Picosecond laser photolysis studies of DMA–DMPP in solution

2 July 1999

Chemical Physics Letters 307 Ž1999. 121–130 www.elsevier.nlrlocatercplett

Picosecond laser photolysis studies of DMA–DMPP in solution Hiroshi Miyasaka a , Akira Itaya a , Krystyna Rotkiewicz

b,)

, Karl Rechthaler

c

a

b

Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606, Japan Institute of Physical Chemistry, Polish Akademy of Sciences, Kasprzaka 44 r 52, 01-224 Warsaw and the Pedagogical UniÕersity, Institute of Chemistry, Che˛cinska ´ 5, 25-020 Kielce, Poland c Institut fur ¨ Theoretische Chemie und Strahlenchemie, UniÕersitat ¨ Wien, Althanstrasse 14, 1090 Vienna, Austria Received 12 April 1999

Abstract Picosecond transient absorption spectra of: 4-Ž4X-N,N-dimethylaminophenyl. –3,5-dimethyl-1,7-diphenyl-bis-pyrazolow3,4-b;4X ,3X-ex-pyridine ŽDMA–DMPP., 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-w3,4-b;4X ,3X-ex-pyridine ŽBPP. and 3,5-dimethyl-1,4,7-triphenyl-bis-pyrazolo-w3,4-b;4X ,3X-ex-pyridine ŽH-DMPP. were measured in solvents of different polarity. The results revealed the previously postulated change of the character of the fluorescing state from a primary excited, low polar state in non-polar solvents to a CT state in highly polar ones. Transient absorption spectra in the equilibrium fluorescent state of DMA–DMPP in polar solvents comprised the band ascribable to the cation radical of the DMA moiety. The charge transfer process is fastest in methanol and acetonitrile; in the former it is not controlled by a dynamic Stokes shift. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction 4-Ž4X-N,N-dimethylaminophenyl.-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-w3,4-b;4X ,3X -ex-pyridine ŽDMA– DMPP. is a representative of compounds with bulky electron acceptor and relatively large electron donor subunits, linked by a single bond Žsee formulae. and able to relax to a charge transfer ŽCT. state. This class of molecules is interesting from a cognitive point of view and from their applicability as promising chromophors for non-linear optics w1,2x. In aprotic solvents, DMA–DMPP shows a single fluorescent band. The analysis of the dependency of fluorescence properties Žthe position of the maximum of the spectrum and the radiative fluorescence lifetime. shows that the emitting state changes with increasing solvent polarity from highly emissive non-CT to medium or weakly emissive CT w3x. In protic solvents, however, dual fluorescence occurs. The long wavelength band was ascribed to an intramolecular CT state w3x. The mechanism of the occurrence of this dual fluorescence is not yet definitely explained.

)

Corresponding author. Fax: q48 22 632 5276; e-mail: [email protected]

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 5 3 7 - 0

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Picosecond transient absorption spectroscopy is a powerful tool for studying the excited CT states. In the present study, we have applied picosecond transient absorption spectroscopy to the direct elucidation of the solvent dependence of the equilibrium fluorescent state of DMA–DMPP as well as the dynamic relaxation processes following the photoexcitation. The degree of the intramolecular CT may be estimated from the absorption band shapes in the fluorescent state. In the case where the full CT is attained, the absorption spectral band shape is simply ascribable to the sum of the anion radical of electron acceptor moiety ŽAyØ. and the cation radical of the donor moiety ŽDqØ.. On the other hand, the deviation from this simple additivity provides information on the delocalization interaction in the excited state. For comparison, the transient spectra of 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-w3,4-b;4X ,3X-ex-pyridine ŽBPP. as well as 3,5-dimethyl-1,4,7-triphenylbis-pyrazolo-w3,4-b;4X ,3X-ex-pyridine ŽH-DMPP. are investigated. Both compounds work as model systems for the acceptor subunit Žsee formulae..

