Systematic features of the energy dependence of radiationless processes in large molecules: The substituted naphthalenes

Abstract

This work examines, both experimentally and theoretically, nonradiative decay processes in a series of substituted naphthalenes. We report single vibronic level fluorescence lifetimes and fluorescence excitation spectra of jet-cooled 2-chloronaphthalene, 1- and 2-fluoronaphthalene and 1- and 2-methylnaphthalene over an energy range of about 0–4000 cm−1 in S1. While the 00 nonradiative rates of these molecules vary by a factor of 30, the energy dependences of the nonradiative rates are quite similar. At low vibrational energies the nonradiative rates depend sensitively on the level excited, but in general they increase with energy. As energy increases, the nonradiative rates become less sensitive to the level excited and eventually become almost independent of vibrational energy. We can qualitatively predict this behavior using a thermodynamic formalism which treats the density of states as an intramolecular entropy and avoids the calculation of vibrational coupling terms. In addition we report the fluorescence lifetime of the S1 vibrationless level of 1-chloronaphthalene, and single vibronic level fluorescence spectra of levels up to 1396 cm−1 in 2-chloronaphthalene. Most of the fluorescence excitation and fluorescence spectra show substantial enhancement of the origin inensity and vibrational mode mixing in S1 compared to naphthalene. We briefly discuss these substituent effects and make some tentative assignments of S1 vibrational levels. We also discuss substituent effects on the 00 fluorescence lifetimes. In particular, the order of magnitude increase in the decay rate of 1-chloronaphthalene relative to 2-chloronaphthalene cannot be explained using CNDO/S calculations.

Resonant two-photon mass-analyzed threshold ionization spectroscopy of 1-fluoronaphthalene and 2-fluoronaphthalene

“…These experimental results provide information about (1) excitation energies of the S 1 S 0 electronic transition (E 1 's), (2) adiabatic ionization energies (IEs), (3) active vibrations of these two positional isomers in the electronically excited S 1 and cationic ground D 0 states. Comparison of these new data with those of naphthalene [3,4,12,15] and 1FN and 2FN [6,[9][10][11][12][13][14][15] allows us to gain knowledge about the fluoro substitution effects on the transition energy and molecular vibrations of naphthalene.…”

“…The S 1 S 0 electronic transitions of these molecules in gaseous phases have been studied by using UV absorption spectroscopy [10][11][12]. Goodman, Pratt, and Rice and their coworkers [13][14][15] performed the supersonic jet-cooled laser induced fluorescence (LIF) experiments to investigate various molecular properties, including the allowed and forbidden vibronic transition, rotational constants, S 1 S 0 transition moments, nonradiative lifetime, and fluorescence decay rate, etc. The vertical ionization energies (IEs) of 1FN and 2FN are reported on the basis of the conventional photoelectron experiments [16].…”

“…The excitation LIF [14,15] and ZEKE photoelectron [17,[19][20][21][22][23] spectroscopic techniques which detect photons or electrons with high sensitivity and spectral resolution can be used to record vibronic and cation spectra. In contrast, the resonance-enhanced multi-photon ionization (REMPI) [24,25] and MATI [18,[26][27][28] methods involve detection of ions and provide similar spectroscopic date with mass information.…”

Time-dependent photoionization of azulene: Competition between ionization and relaxation in highly excited states

“…7 With increasing vibrational energy, the internal conversion rate is expected to increase slightly before tailing off at yet higher energy. 38,41 The overall trend of exponentially increasing IC k versus vibrational energy in S 2 has already been observed [7][8][9]41 for an excess vibrational energy greater than 0.24 eV. 2,8 In the framework of Fermi's Golden Rule, the internal conversion rate can be approximated as: …”

“…In Figure 5 we plot the decay rates measured for different vibrational energy in S 2 state, together with lifetime measurements 9 obtained for low vib E and data deduced from quantum yield measurements. 7,8 The fluorescence rate is usually supposed independent of the vibrational mode, 38 such that any variation in the decay time simply stems from a dependency of the IC rate with the excess energy. At the origin of S 2 , the weak coupling between the S 2 -S 1 state 39 can be described by an electronic energy gap law.…”