersystem crossing rate constant is activated. That is,

kTS = ATS e- Ea/(RT) (8)

where ATS is an Arrhenius prefactor, T is the absolute temperature and E a is the activation energy.

Therefore,

` ln(iDF/iRTP ) = ln(ATS/kP) (Ea/R) (1/T) (9)

The activation energy can be measured by graphing ln(iDF/iRTP) versus 1/T, with T in Kelvin.

Quantum-chemical calculations will complement the spectroscopic measurements. Energies of thetwo singlet states, S0 and S1 will be calculated. Challenging aspects of the calculation are the four

iodine atoms in erythrosin and the need for an excited-state energy. The difficulty with iodine is

that heavy atoms have many electrons, large electron correlation energy, and likely relativistic

effects. Pseudopotentials will be used, which replace iodine core electrons with potential-energy

functions and include relativistic corrections. The difficulty with excited states is that the usual

density-function and Hartree-Fock calculations calculate only ground states. We will use time

dependent density functional theory to calculate the energy of the first excited singlet state. The

excited-state energy will be calculated at the ground-state geometry, which is consistent with the

Franck-Condon principle. After energy difference between S1 and S0 is calculated and convertedto convenient units such as Joules, one may calculate the wavelength simply from = hc/E.



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Materials and Reagents

Solid solution of erythrosin B in PMMA. The dye erythrosin B has the formula C20H6I4Na2O5,

formula mass 880 g/mol. Solid solutions of erythrosin in PMMA were prepared as described

by Lettinga, Zuilhof and van Zandvoort.

6

Choose a samplefrom the drawer below the fluorescence instrument.

Samples E6 and E7 are convenient. Should you choose

sample BT8 note that it is cut to fit the sample

compartment directly, not in a cuvette.

The drawing of erythrosin at right8 is planar, for simplicity.

In three dimensions, the iodine-bearing rings are planar and

are perpendicular to the carboxylate group and its phenyl

ring.

A plastic cuvette. Use a four-side-clear cuvette for fluorescence. A cuvette that is frosted or

ribbed on two sides is not suitable for fluorescence measurements.

Recommended: something to read while waiting for temperature equilibration and

phosphorescence scans.


Figure 3: planar


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Procedure

Record the fluorescence spectrum

Use the Varian Cary Eclipse instrument. Choose SCAN mode. Locate a four-sided

plastic or glass cuvette. Zero the instrument with the water-filled cuvette. Under

Setup, set the excitation wavelength to 500 nm. Select 5 nm emission slit width and

10 nm excitation slit width. The detector sensitivity is set using the photomultiplier

tube (PMT) voltage on the Options tab. I suggest setting PMT to medium. Then one

may switch to Low or High as necessary to bring the peak signal on-scale. Once

could also increase intensity by choosing a larger (e.g., 20 nm) excitation slit.

Transfer a block of erythrosin in PMMA into the cuvette, pushing out water so that the sample is

surrounded by water. The water will give good thermal contact. Record the fluorescence emission

spectrum from 530 to 760 nm. If the spectrum is off scale, go back to Setup, choose Options, andreduce the photomultiplier tube (PMT) voltage and or the excitation slit width.

Record the wavelength, F, of maximum fluorescence intensity.

Record the DF and RTP spectrum

Under Setup on the SCAN application, select Phosphorescence. Set the following:

Table 1. phosphorescence settings.

excitation wavelength 500 nm

start scan 530 nmstop scan 760 nm

excitation slit 10 nm wavelength range for the exciting light

emission slit 10 nm wavelength range for a single emissionintensity

tavg 0.1 seconds time over which to average the emissionintensity

4 nm interval between wavelengths in the spectrum

PMT medium "high" gives greatest intensity but may take

the signal off scale (i.e., above 1000).Turn on the spectrophotometer's temperature control unit. Turn it on with the switch on its back.Start the water pump after making sure it is submerged. Verify that water is circulating throughthe hoses. The circulating water does not heat or cool the sample directly; rather, it is to cool thePeltier temperature unit that controls the sample temperature. If water does not circulate turn offthe temperature control unit and solve the circulation problem.


Figure 4:erythrosin in

cuvette


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Set the temperature to 40C. You can set this manually just by depressing the up- or down- arrow

key on the top of the temperature control unit. Wait until the temperature display reaches 40C,

then wait another 15 minutes. (This might be a good opportunity to use item three in the

Materials and Reagents list.) Then use distilled water to zero the instrument.

