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The infrared and visible absorption spectra of the lowest triplet state of acridine isolated in an argon matrix
CHEMICAL PHYSICS LETTERS
Volume 63. number 3
THE INFRARED
AND VISIBLE
ABSORPTION
George R. SMITH,
June 1979
SPECTRA
OF THE LOWEST TRIPLET STATE OF ACRIDINE hlark B_ MITCHELL.
1
ISOLATED
1N AN ARGON
MATRIX
Kathy JAHSEi\: and W-A. CUILLORY US.4
Depmtznetrt of Chenzi+tr_y,U.ui*ersit. of Utah. Sait take Cip. Utnlt 84112, Received 3 February 1979
The intkwcd and vi&k absorption spccrm of zhe Iowcst 3~* state ofacridinc, produced xi:, cw lacr puoqzing of an ;~rson matrix &oh&* sample are reported. The visibie spectrum agrees%eII with previous studies. in the infrared, abbsorptionsof tlx triplet state are obscrwzdaf 1434, 1104, 1018, 1004 and 75 I cm-‘_
I. Introduction Over the past several years, triple?--triplet (T2z * Tt) absorption studies of large organic molecules have become almost routine. and have provided considerable infomlation on the triplet manifotd. Spectroscopic data about the first triplet (TI) however, is rather sparse. Gas phase studies such as direct triplet absorption, magnetic rotation, or phosphorescence, require well resolved spectra to allow the derivation of the vibrational frequencies of the triplet, and are thus usualiy limited to smaller molecules. In the solid phase, viirational relaxation is almost always fas:er than emission, so that the molecule must be chosen to have a sufficient triplet absorption coefficient for triplet spectroscopy to be perfoned. Often, in these cases. Fran&--Condon considerations limit the number of vibronic transitions and correspondingly the number of vibmtional modes characterized_ An afternative meth$ for obtaining information on Tt is to prepare ELsteady-state population of the triplet via cw laser pumping of the singIet (So) followed by intersystem crossing (ISC). Provided the phosphorescent lifetime is reasonably long, a sufficient concentration of T, can be produced to allow direct spectroscopic investigation. Although this technique has been used prwiously to study T,, + T1 [I] and ESR [2] spectra of large dye molecules, no reports of this application to mediumsized ma:rix-isolated molecules have been published_ The matrix isolation technique offers several advantages in these experiments_ In the case of Tlr + T, spectroscopy,
the quenching of rotational motion provides improved resolution as compared to gas cr solution phase spectraThis improved resolution is even more important when the vibrational spectrum of TI is to be recorded. The narrow absorptions observed in matrix isoiated samples allow the resolution of common vibrational modes (of SO and TI) whose frequencies may be shifted only a few wavenumbek in the two states_ In this Ietter, we report the vislbIe absorption spectrum of triplet acridine, produced in steady state by Iaser irradiation of a matrix-isolated sample- In addition. we h&ve directly observed the infrared spectrum of matrix-isolated triplet acridine using this steady-state pumping technique.
2. Experimental Acridine (Aldrich_ = 97% purity) was recrystakzed from ethanol prior to use- The mrttrkisolnted sampies were prepared by depositing a stream of argon @latheson Grade) simultaneously with the vapor from subliming acridine, onto a CsI or LiF window maintained at ==I3 K by an Air Products Displex. Due to this method of wmple preparation, the acridine concenrration could not be determined. However, the infrared absorption spectra suggested very dilute matrix samples- The relative concentration could be varied by chsnging either the argon flow rate or the sublimation temperature of the solid acridine during deposition. Excitation of the samples w’;ts achieved with a Cohereni
VoIume 63. number3
1 June 1979
CHEMICAL PHYSICS LETTERS
Radiation CR-18 Ar ion laser_ The W output of the laser could be dispersed with a quartz prism to give essentially three lines at 3638,35 14-35 11, and 3336 A for pumping specific absorptions, but in most of these experiments was used undispersedW-visible St + So and T, + T, absorption experiments were performed with a McPherson model 225 vacuum monochromator equipped with gratings blazed at 2000 and 7000 A, an HTV-R928 photomultiplier, and a 4X image magnifier collection lens- The absorption source was either a tungsten or a deuterium lamp, depending upon the spectraI region of interest_ In the case of T,, +- Tt absorption spectra7 the experimental set up was quite similar to that described by Pavl0pou10~ [I ] _ The sampIe was Iocated at the image pIane of the collection lens and the laser directed onto the sample at the collection