Thomas R. Dyke and J. S. Muenter
University of Rochester Rochester, New York 14627
I 1
An Undergraduate Experiment for the Measurement of Phosphorescence Lifetimes
Electronic triplet states of molecules play important roles in many areas of chemistry including theoretical quantum mechanics, radiationless transition theory, and photochemistry in large organic molecules (1-3). Because virtually all molecules have singlet ground states and because electron spin is completely non-classical (4) the origins of the unusual properties of triplet states are not readily grasped by students on first exposure. One of the most important aspects of triplet states is the difficulty of conversion between the ground electronic state and the manifold of excited triplet states. This is made particularly clear by the long lifetime of the lowest triplet state. The purpose of this article is to describe an experiment which is suitable for undergraduate laboratories and which monitors the population of the lowest triplet state by directly observing phosphorescence intensity as a function of time. The experiment is carried out on several chemical systems under different conditions which allow the observation of many of the factors which affect triplet state lifetimes. The experiment is easily set up, is popular with students, and is relatively inexpensive. Theory
The vast majority of stable molecules has an even number of electrons and their ground electronic state is one in which all the electrons have their spins paired. This means that the electron spin angular momentum of the molecule is zero. The molecule will only have a single energy in a magnetic field and is thus called a singlet state. Molecules can be excited to higher electronic states by promoting an electron to a higher lying molecular orbital. If this ~romotiontakes place without reorienting the direction of the spin of the eiectron, the excited statewill still be a singlet state. However, if the spin of the excited electron reorients on excitation, two unpaired electrons with half integral spin angular momentum will have the same orientation resulting in the molecule having a spin angular momentum of unity. In a magnetic field, this type of state will split into three levels of different energy: one having electronic angular momentum pointing with the field, one against the field, and one perpendicular to the field. Because of these three possibilities, this type of electronic state is referred to as a triplet state. The orientation of an electron spin, to a first approximation, is completely independent of the spatial coordinates of the electrons and for this reason it is relatively difficult to change the direction of an electron s ~ i n In . oarticular. at the level of approximation that the spin coordinates and the snatial coordinates of an electron do not interact, radiation fields cannot change the direction of an electron spin (5). For this reason spectroscopic processes, the absorption or emission of a photon, do not readily cause the interconversion of singlet and triplet states. Therefore, it is convenient to group the electronic states of molecules into two manifolds, singlets and triplets. The states are labeled So for the ground state, SIfor the first excited singlet, Sz for the second excited singlet, etc., and T I for the lowest excited triplet, Tz for the second excited triplet, and so forth. Figure 1 is an energy level diagram indicating these electronic states as well as some of the vibrational states within each electronic state. It is quite gener-
SINGLET MANIFOLD
TRIPLET MANIFOLD
Figure 1. Singlet and triplet energy levels with the lower vibrational levels of each electronic state indicated. ally true (6). as is shown in the diagram, that TI is the lowest energy excited electronic state and that, specifically, the energy of TI is less than that of S1. Although interconversion between the singlet and triplet manifolds is difficult, many important physical and chemical processes are affected by this interconversion. Changes of electron spin orientation can take place in two different ways, by radiative or photon processes and by nonradiative processes. The statement that absorption or emission of photons cannot change the multiplicity of an electronic state is true only if the spatial and spin coordinates are completely separated. Any interaction which mixes these coordinates will permit radiative transitions between singlets and triplets. By far the most common interaction of this type is the spin-orbit interaction (7). A rigorous discussion of spin-orbit interaction is beyond the scope of this article but it can crudely he considered as arising from the interaction of the electron magnetic moment (caused by its spin angular momentum) with the magnetic field generated by the orbital motion of the electron. Since the orbital motion is a function of the electron's spatial coordinates and it interacts with radiation fields, this radiation interaction is extended to the electron spin coordinates. When an interaction, such as spinorbit interaction, mixes the coordinates of different states each state takes on some of the characteristics of the other state with which it is mixed. The term "mixine" - is.. therefore, used generally to describe interacting states. The mixine of states bv the soin-orbit interaction is not indiscrimirke but foliows certain requirements imposed by the symmetry of the wave functions describing the states being mixed. In particular, the spin-orbit interaction does not mix states of the same orbital configuration (8). For example, a *a* state can be mixed with an na* state, but an na* state will not mix effectively with another nx* state, nor will two ?is* states be mixed by spin-orbit interactions. An equivalent way of describing this mixing is to say that spin-orbit mixing allows the forbidden singlet-triplet transition to "borrow" intensity from the allowed singletsinglet transition. Radiative transitions between singlet Volume 52. Number 4. April 1975
