Excited-state intramolecular proton transfer followedby cis±trans isomerization of (1-hydroxy-2-naphthyl)-s-triazine

derivatives

Masahiko Moriyama, Masahiro Kosuge, Seiji Tobita 1, Haruo Shizuka *

Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan

Received 27 September 1999

Abstract

The excited-state intramolecular proton transfer (ESIPT) reactions of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)-s-

triazine (NTR) and 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)-1,3-pyrimidine (NPR) were studied by laser photolyses

and ¯uorescence measurements with the aid of MO calculations. Large Stokes-shifted ¯uorescences (D�m� 6400 and

7300 cmÿ1) originating from ESIPT were observed for NTR and NPR. Laser photolyses of NTR and NPR gave long-

lived transients which could be attributed to their trans-keto tautomers (1Ktr) produced by cis±trans isomerization of the

proton-transferred cis-keto form (1K�cis) in the S1 state. Temperature e�ects on the ¯uorescence lifetime and ¯uorescence

quantum yield of 1K�cis and relative formation yield of 1Ktr showed that another temperature-dependent nonradiative

deactivation process competing with the cis±trans isomerization was involved in the relaxation processes of 1K�cis. A

lower apparent activation energy (12 kJ molÿ1) for the total nonradiative deactivation rate of NTR including cis±trans

isomerization was obtained in comparison with that (18 kJ molÿ1) of NPR. Remarkable viscosity e�ects were observed

on the ¯uorescence lifetimes of 1K�cis of NTR and NPR at 293 K, indicating that the main deactivation pathway from1K�cis was the cis±trans isomerization 1K�cis ! 1Ktr. The results of MO calculations on the ground and excited states of

NTR and NPR supported a relaxation scheme including ESIPT followed by cis±trans isomerization. Ó 2000 Elsevier

Science B.V. All rights reserved.

1. Introduction

Since the original work of Weller [1], on theexcited-state intramolecular proton transfer(ESIPT) of methyl salicylate, ESIPT has receivedmuch attention because of its importance in fun-damental aspects of reaction dynamics [2±11] andalso because of the various possibilities for appli-cations, as in e.g., photostabilizer [12±15], proton

transfer laser [16±19], information storage deviceat a molecular level [20], luminescent solar con-centrator [21], and ¯uorescence probe for bio-molecules [22]. Such a variety of applications arebased on unique photophysical properties ofESIPT systems, i.e., large radiationless deactiva-tion e�ciency, a four-level relaxation scheme,spectral sensitivity to environment, large Stokes-shifted ¯uorescence, etc.

We have recently studied the ESIPT of1-naphthol derivatives as shown in Scheme 1 bymeans of the laser photolysis method, time-re-solved thermal lensing method, and time-resolved

Chemical Physics 253 (2000) 91±103

www.elsevier.nl/locate/chemphys

* Corresponding author. Fax: +81-277-301213.1 Also corresponding author.

0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 1 - 0 1 0 4 ( 9 9 ) 0 0 3 7 7 - 8

¯uorometry [23]. Because of relatively small exo-thermicities for the ESIPT reactions of 1-naphtholderivatives compared to those for phenol ana-logues, the occurrence of ESIPT depends stronglyon the electronic nature of the substituent R. Asthe electron-donating substituent destabilizes theS1 state of the proton-transferred keto-form,ESIPT is not observed for compound 1, while thecompounds 2 and 3 show large Stokes-shifted¯uorescences originating from ESIPT. The ESIPThas also been found for a triazine derivative 4 [24]which is a naphthalene analogue of (hydroxyphe-nyl)-s-triazine [25]. The transient absorptionspectra of 2±4 exhibit a long-lived transient whichcan be assigned to the ground-state trans-ketotautomer produced by cis±trans isomerizationfollowing ESIPT [23,24]. The relaxation schemeincluding ESIPT of compounds 2±4 is depicted inScheme 2.

