9270
J. Phys. Chem. 1993,97, 9270-9273
Intramolecular Triplet Energy Transfer. 3. A Carbazole-Naphthalene System Having Short Chain Length Methylene Spacer Units Gregory W. Haggquistt Hideaki Katayama, Akira Tsuchida, Shinzaburo Ito, and Masahide Yamamoto’ Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Sakyo- ku, Kyoto 606, Japan Received: March 10, 1993; In Final Form: June 3, 1993@
Laser flash photolysis was used to measure rates of triplet energy transfer from carbazole (Cz) to naphthalene (Np) in rigid solutions of C Z - ( C H ~ ) ~ - Nwhere ~ , n = 4,5,6, or 8. The results are interpreted in terms of Dexter’s equation and a model which allows for a distribution of donor-acceptor distances for a given n. The distribution was calculated using an arithmetic mean of a center-to-center and closest edge distance between the donor and acceptor chromophores. This model provided the transfer rate for all the compounds tested, and the results indicated a through-space mechanism.
I. Introduction Triplet-triplet (T-T) energy transfer has been observed in a wide variety of industrial, photochemical, and photobiological systems.’ Dexter2 modeled the transfer rate using an electronexchangemechanism, which requires overlapof theelectron clouds for the triplet donor (D) and acceptor (A) chromophores, and assumed that the transfer rate decreases exponentially as the distance between the D and A increases. Dexter’s mechanism has been studied thoroughly using model compounds of the type D-~pacer-A.~ The nature of this spacer unit can range from flexible methylenechains to rigid cyclohexane rings. Two main mechanisms, involving transfer through-bond or through-space, have been adopted depending on the type of spacer units used. Closs et al.4 and Speiser et al.5 have shown that for compounds with rigid spacer units the triplet energy transfer rate can be modeled in terms of a through-bond mechanism. Closs et a1.4stated that the transfer rateisdependent not only on the separation distance but also on the orientation of D and A. Further, they found that the transfer rate drops by 1 order of magnitude for each bond that separates the D and A. Recently, other labs have shown that a through-space mechanism can represent the triplet energy transfer for long-distance transfers.697 Wagner reports that triplet energy transfer proceeds predominantly through-space rather than through-bond for compounds with flexible spacer units exceeding five atoms.* In this lab, the focus of intramolecular triplet energy transfer has been placed on flexible compounds. A recent studyg on compoundscontainingcarbazole (Cz) as D and naphthalene (Np) as A has shown that the transfer rate can be modeled by a throughspace mechanism when the chromophores are connected by methylene chains of 8,9, 10, or 12 methylenes. This model was based on a determination by computational means of the distribution of distances measured between the centers of D and A. By incorporating this distribution into Dexter’s equation, we have been able to simulate the observations. The transfer rate was monitored by the decay of the phosphorescenceof D. Similar experiments have also been performed with other D-A pairs;3 all these results led to the conclusion that a through-space mechanism accounted for the transfer rates in compounds with flexible spacer units. The purpose of this investigation was to apply the same treatment from our previous work to compounds with shorter spacer units. In the present study, we investigated,by nanosecond f Present address: Department of Physics, University of Trondheim,N-7055 Dragvoll, Norway. *Abstract published in Advance ACS Abstructs, August 15, 1993.
0022-3654/93/2097-9270$04.00/0
laser photolysis, a series of Cz-(CHz),-Np pairs separated by shorter methylenechains where n = 4,5,6, or 8. Dexter’s equation was again applied as the model neglecting through-bond coupling between D and A. Computational means yielded a distribution of distances, and the transient absorbance decay of the Cz triplet was used to determine the transfer rate.