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and triplet states will therefore depend on the magnitude of the spin-orbit interaction, the degree of mixing this interaction causes, and the intensity of the singlet-singlet transition from which "borrowing" occurs. Many facton of molecular structure affect the magnitude of the spin-orbit interaction. One of the more important of these is the atomic number of the atoms in the molecule. The larger the atomic number, the larger will be this interaction so molecules containing heavy atoms such as iodine or bromine will have large spin-orbit interactions (9). The degree of mixing caused by the spin-orbit interaction between a specific singlet state labelled S and a specific triplet state labelled T in the molecule depends on the relative configurations and energies of S and T. The greater the mixing between S and T,the greater will be the singlet character of the triplet state, T.The effect this singlet character has on a radiative transition from T to another sindet state., S'.. denends on the intensitv of the S . S' radiative transition. If the S S' transitibn is ve& S' radiative transition can borrow a laree weak. the T S' intensity and still not g z n of the S S' transition is very intense, much. However, if the S borrowing a small fraction of this large intensity, i.e., only a small amount of mixing, can greatly enhance the T S' radiative intensity. By definition, any radiative &ansition in which a photon is emitted and the spin multiplicity of a state is changed is called phos~horescence.Radiative transitions emitting a photon whiih do not change spin multiplicity are defined as fluorescence. Phos~horescenceand fluorescence are the two spontaneous radiative, or photon, processes which allow energy to be lost from molecular states. In general, there will also he nonradiative processes which compete with phosphorescence and fluorescence. There are three types of nonradiative mechanisms which alter electronic energy in molecules that are important in discussing the triplet state: internal conversion (101, intersystem crossing (II), and quenching (12). The first of these, internal conversion, does not involve a change in spin multiplicity but consists of excited singlet or triplet states relaxine within their own s ~ i nmanifold. Fieure 2 shows the vihrational energy levels'of the electronicstates in the sinelet manifold in more detail. The excited vibrational energy levels of one electronic state will generally extend to energies higher than the lowest vihrational level of the next higher electronic state. Thus, as is shown in Figure 2, highly excited vihrational states of SOwill have
-
- - -
-
-VIBRATIONAL
nearly the same absolute energy as low lying vibrational states of SI. Similarly, high vibrational states of SI overlap low states of S2, etc. The same situation occurs in the triplet manifold with high vihrational states of TI overlapping low vihrational states of Tz,etc. The radiationless transition between two electronic states will occur between energy levels having the same energy. These transitions will occur, therefore, from low vibrational states of the higher electronic state to high vibrational levels of the lower electronic state. After theelectronic transition takes place the vihrational energy of the lower electronic state very rapidly relaxes with the energy being dissipated into the surrounding solution or lattice. This vibrational relaxation is also nonradiative. The probability of the change of electronic state through internal conversion depends both on specific details of the two electronic states involved and on how well the vibrational wave functions of the two particular equal energy levels overlap (13). Since the electron spin, or multiplicity, is not changing in internal conversion. the electronic Dart of the transition nrobability can be large. However, Gibrational wave functions of meatlv. differine vibrational auantum number do not have high overlap (13). Since a greater energy difference between the two electronic states involved requires larger vibrational quantum numbers for the lower state, internal conversion will be more probable between more closely spaced electronic states. Internal conversion from highly excited singlet states to the next lower state is usuallv very rapid since these states will he sufficiently close that the lower state vihrational level will not be of particularly high vibrational quantum number, and good vibrational overlap will obtain. It is apparent from these considerations-that it will be the h i s s t energy vibrations, particularly C-H stretching modes, that are most important in internal conversion (14). If internal conversion were the only mechanism for changing electronic energy states, all molecules would eventually end up either in So, the lowest singlet, or TI, the lowest triplet state. The second nonradiative process is the radiationless transition between singlet and triplet states and is called intersystem crossing (11). Intersystem crossing has many similarities to internal conversion, except that now the multiplicity of the state must also change. The two specific levels involved in the intersystem crossing must again have the same absolute energy. The probability of intersystem crossing depends on the product of an electronic transition probability and the vibrational overlap (15). Since the electron spin has to change in the transition, the electronic transition probability can be relatively small but this can be counteracted by a large vibrational overlap. This situation will occur when the vibrational levels of the excited singlet and triplet states are isoener-
CONVERSIOT RELAXATION k>lOiL
-
CONVERSION k=IO0-10''
-
s8
INTERSYSTEM CROSSING
h:IOa-10"
-
-
-
VIBRATIONAL RELAXATION k>10"
So
I
> I
Figure 2. Details of the manifold of singlet states Showing different energy ~ o n v e r ~ i oPrOCeSSeS n and approximate rates tor these processes.
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Figure 3.