The cis±trans isomerization following ESIPThas also been reported for other proton transfersystems such as o-hydroxyphenylbenzazoles[26±29], 2-(20-hydroxyphenyl) oxazole [30], 2,20-bipyridin-3-ol [31], and photochromic salicylide-neaniline [32]. Cis±trans isomerization is sensitiveto the environments, and the ground-state trans-keto tautomer has a large activation energy forback proton transfer. As a result, the ESIPT fol-lowed by cis±trans isomerization systems havepossibilities for the various applications, e.g., a

new information storage device at a molecularlevel [30] and ¯uorescence probe for lipid bilayers[33]. Therefore, elucidation of reaction dynamicsof ESIPT followed by cis±trans isomerization areof essential importance for designing new molec-ular devices.

The aim of this article is to clarify relaxationpathways of the proton-transferred cis-keto tau-tomer including cis±trans isomerization and thereason why cis±trans isomerization occurs just af-ter ESIPT. Spectral and photophysical propertiesof 2,4-dimethoxy-6-(1-hydroxy-2-napthyl)-s-tri-arine (NTR) are compared with those of 2,4-di-methoxy-6-(1-hydroxy-2-naphthyl)-1,3-pyrimidine(NPR), which involves intramolecular H±H re-pulsion as shown in Scheme 1.

2. Experimental and MO calculations

2.1. Materials

Synthesis of NTR was described previously[24]. NPR was synthesized by photochemicalrearrangement [34] of 2,4-dimethoxy-6-(1-naphth-oxy)-1,3-pyrimidine, which was synthesized by thereaction of 1-naphthol with 6-chloro-2,4-dimeth-oxy-1,3-pyrimidine (Aldrich, 99 at.%) according tothe reported procedure [35]. The product waspuri®ed by column chromatography followed by

Scheme 1.

92 M. Moriyama et al. / Chemical Physics 253 (2000) 91±103

recrystallizations twice from acetonitrile and wasidenti®ed by mass spectrometry and NMR spectra.Potassium hydroxide (Kanto), polystyrene (PS;Aldrich, average MW ca. 280,000), 3-methylpen-tane (3MP; Wako, special grade), methylcyclo-hexane (MCH; Aldrich, spectrophotometricgrade), and n-pentane (Kanto, Uvasol) were usedas received. n-Hexadecane (Kanto, special grade)was puri®ed by column chromatography. Ethanol(EtOH; Wako, spectrosol) and acetonitrile (Kish-ida) were puri®ed by fractional distillation.

2.2. Methods

Absorption and ¯uorescence spectra were re-corded on a UV/VIS spectrophotometer (JASCO,Ubest-50) and a spectro¯uorometer (Hitachi,F-4010), respectively. Fluorescence lifetime (sf )was obtained with a time-correlated single-photoncounting ¯uorometer (Edinburgh Analytical In-

struments, FL900). Fluorescence lifetimes lessthan �200 ps were determined by using apicosecond laser system which was based on amode-locked Ti : sapphire laser (Spectra-PhysicsTsunami; 800 nm; 82 MHz; FWHM, �70 fs)pumped by a diode laser (Spectra-Physics Millen-nia V; 532 nm; 4.5 W). The repetition rate wasadjusted to 4 MHz by using a pulse-picker (Spec-tra-Physics Model 3980), and the third harmonic(266 nm; FWHM, �250 fs) was used for the ex-citation source. The monitoring system consistedof a microchannel plate-photomultiplier tube(Hamamatsu, R3809U-51) cooled at )20°C anda single-photon counting module (Becker andHickl, SPC-530). The ¯uorescence was detectedthrough a polarizer placed at ``magic angle'' withrespect to the polarization direction of the excita-tion source. Fluorescence quantum yields weredetermined relative to quinine bisulfate in 1 NH2SO4 [36]. NMR spectra were recorded on a

Scheme 2.

