J . Phys. Chem. 1993, 97, 2491-2493
2491
Dispersed Fluorescence Spectra of Single Molecules of Pentacene in pTerphenyl Paul Tcbhio,+Anne B. Myers,t and W. E. Moerner’ IBM Research Division, AImaden Research Center, 650 Harry Road, K95/801, Son Jose, California 951 20-6099 Received: December 8,1992; In Final Form: January 26, 1993
Dispersed fluorescence spectra of single molecules of pentacene inp-terphenyl a t 1.8 K have been recorded by tuning the excitation laser to single-molecule features a t the long-wavelength edge of the fluorescence excitation spectrum. The main vibrational features in these spectra are similar in frequency and intensity to those obtained from the bulk sample with excitation near the center of the origin band, indicating that the absorption features in the wings of the inhomogeneous distribution probably arise from normal pentacene molecules to within the resolution of this experiment. Prospects for the development of this technique are discussed.
Introduction Recently several groups have demonstrated the feasibility of optically detecting single molecules in low-temperature solids by means of ultrasensitive and ultraselective spectroscopic techniques, chiefly fluorescence excitation.l-1l By tightly focusing a singlemode laser onto a sample of very low chromophore concentration and tuning the excitation source into the wings of the inhomogeneously broadened electronic origin, features in the excitation spectrum due to single absorbers can be identified and studied. Electronic line w i d t h ~ , ~spectral J d i f f u ~ i o n , ~hole * ~ $burning?,’ ~ Stark photon bunchingl3 and antibunching,1° and fluorescence lifetimesI4have all been examined in single molecules and are generating a variety of insights into the different environments occupied by different molecules in a mixed crystal or polymeric matrix a t very low temperatures.15 To date, fluorescence detection of single molecules has involved ‘total” fluorescence, typically observed through a long-pass filter toexcludescattered laser light but otherwise spectrally unresolved. A vibrationally resolved dispersed fluorescence spectrum could provide much more detailed information about the nature of the emitting molecule. The emission spectrum would establish the chemical identity of the emitter and, if obtained with sufficient frequency resolution, its isotopic composition. On a more subtle level, perturbations of the frequencies and/or intensities in the emission spectrum might reveal environmentally induced conformational distortions and their correlation, if any, with shifts in the electronic origin transition frequency. The latter is a particularly interesting issue in view of recent speculations about the role played by conformational distortions in the variable intersystem crossing yields among the four spectroscopically distinct sites in the widely studied pentacenepterphenyl system.l6.l7 A back of the envelope calculation of expected signal levels based on the count rates obtained when detecting total emission indicates that single-molecule dispersed fluorescence spectroscopy at reasonably high resolution should be feasible. Pentacene in p-terphenyl exhibits negligible hole burning at 1.8 K and many molecules (the so-called ‘Class I”) undergo almost no spectral diff~sion,’.~ allowing very long (hours if necessary) data collection times. In this communication we report the first vibrationally resolved emission spectra of single molecules of pentacene in p-terphenyl, demonstrating the feasibility of such measurements even with an optical system that is far from optimized. The shortcomings of Permanent address: Laboratoire A. Cotton, CNRS 11, Universiti Paris 505, 91405 Orsay Cedex, France. Permanent address: Department of Chemistry, University of Rochester, Rochester, N Y 14627-0216. * To whom inquiries should be addressed. +
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the current experimental configuration and the modifications required to improve it are discussed. Finally, we describe what can be learned about the structure and identity of individual absorbers from these experiments, and discuss prospects for the future development of the technique.
Experimental Section The experimental setup was the same as described previously,8 with the following modifications. The fluorescence, after being collected with a paraboloidal reflector and directed out of the cryostat as an approximately collimated beam, was then divided with a 50% beam splitter (Newport BS.1 coating). The reflected light was directed through a long-pass filter (Schott RG630), focused onto a GaAs photomultiplier, and processed with photon counting electronics as in previous experiments.* The transmitted light was focused with a 100-mm focal length lens through an RG630 filter into an ISA HR320 0.32-m single spectrograph equipped with a 600 groove/mm grating blazed at 500 nm and detected with a thinned, back-illuminated, liquid nitrogen cooled CCD detector (Princeton Instruments). The dispersion in the spectral region of interest was approximately 3.1 cm-’/27 pm pixel. This setup allowed simultaneous detection of total and spectrally dispersed fluorescence. As an alignment aid, the LiF plate covering the thin sample crystal was spin-coated with a thin film of perylene in poly(vinylbutyra1). When excited with blue light, this sample produced strong, easily visible blue-green emission, yet it generated essentially no interfering emission at the wavelengths used to excite the pentacene fluorescence. The procedure employed to obtain single-molecule dispersed emission spectra was as follows. First, the sample (thickness on the order of a few microns) was mounted and cooled to liquid He temperature, a HeCd laser at 441 nm was focused onto the sample, and the blue-green perylene emission was used to roughly align the collecting optics by eye. The light source was then switched to the single-mode dye laser, tuned to the peak of the pentacene 0, origin transition at 5923.2 A,and thecollectingoptics adjusted to maximize both the total fluorescence and the dispersed fluorescence signals. With 1 pW of laser power (focused spot size 9 f 3 pm) near the line center, the dispersed fluorescence was easily observed in ‘real time” (