Intersystem crossing between singlet and triplet manifolds
getic and have nearly the same vibrational quantum number. Intersystem crossing is depicted in Figure 3. The final radiationless process is referred to as quenching (12). If a molecule in an excited state collides (16) with a second molecule there are several ways in which the energy of excitation can be lost. One of the most important quenching mechanisms occurs when the excited state is involved in a chemical reaction brought on by the collision. Because of the energy of excitation many reactions can occur in this manner which would normally not take place. This type of reaction is a dominant part of photochemistry. The reaction can result in the excited molecule splitting into products, or i t can involve a solvent molecule, an impurity molecule, or a purposely added reaction partner. Quenching can also occur without any chemical reaction. In this case the energy of excitation is given up to the surrounding medium on collision. The collision partner can again be from the solvent, an impurity, an intended reagent, or an unexcited molecule of the same kind as the excited one. This latter case is called self-quenching. The details of the various quenching mechanisms are not at all well understood. It is geuerally thought that a short-lived collision complex is first formed and the decomposition of the complex determines the type of quenching (17). In this model, the ability with which complexes form as well as how readily the complex leads to the de-excitation will determine how effective the quenching is. Both excited singlet and triplet states can be quenched. The quenching of triplets to the ground electronic state involves a change in spin multiplicity and, as in all processes that reorient electron spin, this requires special consideration. If the quencher has a singlet-triplet transition of lower energy than the triplet being quenched, the initial step of the quenching can be the simultaneous de-excitation of the molecule being quenched and the excitation of a triplet state of the quencher (18). In this manner, spin angular momentum is conserved. Therefore a molecule which is an effective quencher will often have its TI state at a lower energy than TI in the molecule being quenched. Oxygen represents a special case for the quenching of triplet states. The ground statel of Oz is a triplet, 3Z; and the molecule has two low energy singlet states, a 'S and a l A . The triplet ground state, the low lying singlet states, and the complex-forming abilities of oxygen all make it an extraordinarily good quencher of excited triplet electronic states. The effects of the various ways in which electronic energy can be distributed in a molecule can be summarized by considering what happens when molecules are exposed to a short burst of radiation. The sample molecules will initially be in So, the ground electronic state. The molecules that absorb the radiation will be excited almost exclusively to higher singlet states because the singlet-triplet absorptions are forbidden to a first approximation. Several different things can happen to the excited singlet molecules. They can reradiate to the ground state by emitting a hoto on, i.e., fluorescence. They can relax back to So by thk required number of internal conversions followed b; vibrational relaxations, or they can be quenched. Finally, they can transfer to the triplet manifoldby undergoing intersystem crossing. Once in the triplet manifold the molecules can relax to the lowest triplet, TI, by internal conversion-vibrational relaxation. The molecules can return to the singlet manifold by intersystem crossing, phosphorescence, or quenching. In any specific case, the importance of these various processes clearly depends on the relative rates of each competing process. Therefore, the common terminology and formalism of chemical kinetics can be applied to this problem. All of the mechanisms, except For diatomic molecule electronic state notation, see ref. (13). Z*D
=*ISC[K~/(KI, + K ~ s c+ KO)].
quenching, discussed above are unimolecular and can be described by a single unimolecular rate constant for each step that gives rise to a change in the molecule's energy. Quenching can also be described by a single rate constant, a pseudo-first-order constant. At any time, the number of .molecules in excited states will always be much less than the number of those molecules in the ground state and also much less than the number of impurity, solvent, or other reagent molecules. The rate of quenching will be given by K[E][Q], where K is a second-order rate constant, [El is the concentration of the excited state being quenched, and [Q] is the concentration of the quencher. Since [Q] is very much larger than [El the reaction cannot change the value of [Q] and K[Q] will he constant during the reaction. K[Q] is defined as the pseudo-first-order rate constant, K[Q] = k,. Typical ranges for the values of the various rate constants discussed here have been included in the figures. Some of the considerations leading to these values, and the conclusions which can be drawn from them, will be discussed below. After an instantaneous radiative excitation, molecules in a condensed phase will primarily be in the first excited singlet, SI, though internal conversion in a time as short as 10-12 s. The energy spacing between SI and So is normally much larger than that between the higher states and, therefore, internal conversion to the ground electronic state will be much slower and other processes can compete to depopulate & (19). This depopulation can be described by a first-order rate equation.
The left hand side of eqn. (1) is the rate of change of the number of molecules in SI, Nsl. On the right hand side there is a first-order term for each path or channel that removes molecules from SI. k, determines the fluorescence rate to So; k ~ cdetermines the internal conversion rate to So; h'lsc determines the intersystem crossing rate to TI; and h', determines the pseudo-first-order quenching rate to So. hr is defined by the intensity of the S1 to So radiative transition in the. absence of the competing rate constants in eqn. (1). The larger kr the more intense this transition will be and the more intense the fluorescence will be. k ~ is c defined here specifically for the S1 to So internal conversion and is normally much smaller than the rate constants for the higher level internal conversions. In general, the greater the separation between S1 and SOthe smaller k ~ cwill be because of the poorer resulting vibrational overlap. k'rsc, as mentioned earlier, depends on spin-orbit mixing, details of the two electronic states involved, and on the degree of vibrational overlap. As shown in Figure 3, the magnitude of h'lsc can vary from quite small to quite large. k',, of course, will depend on what chemical species are present in the sample. However, because quenching depends on collisions, k', can be made very small by freezing the sample to a rigid glass. The primes on k'lsc and k', are used to differentiate these rate constants from those for the same processes which occur in the triplet state. Eqn. (1) may he integrated to give
Ns,
=
N,,O exp(-tlr0
(2)
where Nsl0 is the number of molecules in SI a t time zero and rr, the observed fluorescence lifetime, is defined as l/(kr + k ~ c+ k'lsc + kcq). For the majority of molecules, ~r is less than s. Another lifetime for molecules in the SI state can also be defined just for the radiative process, kr. This is referred to as the natural radiative fluorescence lifetime or. the intrinsic radiative fluorescence lifetime and is equal to llkr. Molecules that undergo intersystem crossing from & into the triplet manifold will immediately go to the lowest Volume 52, Number 4. April 1975 /
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level of the lowest triplet, TI, through internal conversion and vibrational relaxation. They can then undergo intersystem crossing back to the singlet manifold, they can be quenched, or they can phosphoresce. Equations similar to (1) and ( 2 ) can be written to describe the depopulation of TI.
