Physics and Astronomy

Deethylation and Degradation of DryRhodamine B Films Under Visible Light

Irradiation

Bachelor Thesis

Duncan de Vos

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Supervisors:

Dr. F.T. RabouwUtrecht University

S.O.M. Hinterding, MSc.Utrecht University

January 16, 20191RhB and RhB after five minutes of irradiation

(a) Structure of rhodamine B

(b) Colourchange of rhodamine

Abstract

Rhodamine B is a commonly used fluorescent dye in many different fields of research. It is a dye thatcan change colour by losing ethyl groups, deethylation. It is also possible for rhodamine B to degradeunder irradiation, making it unable to emit light. Previously it was seen that rhodamine B in solutiononly undergoes reactions, that lead to the loss of ethyl groups, when a photocatalyst is present undervisible light irradiating. However recently it has been seen that these reactions can occur without thepresence of a photocatalyst for dry films of rhodamine B. To further understand the reactions that takeplace, we have studied and modeled the photo induced degradation and deethylation rhodamine B undervarious conditions. This was done by illuminating dry films of rhodamine B in contact with glass andor air and recording the spectrum the dye emits at several points in time during irradiation. With thesespectra we modeled the dynamics of deethylation and degradation using a simple rate-equation model.The dye in contact with glass degrades faster than when not in contact with glass, and changes colourwhen in contact with both glass and air. This means dry rhodamine B can react without the presenceof a photocatalyst under visible light irradiation.

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Contents

1 Introduction 3

2 Theory 32.1 Fluorescent molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Fluorescence microscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Rhodamine B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Experimental Section 53.1 Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 Optical setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.1 Setup for measuring spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3 Procedures and Analyses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3.1 Rhodamine films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3.2 Spectral measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3.3 Data analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Results and Discussion 7

5 Conclusions 10

6 Acknowledgments 10

A Appendix IA.1 Rhodamine B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

A.1.1 Rhodamine B on glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IA.1.2 Rhodamine B on PMMA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IVA.1.3 Rhodamine B on glass reacted with HDMS. . . . . . . . . . . . . . . . . . . . . . . . . VIIA.1.4 Rhodamine B on glass under PMMA. . . . . . . . . . . . . . . . . . . . . . . . . . . . XA.1.5 Rhodamine B in between two layors of PMMA. . . . . . . . . . . . . . . . . . . . . . . XIII

A.2 Rhodamine 110. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVIA.2.1 Rhodamine 110 on glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVIA.2.2 Rhodamine 110 on PMMA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVIA.2.3 Rhodamine 110 on glass reacted with HDMS. . . . . . . . . . . . . . . . . . . . . . . . XVIIA.2.4 Rhodamine 110 on glass under PMMA. . . . . . . . . . . . . . . . . . . . . . . . . . . XVIIA.2.5 Rhodamine 110 in between two layors of PMMA. . . . . . . . . . . . . . . . . . . . . . XVIII

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1 Introduction

Many large industries rely on chemical reactionsfor production. The efficiency of these reactions areimportant to both their profit and to the environ-ment. Finding catalytic reactions that can boost thisefficiency could therefore be very valuable. One waywe try to improve catalytic reactions is by doing fun-damental studies on single particles [1]. To modelcatalytic reactions it can be useful to use fluores-cent dyes, they provide a large resolution in bothtime scale and positional scale. Rhodamine B isone of the dyes that can be used, and it has theadded benefit to change colour under certain circum-stances giving us insight in the reactions at hand.It is thus of great importance to us to know underwhat circumstances rhodamine B reacts. Previously,papers have been published looking at the fluores-cent emission and luminescent strength of rhodamineB. Watanabe et al. [2] found that aqueous solutionsof rhodamine B are considerably stable under visiblelight excitation, but when powdered Cadmium Sul-fide is suspended in the solution, rhodamine B under-goes N-deethylation. Rhodamine B changes colourby deethylation. Wu et al. [3] used multiple meth-ods to show that RhB in an aqueous solution of ti-tania undergoes both N-deethylation and degrada-tion of the molecules (bleaching). Qu et al. [4] alsoresearched solutions of RhB and titania. They toofound both N-deethylation and degradation. Theyattribute the N-deethylation to OOH∗ or HO∗ rad-icals. Xeufeng et al. [5] looked at solutions ofRhB with vanadate and/or platinum species present.They found the following; RhB undergoes efficient N-deethylation in the presence of VO+

2 and degradationin the presence of Pt(IV). Zhuang et al. [6] looked atN-deethylation and degradation of aqueous RhB solu-tions in the presence of thin titania films with varyingsurfaces. All these studies have in common that forN-deethylation and degradation of RhB in solutiona photocatalyst needs to be present. Recently how-ever it has been observed by my supervisor that dryrhodamine B changes colour under visible light irra-diation without the presence of a photocatalyst. Thisraises the question: What are the processes involvedin deethylation and degradation of dry rhodamine Bfilms?