II. Experimental Methods (A) Materials. A series of polymethylene compounds having a carbazole group and naphthalene group as chain terminals (Cz( C H Z ) ~ - Nwere ~ ) synthesized by the Grignard reaction of 94wbromoalky1)carbazole with 2-(bromomethy1)naphthalene or 2-bre monaphthalene. To a mixture of sodium hydride and dry THF was added dropwise a solution of carbazole in THF, and the mixture was refluxed at 50 OC for 2 h. After the mixture was cooled to room temperature, a,w-dibromoalkane was added dropwiseand refluxed at 70 OC for 2 h. The product was extracted with ether and washed twice with water and with aqueous saturated sodium chloride. The product mixture was purified by column chromatography on silica gel and eluted with a mixed solvent of hexane and benzene (2: 1) to give 9-(w-bromoalkyl)carbazole. Cz-(CHz)rNp. A solution of 9-(3-bromopropyl)carbazole (2 g) in ether was added to a mixture of magnesium (0.2 g) and dry ether (5 mL). To the solution mixture, 2-(bromomethy1)naphthalene (2 g) and silver bromide (2 g) were added. The mixture was stirred at room temperature for 1 h and then refluxed for 2 h. Then aqueous ammonium chloride was added. The product was extracted with ether and washed with water and with aqueous saturated sodium chloride. Then it was dried over sodium sulfate. After evaporation of the solvent, the product was purified by column chromatography on silica gel eluted with a mixed solvent of hexane and benzene (3:l). Cz-( C H z ) r N p and Cz-( C H 2 ) r N p . These were synthesized and purified by the same method as that used for Cz-(CH&Np. 9-(4-Bromobutyl)carbzoleand9-(5-bromopentyl)car~zole were used instead of 9-(3-bromopropyl)carbazole,respectively. Cz-(CHz)gNp. This was synthesizedfrom 9-(8-bromooctyl)carbazole and 2-bromonaphthalene. Details of the synthetic method were described previously.5 The obtained products were identified by IR and NMR spectra. The spectra for all the obtained products showed the same characteristic spectra. The data were as follows: IR (KBr) 2920,2850,1324,854,8 10,746, and 720 cm-1; NMR (CDC13) 6 1.4-2.1 (m, methylene H), 2.89 (t, methyleneH),4.3 (t, methyleneH), and 7.0-8.1 (m, aromatic H) * 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9271
Intramolecular Triplet Energy Transfer
(B) Sample Preparation. The 2-methyltetrahydrofuran (MTHF) was refluxed and distilled from CaH2just prior to data collection. The samples were dissolved in 4 mL of MTHF and had an absorbance of 0.1-0.4 at 351 nm. Low concentrations (1.2 X 10-4 M) were used so as to prevent intermolecular interactions. A 1-cm quartz cuvette was used as the sample cell. (C) Measurements. Transient absorbance decays were collected using a cross beam alignment. A Lambda Physik EMGlOlMSC excimer laser was used as the excitation source. The excitation wavelength was 351 nm (XeF). Pulse energies were generally kept at 5 mJ/cm2 by using glass and wire attenuation filters. The interrogation source was an Ushio 150 W Xe lamp which was pulsed by a homemade high-voltage pulser. The interrogation source was collimated and passed through the front edge of the sample. A 0.5-mm aperture was used on the detection side of the sample. The sample was placed in a Pyrex Dewar cell and immersed in liquid nitrogen. The monochromator, Ritsu Oyo Kogaku MC-10 N, was placed 30 cm from the sample in order to reduce the collection of the emission from the sample. A Hamamatsu R1477 photomultiplier tube, with a bleeder chain wired so as to recover from an intense burst of prompt fluorescence,Iowas placed at the exit slit of the monochromator. The data were collected on a Hewlett Packard 54510A digital oscilloscope (250 MHz) and transferred to a personal computer. Curve fitting was performed also on a personal computer using a Marquart-Levenburg algorithm. The transient absorption spectra were collected using the same excitation and interrogation source as in the kinetic measurements. The interrogation beam was collected by an optical fiber and directed into an optical multichannel analyzer (Unisoku USP500).
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Separation Distance /A Figure 1. Distribution function of Ri for the Cz-(CH&-Np com unds. The distance between successive marks on the abscissa is 0.2
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111. Computations
Empirical potential energy calculations were used to determine the separation between the Cz and Np chromophores. The probability of each calculated distance was also determined. A detailed explanation of these calculations has already been published, and only the main deviations from those calculations will be explained." Earlier? Ri, the intermolecular distance, was defined as the distance between the centers of D and A. The computation considered the van der Waals interaction energy, calculated by using the Lennard-Jones potential, between neighboring nonbonded atoms and the intrinsic torsional potential energy of the rotation about the C-C bonds. In this report, we calculated two sets of distances and probabilities and then combined the results by giving equal weight to each of the calculated sets. The first set of calculations was done using a center-to-center distance, while the second set was calculated by defining Ri as the distance between the closest atoms of D and A, to be referred to as the edge-teedge separation. Plots of the resultant distribution functions are shown in Figure 1. Dexter's equation was used to model the T-T energy transfer rate, and for each Ri there is a corresponding T-T energy transfer rate (ki)
where the symbols L and ko are the effective average Bohr radius and a constant, respectively. When Ri is governed by a distribution, the time-dependent absorbance of the donor triplets, a(t),decays according to the equation n
a(t) = a ( O ) ~exp[-(ki ,
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where pi is the probability of a separation distance equal to Ri,
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Wavelength /nm Figure 2. Transient absorption spectra in MTHF at 77 K with a delay time of 300 ns for (A) N-ethylcarbazole and (B) Cz-(CH2)4-Np.
kO = l / ~ and , TO is the natural lifetime of D in the absence of A. For the system studied in this paper TO-^