+
k,,, + kJNn N T , = NrIOexp(-t/rJ
dNr,/dt = -tkp
(3) (4)
The terms are similarly defined; Nrl is the number of molecules in TI, hp is the phosphorescence rate constant, klsc is the intersystem crossing constant from TI to So, and h, is the pseudo first-order triplet quenching constant. Nr1° is the number of molecules in TI at time zero and r p is the observed phosphorescence lifetime, l / ( h p h ~ s c k,). There will also be a natural radiative phosphorescence lifetime which does not depend on the radiationless transitions and equals l / h p . Note that an internal conversion t e r n is not needed in eqn. (3) because there are not any triplet levels of lower energy than TI. The factors afkrting the magnitude of k,, a & just those affecting the radiative transitions between triplet and singlet states and have already been discussed. Because TI is lower in energy than SI, intersystem crossing from the triplet can only go to So. The size of h ~ s cdepends on electronic factors such as spin-orbit mixing and also on the amount of vibrational overlap. The greater the energy difference between TI and So, the larger will he the vihrational quantum number of an So level which is isoenergetic with a TI level. Since SI is closer to TI than TI is to So, h'lsc, the intersystem crossing rate from S1 to TI, is always larger than hlsc, the TI to So intersystem crossing rate. This is very important to photochemistry since h'rsc is one of the few means of populating the triplet state. hq will again depend on the chemical species present and will be particularly sensitive to the presence of oxygen. k, can also be made very small by going to a low temperature solid sample. The results of a photochemical experiment depend on the relative magnitude of the various rate constants. In order to get molecules into the triplet state, where the lifetime will he long enough for chemical reactions to occur, h'lsc must be greater than (hr + h'rc). If the molecules which reach the triplet state are to react to form the desired products, the rate constant for this reaction must be greater than the sum of the other quenching rates plus fhp + hlsc). Much of the information available for the many processes being discussed here is obtained by experimentally observing the intensity of the fluorescence and phosphorescence as a function of time to observe rr and T P . Because of the very rapid internal conversion in both the singlet and triplet manifolds, virtually all observations are made on fluorescence from SI and phosphorescence from TI. It is useful to define the term quantum yield, @r, as the number of fluorescence photons divided by the total number of photons absorbed. Similarly, the phosphorescence quantum yield, a,, is the number of phosphorescence photons divided by the total number of absorbed photons. Before discussing the specific molecules of interest to this experiment one additional general point should be mentioned. The foregoing discussion applies to large molecules in condensed media, either fluid or solid solutions. Somewhat different considerations apply to large molecules in the gas phase if the pressure is low enough that each molecule may be considered to be isolated. The main differences in the isolated molecule case are that vibrational energy cannot be given up to a surrounding medium and that quenching cannot be an operative mechanism. If one considers small molecules, and particularly diatomic
+
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+
molecules, the situation is also different. In both internal conversion and intersystem crossing it was assumed that there are always levels of exactly the same energy involved in the transition. For large molecules this is valid because the density of energy levels is so high that individual levels may be considered to overlap one another. This occurs because of the large number of vibrational degrees of freedom in large molecules. In small molecules this degeneracy cannot be assumed and thus the nature of internal conversion and intersystem crossing is quite different. At the present time it is not possible to clearly define whether a molecule is small or large. However, diat o m i c ~are definitely small while benzene is probably large, so the vast majority of molecules will fall under the large molecule description presented here. Specific Molecules Biacetyl The major portion of the experiment presented here concerns biacetyl, (CHsC=0)2. Photochemical and photophysical processes in biacetyl have been the subject of numerous investigations over the past 20 years and, in particular, the work of Noyes and coworkers has made biacetyl one of the best understood photochemical systems. Biacetyl is discussed in review articles (21, 22). and many references are found in two recent papers on gas phase lifetime measurements (23). The exnerimental details employed here, as well as a general dhxssion of the photochemical ropert ties of biacetyl, . . are found in the PbD thesis of ~ o b e rEngel t (24). The attractiveness of biacetyl stems from its high phosphorescence quantum yield, a natural radiative phosphorescence lifetime of ;.lo ms, ease of excitation, and the fact that it is one of the few molecules observed to phosphoresce in solution a t room temperature. The general photochemistry of carbonyl compounds is of interest and the photochemistry of biacetyl is briefly reviewedin reference (24). Specific references for biacetyl are given in references (25-34). For example, the excitation process removes a nonbonding electron from the oxygen and places it in a a* orbital of the carbonyl. The electron deficient oxygen atom then behaves similar to an alkoxy radical (25) and can abstract hydrogen atoms from suitable donors. One of the purposes of the experiment described here is to measure specific electronic properties of biacetyl which are important to its