Here we study dry films of rhodamine B undervisible light irradiation in different environments bymeasuring spectral shifts. We fabricated a series ofsamples of rhodamine B spin coated on glass slides.By coating the slides with a polymer film (polymethylmethacrylate; PMMA), before or after spin coat-ing rhodamine B on them, we can control whether

rhodamine B is in contact with the glass substrateand/or air. We illuminate these samples and mea-sure their spectra at several points during illumina-tion. Using these spectra we observe whether rho-damine B reacts to glass and whether it reacts withair and what these reactions cause: N-deethylation ordegradation. Our findings show that glass allows therhodamine B to degrade faster than when it is not incontact with glass. When rhodamine B is in contactwith both glass and air we see both fast degradationand N-deethylation. When it is not in contact withglass and air, we observed no N-deethylation and aslow degradation.

In the remaining sections we will first providesome background information about the relevantphysics at play here. Following this we describe themethods of measuring the processes. Subsequentlywe present the findings and these will be discussed.Finally we will present our conclusion. In the ap-pendix, spectra of each measurement environmentof rhodamine B as well as all the rates of deethyla-tion and degradation corresponding with these spec-tra can be found. Included will also be the spectra ofrhodamine 110 in the different environments.

2 Theory

In this section we will present an overview of priorknowledge used throughout this thesis. Firstly wewill give a short explanation on fluorescence by or-ganic dye molecules. This is relevant to us becauserhodamine B is a fluorescent molecule and only un-dergoes deethylation and degradation under light ir-radiation. Then we give a short overview of fluores-cence microscopy, which is what we used to do ourmeasurements. Following this we show what we knowabout the degradation and deethylation of rhodamineB from previous research and what reaction mecha-nisms researchers have proposed.

2.1 Fluorescent molecules.

When a photon interacts with a molecule, it canin some cases cause the molecule to emit light. Inthis case one calls the molecule fluorescent. The waythis works is the following: a molecule gets hit by aphoton. If the photon has enough energy to promotean electron to a higher orbital, the molecule absorbsthis photon. One of its electrons will then be put ina state with a higher energy. This is illustrated inthe first two steps of figure 2. The minimum energyof the photon must be the energy needed to bridgethe gap from the Highest Occupied Molecular Orbital

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(HOMO) and the Lowest Unoccupied Molecular Or-bit (LUMO). Some time later this electron can returnto the HOMO and doing so emit a photon, becausethe total energy must be conserved. The energy ofthe emitted photon is that of the difference in en-ergy between the state the electron was in and thestate the electron falls back in. This energy is usu-ally around the same energy as the energy differenceof the HOMO and LUMO of the molecule.

A B C D

Figure 2: The absorption and emission of a fluores-cent molecule: A) a photon hits a molecule, B) thephoton is absorbed by the molecule and one electrongoes to a higher energy state, C) a photon is emittedand the electron falls back to a lower energy state, D)there is no electron in the LUMO and the moleculeis back in its low energy state.

What can also happen is that after getting pro-moted to a high energy state, the electron transfersto another molecule which has a state for the electronwith a lower energy. This is shown in figure 3. Whenthis happens the fluorescent dye will have a charge ofplus one. For rhodamine B having this charge allowsit to undergo reactions that lead to deethylation anddegradation.

A B C

Figure 3: Transfer of an electron from one moleculeto another, A) a photon hits a molecule, B) one elec-tron goes into a higher energy orbital, C) the electrontransfers to another molecule.

2.2 Fluorescence microscopy.

When looking at fluorescent dyes one can do mea-surements on the emitted light. This can be done byredirecting the light emitted by the dye to a cameradirectly or to a camera through a spectrometer. Inthe latter case the spectrometer will separate lightof different wavelengths making it possible to record

the spectrum of the dye. To excite the dye however,one needs to irradiate it with light. It is importantto ensure this light does not reach the camera andinterfere with the emitted light by the sample. Todo this one uses three types of filters; longpass filters(LP), shortpass filters(SP) and dichroic mirrors(DC).Longpass filters reflect light of wavelengths shorterthan a certain value and allows other light to passthrough. Shortpass filters reflect light of wavelengthslonger than a certain value. Dichroic mirrors comein the shortpass and the longpass variant. They aremade to work under an angle of 45°. What one cando using these filters is excite the dye with light origi-nating from a light source. In the path from the lightsource to the dye one can place a shortpass filter fol-lowed by a dichroic mirror that reflects only light inthe transmission range of the shortpass filter and re-flective range of the dichroic mirror to the dye. Thedye then gets excited by this filtered light and emitsits own light. This emitted light and some reflectedscource light then hit the dichroic again and most ofthe source light is reflected away. The wavelengths oflight emitted by the dye that the dichroic allows topass through go through. Behind this dichroic, oneplaces a longpass filter to make sure the light origi-nating from the light source is filtered out. This isschematically depicted in figure 4 with the filters usedin our measurements.

Figure 4: Light from the light scours hits a SP filterwith a 532nm cut-off, passes through and hits a DCmirror with a cut-off at 520nm, then hits a samplewhich emits light that hits the DC mirror and passesthrough a LP with its cut-off at 519nm after which aspectrometer separates light of different wavelengthsand redirects the separated light to a camera.

