13668
J . Phys. Chem. 1993, 97, 13668-13672
Separation of Transient Absorption and Population Lens Effect from the “Thermal Lens” Signal Masahide Terazima,’ Takashi Hara, and Noboru Hirota Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606 Japan Received: August 6, 1993; In Final Form: September 8, 1993”
For calorimetric measurements by a photothermal technique such as the thermal lens method, it is important to ensure that only the thermal effect can be detected. A method for evaluating the contribution of the pure thermal lens (TL) in the observed “TL,, signal” is presented in a case in which the TL signal is contaminated by the transient absorption and/or the population lens effect. This method is based on a solvent dependence of the signal intensity together with the concave-convex lens (CCL) method. As an example, this method is applied to evaluate the quantum yield of the triplet formation of anthracene probed a t a wavelength near the triplet-triplet absorption.
1. Introduction
In the thermal lens (TL) experiment, the change of the refractive index in solution after photoexcitation is detected as the variation of the probe beam light density in the irradiated region. Usually the modulation of the refractive index is assumed to come from the heating effect of the matrix by the radiationless transition from the photoexcited states.14 Since the signal intensity is proportional to the amount of heat as long as the above assumption is correct, it has been used as a sensitive and useful method to trace the photophysical ‘dark” processes (Le., the processes without emission) or chemical reacti~ns.~-l I Because of the high sensitivity of this method, however, the signal could reflect the variation of the refractive index not only due to the heating effect but also due to the different optical properties between the ground and metastable states. In fact, Terazima and Hirota have demonstrated the existence of such a contribution, which is called the population lens, in the observed “TL,, signal”.I2 (In this paper, we use the TL,, signal as the variation of the probe light intensity detected through a pinhole under the normal TL experimental condition. The TL,, signal may containother contributions besides the pure TL signal, which comes from the probe light density variation due to a refractive index lens composed of only the thermal effect.) In some cases, a transient absorption (TA) of the probe light due to the metastable statemay contributein theTL,, signal.I2J3 Thereforeoneshould somehow eliminate the contributions of PL and TA, if we want to use the TL method as one of the photothermal techniques to detect or measure the heat from the photoexcited states. In other words, we should measure only the pure TL signal, which is proportional to the heat energy. Recently we have proposed a new method, called the concave-convex lens (CCL) method, to quantitatively evaluate the contribution of the TA in the TL,, ~igna1.I~However, the separation of the PL contribution from the TL signal has not been made. In this paper we report an experimental method to separate out the PL contribution from the observed TL,, signal. The principle of this method is based on the fact that the pure TL effect depends on the thermal properties of the medium, while the PL effect does not. We choose anthracene as an example for studying this effect. Previously we have shown that the TA and PL signals are included in the observed TLex signal probed at 458 nm.I3 Therefore the photophysical properties such as the quantum yield of the triplet formation (disc)should not be accurately determined from the time profile of the TL,, signal probed at this wavelength. We determine 4iScunder this condition to demonstrate the applicability of the CCL method and the solvent
* Abstract published in Advance ACS Absrructs. November 15,
1993.
dependence method. At the same time, a similar experiment is performed for quinoxaline probed at 633 nm. The TL,, signal observed for this molecule is known to be composed of only the pure TL signal. 2. Metbod
The relative contribution of the PL in the observed TL,, signal is evaluated by the solvent dependence of the time profile. After a molecule absorbs a photon (photon energy hv,,), various photophysical processes may take place. If the molecule is photochemically stable, a typical energy flow diagram can be illustrated, as shown in Figure 1. The vibrational relaxation in the lowest excited singlet (SI) state, kvib, and the decay of the SI state, (kr ki, kisc)-I,are much faster than the time resolution of our experiment (- 100 ns). Therefore the energies coming out from these processes are converted into the thermal energy altogether in our time scale. When the lowest excited triplet (Ti) state is created during the decay of the SI state with a quantum yield of the TI state could live longer than the time resolution. The decays of the T I state (k,,ka,and kz) result in an increase of temperature of the solution with relatively slow time constants. Therefore if we can monitor the pure TL signal, which is proportional to the heat energy, the signal should rise promptly to a certain intensity (U&,) and then slowly to an intensity Ulkow. The relative intensity of in the total signal is given by intensity (U:;, = UILow +
+ +
where ETis the energy of the T I state, Ef is the average photon energy of fluorescence, and & is the quantum yield of the fluorescence. However, since the metastable state (Ti state) exists during the observation time, we expect the contribution of the PL (intensity: UPL) in the signal besides the pure TL signal. Even in such a case, after a time long enough for the decay of the TI state but shorter for the decay of the TL signal due to the thermal diffusion, the total signal intensity (UTOT) is given by that of the TL signal (U;kT). The relative intensity of the slow rising component of the observed TL,, signal (USLOW) in that situation is represented by USLOW -- ulkow + UP, ---G k o w ;
UTOT
G b T
u%T
4.