M. Moriyama et al. / Chemical Physics 253 (2000) 91±103 93

JEOL Alpha-500 FT NMR spectrometer. Chemi-cal shifts (d) in ppm were measured in CDCl3

referred to trimethylsilane as internal standard.Nanosecond laser ¯ash photolysis experiments

were carried out by using the third harmonic(355 nm) of an Nd3�±YAG laser (Spectra Physics,GCR-130, pulse width 5 ns) as excitation source.The monitoring system for the transient speciesconsisted of a 150 W short gap xenon lamp(Hamamatsu, L2274), a monochromator (JOBINYVON, HR320), and a photomultiplier tube(Hamamatsu, R928). The transient signals wererecorded on a digitizing oscilloscope (Tektronix,TDS-540) and transferred to a personal computer(NEC, PC-9801 BX3) to analyze the data.

All measurements were carried out under de-aerated condition unless otherwise noted. Allmeasurements except for temperature e�ect ex-periments were carried out at 293 K. In the tem-perature e�ect experiments, the temperature of thesample was controlled by using an Oxford DN-1704 cryostat.

Molecular orbital calculations were performedby use of the PM3 method (MOPACMOPAC ver. 6.01) [37]and the INDO/S±CI method on an IBM RS/6000workstation (model 41T). Within the CI scheme,all singly excited con®gurations involving the 10highest occupied and the 10 lowest unoccupiedorbitals were included.

3. Results and discussion

3.1. Ground-state geometries of NTR and NPR

In order to obtain an insight into the ground-state geometry of NTR and NPR, their NMRspectra were measured. The chemical shift (d) ofthe hydroxyl proton in the NMR spectra of NTR(d � 13.9 ppm) and NPR (d � 14.7 ppm) showedremarkable increases in comparison with that(d � 5:4 ppm [38]) of 1-naphthol, indicating that astrong intramolecular hydrogen bond is involvedin the ground state of NTR and NPR, and theintramolecular hydrogen bond in NPR is strongerthan that in NTR [39].

Fig. 1 shows the optimized molecular structuresfor the enol form of NTR and NPR obtained by

PM3 calculations. In both compounds, the naph-thalene and heteroaromatic rings are coplanar toeach other, inspite of the steric repulsion betweentwo hydrogen atoms in the ortho position of thetwo aromatic rings in NPR. The equilibrium tor-sional angle of biphenyl has been reported to be�40° [40,41], while in 2-(20-hydroxyphenyl)pyri-dine [42], two aromatic rings have been calculatedto be almost coplanar. The coplanarity in 2-(20-hydroxyphenyl)pyridine has been explained on thebasis of counterbalance among the H±H repulsionin the ortho position of two aromatic rings, thehydrogen-bond stabilization, and electron delo-calization e�ects. As in the case of the abovecompound, destabilization originating from theH±H repulsion in NPR is considered to be com-pensated by stabilization due to the strong hy-drogen bond and also the electron delocalizatione�ects.

3.2. Spectral properties of NTR and NPR

Fig. 2 shows the absorption and ¯uorescencespectra of NTR and NPR in MCH at 293 K. Fromthe relatively large extinction coe�cients, all theultraviolet absorption bands can be assigned to(p,p�) transitions. It is clear from Fig. 2 that theabsorption spectra of NTR and NPR cannot bedescribed as a simple superposition of the ab-sorption spectra of 1-naphthol and heteroaromaticmoieties. This shows that there exists strong elec-tronic interaction between two chromophores,consistent with their coplanarity obtained by MOcalculations. The ®rst absorption band at around�27,000 cmÿ1 corresponds to the pseudo (p,p�)transition associated with the intramolecular hy-drogen bond. The ®rst and second (�33,000 cmÿ1)absorption bands of NTR and NPR are similar inshape and intensity except for a slight shift of thepeak wavelength. The third absorption band at�36,000 cmÿ1 of NTR is shifted to a lower energyfor NPR (�35,000 cmÿ1), and the extinction co-e�cient of the band of NPR decreases signi®-cantly. These spectral properties can be explainedon the basis of MO calculations as describedbelow.

