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Relaxation processes of highly excited naphthalene in solution studied by time-resolved thermal lensing technique
16 I
Chemical Physics North-Holland
( 1992) 447-452
Relaxation processes of highly excited naphthalene in solution studied by time-resolved thermal lensing technique Tadashi Suzuki, Yoshizumi
Kajii, Kazuhiko Shibuya and Kinichi Obi
Department of Chemutty.Tokyo Institute of Technology, 2-12-I Ohokayama, Meguro-ku, Tokyo 152, Japan Received
15 November
199 1
Relaxation dynanncs of naphthalene in the highly excited singlet state has been studied by the time-resolved thermal lensing (TRTL) technique with nanosecond laser excitation in hexane solutlon. Heat converslon efficiency of naphthalene m the first excited smglet (S, ) state IS measured to be 0.80 k 0.0 1, giving the fluorescence quantum yield of 0.19 5 0.0 I By analyzing the time evolution of the TRTL signal, the quantum yield oftriplet formation and the triplet hfetlme are determined to be 0.79 k 0.01 and 35 i 1 ps, respectively. Extinction coeffkent of S, +S, absorption at 308 nm is 850 M-’ cm-’ and the two-photon absorption (S,,+S,tS,) obviously occurs in the laser power region above 6.7~ 102“ photon cm-’ s-’ (20 pJ). Relaxation pathways from the S, state are mvestlgated on the basis of excitation laser power dependence of the TRTL signal, fluorescence intensity, and triplet-tnplet absorption. It is demonstrated that ( 1) 82% of S, relaxes to S,, (2) 18% relaxes to So, and (3) the S,+T, intersystern crossing IS negligible. The observation of the S, +SO internal conversion ( 18%) IS understood by occurrence of prompt S,+S,, followed by accelerated S, *So internal converSion due to large excess energy.
1. Introduction The experimental studies have been extensively carried out on the excitation energy dependency of radiationless transition rates [ l-5 1. Under the collision-free conditions, gaseous aromatic molecules such as naphthalene, anthracene, and tetracene fluoresce in the same spectral range whether they are excited into a highly excited (S,) state or into the first excited (S, ) state [ 11. This implies that internal conversion to the S, state takes place in an isolated molecule as an intramolecular phenomenon. Lim and coworkers [ 2,3] measured the fluorescence lifetimes and quantum yields of naphthalene and its derivatives, and determined the nonradiative decay rates as a function of the excitation energy. The nonradiative decay rate slowly increases with excess energy near the S, origin (32200 cm-’ for naphthalene-h,), and grows exponentially above the excess energy of 2000 cm-’ from the origin. It is accepted that the former dependence near the origin is attributed to the enhancement of the S1-tTl intersystem crossing, while the latter steep dependence is due to the S, -+So internal conversion. The calculation showed that the ex0301-0104/92/$05.00
0 1992 Elsevier Science Publishers
cess energy was not completely redistributed among all the normal modes but rather distributed in a class of selected accepting modes [ 41. In a recent supersonic free jet experiment, it was found that anthracene and its derivatives in the S, state underwent intersystem crossing to the near-lying triplet state (T,)
151. In the condensed and high-pressure gas phases, the spectral shape and the quantum yield of fluorescence are generally independent of the excitation energy [ 61. Even if a molecule is excited into a highly excited state in solution, the S,+S, internal conversion dominates intersystem crossing and fluorescence. The fluorescence quantum yield of a highly excited state is extremely low [ 71 except for azulenes [ 81. As a result, fluorescence and intersystem crossing proceed through the thermally equilibrated S, state. The surrounding solvent molecules control the relaxation mechanism of the excited molecule. Raman scattering [ 9, lo] and picosecond transient absorption measurements [ 111 indicate that the solvent molecules deactivate the vibrationally excited molecules in the time order of 1- 10 ps. The time-resolved thermal lensing (TRTL) tech-
B.V. All nghts reserved.
448
T. Su-_ukl et al /
Relaxationof hlghiy-e.wted naphthalene
nique is powerful to study the nonradiative processes [ 121. In this paper, we investigate the nonradiative processes of naphthalene using the TRTL technique coupled with the nanosecond laser flash excitation, and report the dynamics of the highly excited 64900 cm-‘, corresponding to the photoex(&xc= citation at 154 nm) naphthalene in hexane solution.
