J. Phys. Chem. 1984,88, 3921-3923
3921
Two-Photon-Induced Excited-State Proton Transfer Process in a Methanol Solution of 7-Hydroxyquinoline Kunihiro Tokumura and Michiya Itoh* Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: March 8, 1984; In Final Form: May 7, 1984)
In contrast to the dual fluorescence (A,, = 378 and 525 nm) from the excited-state normal (N*) and tautomer (T*)forms upon one-photon excitation of 7-hydroxyquinoline in methanol solution at room temperature, only a green (525 nm) fluorescence was observed upon two-photon excitation (210-250 nm) of the system. Rapid two-photon-inducedproton transfer, surpassing competitive internal conversion to N*, may be responsible for the appearance of the green T* fluorescence (without risetime), whose spectrum (A,, = 525 nm) and lifetime (2.9 ns) are almost identical with those (525 nm, 2.7 ns) in the one-photon process.
Introduction The normal (violet) and largely Stoke’s shifted (green) fluorescences of 7-hydroxyquinoline (7-HQ) in ethanol solution were examined by Mason et ale1with steady-state fluorimetry. They reported that the phenol group of 7-HQ is more acidic and the ring nitrogen atom more basic in the excited state than in the ground state, and the green fluorescence was ascribed to the zwitterion (tautomer, T*) form, generated through a two-stage prototropic change of 7-HQ in the excited state. Very recently, pico- and nanosecond time-resolved fluorescence measurements24 were carried out for 7-HQ in methanol solution. Itoh et al. reported4 that 7-HQ may form 1:l and 1:2 hydrogen-bonding complexes exhibiting nearly the same violet fluorescence spectra in the 350-450-nm region, and that the excited-state proton transfer to form T*, exhibiting green fluorescence, takes place only from the 1:2 complex. They also detected the relatively long-lived (T 3.5 bs) ground-state tautomer (T) by two-step laser excitation (TSLE) fluorescence of T*, which consists of the formation of T by the first laser excitation and the second laser excitation of T absorption band within the lifetime of T.334 The present paper describes the first observation of a twophoton-induced excited-state proton transfer process of an intermolecular hydrogen-bonding system. Two-photon (hv[k]) excitation of a 1:2 hydrogen-bonding complex (N) between 7-HQ and methanol molecules exhibits no N* fluorescence but only T* fluorescence (A,,, = 525 nm), while one-photon (hv[k/2]) excitation of the complex exhibits dual fluorescence due to N* (378 nm) and T* (525 nm). These facts suggest that the unusually rapid proton transfer, overwhelming the internal conversion to the relaxed fluorescent state of N, takes place to form T* via a two-photon allowed excited state of N.
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Experimental Section 7-Hydroxyquinoline (7-HQ) was purified by repeated recrystallizations. Spectral grade methanol (Nakarai Chemicals) was used without further purification. A nitrogen-laser pumped dye laser (Molectron UV-12 and DL-14) was used for the light pulse in the two-photon excitation. The fluorescence intensity (ZF) was monitored through appropriate cutoff filters by a photomultiplier (HTV 1P28), and a fraction of dye laser output was monitored by a calibrated pyroelectric detector (Molectron J3-02DW). In order to obtain the light intensity (Z,) of a dye laser pulse, the apparent gain of the detector was corrected for the transmission factor (T[A]) of the diffuser window of the detector as well as the photon energy (E[k]). The two-photon fluorescence excitation spectrum (ZF/ZL2) of 7-HQ in methanol was constructed by overlapping spectral segments from ~
various dye-solvent combinations. The total gain of the dye laser output was controlled by adjusting the length of the active medium along the dye laser cavity axis. An estimation of the two-photon absorption cross section (6) was performed with the method reported by Parma and O m e n e t t ~ :Fluorescence ~ intensities ( S , and S,) in the two-photon (440 nm) and one-photon (337 nm) excitations are expressed as the products of fluorescence quantum yields ( f 2 and q l ) and light quanta absorbed, and their ratio is taken in the following equation which includes two-photon and one-photon absorptivity (6 and a): S2/S1 = 7212 (1 - exP[-6C2~~2l)/q,~,(l - exP[-G4) Here, Z,C, and 1 denote incident light intensity, the concentration of 7-HQ, and the light path length, respectively. The values of S2/S1were calculated by evaluating various parameters (v2 = 1.O [assumed], q 1 = 0.027 [determined], Z2 1.1 X photons cm-2 s-l, I, 2.9 X photons cm-2 s-l, etc.) in the equation. Fluorescence lifetimes upon two-photon excitation were determined by a computer simulation of the decay signals, detected by an oscilloscope (Tektronix Model 7904, 500 MHz)-photomultiplier (subnanosecond response time) system described previously.6
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Results and Discussion As mentioned in the introductory section, Itoh et al. examined4 the dual fluorescence of 7-HQ in detail by a combined study of time-resolved and steady-state fluorescence measurements of the compound in various methanol-hexane mixed solvents. They reported that 7-HQ may form 1:l and 1:2 hydrogen-bonding complexes with methanol molecule(s) (N’ and N) exhibiting nearly the same fluorescence spectra in the 350-450-nm region, and that the excited-state proton transfer to form T* exhibiting green = 525 nm) takes place only from the 1:2 fluorescence (A,, complex (N*). The respective fluorescence lifetimes of N*, N’*, and T* were reported to be 0.20, 2.0, and 2.7 m4
a = m = br
HO
ROCH,
p --
--
FIOCH,
H6CH3
N’
a
N
2-
I
Y’B
R
HOCH,
+
-bCH,
~~~
(1) Mason, S. F.; Philp, J.; Smith, B. E. J . Chem. Soc. A 1968, 3051. ( 2 ) Thistlethwaite, P. J.; Corkill, P. J. Chem. Phys. Lett. 1982, 85, 317. (3) Itoh, M.; Adachi, T.; Tokumura, K. J . Am. Chem. SOC.1983, 105, 4828. (4) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Sor. 1984,106,850.