When using this setup however one needs to makesure that the filtered light that reaches the dye, fallsin the absorption range of the dye or the dye will not

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be excited. Also one needs to pay attention not to fil-ter out the light emitted by the dye on the way to thecamera. For instance: If one wants to look at Rho-damine 110 one might do the following. One places a590 nm SP filter and 581 nm DC(LP) mirror in be-tween the light source and the dye and a 590 nm LPfilter in between the dye and the camera. Indeed thedye does get excited by the light and no light fromthe light source will reach the camera and interfere.However because Rhodamine 110 mainly emits lightin the range of wavelengths below 550nm, it gets fil-tered out by the LP filter and thus, no light reachesthe camera.

2.3 Rhodamine B

Rhodamine B is a fluorescent dye. It can be ex-ited and then emit light. The structure of rhodamineB is shown in figure 5. There are four ethyl groupsattached to nitrogen atoms as seen in the the struc-ture. These ethyl groups can under certain circum-stances react and form acetaldehyde, leaving the rho-damine B molecule an ethyl group poorer (deethyla-tion). This changes the structure of the rhodaminemolecule and in doing so it changes the wavelength oflight it emits. For each ethyl group the molecule hasless, the spectrum is slightly shifted towards lowerwavelengths. To get an idea of how large the shift ofthe spectrum is we use a solution of rhodamine B inethanol as an example. For a solution of rhodamineB in ethanol the peak of the spectrum shifts from 571nm to 525 nm[7]. Once all four ethyl groups have re-acted the molecule has become rhodamine 110. Therecan be zero, one, two, three or four groups attached.In the situation where there are two, there can betwo on one side or two on each side. In this The-sis we make no distinction between these. Anotherprocess that can happen is that the molecule breaksdown, making it unable to emit light (degradation).

Figure 5: Structure of rhodamine B

To choose different environments in which we can

study the dye we need to understand what the dyemight react to. To get an idea of what dry rhodamineB films react to we look at the previously proposedreaction mechanisms of rhodamine B in a solutionmentioned in the introduction 1. Firstly it has beenfound that for the initial stage of deethylation rho-damine B must acquire a positive charge. The follow-ing mechanism for acquiring this charge using CdSwas proposed [2]:

[>N-C2H5]ads + hν −→ [>N-C2H5]ads∗

[>N-C2H5]ads∗ −→ [>N-C2H5]ads

+ + e−cond.Or when CdS absorbed a photon:

CdS + hν −→ h+val + e−cond

h+val + [>N-C2H5]ads −→ [>N-C2H5]ads

+.

The function of CdS in these reactions is mak-ing the dye positive, either by donating an electronhole or by accepting an electron. Once the dye has apositive charge, the ethyl group can react and formacetaldehyde C2H4O [2]. For this to happen oxygenneeds to be present. In later research it was foundthat, when using TiO2 as the semiconductor and hav-ing H2O2 present in solution, O2 becomes O−

2 whichthen forms OH. This can react with the ethyl groupsforming acetaldehyde [3] [4].

For us this means we must find out what rho-damine B donates an electron to or receives a posi-tive electron hole from and how OH forms to makedeethylation possible.

3 Experimental Section

3.1 Materials.

Rhodamine B ≥95% (HPLC-grade), Rhodamine110 chloride ≥99.0%, Ethanol absolute (100%),Poly(methyl methacrylate) PMMA 996000 u andHexamethyldisilazane HMDS were purchased fromSigma-Aldrich and used without further purification,cover slips #1.5 and microscope slides from Menzel-Glaser were used throughout this research.

3.2 Optical setup.

3.2.1 Setup for measuring spectra.

The microscope setup is as described in the sectionfluorescence microscopy 2.2. A microscope, NikonEclipse Ti-U, and light source the Sola light engineby Lumencore were used. In the path of the lightto the sample a 532SP filter and 520DC mirror wereplaced. In the path from the sample to the spectrom-eter a 519LP filter was placed. A spectrometer (An-dor, the Kymera) and a camera (Andor, iXon Ultra)were used.

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3.3 Procedures and Analyses.

3.3.1 Rhodamine films.

Microscopy samples were prepared as follows: Thecover slips were prepared by plasmacleaning them for90 seconds. This ensures there are no impurities onthe glass. Then one of three things was done to thecover slips. Some were not treated further. For theothers, either 100 µL HMDS was dropcoated on thecover slip, dried at 150 °C then rinsed with EtOH100%. The Si atoms of HMDS bond to the oxygenatoms on the surface of the glass, making it more hy-drophobic. The effect of this changed chemical struc-ture of the glass on the reactions rhodamine B un-dergoes we can compare to the reactions rhodamineB undergoes on untreated glass. Or 100 µL of 31nM PMMA in toluene was dropcoated on the coverslip and let dry completely. This creates a layer onwhich rhodamine can be applied without it being incontact with glass. To create the films, 10 µL of 50µM rhodamine B in ethanol was spincoated on thecover slips. After spincoating, the sample was left inthe dark for an hour to dry. On some of the samplesthen PMMA was dropcoated to prevent contact withair and they were again allowed to dry in the dark.All films were then stuck on microscope slides withdouble sided tape.

