Dynamics of Excitation Energy Transfer from Biphenylylene Excimers

Jun 20, 2013 - Dynamics of Excitation Energy Transfer from Biphenylylene Excimers in Pore Walls of Periodic Mesoporous Organosilica to Coumarin 1 in t...
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Dynamics of Excitation Energy Transfer from Biphenylylene Excimers in Pore Walls of Periodic Mesoporous Organosilica to Coumarin 1 in the Mesochannels Ken-ichi Yamanaka,†,§ Tadashi Okada,*,‡ Yasutomo Goto,†,§ Masamichi Ikai,†,§ Takao Tani,†,§ and Shinji Inagaki*,†,§ †

Toyota Central R&D Labs., Inc., and ‡Toyota Physical & Chemical Research Institute, Nagakute, Aichi 480-1192, Japan § Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: To understand the energy transfer dynamics of biphenylylene (Bp)-bridged periodic mesoporous organosilica (PMO) powder doped with coumarin 1, picosecond timeresolved fluorescence spectroscopic studies were carried out. The time-resolved fluorescence spectra after excitation of the Bp moieties in the pore walls revealed rapid formation of Bp excimers, followed by transfer of their excitation energy to coumarin 1 placed in the mesochannels. The fluorescence decay curves were analyzed by Monte Carlo simulation on the basis of the Bp excimer distribution and the position of coumarin 1 in the mesochannels of Bp-PMO. The analytical results suggest that Förster-type energy transfer occurs from the three types of Bp excimers to coumarin 1 located in the vicinity of the pore surface, more precisely at the hydrophilic silica layers on the pore walls, while some of the Bp excimers are not affected by any acceptors.

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olate)aluminum(III) (Alq3) amorphous films (0.23 mol % for η = 51%).23 The fluorescence properties were analyzed by Förster-type energy transfer mechanism, and the critical distance (R0) was calculated to be 3.2 nm for the coumarin 1/Bp-PMO system using the fluorescence quantum yield (ΦF) of 0.45 for Bp-PMO and the overlap integral of the fluorescence spectrum of Bp-PMO and UV/vis spectrum of coumarin 1.15 It is also suggested that other mechanisms may contribute to the efficient energy transfer in addition to Förstertype energy transfer particularly for the powder sample; however detailed mechanisms are still unclear. Recently, we reported the excited-state dynamics of a BpPMO powder,24 including (i) the relaxation of the twisted Franck−Condon (FC) state of the Bp group to the lowest singlet excited state (S1) with a time constant of 730 ± 95 fs; (ii) the efficient quenching of the S1 state by the formation of three types of excimers, of which the fluorescence band maxima are 345 nm (E1), 365 nm (E2), and 385 nm (E3), and the rise times are 7.0 ± 0.2 ps (E1: ca. 64%) and 170 ± 47 ps (E2 and E3: ca. 36%); (iii) the decay of these excimers with different time constants of 1.3 ± 0.2 ns (E1), 8.2 ± 0.7 ns (E2), and 27 ± 2 ns (E3); (iv) the estimation of the fluorescence quantum

INTRODUCTION Periodic mesoporous organosilicas (PMOs), synthesized from 100% or less organic-bridged alkoxysilane precursors,1,2 are a new class of organic−inorganic hybrid materials with welldefined mesopores3−5 and high functionalities attributed to the organic moieties in the frameworks.6−9 Recent developments have enabled PMOs to exhibit unique photofunctionalities, such as efficient fluorescence,10−14 light harvesting,15−18 photoinduced electron transfer,19,20 and photoinduced hole transportation.21 The light harvesting antenna property of PMOs has particularly opened their potential to solid-state photoreaction systems that mimic natural photosynthesis. In our first report on the light harvesting of PMOs, we demonstrated that highly efficient energy transfer from biphenylylene (Bp) in the PMO framework to coumarin 1 doped in the mesochannels (Figure 1a).15 Bp-PMO has a crystal-like pore wall structure in which hydrophobic Bp and hydrophilic silica belt-like layers are arranged alternately in the channel direction with an interval of 1.19 nm. The doping amount of coumarin 1 (coumarin 1/Bp ratio) achieving the energy-transfer efficiency (η) of 50% are 0.044 and 0.17 mol % for the Bp-PMO powder and film, which are relatively low compared to those for reported previously for solid-state energy transfer systems, such as dye-doped mesoporous silica containing lanthanide complexes in the framework (6.7 mol % for η = 29.5%)22 and pentacene-doped tris(quinolin-8© 2013 American Chemical Society

Received: May 13, 2013 Revised: June 19, 2013 Published: June 20, 2013 14865

dx.doi.org/10.1021/jp404691c | J. Phys. Chem. C 2013, 117, 14865−14871

The Journal of Physical Chemistry C

Article

Figure 1. (a) Schematic model of Bp-PMO and (b) reaction scheme and lifetimes of excited states for Bp-PMO.

h before filtration, and the recovered powder was then air-dried. The amounts of coumarin 1 doped in Bp-PMO were determined from coumarin 1 concentrations in the filtrates, as determined by UV/vis absorption spectroscopy (Jasco, V670) using an appropriate calibration curve; 0.04, 0.10, 0.17, and 0.36 mol % concentrations of coumarin 1 were obtained in Bp. Nondoped Bp-PMO was also prepared by the same procedure, but without the addition of coumarin 1 in the starting suspension, in order to eliminate the effects of the template surfactant in the mesochannels on the optical and photophysical properties. Measurements and Analysis. Fluorescence decay curves were measured using a time-correlated single photon counting (TCSPC) method (Becker & Hickl, SPC-730 TCSPC Module). The repetition rate (80 MHz) of the mode-locked Ti:sapphire oscillator (Coherent, Vitesse) was reduced to 2 MHz by a pulse-picker (Coherent, Pulse Picker 9200). The output was converted to the third harmonic generation (THG) (266 nm) using a 0.5 mm thick β barium borate (BBO) crystal and a dispersion compensator. The THG pulse ( 50% at 0.05 mol % and more than 90% at 0.5 mol %, the highest efficiency for a solid-state energy transfer system to our present knowledge.15 These findings are expected to be useful for the development of solid state photofunctional materials and their respective devices.

ASSOCIATED CONTENT

* Supporting Information S

Fluorescence decay curves and plots of fitting parameters for nondoped Bp-PMO, fluorescence spectra of each excimer and absorption spectrum of coumarin 1, histograms of the distribution of the distance between the Bp excimer and coumarin 1, and residual sum of squares of the fitting results using eq 6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge Prof. H. Miyasaka (Osaka University) and Dr. S. Shirai (Toyota CRDL) for the useful discussions. This work was partially supported by a Grant-inAid for Scientific Research on Innovative Areas (No. 2406) from the Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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dx.doi.org/10.1021/jp404691c | J. Phys. Chem. C 2013, 117, 14865−14871