0022-3654/84/2088-3921$01.50/0
T
( 5 ) Parma, L.; Omenetto, N. Chem. Phys. Lett. 1978, 54, 541. (6) Itoh, M.; Tokumura, K.; Tanimoto, Y.; Okada, Y.; Takeuchi, H.; Obi, K.; Tanaka, I. J . Am. Chem. SOC.1982, 104, 4146.
0 1984 American Chemical Society
3922
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
Letters TWO-PHOTON WAVELENGTH
a
t*
1 500
400
II
600
-
P O 0
'I
0
a o,o
I
0
0 0 0
0
P o
500 600 WAVELENGTH ( nm)
400
Figure 1. Uncorrected fluorescence spectra, obtained upon 220-nm steady light excitation (-) of 7-HQ (1.2 X 10m5M),N2-laser pulse (337-nm) excitation (e),and N2-laserpumped dye laser pulse (440-nm) excitation (0) of 7-HQ (1.8 X 104M) in methanol at room temperature. I
I
I
I
1
u 1
0.775
480
4r-----l
0
0 0
IF
440
DYE LASER WAVELENGTH
monitored at 530nm 0.95 115 log IL(arb. mit)
Figure 2. The plots of log ZF vs. log Z,, showing dye laser intensity dependence of T*-fluorescenceintensity (I,) at 530 nm.
Both 337- and 365-nm pulse excitations of a methanol solution M) of 7-HQ at room temperature afforded dual (1.8 X = 390 and 525 nm), which are almost fluorescence spectra (A, identical with that upon 220-nm steady-light excitation of more dilute solution of 7-HQ, as shown in Figure la.' Upon 440-nm (or 386-nm) pulse excitation of the methanol solution of 7-HQ, however, almost no violet fluorescence but only a green fluorescence were observed, as shown in Figure lb. Since there are no appreciable absorption at wavelengths longer than 380 nm,8 it seems that the drastic change in fluorescence spectrum on going from 337-nm excitation to 386-nm excitation reflects the alternation in the light-absorbing manner: The former is attributable to onephoton absorption, while the latter to two-photon absorption. In order to confirm the argument that two-photon excitation of 7-HQ results in different fluorescent behavior from that of one-photon excitation, the pulse intensity (ZL[440nm]) dependence of fluorescence intensity (ZF[530 nm]) was examined, as shown (7) The slight shift of the violet fluorescence maximum may be due to fluorescence reabsorption in concentrated solution. (8) Mason et al.' reported that a fluorescent zwitterionic species of 7-HQ exists in neutral aqueous solution. Very weak absorption (A,,, = 409 nm) probably due to the zwitterionic species was detected in a methanol solution of 7-HQ. However, the validity of the two-photon process upon dye laser excitation was supported by many experimental facts, which will be described later.
;c
\-I 240 WAVELENGTH(nrn1
Figure 3. The two-photon fluorescence excitation (TPE) spectrum (upper curve) and one-photon absorption spectrum (lower curve) of 7-HQ in methanol at room temperature. Monitoring and pumping wavelengths for the TPE spectrum of 7-HQ (2.4 X 10-4M) are 530 and 440 nm, respectively.
in Figure 2. The plots of log IFvs. log ZL gave a straight line with a slope of 2 at low laser pulse intensity levels, and a deviation may be attributable to a saturation e f f e ~ t . Therefore, ~ it seems that the green fluorescence originates from a two-photon absorption process, and that it is ascribed to the excited-state tautomer generated through two-photon-induced proton transfer of N*. Very poor violet fluorescencelO in the two-photon excitation may be attributable to the argument that the unusually rapid proton transfer, surpassing competitive internal conversion to relaxed SI*state (N*) takes place from two-photon allowed states, as will be discussed. Upon two-photon excitation of N, the lifetime of the green fluorescence was determined to be 2.9 ns without a fluorescence rise. However, the green fluorescence rise (0.20 ns) and decay (2.7 ns) were determined upon one-photon excitation of N.4 Furthermore, the green fluorescence spectra in both one- and two-photon processes are almost the same within experimental error, as shown in Figure 1. Thus, it is reasonable to consider that a common fluorescent state (T*) is generated by both oneand two-photon excitations of N. Figure 3 shows the two-photon fluorescence excitation (TPE) spectrum, obtained by plotting ZF(A)/ZL2(A) against the two-photon wavelength (210-250 nm) and the dye laser wavelength (420-500 nm). The plots are related to two-photon absorptivity (S[A]) and other parameters, as expressed by
Here, C is a wavelength-independent constant, and CPF(h) is fluorescence quantum yield. By assuming that aFis independent of the two-photon excitation wavelength, the TPE spectrum should reflect the two-photon absorption spectrum. In the two-photoninduced proton transfer, however, the excitation energy dependence must be considered: aPF may be regarded as the apparent fluorescence quantum yield, which is the product of proton transfer quantum yield ((PPT) and fluorescence quantum yield (qF)of T*, as expressed by @F(h)
= 'PPT(X)
(2)
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Upon direct laser (386, 406, and 440 nm) excitations of the ground-state tautomer (T, T 3.5 ps) of 7-HQ in two-step laser (9) Tobita, S.;Tanaka, I. Chem. Phys. Lett. 1983, 96, 517. (10) The contribution of the fluorescence of the 1:l hydrogen-bonding complex to short wavelentgh (350-450 nm) fluorescence is not significant (