lens focaI point_ The tungsten source was focused onto a Princeton Applied Research (PAR) chopper and then collimated and refocused to be coincident with the laser_ In these experiments, the absorption source was chopped rather than the laser since it was found that while the ground state molecute did not have any absorptions in the region of interest_ laser related emission was present_ The signal was processed with a PAR 122 lock-in amplifier and displayed on a strip chart recorder_ For the infrared experiments, the basic instrument was a Perkin-Elmer model I80 spectrophotometer. In some experiments, the instrument was used normally, with the unchopped laser, aiter passage through a beam expmder, being directed onto the sampIe in the infrared beam- In most experiments, however, the internal choppers of the 180 were disabled, and the optics aligned for single beam operation_ The signal was taken from the spectrometer before the demodulator, but after ~140 dB of amplilication, and fed to an lthaco Dynatrac 3 lock-in amplifier. In this case. the Iaser was chopped at 15 Hz to match the tuned amplifier circuits of the specirometer. The slits were fixed at I mm, and the scan rate WIS chosen to be compatibIe with the lock-in time constant. Survey spectra were taken from 1600 to 500 cm-t at -5 cm-t/min with a time constant of 4 s. These spectra were of poor quaIity, and served only to indicate the regions on interest_ Detailed spectra were scanned at ~0.5 cm-t/min with a 12 s time constant_
3_ Results and discussion Several St + So absorption spectra of matrixisolated acridine were obtained, and were found to agree we11 with previously published spectra [3-8]_ The origin of the system is somewhat sensitive to environment: z3922 A in the solid [S] ; -3750 in n-hexane at room temperature; 238 I5 in n-heptane at 4 K [6] ; =3900 in alcohol or alcohol/ether at 77 K [3,4] ; and a3600 in the gas phase [7] _ The observation of the origin at 3697 A in argon suggests that the argon matrix perturbs the electronic structure somewhat less than the other solvents_ The laser induced visible absorption spectrum of matrix isolated acridine from 4400 to 3900 A is shown in rig. I _ SeveraI experiments were performed to ensure that this spectrum was due to the laser interacting with acridine- No absorptions were obtained in this region with the Iaser off_ Experiments with the laser on and with the chopper operating to trigger the lock-in, but with the tungsten source blocked showed we were capable of discriminating against laser induced emission. Experiments with pure argon matrices produced no absorptions. To eliminate the remote possibility that the spectra were due to pureIy thermal effects of the absorbed Iaser power, experiments were also performed with benzophenone, which absorbs the laser strongly. No absorptions were found in this region with this sample- Spectra obtained using the individual 3.51 I-3514 and 3638 A laser lines, adjusted to be at the same power, were proportiona in absorption intensity to the over-
* I
E
f
I 4251
Fig. I-Triplet-triptet absorption spectrumoCmatri.\-isol.rtcd acridincrecorded with cw laserirradiation.wavelengthsin A_ 476
Volume 63, number 3
CHEMICAL
PHYSICS
lap of the fines with the St + So absorption spectrum. Fin&y, the intensity of the induced abrorptions varied Iinearly with laser power_ The T, -+ T, absorption spectrum of a&dine has been reported several times previously [9- 131_ The wavelength of mzxjmum absorption is again dependent on conditions and ranges frotn ~4200 A in the gas phase [9] to -20 I%in benzene [13]- Again, the Ar matrix vaIue of 425 1 A is intermediate, and closer to the gas phase result_ Based on ali of the observations given above, we conclude that it is possibIe to generate a sufficient steady state concentration of acridine in the T, (3mr*) state in zm argon matrix For direct spectroscopic observation_ Prior to attempting to record the infrared spectrum of triplet acridine, we obtained the double beam spectrum of the ground singlet state, So. Despite the acridine purification and the method of deposition, it was found that significant impurities were sometimes present. If the same sample was used for several depositions however, the relative impurity level decreased, and in some experiments, impurities were not observed_ As mentioned briefly before, the acridine samples appeared rather dilute, with the strongest absorption (at 737 cm-1 in Ar) never being stronger than =90% transmittingNevertheless, the si.u strongest absorptions [ 141 of SG at 15S9,906,854,790,737, and 601 cm-t were readily observed. The source signal of the infrared spectrophotomcter used