2. Experimental A XeCl excimer laser (Lambda Physik LPX 105; 20 ns, 130 mJ/pulse) was used as an excitation light source. The excitation laser beam was focused with a 120 mm focal length lens into a flow sample cuvette (NSG T-59FL- 10; 10 mm light path length). The excitation beam waist was determined to be 150 urn by sweeping a 10 urn pinhole. The laser power introduced into the sample cell was monitored by a silicon photodiode (Hamamatsu Photonics S 1336-SBQ), after being attenuated through a variable neutral density filter (Corion 2161). The photodiode was calibrated with a pyroelectric detector (Gentec ED 100 ) . The time profile of the laser pulse was measured with a biplanar tube (Hamamatsu Photonics R-617). A He-Ne laser (NEC GLG5380; 1.5 mW) used as a monitoring light for TRTL signals was focused with a 30 mm focal length lens before the cell and introduced collinearly with the excitation beam. The TRTL signals were obtained through a pinhole (Corion 240 1: 300 urn diameter) and a monochromator (Nikon P250), and detected by a photomultiplier (Hamamatsu Photonics R928 ). The output signals were converted into the voltage with a 50 or 500 Q load resistor, and measured by a transient memory (Iwatsu DM901; 10 ns/word; 2 kwords memory) or a digital storage oscilloscope (Gould 4047; 400 MHz) connected with a personal computer (NEC PC9801F2). The TRTL signals were averaged over 50 shots to improve the S/N ratio. In the transient absorption experiments, a Xe flash lamp (Ushio UXLl50DS; 150 W) used as a monitoring light was introduced into the cell with an optical fiber. The power dependence measurements of transient absorption and fluorescence were carried out under the same excitation condition as that of the TRTL experiments. When the signal was weak, we used a preamplifier (ORTEC 574) and an oscillo-
scope (Tektronix 7904) equipped with a differential comparator (7A13) and a time base (7B92). Absorption spectra were measured with a double beam spectrophotometer (Shimadzu UV-2200). Fluorescence spectra were recorded with a spectrofluorometer (Jasco FP-550A), and were corrected afterward for the spectral response. Naphthalene (Tokyo Kasei) was purified by recrystallization several times in hexane and subsequently in ethanol. Hexane (Kant0 Chemical; GR grade) was used without further purification. The sample solution was deaerated with Ar gas (purity 99.95%) saturated with hexane vapor for half an hour before use.
3. Results and discussion 3.1. Photophysicalproperties of naphthalene exated to the S, state Fig. 1 shows the typical TRTL signal of naphthalene in hexane solution at low laser power (less than 1 uJ ), which sharply rises immediately after laser irradiation, subsequently grows slowly, and becomes a
0
100 Time
201
( ,vs)
Fig. 1. Typlcal TRTL signal of naphthalene in hexane solution at low laser power. The mset shows the time profile of the triplettriplet absorption momtored at 420 nm. The top figure shows residuals of curve fitting and CJis an error of mean square.
T. Suzukr et al. / Relaxation ofhrghly-e.mted naphthalene
plateau after 200 us. After laser irradiation, an excited naphthalene molecule relaxes to the ground state through several photophysical pathways: vibrational relaxation, fluorescence, internal conversion, and intersystem crossing to the triplet manifold. First, we determined the heat conversion efficiency (a), namely the fraction of the energy released as heat against the total energy absorbed. The procedures to derive the a value have been already described elsewhere in detail [ 131. All the photophysical phenomena are completed within 200 us. and therefore the TRTL signal intensity at 200 us is proportional to the total heat (UT) released by the nonradiative relaxation processes. Fig. 2 plots Ur versus laser power (IL) for various naphthalene absorbances (A). The plot for a fixed absorbance shows good linearity in the laser power region examined. To eliminate the experimental factors including solvent properties [ 12,131, the slopes, Ur/I,, are plotted against the light-absorption fraction, l-10- 4 in fig. 3, where A denotes absorbance at 308 nm. Pyridazine, whose LYvalue was established to be unity [ 131, was employed as a calorimetric standard. Caution was taken to keep the experimental conditions to be strictly the same for both calorimetric systems. The solid lines in fig. 3 were determined by the least-squares method. Comparing slopes of these lines, we obtained the a value to be 0.80 * 0.01 for naphthalene. Because naphtha-
449
Fig. 3. Plots of the slopes t;/I, of the solid lines in fig. 2 agamst the light absorption fraction ( l-l O- ‘) of pyrldazine (-•-) and naphthalene (-II-). Both plots show good linear relation.
lene emits fluorescence, a= (&x -@&)I&,
>
the (Yvalue is expressed as (1)
where E,,, Es, and @rare the energy of the excitation light, the S, energy, and the fluorescence quantum yield, respectively. The contribution of phosphorescence can be neglected because of the low quantum yield under this experimental condition. When the fluorescence spectrum spreads widely, Es must be replaced by (E
>
S
=
I zr(f) v dv j”&(F) dv
’
(2)
where cence aged cm-’ Then, with
Zr(Y”) is the spectral distribution of fluoresand v”is the energy in wavenumbers. The aver(Es) value was determined to be 2.95 x lo4 from the measured fluorescence spectrum. eq. ( 1) gives @rto be 0.22 + 0.0 1, which agrees the reported fluorescence quantum yield (q+= 0.19 [ 141). These results are listed in table 1, together with the data of naphthalene in acetonitrile [131.
Laser Power
IL
( p J)
Fig. 2. Plots of UT versus laser power (IL) for various naphthalene absorbances (A) at 308 nm. The solid lines are determmed with the least-squares method. The absorbances are 1.05 1,0.699, 0.494.0.385.0.304,0.192,0.144,0.088,and0.035,downward.