Excited state properties of NTR and NPRwere calculated by the INDO/S±CI method at

94 M. Moriyama et al. / Chemical Physics 253 (2000) 91±103

the ground-state geometries optimized by thePM3 method. The calculated oscillator strengthsfor each absorption band (A±D) are shown inFig. 2. The calculated energy and oscillatorstrength for the Sn S0 transitions of NTR andNPR coincide well with the observed absorptionspectra except for the lowest energy transition A.As the ®rst absorption band corresponds to thepseudo (p,p�) transition due to intramolecularhydrogen bond, we could not obtain satisfactoryresults by MO calculations at an INDO/S±CIlevel. The MO calculations show that transitionC lies close in energy to transition D in NTR,while in NPR transitions C and D are far apart.This is consistent with the fact that the thirdabsorption band is broadened in NPR comparedwith NTR.

Fig. 3 shows molecular orbitals of NTR andNPR obtained by INDO/S±CI calculations. Basedon these orbitals, CI expansions of the wave

functions for the transitions (A±D) are given bythe following equations:

NTR

/�A� � 0:79v�1ÿ 1� � 0:33v�1ÿ 2�; �1�

/�B� � ÿ0:59v�2ÿ 1� � 0:52v�1ÿ 1�ÿ 0:47v�1ÿ 2�; �2�

/�C� � 0:37v�5ÿ 1� ÿ 0:51v�1ÿ 2�� 0:44v�1ÿ 3�; �3�

/�D� � ÿ0:37v�5ÿ 1� ÿ 0:56v�1ÿ 2�ÿ 0:36v�1ÿ 3�; �4�

NPR

/�A� � ÿ0:75v�1ÿ 1� � 0:39v�1ÿ 2�; �5�

Fig. 1. Optimized molecular structures for the enol form of NTR and NPR.

M. Moriyama et al. / Chemical Physics 253 (2000) 91±103 95

/�B� � 0:63v�2ÿ 1� ÿ 0:50v�1ÿ 1�ÿ 0:34v�1ÿ 2�; �6�

/�C� � 0:46v�3ÿ 1� � 0:60v�1ÿ 2�ÿ 0:28v�1ÿ 3�; �7�

/�D� � 0:39v�3ÿ 1� ÿ 0:51v�1ÿ 2�� 0:42v�1ÿ 4�; �8�

where v�xÿ y� denotes the one-electron transitionfrom an occupied orbital x to an unoccupied or-

bital )y. The CI expansions for transitions A andB of NTR are similar to those of NPR, which isconsistent with the similarity between the lowesttwo absorption bands of NTR and NPR. Thedi�erences in shape and intensity of the third ab-sorption band for NTR and NPR would be asso-ciated with the di�erences in the CI expansions fortransitions C and D of NTR and NPR. It shouldbe noted here that the ®rst absorption band cor-responds mainly to the LUMO HOMO transi-tion which has intramolecular charge transfercharacter from the naphthalene moiety to theheteroaromatic ring both for NTR and NPR. Thisintramolecular charge transfer in the S1 state ofNTR and NPR would facilitate ESIPT because ofan increase of the acidity of OH group and thebasicity of the nitrogen atom in the heteroaromaticring.

In Fig. 2, moderately large Stokes-shifted ¯uo-rescences can be seen for both compounds. The¯uorescence spectra have no mirror image rela-tionship with respect to the ®rst absorption band.The ¯uorescence excitation spectra taken at the¯uorescence maxima coincided well with the cor-responding absorption spectra. These observationsare indicative of the occurrence of ESIPT in NTR[24] and NPR, which is also con®rmed fromtransient absorption measurements as describedbelow. The Stokes shift (D�m � 7300 cmÿ1) obser-ved for NPR is greater than that (D�m � 6400 cmÿ1)of NTR. These di�erences can be attributed to anincrease in the stabilization energy accompanyingESIPT in NPR (see Section 3.5).

