Article pubs.acs.org/JPCC
Dynamics of Intramolecular Excited State Proton Transfer in Emission Tunable, Highly Luminescent Imidazole Derivatives Adina I. Ciuciu,† Kamil Skonieczny,‡ Dominik Koszelewski,‡ Daniel T. Gryko,‡,§ and Lucia Flamigni*,† †
Istituto per la Sintesi Organica e Fotoreattivita’ (ISOF), CNR, Via P. Gobetti 101, 40129 Bologna. Italy Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland § Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland ‡
S Supporting Information *
ABSTRACT: The enol−keto excited state dynamics of a series of emission tunable imidazole derivatives undergoing excited state intramolecular proton transfer (ESIPT) were determined by means of steady state and time-resolved spectroscopic techniques in different solvents at room temperature and at 77 K. Examination of the corresponding nonESIPT compounds, with the proton transfer function deliberately blocked, was carried out for comparison. At room temperature, the ESIPT process in the examined samples, determined by picosecond streak camera experiments, had lifetimes ranging from less than 10 ps to ca. 100 ps, and the resulting keto forms deactivated with lifetimes less than 100 ps up to a few nanoseconds. Delayed luminescence detection at 77 K in solid glasses allowed the identification of the phosphorescence of the enolic form and, in a few cases, P-type delayed fluorescence was also seen. The phosphorescence lifetimes were in the range of seconds at 77 K. The enolic triplet excited state absorption at RT, determined by nanosecond laser flash-photolysis, displayed a maximum around 460−500 nm and lifetimes on the order of tens of microseconds. In a few cases, a broad band with a maximum around 420 nm was detected and tentatively ascribed to the triplet excited state of the keto form. Reaction rates with oxygen on the order of (2− 4) × 109 M−1 s−1 were measured.
■
INTRODUCTION Excited state intramolecular proton transfer (ESIPT) is a phototautomerization process that occurs in the electronic excited state of a molecule, where a heterocyclic ring is formed by an intramolecular hydrogen bond between a proton donor group (typically an OH or NH group) and a neighboring proton acceptor. This is a four-level photocycle process involving enol (E) to keto (K) phototautomerization (E → E* → K* →K → E). ESIPT is a complex process and, in broad terms, its efficiency is affected by internal (e.g., substitution pattern) or external (e.g., solvent) parameters.1,2 The characteristics of this process are a large Stokes-shifted luminescence from the keto form and great solvent sensitivity, depending on the proticity of the medium. ESIPT has emerged in recent years as a phenomenon which can be exploited, because of the above characteristics, for interesting applications spanning from laser and imaging materials3 to the most diverse luminescent probes4 and innovative optoelectronic materials for applications in OLED technology.5 For all these applications, high fluorescence quantum yield is a key property. Among various ESIPT compounds, tetraphenylimidazoles have recently attracted significant interest.6 Ingenious work by Park and co-workers6a showed that this scaffold, with the proper choice of substituents, can lead to white-light emitting compounds by the combination of excited-state intramolecular © 2013 American Chemical Society
proton transfer and restricted energy transfer. We have recently reported the synthesis, basic optical spectroscopic properties and two-photon absorption characteristics of a series of new imidazole derivatives and analogues displaying, in most cases, ESIPT properties.7 The luminescence property of each dye was compared to that of the corresponding, non-ESIPT reference compound, in which proton transfer was deliberately blocked. The spectroscopic properties of the new samples compare quite well with those of similar imidazole-based ESIPT compounds.8 In this report, we intend to explore in detail the dynamics of the ESIPT process in the most interesting among these compounds, with a special focus given to phenanthro[9,10d]imidazoles and imidazo[4,5-f ][1,10]phenanthrolines (Figure 1), taking advantage of time-resolved spectroscopic techniques. Both luminescence and transient absorption in different time domains were employed for the experiments performed in solvents of different dielectric constant and proticity, namely toluene (TOL), dichloromethane (DCM), and methanol (MeOH). The use of solvents with different dielectric constants and different hydrogen bonding ability is justified by the effect Received: September 28, 2012 Revised: December 19, 2012 Published: January 8, 2013 791
dx.doi.org/10.1021/jp3096538 | J. Phys. Chem. C 2013, 117, 791−803
The Journal of Physical Chemistry C
Article
see ref 7. Determinations at 77 K on glassy solutions made use of capillary quartz tubes dipped in a homemade quartz Dewar filled with liquid nitrogen. Fluorescence lifetimes in the nanosecond region were detected by a time correlated single photon counting apparatus (IBH) with excitation at 331 nm. For picosecond time-resolved luminescence, a Hamamatsu C1587 Streak Camera equipped with a fast unit M1952 (2 ps resolution) was used after excitation with an Nd:YAG laser at 355 nm (Continuum PY62/10, 35 ps pulse, energy