The slow component of the TRTL signal grows exponentially with a rise time of 35 _+1 us (the leastsquares fit), which accords with the lifetime (36 -t 1 us) of the triplet naphthalene separately determined from the triplet-triplet absorption after the flash photolysis (the inset of fig. 1). The slow rising component is due to the heat released by the triplet relax-
T. Suzuki et al. / Relaxatron of hrghly-emted naphthalene
450
Table 1 Photophyslcal properties of naphthalene in solutions Solvent hexane
0.80
acetomtrile
0.83
0.19 (0.19) 0.18 (0.20)
0.79 (0.82) 0.82 (0.80.0.71)
35
2.95x lo4
(38)
2.99x lo4
a’ Values m parentheses from ref. [ 141. b, Value in parentheses from ref. [ 16 1.
ation to the ground state. The solid curve in fig. 1 presents the best fit of the TRTL time profile. The ratio of the slow component, Us, to the total heat is denoted as c’slu,=~,,,E,l(E,,-~~(Es))
3
(3)
where I&, and ET are the quantum yield of the triplet formation and the triplet energy, respectively. Fig. 1 gives the Us/CT, ratio of 0.65. ET is reported to be 2.13 x lo4 cm-’ [ 141. Therefore, @,,, is determined as 0.79 -t 0.0 1, using eq. (3 ). The total quantum yield of fluorescence and triplet formation is unity, and hence the quantum yield of internal conversion is negligibly small upon the photoexcitation of naphthalene at 308 nm. 3.2. Photophysical singlet state
dynamics of the highly excited
Fig. 4 shows the relation of U, versus laser power in higher region of photon density than fig. 2. The upward deviation from the linear relation implies that appreciable amount of heat originates through twophoton processes, which will be discussed later. There are two possibilities for the two-photon processes due to the high triplet quantum yield: sequential twophoton absorption through the S, or T, state. Only the S,eS, absorption can be concluded to be important in this system. First, the fluorescence lifetime of 100 ns is rather long compared to the duration of the laser pulse (20 ns). Second, the extinction coeffcient of T-T absorption at 308 nm is too small to be detected by the transient absorption. Fig. 5 shows the laser power dependences of fluorescence at 349 nm and of T-T absorption at 4 14 nm. Both plots deviate downward from the linear relation in the higher laser power region. These facts demon-
Laser Power
IL
( YJ)
Fig. 4. Laser power dependence of U,. The laser power is much higher than that m fig. 2.
Laser Power
IL
(,uJ)
Fig. 5. Laser power dependences of fluorescence (-+-) and T-T absorption (-•-). The laser power range is the same as that m fig. 4.
T. Suzuki et ul. /
Relaxation
strate that the two-photon excitation is not a coherent but a stepwise process into the S, state. If one assumes a general rule that naphthalene molecules, excited to the S, state, relax to the bottom in the S, state with a quantum yield of unity, one cannot rationalize the experimental results described above. This indicates that the S, molecules generated by twophoton absorption partially undergo photoreactions or photoionization, or disappear by nonradiative transition not through the bottom of S,. The possibility of photoreactions is eliminated because the absorption spectrum did not change with 1O5laser shots: The reaction quantum yield from the S, state is estimated to be less than 10P5. The quantum yield for free ions in the two-photon ionization of anthracene (Z,=7.43 eV in the gas phase) at 308 nm was reported to be as low as 0.0023 in n-hexane [ 15 1. The two-photon ionization quantum yield for naphthalene with higher ionization potential (ZP= 8.12 eV in the gas phase) is expected to be even less than that for anthracene. The diminishing tendency of fluorescence and TT absorption in the high energy region is treated in consequence of enhanced nonradiative transition due to large excess energy. The following relaxation pathway must be considered to explain the present experimental results: in addition to the normal internal conversion to the bottom of the S, state, the S, state relaxes to the So and/or to the triplet states without passing through the bottom of the S, state. The vibrational relaxation is not considered because 308 nm almost corresponds to the origin of the S, +SO transition. Based on this assumption, the following differential equations hold: d[S, lldt=Z(t)
0, [Sol
- (z(t)% +~f+kc+k,c) d[Snlldt=Z(t)
[S, 1+K[S,l
>
(5)
d[T, lldt=ksc[S, 1+k,[Snl -kT[TI 1 / -Z(t) 61[Sol + (h+k,)
+~,[T,l+k,[S,l
(4)
~Is,l
-(~:c+k,+k,)P,l d[%lldf=
1
/
(6)
[S, 1 (7)
where Z(t) is laser flux, (T,, and 0, represent the absorption cross sections of the S, t S,, and S,eS, transitions, respectively. k:,, k,,, and k,, are relaxation rate
ofhtghly-e.n~lted naphthalene
451
constants of the S, to the bottom of S, state, the S, to So state, and the S, to T, state, respectively. It is impossible to solve these equations analytically because the time profile of the laser, Z(t). is not expressed in an analytical form. Therefore, we carried out numerical calculation for the set of differential equations (4 )- ( 7 ) . The kinetic parameters used are as follows: 6, =3.82x 10-‘9cm’, k,=2.Ox 10’s_‘, k,,=O.2x lo6 s-‘arereportedvalues [8],k,,,= 8.5x10’s_‘iscalculated from the fluorescence lifetime and the triplet quantum yield, and kT= 36 ps is the value described before. As recognized from fig. 5, the ratio of fluorescence to T-T absorption intensities is constant. This fact indicates that k:, z+ X-,, and k,, x- kst are good approximations; k,, can be neglected. The fitting parametersusedarea,and@(S,+S,),where@(S,+S,) is the quantum yield of the S,+S, internal conversion defined as qqspS,)=
--!!Gk:, + k,, .