3.3. Cis±trans isomerization following ESPITof NTR and NPR

Fig. 4(a) shows the transient absorption spectraobtained by 355 nm laser photolysis of NPR indegassed MCH at 293 K. A broad absorptionband around 470 nm is seen in addition to ground-state bleaching around 360 nm. The recovery ofthe parent enol molecule is found to occur with adecay of the transient at 470 nm. A remarkablefeature is that the transient absorption band at470 nm is not quenched by dissolved oxygen, in-dicating the transient to be a ground-state species.As an NPR anion produced by proton ejection of

Fig. 2. Absorption and ¯uorescence spectra, and calculated

oscillator strengths for the Sn S0 transitions of (a) NTR and

(b) NPR in MCH at 293 K. The calculated values for the

oscillator strength are 0.24, 0.24, 0.39, and 0.55 for transitions

A±D of NTR, and 0.30, 0.33, 0.16, and 0.53 for transitions A±D

of NPR.

96 M. Moriyama et al. / Chemical Physics 253 (2000) 91±103

NPR upon laser excitation would be a possiblecandidate for such a transient, we compared thetransient absorption spectra in Fig. 4(a) with thedi�erence absorption spectra obtained by sub-tracting a steady-state absorption spectrum ofNPR in EtOH from the absorption spectrum ofNPR in a 10ÿ2 M KOH/EtOH solution. However,an agreement in the spectral pro®les was not ob-tained.

The transient absorption spectra of NPR de-pended strongly on environment as shown inFig. 4. In polystyrene (PS) ®lms at 293 K, the

transient absorption spectra taken by 355 nm laserphotolysis of NPR consist, almost exclusively, ofthe Tn T1 absorption bands, and the 470 nmband due to the keto form almost disappears. Thisindicates that large structural changes are includedin the course of the formation of the long-livedtransient. In general, the ESIPT process does notsubject solvent-viscosity e�ects; therefore, thestructural change would occur after the ESIPT.From these facts, the transient at 470 nm can beattributed to the trans-keto tautomer (1Ktr) pro-duced by cis±trans isomerization of the cis-keto

Fig. 3. Molecular orbitals for the enol form of NTR and NPR.

M. Moriyama et al. / Chemical Physics 253 (2000) 91±103 97

tautomer (1K�cis) following ESIPT as in the case ofNTR. Hence, similar relaxation pathways includ-ing ESIPT followed by cis±trans isomerization canbe seen both for NTR and NPR as shown inScheme 2. The increase in the Tn T1 absorptionintensity relative to that of the keto form in PS ®lmsuggests that the formation of the keto form com-petes with intersystem crossing to the triplet state.

3.4. Temperature e�ects on thephotorelaxation properties of NTR and NPR

Fig. 5 shows plots of the initial absorbance ratio(DA(T)/DA(293 K)) measured at the transient ab-sorption maxima (450 and 470 nm for NTR andNPR, respectively), the relative ¯uorescencequantum yield (Uf (T)/Uf (77 K)), and the relative¯uorescence lifetime (sf (T)/sf (77 K)) of NTR andNPR in 3MP as a function of Tÿ1. Here the DA(T)value is proportional to the trans-keto formationyield, and the value of DA(T)/DA(293 K) corre-sponds to the relative yield of the trans-keto form,normalized at 293 K. It can be seen that the rela-tive formation yield of the trans-keto form de-creases with decreasing temperature. In contrast,the relative ¯uorescence lifetime and ¯uorescencequantum yield increase with a decrease in tem-perature, and become almost temperature-inde-pendent below �125 and �143 K for NTR andNPR, respectively. The signi®cant increases in¯uorescence lifetime and quantum yield while

decreasing the keto formation yield is also amanifestation of the cis±trans isomerization reac-tion [43,44] following ESIPT. Similar temperaturee�ects have also been reported for other ESIPTfollowed by cis±trans isomerization systems, 2-(20-hydroxyphenyl)-benzothiazole [27] and 2-(20-hy-droxyphenyl)-3-H-indole [28].