2. Experimental The three investigated compounds BPP, H-DMPP, and DMA–DMPP were synthesized in the Department of Organic Chemistry of the Pedagogical University in Kielce ŽPoland. and purified w3,4x. The solvents used for the stationary fluorescence spectra and for the decay measurements were of spectroscopic grade ŽMerck Uvasol.. For picosecond experiments, hexane, ethyl acetate, methanol ŽDotite, Spectrosol., and acetonitrile ŽNakarai, fluorometry reagent. were used as received. All sample solutions Žfor picosecond measurements a concentration of DMA–DMPP ; 3 = 10y4 M was used. were deaerated by purging with a stream of dry nitrogen. A sample cell with a 1 cm optical path length was used for the measurements. All measurements were performed at 21 " 28C under O 2 free conditions. Absorption spectra were measured with a Shimadzu or Hitachi U3300 spectrophotometer. Stationary and nanosecond time-resolved fluorescence measurements were performed as described in Ref. w3x. A microcomputer-controlled picosecond laser photolysis system with a custom-built repetitive mode-locked Nd 3q:YAG laser was used for all measurements. The optical alignments were almost the same as those developed previously w5x. The third harmonic pulse at 355 nm with 16 ps FWHM and 0.5 mJ output power was used for exciting the samples. Monitoring white light was generated by focusing the fundamental light into a 10 cm D 2 O–H 2 O Ž3:1. cell. Two sets of the multichannel photodetectors ŽHamamatsu, S4874. combined with a polychromator were used for the detection of the monitoring light. Circular polarized probe light was used for the absorption spectroscopy. The repetition rate of the excitation light was kept low Ž- 0.5 Hz.. Most of the

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data were accumulated over 4 measurements. Chirping of the monitoring white continuum light Ždispersion. was corrected for the transient spectra.

3. Results and discussion 3.1. Spectroscopic characteristics of BPP, H-DMPP, and DMA–DMPP In the case of BPP, the absorption and fluorescence spectra are weakly dependent on the solvent ŽFig. 1a.. The fluorescence quantum yield is near unity, as previously reported w6x. H-DMPP shows a small blue-shift in absorption and a small red-shift ŽFig. 1b. in fluorescence. This effect was explained as the result of an increase in the dipole moment and a change of its direction w7,8x. The absorption and fluorescence spectra of DMA–DMPP in solvents used for transient absorption measurements are depicted in Fig. 1c. The spectral position and fluorescence quantum yield is strongly solvent-dependent. The quantum yield is large in non-polar solvents Ž0.78 in hexane., but decreases in polar ones Ž0.26 in acetonitrile, 0.017 in methanol. w3x. The plot of the transition dipole moment of fluorescence versus the Lippert–Mataga w9–12x solvent-polarity function D f s Ž D y 1.rŽ2 D q 1. y Ž n 2 y 1.rŽ2 n 2 q 1., depicted in Fig. 1d, shows a dramatic change in the slope in the region of medium polarity Ž D f s 0.2., similarly as was previously found for fluorescence maxima and radiative fluorescence lifetimes w3x. This confirms the previously postulated change of emitting state character from a weakly polar to a highly polar one with a dipole moment of 97 = 10y3 0 Cm w3x. At low polarities, emission originates from a state localized on the bis-pyrazolopyridine moiety, whereas at high polarities the excited state is of CT character, in which an electron is promoted from the donor to the bis-pyrazolopyridine moiety. According to semi-empirical calculations including the solvent reaction field w13,14x, this state is the S 6 in the gas phase, but becomes the lowest excited singlet in polar solvents. Its transition moment decreases ŽFig. 1d. with increasing solvent polarity. This effect can be explained as the result of decreasing mixing of CT and primary excited state due to the increasing gap between these states. 3.2. Picosecond measurements DMA–DMPP solutions in solvents of different polarity, excited with a picosecond 355 nm laser pulse, show transient absorption between 400 and 1000 nm, i.e. in the spectral region available in our experiment. These spectra appear immediately after excitation. The rise of the transient absorbance is identical with the response of the apparatus, within experimental error. In the case of solution in ethyl acetate, the transient absorption present at soon after the pump pulse undergoes further transformation. Fig. 2a shows the transient absorption spectra of DMA–DMPP in an n-hexane solution, excited with a picosecond 355 nm laser pulse. The absorption spectra with maxima at ca. 500 and 630 nm appear very rapidly following the excitation. The transient spectrum is produced almost within the time resolution of the apparatus, i.e. within a few picoseconds. It decays with a time constant of 6.3 ns. Since the decay time constant of the absorption bands is identical with the fluorescence lifetime of DMA–DMPP in this solvent Ž6.7 ns; Table 1. within the experimental error, the transient spectrum in Fig. 2a can safely be assigned to the S n § S 1 absorption of DMA–DMPP in n-hexane. On the other hand, in acetonitrile solution where the steady-state emission shows a broad CT fluorescence peaking at 550 nm, the transient absorption spectra of DMA–DMPP following the 355 nm excitation in Fig. 2b show quite a different band shape from those in n-hexane. The appearance of the band at 470 nm is almost identical with the response function of the apparatus. In the time window available in the present measurements Ž0–6 ns., apparent evolution of the spectral band shape was not observed. The decay time constant of the present absorption band was estimated to be equal to, or greater than, 15–20 ns, which is similar to the

124 H. Miyasaka et al.r Chemical Physics Letters 307 (1999) 121–130 Fig. 1. Absorption and corrected fluorescence spectra of: BPP Ža., H-DMPP Žb., and DMA–DMPP Žc., in different solvents. The plot of the transition depole moment in the fluorescence of DMA–DMPP vs. the solvent-polarity function Žd.. The solvents are: hexane Ž1., benzene Ž2., dibutylether Ž3., 1-chlorobutane Ž4., tetrahydrofuran Ž5., dichloromethane Ž6., pentanol-1 Ž7., butanol-1 Ž8., cyclohexanone Ž9., propanol-1 Ž10., ethanol Ž11., acetonitrile Ž12., methanol Ž13., dimethyl sulfoxide Ž14., propylene carbonate Ž15., ethyl acetate Ž16..

