New Two-Photon Absorbing BODIPY-Based Fluorescent Probe: Linear Photophysics, Stimulated Emission, and Ultrafast Spectroscopy Binglin Sui, † Mykhailo V. Bondar, ‡ Dane Anderson, §,∥ Hector J. Rivera-Jacquez, §,∥ Artë m E. Masunov, §,∥,⊥ and Kevin D. Belfield* ,†,# † College of Science and Liberal Arts, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102, United States ‡ Institute of Physics, Prospect Nauki, 46, Kiev-28, 03028, Ukraine § Department of Chemistry, University of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366, United States ∥ NanoScienece Technology Center and Department of Physics, University of Central Florida, Orlando, Florida 32816, United States ⊥ Photochemistry Center RAS, ul. Novatorov 7a, Moscow, 119421, Russia # School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, 710062, P.R. China ABSTRACT: The synthesis and comprehensive linear spec- troscopic and nonlinear optical properties of a new BODIPY- based fluorene-containing derivative 4,4-difluoro-8-(4-{2-[2- (2-hydroxyethoxy)ethoxy]ethoxy}phenyl)-1,3,5,7-tetramethyl- 2,6-di[(9,9-di{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-9H-fluo- ren-2-yl)ethynyl]-4-bora-3a,4a-diaza-s-indacene (1) are re- ported. The steady-state absorption, emission, and excitation anisotropy spectra along with fluorescence lifetimes of 1 were analyzed in organic solvents of different polarity. The degenerate two-photon absorption (2PA) spectrum of 1 was obtained over a broad spectral range by the open aperture Z- scan method using 1 kHz femtosecond excitation, and the 2PA cross section had a maximum value of ∼400 GM. The one- photon stimulated emission depletion spectrum of 1 was determined by a fluorescence quenching method with values of stimulated emission cross sections close to the corresponding ones of the ground state linear absorption. The nature of ultrafast relaxation processes in 1 was analyzed by a transient absorption femtosecond pump−probe technique, and the characteristic times of intramolecular relaxations between the excited electronic states (<150 fs) and solvation dynamics (4−6 ps) were determined. Efficient superluminescence emission of 1 was observed in solution under one- and two-photon femtosecond pumping. Detailed analysis of the singlet excitations in 1 was performed by a time-dependent density functional theory (TD- DFT) method. Three one-photon and two two-photon absorbing states were predicted in the wavelength range investigated. A reactive handle was included in the meso-position of the BODIPY chromophore to facilitate future bioconjugation or functionalization for bioimaging applications. 1. INTRODUCTION Dipyrromethene boron difluoride (BODIPY) derivatives are a well-studied class of chromophores with high fluorescence quantum yields 1−3 and exceptional photostability. 4−7 They are widely used in a large number of applications, including numerous sensing technologies, 8−14 photodynamic ther- apy, 15−17 lasing, 18−21 optical power limiting, 22,23 and one- and two-photon fluorescence bioimaging, 24−27 to name a few. One application of BODIPY dyes is in two-photon fluorescence microscopy (2PFM), a powerful tool in biomedical research that allows noninvasive visualization of biological tissues with high spatial resolution. 28−30 In this content, the synthesis and investigation of new BODIPY-based two-photon fluorescence probes attracts great interest due to their versatile linear and nonlinear optical properties and well-developed derivatization strategies. 2,3,31−34 A comprehensive analysis of the structure−property relation- ships of the manifold BODIPY derivatives revealed the specific electronic and steric effects 2,23,35−40 that can dramatically modify their linear photophysical and photochemical proper- ties, including the spectral position of the main absorption bands, 24,37 fluorescence quantum yields, 38,39 and photostabil- ity, 4, 5, 40 that are of great importance for bioimaging applications. The nature of fast relaxations in the ground and excited electronic states was investigated for BODIPYs with Received: May 2, 2016 Revised: June 6, 2016 Published: June 22, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 14317 DOI: 10.1021/acs.jpcc.6b04426 J. Phys. Chem. C 2016, 120, 14317−14329