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T-T′ absorption of molecules with extremely high extinction coefficients
Volume
15, number
15 July 1972
CHEhIICAL PHYSICS LETTERS
1
T-T’
ABSORPTION
WITH EXTREMELY
HIGH
OF MOLECULES
EXTINCTION
COEFFICIENTS
J.S. BRINEN
ReceiMd 7 April 1972
Extremely intense triplet-triplet absorption spectra have been obserbvd for a series of aryl substituted triphenylenes and phcnanthrcnes. These transitions are a result of escitcd State resonance interactions Thin rhc triplet manifold. Polarization measurements contirm that the linear polyphenyl-like states in these mokcules are responsible for these strong transitions.
1. Introduction
Excited state resonance interactions can markedly change the nature of the lowest trip!et state of organic molecules.
Such resonance
interactions
were observ-
ed when substitution in the Z-position of triphenylene by phenyl, biphenylyl and triphenylyl groups changed the nature
of the lowest triplet
state from that of a per-
turbed triphenylene to that of a linear polyphenyl [ 11. In this paper further consequences of these interactions are presented in terms of changes in the multiplicity allowed transitions between the lowest triplet state and higher lying triplet states. In addition to the aryltriphenylenes, similar effects are reported for aryl substituted phenanthrenes.
2. Experimental Triplet-triplet absorption spectra were obtained at 77°K (in 2-methyltetrahydrofuran) under conditions of steady state excitation using a 200 W Hg arc. The apparatus used fcr these measurements and for the determination of the polarization of these tnnsilions has been previously reported [2] . The simuitan-
eous ESR-optical method was used to obtain values for the extinction coefficients of the triplet--triplet absorption bands [3j. Neutral density scree!ls were used to attenuate the exciting radiation so that accurate determinations of optical densities could be made. Under these conditions low triplet concentrations were obtained and the following procedure was necessary to arrive at an accurate extincticn coefficient for the most intense absorption band. The T-T’ spectrum was measured with sufficient attenuation of the exciting light to obtain an accurate ratio of optical densities for two bands in the spectrum (one being the most intense band). Then the optical density of the weak band was monitored, together with the ESR signal, as a function of increasing excitation intensity. Lineqity between ESR concentration and optical density was obtained. The extinction coefficient of the intense band was then obtzined from this ratio and the extinction coefficient of the weaker band. 2-phenyltriphenylene (lj wasprepared from cyclohexanone and 2-lithiotriphenylene. The intermediate cyclohexanol was refluxed over palladium-on-charcoal to give the desired product. The same procedure was followed to prepare 2<4_biphenylyljtriphenylene (11) using 4-phenylcyclohexanone in place of cyclohexa69
Volume 15,number 1
15 fulr 1972
CHEMICALPHYSICS LETTERS
none. 2, ~‘-bitriphenyien~ (III) was prepared f4] by treating 2 moles of 2-~t~(~t~phenylene xvith CoBr, and n-butylbromide. The method of Mandelbaum and Cais f5] was used to prepare 2,7-diphenyltriph&ylene (IV) in very low yields. The 2,7-di~ylphen~t~r~nes were prepared by the same generaI procedure. ?,7-diiodo9, IO-dihydrophenanthrene ~3s prepared by iodination of the 9,lO. dihydrophenanthrens. The conesponding 2Jdilithio compound was obtained by reaction with n-butylithiurn. The dilithio compound was then refluxed with the appropriate ~ylcyc~o~exaIloR~ to give the corresposding 9, IO-dihydro-2,%phenanthrylenebis(aryicyclohexano1). The latter was then dehydrated and dehydro~enated to give the desired product. The substituted phenanthrenes reported in this study are 2,7diphenylphenanthrene (V); 2,7-di(3-n-hexylphenyi)phena~thr~ne (VI); 2,?-di(4ethylphenyl)phenanthrene (VII) and 2,7-di(4ethyi-l,3*c~clohexadi~ny~)phenanthrene (VIII). All the compounds had satisfactory elemental analyses and were characterized by their ultraviolet, infrared and _NblRspectra.