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Fig. 2. The transient absorption spectrum S n §S1 of DMA-DMPP in hexane Ža., and in acetonitrile Žb.. Insert: The absorption of the DMA radical cation in a freon matrix at 77 K w15x.

fluorescence lifetime of DMA–DMPP in acetonitrile Ž21 ns; Table 1.. A decay constant of 13.0 ns, reported in Ref. w3x, was obtained previously for DMA–DMPP solution in acetonitrile with a small amount Ža few percent. of 1-chlorobutane. It was found that even a small admixture of a less polar solvent to acetonitrile causes a remarkable decrease of the fluorescence decay time. The band observed in the present picosecond measurement, hence, is ascribable to the S n § S 1 absorption of DMA–DMPP in acetonitrile. It was already reported that the characteristic absorption of N,N-dimethylaniline radical cation has a maximum at 470 nm w15x Žsee insert Fig. 2b.. On the other hand, the peak position of the anion radical of BPP Table 1 Decay times of the transient absorption and fluorescence of DMA–DMPP in some solvents Solvent

Hexane

Acetonitrile

Methanol

Ethyl acetate

trns from transient abs. fluorescence

6.3 6.7 a

15–20 21.0

1.25 1.4 a

10.7 13.5

a

Data from Ref. w3x. The accuracy of the fluorescence lifetime determination is at least 0.2 ns.

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Fig. 3. Transient absorption spectrum of the BPP solution Ž cs 3.2=10 -4 M. with diphenylamine ŽDPA, cs 0.25 M. in ethyl acetate at t s100 ps after the pump laser pulse Ža.; and without DPA Žb.. Transient absorption spectrum of H-DMPP in ethyl acetate at t s100 ps after pump laser pulse Žc..

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was unclear. In order to obtain the electronic spectrum of the BPP anion radical, we measured the transient absorption spectra of BPP with and without diphenylamine ŽDPA. as an electron donor in ethyl acetate solution. The addition of diphenylamine ŽDPA. to BPP causes the appearance of an absorption 100 ps after the pump pulse with maxima at 600 and 675 nm ŽFig. 3a. which disappear after 1 ns. The band, at 675 nm, characteristic for absorption of DPA radial cation w15x, is absent in BPP solution without DPA ŽFig. 3b.. This result clearly indicates that the intermolecular electron transfer actually takes place between the excited BPP and DPA. However, we found no other absorption bands except for the S n § S1 transition of the BPP and DPA cation radical. Thus, it is concluded the absorption of the BPP radical anion has no strong and characteristic absorption bands in the spectral region under study. The transient absorption spectra of DMA–DMPP in ethyl acetate solution are shown in Fig. 4a. Clear time evolution of the absorption spectrum was observed in the initial several tens of picoseconds time region. In the early stage after the excitation, the band shape and absorption maxima in the spectrum are close to those observed in n-hexane solution. This spectrum evolves in time into the absorption, which has a strong band in the 400–500 nm region. This absorption spectrum decays with a time constant of 11 ns ŽFig. 4b.. Since this time constant was identical with the fluorescence lifetime Ž13.5 ns; Table 1. within the experimental error, the present spectrum is assigned to the S n § S1 absorption of equilibrium S1 state in ethyl acetate. Although the absorption spectral shape after several tens of picoseconds following the excitation is close to that in acetonitrile, a small difference between them is observed. This suggests that mixing of the CT excited

Fig. 4. The transient absorption spectrum S n § S1 of DMA–DMPP in ethyl acetate Ža., the decay curve monitored at 470 nm, Žb. the time profile of the transient absorption at 545 nm Žc., and at 456 nm Žd..