3.3.2 Spectral measurements.

The samples were then studied using the micro-scope setup as described in section 3.2. We broughta random part of the film in focus using a low lightintensity. Once in focus the light was turned off andthe film was moved to a fresh area right outside thearea we used to put the film in focus. Then the lightwas turned on and a measurement of the emissionspectra was started, recording at one frame per sec-ond, where each frame was the spectrum at that time.This was repeated multiple times for each film.

3.3.3 Data analysis.

From the measured spectra the speed of degrada-tion, speed of N-deethylation and composition changewere extracted. To find these properties of the filmsthe following was done. The data corresponding towavelengths between 519 nm and 700 nm was ex-tracted from the measurements. This was done be-cause of the LP filter of 519 nm and because above700 nm there is no useful information. Following thisa background was subtracted from each frame. Thisbackground was determined by averaging over the in-tensities below 519 nm and above 700 nm. The total

intensity of each frame was then calculated by inte-grating over the intensities of wavelengths between519 nm and 700 nm. Next, time t = 0 was definedas the first frame after turning the light on. Sinceduring each experiment the films slowly degrade intime and thus the intensity decreases, a tend is de-fined as the frame at which the intensity goes below5% of the highest recorded intensity for that mea-surement. This will allow us to compare the dura-tion of degradation in different films. The wavelengthassociated with the highest intensity for each framewas determined, λmax, for each frame between t = 0and t = tend. Through these values a line was fit ofthe form λmaxfit = a + bt where t gives the time inframes, a gives the estimated value of λmax at t = 0and b gives the speed of change in λmax in units of[nm/frame]. We record one frame per second, thusthe parameter b gives us the change in wavelength inunits of [nm/s].

We also look at the composition change in time ofthe films. To do this we fit a combination of the spec-tra of rhodamine B, 1,2,3-deethylated rhodamine Band rhodamine 110 throught the spectra we measure.The spectra of the intermediates are approximatedin the following way: first we take the spectrum att = 0 for each measurement. From this spectrumwe subtract the background and normalise it. Thiswill be used as the spectrum of rhodamine B. To es-timate the spectra of 1,2,3-deethylated rhodamine Band rhodamine 110 we simply shift this spectrum, inwavelength scale, towards the found value of λmax ofrhodamine 110 for that environment in four equallylarge steps, which could be expected when looking atthe absorption spectra of these species as found byWatanabe et al. [2]. One can also analyse the spec-tra in energy scale, but because the range of wave-lengths is so small there is no significant differencewhen doing so. Then for fifty evenly spread framesbetween t = 0 and t = tend a fit was made to find thebest combination of contributions of each rhodaminemolecule to the total emitted spectrum.

To the found contributions of population in timewe fit a model for a rate equation where we assumethe following reactions can take place; The first reac-tion is degradation for which we assume the rate con-stants are the same for all rhodamine species. Thesecond reaction is the deethylation from rhodamine Bto 1-deethylated rhodamine B, for which we assumethe rate constant is k1. The last reactions are thedeethylation reactions from 1-deethylated rhodamineB to rhodamine 110 for which we assume the rateconstant is the same, k2. We do this to minimise thenumber of parameters used for the fit. The model we

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fit takes the following form:

dyRhB

dt= −k1yRhB − kdegyRhB (1)

dyRh3

dt= k1yRhB − k2yRh3 − kdegyRh3 (2)

dyRh2

dt= k2yRh3 − k2yRh2 − kdegyRh2 (3)

dyRh1

dt= k2yRh1 − k2yRh1 − kdegyRh1 (4)

dyRh110

dt= k2yRh1 − kdegyRh110 (5)

Where yRhB, yRh3, yRh2, yRh1, yRh110 are respec-tively the amount of rhodamine B, rhodamine withthree, two and one ethyl groups, and rhodamine 110.k1 is the rate constant for rhodamine B losing anethyl group, k2 is the rate constant for the other in-termediates losing one ethyl group and kdeg is therate constant of degradation. We assume there is onlyrhodamine B when the reactions start which we usewhen solving the differential equations. This gives ussolutions that depend on yRhB(0). After fitting thismodel through the values found for the intermediates,we use k1 and kdeg to compare the rate of degradationand deethylation in different environments.

4 Results and Discussion

Firstly we look at how the emission spectrum ofa dry film of rhodamine B on glass in air changes intime. This is shown in figure 6a. The first shownspectrum at t = 10 has an intensity peak at 580nm.The total intensity of the following spectra decreasesto a small amount and their peak shifts towards theshorter wavelengths. This behaviour was observedbefore for the absorption spectra of RhB in solutionwith photocatalysts present[2].

520 540 560 580 600 620 6400.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Time (s)10 60 110 160 210 260 310

(a)

520 540 560 580 600 620 6400.0

0.1

0.2

0.3

0.4

0.5

λ(nm)

I(counts)

Time (s)10 210 410 610 810

1010 1410 1610 1810 2010

(b)

Figure 6: Emission spectrum at several points intime under continuous excitation with broadbandblue light of (a) a dry rhodamine B film on glass,and (b) a dry rhodamine B film on PMMA.