On the basis of Scheme 2, the observed ¯uo-rescence quantum yield Uf can be written as

Uf � UPT/f

� UPT

kf

kf � kd � kiso

� UPTkfsf ;

�9�

where UPT and /f ; respectively, are the quantumyield of intramolecular proton transfer from thecis-enol form to the cis-keto form (1E�cis ! 1K�cis)and the ¯uorescence quantum e�ciency of 1K�cis,

Fig. 5. Plots of the initial absorbance ratio DA(T)/DA(293 K)

(�), ¯uorescence quantum yield ratio Uf (T)/Uf (77 K) (M), and

¯uorescence lifetime ratio sf (T)/sf (77 K) ( ) of (a) NTR and

(b) NPR in 3MP as a function of Tÿ1.

Fig. 4. (a) Transient absorption spectra obtained by 355 nm

laser photolysis of NPR in MCH at 293 K. (b) Transient ab-

sorption spectra obtained by 355 nm laser photolysis of NPR in

PS ®lms at 293 K.

98 M. Moriyama et al. / Chemical Physics 253 (2000) 91±103

and kf , kiso, and kd are the rate constants for ¯u-orescence, cis±trans isomerization, and nonradia-tive deactivation process (except for the cis±transisomerization), respectively. From Eq. (9), one canexamine the temperature dependence of the ESIPTprocess. We calculated the Uf /sf value for eachtemperature from the data in Fig. 5, and obtainedalmost constant values for the whole temperaturerange investigated. Since the kf value can be as-sumed to be insensitive to temperature change, thevalue of UPT is found to be independent of tem-perature. These observations show that the ESIPTprocesses of NTR and NPR have no potentialbarrier, and that there are no nonradiative pro-cesses competing with intramolecular protontransfer from the S1 state of the parent cis-enolform (1E�cis) probably due to the ultra-fast natureof ESIPT processes in NTR and NPR. From theseresults, the value of UPT can be assumed to beunity, and therefore Uf /sf corresponds to kf . Thekf values are summarized in Table 1 together withthe Uf and sf values at 293 and 77 K. At 293 K, theUf and sf values of NTR and NPR are very smallin 3MP, on the contrary the sf values of NTR andNPR in PS ®lms are much larger than those in3MP. These results indicate that the main deacti-vation pathway from the S1 state of the cis-ketoform in ¯uid media at room temperature is the cis±trans isomerization which requires large amplitudemotions.

On the basis of Scheme 2, the nonradiative de-activation rate knr(T) of the 1K�cis and the quantumyield Uiso(T) for cis±trans isomerization (1K�cis !1Ktr) at temperature T can be written as

knr�T � � kiso�T � � kd�T �� fsf�T �gÿ1 ÿ fsf�77K�gÿ1

; �10�

Uiso�T � � UPT

knr�T � ÿ kd�T �fsf�77K�gÿ1 � knr�T �

� CUPTDA�T �; �11�

where kiso(T) and kd(T) denote the cis±trans iso-merization rate and the temperature-dependentnonradiative deactivation rate other than cis±transisomerization, and sf (77 K) is the ¯uorescencelifetime at 77 K. Since the initial absorbanceDA(T) of 1Ktr observed at 450 nm (for NTR) or470 nm (for NPR) should be proportional to theisomerization yield Uiso(T), the relation in Eq. (11)can be obtained. From Eq. (10), one can calculatethe knr(T) value from those of sf (T) and sf (77 K).If the kd(T) value is negligibly small, the value ofUiso(T) is obtained from Eq. (11) based on thecalculated value of knr(T) because UPT can be as-sumed to be unity. Therefore, if kd(T) is negligible,the ratio of the isomerization yields at tempera-tures T and 293 K should be equal to that of theinitial absorbances at temperatures T and 293K.Table 2 shows the calculated value of Uiso(T)/Uiso(293 K) based on Eq. (11) and the ratio of theobserved initial absorbance, DA(T)/DA(293 K). Itcan be seen from Table 2 that the calculated valuesof Uiso(T)/Uiso(293 K) tend to become larger thanDA(T)/DA (293 K) at lower temperatures both forNTR and NPR. These indicate that in addition tothe cis±trans isomerization, another temperature-dependent nonradiative deactivation process suchas internal conversion is involved in the relaxationprocesses of 1K�cis. The involvement of nonradia-tive deactivation via an (n,p�) excited state com-peting with cis±trans isomerization followingESIPT has been reported for 2-(20-hydroxyphe-nyl)pyridine [42]. The nonradiative decay process,which is induced by out-of-plane bending and/or