(8)
First, using eqs. (4)-( 7) and each numerical set of a2 and @(S,-tS,), the time profiles of [S,]. [S,], [T, 1, and [So] were simulated for a fixed laser power, and then the total heat ( U, ) was estimated. Second, the observed laser power dependences of fluorescence intensity, triplet concentration, and total heat (U,) can be analyzed with the laser power dependency, y=aZ2+ bZ. The best fit was obtained for a?=3.2~10-” cm’ (corresponding to ~=850 M-’ cm-‘),and@(S,+S,)=0.82. Thesolidlinesinfigs. 4 and 5 are the simulated curves. The value of @(S,+SO) is estimated to be 0.18 from @(S,,+&,) = 1 -$(S,-S,): It is demonstrated that 18% of naphthalene molecules in the S, state relax through the ground state, not through the bottom of the S1 state, even in the condensed phase. As mentioned in section 1, the relation between the internal conversion rate and excitation energy under collision-free conditions was experimentally studied and theoretically analyzed by Lim et al. [ 3,4]. As the excitation energy increases, the internal conversion rate grows exponentially and becomes larger than the intersystem-crossing rate in the excess energy region above 2000 cm-’ from the S, origin. In the condensed phase, when a naphthalene molecule is excited to a relatively low excess energy level in the S, state, the cooling by the surrounding solvent mole-
452
T. Suzuki et al. / Relaxatron ofhrghly-excited naphthalene
cules is known to be fastest among the relaxation processes. On the other hand, the relaxation mechanism of highly excited naphthalene has not been established yet to the best of our knowledge. The highly excited state (S,) prepared by the two-photon absorption at 308 nm will quickly relax to the S, state ( S,+ S, internal conversion), which causes accumulation of vibrational energy at most 32000 cm-’ within S1. The S, +S, internal conversion of vibrationally excited S, molecules is expected to be fast enough to compete with the cooling. The internal conversion rate of isolated molecules with vibrational energy of 32000 cm-’ is roughly estimated to be on the order of 1Oi3 s- ’ [ 4 1. The vibrationally excited molecules in the condensed phase will lose the excess energy through the interaction with the surrounding solvent molecules (cooling). Typical cooling time by hydrocarbon solvents is reported to be 15 ps [ 111. It is, therefore, reasonable that the S, -So internal conversion proceeds at a rate comparable with the cooling rate when the S, molecule has a large vibrational energy like 32000 cm-‘.
5. Conclusion We successfully applied the TRTL technique to the study of relaxation dynamics of highly excited naphthalene prepared by the two-photon absorption at 308 nm in the condensed phase. It is demonstrated that naphthalene molecules in the S, state relax to the bottom of the S, state (82%) and to the highly vibrational excited state of S, followed by internal conver-
sion to the So state ( 18%). This result is interpreted by the accelerated S, + So internal conversion rate with large excess energy.
References [ 1] J.O. Uy and E.C. Lim, Chem. Phys. Letters 7 (1970) 306; B. Stevens and E. Hutton, Mol. Phys. 3 ( 1960) 7 1: R. Wilhams and G.J. Goldsmtth, J. Chem. Phys. 39 (1963) 2008. [2] E.C. Lim and C.S. Huang, J. Chem. Phys. 58 (1973) 1247. [ 31 J.C. Hsteh, C.S. Huang and E.C. Lim, J. Chem. Phys. 60 (1974) 4345. (41 SF. Fischer, A.L. Stanford and E.C. Lim. J. Chem. Phys. 61 (1974) 582. [ 51 F. Tanaka, S. Hirayama and K. Shobatake. Chem. Phys. Letters 164 (1989) 335. [ 61 M. Kasha, Dtscusstons Faraday Sot. 9 ( 1950) 14. [ 71F. Hirayama, T.A. Gregory and S. Lipsky. J. Chem. Phys. 58 (1973) 4696. [ 81 J.B. Birks, Organic molecular photophysics (Wiley, New York, 1979). [ 91 A. Laubereau, D. von der Lmde and W. Kaiser, Phys. Rev. Letters 28 (1972) 1162. [ lo] J.E. Griffths, M. Clerc and P.M. Rentzepis, J. Chem. Phys. 60 (1974) 3824. [ 11 ] U. Sukowski, A. Seilmeier. T. Elsaesser and S.F. Fischer, J. Chem. Phys. 93 ( 1990) 4094. [ 121 S.E. Braslavsky and K. Heihoff, eds.. Handbook of organic photochemtstry (CRC Press, Boca Raton. 1988), and references therem. ]13 T. Suzukt, Y. Kaju, K. Shibuya and K. Obi, Res. Chem. Intermediates 15 (1991) 261. (Dekker, New 114 S.L. Murov, Handbook of photochemtstry York. 1973). [15 1K.H. Schmidt. M.C. Sauer Jr., Y. Lu and A. Liu. J. Phys. Chem. 94 ( 1990) 244. 116 D.A Head. A. Singh, M.G. Cook and M.J. Quinn, Can. J Chem. 51 (1973) 1624.