When we prevent the rhodamine B from being incontact with glass by coating the glass with PMMAwe observe the spectra shown in figure 6b. (For allother samples the spectra can be found in the ap-pendix.) Here we see the λmax starts at 570nm anddecreases in intensity over time. There are threedifferences with the previous experiment where rho-damine B is in contact with glass. Firstly the value ofλmax does not decrease in value when rhodamine B isnot in contact with glass. Secondly it takes substan-tially more time for rhodamine B to degrade when itis not in contact with glass. Lastly the value of λmax

is different for the two samples.The first difference indicates that for deethylation

glass needs to be present. The second difference indi-cates the glass has some effect on the rate of degrada-tion. The last difference can be explained in the fol-lowing way: Rhodamine dyes have different emissionspectra based on the environment[7], thus allowingrhodamine B to have a different spectrum in thesetwo samples. To make sure these differences in λmax

of the spectra of the samples at t = 10 are not dueto deethylation before the measurement started, welooked at the value of λmax for rhodamine 110 in all

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of the environmental conditions in which we do mea-surements. This is done with rhodamine 110 becauseit has no ethyl groups left, which means no deethyla-tion is possible for rhodamine 110, and thus we knowwhat dye we are looking at. In figure 7 we see thatindeed rhodamine 110 has a different λmax in the dif-ferent environments. So for the rest of this thesis wewill assume that the spectrum corresponding to thestart of an experiment originates from RhB and notany deethylated products of rhodamine B. This couldbe checked in future research by NMR spectroscopy,infrared spectroscopy or measurements on the fluo-rescent lifetime.

●●

■■

◆◆

▲▲▼▼

524

526

528

530

532

534

λmax(nm)

Film

● Rh110 on PMMA ■ Rh110 on HMDS reacted glass

◆ Rh110 on glass ▲ Rh110 on glass under PMMA

▼ Rh110 sandwiched between PMMA

Figure 7: Wavelength with highest intensity for dryrhodamine 110 films in different environments.

We then compare the change in total intensityand change of maximum wavelength of RhB on glasswith the changes of RhB on a layer of PMMA. Thisis shown in figure 8a. Here you see the the total in-tensity in time 8a and maximum wavelengths in time8b. We see that when rhodamine B is in contactwith glass there is a strong N-deethylation and fasterdegradation then for the situation where rhodamineB is not in contact with glass. We also see an increasein intensity during the first 100 s for the situation ofrhodamine B on glass. This is likely caused becausethe light used to excite the dye does not have thesame intensity for every wavelength, but has a spec-trum that overlaps better with the absorption spectraof the deethylated products.

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

I/Imax

FilmRhB on a layer of PMMA RhB on glass

(a)

0 500 1000 1500 2000

545

550

555

560

565

570

575

Time (s)

λmax

(nm)

FilmRhB on a layer of PMMA RhB on glass

Fit through data of RhB on PMMA Fit through data of RhB on glass

(b)

Figure 8: Differences in intensity and emitted wave-length in time under continuous excitation withbroadband blue light of rhodamine B on glass versusrhodamine B on PMMA, where (a) shows the changein the integrated emission intensity normalized to themaximum as a function of time, and (b) shows themeasured changes in wavelength as a function of timeand the fitted line through these measured points foreach sample.

We have also looked at samples where rhodamineB is on glass with a layer of PMMA over it preventingit from being in contact with air, samples where rho-damine B is sandwiched between two layers of PMMApreventing it from being in contact with glass and airand samples where rhodamine B is coated on glassthat has reacted with HDMS. For all these measure-ments we show the speed of n-deethylation in [nm/s]in one figure 9. In the same figure we show the timeit took the film to reach 5% total intensity in [s].

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●

●

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●

■■ ■■

■■■

◆◆ ◆

◆ ◆◆

◆▲▲ ▲

▲▲▲

▼

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-0.10 -0.08 -0.06 -0.04 -0.02 0.00 0.020

500

1000

1500

2000

Speed of deethylation (nm/s)

Timeofdegradation(s)

Film

● RhB on PMMA ■ RhB on HMDS reacted glass

◆ RhB on glass ▲ RhB on glass under PMMA

▼ RhB sandwiched between PMMA

Figure 9: The change in wavelength per time on thex-axis and the time it takes for the total intensity ofa spectrum to reach five percent on the y-axis for thedifferent environments.

We see that for the case where rhodamine B is incontact with glass and air there is stronger deethyla-tion and degradation than all other cases, with the ex-ception of rhodamine B on glass with PMMA coatedon the rhodamine B which has a faster degradation.We then compare this to rhodamine B on glass re-acted with HDMS. In this case we see a slightly slowerdeethylation and degradation. For the case whererhodamine B is on PMMA or the case where it issandwiched between two layers of PMMA, there isno deethylation and much slower degradation. Thecase where rhodamine B is under PMMA there isfast degradation in comparison with the other sam-ples, but no deethylation. In short we see that for allsamples where the rhodamine B was in contact withglass there is a fast degradation of the dye but onlywhen the dye is in contact with both air and glass adeethylation occurs.

We can compare this to the parameters foundwhen fitting the analytical model discussed in thedata analysis section for a more numeric analysis. Asan example of how we do this the results of our fit,that gave the amount of intermediates in time, of oneof the samples of rhodamine B on glass is shown infigure 10.