(2)
u%T
Under a usual TL experimental configuration, the TL signal intensity after depositing a certain amount of thermal energy
0022-3654/93/2097-13668$04.00/0 0 1993 American Chemical Society
Thermal Effect Detection in the Thermal Lens Method
The Journal of Physical Chemistry, Vol. 97,No. 51, 1993 13669
F v 1. Photophysical processesafter an excitation of a photochemically stable molecule. ( H ) is given
(3) where A is a constant which includes the parameters of the experimental configuration, p is the density, C, is the specific heat capacity at a constant pressure, n is the refractive index, T is the temperature, and S is defined by (l/pC,)(dn/dT). The second equation comes from the fact that the thermal energy H should be proportional to the concentration of the excited state (ex) and A'= AE4,where E is the energy difference between thestates and C$ is thequantum yield of the radiationlesstransition. Combining eqs 2 and 3, one can obtain USLOW/UTOT as
(4)
where A" is a parameter of the experimental configuration for the PL, D = A"/A', and
Further, f;. is the oscillator strength of an ith absorption band with a central frequency oi and a width yi. The first term in eq 4 is determined solely by the photophysical properties of the solute. The factor l/nS in the second term can be varied by changing the temperature of the medium or by changing the solvent. When we measure USLOw/UTOT under various conditions, the plot of the observed USLOW/UTOT value against 1/Sn should give a straight line with an intercept of U&,w/UT&T. In this way, the useful quantity U & w / U T & T can be determined experimentally. In this work, we use various solvents (a series of alkanes) for changing the value of l/nS. A similar method using the solvent dependence has been applied to determine the reaction volume change in the photoacoustic experiment.I4J5 However there is a difference in this TL experiment from the PA experiments besides the purpose of the measurements. Here, we measure the relative signal intensity of USLOW against UTOT, not the absolute signal intensity. Therefore, the uncertainties due to the factors such as the fluctuation of the excitation laser power, a slight misalignment of the sample cell, and a concentration variation from a sample to another are expected to be less serious as long as the signal intensity is proportional to the thermal energy during a single experiment. There are several assumptions which should be satisfied to give a reliable value by this method. First, the basic assumptions for the TL experiment should be satisfied: the variation of the refractive index should be small, multiphoton process does not
participate in the excitation process, and so on. The validity of these assumptions should be confirmed under the experimental configuration by checking the linearity of the TL signal against the input thermal energy. Second, the optical properties of the solute should not depend on the solvent, at least in a series of solvents we use. For example, the absorbance and the wavelength of the transient absorption, 4iscrC$f and ET,Ef,should be constant in all the solvents. If the absorbance from the ground state of the solute depends on the solvents, the signal intensity (UTOT) should be corrected for that variation, but it will not affect the U s L o w / U r o T value.
3. Experiments The excited state of a photochemically stable molecule was created by irradiation of the laser light (A = 308 nm) from an excimer laser (Lumonics Hyper 400). After shaping the laser beam by a pinhole, the beam was focused into a quartz sample cell (path length 5 mm) with a convex lens cf = 20 cm). A He-Ne laser or an Ar ion laser was used as a probe beam. The beam was combined and made collinear with the excitation beam on a surface of a quartz plate. A 458-nm line of the Ar ion laser was selected by a monochromator. The probe beam was expanded by a concave or a convex lens (focal length of both of the lenses was 20 cm) after passing through the sample. The probe beam was detected by a photomultipliertube (Hamamatsu R-928)after the excitation light was removed by a solution filter and a glass filter. The signal was fed into a digital oscilloscope (Tektronix 2430A) and averaged by a microcomputer. The excitation laser power was adjusted by a neutral density filter so that the signal intensity is in the linear response range to the input power of the laser (