Table 1

Photophysical properties of NTR and NPR in 3MP and PS ®lms at 293 and 77 K

Sample Uf sf (ns) kf (sÿ1)a

293 Ka 77 Ka 293 Ka 293 Kb 77 Ka

NTR 6.4 ´ 10ÿ3 0.57 0.053 1.7 (24%) 5.9 9.7 ´ 107

4.4 (76%)

NPR 4.4 ´ 10ÿ3 0.33 0.080 1.4 (32%) 4.7 7.0 ´ 107

3.3 (68%)

a In 3MP.b In PS ®lms, two components were observed.

M. Moriyama et al. / Chemical Physics 253 (2000) 91±103 99

torsional motion involving the carbonyl group, is apossible candidate for the nonradiative process[45,46].

In order to evaluate the activation energy forthe temperature-dependent nonradiative deactiva-tion process, the value of ln (sf (T)ÿ1±sf (77 K)ÿ1)was plotted as a function of Tÿ1 as shown in Fig. 6.Linear relations are seen both for NTR and NPR,and the activation energies are obtained to be 12and 18 kJ molÿ1 for NTR and NPR, respectively.As described above, these activation energies in-volve contribution from not only cis±trans iso-merization but also from other nonradiativeprocesses. Further, the dynamics of cis±trans iso-merization usually depend on viscosity and tem-perature [47±50], so that the activation energyobtained by the plot in Fig. 6 would re¯ect thee�ects of both temperature and viscosity.

Viscosity e�ects on the ¯uorescence lifetimes ofNTR and NPR were examined at 293 K in severalalkanes which have di�erent viscosity as shown inTable 3. As the viscosity of the solvent becomeslarger (n-pentane to n-hexadecane), the ¯uores-

cence lifetime of NTR and NPR increases signi®-cantly. This remarkable viscosity e�ect on the¯uorescence lifetime supports the conclusion thatthe main deactivation path from the S1 state of thecis-keto form is cis±trans isomerization (1K�cis! 1Ktr) which requires large structural changes atambient temperature. In order to separate the ef-fects of temperature and viscosity, further studiesby using a series of n-alkanes will be needed infuture.

3.5. Theoretical considerations for theintramolecular proton transfer cycleof NTR and NPR

The energy state diagram calculated for the re-laxation processes of excited NTR and NPR isshown in Fig. 7. Here the ground-state structuresare optimized by the PM3 method, and the energydi�erence between ground and excited states arederived by the INDO/S±CI calculations. Theelectronic character of the lowest excited singlet

Fig. 6. Arrhenius plots of nonradiative deactivation rates for

NTR (�) and NPR ( ) in 3MP.

Table 3

Fluorescence lifetimes of NTR and NPR in various solvents at 293 K

Solvent Dielectric constant g/cP sf (NTR)/ps sf (NPR)/ps

n-Pentane 1.84 [52] 0.234 (20°C) [53] 44 80

3-Methylpentane 1.90 [52] 0.307 (25°C) [52] 53 80

Methylcyclohexane 2.02 [52] 0.734 (20°C) [52] 58 83

n-Hexadecane 2.05 [54] 3.474 (20°C) [53] 172 155

Acetonitrile 37.5 [52] 0.375 (15°C) [52] 22 22

Table 2

Initial DA ratio and calculated Uiso ratio for NTR and NPRa

Sample 293 K 250 K 233 K

NTR DA(T)/DA(293 K)b 1.0 0.76 0.59

Uiso(T)/Uiso(293 K)c 1.0 0.97 0.95

NPR DA(T)/DA(293 K)b 1.0 0.81 0.56

Uiso(T)/Uiso(293 K)c 1.0 0.94 0.87

a Since at temperatures below �220 K, precipitation of the

solute occurred in sample solutions in the transient absorption

measurements, the values of DA(T)/DA(293 K) were determined

above 233 K.b Absorbances at lower temperatures were corrected for solvent

contraction.c Calculated values based on Eq. (11), assuming that kd(T) was

negligible.