in these studies is entirely ac coupled up to the point where it was removed for external amplification. Thus when the choppers were disabled, no signal wxs observed. However, when the chopped laser besm was directed onto the sample, the concentration of So and Tt of acridine changed at the chopping frequencySince the spectrometer is sensitive to varying signals, we therefore expect to qbserve only So and the metastable. For relatively weak double beam signals (< 50% T) the absorptions in the singIe beam spectrum of So are expected to appear in the same order OF intensity as observed in double beam spectra_ given that the source intensity is rekJonabIy constant over the wvelength interval. Furthermore. under the conditions present here, the intensity of the single be,ml spectrum of the metastabIe state shouId always be in rough proportion to that of the depIcted So ground state, where the proportionality factor LSthe ratio of the b.md absorption coefficients_ Indeed, this is exxtly whnt is
LETTERS
1 June 1979
r‘iz. 1.. Portion of the singk beam 1x11-mredspectrum of msrriz~tsol-ltcd aridme recorded x\it.h chopped her irndutton. 1-w yuencies in ctri’ _
obsertcd_ Fig. 2 shows a portion of the single beam spectrum. The feature at 737 cm-t is essdy identified as being due to the strongest So infrared absorption. AH six of the previously mentioned parent features dre observed in the single beam spectrum in the same order of intensity as in double beam spectrrl. The remaining single beam absorptions. at 1434, 1104. 104s. 1003 and 75 1 cm-1 were always found to be proportional to the So features regardless of its concentration or the impurity level_ Two of these features (1104 and 751 cm-t) are more intense than the most intense So feature. and certainly could not be due to the ground state_ No absorptions were present in normal double beam spectra at these frequencies_ To further ensure that these features were due to infrared absorption of a species produced by the IJser. spectra were obtained with the laser off, with the infrared source blocked, and ofpcre argon matrices. in none of these experiments \iete the absorptions srrnbuted to T1 observed. Fin& ly, once the positions of the transient absorptions were known we were able to record them in normal double beam mode, under constant laser irradiation. The feature at 75 1 cm-t was always observed with the laser on, while the next most intense. but still considerably weaker, absorptions at 1101- and 1048 cm-l could be observed depending on experimental conditions. There appesrs to be IittIe doubt that the above absorptions are due to a metastable electromc state produced by -177
VoIume 63. numba 3
CHEBIICAL PHYSICS LETTERS
Iaser pumping ofacridinc_ Given the results of the Wt&s is most Iikefy the lowest 3mr* state. The infrared spectrum of So is not well understood [ 141, and thus we have not assigned the tripfet spectrum. We note however, that the r&ted mofccutc anthracene aIs has one of its strongest absorptions in this region (725 cm-*), and it has been assigned as one of the Cff out-of-plane bending modes (15] _A simikrr nsslgnment would be quite appropriate here for the 737 and 75L cm-1 absorptions of ground singlet and triplet acridine respectively-I-hisis ciearly a preliminary study of a cornpIes system, and further experiments are in progress_ The primary trust is to improve the SfrV of the spectra, via changing deposition conditions, matrix mate&Is_ em_, SO that a wider range of moIecu1e-ecrm be investigated. In the particular case of acridine, it is hoped that different eIectronics \vilI significzriitly improve the S/3 and therefore, the resohrtion of fig_ I * and that different matrices wviIIsharpen the transitions_ Shpol‘skii type matrices are anticipated to be mast significant in this regard fI6,17) and with the single beam technique, it i3 possibie they could be used in infrared studies. The range of the infrared study reported here wih be estended, and we hope to present more definitive vibrational assignments of both the ground and excited state spectra. As far as other systems are concerned_ moIecuIes closely r&ted to acridinc are obvious Lzmdidates, and preIiminary T,z - Tl spectra of phenazine have aheady been obtained_ The direction of most interest to us however_ is to study smaller moiecufes wherein the shifts of the infrared frequencies between the two ekxtronic states can be better correbted lvith the etectronic structure ofthe excited state. As we observe the vibrational spectrum uncomplicated by Franck-Condon considerations. the intensities in these spectra give direct information about the potential surface of the triplet slate.
