J. Phys. Chem. 1996, 100, 10015-10020
10015
Excited State Dynamics of 9,10-Diazaphenanthrene Studied by the Time-Resolved Transient Grating Method Makoto Takezaki, Noboru Hirota, and Masahide Terazima* Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606, Japan ReceiVed: January 25, 1996; In Final Form: April 17, 1996X
The heat-releasing processes of the photoexcited states of 9,10-diazaphenanthrene (DAP) in ethanol have been directly detected using the picosecond time-resolved transient grating (TG) method. From the heatreleasing rates, a surprisingly short lifetime of the lowest excited triplet state (∼13 ns) and a relatively long lifetime of the lowest excited singlet state (4.3 ns) are detected. By measuring the heat energy coming from the excited state of DAP together with the fluorescence quantum yield measurement, the quantum yield of the triplet formation is found to be about unity. The unique photophysical properties of DAP are also discussed.
1. Introduction Since the energy relaxation of excited molecules after photoexcitation is one of the main issues in molecular spectroscopy, various detection methods (such as emission detection and transient absorption techniques) have been used so far to elucidate the photophysical processes of many molecules. However, there remain several fundamental molecules for which excited state dynamics have not been clarified, because these traditional techniques are hard to apply. For example, although the character and dynamics of the photoexcited states of most diazaphenanthrenes have been clarified,1-4 the excited state properties of 9,10-diazaphenanthrene (DAP) are quite different from those of the other diazaphenanthrenes, and they are not well understood. Although the lowest excited singlet (S1) state of DAP is 1nπ* in character,5 the fluorescence can be detected easily. Once the lowest excited triplet (T1) state of DAP was understood to be 3ππ* in character, the T1 state was reassigned to a 3nπ* state from the time-resolved EPR studies.6,7 Despite the 3nπ* character, which is generally considered to cause strong phosphorescence, DAP does not phosphoresce in a neat crystal or in organic solvents at any temperature. In highly polar solvents (such as 2,2,2-trifluoroethanol8) and polyhydroxy alcohols (such as ethylene glycol9), the T1 state is a 3ππ* state and it phosphoresces. Surprisingly, the triplet lifetime (τT) was found to be very short (100 ns) in a solid phase at 3.0 K by the time-resolved EPR method.10 These properties (characters) observed at low temperatures in the solid phase are very unusual and unique. Although the time-resolved EPR method is powerful enough to reveal these features, this method can only detect the paramagnetic state. It is desirable to study the entire energy relaxation processes after the photoexcitation by another spectroscopic method. Furthermore, the excited state dynamics of DAP have not been clear at high temperatures in solution. One reason for the ambiguity is that the dominant relaxation process of this molecule is nonradiative transition. If we can study the excited state dynamics by detecting the nonradiative transition, the dynamics will be much clearer. For this reason, photothermal methods, for example, transient grating (TG) and thermal lens (TL) methods, are suitable for the study of the energy relaxation processes. In particular, the TG method is highly sensitive and useful for studying the temporal profile of the heat-releasing processes from very short to rather long time ranges. In fact, the enthalpies of formation of diradicals,11-13 X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00254-7 CCC: $12.00
the vibrational relaxation processes of excited proteins in water,14 the energy of the excited twisted phantom state, the volume change between the ground and the twisted excited state of tetraphenylethylene,15,16 the heat deposition yield of the radiationless process,17 and the quantum yield of the triplet formation (ΦISC)18-21 have been studied by the time-resolved TG method over a very wide time range. In this study we have applied the TG method to the dynamics of the photoexcited states of DAP from picosecond to nanosecond time scales. Combining the result obtained by the fluorescence detection method, we can clarify the excited state dynamics of this unique molecule. 