Chemical Physics North-Holland
( 1992) 447-452
Relaxation processes of highly excited naphthalene in solution studied by time-resolved thermal lensing technique Tadashi Suzuki, Yoshizumi
Kajii, Kazuhiko Shibuya and Kinichi Obi
Department of Chemutty.Tokyo Institute of Technology, 2-12-I Ohokayama, Meguro-ku, Tokyo 152, Japan Received
15 November
199 1
Relaxation dynanncs of naphthalene in the highly excited singlet state has been studied by the time-resolved thermal lensing (TRTL) technique with nanosecond laser excitation in hexane solutlon. Heat converslon efficiency of naphthalene m the first excited smglet (S, ) state IS measured to be 0.80 k 0.0 1, giving the fluorescence quantum yield of 0.19 5 0.0 I By analyzing the time evolution of the TRTL signal, the quantum yield oftriplet formation and the triplet hfetlme are determined to be 0.79 k 0.01 and 35 i 1 ps, respectively. Extinction coeffkent of S, +S, absorption at 308 nm is 850 M-’ cm-’ and the two-photon absorption (S,,+S,tS,) obviously occurs in the laser power region above 6.7~ 102“ photon cm-’ s-’ (20 pJ). Relaxation pathways from the S, state are mvestlgated on the basis of excitation laser power dependence of the TRTL signal, fluorescence intensity, and triplet-tnplet absorption. It is demonstrated that ( 1) 82% of S, relaxes to S,, (2) 18% relaxes to So, and (3) the S,+T, intersystern crossing IS negligible. The observation of the S, +SO internal conversion ( 18%) IS understood by occurrence of prompt S,+S,, followed by accelerated S, *So internal converSion due to large excess energy.
1. Introduction The experimental studies have been extensively carried out on the excitation energy dependency of radiationless transition rates [ l-5 1. Under the collision-free conditions, gaseous aromatic molecules such as naphthalene, anthracene, and tetracene fluoresce in the same spectral range whether they are excited into a highly excited (S,) state or into the first excited (S, ) state [ 11. This implies that internal conversion to the S, state takes place in an isolated molecule as an intramolecular phenomenon. Lim and coworkers [ 2,3] measured the fluorescence lifetimes and quantum yields of naphthalene and its derivatives, and determined the nonradiative decay rates as a function of the excitation energy. The nonradiative decay rate slowly increases with excess energy near the S, origin (32200 cm-’ for naphthalene-h,), and grows exponentially above the excess energy of 2000 cm-’ from the origin. It is accepted that the former dependence near the origin is attributed to the enhancement of the S1-tTl intersystem crossing, while the latter steep dependence is due to the S, -+So internal conversion. The calculation showed that the ex0301-0104/92/$05.00
0 1992 Elsevier Science Publishers
cess energy was not completely redistributed among all the normal modes but rather distributed in a class of selected accepting modes [ 41. In a recent supersonic free jet experiment, it was found that anthracene and its derivatives in the S, state underwent intersystem crossing to the near-lying triplet state (T,)
151. In the condensed and high-pressure gas phases, the spectral shape and the quantum yield of fluorescence are generally independent of the excitation energy [ 61. Even if a molecule is excited into a highly excited state in solution, the S,+S, internal conversion dominates intersystem crossing and fluorescence. The fluorescence quantum yield of a highly excited state is extremely low [ 71 except for azulenes [ 81. As a result, fluorescence and intersystem crossing proceed through the thermally equilibrated S, state. The surrounding solvent molecules control the relaxation mechanism of the excited molecule. Raman scattering [ 9, lo] and picosecond transient absorption measurements [ 111 indicate that the solvent molecules deactivate the vibrationally excited molecules in the time order of 1- 10 ps. The time-resolved thermal lensing (TRTL) tech-
B.V. All nghts reserved.
448
T. Su-_ukl et al /
Relaxationof hlghiy-e.wted naphthalene
nique is powerful to study the nonradiative processes [ 121. In this paper, we investigate the nonradiative processes of naphthalene using the TRTL technique coupled with the nanosecond laser flash excitation, and report the dynamics of the highly excited 64900 cm-‘, corresponding to the photoex(&xc= citation at 154 nm) naphthalene in hexane solution.