0 10 20 30 40 50

0

5000

10000

15000

20000

25000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 10: The amount of each of the intermediatesshown in time, where each frame is a set time apart,found by fitting a combination of rhodamine speciesthrough the total spectra.

Through these amounts of intermediates in timewe fit the model found in the section: data analy-sis.From this fit we get parameters for rate of deethy-lation and degradation. We plot the amounts of in-termediates from analytical model over the amountsseen above in figure 11.

10 15 20 25 30 35 40 450

50000

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150000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 11: The amount of intermediates found fromthe fits through measurements as dots and theamount of intermediates found through the model aslines.

We see that the model overlaps reasonably wellwith the found amounts. We then do this for all sam-ples and compare the parameters that the fits give us.The parameters for rate of degradation and deethy-lation are shown in figure 12.

9

●●

●●

●

●

●■ ■

■■■ ■■

◆

◆◆

◆◆

◆◆▲

▲▲

▲▲

▲

▼

▼ ▼

▼▼▼

▼

▼

0.00 0.02 0.04 0.06 0.08 0.100.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Deethylation rate (s-1)

Degradationrate

(s-1)

Film

● RhB on PMMA ■ RhB on HMDS reacted glass

◆ RhB on glass ▲ RhB on glass under PMMA

▼ RhB sandwiched between PMMA

Figure 12: The rate of deethylation on the x-axis andrate of degradation on the y-axis for all the differentfilms of rhodamine B.

We once again observe that the rate of deethy-lation is fastest for the film on glass and in contactwith air. The films on PMMA show slow deethy-lation and degradation. The film on glass reactedwith HDMS has a faster degradation and deethy-lation than rhodamine on PMMA but slower thanrhodamine on glass. The sample of rhodamine onglass under PMMA shows fast degradation and vary-ing rates of deethylation. In short these results agreewith the results before where there is fast degradationwhen rhodamine is in contact with glass and there isalso deethylation when it is in contact with both airand glass.

To explain what the reaction mechanisms at workare, we note again that in previous research it hasbeen found that one of the reaction products ofdeethylation was acetaldehyde[2]. This means anoxygen atom has reacted with an ethyl group dur-ing deethylation. However for oxygen to react itfirst needs to form reactive oxygen species OH. Thismeans hydrogen needs to be present. Using thesefacts we propose the following for the deethylation ofrhodamine B: First rhodamine B gets irradiated byvisible light causing an electron to go into an exitedstate. This electron can be transferred to oxygenforming O−

2 . The point of zero charge of the surfaceof the glass is at a pH level of approximately 3 [8].This means that on the surface of the glass there arehydrogen atoms present that are able to react in oursituation. These hydrogen atoms react with the O−

2

and form the reactive oxygen species OH∗. These canthen react with the dye and form acetaldehyde and1-deethylated rhodamine. The one hydrogen atomthat is left then returns to the surface of the glass.This can be summarised in the following reaction

diagrams.

RhBhν−−→ RhB*

O2 + RhB* −−→ O2•– + RhB+•

Surf–Si–OH + O2•– −−→ OH• + OH–

RhB+• + OH• −−→ Rh(C2H5)3 + C2H4O + H+

Where RhB is rhodamine B, Surf-Si-OH is thegroup on the surface of the glass that contains thehydrogen atoms that react and Rh(C2H5)3 is 1-deethylated rhodamine B. This process can also oc-cur from 1-deethylated rhodamine B to 2-deethylatedrhodamine B, all the way to rhodamine 110.

This means the requirements for deethylation arethat there needs to be air and glass present and thatthe dye needs to be irradiated by visible light.

5 Conclusions

We have studied dry rhodamine B films undervisible light irradiation using fluorescence microscopyand preformed rate equation modeling to analyse thedata. We found that dry rhodamine B films in airon glass undergo both deethylation and degradation.A prerequisite for the deethylation is the creation ofOH which forms by oxygen reacting with hydrogenon the glass. Degradation is accelerated by the pres-ence hydrogen on the surface of the glass. This meansno photocatalyst is needed to deethylate or degradedry rhodamine B by visible light. In future researchone might want to look at whether there are differ-ences in the cis- and tans 2-deethylated rhodamineB. This could reveal what ethyl group is more likelyto react. If there is a difference in when cis or transethyl groups are more likely to be present depend-ing on environmental factors, this could be useful inidentifying products that form during reactions whichcould be used to get insight in certain catalytic reac-tions and hopefully allow us to improve them. Onecan also use the knowledge we now have on the reac-tions of rhodamine B to study fundamental catalyticreactions with the goal to improve efficiency of manychemical processes.

6 Acknowledgments

I would like to give special thanks to my mainsupervisor Dr. F.T. Rabouw and daily supervisorS.O.M. Hinterding MSc. for guiding me throughoutthe project, my student colleagues for the pleasantworking environment, my parents for proofreadingmy work and their support, and the UU SCM groupfor supplying the needed information and materials.

10

References

[1] Liu Yijin et al. “Relating structure and com-position with accessibility of a single cat-alyst particle using correlative 3-dimensionalmicro-spectroscopy”. In: Nat. Commun. 7.12634(2016).