1 June 1979
AcknowIedgement
visibk experiments,
478
We gratefuIIy acknowIedSe support for this research by the National Science Foundation through Grant No. CITE-77-09 175 and Equipment Grant No. ME-76-05 17; and the Biomedicf Sciences Support Grant through the University of Utah, PI-IS Grant No- RR07092 We also wish to thank Grady-hfoore Associates for the loan of the Ithaca lock-in amplifier, and Dr_ M_L_ Lesiecki and Mr- J- Warren for assistance in some of the experiments_
References [t I T-G. P;lrfopoulos,J. Opt. Sot- Am. 63 (1973) 1SO. [Z] .\I. Ymnashita and Ii. Kashiw& J. Ph)s. Cbem. 78 (1973) 2006. 131 V. Zanker and W. Schmid, Chem. Brr. 90 (1957) X253_ (41 A. \vittw;crand V. Zanker, Z- Physk Chem. NF 22 (1959) 417. fS1 H.H. Pcrktmpus and R. Kortum, 2. Ph_vsrk-Chem. NF 56 ( f 967) 73. (61 R-31. JhcNab and K. Saucr. J. Chem. Phys- 53 (1970) 2805.. [ 7 J J.P. Byme sod LG. Ross. Australi.m J_ Chcm- 24 ( 1971) 1107. [S] R.P. Steiner and J. Michl. J. Am. Chem. Sot. 1GO(1978). to bc published. [9] C_t’. Ashpolc and S-J. Formosinho, J_ Mol. Spwtry. 53 fi974) 489_ [ I Oj- Y. Yimta and L Tzmz~ftl,Chum.. Phyr Lrrtters4 1 (2 976) 336. [ I I] V_ Sut&trcm and P.M. Rentzcpis, J. Chcm. Ph>s_ 66 (1977) 4x7_ f I-?] Y. lifirztzand LTan&a,Chcm. PIqs_ 25 (1977) 381. j13f A.. Bellman, J. Ph>s. Chem_ Sl (1977) 1193. [ 141 SL Etrigadiotand J.M. Lnbss. J. Chim- P&s. Physicochim. Biol. 69 <1971) 964. [ 151 J. Vod~hnaI and V. Stcpsn. Collection Czwh. Chem. Commun. 36 (1971) 3930. [ t 6 j C-V. Shpol’skii, Soviet Plq s. Uaqxkhi 3 ( 1960) 372. f I7f P- Tokousbdidcs. EL. Wehr) and G. JIamanto;, J. P&s. Chem. 81 (1977) i769.
Volume 63. number 3
THE INFRARED
AND VISIBLE
ABSORPTION
George R. SMITH,
June 1979
SPECTRA
OF THE LOWEST TRIPLET STATE OF ACRIDINE hlark B_ MITCHELL.
1
ISOLATED
1N AN ARGON
MATRIX
Kathy JAHSEi\: and W-A. CUILLORY US.4
Depmtznetrt of Chenzi+tr_y,U.ui*ersit. of Utah. Sait take Cip. Utnlt 84112, Received 3 February 1979
The intkwcd and vi&k absorption spccrm of zhe Iowcst 3~* state ofacridinc, produced xi:, cw lacr puoqzing of an ;~rson matrix &oh&* sample are reported. The visibie spectrum agrees%eII with previous studies. in the infrared, abbsorptionsof tlx triplet state are obscrwzdaf 1434, 1104, 1018, 1004 and 75 I cm-‘_
I. Introduction Over the past several years, triple?--triplet (T2z * Tt) absorption studies of large organic molecules have become almost routine. and have provided considerable infomlation on the triplet manifotd. Spectroscopic data about the first triplet (TI) however, is rather sparse. Gas phase studies such as direct triplet absorption, magnetic rotation, or phosphorescence, require well resolved spectra to allow the derivation of the vibrational frequencies of the triplet, and are thus usualiy limited to smaller molecules. In the solid phase, viirational relaxation is almost always fas:er than emission, so that the molecule must be chosen to have a sufficient triplet absorption coefficient for triplet spectroscopy to be perfoned. Often, in these cases. Fran&--Condon considerations limit the number of vibronic transitions and correspondingly the number of vibmtional modes characterized_ An afternative meth$ for obtaining information on Tt is to prepare ELsteady-state population of the triplet via cw laser pumping of the singIet (So) followed by intersystem crossing (ISC). Provided the phosphorescent lifetime is reasonably long, a sufficient concentration of T, can be produced to allow direct spectroscopic investigation. Although this technique has been used prwiously to study T,, + T1 [I] and ESR [2] spectra of large dye molecules, no reports of this application to mediumsized ma:rix-isolated molecules have been published_ The matrix isolation technique offers several advantages in these experiments_ In the case of Tlr + T, spectroscopy,
the quenching of rotational motion provides improved resolution as compared to gas cr solution phase spectraThis improved resolution is even more important when the vibrational spectrum of TI is to be recorded. The narrow absorptions observed in matrix isoiated samples allow the resolution of common vibrational modes (of SO and TI) whose frequencies may be shifted only a few wavenumbek in the two states_ In this Ietter, we report the vislbIe absorption spectrum of triplet acridine, produced in steady state by Iaser irradiation of a matrix-isolated sample- In addition. we h&ve directly observed the infrared spectrum of matrix-isolated triplet acridine using this steady-state pumping technique.