2. Experimental A XeCl excimer laser (Lambda Physik LPX 105; 20 ns, 130 mJ/pulse) was used as an excitation light source. The excitation laser beam was focused with a 120 mm focal length lens into a flow sample cuvette (NSG T-59FL- 10; 10 mm light path length). The excitation beam waist was determined to be 150 urn by sweeping a 10 urn pinhole. The laser power introduced into the sample cell was monitored by a silicon photodiode (Hamamatsu Photonics S 1336-SBQ), after being attenuated through a variable neutral density filter (Corion 2161). The photodiode was calibrated with a pyroelectric detector (Gentec ED 100 ) . The time profile of the laser pulse was measured with a biplanar tube (Hamamatsu Photonics R-617). A He-Ne laser (NEC GLG5380; 1.5 mW) used as a monitoring light for TRTL signals was focused with a 30 mm focal length lens before the cell and introduced collinearly with the excitation beam. The TRTL signals were obtained through a pinhole (Corion 240 1: 300 urn diameter) and a monochromator (Nikon P250), and detected by a photomultiplier (Hamamatsu Photonics R928 ). The output signals were converted into the voltage with a 50 or 500 Q load resistor, and measured by a transient memory (Iwatsu DM901; 10 ns/word; 2 kwords memory) or a digital storage oscilloscope (Gould 4047; 400 MHz) connected with a personal computer (NEC PC9801F2). The TRTL signals were averaged over 50 shots to improve the S/N ratio. In the transient absorption experiments, a Xe flash lamp (Ushio UXLl50DS; 150 W) used as a monitoring light was introduced into the cell with an optical fiber. The power dependence measurements of transient absorption and fluorescence were carried out under the same excitation condition as that of the TRTL experiments. When the signal was weak, we used a preamplifier (ORTEC 574) and an oscillo-
scope (Tektronix 7904) equipped with a differential comparator (7A13) and a time base (7B92). Absorption spectra were measured with a double beam spectrophotometer (Shimadzu UV-2200). Fluorescence spectra were recorded with a spectrofluorometer (Jasco FP-550A), and were corrected afterward for the spectral response. Naphthalene (Tokyo Kasei) was purified by recrystallization several times in hexane and subsequently in ethanol. Hexane (Kant0 Chemical; GR grade) was used without further purification. The sample solution was deaerated with Ar gas (purity 99.95%) saturated with hexane vapor for half an hour before use.
3. Results and discussion 3.1. Photophysicalproperties of naphthalene exated to the S, state Fig. 1 shows the typical TRTL signal of naphthalene in hexane solution at low laser power (less than 1 uJ ), which sharply rises immediately after laser irradiation, subsequently grows slowly, and becomes a
0
100 Time
201
( ,vs)
Fig. 1. Typlcal TRTL signal of naphthalene in hexane solution at low laser power. The mset shows the time profile of the triplettriplet absorption momtored at 420 nm. The top figure shows residuals of curve fitting and CJis an error of mean square.
T. Suzukr et al. / Relaxation ofhrghly-e.mted naphthalene
plateau after 200 us. After laser irradiation, an excited naphthalene molecule relaxes to the ground state through several photophysical pathways: vibrational relaxation, fluorescence, internal conversion, and intersystem crossing to the triplet manifold. First, we determined the heat conversion efficiency (a), namely the fraction of the energy released as heat against the total energy absorbed. The procedures to derive the a value have been already described elsewhere in detail [ 131. All the photophysical phenomena are completed within 200 us. and therefore the TRTL signal intensity at 200 us is proportional to the total heat (UT) released by the nonradiative relaxation processes. Fig. 2 plots Ur versus laser power (IL) for various naphthalene absorbances (A). The plot for a fixed absorbance shows good linearity in the laser power region examined. To eliminate the experimental factors including solvent properties [ 12,131, the slopes, Ur/I,, are plotted against the light-absorption fraction, l-10- 4 in fig. 3, where A denotes absorbance at 308 nm. Pyridazine, whose LYvalue was established to be unity [ 131, was employed as a calorimetric standard. Caution was taken to keep the experimental conditions to be strictly the same for both calorimetric systems. The solid lines in fig. 3 were determined by the least-squares method. Comparing slopes of these lines, we obtained the a value to be 0.80 * 0.01 for naphthalene. Because naphtha-
449
Fig. 3. Plots of the slopes t;/I, of the solid lines in fig. 2 agamst the light absorption fraction ( l-l O- ‘) of pyrldazine (-•-) and naphthalene (-II-). Both plots show good linear relation.
lene emits fluorescence, a= (&x -@&)I&,
>
the (Yvalue is expressed as (1)
where E,,, Es, and @rare the energy of the excitation light, the S, energy, and the fluorescence quantum yield, respectively. The contribution of phosphorescence can be neglected because of the low quantum yield under this experimental condition. When the fluorescence spectrum spreads widely, Es must be replaced by (E
>
S
=
I zr(f) v dv j”&(F) dv
’
(2)
where cence aged cm-’ Then, with
Zr(Y”) is the spectral distribution of fluoresand v”is the energy in wavenumbers. The aver(Es) value was determined to be 2.95 x lo4 from the measured fluorescence spectrum. eq. ( 1) gives @rto be 0.22 + 0.0 1, which agrees the reported fluorescence quantum yield (q+= 0.19 [ 141). These results are listed in table 1, together with the data of naphthalene in acetonitrile [131.
Laser Power
IL
( p J)
Fig. 2. Plots of UT versus laser power (IL) for various naphthalene absorbances (A) at 308 nm. The solid lines are determmed with the least-squares method. The absorbances are 1.05 1,0.699, 0.494.0.385.0.304,0.192,0.144,0.088,and0.035,downward.