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state with the LE ones takes place, leading to the electronic delocalization interaction between donor and acceptor in the equilibrium fluorescent state in ethyl acetate. Time profiles of transient absorbance at 545 and 456 nm in the early stages after the excitation are exhibited in Fig. 4c and d. Solid lines in these figures present the curves calculated with pulse duration and the time constant of the evolution of 17 ps. As shown in these figures, both curves, the decay at 545 nm ŽFig. 4c. and the rise at 456 nm ŽFig. 4d., were reproduced with a time constant of 17 ps. This result indicates that the intramolecular CT to an equilibrium fluorescent state in ethyl acetate takes place with a much longer time constant than in acetonitrile. The shape of the transient absorption spectra in methanol solution shown in Fig. 5a is very close to that in acetonitrile. The appearance of the characteristic band at 470 nm also takes place almost within the time resolution of the apparatus ŽFig. 5b. and decreases with a time constant of 1.3 ns ŽFig. 5c., which was almost identical with the fluorescent lifetime of 1.4 ns ŽTable 1.. These results indicate that CT separation is very rapid and the degree of the CT in the equilibrium excited state is very large. Although the rise of the main transient absorption band of DMA–DMPP in methanol at 470 nm appears very fast, the position of the valley region which is partially attributed to the stimulated emission, shifts to red in several tens of picoseconds after excitation. Fig. 5d shows the transient absorption spectra in the range of 550–700 nm, whereas in Fig. 5e the temporal shift of the transient spectrum minimum is depicted. This shift may be described by a bi-exponential function. The fast main component decays with a 34 ps time constant. It is followed by the slower one with a time constant larger than 1 ns. The longer time constant may be attributed to

Fig. 5. The transient absorption spectrum S n § S1 of DMA–DMPP in methanol Ža., its rise time profile Žb., the decay profile monitored at 470 nm Žc., dynamic solvent shift Žd., and time dependence of transient spectrum minimum Že..

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the spectral evolution such as the decay of the fluorescent state and appearance of the long-lived species such as the triplet state. It should be mentioned that the characteristic absorption indicating the almost full CT was observed first and is followed by the relaxation with the time constant of 34 ps which is closely related to the dynamic Stokes shift of the fluorescence. This result indicates that the charge separation is not regulated by this time constant. In addition, the present time constant of 34 ps is much longer than the longitudinal relaxation time of the solvent, i.e. 9.2 ps, estimated from Debye’s relaxation time equal to 55.6 ps w16x. Although it is rather difficult at the present stage of the investigation to derive a clear conclusion on this spectral evolution in methanol, this dynamic behaviour may be related to the characteristic behaviour in the fluorescence spectrum in this solvent. The steady-state fluorescence spectrum in methanol shows the two bands indicating the existence of at least two different species w3,17x. In addition, the dynamic rearrangement of the hydrogen bonding between the solute and the solvent after the charge separation may also affect the present dynamic behaviour. The charge separation in methanol takes place very rapidly, as in acetonitrile, and the degree of the CT is very large. Although in ethyl acetate the CT process is remarkably slower than in methanol, dual fluorescence is not observed in the first solvent. It is also not observed in acetonitrile, in which the relaxation to the CT state is comparably fast, as in methanol. Thus, dual fluorescence has not been ascribed to an adiabatic relaxation to the CT state. The appearance of two fluorescence bands has to be due to a specific solute–solvent interaction and to the existence of two different species in the ground state w17x. The picosecond transient absorption in methanol indicates only that the main fraction of DMA–DMPP molecules exist in the form undergoing a fast Žof the order of a few picoseconds. CT process in excited state. The transient absorption spectrum of H-DMPP in ethyl acetate, depicted on Fig. 3c, differs from that of DMA–DMPP in hexane and BPP in ethyl acetate. The difference can be ascribed to the superposition of the S n § S 1 absorption of the different subunits of these molecules.

4. Conclusions Ž1. The results of the measurements of picosecond transient absorption spectra of DMA–DMPP indicate a fast irreversible relaxation process to the CT state in polar solvents. The time constant of the decay of the transient absorption closely resembles those obtained from fluorescence decay analysis ŽTable 1.. These kinetics, together with the transient absorption spectra in polar solvents, whose peak at 470 nm corresponds to the cation of N, N-dimethylaniline, give a direct proof that the fluorescing state is charge-separated. The absorption of the radical anion of bis-pyrazolo-pyridine is beyond the spectral region available in our experiments. Ž2. The charge separation process is not rate-determined by the dynamic solvent shift in methanol. Ž3. The relaxation to the CT state in the medium polar ethyl acetate is slower than in the case of the strongly polar methanol or acetonitrile.

Acknowledgements HM appreciates the Grant-in-Aid from the Ministry of Education, Science, Sports and Culture Ž10640490.. KR is indebted to Committee for Scientific Research in Warsaw for financial support within Polish–Austrian Projects 01-003 and 060rR98rR99 and Grant 8T11b00510. KR would like to thank the Fonds zur Forderung ¨ ¨ Žproject nr. P11880-CHE. for financial support. der wissenschaftlichen Forschung in Osterreich

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