photochemistry and in general to observe and understand the molecular properties which are important to photochemistry. One of the reasons for the above mentioned advantages of biacetyl is that the intersystem crossing from SI to TI is extremely efficient. When the molecules are optically excited into the singlet manifold they will very quickly cascade to SI and then over 90% of them will cross over into TI. This efficient singlet-triplet conversion occurs because of the relative magnitude of the rate constants in eqn. ( 1 ) . kr is rather small indicating that the SI to So radiative transition is not very intense. K I Cis not particularly large, and h', can he made very small by freezing the sample to a solid glass. In contrast, h'lsc is relatively large. This occurs because the spin-orbit interaction effectively mixes the singlet character of aa* excited singlets into the na* triplet state. Therefore, thedepopulation of SI described in eqn. ( 1 ) is dominated by h'rsc. The fate of the triplet molecules, after thev vibrationallv relax to the bottom of TI, depends on the environment i f the sample. If the biacetyl is dissolved in a solution of ethvl ether. isopentane, and ethanol (EPA) in a 5:5:2 ratio and cooled to liquid nitrogen temperature a transparent rigid glass obtains. Under these conditions Kq will be very small and 'quenching will not affect the observed phosphorescence lifetime, 7,. As has been already mentioned intersystem
crossing from T I to So is much slower than that which populates T I from SI and the quantum yield for phosphorescence is about 25% under these conditions. The phosphorescence lifetime is observed to he 2.4 ms (24,25). The triplet lifeti,me of hiacetyl has also been measured a t room temperature in the gas phase (24). If care is taken to exclude oxygen the lifetime obtained is found to he 1.7 ms. At first glance, this shorter lifetime is readily interpreted as due to triplet quenching from collisions between excited hiacetyl and ground state biacetyl. However, a careful study of the pressure dependence of this lifetime shows the lifetime to he independent of pressure (23). This requires k, to he zero or too small to he measured under these conditions. k ~ s chas been experimentally ohserved to be temperature dependent in many systems, and this is the source of the shorter lifetime a t room temperature. This observed temperature dependence of krsc is exponential in form and can he associated with thermal population of excited vibrational states in TI. These excited vihrational states, while representing only a very small fraction of the triplet molecules at room temperature, will have better vibrational overlap with the So states of the same energy and thus the intersystem crossing rate will increase. The importance of vihrational overlap in determining the intersystem crossing rate from T I to So is also illustrated by the triplet lifetime of perdeuterohiacetyl in EPA a t -196°C (24). Here the lifetime increases to 3.3 ms, and the phosphorescence quantum yield increases to 0.32. The deuteration does not effect the electronic energy levels of the molecule to any significant extent hut the energy of the carhon-hydrogen vibrations decreases on deuteration by approximately 1 / 4 2 . The energy of T I above So corresponds to between 6 and 7 quanta of a C-H stretching vibration while approximately 9 quanta of a C-D stretch are required for the same energy. Therefore, the vihrational levels of So which have the same energy a t T I require higher vibrational quantum numbers in deuterated hiacetyl and poorer vihrational overlap results (36). This lowers krsc and both the quantum yield and the lifetime increase. If it is assumed that all of the originally excited molecules reach the triplet state through the efficient S1 to TI intersystem crossing, then the natural radiative phosphorescence lifetime is just 1,pp.2This gives an approximation for this radiative lifetime (the lifetime which would result if all the nonradiative mechanisms to depopulate the triplet were zero) of 0.01 s. Therefore, k, == 100 s-1. This radiative lifetime is not long when compared to many aromatic hydrocarbons as will be seen below. The effects of quenching on the hiacetyl triplet can be seen if the phosphorescence lifetime is measured a t room temperature in henzene solution (24). The pressure independence of the lifetime in the gas phase measurements indicates that the henzene should not quench the triplet; yet no phosphorescence is visible in a freshly prepared room temperature benzene solution. If this sample is carefully degassed by freezing and thawing for many cycles under high vacuum, phosphorescence becomes observable. The better the degassing procedure, the longer the measured triplet lifetime will be. However, it appears that all the oxygen cannot he removed since the longest observed lifetime for a room temperature solution is 1 ms. Calvert and coworkers have measured the second-order rate constant for oxygen quenching to he 5 X lo8 l/(mole-s) (23). From the room temperature gas phase lifetime and the phosphorescence rate constant, h,,, kxsc may be obtained. Then, using the second-order quenching constant for oxygen and the observed room temperature solution lifetime the oxygen concentration in the solution may he obtained. The second-order rate constants for quenching may he obtained by observing the lifetimes and quantum yields
for phosphorescence, *,, as a function of the quencher concentration. In hiacetvl. . . the ratio of a, in the absence of quencher to the quantum yield with quencher, is eaual to the ratio of the emission intensities. PII. wh~ch can be measured using .a spectrofluorimeter. he quantum vield ratios or intensity vield ratios can he plotted as a function of quencher &ncentration using the Stern-Volmer relation,