The slow component of the TRTL signal grows exponentially with a rise time of 35 _+1 us (the leastsquares fit), which accords with the lifetime (36 -t 1 us) of the triplet naphthalene separately determined from the triplet-triplet absorption after the flash photolysis (the inset of fig. 1). The slow rising component is due to the heat released by the triplet relax-
T. Suzuki et al. / Relaxatron of hrghly-emted naphthalene
450
Table 1 Photophyslcal properties of naphthalene in solutions Solvent hexane
0.80
acetomtrile
0.83
0.19 (0.19) 0.18 (0.20)
0.79 (0.82) 0.82 (0.80.0.71)
35
2.95x lo4
(38)
2.99x lo4
a’ Values m parentheses from ref. [ 141. b, Value in parentheses from ref. [ 16 1.
ation to the ground state. The solid curve in fig. 1 presents the best fit of the TRTL time profile. The ratio of the slow component, Us, to the total heat is denoted as c’slu,=~,,,E,l(E,,-~~(Es))
3
(3)
where I&, and ET are the quantum yield of the triplet formation and the triplet energy, respectively. Fig. 1 gives the Us/CT, ratio of 0.65. ET is reported to be 2.13 x lo4 cm-’ [ 141. Therefore, @,,, is determined as 0.79 -t 0.0 1, using eq. (3 ). The total quantum yield of fluorescence and triplet formation is unity, and hence the quantum yield of internal conversion is negligibly small upon the photoexcitation of naphthalene at 308 nm. 3.2. Photophysical singlet state
dynamics of the highly excited
Fig. 4 shows the relation of U, versus laser power in higher region of photon density than fig. 2. The upward deviation from the linear relation implies that appreciable amount of heat originates through twophoton processes, which will be discussed later. There are two possibilities for the two-photon processes due to the high triplet quantum yield: sequential twophoton absorption through the S, or T, state. Only the S,eS, absorption can be concluded to be important in this system. First, the fluorescence lifetime of 100 ns is rather long compared to the duration of the laser pulse (20 ns). Second, the extinction coeffcient of T-T absorption at 308 nm is too small to be detected by the transient absorption. Fig. 5 shows the laser power dependences of fluorescence at 349 nm and of T-T absorption at 4 14 nm. Both plots deviate downward from the linear relation in the higher laser power region. These facts demon-
Laser Power
IL
( YJ)
Fig. 4. Laser power dependence of U,. The laser power is much higher than that m fig. 2.
Laser Power
IL
(,uJ)
Fig. 5. Laser power dependences of fluorescence (-+-) and T-T absorption (-•-). The laser power range is the same as that m fig. 4.
T. Suzuki et ul. /
Relaxation
strate that the two-photon excitation is not a coherent but a stepwise process into the S, state. If one assumes a general rule that naphthalene molecules, excited to the S, state, relax to the bottom in the S, state with a quantum yield of unity, one cannot rationalize the experimental results described above. This indicates that the S, molecules generated by twophoton absorption partially undergo photoreactions or photoionization, or disappear by nonradiative transition not through the bottom of S,. The possibility of photoreactions is eliminated because the absorption spectrum did not change with 1O5laser shots: The reaction quantum yield from the S, state is estimated to be less than 10P5. The quantum yield for free ions in the two-photon ionization of anthracene (Z,=7.43 eV in the gas phase) at 308 nm was reported to be as low as 0.0023 in n-hexane [ 15 1. The two-photon ionization quantum yield for naphthalene with higher ionization potential (ZP= 8.12 eV in the gas phase) is expected to be even less than that for anthracene. The diminishing tendency of fluorescence and TT absorption in the high energy region is treated in consequence of enhanced nonradiative transition due to large excess energy. The following relaxation pathway must be considered to explain the present experimental results: in addition to the normal internal conversion to the bottom of the S, state, the S, state relaxes to the So and/or to the triplet states without passing through the bottom of the S, state. The vibrational relaxation is not considered because 308 nm almost corresponds to the origin of the S, +SO transition. Based on this assumption, the following differential equations hold: d[S, lldt=Z(t)
0, [Sol
- (z(t)% +~f+kc+k,c) d[Snlldt=Z(t)
[S, 1+K[S,l
>
(5)
d[T, lldt=ksc[S, 1+k,[Snl -kT[TI 1 / -Z(t) 61[Sol + (h+k,)
+~,[T,l+k,[S,l
(4)
~Is,l
-(~:c+k,+k,)P,l d[%lldf=
1
/
(6)
[S, 1 (7)
where Z(t) is laser flux, (T,, and 0, represent the absorption cross sections of the S, t S,, and S,eS, transitions, respectively. k:,, k,,, and k,, are relaxation rate
ofhtghly-e.n~lted naphthalene
451
constants of the S, to the bottom of S, state, the S, to So state, and the S, to T, state, respectively. It is impossible to solve these equations analytically because the time profile of the laser, Z(t). is not expressed in an analytical form. Therefore, we carried out numerical calculation for the set of differential equations (4 )- ( 7 ) . The kinetic parameters used are as follows: 6, =3.82x 10-‘9cm’, k,=2.Ox 10’s_‘, k,,=O.2x lo6 s-‘arereportedvalues [8],k,,,= 8.5x10’s_‘iscalculated from the fluorescence lifetime and the triplet quantum yield, and kT= 36 ps is the value described before. As recognized from fig. 5, the ratio of fluorescence to T-T absorption intensities is constant. This fact indicates that k:, z+ X-,, and k,, x- kst are good approximations; k,, can be neglected. The fitting parametersusedarea,and@(S,+S,),where@(S,+S,) is the quantum yield of the S,+S, internal conversion defined as qqspS,)=
--!!Gk:, + k,, .