100 M. Moriyama et al. / Chemical Physics 253 (2000) 91±103

state of NTR and NPR is calculated to be (p,p�)both for 1Ecis and 1Kcis. The energy level of 1K�cis islocated at a slightly lower energy relative to that of1E�cis both in NTR and NPR, which is consistencewith the occurrence of the ESIPT. The stabiliza-tion energy (22 kJ molÿ1) accompanying ESIPT ofNPR is larger than that (12 kJ molÿ1) of NTR,whereas, the destabilization energy in the Franck±Condon ground state is almost the same. This re-sult supports the larger Stokes shift observed forNPR. The S1 S0 transition energies for 1Kcis and1Ktr of NPR are smaller than those of NTR, whichis in agreement with the experimental results in the

¯uorescence spectra of 1K�cis and transient ab-sorption of 1Ktr. It is worth noting here that theenergy di�erence between the cis-keto form andtrans-keto form of NPR is much smaller than thatof NTR in both ground and excited states. Thisdi�erence can be explained as follows: In NTR,cis±trans isomerization (1K�cis ! 1Ktr) results insteric hindrance between the ortho hydrogen atomsand electronic repulsion between the lone pair oforbitals on the carbonyl oxygen and the nitrogenatom, whereas in NPR, steric hindrance betweenthe ortho hydrogen atoms already exists in the cis-keto form.

Fig. 7. Calculated energy-state diagram for the intramolecular proton transfer cycle of NTR and NPR. The calculated dipole moment

in Debye is shown in parentheses for each state.

M. Moriyama et al. / Chemical Physics 253 (2000) 91±103 101

Fig. 8 shows the HOMO and LUMO of 1Kcis

and its 90° twisted state (1Ktw) for NTR and NPR,calculated by the INDO/S±CI method. As de-scribed in Section 3.2, the S1 S0 transition of1Kcis is mainly due to the LUMO HOMO tran-sition. The LUMO of the 1Kcis form possessesantibonding character on the central C±C bond asshown in Fig. 8, which is favorable for the cis±trans isomerization in 1K�cis. The bond orders ofthe central C±C bond of 1Kcis of NTR were cal-culated to be 1.50 and 1.15 for the S0 and S1 states,respectively. For the 1Kcis of NPR, the bond or-ders were obtained to be 1.43 and 1.17 for the S0

and S1 states, respectively. The signi®cant decreasein the bond order in the S1 state is also consistentwith the occurrence of cis±trans isomerizationfollowing ESIPT.

The calculated dipole moment of each species isshown in Fig. 7. The 1Ktw form has a large dipolemoment (14.6 and 13.1 Debye for NTR and NPR,respectively) originating from intramolecularcharge transfer from the heteroaromatic ring tothe naphthalene moiety. The electronic transitionsto 1K�tw cause a charge migration to an oppositedirection, leading to a decrease in the dipole mo-ment (7.2 and 6.4 Debye for NTR and NPR, re-spectively). These dipole moments of 1K�tw are stillhigher than those of 1K�cis. The more polar natureof 1K�tw, compared to 1K�cis was consistent with thedecrease of ¯uorescence lifetime in going fromnonpolar MCH (sf � 58 and 83 ps for NTR andNPR) to polar acetonitrile (sf � 22 ps both forNTR and NPR, Table 3). Potter et al., have re-

ported the role of twisted intramolecular charge-transfer states in the decay of 2-(20-hydroxy-phenyl)benzothiazole following ESIPT [51].LeGourri�erec et al., have suggested the presence ofa weakly polar biradicaloid 1K�tw state in the courseof ESIPT followed by cis±trans isomerization of2-(20-hydroxyphenyl)pyridine [42]. In the cis±transisomerization following ESIPT of NTR and NPR,intramolecular charge transfer character is partlyretained in 1K�tw, thus facilitating the cis±transisomerization in polar media.

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