2. Experimental Acridine (Aldrich_ = 97% purity) was recrystakzed from ethanol prior to use- The mrttrkisolnted sampies were prepared by depositing a stream of argon @latheson Grade) simultaneously with the vapor from subliming acridine, onto a CsI or LiF window maintained at ==I3 K by an Air Products Displex. Due to this method of wmple preparation, the acridine concenrration could not be determined. However, the infrared absorption spectra suggested very dilute matrix samples- The relative concentration could be varied by chsnging either the argon flow rate or the sublimation temperature of the solid acridine during deposition. Excitation of the samples w’;ts achieved with a Cohereni
VoIume 63. number3
1 June 1979
CHEMICAL PHYSICS LETTERS
Radiation CR-18 Ar ion laser_ The W output of the laser could be dispersed with a quartz prism to give essentially three lines at 3638,35 14-35 11, and 3336 A for pumping specific absorptions, but in most of these experiments was used undispersedW-visible St + So and T, + T, absorption experiments were performed with a McPherson model 225 vacuum monochromator equipped with gratings blazed at 2000 and 7000 A, an HTV-R928 photomultiplier, and a 4X image magnifier collection lens- The absorption source was either a tungsten or a deuterium lamp, depending upon the spectraI region of interest_ In the case of T,, +- Tt absorption spectra7 the experimental set up was quite similar to that described by Pavl0pou10~ [I ] _ The sampIe was Iocated at the image pIane of the collection lens and the laser directed onto the sample at the collection lens focaI point_ The tungsten source was focused onto a Princeton Applied Research (PAR) chopper and then collimated and refocused to be coincident with the laser_ In these experiments, the absorption source was chopped rather than the laser since it was found that while the ground state molecute did not have any absorptions in the region of interest_ laser related emission was present_ The signal was processed with a PAR 122 lock-in amplifier and displayed on a strip chart recorder_ For the infrared experiments, the basic instrument was a Perkin-Elmer model I80 spectrophotometer. In some experiments, the instrument was used normally, with the unchopped laser, aiter passage through a beam expmder, being directed onto the sampIe in the infrared beam- In most experiments, however, the internal choppers of the 180 were disabled, and the optics aligned for single beam operation_ The signal was taken from the spectrometer before the demodulator, but after ~140 dB of amplilication, and fed to an lthaco Dynatrac 3 lock-in amplifier. In this case. the Iaser was chopped at 15 Hz to match the tuned amplifier circuits of the specirometer. The slits were fixed at I mm, and the scan rate WIS chosen to be compatibIe with the lock-in time constant. Survey spectra were taken from 1600 to 500 cm-t at -5 cm-t/min with a time constant of 4 s. These spectra were of poor quaIity, and served only to indicate the regions on interest_ Detailed spectra were scanned at ~0.5 cm-t/min with a 12 s time constant_
3_ Results and discussion Several St + So absorption spectra of matrixisolated acridine were obtained, and were found to agree we11 with previously published spectra [3-8]_ The origin of the system is somewhat sensitive to environment: z3922 A in the solid [S] ; -3750 in n-hexane at room temperature; 238 I5 in n-heptane at 4 K [6] ; =3900 in alcohol or alcohol/ether at 77 K [3,4] ; and a3600 in the gas phase [7] _ The observation of the origin at 3697 A in argon suggests that the argon matrix perturbs the electronic structure somewhat less than the other solvents_ The laser induced visible absorption spectrum of matrix isolated acridine from 4400 to 3900 A is shown in rig. I _ SeveraI experiments were performed to ensure that this spectrum was due to the laser interacting with acridine- No absorptions were obtained in this region with the Iaser off_ Experiments with the laser on and with the chopper operating to trigger the lock-in, but with the tungsten source blocked showed we were capable of discriminating against laser induced emission. Experiments with pure argon matrices produced no absorptions. To eliminate the remote possibility that the spectra were due to pureIy thermal effects of the absorbed Iaser power, experiments were also performed with benzophenone, which absorbs the laser strongly. No absorptions were found in this region with this sample- Spectra obtained using the individual 3.51 I-3514 and 3638 A laser lines, adjusted to be at the same power, were proportiona in absorption intensity to the over-