2. Experimental Section The experimental setup for the picosecond time-resolved TG experiment is shown in Figure 1. We used two laser systems in the picosecond time region. One of them consists of a dye laser (Coherent Satori 774) pumped by the SHG output from a Nd:YAG laser (Coherent Antares 76-S). The pulse (∼180 fs) was amplified with a dye amplifier system (Continuum PTA60) pumped by a regenerative pulsed Nd:YAG laser (Continuum RGA60). The wavelength of the dye laser was 640 nm. The amplified dye beam was split into three with two beam splitters. Two of these beams were focused by lenses (f ) 25 cm), after the frequency was doubled to a wavelength of 320 nm by BBO crystals. These UV beams, having power and spot size of ∼1 µJ/pulse and ∼1 mm φ, respectively, were crossed at about 30° inside a quartz sample cell to generate an optical interference pattern. The other beam (a probe beam), after passing through an optical delay line, was focused by a lens (f ) 20 cm) and brought into the sample cell at an angle that satisfied the Bragg condition. The diffracted probe beam, TG signal, was separated from other pulse beams using a pinhole and a glass filter (Toshiba R-60) and was detected by a photomultiplier tube. The detected signal was averaged with a boxcar-integrator (EG&G Model 4400 Series) and with an Apple-Macintosh computer (Quadra-950). The other laser system consists of a dye laser system (Continuum PD10) pumped by a Nd:YAG laser (Continuum PY61c-10). The wavelength of the dye laser, which was used as a probe beam, was 640 nm. The third harmonic (355 nm) of the Nd:YAG laser beam was used as the pump beam. The width of these pulses was ∼20 ps. The pump beam was split with a beam splitter and crossed at about 30° inside the sample cell (the laser power was ∼2 µJ/pulse and the spot size was © 1996 American Chemical Society
10016 J. Phys. Chem., Vol. 100, No. 24, 1996
Takezaki et al. interference pattern with a fringe spacing Λ22,23
Λ)
λex 2 sin(θ/2)
(1)
where λex is the excitation light wavelength. Molecules are excited in the bright region of the optical interference pattern and deposit the energy by radiationless relaxation processes. By the sinusoidal thermal expansion of the medium, an acoustic (sound) wave is generated. The probe light is diffracted by the optically induced grating. According to the diffraction theory in optics, the diffraction efficiency, η, for a thick grating is given by24
η ) A[(∆n)2 + (∆k)2] Figure 1. Schematic illustration of the picosecond time-resolved TG experiment setup using the subpicosecond laser system: L, lens; C, SHG crystal; S, sample; P, pinhole; F, filter; PM, photomultiplier tube; CPU, computer.
∼1 mm φ). The probe beam was crossed at the Bragg angle. The signal detection and analyzing systems were the same as those mentioned above. For the TG experiment in the nanosecond time scale, an excimer laser (Lumonics HyperEX-400 XeCl; λ ) 308 nm) with a 10 ns pulse width was used as an excitation beam and a HeNe laser beam as a probe beam. The diffracted probe beam was isolated from the excitation laser beams with a glass filter (Toshiba R-60) and a pinhole, detected by a photomultiplier tube (Hamamatsu R-928) and fed into a digital oscilloscope (Tektronix TDS-520). The TG signal was averaged by a microcomputer to improve the S/N ratio. For a steady state fluorescence measurement, a conventional fluorometer (Shimadzu RF502A) was used. Quinine sulfuric acid solution (1.0 × 10-5 M) was used for calibrating the sensitivity of the spectrometer. The concentration of the DAP ethanol solution was 1.5 × 10-5 M. The optical densities were measured by using a UV-visible absorption spectrometer (Shimadzu UV-3000). The lifetime of the fluorescence of DAP was measured by a time-correlated single photon counting system. The laser system for the fluorescence lifetime measurement consists of a modelocked Ti:sapphire laser (Spectra-Physics Tsunami) pumped by an Ar+ laser beam (Spectra-Physics Lok2060). The pulse width of the laser beam was ∼2 ps. The excitation beam at 280 nm was the third harmonic of the laser beam generated by using LiIO3 and β-BBO crystals. The excitation beam was focused by a lens (f ) 25 cm) inside a quartz sample cell. The fluorescence was dispersed by a monochromator (Nikon PD250) and detected by a microchannel plate (Hamamatsu 2049U). The fluorescence signal was averaged using a microcomputer. All experiments were carried out at room temperature. DAP (Aldrich), ethanol (spectrophotometric grade of Nacalai Tesque Inc.), and sulfuric acid (guaranteed reagents of Nacalai Tesque Inc.) were used without further purification. Quinine (Nacalai Tesque Inc.) was purified by precipitation from its sulfate solution using ammonia water and then recrystallized. Azobenzene and trans-stilbene were obtained from Nacalai Tesque Inc. and purified by recrystallization from ethanol. 3. Analysis The time dependence of the thermally induced TG signal has been well characterized so far. Here we briefly summarize the principle of the measurement and the method of analysis. Two excitation laser beams, spatially and temporally overlapped inside a sample at an angle θ, produce an optical
(2)
where A is a constant that is determined by the experimental condition, such as the crossing angle, the optical density at the excitation wavelength, or the grating thickness. ∆n and ∆k denote the peak-null differences in the refractive index and extinction coefficient, respectively, which are induced by the optical interference pattern. If there is no absorption and no refractive index change from excited molecules at the probe wavelength, the dominant contribution comes from the modulation of the refractive index caused by the variation of density (F).14 (The so-called temperature grating component is much weaker in organic solvents.25) The time dependent ∆n is described by14