3. Results and discussion The triplet-triplet absorptiitn spectrum oftriphenylens is characterized by a series of bands with I~ZLXimum intensity at 23200 cm-‘, E = 1.6 X IO4 I/M cm. The oscillator strength (f) for thrs transition is 0.08. T-T’ absorption bands of approximately the same energy and intensity have been Goserved for 2methyl and 2-methoxytriphenyfene 161,In the T-T’ spectrum of 2-phenylt~pllenyiene, the most intense absorption band occurs at 20700 cm-l with E = 1.92 X lo5 l/M cm. The f value for this electronic transition is 1.5. For 2,2’-bitriphenylene (!!I) the T-T’ absorption maximum is shifted further to lower energy with E = 1.80 X 105 l/M cm at 17750 cm-l. The T-T.’ transition observed for II is similar to that observed for III, with e = 1.54 X I@ l]M cm at 18 100 cm-l. A comparison of the observed T-T’ bands of triphenyiene, I and III is shown in fig. I, The T-T’ spectrum ofpterphenyl-d14 has previously been reported f7f. This spectrum is characterized by a strong O-O band with t = 1.29 X 105 I/M cm at 21700 cm- f (f= 0.9), very simiI*q in energy, 70
3f
r
y
r24
:*
22
1
1.8
\ 1
1.G
IO” cm-’
Fig. 1. T-T’ absorption spzctn of - - - triphenylene, pI~enykriphenyione and -2,2-bifriphenylene thyltetrahydrofuran at 77’I;.
2in 2-me-
and shape co the analogous transition observed for 3-phenyltriphenylene. The lower energy transitions observed for II and III occur in the same region as the reported spectrum for ~4uaterphenyI intensity
PI * 2,7-djplzenylrrjpfzenylene
also exhibits T-T’ ab-
sorption in the pquaterphenyl
region. This con~pound
in this study with E = 6.0 X 10” l/M cm at 18 100 mm1 ( identical rvirh the absorption maximum found in II. The E of the O-I band (19600 cm-l) is 0.8 X lUS i/M cm. The high iniensity of these transitions coupled with the relatively favorable intersystem crossing [I 1 in these molecules facilitates the measurement of the polarization of these transitions. Such measurements in these systems are of importance because they serve 10 verify the existence u~~o~y~h~nyi-~ike singtr3t states in the singlet manifold as suggested by ~fandeIbaum and Cais [S] . Polarization measurements were performed by monitoring the O-O band of the T-T’ transition whilt: exciting at 313 nm. For IV, this trirnsition was found to be strongly positively poiarized (-t-3@,), parallel to the direction of the absorption band at has the strongest
bands
observed
Volume 15, number
1
CHEMICAL PHYSICS LETTERS
313 nm. As in the case of p-terphenyl, if the T-T’ band is long axis (polyphenyl axis) polarized, the singlet-singlet transition must also be. Similar intensification is observed in the T-T’ spectra oi the 2,7-diary1 substituted phenanthrenes. The T--T’ absorption spectrum of 2,7-diphenylphenanthrene (V) is similar to that observed for IV. The extinction coefficient for V at the band nxaimum (18000 WI-~) is 2.93 X lo5 l/M cm. Substitution of an Aexyl group in the meta position of each phenyl ring (VI) does not severely alter the spectrum. A slight red shift is observed with e,nas = 2.20 X lo5 l/M cm. ‘The polarization of the T-T’ transition in VI measured at the O-O band with respect to 3 13 nm excitation was found to be +39%. This verifies the existence of polyphenyl-like absorption bands in the singlet-singlet absorption spectrum. The high intensity of the T-T’ transitions observed for these systems may then be attributed to the inphase addition of transition moments which are polarized along the long axis of the molecule as is the case for linear conjugated polyenes. The effect of the degree of’ conjugation in the substituent groups on the 2,7-positions of phenanthrene was also investigated. Instead of phenyl substituents, 4 ethyl- 1,3-cyclohe.uadienyl groups were placed at the 2- and 7-positions (VIII). The maximum intensity of the T-T’ absorption band is at 20800 cm-I (E = 1.14 X 10” l/M cm) while for the corresponding 4-. ethyl phenyl compound (VII) the T-T’ intensity maximum is at 17 500 cnl-l (E = 1.90 X lo5 l/M cm). For phenanthrene [3] itself, the intensity maximum of the T-T’ absorption is at 20400 cm-l with E = 0.42 X lo5 l/M cm. The reduction in conjugation on going from pheny! to cyclohexadienyl substituents results in a decrease in both the energy and the intensity of the T-T’ absorption bands. The triplet-triplet absorption measurements are summarized in table 1.