* I
E
f
I 4251
Fig. I-Triplet-triptet absorption spectrumoCmatri.\-isol.rtcd acridincrecorded with cw laserirradiation.wavelengthsin A_ 476
Volume 63, number 3
CHEMICAL
PHYSICS
lap of the fines with the St + So absorption spectrum. Fin&y, the intensity of the induced abrorptions varied Iinearly with laser power_ The T, -+ T, absorption spectrum of a&dine has been reported several times previously [9- 131_ The wavelength of mzxjmum absorption is again dependent on conditions and ranges frotn ~4200 A in the gas phase [9] to -20 I%in benzene [13]- Again, the Ar matrix vaIue of 425 1 A is intermediate, and closer to the gas phase result_ Based on ali of the observations given above, we conclude that it is possibIe to generate a sufficient steady state concentration of acridine in the T, (3mr*) state in zm argon matrix For direct spectroscopic observation_ Prior to attempting to record the infrared spectrum of triplet acridine, we obtained the double beam spectrum of the ground singlet state, So. Despite the acridine purification and the method of deposition, it was found that significant impurities were sometimes present. If the same sample was used for several depositions however, the relative impurity level decreased, and in some experiments, impurities were not observed_ As mentioned briefly before, the acridine samples appeared rather dilute, with the strongest absorption (at 737 cm-1 in Ar) never being stronger than =90% transmittingNevertheless, the si.u strongest absorptions [ 141 of SG at 15S9,906,854,790,737, and 601 cm-t were readily observed. The source signal of the infrared spectrophotomcter used in these studies is entirely ac coupled up to the point where it was removed for external amplification. Thus when the choppers were disabled, no signal wxs observed. However, when the chopped laser besm was directed onto the sample, the concentration of So and Tt of acridine changed at the chopping frequencySince the spectrometer is sensitive to varying signals, we therefore expect to qbserve only So and the metastable. For relatively weak double beam signals (< 50% T) the absorptions in the singIe beam spectrum of So are expected to appear in the same order OF intensity as observed in double beam spectra_ given that the source intensity is rekJonabIy constant over the wvelength interval. Furthermore. under the conditions present here, the intensity of the single be,ml spectrum of the metastabIe state shouId always be in rough proportion to that of the depIcted So ground state, where the proportionality factor LSthe ratio of the b.md absorption coefficients_ Indeed, this is exxtly whnt is
LETTERS
1 June 1979
r‘iz. 1.. Portion of the singk beam 1x11-mredspectrum of msrriz~tsol-ltcd aridme recorded x\it.h chopped her irndutton. 1-w yuencies in ctri’ _
obsertcd_ Fig. 2 shows a portion of the single beam spectrum. The feature at 737 cm-t is essdy identified as being due to the strongest So infrared absorption. AH six of the previously mentioned parent features dre observed in the single beam spectrum in the same order of intensity as in double beam spectrrl. The remaining single beam absorptions. at 1434, 1104. 104s. 1003 and 75 1 cm-1 were always found to be proportional to the So features regardless of its concentration or the impurity level_ Two of these features (1104 and 751 cm-t) are more intense than the most intense So feature. and certainly could not be due to the ground state_ No absorptions were present in normal double beam spectra at these frequencies_ To further ensure that these features were due to infrared absorption of a species produced by the IJser. spectra were obtained with the laser off, with the infrared source blocked, and ofpcre argon matrices. in none of these experiments \iete the absorptions srrnbuted to T1 observed. Fin& ly, once the positions of the transient absorptions were known we were able to record them in normal double beam mode, under constant laser irradiation. The feature at 75 1 cm-t was always observed with the laser on, while the next most intense. but still considerably weaker, absorptions at 1101- and 1048 cm-l could be observed depending on experimental conditions. There appesrs to be IittIe doubt that the above absorptions are due to a metastable electromc state produced by -177