to obtain the second-order rate constant h, (37) if r , is known. Naphthalenes The study of the electronic spectra, fluorescence, phosphorescence, and photochemistry of aromatic hydrocarbons is a very active field of research. In this experiment the phosphorescence lifetimes of several halogen suhstituted naphthalenes are investigated. As background for these measurements, some of the properties of naphthalenes will be discussed here. The differences in the behavior of the excited states of naphthalenes from biacetyl stem from the different molecular orbital configurations involved. In the biacetyl ground electronic state the highest occupied orhital is nonbonding and the lowest singlet and triplet transitions are na*. The So-TI energy spacing is about 19,500 cm-1 while the So& energy spacing is about 23,000 cm-1. In contrast, the highest filled orbital in naphthalene is a a orbital, and the lowest excited singlet and triplet states involve excitations to a* orbitals of 31,500 cm-l and 20,500 cm-' energy of excitation, respectively. The effective mixing of singlet character into the na* orbital configuration of T I in carbonyl compounds is from singlet aa* orhital configurations. The existence of these orbital configurations suitable for spin-orbit mixing into the ns* triplet and the fact that the ar* sinelet to mound state radiative transitions are intense account for tYhe relatively short triplet lifetimes and laree auantum - ohos~horescence . . yields in carbonyls. However, in aromatic hyd;ocarhon compounds T I is a ar* state and there are no na* states in the singlet manifold. All of the strong singlet transitions are from aa* states and these will not spio-orbit mix into the triplet of the same symmetry. The only available singlet states. that can he used to mix singlet character into the triplet are mr*, and they have only very weak transitions to the ground state. Therefore, the value of kp will he small for the naphthalenes. The fluorescence quantum yield is very much larger in naphthalene than in hiacetyl because of the much greater energy difference between & and T I . This means that intersystem crossing from low vibrational states of SI will have to go to much higher vibrational states of T I in naphthalene as compared to hiacetyl. The poorer resulting vibrational overlap means that k'rsc will he smaller, contributing to the large fluorescence quantum yield. By substituting halogens onto naphthalene the effect of increased spin-orbit interaction can he readily observed. Table 1 lists the quantum yields, triplet lifetimes, and approximate intersystem crossing rates for naphthalene and the halogenated naphthalenes. The effects of increasing spiu-orbit interaction in going from the fluoro to the iodo compound is obvious. Table 1. Luminescence Prouerties of NaohthalenesS
~ e f (I), . p. 272.
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1
TRIGGER
Figure 4. Block diagram for experimental apparatus. 1. Strobe lamp; 2. excitati~nfilter; 3, sample holding block: 4. sample and Dewar; 5, phosphorescence filter; 6, photomultiplier tube and housing; 7, photomultiplier power supply; 8, oscilloscope.
Experimental Apparatus This experiment consists of measuring the phosphorescence lifetimes of biacetyl and halogenated naphthalenes. Biacetyl lifetimes are measured in benzene solution and in the gas phase a t room temperature, and a t -196°C in EPA glass. Chloro, hromo, and iodouaphthalene lifetimes are measured in EPA glass a t liquid nitrogen temperature. Figure 4 gives the hlock diagram for the apparatus used for these lifetime measurements (24). The flashlamp is a General Radio 1531AB Strobotac strobe lamp. This lamp puts out high intensity flashes with half widths of approximately 3 fis. It will internally trigger these flashes at rates adjustable down to 110 flasheslmin, which is slow enough for all the samples except the chlorohenzene for which manual triggering is used. The lamp provides a trieeer oulse for initiatine the oscilloscooe sweeo. .. or the oscilloscope may he internally triggered on the leading edge of the scattered light signal from the photomultiplier. The lamp pulse is filtered with a 3% X 3%-in. Coming CS-7-59 filter which transmits only in the wavelen&h region of 3200-4400 A. The sample & held in an aluminum block which is descrihed in detail below. The phosphorescence radiation is filtered with the same size Coming CS-3-71 filter which transmits wavelengths longer than 4800 A and absorbs shorter wavelengths. The two filters combine to greatly attenuate the light which is scattered from the lamp to the photomultiplier. The filters are only necessary for the observation of the relatively weak phosphorescence from the biacetyl in benzene solution, and the exciting filter must be removed for the naphthalene experiments since shorter wavelengths are necessary to excite the higher energy S1 in these molecules. The photomultiplier and housing are combined in a Heath EU 70130 module. This module can he mounted directly to the sample hlock by removing the large square flange from the multiplier housing. Then the phosphorescence filter is simply clamped between the multiplier housing and the sample hlock. A Heath EU-42A high voltage power supply is used with the multiplier. The requirements placed on the oscilloscope are not high, but it must have a calibrated time base and calibrated and dc-coupled vertical amplifier. The sweep must either he triggerable from the signal itself or by the trigger pulses from the lamp so that a stable display results. The frequency response of the oscilloscope should be a t least 500 KHz. We have used both a Hewlett-Packard 120B and a Tektronix 5103N oscilloscope with this experiment. While it is not at all necessary, an oscilloscope camera is convenient to record the slow decay of the chloronaphthalene phosphorescence, and we have used a T e k t r o ~ i xC-5 camera for this purpose. The faster decay data are readily taken directly from the oscilloscope