Triplet-tiplet
I5 July 1972 Table i extinction
cwfficientsa)
Compound
ET-T’ (104 cm-‘)
ET-T’ (lo-’ I/M cm)
2-phenyltriphenylenc
1.80 2.07 2.20
0.24 1.92 0.60
2-biphenylyltripllenylcne
1.81 1.94 sh
1.71 0.57
2,2-hitriphenylene
1.74 1.94 sh
0.57
2.7diphenyltriphenylene
1.81 1.96
6.0 0.8
2,7diphenylphenantIuene
i.80 1.94
0.33
2.14
0.06
1.I0 sh 1.78 1.92 2.03 sh
0.13 2.2 0.35 0.13
2,7ili(4ethylphenyi)phenanthrene
1.75 1.93 2.04
1.90 0.28 0.01
2,7di(4-ethyl1,3cyclokxadienyl)-
2.01
0.75
7.08 2.17
0.50
2,7di(3-n-hexylphcnyl)phenanthrcnc
phenanthccnc a)Thesc measurements\~ere at 77OK.
1.80
2.93
1.14
made in 2-rncthyltctrahydrofur~n
triplet spectra [l] ~ the absorption properties of the lowest triplet state parallel those of the corresponding linear polyphenyIs. Transitions with high extinction coefficients and osciltator strengths are observed which are considerably shifted in energy from the parent triphenylene. Similar observations are observed for the 2,7-diarylphenanthrenes as previously predicted [l] on the basis of the relatively high triplet state energy of phenanthrene. ‘These observations demonstrate excited state state interactions in the triplet manifold.
4. Conclusion It is evident that the change in the eIectronic structure of the lowest triplet state resulting Som aryl substitution in the 2- and 7-positions of triphenylene drastically affects the triplet-triplet absorption behavior. AS was the case for the luminescence and ESR
References [l] J.S. Brinen, J. Luminescence 5 (1972) 73. [2] J.S. GAlivanand J.S. Brinen, J. Chem. Phys. 50 (1969) 1950.
71
Volume 15, number
1
CHEMICAL PHYSICS LETTERS
[3 J J.S. %-inen, 3. Chcm. Phys. 49 f1968) 586. [4] J.A. Cade and A. Pilbeam, J. Chem. Sot. (1964) 114. IS] A. Sfandelbauzn and M. &is, J. Org. Chem. 26 (1961) 2633. [6] J.S. Brinen, unpub!ished work.
15 July 1972
[7] J.S. Brinen end h1.K. Orloff, J. Chem. Phys. 51 (1969) 527. 181 LA. Ramsey and 1.H. Munro, in: The triplet state, ed. A.B. Zahlan (Cambrdi@ Univ. Pr%a, London, 1967) p. 415.
15, number
15 July 1972
CHEhIICAL PHYSICS LETTERS
1
T-T’
ABSORPTION
WITH EXTREMELY
HIGH
OF MOLECULES
EXTINCTION
COEFFICIENTS
J.S. BRINEN
ReceiMd 7 April 1972
Extremely intense triplet-triplet absorption spectra have been obserbvd for a series of aryl substituted triphenylenes and phcnanthrcnes. These transitions are a result of escitcd State resonance interactions Thin rhc triplet manifold. Polarization measurements contirm that the linear polyphenyl-like states in these mokcules are responsible for these strong transitions.
1. Introduction
Excited state resonance interactions can markedly change the nature of the lowest trip!et state of organic molecules.
Such resonance
interactions
were observ-
ed when substitution in the Z-position of triphenylene by phenyl, biphenylyl and triphenylyl groups changed the nature
of the lowest triplet
state from that of a per-
turbed triphenylene to that of a linear polyphenyl [ 11. In this paper further consequences of these interactions are presented in terms of changes in the multiplicity allowed transitions between the lowest triplet state and higher lying triplet states. In addition to the aryltriphenylenes, similar effects are reported for aryl substituted phenanthrenes.