VoIume 63. numba 3
CHEBIICAL PHYSICS LETTERS
Iaser pumping ofacridinc_ Given the results of the Wt&s is most Iikefy the lowest 3mr* state. The infrared spectrum of So is not well understood [ 141, and thus we have not assigned the tripfet spectrum. We note however, that the r&ted mofccutc anthracene aIs has one of its strongest absorptions in this region (725 cm-*), and it has been assigned as one of the Cff out-of-plane bending modes (15] _A simikrr nsslgnment would be quite appropriate here for the 737 and 75L cm-1 absorptions of ground singlet and triplet acridine respectively-I-hisis ciearly a preliminary study of a cornpIes system, and further experiments are in progress_ The primary trust is to improve the SfrV of the spectra, via changing deposition conditions, matrix mate&Is_ em_, SO that a wider range of moIecu1e-ecrm be investigated. In the particular case of acridine, it is hoped that different eIectronics \vilI significzriitly improve the S/3 and therefore, the resohrtion of fig_ I * and that different matrices wviIIsharpen the transitions_ Shpol‘skii type matrices are anticipated to be mast significant in this regard fI6,17) and with the single beam technique, it i3 possibie they could be used in infrared studies. The range of the infrared study reported here wih be estended, and we hope to present more definitive vibrational assignments of both the ground and excited state spectra. As far as other systems are concerned_ moIecuIes closely r&ted to acridinc are obvious Lzmdidates, and preIiminary T,z - Tl spectra of phenazine have aheady been obtained_ The direction of most interest to us however_ is to study smaller moiecufes wherein the shifts of the infrared frequencies between the two ekxtronic states can be better correbted lvith the etectronic structure ofthe excited state. As we observe the vibrational spectrum uncomplicated by Franck-Condon considerations. the intensities in these spectra give direct information about the potential surface of the triplet slate.
1 June 1979
AcknowIedgement
visibk experiments,
478
We gratefuIIy acknowIedSe support for this research by the National Science Foundation through Grant No. CITE-77-09 175 and Equipment Grant No. ME-76-05 17; and the Biomedicf Sciences Support Grant through the University of Utah, PI-IS Grant No- RR07092 We also wish to thank Grady-hfoore Associates for the loan of the Ithaca lock-in amplifier, and Dr_ M_L_ Lesiecki and Mr- J- Warren for assistance in some of the experiments_
References [t I T-G. P;lrfopoulos,J. Opt. Sot- Am. 63 (1973) 1SO. [Z] .\I. Ymnashita and Ii. Kashiw& J. Ph)s. Cbem. 78 (1973) 2006. 131 V. Zanker and W. Schmid, Chem. Brr. 90 (1957) X253_ (41 A. \vittw;crand V. Zanker, Z- Physk Chem. NF 22 (1959) 417. fS1 H.H. Pcrktmpus and R. Kortum, 2. Ph_vsrk-Chem. NF 56 ( f 967) 73. (61 R-31. JhcNab and K. Saucr. J. Chem. Phys- 53 (1970) 2805.. [ 7 J J.P. Byme sod LG. Ross. Australi.m J_ Chcm- 24 ( 1971) 1107. [S] R.P. Steiner and J. Michl. J. Am. Chem. Sot. 1GO(1978). to bc published. [9] C_t’. Ashpolc and S-J. Formosinho, J_ Mol. Spwtry. 53 fi974) 489_ [ I Oj- Y. Yimta and L Tzmz~ftl,Chum.. Phyr Lrrtters4 1 (2 976) 336. [ I I] V_ Sut&trcm and P.M. Rentzcpis, J. Chcm. Ph>s_ 66 (1977) 4x7_ f I-?] Y. lifirztzand LTan&a,Chcm. PIqs_ 25 (1977) 381. j13f A.. Bellman, J. Ph>s. Chem_ Sl (1977) 1193. [ 141 SL Etrigadiotand J.M. Lnbss. J. Chim- P&s. Physicochim. Biol. 69 <1971) 964. [ 151 J. Vod~hnaI and V. Stcpsn. Collection Czwh. Chem. Commun. 36 (1971) 3930. [ t 6 j C-V. Shpol’skii, Soviet Plq s. Uaqxkhi 3 ( 1960) 372. f I7f P- Tokousbdidcs. EL. Wehr) and G. JIamanto;, J. P&s. Chem. 81 (1977) i769.