[2] Tadashi Watanabe, Takuo Takirawa, andKenichi Honda. “Photocatalysis through Ex-citation of Adsorbates. 1. Highly Efficient N-Deethylation of Rhodamine B Adsorbed toCdS”. In: J. Phys. Chem. 81 (1977), pp. 1845–1851.

[3] Taixing Wu et al. “Photoassisted Degradationof Dye Pollutants. V. Self-Photosensitized Ox-idative Transformation of Rhodamine B underVisible Light Irradiation in Aqueous TiO2 Dis-persions”. In: J. Phys. Chem. B 102 (1998),pp. 5845–5851.

[4] Ping Qu et al. “TiO -assisted photodegradationof dyes: A study of two 2 competitive primaryprocesses in the degradation of RB in an aque-ous TiO colloidal solution”. In: J. Mol. Catal.Chem. 129 (1998), pp. 257–268.

[5] Xuefeng Hu et al. “Oxidative Decompositionof Rhodamine B Dye in the Presence of VO+

2

and/or Pt(IV) under Visible Light Irradia-tion: N-Deethylation, Chromophore Cleavage,and Mineralization”. In: J. Phys. Chem. B 110(2006), pp. 26012–26018.

[6] Jiandong Zhuang et al. “Photocatalytic Degra-dation of RhB over TiO2 Bilayer Films: Effectof Defects and Their Location”. In: Langmuir26 (2010), pp. 9686–9694.

[7] Xian-Fu Zhang, Yakui Zhang, and Limin Liu.“Fluorescence lifetimes and quantum yields often rhodamine derivatives: Structural effect onemission mechanism in different solvents”. In: J.Lumin. 145 (2014), pp. 448–453.

[8] Mumuni Amadu and Adango Miadonye. “De-termination of the Point of Zero Charge pH ofBorosilicate Glass Surface Using Capillary Im-bibition Method”. In: International Journal ofChemistry 9.3 (2017).

11

A Appendix

In the appending spectra of one sample in each different environment are enclosed with their peak wavelengthin time, intensity in time, the concentrations of the intermediates in time found by fitting and the fit of theanalytical model through these concentrations. Also the spectrum of one sample in each environment ofrhodamine 110 in different environments will be enclosed.

A.1 Rhodamine B.

A.1.1 Rhodamine B on glass.

520 540 560 580 600 620 6400.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Time (s)10 60 110 160 210 260 310

Figure 13: Emission spectrum at several points in time under continuous excitation with broadband bluelight of a dry rhodamine B film on glass.

I

0 50 100 150 200 250 300 350545550555560565570575

Time (s)

λmax

(nm)

Figure 14: The measured changes in wavelength as a function of time under continuous excitation withbroadband blue light of a dry rhodamine B film on glass.

0 100 200 300 400 5000.00.20.40.60.81.0

Time (s)

I/Imax

Figure 15: The change in the integrated emission intensity normalized to the maximum as a function of timeof a dry film of rhodamine B on glass under continuous excitation with broadband blue light.

II

0 10 20 30 40 50

0

5000

10000

15000

20000

25000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 16: The amount of each of the intermediates shown in time, where each frame is a set time apart,found by fitting a combination of rhodamine species through the total spectra of a dry film of rhodamine Bon glass under continuous excitation with broadband blue light.

10 15 20 25 30 35 40 450

50000

100000

150000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 17: The amount of intermediates found from the fits through measurements as dots and the amountof intermediates found through the model as lines of a dry film of rhodamine B on glass under continuousexcitation with broadband blue light.

III

A.1.2 Rhodamine B on PMMA.

520 540 560 580 600 620 6400.0

0.1

0.2

0.3

0.4

0.5

λ(nm)

I(counts)

Time (s)10 210 410 610 810

1010 1410 1610 1810 2010

Figure 18: Emission spectrum at several points in time under continuous excitation with broadband bluelight of a dry rhodamine B film on glass coated with PMMA.

IV

0 500 1000 1500 2000550

555

560

565

570

575

580

Time (s)

λmax

(nm)

Figure 19: The measured changes in wavelength as a function of time under continuous excitation withbroadband blue light of a dry rhodamine B film on PMMA.

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

I/Imax

Figure 20: The change in the integrated emission intensity normalized to the maximum as a function of timeof a dry film of rhodamine B on PMMA under continuous excitation with broadband blue light.

V

0 10 20 30 40 50

0

5000

10000

15000

20000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 21: The amount of each of the intermediates shown in time, where each frame is a set time apart,found by fitting a combination of rhodamine species through the total spectra of a dry film of of rhodamineB on PMMA under continuous excitation with broadband blue light.

10 15 20 25 30 35 40 450

10000

20000

30000

40000

50000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 22: The amount of intermediates found from the fits through measurements as dots and the amountof intermediates found through the model as lines of a dry film of rhodamine B on PMMA under continuousexcitation with broadband blue light.

VI

A.1.3 Rhodamine B on glass reacted with HDMS.

520 540 560 580 600 620 6400.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Time (s)10 60 110 160 210 260 310 360 410

Figure 23: Emission spectrum at several points in time under continuous excitation with broadband bluelight of a dry rhodamine B film on glass reacted with HMDS.