(8)
First, using eqs. (4)-( 7) and each numerical set of a2 and @(S,-tS,), the time profiles of [S,]. [S,], [T, 1, and [So] were simulated for a fixed laser power, and then the total heat ( U, ) was estimated. Second, the observed laser power dependences of fluorescence intensity, triplet concentration, and total heat (U,) can be analyzed with the laser power dependency, y=aZ2+ bZ. The best fit was obtained for a?=3.2~10-” cm’ (corresponding to ~=850 M-’ cm-‘),and@(S,+S,)=0.82. Thesolidlinesinfigs. 4 and 5 are the simulated curves. The value of @(S,+SO) is estimated to be 0.18 from @(S,,+&,) = 1 -$(S,-S,): It is demonstrated that 18% of naphthalene molecules in the S, state relax through the ground state, not through the bottom of the S1 state, even in the condensed phase. As mentioned in section 1, the relation between the internal conversion rate and excitation energy under collision-free conditions was experimentally studied and theoretically analyzed by Lim et al. [ 3,4]. As the excitation energy increases, the internal conversion rate grows exponentially and becomes larger than the intersystem-crossing rate in the excess energy region above 2000 cm-’ from the S, origin. In the condensed phase, when a naphthalene molecule is excited to a relatively low excess energy level in the S, state, the cooling by the surrounding solvent mole-
452
T. Suzuki et al. / Relaxatron ofhrghly-excited naphthalene
cules is known to be fastest among the relaxation processes. On the other hand, the relaxation mechanism of highly excited naphthalene has not been established yet to the best of our knowledge. The highly excited state (S,) prepared by the two-photon absorption at 308 nm will quickly relax to the S, state ( S,+ S, internal conversion), which causes accumulation of vibrational energy at most 32000 cm-’ within S1. The S, +S, internal conversion of vibrationally excited S, molecules is expected to be fast enough to compete with the cooling. The internal conversion rate of isolated molecules with vibrational energy of 32000 cm-’ is roughly estimated to be on the order of 1Oi3 s- ’ [ 4 1. The vibrationally excited molecules in the condensed phase will lose the excess energy through the interaction with the surrounding solvent molecules (cooling). Typical cooling time by hydrocarbon solvents is reported to be 15 ps [ 111. It is, therefore, reasonable that the S, -So internal conversion proceeds at a rate comparable with the cooling rate when the S, molecule has a large vibrational energy like 32000 cm-‘.
5. Conclusion We successfully applied the TRTL technique to the study of relaxation dynamics of highly excited naphthalene prepared by the two-photon absorption at 308 nm in the condensed phase. It is demonstrated that naphthalene molecules in the S, state relax to the bottom of the S, state (82%) and to the highly vibrational excited state of S, followed by internal conver-
sion to the So state ( 18%). This result is interpreted by the accelerated S, + So internal conversion rate with large excess energy.
References [ 1] J.O. Uy and E.C. Lim, Chem. Phys. Letters 7 (1970) 306; B. Stevens and E. Hutton, Mol. Phys. 3 ( 1960) 7 1: R. Wilhams and G.J. Goldsmtth, J. Chem. Phys. 39 (1963) 2008. [2] E.C. Lim and C.S. Huang, J. Chem. Phys. 58 (1973) 1247. [ 31 J.C. Hsteh, C.S. Huang and E.C. Lim, J. Chem. Phys. 60 (1974) 4345. (41 SF. Fischer, A.L. Stanford and E.C. Lim. J. Chem. Phys. 61 (1974) 582. [ 51 F. Tanaka, S. Hirayama and K. Shobatake. Chem. Phys. Letters 164 (1989) 335. [ 61 M. Kasha, Dtscusstons Faraday Sot. 9 ( 1950) 14. [ 71F. Hirayama, T.A. Gregory and S. Lipsky. J. Chem. Phys. 58 (1973) 4696. [ 81 J.B. Birks, Organic molecular photophysics (Wiley, New York, 1979). [ 91 A. Laubereau, D. von der Lmde and W. Kaiser, Phys. Rev. Letters 28 (1972) 1162. [ lo] J.E. Griffths, M. Clerc and P.M. Rentzepis, J. Chem. Phys. 60 (1974) 3824. [ 11 ] U. Sukowski, A. Seilmeier. T. Elsaesser and S.F. Fischer, J. Chem. Phys. 93 ( 1990) 4094. [ 121 S.E. Braslavsky and K. Heihoff, eds.. Handbook of organic photochemtstry (CRC Press, Boca Raton. 1988), and references therem. ]13 T. Suzukt, Y. Kaju, K. Shibuya and K. Obi, Res. Chem. Intermediates 15 (1991) 261. (Dekker, New 114 S.L. Murov, Handbook of photochemtstry York. 1973). [15 1K.H. Schmidt. M.C. Sauer Jr., Y. Lu and A. Liu. J. Phys. Chem. 94 ( 1990) 244. 116 D.A Head. A. Singh, M.G. Cook and M.J. Quinn, Can. J Chem. 51 (1973) 1624.