∆ns ) ∑2qBQi(b2 + ω2)-1[(b/ω) sin(ωt) exp(-dt) i
cos(ωt) exp(-dt) + exp(-bt)] 2qBQi(γi2 + ω2)-1 [(γi/ω) sin(ωt) exp(-dt) cos(ωt) exp(-dt) + exp(-γit)] (3) where q ) 2π/Λ, ω ) Vq (V; the sound velocity), d is the acoustic attenuation rate constant, b ) λwq2/FCp (λw is the thermal conductivity, and Cp is the specific heat), B is a constant determined by the physical properties of the solvent, 1/γi is the lifetime of the excited state i, and Qi is the thermal energy in the relaxation processes from the excited state i. Since the experimentally observed TG signal is represented by the convolution of the impulsive response with the heat-releasing process, a time dependence of the nonradiative transition much faster than the acoustic period cannot be determined accurately. Therefore, the time resolution of the TG signal becomes higher with shortened acoustic period or, equivalently, fringe spacing. To make the time resolution better, we performed the picosecond TG experiment at a large crossing angle, ∼30°. For the nanosecond pulsed laser excitation, the oscillation period is much shorter than the excitation pulse width, and eq 4 convoluted with the pulse shape can be reduced to21
∆ns ≈ ∑2qBQiC[exp(-bt) - exp(-γit)]
(4)
i
where C ) 1/[b2 + (Vq)2]. If excited molecules contribute to the refractive index at the probe wavelength, the change of the refractive index due to excited molecules is described by
∆np ) ∑RiCi(t)
(5)
i
where Ri is a constant that is determined by the refractive index of the excited state and Ci(t) the concentration of the excited molecule. In this case, the time dependent TG signal ITG is
Excited State Dynamics of DAP
J. Phys. Chem., Vol. 100, No. 24, 1996 10017
Figure 2. (a) TG signal of trans-stilbene/ethanol with the nanosecond excimer laser excitation (dotted), response curve (broken line), and the best fitted curve (solid line). (b) TG signal of 9,10-DAP/ethanol (dotted), best fitted curve (solid line), fitted curve of (a) (broken line), and response curve (dotted line).
described by
ITG ∝ (∆ns + ∆np)2
(6)
4. Results 4.1. Time-Resolved TG Measurement. First, the relatively slow dynamics of DAP was studied by using the nanosecond laser pulse for the excitation (excimer laser) and the He-Ne laser for the probe. To determine the response function of our experimental setup and the rate of thermal conduction, the temporal profile of the TG signal after the photoexcitation of trans-stilbene in ethanol was measured (Figure 2a). It is already known that the excited state of trans-stilbene in ethanol relaxes with a very short lifetime (∼40 ps26) compared with the pulse width of the excimer laser. The response function of our system was measured by monitoring the fluorescence of trans-stilbene in ethanol. The observed TG signal after the photoexcitation of trans-stilbene can be fitted well with a single-exponential decay function (the lifetime is 6.54 µs) with a very fast rise [within the time resolution of the experimental setup (5 ns)], which is convoluted with the response function. The observed very fast rise time of the signal is consistent with the short lifetime of trans-stilbene. The observed TG decay is due to the thermal conduction (diffusion) between the fringes (1/b ) 6.54 µs). The temporal profile of the TG signal after the photoexcitation of DAP in ethanol is shown in Figure 2b. This profile was best fitted by two exponential curves and the thermal conduction using eqs 4 and 6 convoluted with the response function. The relaxation lifetimes (1/γ2n and 1/γ3n) are e5 and 13.4 ( 0.3 ns, respectively. The ratio of the heats released with these relaxation times, |Qn(γ2n)|:|Qn(γ3n)|, is 0.45 ( 0.01:0.55 ( 0.01. To investigate faster dynamics, the TG signal was measured with the picosecond laser excitation. In the fast time scale, the
Figure 3. (a) Picosecond time-resolved TG signal of azobenzene/ ethanol obtained by using the subpicosecond pulsed laser system (pump 320 nm, probe 640 nm) (circles) and fitted curve (solid line). (b) TG signal of 9,10-DAP/ethanol (circles) and fitted curve (solid line). (c) TG signal of 9,10-DAP/ethanol obtained by using the picosecond laser system (pump 355 nm, probe 640 nm) (circles) and fitted curve (solid line).
acoustic oscillation appears and the fast dynamics should be extracted from the observed TG signal. Before the temporal profile of the TG signal from DAP is analyzed, it is desirable to have the acoustic signal under the impulsive heat dispersion condition for determining the sound speed and the acoustic attenuation constant of the solution. We used azobenzene in ethanol for this purpose. Figure 3a shows the time profile of the picosecond time-resolved TG signal of azobenzene in ethanol excited at 320 nm by the subpicosecond pulse laser system. The TG signal after the picosecond pulse laser excitation is similar to that shown in Figure 3a. It is already known that the excited state of azobenzene relaxes with very short lifetimes (25 ( 5 ps for the S1 state27 and