Put your sample in the sample holder. Click START

to start the scan. A phosphorescence scan takes longer

than a fluorescence scan so be patient. You may need

to autoscale the Y axis during the run to see the

spectrum if its intensity is low. You should see two

peaks. The longer-wavelength peak is room

temperature phosphorescence (RTP), the shorter is

delayed fluorescence (DF). Note max for each. That

for DF isDF; that for RTP is RTP. You should find that DF is the nearly the same as F, the max

for fluorescence.

Measure lifetimes

Choose the Varian/Cary Eclipse "lifetimes" application. Set the following:

Table 2. Cary lifetime settings

excitation wavelength 500 nmemission wavelength RTP. to measure RTPflashes 25 25 flashes of the excitation lamp

slits 20 nm both slits wide to increase intensity

delay time 0.2 ms This waits until the lamp is off (about 0.1 ms) be-fore collecting emission data

gate time 0.05 ms time between measurementstotal time 3 ms

number of cycles 50 averaging over many cycles improves signal tonoise ratio

on the "Options" tab set PMT voltage "high" for maximum intensity, reduce tomedium if signals is off scale.

on the "analyze" tab stop = total time, single exponential, check "auto calculate"and check "Lifetimes"

Click on START. After all 50 cycles, the lifetime ("tau") will be calculated and printed in the text

window. The fitted line should also be drawn through the data points. If the fit is not displayed on

the graph, go to "Trace Preferences" and check the "SingleExpFit" trace.

Repeat the procedure at DF to measure the lifetime of the delayed fluorescence, DF. One expects

to find DF RTP .


Figure 5: schematic spectrum


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Temperature Dependence of DF and RTP intensities

Return to the SCAN application. Check that the parameters are reset to the values you used to

record the DF and RTP spectra. In order to display multiple spectra on a single graph you may

check the "Overlay Traces" option under Setup. Zero the instrument with a cuvette of distilled

water.

Record DF and RTP spectra at the following

temperatures: 40, 50, 60, and 70C. Changing

temperature 10 takes about 15 minutes, based on

trials run with a 3-mL sample of liquid water. The

graph at right shows the instruments displayed

temperature and the actual sample temperatures when

a sample initially at 20C was heated or cooled by 10.

Based on these data, a 20-minute wait is suggestedafter calling for a 10 temperature change.

On each temperatures spectrum, measure iDF and iRTP. Measure these intensities at DF and RTP,

the same maximum-intensity wavelengths that you identified when you first recorded DF and RTP

spectra at 40C.

Print the spectra.

When you are done with the fluorescence instrument, please

Remove the erythrosin/PMMA sample from the fluorescence instrument, being careful

because the final temperature of 70C is uncomfortably hot.

Turn off the temperature-control unit (switch is on the back)

Turn off the water pump

Turn off the spectrophotometer

Calculate the activation energy for T1 to S1 back intersystem crossing.


Figure 6: temperature equilibration


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Quantum-Chemical Calculations

For this lab, calculations are conveniently done using GAMESS, which is installed on several

computers in room 338 and can also easily be installed on a personal computer. The calculations

require several hours. You may want to start calculations and then let them run overnight. Sharingcalculations with your lab partner could also be good.

Step-by-step instructions follow.

1. Draw the erythrosin molecule in the form of the di-sodium

salt but with H atoms in place of the Na atoms. Start with H in

place of Na to make the initial optimization fast.

2. Optimize using the semi-empirical PM3 method.

$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZE $END$BASIS GBASIS=PM3 $END$STATPT OPTTOL=0.0001 NSTEP=400 $END

You may find energy approximately -188 Hartree, the heat

of formation about 50 kJ/mol.

3. Replace two H atoms with Na. Make the O-Na bond length 2.05 Angstroms, which is the O-

Na distance in diatomic NaO. Do not optimize this structure with PM3, because PM3 handles the

sodium atoms poorly. Rather, set up an equilibrium-

geometry calculation using Hartree-Fock Theory, the Hay-

Wadt basis set (with d polarization functions), and Hay-

Wadt pseudopotentials.

$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZEMAXIT=30 MULT=1 PP=HW $END

$SYSTEM MWORDS=100 $END$BASIS GBASIS=HW NDFUNC=1 $END

$SCF DIRSCF=.TRUE. $END$STATPT OPTTOL=0.001 NSTEP=200 $END

It is the command PP-HW that selects Hay-Wadt

pseudopotentials to replace core electrons. The command

OPTTOL=0.001 relaxes the default tolerance for geometry

optimization. Submit the job. Expect about 6 hours to optimize the geometry.

Use the optimized geometry (i.e., the coordinates below EQUILIBRIUM GEOMETRY in the

output file) for the TDDFT calculation below.


Figure 7: Erythrosin B.R. T. Bailey, et al.,

Analytica Chimica Acta, 2003

Figure 8.