2. Experimental Triplet-triplet absorption spectra were obtained at 77°K (in 2-methyltetrahydrofuran) under conditions of steady state excitation using a 200 W Hg arc. The apparatus used fcr these measurements and for the determination of the polarization of these tnnsilions has been previously reported [2] . The simuitan-
eous ESR-optical method was used to obtain values for the extinction coefficients of the triplet--triplet absorption bands [3j. Neutral density scree!ls were used to attenuate the exciting radiation so that accurate determinations of optical densities could be made. Under these conditions low triplet concentrations were obtained and the following procedure was necessary to arrive at an accurate extincticn coefficient for the most intense absorption band. The T-T’ spectrum was measured with sufficient attenuation of the exciting light to obtain an accurate ratio of optical densities for two bands in the spectrum (one being the most intense band). Then the optical density of the weak band was monitored, together with the ESR signal, as a function of increasing excitation intensity. Lineqity between ESR concentration and optical density was obtained. The extinction coefficient of the intense band was then obtzined from this ratio and the extinction coefficient of the weaker band. 2-phenyltriphenylene (lj wasprepared from cyclohexanone and 2-lithiotriphenylene. The intermediate cyclohexanol was refluxed over palladium-on-charcoal to give the desired product. The same procedure was followed to prepare 2<4_biphenylyljtriphenylene (11) using 4-phenylcyclohexanone in place of cyclohexa69
Volume 15,number 1
15 fulr 1972
CHEMICALPHYSICS LETTERS
none. 2, ~‘-bitriphenyien~ (III) was prepared f4] by treating 2 moles of 2-~t~(~t~phenylene xvith CoBr, and n-butylbromide. The method of Mandelbaum and Cais f5] was used to prepare 2,7-diphenyltriph&ylene (IV) in very low yields. The 2,7-di~ylphen~t~r~nes were prepared by the same generaI procedure. ?,7-diiodo9, IO-dihydrophenanthrene ~3s prepared by iodination of the 9,lO. dihydrophenanthrens. The conesponding 2Jdilithio compound was obtained by reaction with n-butylithiurn. The dilithio compound was then refluxed with the appropriate ~ylcyc~o~exaIloR~ to give the corresposding 9, IO-dihydro-2,%phenanthrylenebis(aryicyclohexano1). The latter was then dehydrated and dehydro~enated to give the desired product. The substituted phenanthrenes reported in this study are 2,7diphenylphenanthrene (V); 2,7-di(3-n-hexylphenyi)phena~thr~ne (VI); 2,?-di(4ethylphenyl)phenanthrene (VII) and 2,7-di(4ethyi-l,3*c~clohexadi~ny~)phenanthrene (VIII). All the compounds had satisfactory elemental analyses and were characterized by their ultraviolet, infrared and _NblRspectra.
3. Results and discussion The triplet-triplet absorptiitn spectrum oftriphenylens is characterized by a series of bands with I~ZLXimum intensity at 23200 cm-‘, E = 1.6 X IO4 I/M cm. The oscillator strength (f) for thrs transition is 0.08. T-T’ absorption bands of approximately the same energy and intensity have been Goserved for 2methyl and 2-methoxytriphenyfene 161,In the T-T’ spectrum of 2-phenylt~pllenyiene, the most intense absorption band occurs at 20700 cm-l with E = 1.92 X lo5 l/M cm. The f value for this electronic transition is 1.5. For 2,2’-bitriphenylene (!!I) the T-T’ absorption maximum is shifted further to lower energy with E = 1.80 X 105 l/M cm at 17750 cm-l. The T-T.’ transition observed for II is similar to that observed for III, with e = 1.54 X I@ l]M cm at 18 100 cm-l. A comparison of the observed T-T’ bands of triphenyiene, I and III is shown in fig. I, The T-T’ spectrum ofpterphenyl-d14 has previously been reported f7f. This spectrum is characterized by a strong O-O band with t = 1.29 X 105 I/M cm at 21700 cm- f (f= 0.9), very simiI*q in energy, 70
3f
r
y
r24
:*
22
1
1.8
\ 1
1.G
IO” cm-’
Fig. 1. T-T’ absorption spzctn of - - - triphenylene, pI~enykriphenyione and -2,2-bifriphenylene thyltetrahydrofuran at 77’I;.