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face since the flash rate is high enough to give a continuous stable display. I t is necessary to lower the input impedance of the oscilloscope for the rapid decay measurements of biacetyl in benzene since the RC time constant of the input is approximately 0.1 ms. This is easily done by connecting a 100 kohm resistor from the vertical input of the oscilloscope to ground. The only parts of the apparatus that are not commercially available are the sample block and Dewar. The Dewar was made at a local glass blowing shop from 25 mm diameter Pyrex tubing for the outer wall and 12 mm diameter Pyrex tubing for the inner wall. The Dewar is 6 in. long. Details of the sample hlock are shown in Figure 5. It is made from a 6 i n . length of 3-in. square aluminum bar stock. The tapped holes match those on the photomultiplier housing, and similar holes are used to hold the exciting light filter on the other side of the block. The inside surfaces of the hlock are painted with flat black Krylon spray paint to reduce scattered light. For room temperature samples, Eck and Krehs 7544 sample bulbs are convenient to use with this sample block. Chemicals Spectrograde solvents are used without purification. EPA can he purchased already prepared fmm Matheson, Coleman, and Bell. Eastman hiaeetyl and 1-halonaphthaleneswere vacuum distilled before use. A 0.05 M solution of biacetyl in benzene is used for the room temprature measurements. The EPA solutions of 0.05 M concentration were made up and vacuum degassed and then sealed under vacuum in small ampules convenient for use in the Dewar. A small pad of Pyrex wool in the bottom of the Dewar eliminates hreakage . and can be used to adiust the height of the sample ampoule. Procedure The actual measurements of the phosphorescence lifetimes are straightforward and surprisingly accurate considering the simplicity of the apparatus. With the exception of the langer-lived chloronaphthalene, the measurements on the EPA glasses at liquid nitrogen temperatures are all very similar. With the sample
Figure 5. Construatian details of sample block. Dimensions are in inches. Dimensions for the two pairs of 3 X 6 faces are identical. The small holes are 'kin. deep and tapped for 6-32 screws
pump-thaw cycles followed by a lifetime measurement. The degassing is then repeated and the lifetime again measured. This process is continued until the lifetime no longer increases on further degassing. We believe that this procedure could be shortened and that better lifetimes would be obtained if the degassing involved a bulb-to-bulb distillation under vacuum rather than just the freezing, pumping, and thawing cycle, and we plan to use this procedure in the future. We have also had best results with freshly prepared solutions, although the biacetyl need not be redistilled if refrigerated. Because of the shorter lifetimes involved here, it is necessary to reduce the input RC time constant of the oscilloscope as mentioned, and the first measurement should not be taken until 100 rns after the flash. Data Analysis
I
TIME
-
Figure 6. Semilog data plot for biacetyl in EPA at -196°C
cmled down and the Dewar in the sample block, a black cloth is put over the Dewar to exclude room light from the phototube. The photomultiplier voltage and vertical oscilloscope gain should he adjusted such that the scattered light signal is off scale so the weaker phosphorescence signal fills a large percentage of the oscilloscope face. The flash rate and oscilloscope sweep rate should initially be adjusted so the phosphorescence has completely decayed before the next flash occurs. Thus the oscilloscope base line in the absence of signal can be set to the topmost ruling on the oscilloscope. Then the sweep rate should be increased sa the phosphorescence once again fills the screen. This will typically be 1-2 ms/cm. The first measurements should be taken at least 1 ms after the flash to avoid any effects from the large scattered light signal decaying at the 0.1 ms rate determined by the oscilloscope input RC time constant. To obtain accurate measurements for longer times after the flash, the vertical gain should be increased and the sweep rate decreased. Figure 6 shows a plot of the logarithm of the signal voltage versus time for a typical set of data from this measurement. A careful student can obtain signal voltage measurements over a two decade range which accurately lie an a straight line by this procedure. Table 2 lists the results a class of students obtained for the triplet lifetime of biacetyl in EPA glass at -196°C. The sealed sample ampoules are provided already prepared to the students for the law temperature measurements. Once prepared, these ampoules last at least six months with no measurable degradation. Room temperature samples are in a bulb fitted with a stopcock, and students must degas these. This is easily done for the gas phase sample, which is a sample bulb with just a small amount of pure biacetyl in it. The vacuum system used for Torr region and, degassing must achieve pressures in the therefore. a diffusion pump is required. We have used a simple glass manifold pumped with a glass, single-stage, air-cooled diffusion pump. The pure hiacetyl can be degassed by two freeze-pumpthaw cycles. By far the most difficult part of the experiment is the room temperature benzene solution of biacetyl. As is indicated by a very wide spread of literature values for this measurement, it is very difficult to eliminate all the quenchers from this sample. The results of Engel indicate that this is primarily due to dissolved oxygen and not solvent impurities. The procedure we have adopted is for the degassing to initially begin with four freezeTable 2. Class Results for Biacetvl in EPA at -196°C.