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4. Calculate the excitation energy using TDDFT. Calculations involving excited states should

include electron correlation, so we will use the B3LYP density functional. Correlated

calculations, especially those for an excited state, require larger basis sets. We will use the MCP-

DZP basis set and its associated MCP pseudopotentials. Coordinates of the atoms should be

taken from the previous geometry optimization.

$CONTRL SCFTYP=RHF RUNTYP=ENERGY MAXIT=50TDDFT=EXCITE DFTTYP=B3LYP PP=MCP ISPHER=1 $END

$BASIS GBASIS=MCP-DZP $END$SCF DIRSCF=.TRUE. $END$SYSTEM MWORDS=100 $END

The command ISPHER=1 tells GAMESS to use spherical-harmonic functions for angle-

dependence of the basis functions, rather than the Cartesian functions (e.g., x for a p x orbital) that

are used by default. The directive MWORDS=100 sets aside nearly a gigabyte of RAM for the

calculation.

Expect the calculation to take about 7 hours. In the output file, near the end, should be a small

table of results. E will be given in eV.

SUMMARY OF TDDFT RESULTSSTATE ENERGY EXCITATION TRANSITION DIPOLE, A.U. OSCILLATOR

HARTREE EV X Y Z STRENGTH0 A -751.XXXXXXXXXX 0.0001 A -751.XXXXXXXXXX X.XXX -0.0290 -0.0304 -0.1943 0.002

SELECTING EXCITED STATE IROOT= 1 AT E= -751.XXXXXXXXXX AS THE STATE OF INTEREST.

5. Convert E to in nm.



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Lab Report

Include all spectra.

Compare your fluorescence, DF and RTP wavelengths to literature values (which may be

for matrices other than PMMA).

Compare your DF and RTP lifetimes to each other.

Compare Ea to a literature value for erythrosin in some solid matrix.

Report your DFT B3LYP energies of the ground and excited states, your E, and your

wavelength. Compare your calculated wavelength to an experimental or literature max.

References

1. Engel, Thomas Quantum Chemistry and Spectroscopy, Pearson Benjamin-Cummings, San Francisco,

2006, Sections 15.5-15.8.Silbey, R. J.; Alberty, R. A.; Bawendi, M. G.Physical Chemistry, 4th ed., John Wiley & Sons, Inc.:

New York, 2005; Section 14.8.

2. Wayne, R. P.Principles and Applications of Photochemistry; Oxford University Press: Oxford, 1988;

pages 91-92.

3. Levy, D.; Avnir, D. Room temperature phosphorescence and delayed fluorescence of organic molecules

trapped in silica sol-gel glasses. J. Photochem. Photobiol. A: Chem.1991, 57, 41-63.

4. Lam, S. K.; Lo, D. Time-resolved spectroscopic study of phosphorescence and delayed fluorescence ofdyes in silica-gel glasses. Chemical Physics Letters1997, 281, 35-43. Here is a link to the article.

doi:10.1016/j.physletb.2003.10.071 (UMD username and password may be required. If that does not

work, see the link under CHEM 4644 lab instructions.) Pertinent values from the paper are listed here.

All values are for erythrosin B in sol-gel silica.

F = DF = 5613 nm. RTP = 6833 nm. E = 0.40 eV (Ea)

F = 1.1 ns DF = 230100 s RTP = 23010 s on page 3701.

5. Duchowicz, R.; Ferrer, M. L.; Acuna, A. U. Kinetic spectroscopy of erythrosin phosphorescence and

delayed fluorescence in aqueous solution at room temperature. Photochemistry and Photobiology1998,

68(4), 494-501. Table 1 give RTP in PMMA.

6. Letinga, Minne Paul; Zuilhof, Han; van Zandvoort, Marc A. M. Phosphorescence and fluorescence

characterization of fluorescein derivatives immobilized in various polymer matrices. Physical

Chemistry Chemical Physics2000, 2, 3697-3707. PMMA preparation is briefly described.

7. Pravinata, Linda C.; You, Yumin, Ludescher, Richard D. Erythrosin B phosphorescence monitors

molecular mobility and dynamic site heterogeneity in amorphous sucrose. Biophysical Journal, 2005,

88, 3551-3561.

8. Bailey, R.T.; Cruickshank, F.R.; Deans, G.; Gillanders, R.N.; Tedford, M.C. Characterization of afluorescent sol-gel encapsulated erythrosin B dissolved oxygen sensor.Analytica Chimica Acta, 2003,

487, 101-108.

http://dx.doi.org/10.1016/S0009-2614(97)01172-Xhttp://dx.doi.org/10.1016/S0009-2614(97)01172-Xhttp://dx.doi.org/10.1016/S0009-2614(97)01172-X

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Phosphorescence ErythrosinPMMA