0 100 200 300 400 500 600545550555560565570575

Time (s)

λmax

(nm)

Figure 24: The measured changes in wavelength as a function of time under continuous excitation withbroadband blue light of a dry rhodamine B film on glass reacted with HDMS.

VII

0 200 400 600 8000.00.20.40.60.81.0

Time (s)

I/Imax

Figure 25: The change in the integrated emission intensity normalized to the maximum as a function of timeof a dry film of rhodamine B on glass reacted with HMDS under continuous excitation with broadband bluelight.

0 10 20 30 40 50

0

5000

10000

15000

20000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 26: The amount of each of the intermediates shown in time, where each frame is a set time apart,found by fitting a combination of rhodamine species through the total spectra of a dry film of rhodamine Bon glass reacted with HMDS under continuous excitation with broadband blue light.

VIII

10 15 20 25 30 35 40 450

20000

40000

60000

80000

100000

120000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 27: The amount of intermediates found from the fits through measurements as dots and the amountof intermediates found through the model as lines of a dry film of rhodamine B on glass reacted with HMDSunder continuous excitation with broadband blue light.

IX

A.1.4 Rhodamine B on glass under PMMA.

520 540 560 580 600 620 6400.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Time (s)5 10 15 20 25 30 35 40 45 50

Figure 28: Emission spectrum at several points in time under continuous excitation with broadband bluelight of a dry rhodamine B film on glass under a coating of PMMA.

0 20 40 60 80 100 120 140560

565

570

575

580

Time (s)

λmax

(nm)

Figure 29: The measured changes in wavelength as a function of time under continuous excitation withbroadband blue light of a dry rhodamine B film on glass under a coating of PMMA.

X

0 50 100 150 2000.00.20.40.60.81.0

Time (s)

I/Imax

Figure 30: The change in the integrated emission intensity normalized to the maximum as a function of timeof a dry film of rhodamine B on glass under a coating of PMMA under continuous excitation with broadbandblue light.

0 10 20 30 40 50

0

5000

10000

15000

20000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 31: The amount of each of the intermediates shown in time, where each frame is a set time apart,found by fitting a combination of rhodamine species through the total spectra of a dry film of rhodamine Bon glass under a coating of PMMA under continuous excitation with broadband blue light.

XI

10 15 20 25 30 35 40 450

20000

40000

60000

80000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 32: The amount of intermediates found from the fits through measurements as dots and the amountof intermediates found through the model as lines of a dry film of rhodamine B on glass under a coating ofPMMA under continuous excitation with broadband blue light.

XII

A.1.5 Rhodamine B in between two layors of PMMA.

520 540 560 580 600 620 6400.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Time (s)10 60 110 160 210

260 310 360 410 460

Figure 33: Emission spectrum at several points in time under continuous excitation with broadband bluelight of a dry rhodamine B film in between two layers of PMMA.

XIII

0 200 400 600 800 1000550

555

560

565

570

575

580

Time (s)

λmax

(nm)

Figure 34: The measured changes in wavelength as a function of time under continuous excitation withbroadband blue light of a dry rhodamine B film in between two layers of PMMA.

0 200 400 600 800 1000 1200 14000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

I/Imax

Figure 35: The change in the integrated emission intensity normalized to the maximum as a function of timeof a dry film of rhodamine B in between two layers of PMMA under continuous excitation with broadbandblue light.

XIV

0 10 20 30 40 50

0

5000

10000

15000

20000

25000

30000

35000

Frame

I(counts)

Samplerhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 36: The amount of each of the intermediates shown in time, where each frame is a set time apart,found by fitting a combination of rhodamine species through the total spectra of a dry film of rhodamine Bin between two layers of PMMA under continuous excitation with broadband blue light.

10 15 20 25 30 35 40 450

20000

40000

60000

80000

100000

120000

140000

Frame number

Amount

Speciesrhodamine B 1-deethylated rhodamine B 2-deethylated rhodamine B

3-deethylated rhodamine B rhodamine 110

Figure 37: The amount of intermediates found from the fits through measurements as dots and the amountof intermediates found through the model as lines of a dry film of rhodamine B in between two layers ofPMMA under continuous excitation with broadband blue light.

XV

A.2 Rhodamine 110.

A.2.1 Rhodamine 110 on glass.

450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Figure 38: Spectrum of rhodamine 110 spincoated on glass under excitation of a 405 nm laser.

A.2.2 Rhodamine 110 on PMMA.

450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Figure 39: Spectrum of rhodamine 110 spincoated on PMMA under excitation of a 405 nm laser.

XVI

A.2.3 Rhodamine 110 on glass reacted with HDMS.

450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Figure 40: Spectrum of rhodamine 110 spincoated on glass, that has reacted with HMDS, under excitationof a 405 nm laser.

A.2.4 Rhodamine 110 on glass under PMMA.

450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

λ(nm)

I(counts)

Figure 41: Spectrum of rhodamine 110 spincoated on glass under a coating of PMMA under excitation of a405 nm laser.

XVII

A.2.5 Rhodamine 110 in between two layors of PMMA.

450 500 550 600 6500.00.20.40.60.81.0

λ(nm)

I(counts)

Figure 42: Spectrum of rhodamine 110 in between two layers of PMMA under excitation of a 405 nm laser.

XVIII