2in 2-me-
and shape co the analogous transition observed for 3-phenyltriphenylene. The lower energy transitions observed for II and III occur in the same region as the reported spectrum for ~4uaterphenyI intensity
PI * 2,7-djplzenylrrjpfzenylene
also exhibits T-T’ ab-
sorption in the pquaterphenyl
region. This con~pound
in this study with E = 6.0 X 10” l/M cm at 18 100 mm1 ( identical rvirh the absorption maximum found in II. The E of the O-I band (19600 cm-l) is 0.8 X lUS i/M cm. The high iniensity of these transitions coupled with the relatively favorable intersystem crossing [I 1 in these molecules facilitates the measurement of the polarization of these transitions. Such measurements in these systems are of importance because they serve 10 verify the existence u~~o~y~h~nyi-~ike singtr3t states in the singlet manifold as suggested by ~fandeIbaum and Cais [S] . Polarization measurements were performed by monitoring the O-O band of the T-T’ transition whilt: exciting at 313 nm. For IV, this trirnsition was found to be strongly positively poiarized (-t-3@,), parallel to the direction of the absorption band at has the strongest
bands
observed
Volume 15, number
1
CHEMICAL PHYSICS LETTERS
313 nm. As in the case of p-terphenyl, if the T-T’ band is long axis (polyphenyl axis) polarized, the singlet-singlet transition must also be. Similar intensification is observed in the T-T’ spectra oi the 2,7-diary1 substituted phenanthrenes. The T--T’ absorption spectrum of 2,7-diphenylphenanthrene (V) is similar to that observed for IV. The extinction coefficient for V at the band nxaimum (18000 WI-~) is 2.93 X lo5 l/M cm. Substitution of an Aexyl group in the meta position of each phenyl ring (VI) does not severely alter the spectrum. A slight red shift is observed with e,nas = 2.20 X lo5 l/M cm. ‘The polarization of the T-T’ transition in VI measured at the O-O band with respect to 3 13 nm excitation was found to be +39%. This verifies the existence of polyphenyl-like absorption bands in the singlet-singlet absorption spectrum. The high intensity of the T-T’ transitions observed for these systems may then be attributed to the inphase addition of transition moments which are polarized along the long axis of the molecule as is the case for linear conjugated polyenes. The effect of the degree of’ conjugation in the substituent groups on the 2,7-positions of phenanthrene was also investigated. Instead of phenyl substituents, 4 ethyl- 1,3-cyclohe.uadienyl groups were placed at the 2- and 7-positions (VIII). The maximum intensity of the T-T’ absorption band is at 20800 cm-I (E = 1.14 X 10” l/M cm) while for the corresponding 4-. ethyl phenyl compound (VII) the T-T’ intensity maximum is at 17 500 cnl-l (E = 1.90 X lo5 l/M cm). For phenanthrene [3] itself, the intensity maximum of the T-T’ absorption is at 20400 cm-l with E = 0.42 X lo5 l/M cm. The reduction in conjugation on going from pheny! to cyclohexadienyl substituents results in a decrease in both the energy and the intensity of the T-T’ absorption bands. The triplet-triplet absorption measurements are summarized in table 1.
Triplet-tiplet
I5 July 1972 Table i extinction
cwfficientsa)
Compound
ET-T’ (104 cm-‘)
ET-T’ (lo-’ I/M cm)
2-phenyltriphenylenc
1.80 2.07 2.20
0.24 1.92 0.60
2-biphenylyltripllenylcne
1.81 1.94 sh
1.71 0.57
2,2-hitriphenylene
1.74 1.94 sh
0.57
2.7diphenyltriphenylene
1.81 1.96
6.0 0.8
2,7diphenylphenantIuene
i.80 1.94
0.33
2.14
0.06
1.I0 sh 1.78 1.92 2.03 sh
0.13 2.2 0.35 0.13
2,7ili(4ethylphenyi)phenanthrene
1.75 1.93 2.04
1.90 0.28 0.01
2,7di(4-ethyl1,3cyclokxadienyl)-
2.01
0.75
7.08 2.17
0.50
2,7di(3-n-hexylphcnyl)phenanthrcnc
phenanthccnc a)Thesc measurements\~ere at 77OK.
1.80
2.93
1.14
made in 2-rncthyltctrahydrofur~n
triplet spectra [l] ~ the absorption properties of the lowest triplet state parallel those of the corresponding linear polyphenyIs. Transitions with high extinction coefficients and osciltator strengths are observed which are considerably shifted in energy from the parent triphenylene. Similar observations are observed for the 2,7-diarylphenanthrenes as previously predicted [l] on the basis of the relatively high triplet state energy of phenanthrene. ‘These observations demonstrate excited state state interactions in the triplet manifold.
4. Conclusion It is evident that the change in the eIectronic structure of the lowest triplet state resulting Som aryl substitution in the 2- and 7-positions of triphenylene drastically affects the triplet-triplet absorption behavior. AS was the case for the luminescence and ESR
References [l] J.S. Brinen, J. Luminescence 5 (1972) 73. [2] J.S. GAlivanand J.S. Brinen, J. Chem. Phys. 50 (1969) 1950.
71
Volume 15, number
1
CHEMICAL PHYSICS LETTERS
[3 J J.S. %-inen, 3. Chcm. Phys. 49 f1968) 586. [4] J.A. Cade and A. Pilbeam, J. Chem. Sot. (1964) 114. IS] A. Sfandelbauzn and M. &is, J. Org. Chem. 26 (1961) 2633. [6] J.S. Brinen, unpub!ished work.
15 July 1972
[7] J.S. Brinen end h1.K. Orloff, J. Chem. Phys. 51 (1969) 527. 181 LA. Ramsey and 1.H. Munro, in: The triplet state, ed. A.B. Zahlan (Cambrdi@ Univ. Pr%a, London, 1967) p. 415.