Lifetimes are obtained from the experimental data from semi-log plots, plotting the logarithm of the signal versus time, or by using a n appropriate exponential least-squares fitting procedure. The results for the two mom temperature measurements on biacetyl are combined with the literature value for the second-order oxygen quenching rate constant and the radiative lifetime to obtain a semiquantitative measurement of the oxygen content of the benzene solution. Typical results for this are approximately 4 X 10-5 M, clearly demonstrating the effectiveness of oxygen quenching and the sensitivity of t h e method. All of the lifetime measurements taken together provide a very g w d insight to the physical and chemical properties t h a t affect radiative and non-radiative processes of large molecules. Discussion This experiment can be treated on several levels, t h e simplest df which is just a n attractive example of firstorder kinetics. There is ample opportunity t o emphasize the relationship of this experiment to both organic-and physical chemistry. At the most sophisticated level, the results and discussion of this experiment can be related to current research into understanding nonradiative processes in molecules. The experiment integrates well with other experiments in the laboratory. In particular, the concept and effects of spin-orbit coupling are a part of several experiments in the University of Rochester's physical chemistry laboratory including the splitting of doublets in alkali metal emission spectra, broadline nmr, and magnetic susce~tihilitvmeasurements. T h e experiment introduces many useful concepts in electronic and optical instrumentation and reauires students t o be familiar with important vacuum techniques. T h e instrumentation, the nature of the d a t a display, and the opportunity t o obtain precise information make this experiment very well received by students. An average student can complete the work outlined here in 4-5 h r and all the measurements with the exception of the biacetyl in benzene solution can be made easily in one 3-hr laboratory period. There are many additional projects or extensions for this experiment. If a spectrofluorimeter is available, organlc photochemistry can be investigated in detail by measuring quantum yields and making Stern-Volmer quenching plots for a variety of compounds. Many such experiments are discussed in reference (25) for biacetyl. In a laboratory program which integrates organic and physical chemistry a very worthwhile project would he the synthesis of perdeuterohiacetyl. A synthesis requiring only D20 is also given in reference (24). DC1 gas is a n intermediate and can therefore integrate well into the standard infrared experiment on the gas phase vibration-rotation spectra of HCI and DC1. If a small monochromator is available, the low temperature phosphorescence is intense enough for wavelength analysis. From a practical point of view. the experiment is easilv Volume 52, Number4. Apnl 1975 / 257
set up, reliable, and relatively inexpensive. If a suitable oscilloscope and vacuum system are available, the experiment can be established for well under $lM)O. Acknowledgment
We would like to thank Chris Dalton for originally suggesting this experiment and for many useful conversations dealing with its implementation. One of us (JSM) would like to acknowledge many useful discussions with Annabel A. Muenter which were crucial in educating a small molecule spectroscopist in some of the ways of the world of large molecules. Literature Cited I11 MeGlpn. S. P.. Azurni. T., and Kinmhita. M., "Molecular S p c t m c o p y of the Triplet Stab,'Prentice-Hall. E n g l ~ u d C l i f f s N.J., . 1969. I21 Heller, D. F., Freed, K. F., and Gelbart. W. M., J. Chrm. Phys.. 56, 2309, 1872: and Jortncr, J.. Rice. S. A,, and Hachstraszer. R. M.. "Advances in Photoehemirtry." Vol. 7, (Editam: N q e . Jr.. W. A,. Pitts, J. N.. and Hsmmond. G I Wile?-lnkmckmr NewYork, 1969. (31 Turro, N. J., "Molecular Photoehemiafry,' W. A. Benjamin. New York. 1967, and Calien, J. G.. and Pitfs, J . N.. "Photoebemistry," Wiloy-lnfe~cience.New York, 1966. %.%""a, M. W., -Quantum Mechsnies in Chemistry." W. A. Benjamin. N",. Ymk. C h a ~ t . 5see. . I.
..CChapt.3. h~~t.5. .Chap,.?.
, Chapt. 8. Sec. 5.6.
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~
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